U.S. patent application number 13/456854 was filed with the patent office on 2013-10-31 for purification of carbon dioxide.
This patent application is currently assigned to AIR PRODUCTS AND CHEMICALS, INC.. The applicant listed for this patent is Paul Higginbotham, John Eugene Palamara. Invention is credited to Paul Higginbotham, John Eugene Palamara.
Application Number | 20130283851 13/456854 |
Document ID | / |
Family ID | 48190198 |
Filed Date | 2013-10-31 |
United States Patent
Application |
20130283851 |
Kind Code |
A1 |
Higginbotham; Paul ; et
al. |
October 31, 2013 |
Purification of Carbon Dioxide
Abstract
Impurities that are less volatile than carbon dioxide, e.g.
hydrogen sulphide, are removed from crude carbon dioxide by
processes involving sub-ambient distillation of said crude carbon
dioxide in a distillation column system operating at
super-atmospheric pressure(s) to produce carbon dioxide-enriched
overhead vapour and bottoms liquid enriched with said impurities.
Where such processes involve at least one heat pump cycle,
significant savings in power consumption are realised when the
process uses more than one recycle pressure in the heat pump
cycle(s).
Inventors: |
Higginbotham; Paul; (Surrey,
GB) ; Palamara; John Eugene; (Macungie, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Higginbotham; Paul
Palamara; John Eugene |
Surrey
Macungie |
PA |
GB
US |
|
|
Assignee: |
AIR PRODUCTS AND CHEMICALS,
INC.
Allentown
PA
|
Family ID: |
48190198 |
Appl. No.: |
13/456854 |
Filed: |
April 26, 2012 |
Current U.S.
Class: |
62/606 |
Current CPC
Class: |
B01D 2257/304 20130101;
F25J 2200/30 20130101; F25J 2230/80 20130101; Y02C 20/40 20200801;
Y02P 20/152 20151101; F25J 2200/02 20130101; F25J 2200/04 20130101;
F25J 2210/06 20130101; F25J 2200/74 20130101; F25J 2200/76
20130101; F25J 2200/78 20130101; F25J 2290/40 20130101; B01D 53/002
20130101; F25J 2235/02 20130101; F25J 2235/80 20130101; F25J
2260/20 20130101; F25J 3/08 20130101; Y02C 10/12 20130101; B01D
2256/22 20130101; F25J 2200/40 20130101; C01B 17/167 20130101; F25J
2240/30 20130101; F25J 2260/80 20130101; F25J 2245/02 20130101;
F25J 2200/50 20130101; F25J 2270/02 20130101; Y02P 20/129 20151101;
F25J 2220/84 20130101; F25J 2240/02 20130101; F25J 2270/80
20130101; F25J 2270/88 20130101; Y02P 20/151 20151101; F25J 3/0266
20130101; F25J 2215/80 20130101 |
Class at
Publication: |
62/606 |
International
Class: |
F25J 3/02 20060101
F25J003/02 |
Claims
1. A process for purifying crude carbon dioxide comprising at least
one impurity that is less volatile than carbon dioxide, said
process comprising: feeding crude carbon dioxide feed at
sub-ambient temperature to a distillation column system operating
at super-atmospheric pressure(s) for separation to produce carbon
dioxide-enriched overhead vapour and bottoms liquid enriched with
said at least one impurity; providing carbon dioxide-enriched
liquid as reflux for said distillation column system; at least
partially re-boiling a portion of said bottoms liquid by indirect
heat exchange to provide vapour for said distillation column
system; removing carbon dioxide-enriched overhead vapour from said
distillation column system; and removing a further portion of said
bottoms liquid, or a liquid derived from bottoms liquid, from said
distillation column system, wherein re-boiling duty for said
distillation column system is provided at least in part by indirect
heat exchange against recycle fluids from at least one heat pump
cycle using a carbon dioxide-containing fluid from said
distillation column system as working fluid, at least one of said
recycle fluids having a different pressure from the other recycle
fluid(s).
2. The process of claim 1 comprising a single heat pump cycle
comprising a least a first recycle fluid and a second recycle
fluid, said second recycle fluid having a pressure that is greater
than that of said first recycle fluid.
3. The process of claim 2, wherein the pressure of said second
recycle fluid is at least 10% greater than the pressure of said
first recycle fluid.
4. The process of claim 2, wherein the pressure of said first
recycle fluid is from about 15 bar to about 30 bar.
5. The process of claim 2, wherein the pressure of said second
recycle fluid is from about 20 bar to about 70 bar.
6. The process of claim 2, wherein said working fluid comprises
carbon dioxide-enriched gas generated by warming said carbon
dioxide-enriched overhead vapour by indirect heat exchange.
7. The process of claim 6, wherein at least a portion of the duty
required to warm said carbon dioxide-enriched overhead vapour is
provided by indirect heat exchange against at least one of said
recycle streams.
8. The process of claim 2, wherein said recycle fluids are recycled
to an appropriate location in said distillation column system after
suitable pressure reduction.
9. The process of claim 2, wherein ratio of molar flow of said
first recycle fluid to said second recycle fluid is from about 0.1
(i.e. 1:10) to about 10 (i.e. 10:1).
10. The process of claim 9, wherein said ratio is from about 3
(i.e. 3:1) to about 10 (i.e. 10:1).
11. The process of claim 2, wherein said working fluid comprises
crude carbon dioxide gas generated by evaporating "intermediate"
liquid taken from an intermediate location in said distillation
column system by indirect heat exchange after suitable pressure
reduction.
12. The process of claim 11, comprising at least partially
condensing carbon dioxide-enriched overhead vapour by indirect heat
exchange to produce at least partially condensed carbon
dioxide-enriched overhead vapour as said reflux for said
distillation column system.
13. The process of claim 12, wherein at least a portion of the duty
required to at least partially condense said carbon
dioxide-enriched overhead vapour is provided by indirect heat
exchange against at least one "cold" process stream.
14. The process of claim 11, wherein at least a portion of the duty
required to evaporate said "intermediate" liquid is provided by
indirect heat exchange against at least one of said recycle
fluids.
15. The process of claim 11, wherein said "intermediate" liquid has
a composition that is at least substantially identical to said
crude carbon dioxide feed.
16. The process of claim 11, wherein said first recycle fluid is
recycled as part of said feed to said distillation column
system.
17. The process of claim 11, wherein said second recycle fluid is
recycled as part of said working fluid for said heat pump cycle
after suitable pressure reduction.
18. The process of claim 1 comprising at least a first heat pump
cycle and a second heat pump cycle, each heat pump cycle comprising
at least one recycle fluid, said recycle fluid of said first heat
pump cycle or, where the first heat pump cycle has more than one
recycle fluid, at least one of said recycle fluids, having a
pressure that is greater than that of a recycle fluid of said
second heat pump cycle.
19. The process of claim 18, wherein the pressure of said recycle
fluid of said first heat pump cycle is at least 10% greater than
the pressure of said recycle fluid of said second heat pump
cycle.
20. The process of claim 18, wherein the pressure of said recycle
fluid of said first heat pump cycle is from about 15 bar to about
60 bar.
21. The process of claim 18, wherein said working fluid of said
first heat pump cycle comprises carbon dioxide-enriched gas
generated by warming said carbon dioxide-enriched overhead vapour
by indirect heat exchange.
22. The process of claim 21, wherein at least a portion of the duty
required to warm said carbon dioxide-enriched overhead vapour is
provided by indirect heat exchange against at least one of said
recycle streams.
23. The process of claim 18, wherein the pressure of said recycle
fluid of said second heat pump cycle is from about 10 bar to about
25 bar.
24. The process of claim 18, wherein said working fluid of said
second heat pump cycle comprises crude carbon dioxide gas generated
by warming "intermediate" vapour taken from an intermediate
location of said distillation column system by indirect heat
exchange.
25. The process of claim 24, wherein at least a portion of the duty
required to warm said "intermediate" vapour is provided by indirect
heat exchange against at least one of said recycle fluids.
26. The process of claim 24, wherein said crude carbon dioxide gas
has a composition that is at least substantially identical to said
crude carbon dioxide feed.
27. The process of claim 18 wherein said recycle streams are
recycled to appropriate locations in said distillation column
system after suitable pressure reduction if required.
28. The process of claim 1, wherein liquid from an intermediate
location in said distillation column system is at least partially
re-boiled by indirect heat exchange to provide additional vapour
for said distillation column system.
29. The process of claim 28, wherein at least a portion of the
re-boiling duty is provided by indirect heat exchange against at
least one of said recycle fluids.
30. The process of claim 1, wherein a portion of said working fluid
is purged from said process.
31. The process of claim 1, wherein said reflux for said
distillation column system is provided by at least one recycle
fluid condensate after suitable pressure reduction.
32. The process of claim 31, wherein the refrigeration duty
required to cool and at least partially condense at least one
recycle fluid is provided by indirect heat exchange against at
least one "cold" process stream.
33. The process of claim 1, wherein said reflux for said
distillation column system is provided by condensed overhead
vapour.
34. The process of claim 33, wherein the refrigeration duty
required to cool and at least partially condense overhead vapour is
provided by indirect heat exchange against at least one "cold"
process stream.
35. The process of claim 1, wherein said crude carbon dioxide feed
is crude carbon dioxide fluid derived from a natural source of
carbon dioxide and expanded prior to feeding to said distillation
column system.
36. The process of claim 35, wherein, prior to said expansion, said
crude carbon dioxide fluid is at a super-critical pressure and a
sub-critical temperature.
37. The process of claim 35, wherein said crude carbon dioxide
fluid is cooled by indirect heat exchange prior to expansion.
38. The process of claim 37, wherein at least a portion of the duty
required to cool said crude carbon dioxide fluid is provided by
indirect heat exchange against at least one "cold" process
stream.
39. The process of claim 35, wherein said expanded crude carbon
dioxide is used as a "cold" process stream to provide refrigeration
duty for said process.
40. The process of claim 35, wherein said expanded crude carbon
dioxide fluid is fed directly to said distillation column
system.
41. The process of claim 1, wherein said feed is derived from
supercritical crude carbon dioxide fluid and carbon
dioxide-enriched liquid is produced as a product.
42. The process of claim 41, wherein said carbon dioxide-enriched
liquid is removed from said distillation column system, pumped and
warmed by indirect heat exchange to produce warmed carbon
dioxide-enriched liquid as said product.
43. The process of claim 42, wherein at least a portion of the duty
required to warm said pumped carbon dioxide-enriched liquid is
provided by indirect heat exchange against at least one of said
recycle fluids.
44. The process of claim 42, wherein said pumped carbon
dioxide-enriched liquid is used as a "cold" process stream to
provide refrigeration duty for the process.
45. The process of claim 1, wherein said feed is derived from crude
carbon dioxide vapour and carbon dioxide-enriched gas is produced
as a product.
46. The process of claim 45, wherein a portion of said carbon
dioxide-enriched vapour is warmed by indirect heat exchange to
produce said carbon dioxide-enriched gas.
47. The process of claim 46, wherein at least a portion of the duty
required to warm said carbon dioxide-enriched overhead vapour is
provided by indirect heat exchange against at least one of said
recycle fluids.
48. The process of claim 46, wherein said carbon dioxide-enriched
overhead vapour is used as a "cold" process stream to provide
refrigeration duty for the process.
49. The process of claim 1, wherein said further portion of bottoms
liquid, or said liquid derived from bottoms liquid, is pumped and
warmed by indirect heat exchange to provide impurity-rich waste
liquid.
50. The process of claim 49, wherein at least a portion of the duty
required to warm said pumped bottoms liquid is provided by indirect
heat exchange against at least one of said recycle fluids.
51. The process of claim 49, wherein said further portion of said
bottoms liquid, or said liquid derived from bottoms liquid, is used
as a "cold" process stream to provide refrigeration duty for the
process.
52. The process of claim 1, wherein the operating pressure(s) said
distillation column system is from about 10 bar to about 25
bar.
53. The process of claim 1, wherein said at least one impurity is
hydrogen sulphide (H.sub.2S).
54. The process of claim 1, wherein said process is
auto-refrigerated.
55. A process for purifying crude carbon dioxide comprising at
least one impurity that is less volatile than carbon dioxide, said
process comprising: feeding crude carbon dioxide feed at
sub-ambient temperature to a distillation column system operating
at super-atmospheric pressure(s) for separation to produce carbon
dioxide-enriched overhead vapour and bottoms liquid enriched with
said at least one impurity; removing said carbon dioxide-enriched
overhead vapour from said distillation column system and warming at
least a portion of said carbon dioxide-enriched overhead vapour by
indirect heat exchange to produce warmed carbon dioxide-enriched
gas; compressing a first working fluid comprising said warmed
carbon dioxide-enriched gas to produce at least one compressed
carbon dioxide-enriched gas; cooling and at least partially
condensing at least a portion of said compressed carbon
dioxide-enriched gas as a first recycle fluid by indirect heat
exchange to produce carbon dioxide-enriched fluid; expanding said
carbon dioxide-enriched fluid to produce expanded carbon
dioxide-enriched fluid and feeding said expanded carbon
dioxide-enriched fluid to said distillation column system, at least
a portion of which being used as reflux; compressing a second
working fluid comprising carbon dioxide-rich gas from said
distillation column system to produce at least one second recycle
fluid; cooling and optionally condensing at least a portion of said
second recycle fluid by indirect heat exchange to produce cooled
carbon dioxide-rich fluid; after expansion as required, feeding at
least a portion of said cooled carbon dioxide-rich fluid to said
distillation column system; at least partially re-boiling a portion
of said bottoms liquid by indirect heat exchange to produce vapour
for said distillation column system; and removing a further portion
of said bottoms liquid, or a liquid derived from bottoms liquid,
from said distillation column system, wherein re-boiling duty for
said distillation column system is provided at least in part by
indirect heat exchange against said first and second recycle
fluids, said first recycle fluid having a different pressure from
said second recycle fluid.
56. The process of claim 55, wherein said compressed carbon
dioxide-enriched gas is divided into at least a first portion and a
second portion, wherein said first portion is said first recycle
fluid(s), and wherein said second portion is said carbon
dioxide-rich gas for compression to produce said second recycle
fluid(s).
57. The process of claim 55, wherein carbon dioxide-rich vapour is
removed from an intermediate location in the distillation column
system and warmed by indirect heat exchange to produce said carbon
dioxide-rich gas for compression to produce said second recycle
fluid(s).
58. The process of claim 55, wherein liquid from an intermediate
location in said distillation column system is at least partially
re-boiled by indirect heat exchange to provide additional vapour
for said distillation column system.
59. A process for purifying crude carbon dioxide comprising at
least one impurity that is less volatile than carbon dioxide, said
process comprising: feeding crude carbon dioxide feed at
sub-ambient temperature to a distillation column system operating
at super-atmospheric pressure(s) for separation to produce carbon
dioxide-enriched overhead vapour and bottoms liquid enriched with
said at least one impurity; condensing a portion of said carbon
dioxide-enriched overhead vapour by indirect heat exchange to
provide reflux for said distillation column system; removing a
further portion of said carbon dioxide-enriched overhead vapour
from said distillation system; removing carbon dioxide-rich liquid
from an intermediate location in said distillation column system
and expanding said liquid to produce expanded carbon dioxide-rich
liquid; warming and evaporating said expanded carbon dioxide-rich
liquid by indirect heat exchange to provide warmed carbon
dioxide-rich gas; compressing a working fluid comprising said
warmed carbon dioxide-rich gas to produce at least one compressed
carbon dioxide-rich gas as a first recycle fluid and at least one
further compressed carbon dioxide-rich gas as a second recycle
fluid; cooling and optionally at least partially condensing said
first recycle fluid by indirect heat exchange to produce cooled
first carbon dioxide-rich fluid; combining said cooled first carbon
dioxide-rich fluid with crude carbon dioxide fluid to produce said
crude carbon dioxide feed for the distillation column system;
cooling and at least partially condensing said second recycle fluid
by indirect heat exchange to produce cooled second carbon
dioxide-rich fluid; expanding said cooled second carbon
dioxide-rich fluid to produce expanded carbon dioxide-rich fluid;
combining said expanded carbon dioxide-rich fluid with a fluid
selected from the group consisting of said carbon dioxide-rich
liquid, said expanded carbon dioxide-rich liquid, and said warmed
carbon dioxide-rich gas; at least partially re-boiling a portion of
said bottoms liquid by indirect heat exchange against at least one
"warm" process stream to produce vapour for said distillation
column system; and removing a further portion of said bottoms
liquid, or a liquid derived from bottoms liquid, from said
distillation column system, wherein said re-boiling duty is
provided at least in part by indirect heat exchange against said
first and second recycle fluids and wherein, in embodiments in
which said expanded carbon dioxide-rich fluid is combined with said
warmed carbon dioxide-rich gas, said expanded carbon dioxide-rich
fluid is first warmed and evaporated by indirect heat exchange to
produce further warmed carbon dioxide-rich gas for said combination
with said warmed carbon dioxide-rich gas.
60. Apparatus for carrying out the process of claim 55, said
apparatus comprising: a distillation column system for operation at
super-atmospheric pressure(s) for separating crude carbon dioxide
feed at sub-ambient temperature to produce carbon-dioxide-enriched
vapour and bottoms liquid enriched with said at least one impurity;
a first heat exchanger arrangement for warming at least a portion
of said carbon dioxide-enriched overhead vapour by indirect heat
exchange to produce warmed carbon dioxide-enriched gas; a conduit
arrangement for removing carbon dioxide-enriched overhead vapour
from said distillation column system and feeding said vapour to
said first heat exchanger arrangement; a first compressor system
for compressing said warmed carbon dioxide-enriched gas to produce
at least one compressed carbon dioxide-enriched gas; a second heat
exchanger arrangement for cooling and at least partially condensing
at least a portion of said compressed carbon dioxide-enriched gas
as a first recycle fluid by indirect heat exchange to produce
carbon dioxide-enriched fluid; a first expansion device for
expanding said carbon dioxide-enriched fluid to produce expanded
carbon dioxide-enriched fluid for feeding to said distillation
column system as reflux; a second compressor system for compressing
a carbon dioxide-rich gas from said distillation column system to
produce at least one second recycle fluid; a third heat exchanger
arrangement for cooling and optionally condensing at least a
portion of said second recycle fluid by indirect heat exchange to
produce cooled carbon dioxide-rich fluid for feeding to said
distillation column system; an optional expansion device for
expanding said cooled carbon dioxide-rich fluid to produce expanded
carbon dioxide-rich fluid prior to being fed to said distillation
column system; a fourth heat exchanger arrangement for at least
partially re-boiling said bottoms liquid by indirect heat exchange
against at least one of said recycle streams to produce vapour for
said distillation column system; and a conduit arrangement for
removing a further portion of said bottoms liquid, or a liquid
derived from bottoms liquid, from said distillation column system,
wherein said first and second compression systems are capable of
compressing said warmed carbon dioxide-enriched gas and said carbon
dioxide-rich gas respectively to different pressures.
61. Apparatus of claim 60 comprising a conduit arrangement for
feeding compressed carbon dioxide enriched-gas from said first
compressor system as feed to said second compressor system.
62. Apparatus of claim 60 comprising: a fifth heat exchanger
arrangement for warming a carbon dioxide-rich vapour by indirect
heat exchange to produce warmed carbon dioxide-rich gas; a conduit
arrangement for feeding carbon dioxide-rich vapour from an
intermediate location in said distillation column system to said
fifth heat exchanger arrangement; and a conduit arrangement for
feeding warmed carbon dioxide-rich gas from said fifth heat
exchanger arrangement to said second compressor system.
63. Apparatus of claim 60, comprising a sixth heat exchanger
arrangement for at least partially re-boiling liquid from an
intermediate location in said distillation column system to provide
additional vapour for said distillation column system.
64. Apparatus for carrying out the process of claim 59, said
apparatus comprising: a distillation column system for operation at
super-atmospheric pressure(s) for separating crude carbon dioxide
feed at sub-ambient temperature to produce carbon-dioxide-enriched
vapour and bottoms liquid enriched with said at least one impurity;
a first heat exchanger arrangement for cooling and partially
condensing said carbon dioxide-enriched overhead vapour by indirect
heat exchange to produce at least partially condensed carbon
dioxide-enriched overhead vapour as reflux for said distillation
column system; a conduit arrangement for removing carbon
dioxide-enriched overhead vapour from said distillation column
system; a first expansion device for expanding carbon dioxide-rich
liquid to produce expanded carbon dioxide-rich liquid; a conduit
arrangement for feeding carbon dioxide-rich liquid from an
intermediate location in said distillation column system to said
first expansion device; a second heat exchange arrangement for
warming and evaporating said expanded carbon dioxide-rich liquid by
indirect heat exchange to provide warmed carbon dioxide-rich gas; a
compressor system for compressing a working fluid comprising said
combined carbon dioxide-rich gas to produce compressed carbon
dioxide-rich gas as a first recycle fluid and at least one further
compressed carbon dioxide-rich gas as a second recycle fluid; a
third heat exchange system for cooling and optionally at least
partially condensing said first recycle fluid by indirect heat
exchange to produce cooled first carbon dioxide-rich fluid; a
conduit arrangement for combining said cooled first carbon
dioxide-rich fluid with crude carbon dioxide fluid to produce said
crude carbon dioxide feed for the distillation column system; a
fourth heat exchange arrangement for cooling said second recycle
fluid by indirect heat exchange to produce cooled second carbon
dioxide-rich fluid; a second expansion device for expanding said
cooled second carbon dioxide-rich fluid to produce expanded carbon
dioxide-rich fluid; a conduit arrangement for combining said
expanded carbon dioxide-rich fluid with a fluid selected from the
group consisting of said carbon dioxide-rich liquid, said expanded
carbon dioxide-rich liquid, and said warmed carbon dioxide-rich
gas; a fifth heat exchanger arrangement for at least partially
re-boiling a portion of said bottoms liquid by indirect heat
exchange against at least one of said recycle streams to produce
vapour for said distillation column system; and a conduit
arrangement for removing a further portion of said bottoms liquid,
or a liquid derived from bottoms liquid, from said distillation
column system, wherein, in embodiments in which said expanded
carbon dioxide-rich fluid is combined with said warmed carbon
dioxide-rich gas, said apparatus comprises sixth heat exchanger
arrangement for warming expanded carbon dioxide-rich fluid by
indirect heat exchange to produce further warmed carbon
dioxide-rich gas for said combination with said warmed carbon
dioxide-rich gas.
65. Apparatus of any of claims 60 to 64, wherein said heat
exchanger arrangements are passages within a single main heat
exchanger.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to processes and apparatus for
the purification of carbon dioxide. In particular, the invention
relates to the removal of impurities that are less volatile than
carbon dioxide from crude carbon dioxide comprising at least one
such impurity by distillation at sub-ambient temperatures and
super-atmospheric pressures. The invention has particular
application to the removal of hydrogen sulphide from carbon
dioxide.
[0002] Carbon dioxide from naturally occurring carbon dioxide
sources, such as natural carbon dioxide fields and natural gas
deposits, is used for enhanced oil recovery (EOR) in some areas of
the world. Some of these sources contain hydrogen sulphide, which
is undesirable for pipeline transport since hydrogen sulphide is
toxic and corrosive in the presence of water. In addition, it is
not desirable to introduce hydrogen sulphide to the crude oil that
is being extracted by the EOR process.
[0003] Processes for the removal of hydrogen sulphide from carbon
dioxide are known. For example, U.S. Pat. No. 3,417,572 (Pryor,
1968) discloses a method of treating hydrogen-rich gas comprising
carbon dioxide and hydrogen sulphide. The hydrogen sulphide and
carbon dioxide are condensed and separated from the hydrogen-rich
gas. The condensed gases are then fed to a distillation column for
separation into an essentially hydrogen sulphide-free carbon
dioxide overhead vapour and a bottoms liquid containing at least 10
vol. % hydrogen sulphide. The separated hydrogen-rich gas is
scrubbed to remove any residual carbon dioxide and hydrogen
sulphide which is then also fed to the distillation column.
Overhead vapour is condensed using an external closed cycle of
propane refrigerant and bottoms liquid is re-boiled using process
cooling water. The distillation column has 100 trays and operates
at about 590 psia (-41 bar) so that the overhead temperature is
42.degree. F. (-6.degree. C.) and the bottom temperature is about
45.degree. F. (-7.degree. C.).
[0004] U.S. Pat. No. 3,643,451 (Foucar, 1972) discloses a method of
producing high purity, high pressure carbon dioxide from a
concentrated low pressure mixture of acid gases. The gaseous
mixture is compressed, cooled and condensed and fed to a
distillation column where it is separated into a high purity (at
least 99.95%) carbon dioxide overhead vapour and a bottoms liquid
containing condensed sulphur-containing gases. The overhead vapour
is condensed using an external closed cycle of ammonia refrigerant
and refrigeration duty for cooling and condensing the feed is
provided by vaporising bottoms liquid, carbon dioxide overhead
liquid and the external refrigerant. The distillation column system
operates at about 300 to 350 psia (.about.21 to 24 bar) so that the
overhead temperature is -5 to -10.degree. F. (.about.-21 to
-24.degree. C.) and the bottoms temperature is 40 to 70.degree. F.
(.about.5 to 21.degree. C.). A bottoms product of 97% hydrogen
sulphide is produced in the example.
[0005] WO 81/02291 (Schuftan, 1981) discloses a method for
separating a gas mixture comprising carbon dioxide, at least one
gas having a lower boiling point than carbon dioxide and at least
one impurity (typically hydrogen sulphide) having a higher boiling
point than carbon dioxide. The gas mixture is cooled and distilled
in a first column to a product gas free of the impurity and a
liquid fraction containing the impurity. Pure carbon dioxide is
obtained in a second distillation column, which operates slightly
above the triple point pressure (.about.518 kPa) of carbon dioxide.
Liquid product from the first column is flashed at an intermediate
pressure to remove dissolved light impurities, then further reduced
in pressure and evaporated before being fed to the second column as
vapour. The carbon dioxide overhead vapour is practically free of
impurities and the bottoms liquid fraction is rich in impurities,
typically containing sulphur compounds (primarily hydrogen
sulphide) at a purity of up to 50 vol. %. Reflux and re-boil are
effected by a heat pump cycle which uses purified carbon dioxide as
the working fluid. The working fluid is passed through a
compressor, a heat exchanger and a re-boiler immersed in the
bottoms liquid, where it is condensed before being fed back to the
top of the column as reflux. A substantially pure carbon dioxide
product is withdrawn from the circulating carbon dioxide
immediately upstream of the compressor at a pressure of about 5
atm. and at near ambient temperature.
BRIEF SUMMARY OF THE INVENTION
[0006] It is an objective of preferred embodiments of the present
invention to reduce the energy consumed in distillation processes
for the removal of less volatile impurities such as hydrogen
sulphide from crude carbon dioxide, that involve a heat pump cycle
to provide re-boil duty.
[0007] According to a first aspect of the present invention, there
is provided a process for purifying crude carbon dioxide comprising
at least one impurity that is less volatile than carbon dioxide,
said process comprising feeding crude carbon dioxide feed at
sub-ambient temperature to a distillation column system operating
at super-atmospheric pressure(s) for separation to produce carbon
dioxide-enriched overhead vapour and bottoms liquid enriched with
said at least one impurity; providing carbon dioxide-enriched
liquid as reflux for the distillation column system; at least
partially re-boiling a portion of the bottoms liquid by indirect
heat exchange to provide vapour for the distillation column system;
removing carbon dioxide-enriched overhead vapour from the
distillation column system; and removing a further portion of the
bottoms liquid, or a liquid derived from bottoms liquid, from the
distillation column system. Re-boiling duty for the distillation
column system is provided at least in part by indirect heat
exchange against recycle fluids from at least one heat pump cycle
using a carbon dioxide-containing fluid from the distillation
column system as working fluid. At least one of the recycle fluids
has a different pressure from the other recycle fluid(s).
[0008] In preferred embodiments, liquid from an intermediate
location in the distillation column is at least partially re-boiled
by indirect heat exchange, preferably against at least one of said
recycle fluids, to provide additional vapour for said distillation
column.
[0009] The first aspect of the present invention embraces a process
comprising feeding crude carbon dioxide feed at sub-ambient
temperature to a distillation column system operating at
super-atmospheric pressure(s) for separation to produce carbon
dioxide-enriched overhead vapour and bottoms liquid enriched with
said at least one impurity; removing the carbon dioxide-enriched
overhead vapour from the distillation column system and warming at
least a portion of the carbon dioxide-enriched overhead vapour by
indirect heat exchange to produce warmed carbon dioxide-enriched
gas; compressing a first working fluid comprising the warmed carbon
dioxide-enriched gas to produce at least one compressed carbon
dioxide-enriched gas; cooling and at least partially condensing at
least a portion of the compressed carbon dioxide-enriched gas as a
first recycle fluid by indirect heat exchange to produce carbon
dioxide-enriched fluid; expanding the carbon dioxide-enriched fluid
to produce expanded carbon dioxide-enriched fluid and feeding the
expanded carbon dioxide-enriched fluid to said distillation column
system, at least a portion of which being used as reflux;
compressing a second working fluid comprising carbon dioxide-rich
gas from said distillation column system to produce at least one
second recycle fluid; cooling and optionally condensing at least a
portion of the second recycle fluid by indirect heat exchange to
produce cooled carbon dioxide-rich fluid; after expansion as
required, feeding at least a portion of the cooled carbon
dioxide-rich fluid to the distillation column system; at least
partially re-boiling a portion of the bottoms liquid by indirect
heat exchange to produce vapour for the distillation column system;
and removing a further portion of the bottoms liquid, or a liquid
derived from bottoms liquid, from the distillation column system.
Re-boiling duty for the distillation column system is provided at
least in part by indirect heat exchange against the first and
second recycle fluids, the first recycle fluid having a different
pressure from the second recycle fluid.
[0010] In some preferred embodiments, the compressed carbon
dioxide-enriched gas is divided into at least a first portion and a
second portion, wherein the first portion is the first recycle
fluid(s), and wherein the second portion is the carbon dioxide-rich
gas for compression to produce the second recycle fluid(s).
[0011] In other preferred embodiments, carbon dioxide-rich vapour
is removed from an intermediate location in the distillation column
system and warmed by indirect heat exchange to produce the carbon
dioxide-rich gas for compression to produce the second recycle
fluid(s).
[0012] Preferably, liquid from an intermediate location in said
distillation column system is at least partially re-boiled by
indirect heat exchange to provide additional vapour for said
distillation column system.
[0013] The first aspect of the present invention also embraces a
process comprising feeding crude carbon dioxide feed at sub-ambient
temperature in a distillation column system operating at
super-atmospheric pressure(s) for separation to produce carbon
dioxide-enriched overhead vapour and bottoms liquid enriched with
said at least one impurity; condensing a portion of the carbon
dioxide-enriched overhead vapour by indirect heat exchange to
provide reflux for the distillation column system; removing a
further portion of the carbon dioxide-enriched overhead vapour from
said distillation system; removing carbon dioxide-rich liquid from
an intermediate location in the distillation column system and
expanding the liquid to produce expanded carbon dioxide-rich
liquid; warming and evaporating the expanded carbon dioxide-rich
liquid by indirect heat exchange to provide warmed carbon
dioxide-rich gas; compressing a working fluid comprising the warmed
carbon dioxide-rich gas to produce at least one compressed carbon
dioxide-rich gas as a first recycle fluid and at least one further
compressed carbon dioxide-rich gas as a second recycle fluid;
cooling and optionally at least partially condensing the first
recycle fluid by indirect heat exchange to produce cooled first
carbon dioxide-rich fluid; combining the cooled first carbon
dioxide-rich fluid with crude carbon dioxide fluid to produce the
crude carbon dioxide feed for the distillation column system;
cooling and at least partially condensing the second recycle fluid
by indirect heat exchange to produce cooled second carbon
dioxide-rich fluid; expanding the cooled second carbon dioxide-rich
fluid to produce expanded carbon dioxide-rich fluid; combining the
expanded carbon dioxide-rich fluid with a fluid selected from the
group consisting of the carbon dioxide-rich liquid, the expanded
carbon dioxide-rich liquid, and the warmed carbon dioxide-rich gas;
at least partially re-boiling a portion of the bottoms liquid by
indirect heat exchange to produce vapour for the distillation
column system; and removing a further portion of the bottoms
liquid, or a liquid derived from bottoms liquid, from the
distillation column system. Re-boiling duty is provided at least in
part by indirect heat exchange against the first and second recycle
fluids. In addition, in embodiments in which the expanded carbon
dioxide-rich fluid is combined with the warmed carbon dioxide-rich
gas, the expanded carbon dioxide-rich fluid is first warmed and
evaporated by indirect heat exchange, for example against overhead
vapour, to produce further warmed carbon dioxide-rich gas for the
combination with the warmed carbon dioxide-rich gas.
[0014] Processes according to the present invention may be
integrated with an up-stream separation, in which a crude carbon
dioxide stream containing hydrogen sulphide as an impurity is
produced. Suitable up-stream separations are disclosed in U.S. Pat.
No. 7,883,569 (and related patents) and WO 81/02291.
[0015] According to a second aspect of the present invention there
is provided apparatus for carrying out a process of the first
aspect. The apparatus comprises a distillation column system for
operation at super-atmospheric pressure(s) for separating crude
carbon dioxide feed at sub-ambient temperature to produce
carbon-dioxide-enriched vapour and bottoms liquid enriched with
said at least one impurity; a first heat exchanger arrangement for
warming at least a portion of the carbon dioxide-enriched overhead
vapour by indirect heat exchange to produce warmed carbon
dioxide-enriched gas; a conduit arrangement for removing carbon
dioxide-enriched overhead vapour from the distillation column
system and feeding the vapour to the first heat exchanger
arrangement; a first compressor system for compressing the warmed
carbon dioxide-enriched gas to produce at least one compressed
carbon dioxide-enriched gas; a second heat exchanger arrangement
for cooling and at least partially condensing at least a portion of
the compressed carbon dioxide-enriched gas as a first recycle fluid
by indirect heat exchange to produce carbon dioxide-enriched fluid;
a first expansion device for expanding the carbon dioxide-enriched
fluid to produce expanded carbon dioxide-enriched fluid for feeding
to the distillation column system as reflux; a second compressor
system for compressing a carbon dioxide-rich gas from said
distillation column system to produce at least one second recycle
fluid; a third heat exchanger arrangement for cooling and
optionally condensing at least a portion of the second recycle
fluid by indirect heat exchange to produce cooled carbon
dioxide-rich fluid for feeding to the distillation column system;
an optional expansion device for expanding the cooled carbon
dioxide-rich fluid to produce expanded carbon dioxide-rich fluid
prior to being fed to the distillation column system; a fourth heat
exchanger arrangement for at least partially re-boiling the bottoms
liquid by indirect heat exchange against at least one of the
recycle streams to produce vapour for the distillation column
system; and a conduit arrangement for removing a further portion of
the bottoms liquid, or a liquid derived from bottoms liquid, from
the distillation column system. The first and second compressor
systems are capable of compressing the warmed carbon
dioxide-enriched gas and the carbon dioxide-rich gas respectively
to different pressures.
[0016] In some preferred embodiments, the apparatus comprises a
conduit arrangement for feeding compressed carbon dioxide
enriched-gas from the first compressor system as feed to the second
compressor system.
[0017] In other preferred embodiments, the apparatus comprises a
fifth heat exchanger arrangement for warming a carbon dioxide-rich
vapour by indirect heat exchange to produce warmed carbon
dioxide-rich gas; a conduit arrangement for feeding carbon
dioxide-rich vapour from an intermediate location in said
distillation column system to the fifth heat exchanger arrangement;
and a conduit arrangement for feeding warmed carbon dioxide-rich
gas from the fifth heat exchanger arrangement to the second
compressor system.
[0018] Preferably, the apparatus comprises a sixth heat exchanger
arrangement for at least partially re-boiling liquid from an
intermediate location in the distillation column system to provide
additional vapour for the distillation column system.
[0019] Also according to the second aspect of the present
invention, there is provided apparatus comprising a distillation
column system for operation at super-atmospheric pressure(s) for
separating crude carbon dioxide feed at sub-ambient temperature to
produce carbon-dioxide-enriched vapour and bottoms liquid enriched
with said at least one impurity; a first heat exchanger arrangement
for cooling a partially condensing carbon dioxide-enriched overhead
vapour by indirect heat exchange to produce at least partially
condensed carbon dioxide-enriched overhead vapour as reflux for
said distillation column system; a conduit arrangement for removing
carbon dioxide-enriched overhead vapour from the distillation
column system; a first expansion device for expanding carbon
dioxide-rich liquid to produce expanded carbon dioxide-rich liquid;
a conduit arrangement for feeding carbon dioxide-rich liquid from
an intermediate location in said distillation column system to the
first expansion device; a second heat exchange arrangement for
warming and evaporating the expanded carbon dioxide-rich liquid by
indirect heat exchange to provide warmed carbon dioxide-rich gas; a
compressor system for compressing a working fluid comprising warmed
carbon dioxide-rich gas to produce compressed carbon dioxide-rich
gas as a first recycle fluid and at least one further compressed
carbon dioxide-rich gas as a second recycle fluid; a third heat
exchange system for cooling and optionally at least partially
condensing the first recycle fluid by indirect heat exchange to
produce cooled first carbon dioxide-rich fluid; a conduit
arrangement for combining the cooled first carbon dioxide-rich
fluid with crude carbon dioxide fluid to produce the crude carbon
dioxide feed for the distillation column system; a fourth heat
exchange arrangement for cooling the second recycle fluid by
indirect heat exchange to produce cooled second carbon dioxide-rich
fluid; a second expansion device for expanding the cooled second
carbon dioxide-rich fluid to produce expanded carbon dioxide-rich
fluid; a conduit arrangement for combining the expanded carbon
dioxide-rich fluid with a fluid selected from the group consisting
of the carbon dioxide-rich liquid, the expanded carbon dioxide-rich
liquid, and the warmed carbon dioxide-rich gas; a fifth heat
exchanger arrangement for at least partially re-boiling a portion
of the bottoms liquid by indirect heat exchange against at least
one of the recycle streams to produce vapour for the distillation
column system; and a conduit arrangement for removing a further
portion of the bottoms liquid, or a liquid derived from bottoms
liquid, from the distillation column system. In embodiments in
which the expanded carbon dioxide-rich fluid is combined with the
warmed carbon dioxide-rich gas, the apparatus comprises sixth heat
exchanger arrangement for warming expanded carbon dioxide-rich
fluid by indirect heat exchange to produce further warmed carbon
dioxide-rich gas for the combination with the warmed carbon
dioxide-rich gas.
[0020] In preferred embodiments, the apparatus comprising a seventh
heat exchanger arrangement for at least partially re-boiling liquid
from an intermediate location in the distillation column system to
provide additional vapour for the distillation column system.
[0021] The heat exchanger arrangements are preferably passages
within a single main heat exchanger, although other arrangements
involving a network of heat exchangers in series or in parallel to
achieve an equivalent overall effect are envisaged.
[0022] One advantage of preferred embodiments of the present
invention is that overall power consumption is reduced to a
significant extent which results in an associated reduction in
capital and operating costs.
[0023] In addition, the use of an intermediate re-boiler has the
effect of reducing the diameter the of the distillation column
system provided below the reboiler due to a significant increase in
concentration of the impurity and therefore an associated
significant reduction in bottoms liquid inventory within the
column. The reductions in column diameter provides a further saving
in capital cost and enables the use of apparatus having a smaller
footprint which may be critical in situations where space is at a
premium.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
[0024] FIG. 1 is a flow sheet depicting a process for purifying
crude carbon dioxide comprising a single heat pump cycle involving
carbon dioxide-enriched overhead vapour as a working fluid and
operating at a single recycle pressure;
[0025] FIG. 2 is a flow sheet depicting a first embodiment of the
present invention comprising a single heat pump cycle using carbon
dioxide-enriched overhead vapour as a working fluid and operating
at two different recycle pressures;
[0026] FIG. 3 is a flow sheet depicting a modified arrangement of
the embodiment depicted in FIG. 2 in which the distillation column
system comprises an intermediate re-boiler;
[0027] FIG. 4 is a flow sheet depicting a modified arrangement of
the embodiment depicted in FIG. 3 in which separators S1 and S2 of
the distillation column system have been eliminated;
[0028] FIG. 5 is a flow sheet depicting a modified arrangement of
the embodiment depicted in FIG. 3 involving a split column;
[0029] FIG. 6 is a flow sheet depicting a modified arrangement of
the embodiment depicted in FIG. 5 involving a different re-boiler
system;
[0030] FIG. 7 is a flow sheet depicting a modified arrangement of
the embodiment depicted in FIG. 3 in which the purge has been
eliminated;
[0031] FIG. 8 is a flow sheet depicting a modified arrangement of
the embodiment depicted in FIG. 7 in which the feed to the process
is a saturated vapour rather than a liquid;
[0032] FIG. 9 is a flow sheet depicting a different modified
arrangement of the embodiment depicted in FIG. 8 in which the feed
to the process is a vapour;
[0033] FIG. 10 is a flow sheet depicting a further modified
arrangement of the embodiment depicted in FIG. 8 in which the feed
to the process is a vapour;
[0034] FIG. 11 is a flow sheet depicting a modified arrangement of
the embodiment depicted in FIG. 3 in which the distillation column
system comprises a double column;
[0035] FIG. 12 is a flow sheet depicting a second embodiment of the
present invention comprising a first heat pump cycle using carbon
dioxide-enriched overhead vapour as a working fluid and a second
heat pump cycle using a crude carbon dioxide working fluid; and
[0036] FIG. 13 is a flow sheet depicting a third embodiment of the
present invention comprising a single heat pump cycle using a crude
carbon dioxide working fluid operating at two different recycle
pressures.
DETAILED DESCRIPTION OF THE INVENTION
[0037] The present invention involves a process for purifying crude
carbon dioxide comprising at least one impurity that is less
volatile than carbon dioxide.
[0038] The crude carbon dioxide typically comprises at least 50
mol. %, e.g. at least 65 mol. % and preferably at least 80 mol. %
carbon dioxide. The crude carbon dioxide typically comprises no
more than 99 mol. %, e.g. no more than 95 mol. %, carbon dioxide.
In preferred embodiments, the crude carbon dioxide comprises from
about 85 mol. % to about 95 mol. % carbon dioxide.
[0039] Typical impurities that are less volatile than carbon
dioxide include hydrogen sulphide, propane and methanol. However,
the present invention has particular application in the removal of
hydrogen sulphide from crude carbon dioxide. The total
concentration of the less volatile impurities in the crude carbon
dioxide is typically significantly less than 50 mol. %, e.g. less
than 20 mol % and usually less than 10 mol. %.
[0040] The crude carbon dioxide feed may contain impurities that
are more volatile than carbon dioxide. Such impurities would
include methane and non-condensable gases such as nitrogen. These
impurities tend to concentrate in the carbon dioxide product of the
present process, which may be further processed to remove these
impurities.
[0041] In its broadest aspect, the process comprises feeding crude
carbon dioxide feed at sub-ambient temperature to a distillation
column system operating at super-atmospheric pressure(s) for
separation to produce carbon dioxide-enriched overhead vapour and
bottoms liquid enriched with said at least one impurity.
[0042] By "sub-ambient temperature", the Inventors mean that the
temperature is below normal ambient temperature, which is typically
from about -10.degree. C. to about 40.degree. C. depending on the
time of year and the geographical location of the plant. The
temperature of the feed is typically below 0.degree. C., e.g. no
more than -10.degree. C. and preferably below -20.degree. C. The
temperature of the feed is typically no lower than -55.degree. C.,
e.g. no lower than -40.degree. C.
[0043] By "super-atmospheric pressure", the Inventors mean that the
pressure is above atmospheric pressure, which is typically about 1
bar. Typically, the operating pressure(s) of the distillation
column system must be above the triple point pressure of carbon
dioxide, which is about 5.2 bar, and preferably, the operating
pressure(s) of the distillation column system is at least 10 bar.
The operating pressure(s) is typically no more than 40 bar, e.g. no
more than 30 bar. In preferred embodiments, the operating
pressure(s) is from about 10 bar to about 25 bar.
[0044] The preferred lower limit of 10 bar for the operating
pressure(s) ensures that the distillation column system does not
need to operate at a temperature that is too cold and light
impurities such as methane tend to be more soluble in the overhead
liquid. The preferred upper limit of 25 bar means that the
distillation column system operates sufficiently far from the
critical pressure for the hydraulic parameters within the column to
be comfortable.
[0045] All references herein to pressure are references to absolute
pressure and not gauge pressure unless expressly stated
otherwise.
[0046] The distillation column system may comprise a single column,
a split column where both parts of the column operate at the same
pressure, or multiple columns where the columns operate at
different pressures. In the latter case, all of the operating
pressures fall within the preferred ranges given above.
[0047] The distillation column system may also comprise at least
one vapour/liquid separator to separate a vapour component from
reflux liquid for the column system, and/or to separate a liquid
component from vapour for the column system generated from
partially re-boiled liquid taken from the column system.
[0048] The carbon dioxide-enriched overhead vapour has a greater
concentration of carbon dioxide than the crude carbon dioxide feed.
The concentration of carbon dioxide in the overhead vapour is
typically at least 90 mol. %, e.g. at least 95 mol. % and
preferably at least 98 mol. %. The overhead vapour is preferably
substantially pure carbon dioxide containing zero, or essentially
zero, of any less volatile impurity.
[0049] The bottoms liquid comprises at least substantially all, and
preferably all, of any less volatile impurity present in the crude
carbon dioxide feed. In preferred embodiments, the vapour flow in
the bottom section of the distillation column system is reduced
resulting in a reduction in the diameter of the bottom section of
the column system. The total inventory of bottoms liquid is thereby
reduced significantly where there is a higher concentration of the
volatile impurities. A reduction in the amount of liquid inventory
means that there is less liquid inventory to escape in the event of
a catastrophic failure of the plant. This advantage is particularly
important where the less volatile impurity or, where there is more
than one, at least one of the less volatile impurities is toxic,
for example, in cases where the impurity is hydrogen sulphide.
[0050] The process also provides carbon dioxide-enriched liquid for
use as reflux for the distillation column system, and a portion of
the bottoms liquid is at least partially re-boiled by indirect heat
exchange to provide vapour for the distillation column system.
Carbon dioxide-enriched overhead vapour is removed from the
distillation column system, as is a further portion of the bottoms
liquid, or a liquid derived from bottoms liquid.
[0051] Re-boiling duty for the distillation column system is
provided at least in part by indirect heat exchange against recycle
fluids from at least one heat pump cycle using a carbon
dioxide-containing fluid from the distillation column system as
working fluid. At least one of the recycle fluids has a different
pressure from the other recycle fluid(s).
[0052] By "heat pump cycle", the Inventors are referring to a cycle
by which thermal energy is transferred from a heat source, which is
at lower temperature, to a heat sink, which is at higher
temperature. The heat pump cycle uses a working fluid which in this
case is a carbon dioxide-containing fluid from the distillation
column system. Typically, the working fluid is removed from the
distillation column system, warmed, compressed and recycled to the
distillation column system after suitable cooling and pressure
reduction. The compressed fluid, or recycle fluid, is used to
provide re-boil duty by indirect heat exchange with liquid(s) taken
from the distillation column system. The recycle fluid(s) are
cooled to a certain extent as a result of providing the re-boil
duty but typically require further cooling before being returned to
the distillation column system.
[0053] In preferred embodiments, the heat source is the overhead
vapour that would condense at a lower temperature that the
re-boiler (the heat sink). However, the Inventors have observed
that, by compressing the overhead vapour in the heat pump cycle,
the vapour transfers heat to the re-boiler and is condensed at a
higher temperature than the reboiler.
[0054] The working fluid is typically selected from the group
consisting of carbon dioxide enriched overhead fluid or crude
carbon dioxide fluid taken from an intermediate location in the
distillation column system.
[0055] The present invention involves at least two recycle fluids
at different pressures. The pressure differential is significant,
typically of the order of at least 10%, e.g. at least 25% or even
at least 50%, although the pressure differential is usually no more
than 200%, e.g. no more than 100%. In absolute terms, the pressure
differential may be at least 2 bar, e.g. at least 5 bar and
preferably at least 10 bar. The pressure differential is usually no
more than 50 bar and preferably no more than 30 bar.
[0056] In some preferred embodiments, the process comprises a
single heat pump cycle comprising a least a first recycle fluid and
a second recycle fluid, the second recycle fluid having a pressure
that is greater than that of the first recycle fluid.
[0057] The pressure of the first recycle fluid is typically from
about 15 bar to about 30 bar.
[0058] The pressure of the second recycle fluid is typically about
20 bar to about 70 bar.
[0059] In some embodiments, the working fluid comprises carbon
dioxide-enriched gas generated by warming the carbon
dioxide-enriched overhead vapour by indirect heat exchange. At
least a portion of the duty required to warm the carbon
dioxide-enriched overhead vapour may be provided by indirect heat
exchange against any suitable "warm" process stream but is
preferably provided by indirect heat exchange against at least one
of the recycle fluids. In preferred embodiments, the compressor
feed is warmed against the compressor products so that the flows on
both sides of the heat exchanger are the same. In these
embodiments, both the first and second recycle fluids are used to
warm the overhead vapour.
[0060] The recycle fluids are typically recycled to an appropriate
location in the distillation column system after suitable pressure
reduction. The appropriate location in the distillation column
system is typically where the composition in the column matches the
composition of the recycle fluids. Where the working fluid is
carbon dioxide-enriched fluid, condensed recycle fluid is typically
recycled as reflux to the distillation column system.
[0061] The ratio of molar flow of the first recycle fluid to the
second recycle fluid is determined by the duty required of the
fluids. Typically, the molar flow ratio is from about 0.1 (i.e.
1:10) to about 15 (i.e. 15:1). In some preferred embodiments, this
ratio is from about 3 (i.e. 3:1) to about 12 (i.e. 12:1). In other
preferred embodiments, the ratio is from about 0.2 (i.e. 1:5) to
about 1 (i.e. 1:1).
[0062] In other embodiments, the working fluid comprises crude
carbon dioxide gas generated by evaporating "intermediate" liquid
taken from an intermediate location in said distillation column
system by indirect heat exchange after suitable pressure reduction.
The "intermediate" liquid is a crude carbon dioxide stream. In
preferred embodiments, the intermediate liquid is removed from a
location that is at least substantially level with the location of
the main feed to the column system. In such embodiments, the
composition of the intermediate liquid is usually at least
substantially identical to that of the crude carbon dioxide feed.
In these embodiments, the working fluid may also comprise carbon
dioxide-enriched gas generated by warming the carbon
dioxide-enriched overhead vapour by indirect heat exchange.
[0063] At least a portion of the duty required to evaporate said
"intermediate" liquid may also be provided by any suitable "warm"
process stream. Preferably, the intermediate liquid is evaporated
by indirect heat exchange against condensing overhead vapour from
the distillation column system.
[0064] In these other embodiments, the first recycle fluid is
preferably recycled as part of the feed to the distillation column
system and, additionally or alternatively, the second recycle fluid
is preferably recycled as part of the working fluid for the heat
pump cycle after suitable pressure reduction.
[0065] The process may comprise at least a first heat pump cycle
and a second heat pump cycle, each heat pump cycle comprising at
least one recycle fluid. In these embodiments, the recycle fluid of
the first heat pump cycle or, where the first heat pump cycle has
more than one recycle fluid, at least one of the recycle fluids,
has a pressure that is greater than that of a recycle fluid of the
second heat pump cycle.
[0066] The working fluid of the first heat pump cycle preferably
comprises carbon dioxide-enriched gas generated by warming the
carbon dioxide-enriched overhead vapour by indirect heat exchange.
At least a portion of the duty required to warm the carbon
dioxide-enriched overhead vapour may be provided by indirect heat
exchange against any suitable "warm" process stream although, in
preferred embodiments, it is provided by indirect heat exchange
against at least one of the recycle fluids. The pressure of the
recycle fluid of the first heat pump cycle is typically from about
15 bar to about 60 bar.
[0067] The working fluid of the second heat pump cycle preferably
comprises crude carbon dioxide gas generated by warming
"intermediate" vapour taken from an intermediate location of the
distillation column system by indirect heat exchange. The
"intermediate" vapour is a crude carbon dioxide fluid. In preferred
embodiments, the intermediate vapour is removed from a location
that is at least substantially level with the location of the main
feed to the column system. In such embodiments, the composition of
the intermediate vapour is usually at least substantially identical
to that of the crude carbon dioxide feed.
[0068] At least a portion of the duty required to warm said
"intermediate" vapour may be provided by indirect heat exchange
against any suitable "warm" process stream although, in preferred
embodiments, it is provided by indirect heat exchange against at
least one of the recycle fluids.
[0069] As in the other embodiments, the recycle streams are usually
recycled to appropriate locations in the distillation column system
after suitable pressure reduction if required. In this connection,
the first recycle fluid is preferably condensed and recycled after
pressure reduction to the top of the distillation column system to
provide reflux. The second recycle fluid is usually recycled after
suitable pressure reduction if required to an intermediate location
in the column system that is at least substantially level with the
location of the main feed to the column system. In preferred
embodiments in which the distillation column system comprises a
dual column arrangement, the working fluid for the second heat pump
cycle is intermediate overhead vapour from the lower pressure
column and is recycled without pressure reduction to the bottom of
the higher pressure column.
[0070] The pressure of the recycle fluid of the second heat pump
cycle is preferably from about 10 bar to about 25 bar, e.g. the
operating pressure of the part of the distillation column system to
which the recycle fluid is recycled.
[0071] Liquid from an intermediate location in the distillation
column system is at least partially re-boiled by indirect heat
exchange to provide additional vapour for the distillation column
system. At least a portion of the re-boiling duty may be provided
by indirect heat exchange against any suitable "warm" process
stream although, in preferred embodiments, it is provided by
indirect heat exchange against at least one of the recycle fluids,
e.g. the first recycle fluid which is at least partially condensed
as a result.
[0072] The advantage of an intermediate re-boiler is that the power
consumption is significantly reduced by only needing to compress a
fraction (typically <10%) of the overhead vapour to the higher
pressure required to heat the bottom re-boiler, whilst the rest
only needs to be compressed to the lower pressure. A further
advantage of the intermediate re-boiler is that the column diameter
below it, where the hydrogen sulphide concentration increases
rapidly, can be significantly reduced so that the inventory of
highly toxic hydrogen sulphide can be reduced.
[0073] A portion of the working fluid may be purged from the
process to prevent the build up of more volatile impurities, e.g.
methane and non-condensable gases such as nitrogen.
[0074] In some preferred embodiments, the reflux for the
distillation column system is preferably provided by at least one
recycle fluid condensate, typically condensed overhead vapour,
after suitable pressure reduction. In other embodiments, the reflux
for the column is provided by an overhead condenser arrangement in
which overhead vapour is at least partially condensed by indirect
heat exchange against at least one "cold" process stream, e.g.
re-boiling bottoms liquid, and returned to the column system. The
refrigeration duty required to cool and at least partially condense
at least one recycle fluid may be provided by indirect heat
exchange against any suitable "cold" process stream.
[0075] By "refrigeration duty", the Inventors mean the cooling duty
and, if applicable, the condensing duty required by the
process.
[0076] By "cold process stream", the Inventors mean any fluid
stream within the process whose temperature is lower than that of
the fluid to be cooled and, where appropriate, condensed and whose
pressure is suitable to provide the necessary indirect heat
exchange. Suitable "cold" process streams include streams entering
a main heat exchange at the cold end. In preferred embodiments, the
duty is provided by indirect heat exchange against at least one
fluid selected from the group consisting of carbon dioxide-enriched
liquid; bottoms liquid; liquid derived from bottoms liquid; and
expanded crude carbon dioxide fluid.
[0077] The crude carbon dioxide feed is preferably crude carbon
dioxide fluid derived from a natural source of carbon dioxide and
expanded prior to feeding to the distillation column system. Prior
to expansion, the crude carbon dioxide fluid is usually at a
super-critical pressure and a sub-critical temperature.
[0078] By "super-critical pressure", the Inventors mean a pressure
that is greater than the critical pressure of carbon dioxide, i.e.
73.9 bar. The pressure of the crude carbon dioxide fluid may be
from about 100 bar to about 200 bar.
[0079] By "sub-critical temperature", the Inventors mean a
temperature below the critical temperature of carbon dioxide, i.e.
31.1.degree. C. The temperature of the crude carbon dioxide fluid
is typically no more than 30.degree. C., e.g. no more than
20.degree. C. and preferably no more than 15.degree. C. The
temperature is usually no less than -20.degree. C., e.g. no less
than -10.degree. C. and preferably no less than 0.degree. C. In
some embodiments, the temperature is about the "bubble point" of
carbon dioxide, i.e. the temperature at which the carbon dioxide
begins to boil at a given pressure. In other embodiments, the
temperature is at or above the dew point of carbon dioxide.
[0080] The crude carbon dioxide fluid may be cooled by indirect
heat exchange prior to expansion. At least a portion of the
refrigeration duty required to cool the crude carbon dioxide fluid
may be provided by indirect heat exchange with any suitable
refrigerant stream although, in preferred embodiments, it is
provided by indirect heat exchange against at least one "cold"
process stream selected from the group consisting of carbon
dioxide-enriched liquid; bottoms liquid; liquid derived from
bottoms liquid; and expanded crude carbon dioxide fluid.
[0081] The expanded crude carbon dioxide is preferably used as a
"cold" process stream to provide refrigeration duty for the
process. Alternatively, the expanded crude carbon dioxide fluid may
be fed directly to the distillation column stream without providing
refrigeration duty by indirect heat exchange.
[0082] The feed is typically derived from supercritical crude
carbon dioxide liquid and carbon dioxide-enriched liquid is
produced as a product. In these embodiments, the carbon
dioxide-enriched liquid is typically removed from the distillation
column system, pumped and warmed by indirect heat exchange to
produce warmed carbon dioxide-enriched liquid as a product. At
least a portion of the duty required to warm the pumped carbon
dioxide-enriched liquid may be provided by indirect heat exchange
against any suitable "warm" process stream although, in preferred
embodiments, it is provided by indirect heat exchange against at
least one of the recycle fluids.
[0083] The pumped carbon dioxide-enriched liquid is preferably used
as a "cold" process stream to provide refrigeration duty for the
process.
[0084] The feed may be derived from crude carbon dioxide vapour and
carbon dioxide-enriched gas is produced as a product. In these
embodiments, a portion of the carbon dioxide-enriched vapour is
typically warmed by indirect heat exchange to produce the carbon
dioxide-enriched gas. At least a portion of the duty required to
warm said carbon dioxide-enriched overhead vapour may be provided
by indirect heat exchange with any suitable "warm" process stream
although, in preferred embodiments, it is provided by indirect heat
exchange against at least one of the recycle fluids.
[0085] The carbon dioxide-enriched overhead vapour is preferably
used as a "cold" process stream to provide refrigeration duty for
the process.
[0086] The further portion of bottoms liquid, or the liquid derived
from bottoms liquid, is usually pumped and warmed by indirect heat
exchange to provide impurity-rich waste liquid. At least a portion
of the duty required to warm the pumped bottoms liquid may be
provided by indirect heat exchange against any "warm" process
stream although, in preferred embodiments, it is provided by
indirect heat exchange against at least one of the recycle
fluids.
[0087] The further portion of the bottoms liquid, or the liquid
derived from bottoms liquid, is typically used as a "cold" process
stream to provide refrigeration duty for the process.
[0088] An external refrigeration cycle may be used to provide at
least a portion of the refrigeration duty required by the process.
However, in preferred embodiments, the process is
auto-refrigerated, i.e. none of the refrigeration duty is provided
by an external refrigeration cycle.
[0089] Particularly preferred embodiments of the process comprise:
[0090] feeding crude carbon dioxide feed at sub-ambient temperature
to a distillation column system operating at super-atmospheric
pressure(s) for separation to produce carbon dioxide-enriched
overhead vapour and bottoms liquid enriched with said at least one
impurity; [0091] removing said carbon dioxide-enriched overhead
vapour from said distillation column system and warming at least a
portion of said carbon dioxide-enriched overhead vapour by indirect
heat exchange to produce warmed carbon dioxide-enriched gas; [0092]
compressing a first working fluid comprising said warmed carbon
dioxide-enriched gas to produce at least one compressed carbon
dioxide-enriched gas; cooling and at least partially condensing at
least a portion of said compressed carbon dioxide-enriched gas as a
first recycle fluid by indirect heat exchange to produce carbon
dioxide-enriched fluid; [0093] expanding said carbon
dioxide-enriched fluid to produce expanded carbon dioxide-enriched
fluid and feeding said expanded carbon dioxide-enriched fluid to
said distillation column system, at least a portion of which being
used as reflux; [0094] compressing a second working fluid
comprising carbon dioxide-rich gas from said distillation column
system to produce at least one second recycle fluid; [0095] cooling
and optionally condensing at least a portion of said second recycle
fluid by indirect heat exchange to produce cooled carbon
dioxide-rich fluid; [0096] after expansion as required, feeding at
least a portion of said cooled carbon dioxide-rich fluid to said
distillation column system; [0097] at least partially re-boiling a
portion of said bottoms liquid by indirect heat exchange to produce
vapour for said distillation column system; and [0098] removing a
further portion of said bottoms liquid, or a liquid derived from
bottoms liquid, from said distillation column system, wherein
re-boiling duty for said distillation column system is provided at
least in part by indirect heat exchange against said first and
second recycle fluids, said first recycle fluid having a different
pressure from said second recycle fluid.
[0099] In these embodiments, the compressed carbon dioxide-enriched
gas may be divided into at least a first portion and a second
portion, wherein the first portion is the first recycle fluid(s),
and wherein the second portion is the carbon dioxide-rich gas for
compression to produce the second recycle fluid(s).
[0100] Alternatively, carbon dioxide-rich vapour may be removed
from an intermediate location in the distillation column system and
warmed by indirect heat exchange to produce the carbon dioxide-rich
gas for compression to produce the second recycle fluid(s).
[0101] Preferably, liquid from an intermediate location in the
distillation column system is at least partially re-boiled by
indirect heat exchange to provide additional vapour for the
distillation column system.
[0102] In other particularly preferred embodiments, the process
comprises: [0103] feeding crude carbon dioxide feed at sub-ambient
temperature to a distillation column system operating at
super-atmospheric pressure(s) for separation to produce carbon
dioxide-enriched overhead vapour and bottoms liquid enriched with
said at least one impurity; [0104] condensing a portion of said
carbon dioxide-enriched overhead vapour by indirect heat exchange
to provide reflux for said distillation column system; [0105]
removing a further portion of said carbon dioxide-enriched overhead
vapour from said distillation system; [0106] removing carbon
dioxide-rich liquid from an intermediate location in said
distillation column system and expanding said liquid to produce
expanded carbon dioxide-rich liquid; [0107] warming and evaporating
said expanded carbon dioxide-rich liquid by indirect heat exchange
to provide warmed carbon dioxide-rich gas; [0108] compressing a
working fluid comprising said warmed carbon dioxide-rich gas to
produce at least one compressed carbon dioxide-rich gas as a first
recycle fluid and at least one further compressed carbon
dioxide-rich gas as a second recycle fluid; [0109] cooling and
optionally at least partially condensing said first recycle fluid
by indirect heat exchange to produce cooled first carbon
dioxide-rich fluid; [0110] combining said cooled first carbon
dioxide-rich fluid with crude carbon dioxide fluid to produce said
crude carbon dioxide feed for the distillation column system;
[0111] cooling and at least partially condensing said second
recycle fluid by indirect heat exchange to produce cooled second
carbon dioxide-rich fluid; [0112] expanding said cooled second
carbon dioxide-rich fluid to produce expanded carbon dioxide-rich
fluid; [0113] combining said expanded carbon dioxide-rich fluid
with a fluid selected from the group consisting of said carbon
dioxide-rich liquid, said expanded carbon dioxide-rich liquid, and
said warmed carbon dioxide-rich gas; [0114] at least partially
re-boiling a portion of said bottoms liquid by indirect heat
exchange against at least one "warm" process stream to produce
vapour for said distillation column system; and [0115] removing a
further portion of said bottoms liquid, or a liquid derived from
bottoms liquid, from said distillation column system, wherein said
re-boiling duty is provided at least in part by indirect heat
exchange against said first and second recycle fluids and wherein,
in embodiments in which said expanded carbon dioxide-rich fluid is
combined with said warmed carbon dioxide-rich gas, said expanded
carbon dioxide-rich fluid is first warmed and evaporated by
indirect heat exchange to produce further warmed carbon
dioxide-rich gas for said combination with said warmed carbon
dioxide-rich gas.
[0116] All features described in connection with any aspect of the
invention can be used with any other aspect of the invention.
[0117] The invention will now be further described with reference
to the comparative process depicted in FIG. 1 and preferred
embodiments of the present invention depicted in FIGS. 2 to 13.
[0118] FIG. 1 depicts a comparative process involving a heat pump
cycle having a single recycle fluid. The feed to the process is a
liquid containing about 91 mol. % carbon dioxide and about 7 mol. %
hydrogen sulphide with the rest being made up of methane, propane
and methanol. The liquid has a pressure of about 45 bar and a
temperature at about its bubble point, 9.degree. C. Such a feed
stream may be produced in a process to remove acid gases, e.g.
carbon dioxide and hydrogen sulphide, from natural gas.
[0119] The feed may be dried (e.g. in a bed containing silica gel
or alumina) prior to entering the cold process if necessary. A
stream 100 of liquid feed is sub-cooled (to make the process more
efficient) in heat exchanger HE1 to produce a stream 102 of cooled
feed liquid which is expanded in a dense fluid expander E1 to
recover energy and to produce a stream 104 of expanded fluid. The
expander E1 may be replaced with a throttling valve, or may have a
valve in series or parallel with it.
[0120] The expanded fluid is used to provide refrigeration duty in
heat exchanger HE 1 thereby producing a stream 106 of crude carbon
dioxide gas which is fed to the column C1 a distillation column
system where it is separated into carbon dioxide-enriched overhead
vapour containing about 99 mol. % carbon dioxide and bottoms liquid
containing about 72 mol. % hydrogen sulphide. Any light impurities,
such as methane, from the feed will concentrate in the overhead
vapour. In this process, the column is operating at a pressure of
about 13 bar.
[0121] A portion 180 of the bottoms liquid is partially re-boiled
by indirect heat exchange in heat exchanger HE1 to produce a
two-phase stream 182. In the arrangement depicted, the re-boiler is
an external, once-through re-boiler. However, other types of
re-boiler such as thermosyphon or downflow boilers may be used, and
may be located in the column C1.
[0122] The two-phase stream could be fed directly back to the
column C1 to provide ascending vapour for the distillation process.
However, in the embodiment depicted in the figure, the distillation
column system comprises a vapour/liquid separator S2 and the
two-phase stream 182 is fed to this separator where the vapour and
liquid components are separated. The vapour component is fed as
stream 184 to column C1 and the liquid component derived from the
bottoms liquid is fed as stream 186 to a pump P3 where it is pumped
to a pressure of about 48 bar. A stream 188 of pumped liquid is
then warmed by indirect heat exchange in heat exchanger HE1 to
produce a stream 190 of warmed liquid. The liquid does not
evaporate in the heat exchanger HE1 as it has been pumped. The
warmed liquid is pumped further in pump P4 to provide a stream 192
of pumped waste liquid at a pressure of about 208 bar. The
composition of the waste liquid is about 94 mol. % hydrogen
sulphide and about 6 mol. % carbon dioxide with trace amounts of
propane and methanol.
[0123] The working fluid of the heat pump cycle in this process is
taken from the column C1 as carbon dioxide-enriched overhead
vapour. In this connection, overhead vapour is removed from the
column C1 and warmed in heat exchanger HE1 to produce a stream 114
of warmed carbon dioxide-enriched overhead gas. In this process, a
first stream 110 of overhead vapour is removed from the top of the
column C1 and warmed by indirect heat exchange in the heat
exchanger HE1 to produce a first stream 112 of warmed overhead gas.
A second stream 140 of overhead vapour is removed from a further
vapour/liquid separator S1 and warmed by indirect heat exchange in
the heat exchanger HE1 to produce a second stream 142 of warmed
overhead gas. At least a portion 144 of the second stream 142 of
warmed overhead gas is combined with the first stream 112 of
overhead gas to form the stream 114 of warmed carbon
dioxide-enriched overhead gas. However, the skilled person would
readily appreciate that (i) the combination of the vapour component
from separator S1 with the overhead vapour from the column C1 could
take place at the cold end of the heat exchanger HE1, and (ii) the
separator S1 could easily be eliminated and all of the overhead
vapour could be removed from the top of the column C1 (e.g. see
FIG. 4). A portion 146 of the second stream 142 of warmed overhead
gas may be purged from the process to prevent an undesirable
build-up of the more volatile impurities such as methane.
[0124] The warmed overhead gas is compressed in a compressor system
to produce a recycle stream 130 of compressed carbon
dioxide-enriched gas.
[0125] It should be noted that, while the compressor system in FIG.
1 is depicted as having two stages, CP1 and CP2, other compressor
systems having a single stage or multiple stages could be used. The
important point is that, irrespective of how the stream is
compressed, the heat pump cycle of this comparative process has
only a single recycle fluid and hence only a single recycle
pressure which, in this case, is about 33 bar.
[0126] It should also be noted that the compressor systems depicted
not only in FIG. 1 but also in FIGS. 2 to 13 may include
intercoolers and/or aftercoolers, even though these features are
not explicitly shown in the figures.
[0127] Recycle fluid 130 is used to provide re-boil duty by
indirect heat exchange in heat exchanger HE1, thereby at least
partially re-boiling stream 180 of bottoms liquid. The recycle
fluid is further cooled and condensed by indirect heat exchange in
heat exchanger HE1 to produce a stream 132 of condensed recycle
fluid which is then expanded across expansion valve V2 to produce a
stream 134 of expanded carbon dioxide-enriched fluid having a
vapour component and a liquid component. As mentioned above, this
stream could be fed directly to the column C1 to provide reflux
(e.g. see FIG. 4). However, the distillation column system depicted
in FIG. 1 comprises the further separator S1 which is used to
separate the vapour and liquid components. The vapour component 140
is used to provide a portion of the working fluid for the heat pump
cycle (see above) and a portion 152 of the liquid component is used
to provide reflux to the column C1.
[0128] Use of a reflux separator with purge allows the power
consumption to be reduced by increasing the required condenser
temperature and reducing the pressure required to drive the
intermediate reboiler. Any purged vapour may be recompressed into
the carbon dioxide product stream and recovered, depending on the
value of carbon dioxide recovered compared to the power cost for
the recompression.
[0129] A further portion 154 of the carbon dioxide-enriched liquid
is removed from the distillation column system and pumped in pump
P1 to produce a stream 156 of pumped carbon dioxide-enriched liquid
at a pressure of about 80 bar. The pumped liquid 156 is then warmed
by indirect heat exchange in heat exchanger HE1 to produce a stream
158 of warmed carbon dioxide-enriched liquid which is further
pumped in pump P2 to produce a stream 160 of liquid carbon dioxide
product at a pressure of about 153 bar. The liquid carbon dioxide
product is substantially pure carbon dioxide (about 99 mol. %)
representing a carbon dioxide recovery of about 99.5%. Carbon
dioxide product 160 is in a form suitable for transport by pipeline
or for use in EOR.
[0130] Refrigeration duty required to cool and, where appropriate,
condense the recycle fluid 130 and the liquid feed 100 is provided
by indirect heat exchange against the carbon dioxide-enriched
overhead vapour(s) (streams 110 & 140), the pumped carbon
dioxide-enriched liquid (stream 156), the pumped liquid derived
from bottoms liquid (stream 188), the expanded fluid feed (stream
104) and the bottoms liquid (stream 180). No external refrigeration
is used in the process, hence the process may be described as
"auto-refrigerated".
[0131] While all of the indirect heat exchange between fluids is
indicated as taking place in a single heat exchanger HE1 (e.g. an
aluminium plate fin heat exchanger), the skilled person would
appreciate that more than one heat exchanger could be used to
effect the necessary heat transfers between particular process
streams.
[0132] Preferred embodiments of the present invention are depicted
in FIGS. 2 to 13. These embodiments may be viewed as modifications
of the comparative process depicted in FIG. 1. The exemplified
embodiments have many features in common with the comparative
process depicted in FIG. 1 and other embodiments depicted in FIGS.
2 to 13. The common features between the processes have been
assigned the same reference numerals. For convenience, a further
discussion of the common features in not provided. The following is
a discussion of the distinguishing features.
[0133] In FIG. 2, stream 114 of warmed carbon dioxide-enriched gas
is compressed in a first compressor system CP1 to produce a stream
116 of compressed carbon dioxide-enriched gas at about 16 bar.
Stream 116 is divided into sub-streams 118 and 120. Sub-stream 120
is used as a first recycle fluid. Sub-stream 118 is further
compressed in a second compressor system CP2 to provide a stream
130 of a second recycle fluid at a pressure of about 28 bar. The
ratio of molar flow in streams 120 and 130 is about 2:5 (i.e. about
0.4).
[0134] Streams 120 and 130 of recycle fluids are both used to
provide re-boil duty by indirect heat exchange in heat exchanger
HE1 and are both cooled and condensed to form streams 122 and 132
respectively of condensed carbon dioxide-enriched fluids. The
fluids are expanded to the same pressure, i.e. the operating
pressure of the column C1, across expansion valves V1 and V2
respectively to produce expanded fluids 124 and 134 respectively.
As before, the expanded fluids could be fed directly to the column
C1 to provide reflux. However, in this embodiment, the expanded
fluids are combined to form stream 126 which is then fed to
separator S1 to separate the vapour and liquid components.
[0135] In this embodiment, recycle fluid 120 is cooled and
condensed by indirect heat exchange mainly against expanded feed
liquid 104, and recycle fluid 130 is cooled and condensed by
indirect heat exchange mainly against re-boiling bottoms liquid
180.
[0136] In FIG. 3, a stream 170 of liquid taken from an intermediate
location in the column C1 is removed from the column C1 and at
least partially re-boiled in heat exchanger HE1. The re-boiler is
depicted as an external, once through re-boiler. However, the
re-boiler may be internal in the column and/or other types of
re-boiler such a thermosyphon or downflow re-boiler, may be
used.
[0137] The duty for re-boiling the intermediate liquid is again
provided by the recycle fluids 122 and 132. The molar flow ratio of
streams 122 and 132 is about 10:1 (i.e. about 10). The bulk of the
boil-up duty for the column C1 is provided by this intermediate
re-boiler.
[0138] The products generated in the flow sheet of FIG. 3 are
pumped to a high enough pressure that they do not evaporate. They
are warmed in the main heat exchanger but not at such a high
pressure that the cost of the heat exchanger is adversely affected.
In an alternate configuration, the feed may be vapour and, in that
case, part of it may be expanded to the column and part may be
condensed and expanded as liquid. In this case, the pumped products
from the column may be evaporated in the heat exchanger at
appropriate pressures and temperatures to condense the feed.
[0139] While the feed is shown as being evaporated in the main
exchanger HE1 before being fed to the column C1, this is not
essential, and it may be only partially vaporised or fed as a
liquid. In this case, the intermediate reboiler duty would increase
in order to provide the equivalent boil-up within the column.
[0140] The flow sheet depicted in FIG. 4 is a modified version of
that depicted in FIG. 3 in which separators S1 and S2 are omitted
from the distillation column system. Accordingly, re-boiled bottoms
liquid 182 and expanded carbon dioxide-enriched fluid 126 are fed
directly to column C1.
[0141] The flow sheet depicted in FIG. 5 is a modified version of
that depicted in FIG. 3 in which the distillation column system
involves a split column C1, both parts of the column having the
same operating pressure. The vapour feed 106 is fed to the upper
part of the column C1. Bottoms liquid from the upper part of the
column C1 is used not only as "intermediate" liquid 170 being
re-boiled in heat exchanger HE1 but also to provide reflux in the
lower part of the column C1. Overhead vapour from the lower part of
the column C1 is fed to the upper part of the column C1 with the
feed vapour 106.
[0142] The flow sheet depicted in FIG. 6 is a modified version of
that depicted in FIG. 5 in which the arrangement for the
intermediate re-boiler is different. In this connection, bottoms
liquid from the upper part of the column C1 is partially re-boiled
by indirect heat exchange in the heat exchanger HE1. Stream 172
therefore has a liquid component and a vapour component which are
separated in a third vapour/liquid separator S3. The liquid
component is fed to the lower part of the column C1 as reflux and
the vapour component is fed to the upper part of the column C1.
Overhead vapour from the lower part of the column C1 is fed to the
upper part of the column C1.
[0143] The flow sheet depicted in FIG. 7 is a modified version of
that depicted in FIG. 3 in which the purge stream 146 is omitted.
The overhead vapour from separator S1 is combined with overhead
vapour from column C1 to form stream 140 of carbon dioxide-enriched
overhead vapour.
[0144] The flow sheet depicted in FIG. 8 is a modified version to
that depicted in FIG. 3 in which the feed 100 to the process is in
the form of a saturated vapour. The feed 100 is expanded in
expander E1 without first cooling the feed by indirect heat
exchange in the heat exchanger HE1. Expanded feed 106 to the column
C1 is about 13% condensate. Overhead vapour from the column C1 is
used as working fluid for the heat pump cycle as in FIG. 3.
However, all of the recycled carbon dioxide-enriched liquid is fed
to the column C1 as reflux and none is removed from the
distillation column system to form a liquid carbon dioxide product.
In contrast, a portion 111 of overhead vapour from the column C1 is
combined with the vapour component separated in separator S1 to
form stream 156 of carbon dioxide-enriched vapour which is warmed
by indirect heat exchange in heat exchanger HE1 to produce a stream
158 of warmed carbon dioxide-enriched gas. The gas is compressed in
compressor system CP3 and pumped in pump P2 to form a stream 160 of
gaseous carbon dioxide product at a pressure of about 110 bar.
[0145] The flow sheet depicted in FIG. 9 is a modified version of
that depicted in FIG. 8 in which the vapour feed 100 to the process
is cooled by indirect heat exchange in heat exchanger HE1 prior to
expansion and the expanded stream 104 is then warmed by indirect
heat exchange in heat exchanger HE1 prior to being fed to the
column C1.
[0146] The flow sheet depicted in FIG. 10 is a modified version of
that depicted in FIG. 9 in which the expanded stream 104 is fed
directly to the column C1 without first being warmed in the heat
exchanger HE1.
[0147] The flow sheet depicted in FIG. 11 is a modified version of
that depicted in FIG. 3 in which the distillation column system
comprises a dual column arrangement with a first column C1
operating at a higher pressure than a second column C2. The feed
106 is at the operating pressure of the first column C1. The feed
is split with a first portion being fed directly to the first
column C1 and the second portion being reduced in pressure across
expansion valve V6 and then fed to the second column C2.
[0148] The feed to the first column C1 is separated into an
intermediate overhead vapour and the bottoms liquid. The
intermediate overhead vapour used to re-boil bottoms liquid in the
second column C1 and as a result is itself condensed. The condensed
stream is reduced in pressure across expansion valve V3 and is then
used to provide reflux to the second column C2.
[0149] The feed to the second column C2 is separated into the
carbon dioxide-enriched overhead vapour and an intermediate bottoms
liquid which is fed to the first column C1. Second column C2 is
elevated in relation to the first column C1. Therefore, the
pressure of the intermediate bottoms liquid is raised by static
head. However, a pump (not shown) may be used to raise the pressure
of the intermediate bottoms liquid.
[0150] The flow sheet depicted in FIG. 12 is similar to that
depicted in FIG. 6 although, rather than a split column C1 in FIG.
6, the flow sheet in FIG. 12 involves a dual column with a first
column C1 operating at a higher pressure than a second column C2.
In addition, rather than a single heat pump cycle generating two
recycle fluids as in FIG. 6, the flow sheet of FIG. 12 involves two
separate heat pump cycles, each generating a single recycle
fluid.
[0151] The working fluid for the first heat pump cycle is carbon
dioxide-enriched overhead vapour from the first column C1 which is
warmed by indirect heat exchange in heat exchanger HE1 and then
compressed in a first compressor system CP1 to produce a first
recycle fluid 120.
[0152] Intermediate overhead vapour from the second column C2 is
not fed to the first column C1 as in FIG. 6. In contrast, the
working fluid for the second heat pump cycle is the intermediate
overhead vapour 200 taken from the second column C2 which is warmed
by indirect heat exchange in heat exchanger HE 1 and then
compressed in a second compressor system CP4 to produce a second
recycle fluid at the operating pressure of the first column C1.
[0153] Re-boil duty is provided by indirect heat exchange in heat
exchanger HE1 between the recycle fluids and the bottoms liquids
from the two columns in the distillation column system.
[0154] The recycle fluids are further cooled by indirect heat
exchange in heat exchanger HE1. The carbon dioxide-enriched liquid
122 from the first recycle stream is expanded across expansion
valve V1 and, after vapour/liquid separation, the liquid component
156 is fed to the first column C1 as reflux. The crude carbon
dioxide fluid 206 of the second recycle fluid after cooling is fed
to the first column.
[0155] The pressure of the liquid component 176 from separator S3
is dropped across expansion valve V7 before the liquid is fed as
reflux to the second column C2.
[0156] The flow sheet depicted in FIG. 13 is a modified version of
that depicted in FIG. 3. In FIG. 3, the working fluid in the heat
pump cycle is carbon dioxide-enriched overhead vapour. In FIG. 13,
crude carbon dioxide fluid is used. In this connection, a stream
210 of crude carbon dioxide liquid is removed from an intermediate
location of the column C1 and is combined with a recycle stream 236
(see below) to form combined stream 212 which is reduced in
pressure across expansion valve V4 to produce expanded stream 214.
The expanded stream 214 is warmed and evaporated by indirect heat
exchange in heat exchanger HE1 to produce crude carbon dioxide gas
stream 216 which is combined with warmed carbon dioxide-enriched
overhead vapour 144 (see below) to produce a stream 218 of carbon
dioxide-rich gas.
[0157] The carbon dioxide-rich gas 218 is compressed in first
compressor system CP4 to produce compressed gas 220 which is
divided into a first recycle stream 222 and a second portion 230.
The second portion is compressed further in a second compressor
system CP5 to produce a second recycle stream 232.
[0158] Re-boil duty for the column C1 is provided by indirect heat
exchange in heat exchanger HE1 against the two recycle streams
which are then further cooled by indirect heat exchange. The
further cooled first recycle stream 224 is combined with a crude
carbon dioxide feed 106 to provide a combined feed 226 for the
column C1. The further cooled second recycle stream 234 is expanded
across expansion valve V5 and the expanded liquid 236 combined with
the crude carbon dioxide liquid 210 taken from the column C1.
[0159] The distillation column system comprises an overhead
condenser arrangement in which overhead vapour 110 is removed and
partially condensed by indirect heat exchange in the heat exchanger
HE1. The partially condensed stream 111 is fed to a vapour/liquid
separator S1 where the vapour and liquid components are separated.
The liquid component is used to provide reflux to the column C1
(stream 152) and to provide the liquid carbon dioxide product
(streams 154 to 160). The vapour component 140 is warmed by
indirect heat exchange in heat exchanger HE1 to produce a stream
142 to overhead gas. A portion 144 of the overhead gas is combined
with the warmed crude carbon dioxide gas 216. A further portion 146
may be purged from the process.
[0160] Aspects of the present invention include:
#1. A process for purifying crude carbon dioxide comprising at
least one impurity that is less volatile than carbon dioxide, said
process comprising: [0161] feeding crude carbon dioxide feed at
sub-ambient temperature to a distillation column system operating
at super-atmospheric pressure(s) for separation to produce carbon
dioxide-enriched overhead vapour and bottoms liquid enriched with
said at least one impurity; [0162] providing carbon
dioxide-enriched liquid as reflux for said distillation column
system; [0163] at least partially re-boiling a portion of said
bottoms liquid by indirect heat exchange to provide vapour for said
distillation column system; [0164] removing carbon dioxide-enriched
overhead vapour from said distillation column system; and [0165]
removing a further portion of said bottoms liquid, or a liquid
derived from bottoms liquid, from said distillation column system,
wherein re-boiling duty for said distillation column system is
provided at least in part by indirect heat exchange against recycle
fluids from at least one heat pump cycle using a carbon
dioxide-containing fluid from said distillation column system as
working fluid, at least one of said recycle fluids having a
different pressure from the other recycle fluid(s). #2. A process
according to #1 comprising a single heat pump cycle comprising a
least a first recycle fluid and a second recycle fluid, said second
recycle fluid having a pressure that is greater than that of said
first recycle fluid. #3. A process according to #2, wherein the
pressure of said second recycle fluid is at least 10% greater than
the pressure of said first recycle fluid. #4. A process according
to #2 or #3, wherein the pressure of said first recycle fluid is
from about 15 bar to about 30 bar. #5. A process according to any
of #2 to #4, wherein the pressure of said second recycle fluid is
from about 20 bar to about 70 bar. #6. A process according to any
of #2 to #5, wherein said working fluid comprises carbon
dioxide-enriched gas generated by warming said carbon
dioxide-enriched overhead vapour by indirect heat exchange. #7. A
process according to #6, wherein at least a portion of the duty
required to warm said carbon dioxide-enriched overhead vapour is
provided by indirect heat exchange against at least one of said
recycle fluids. #8. A process according to any of #2 to #7, wherein
said recycle fluids are recycled to an appropriate location in said
distillation column system after suitable pressure reduction. #9. A
process according to any of #2 to #8, wherein ratio of molar flow
of said first recycle fluid to said second recycle fluid is from
about 0.1 (i.e. 1:10) to about 10 (i.e. 10:1). #10. A process
according to #9, wherein said ratio is from about 3 (i.e. 3:1) to
about 10 (i.e. 10:1). #11. A process according to #2, wherein said
working fluid comprises crude carbon dioxide gas generated by
evaporating "intermediate" liquid taken from an intermediate
location in said distillation column system by indirect heat
exchange after suitable pressure reduction. #12. A process
according to #11, comprising at least partially condensing carbon
dioxide-enriched overhead vapour by indirect heat exchange to
produce at least partially condensed carbon dioxide-enriched
overhead vapour as said reflux for said distillation column system.
#13. A process according to #12, wherein at least a portion of the
duty required to cool and at least partially condense said carbon
dioxide-enriched overhead vapour is provided by indirect heat
exchange against at least one "cold" process stream. #14. A process
according to any of #11 to #13, wherein at least a portion of the
duty required to evaporate said "intermediate" liquid is provided
by indirect heat exchange against at least one of said recycle
fluids. #15. A process according to any of #11 to #14, wherein said
"intermediate" liquid has a composition that is at least
substantially identical to said crude carbon dioxide feed. #16. A
process according to any of #11 to #15, wherein said first recycle
fluid is recycled as part of said feed to said distillation column
system. #17. A process according to any of #11 to #16, wherein said
second recycle fluid is recycled as part of said working fluid for
said heat pump cycle after suitable pressure reduction. #18. A
process according to #1 comprising at least a first heat pump cycle
and a second heat pump cycle, each heat pump cycle comprising at
least one recycle fluid, said recycle fluid of said first heat pump
cycle or, where the first heat pump cycle has more than one recycle
fluid, at least one of said recycle fluids, having a pressure that
is greater than that of a recycle fluid of said second heat pump
cycle. #19. A process according to #18, wherein the pressure of
said recycle fluid of said first heat pump cycle is at least 10%
greater than the pressure of said recycle fluid of said second heat
pump cycle. #20. A process according to #18 or #19, wherein the
pressure of said recycle fluid of said first heat pump cycle is
from about 15 bar to about 60 bar. #21. A process according to any
of #18 to #20, wherein said working fluid of said first heat pump
cycle comprises carbon dioxide-enriched gas generated by warming
said carbon dioxide-enriched overhead vapour by indirect heat
exchange. #22. A process according to #21, wherein at least a
portion of the duty required to warm said carbon dioxide-enriched
overhead vapour is provided by indirect heat exchange against at
least one of said recycle streams. #23. A process according to any
of #18 to #22, wherein the pressure of said recycle fluid of said
second heat pump cycle is from about 10 bar to about 25 bar. #24. A
process according to any of #18 to #23, wherein said working fluid
of said second heat pump cycle comprises crude carbon dioxide gas
generated by warming "intermediate" vapour taken from an
intermediate location of said distillation column system by
indirect heat exchange. #25. A process according to #24, wherein at
least a portion of the duty required to warm said "intermediate"
vapour is provided by indirect heat exchange against at least one
of said recycle fluids. #26. A process according to #24 or #25,
wherein said crude carbon dioxide gas has a composition that is at
least substantially identical to said crude carbon dioxide feed.
#27. A process according to #18 to #26 wherein said recycle streams
are recycled to appropriate locations in said distillation column
system after suitable pressure reduction if required. #28. A
process according to any of #1 to #27, liquid from an intermediate
location in said distillation column system is at least partially
re-boiled by indirect heat exchange to provide additional vapour
for said distillation column system. #29. A process according to
#28, wherein at least a portion of the re-boiling duty is provided
by indirect heat exchange against at least one of said recycle
fluids. #30. A process according to any of #1 to #29, wherein a
portion of said working fluid is purged from said process. #31. A
process according to any of #1 to #30, wherein said reflux for said
distillation column system is provided by at least one recycle
fluid condensate after suitable pressure reduction. #32. A process
according to #31, wherein the refrigeration duty required to cool
and at least partially condense at least one recycle fluid is
provided by indirect heat exchange against at least one "cold"
process stream. #33. A process according to any of #1 to #32,
wherein said reflux for said distillation column system is provided
by condensed overhead vapour. #34. A process according to #33,
wherein the refrigeration duty required to cool and at least
partially condense overhead vapour is provided by indirect heat
exchange against at least one "cold" process stream. #35. A process
according to any of #1 to #34, wherein said crude carbon dioxide
feed is crude carbon dioxide fluid derived from a natural source of
carbon dioxide and expanded prior to feeding to said distillation
column system. #36. A process according to #35, wherein, prior to
said expansion, said crude carbon dioxide fluid is at a
super-critical pressure and a sub-critical temperature. #37. A
process according to #35 or #36, wherein said crude carbon dioxide
fluid is cooled by indirect heat exchange prior to expansion. #38.
A process according to #37, wherein at least a portion of the duty
required to cool said crude carbon dioxide fluid is provided by
indirect heat exchange against at least one "cold" process stream.
#39. A process according to any of #35 to #38, wherein said
expanded crude carbon dioxide is used as a "cold" process stream to
provide refrigeration duty for said process. #40. A process
according to any of #35 to #38, wherein said expanded crude carbon
dioxide fluid is fed directly to said distillation column system.
#41. A process according to any of #1 to #40, wherein said feed is
derived from supercritical crude carbon dioxide liquid and carbon
dioxide-enriched liquid is produced as a product. #42. A process
according to #41, wherein said carbon dioxide-enriched liquid is
removed from said distillation column system, pumped and warmed by
indirect heat exchange to produce warmed carbon dioxide-enriched
liquid as said product. #43. A process according to #42, wherein at
least a portion of the duty required to warm said pumped carbon
dioxide-enriched liquid is provided by indirect heat exchange
against at least one of said recycle fluids. #44. A process
according to #42 or #43, wherein said pumped carbon
dioxide-enriched liquid is used as a "cold" process stream to
provide refrigeration duty for the process. #45. A process
according to any of #1 to #40, wherein said feed is derived from
crude carbon dioxide vapour and carbon dioxide-enriched gas is
produced as a product. #46. A process according to #45, wherein a
portion of said carbon dioxide-enriched vapour is warmed by
indirect heat exchange to produce said carbon dioxide-enriched gas.
#47. A process according to #46, wherein at least a portion of the
duty required to warm said carbon dioxide-enriched overhead vapour
is provided by indirect heat exchange against at least one of said
recycle fluids. #48. A process according to #46 or #47, wherein
said carbon dioxide-enriched overhead vapour is used as a "cold"
process stream to provide refrigeration duty for the process. #49.
A process according to #1 to #48, wherein said further portion of
bottoms liquid, or said liquid derived from bottoms liquid, is
pumped and warmed by indirect heat exchange to provide
impurity-rich waste liquid. #50. A process according to #49,
wherein at least a portion of the duty required to warm said pumped
bottoms liquid is provided by indirect heat exchange against at
least one of said recycle fluids. #51. A process according to #49
or #50, wherein said further portion of said bottoms liquid, or
said liquid derived from bottoms liquid, is used as a "cold"
process stream to provide refrigeration duty for the process. #52.
A process according to any of #1 to #51, wherein the operating
pressure(s) said distillation column system is from about 10 bar to
about 25 bar. #53. A process according to any of #1 to #52, wherein
said at least one impurity is hydrogen sulphide (H.sub.2S). #54. A
process according to any of #1 to #53, wherein said process is
auto-ref rigerated. #55. A process for purifying crude carbon
dioxide comprising at least one impurity that is less volatile than
carbon dioxide, said process comprising: [0166] feeding crude
carbon dioxide feed at sub-ambient temperature to a distillation
column system operating at super-atmospheric pressure(s) for
separation to produce carbon dioxide-enriched overhead vapour and
bottoms liquid enriched with said at least one impurity; [0167]
removing said carbon dioxide-enriched overhead vapour from said
distillation column system and warming at least a portion of said
carbon dioxide-enriched overhead vapour by indirect heat exchange
to produce warmed carbon dioxide-enriched gas; [0168] compressing a
first working fluid comprising said warmed carbon dioxide-enriched
gas to produce at least one compressed carbon dioxide-enriched gas;
[0169] cooling and at least partially condensing at least a portion
of said compressed carbon dioxide-enriched gas as a first recycle
fluid by indirect heat exchange to produce carbon dioxide-enriched
fluid; [0170] expanding said carbon dioxide-enriched fluid to
produce expanded carbon dioxide-enriched fluid and feeding said
expanded carbon dioxide-enriched fluid to said distillation column
system, at least a portion of which being used as reflux; [0171]
compressing a second working fluid comprising carbon dioxide-rich
gas from said distillation column system to produce at least one
second recycle fluid; [0172] cooling and optionally condensing at
least a portion of said second recycle fluid by indirect heat
exchange to produce cooled carbon dioxide-rich fluid; [0173] after
expansion as required, feeding at least a portion of said cooled
carbon dioxide-rich fluid to said distillation column system;
[0174] at least partially re-boiling a portion of said bottoms
liquid by indirect heat exchange to produce vapour for said
distillation column system; and [0175] removing a further portion
of said bottoms liquid, or a liquid derived from bottoms liquid,
from said distillation column system, wherein re-boiling duty for
said distillation column system is provided at least in part by
indirect heat exchange against said first and second recycle
fluids, said first recycle fluid having a different pressure from
said second recycle fluid. #56. A process according to #55, wherein
said compressed carbon dioxide-enriched gas is divided into at
least a first portion and a second portion, wherein said first
portion is said first recycle fluid(s), and wherein said second
portion is said carbon dioxide-rich gas for compression to produce
said second recycle fluid(s). #57. A process according to #55 or
#56, wherein carbon dioxide-rich vapour is removed from an
intermediate location in the distillation column system and warmed
by indirect heat exchange to produce said carbon dioxide-rich gas
for compression to produce said second recycle fluid(s). #58. A
process according to any of #55 to #57, wherein liquid from an
intermediate location in said distillation column system is at
least partially re-boiled by indirect heat exchange to provide
additional vapour for said distillation column system. #59. A
process for purifying crude carbon dioxide comprising at least one
impurity that is less volatile than carbon dioxide, said process
comprising: [0176] feeding crude carbon dioxide feed at sub-ambient
temperature to a distillation column system operating at
super-atmospheric pressure(s) for separation to produce carbon
dioxide-enriched overhead vapour and bottoms liquid enriched with
said at least one impurity; [0177] condensing a portion of said
carbon dioxide-enriched overhead vapour by indirect heat exchange
to provide reflux for said distillation column system;
[0178] removing a further portion of said carbon dioxide-enriched
overhead vapour from said distillation system; [0179] removing
carbon dioxide-rich liquid from an intermediate location in said
distillation column system and expanding said liquid to produce
expanded carbon dioxide-rich liquid; [0180] warming and evaporating
said expanded carbon dioxide-rich liquid by indirect heat exchange
to provide warmed carbon dioxide-rich gas; [0181] compressing a
working fluid comprising said warmed carbon dioxide-rich gas to
produce at least one compressed carbon dioxide-rich gas as a first
recycle fluid and at least one further compressed carbon
dioxide-rich gas as a second recycle fluid;
[0182] cooling and optionally at least partially condensing said
first recycle fluid by indirect heat exchange to produce cooled
first carbon dioxide-rich fluid; [0183] combining said cooled first
carbon dioxide-rich fluid with crude carbon dioxide fluid to
produce said crude carbon dioxide feed for the distillation column
system; [0184] cooling and at least partially condensing said
second recycle fluid by indirect heat exchange to produce cooled
second carbon dioxide-rich fluid; [0185] expanding said cooled
second carbon dioxide-rich fluid to produce expanded carbon
dioxide-rich fluid; [0186] combining said expanded carbon
dioxide-rich fluid with a fluid selected from the group consisting
of said carbon dioxide-rich liquid, said expanded carbon
dioxide-rich liquid, and said warmed carbon dioxide-rich gas;
[0187] at least partially re-boiling a portion of said bottoms
liquid by indirect heat exchange against at least one "warm"
process stream to produce vapour for said distillation column
system; and [0188] removing a further portion of said bottoms
liquid, or a liquid derived from bottoms liquid, from said
distillation column system, wherein said re-boiling duty is
provided at least in part by indirect heat exchange against said
first and second recycle fluids and wherein, in embodiments in
which said expanded carbon dioxide-rich fluid is combined with said
warmed carbon dioxide-rich gas, said expanded carbon dioxide-rich
fluid is first warmed and evaporated by indirect heat exchange to
produce further warmed carbon dioxide-rich gas for said combination
with said warmed carbon dioxide-rich gas. #60. Apparatus for
carrying out a process according to #55, said apparatus comprising:
[0189] a distillation column system for operation at
super-atmospheric pressure(s) for separating crude carbon dioxide
feed at sub-ambient temperature to produce carbon-dioxide-enriched
vapour and bottoms liquid enriched with said at least one impurity;
[0190] a first heat exchanger arrangement for warming at least a
portion of said carbon dioxide-enriched overhead vapour by indirect
heat exchange to produce warmed carbon dioxide-enriched gas; [0191]
a conduit arrangement for removing carbon dioxide-enriched overhead
vapour from said distillation column system and feeding said vapour
to said first heat exchanger arrangement; [0192] a first compressor
system for compressing said warmed carbon dioxide-enriched gas to
produce at least one compressed carbon dioxide-enriched gas; [0193]
a second heat exchanger arrangement for cooling and at least
partially condensing at least a portion of said compressed carbon
dioxide-enriched gas as a first recycle fluid by indirect heat
exchange to produce carbon dioxide-enriched fluid; [0194] a first
expansion device for expanding said carbon dioxide-enriched fluid
to produce expanded carbon dioxide-enriched fluid for feeding to
said distillation column system as reflux; [0195] a second
compressor system for compressing a carbon dioxide-rich gas from
said distillation column system to produce at least one second
recycle fluid; [0196] a third heat exchanger arrangement for
cooling and optionally condensing at least a portion of said second
recycle fluid by indirect heat exchange to produce cooled carbon
dioxide-rich fluid for feeding to said distillation column system;
[0197] an optional expansion device for expanding said cooled
carbon dioxide-rich fluid to produce expanded carbon dioxide-rich
fluid prior to being fed to said distillation column system; [0198]
a fourth heat exchanger arrangement for at least partially
re-boiling said bottoms liquid by indirect heat exchange against at
least one of said recycle streams to produce vapour for said
distillation column system; and [0199] a conduit arrangement for
removing a further portion of said bottoms liquid, or a liquid
derived from bottoms liquid, from said distillation column system,
wherein said first and second compression systems are capable of
compressing said warmed carbon dioxide-enriched gas and said carbon
dioxide-rich gas respectively to different pressures. #61.
Apparatus according to #60 comprising a conduit arrangement for
feeding compressed carbon dioxide enriched-gas from said first
compressor system as feed to said second compressor system. #62.
Apparatus according to #60 comprising: [0200] a fifth heat
exchanger arrangement for warming a carbon dioxide-rich vapour by
indirect heat exchange to produce warmed carbon dioxide-rich gas;
[0201] a conduit arrangement for feeding carbon dioxide-rich vapour
from an intermediate location in said distillation column system to
said fifth heat exchanger arrangement; and [0202] a conduit
arrangement for feeding warmed carbon dioxide-rich gas from said
fifth heat exchanger arrangement to said second compressor system.
#63. Apparatus according to any of #60 to #62, comprising a sixth
heat exchanger arrangement for at least partially re-boiling liquid
from an intermediate location in said distillation column system to
provide additional vapour for said distillation column system. #64.
Apparatus for carrying out a process according to #59, said
apparatus comprising: [0203] a distillation column system for
operation at super-atmospheric pressure(s) for separating crude
carbon dioxide feed at sub-ambient temperature to produce
carbon-dioxide-enriched vapour and bottoms liquid enriched with
said at least one impurity; [0204] a first heat exchanger
arrangement for cooling and partially condensing said carbon
dioxide-enriched overhead vapour by indirect heat exchange to
produce at least partially condensed carbon dioxide-enriched
overhead vapour as reflux for said distillation column system;
[0205] a conduit arrangement for removing carbon dioxide-enriched
overhead vapour from said distillation column system; [0206] a
first expansion device for expanding carbon dioxide-rich liquid to
produce expanded carbon dioxide-rich liquid; [0207] a conduit
arrangement for feeding carbon dioxide-rich liquid from an
intermediate location in said distillation column system to said
first expansion device; [0208] a second heat exchange arrangement
for warming and evaporating said expanded carbon dioxide-rich
liquid by indirect heat exchange to provide warmed carbon
dioxide-rich gas; [0209] a compressor system for compressing a
working fluid comprising said combined carbon dioxide-rich gas to
produce compressed carbon dioxide-rich gas as a first recycle fluid
and at least one further compressed carbon dioxide-rich gas as a
second recycle fluid; [0210] a third heat exchange system for
cooling and optionally at least partially condensing said first
recycle fluid by indirect heat exchange to produce cooled first
carbon dioxide-rich fluid; [0211] a conduit arrangement for
combining said cooled first carbon dioxide-rich fluid with crude
carbon dioxide fluid to produce said crude carbon dioxide feed for
the distillation column system; [0212] a fourth heat exchange
arrangement for cooling said second recycle fluid by indirect heat
exchange to produce cooled second carbon dioxide-rich fluid; [0213]
a second expansion device for expanding said cooled second carbon
dioxide-rich fluid to produce expanded carbon dioxide-rich fluid;
[0214] a conduit arrangement for combining said expanded carbon
dioxide-rich fluid with a fluid selected from the group consisting
of said carbon dioxide-rich liquid, said expanded carbon
dioxide-rich liquid, and said warmed carbon dioxide-rich gas;
[0215] a fifth heat exchanger arrangement for at least partially
re-boiling a portion of said bottoms liquid by indirect heat
exchange against at least one of said recycle streams to produce
vapour for said distillation column system; and [0216] a conduit
arrangement for removing a further portion of said bottoms liquid,
or a liquid derived from bottoms liquid, from said distillation
column system, wherein, in embodiments in which said expanded
carbon dioxide-rich fluid is combined with said warmed carbon
dioxide-rich gas, said apparatus comprises sixth heat exchanger
arrangement for warming expanded carbon dioxide-rich fluid by
indirect heat exchange to produce further warmed carbon
dioxide-rich gas for said combination with said warmed carbon
dioxide-rich gas. #65. Apparatus according to any of #60 to #64,
wherein said heat exchanger arrangements are passages within a
single main heat exchanger.
COMPARATIVE EXAMPLE
[0217] The flow sheet depicted in FIG. 1 was modelled by computer
using Aspen Plus (version 7.2) software and the heat and mass
balance data for key streams are provided in Table 1. In the model,
the purge stream 146 had zero flow.
[0218] According to the modelling, the process of the comparative
example consumes in total 17,054 kW of energy. This figure is the
sum of the power required for compressors CP1 and CP2 (15,278 kW)
and pumps P1 to P4 (1935 kW) less the power recovered by the feed
expander E1 (160 kW).
TABLE-US-00001 TABLE 1 COMPARATIVE EXAMPLE Stream No. 100 102 104
106 154 156 158 160 186 188 190 192 Tem- .degree. C. 8.6 -35.8
-37.7 -31.5 -40.9 -37.6 32.0 54.0 -3.8 -0.6 32.0 49.6 perature
Pressure Bar 44.8 44.7 12.8 12.7 12.6 80.0 79.9 153.0 12.7 48.1
48.0 208.0 Molar kmol/ 1.798 1.798 1.798 1.798 1.657 1.657 1.657
1.657 0.142 0.142 0.142 0.142 Flow s Vapour 0.00 0.00 0.01 0.99
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fraction Mole 0.9140 0.9140
0.9140 0.9140 0.9873 0.9873 0.9873 0.9873 0.0578 0.0578 0.0578
0.0578 fraction CO.sub.2 Mole 0.0740 0.0740 0.0740 0.0740 0.0000
0.0000 0.0000 0.0000 0.9380 0.9380 0.9380 0.9380 fraction H.sub.2S
Mole 0.0100 0.0100 0.0100 0.0100 0.0109 0.0109 0.0109 0.0109 0.0000
0.0000 0.0000 0.0000 fraction Methane Mole 0.0018 0.0018 0.0018
0.0018 0.0018 0.0018 0.0018 0.0018 0.0016 0.0016 0.0016 0.0016
fraction Propane Mole 0.0002 0.0002 0.0002 0.0002 0.0000 0.0000
0.0000 0.0000 0.0025 0.0025 0.0025 0.0025 fraction Methanol Stream
Number 110 112 140 142 120 122 130 132 180 182 170 172 Temperature
.degree. C. -34.1 -6.1 -40.9 -2.7 38.0 -35.7 -19.7 -3.8 Pressure
Bar 12.6 12.4 12.6 12.4 32.7 32.6 12.7 12.7 Molar Flow kmol/s 5.725
5.725 0.253 0.253 5.977 5.977 3.644 3.644 Vapour Fraction 1.00 1.00
1.00 1.00 1.00 0.00 0.00 0.96 Mole fraction Carbon 0.9873 0.9873
0.7932 0.7932 0.9791 0.9791 0.2812 0.2812 Dioxide Mole fraction
Hydrogen 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.7155 0.7155
Sulphide Mole fraction Methane 0.0109 0.0109 0.2052 0.2052 0.0191
0.0191 0.0000 0.0000 Mole fraction Propane 0.0018 0.0018 0.0016
0.0016 0.0018 0.0018 0.0031 0.0031 Mole fraction Methanol 0.0000
0.0000 0.0000 0.0000 0.0000 0.0000 0.0001 0.0001 CP1 12666 kW CP2
2612 kW P1 573 kW P2 1128 kW P3 40 kW P4 194 kW E1 -160 kW Total
power 17054 kW
Example 1
[0219] The flow sheet depicted in FIG. 2 was also modelled by
computer using Aspen Plus (version 7.2) and the heat and mass
balance data for key streams is provided in Table 2. In the model,
the purge stream had zero flow.
[0220] According to the modelling, this embodiment of the present
invention consumes a total of 15,671 kW, indicating an overall
power saving compared to the comparative example of 1,383 kW or
about 8.1%. The bulk of this saving is achieved by the reduction in
the power required to compress the carbon dioxide gas as working
fluid for the heat pump cycle due to the reduction in flow of gas
through the second compressor system CP2.
TABLE-US-00002 TABLE 2 EXAMPLE 1 Stream No. 100 102 104 106 154 156
158 160 186 188 190 192 Tem- .degree. C. 8.6 -40.7 -43.9 -36.5
-48.7 -45.6 29.2 44.8 -11.6 -8.4 29.2 46.6 perature Pressure Bar
44.8 44.7 10.2 10.1 10.0 79.9 79.8 153.0 10.1 46.8 46.7 208.0 Molar
kmol/ 1.798 1.798 1.798 1.798 1.657 1.657 1.657 1.657 0.142 0.142
0.142 0.142 Flow s Vapour 0.00 0.00 0.02 1.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 Fraction Mole 0.9140 0.9140 0.9140 0.9140
0.9873 0.9873 0.9873 0.9873 0.0579 0.0579 0.0579 0.0579 fraction
CO.sub.2 Mole 0.0740 0.0740 0.0740 0.0740 0.0000 0.0000 0.0000
0.0000 0.9386 0.9386 0.9386 0.9386 fraction H.sub.2S Mole 0.0100
0.0100 0.0100 0.0100 0.0109 0.0109 0.0109 0.0109 0.0000 0.0000
0.0000 0.0000 fraction Methane Mole 0.0018 0.0018 0.0018 0.0018
0.0019 0.0019 0.0019 0.0019 0.0010 0.0010 0.0010 0.0010 fraction
Propane Mole 0.0002 0.0002 0.0002 0.0002 0.0000 0.0000 0.0000
0.0000 0.0025 0.0025 0.0025 0.0025 fraction Methanol Stream No. 110
112 140 142 120 122 130 132 180 182 170 172 Temperature .degree. C.
-40.5 5.9 -48.7 6.0 38.0 -42.1 38.0 -42.6 -27.4 -11.6 Pressure bar
10.0 9.8 10.0 9.8 16.3 16.1 27.7 27.6 10.1 10.1 Molar Flow kmol/s
5.273 5.273 0.282 0.282 1.593 1.593 3.963 3.963 3.280 3.280 Vapour
1.00 1.00 1.00 1.00 1.00 0.00 1.00 0.00 0.00 0.96 Fraction Mole
fraction 0.9873 0.9873 0.7376 0.7376 0.9746 0.9746 0.9746 0.9746
0.2983 0.2983 CO.sub.2 Mole fraction 0.0000 0.0000 0.0000 0.0000
0.0000 0.0000 0.0000 0.0000 0.6996 0.6996 H.sub.2S Mole fraction
0.0109 0.0109 0.2607 0.2607 0.0236 0.0236 0.0236 0.0236 0.0000
0.0000 Methane Mole fraction 0.0019 0.0019 0.0017 0.0017 0.0019
0.0019 0.0019 0.0019 0.0019 0.0019 Propane Mole fraction 0.0000
0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0001 0.0001
Methanol CP1 7759 kW CP2 6293 kW P1 580 kW P2 970 kW P3 41 kW P4
194 kW E1 -166 kW Total power 15671 kW
Example 2
[0221] The flow sheet depicted in FIG. 3 was also modelled by
computer using Aspen Plus (version 7.2) and the heat and mass
balance data for key streams is provided in Table 3. In the model,
the purge stream had zero flow.
[0222] According to the modelling, this embodiment of the present
invention consumes a total of 9,933 kW, indicating an overall power
saving compared to the comparative example of 7,121 kW or about
41.8%. The bulk of this saving is achieved by the reduction in the
power required to compress the carbon dioxide gas as working fluid
for the heat pump cycle due to the introduction of the intermediate
re-boiler which reduces significantly the amount of overhead vapour
that needs to be further compressed in the second compressor
system, CP2.
TABLE-US-00003 TABLE 3 EXAMPLE 2 Stream No. 100 102 104 106 154 156
158 160 186 188 190 192 Tem- .degree. C. 8.6 -22.2 -26.9 -22.2
-30.3 -26.8 29.8 46.1 7.4 10.3 29.8 47.2 perature Pressure bar 44.8
44.7 17.2 17.1 17.0 80.0 79.9 153.0 17.1 47.7 47.6 208.0 Molar
kmol/ 1.798 1.798 1.798 1.798 1.656 1.656 1.656 1.656 0.142 0.142
0.142 0.142 Flow s Vapour 0.00 0.00 0.03 1.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 Fraction Mole 0.9140 0.9140 0.9140 0.9140
0.9876 0.9876 0.9876 0.9876 0.0577 0.0577 0.0577 0.0577 fraction
CO.sub.2 Mole 0.0740 0.0740 0.0740 0.0740 0.0000 0.0000 0.0000
0.0000 0.9351 0.9351 0.9351 0.9351 fraction H.sub.2S Mole 0.0100
0.0100 0.0100 0.0100 0.0109 0.0109 0.0109 0.0109 0.0000 0.0000
0.0000 0.0000 fraction Methane Mole 0.0018 0.0018 0.0018 0.0018
0.0015 0.0015 0.0015 0.0015 0.0048 0.0048 0.0048 0.0048 fraction
Propane Mole 0.0002 0.0002 0.0002 0.0002 0.0000 0.0000 0.0000
0.0000 0.0025 0.0025 0.0025 0.0025 fraction Methanol Stream No. 110
112 140 142 120 122 130 132 180 182 170 172 Temperature .degree. C.
-25.1 14.9 -30.3 15.2 38.0 -25.0 38.0 -24.6 -6.5 7.4 -21.4 -18.9
Pressure bar 17.0 16.8 17.0 16.8 24.6 24.4 44.9 44.8 17.1 17.1 17.1
17.1 Molar Flow kmol/ 6.765 6.765 0.316 0.316 6.457 6.457 0.624
0.624 0.619 0.619 5.077 5.077 s Vapour 1.00 1.00 1.00 1.00 1.00
0.00 1.00 0.00 0.00 0.77 0.00 0.86 Fraction Mole 0.9876 0.9876
0.8502 0.8502 0.9815 0.9815 0.9815 0.9815 0.2153 0.2153 0.7775
0.7775 fraction CO.sub.2 Mole 0.0000 0.0000 0.0000 0.0000 0.0000
0.0000 0.0000 0.0000 0.7766 0.7766 0.2198 0.2198 fraction H.sub.2S
Mole 0.0109 0.0109 0.1484 0.1484 0.0170 0.0170 0.0170 0.0170 0.0000
0.0000 0.0002 0.0002 fraction Methane Mole 0.0015 0.0015 0.0014
0.0014 0.0015 0.0015 0.0015 0.0015 0.0074 0.0074 0.0024 0.0024
fraction Propane Mole 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000
0.0000 0.0000 0.0006 0.0006 0.0001 0.0001 fraction Methanol CP1
7214 kW CP2 1085 kW P1 554 kW P2 985 kW P3 36 kW P4 195 kW E1 -134
kW Total power 9933 kW
Example 3
[0223] The flow sheet depicted in FIG. 8 was also modelled by
computer using Aspen Plus (version 7.2) and the heat and mass
balance data for key streams is provided in Table 4. In the model,
the purge stream had zero flow.
[0224] According to the modelling, this embodiment of the present
invention consumes a total of 14,334 kW, indicating an overall
power saving compared to the comparative example of 2,720 kW or
about 15.9%. The additional power requirement for the carbon
dioxide product compressor is more than off-set by the reduction in
power arising from the modified heat pump cycle.
TABLE-US-00004 TABLE 4 EXAMPLE 3 Stream No. 100 102 104 106 154 111
158 160 186 188 190 192 Temperature .degree. C. 11.1 -23.3 -24.8
-24.8 10.8 38.0 7.4 9.5 29.8 48.2 Pressure bar 44.8 17.1 17.0 17.0
16.9 110.0 17.1 39.2 39.1 208.0 Molar Flow kmol/s 1.798 1.798 6.858
1.248 1.656 1.656 0.142 0.142 0.142 0.142 Vapour Fraction 1.00 0.87
1.00 1.00 1.00 1.00 0.00 0.00 0.00 0.00 Mole fraction 0.9140 0.9140
0.9941 0.9941 0.9875 0.9875 0.0578 0.0578 0.0578 0.0578 CO.sub.2
Mole fraction 0.0740 0.0740 0.0000 0.0000 0.0000 0.0000 0.9354
0.9354 0.9354 0.9354 H.sub.2S Mole fraction 0.0100 0.0100 0.0044
0.0044 0.0109 0.0109 0.0000 0.0000 0.0000 0.0000 Methane Mole
fraction 0.0018 0.0018 0.0016 0.0016 0.0016 0.0016 0.0043 0.0043
0.0043 0.0043 Propane Mole fraction 0.0002 0.0002 0.0000 0.0000
0.0000 0.0000 0.0025 0.0025 0.0025 0.0025 Methanol Stream No. 110
112 140 142 120 122 130 132 180 182 170 172 Temperature .degree. C.
-24.8 14.3 38.0 -17.2 38.0 -8.6 -7.0 7.4 -21.3 -18.8 Pressure bar
17.0 16.8 22.8 22.6 37.6 37.5 17.1 17.1 17.1 17.1 Molar Flow kmol/s
5.610 5.610 5.035 5.035 0.576 0.576 0.799 0.799 5.062 5.062 Vapour
Fraction 1.00 1.00 1.00 0.00 1.00 0.00 0.00 0.82 0.00 0.87 Mole
fraction CO.sub.2 0.9941 0.9941 0.9941 0.9941 0.9941 0.9941 0.2266
0.2266 0.7759 0.7759 Mole fraction H.sub.2S 0.0000 0.0000 0.0000
0.0000 0.0000 0.0000 0.7661 0.7661 0.2214 0.2214 Mole fraction
0.0044 0.0044 0.0044 0.0044 0.0044 0.0044 0.0000 0.0000 0.0002
0.0002 Methane Mole fraction 0.0016 0.0016 0.0016 0.0016 0.0016
0.0016 0.0069 0.0069 0.0025 0.0025 Propane Mole fraction 0.0000
0.0000 0.0000 0.0000 0.0000 0.0000 0.0005 0.0005 0.0000 0.0000
Methanol CP1 4541 kW CP2 833 kW CP3 10190 kW P2 565 kW P3 26 kW P4
206 kW E1 -2027 kW Total power 14334 kW
[0225] While the invention has been described with reference to the
preferred embodiments depicted in the figures, it will be
appreciated that various modifications are possible within the
spirit or scope of the invention.
[0226] In this specification, unless expressly otherwise indicated,
the word `or` is used in the sense of an operator that returns a
true value when either or both of the stated conditions are met, as
opposed to the operator `exclusive or` which requires only that one
of the conditions is met. The word `comprising` is used in the
sense of `including` rather than to mean `consisting of`. All prior
teachings above are hereby incorporated herein by reference. No
acknowledgement of any prior published document herein should be
taken to be an admission or representation that the teaching
thereof was common general knowledge in Australia or elsewhere at
the date thereof.
* * * * *