U.S. patent application number 09/755200 was filed with the patent office on 2001-07-19 for hydrocarbon separation process and apparatus.
Invention is credited to Finn, Adrian Joseph, Johnson, Grant Leigh.
Application Number | 20010008073 09/755200 |
Document ID | / |
Family ID | 9883338 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008073 |
Kind Code |
A1 |
Finn, Adrian Joseph ; et
al. |
July 19, 2001 |
Hydrocarbon separation process and apparatus
Abstract
A process is described for separating the heavier hydrocarbons
from a gaseous hydrocarbon feed wherein a first separator is
employed to separate partially condensed gaseous feed and wherein
the vapour portion undergoes work expansion and is fed to a
fractionation column. The liquid portion is subcooled in heat
exchange with the overhead vapour from the fractionation column,
expanded, evaporated to provide refrigeration at a low temperature
level, and fed to the fractionation column. The rewarmed residual
vapour is subsequently compressed to a pressure suitable for
export, with a portion of the compressed gas being cooled,
condensed and recycled back to reflux the top section of the
fractionation column. Also described is a process wherein a first
separator is employed to separate partially condensed gaseous feed
and wherein the vapour portion undergoes work expansion and is fed
to a high pressure wash column. The liquid portion is expanded and
fed to the base of the high pressure wash column. Bottoms liquid
from the wash column is subcooled in heat exchange with the
overhead vapour from a fractionation column, expanded, evaporated
to provide refrigeration at a low temperature level, and fed to the
fractionation column. Vapour from the high pressure wash column is
partially condensed, with the liquid portion used to provide reflux
to the high pressure wash column and the fractionation column. The
processes are especially applicable to recovery of ethane and
heavier components from natural gas. Overall process power
requirements are reduced, recovery of the desired heavy
hydrocarbons is increased or both of these effects are
realised.
Inventors: |
Finn, Adrian Joseph; (West
Yorkshire, GB) ; Johnson, Grant Leigh; (Cheshire,
GB) |
Correspondence
Address: |
STERNE, KESSLER, GOLDSTEIN & FOX PLLC
1100 NEW YORK AVENUE, N.W., SUITE 600
WASHINGTON
DC
20005-3934
US
|
Family ID: |
9883338 |
Appl. No.: |
09/755200 |
Filed: |
January 8, 2001 |
Current U.S.
Class: |
62/621 ;
62/623 |
Current CPC
Class: |
C10G 5/06 20130101; F25J
3/0209 20130101; F25J 2200/02 20130101; F25J 2200/74 20130101; F25J
2270/90 20130101; F25J 2270/02 20130101; F25J 2245/02 20130101;
F25J 3/0219 20130101; C07C 7/005 20130101; F25J 2240/02 20130101;
F25J 3/0238 20130101; F25J 2230/60 20130101; F25J 2200/76 20130101;
F25J 2200/78 20130101; F25J 2200/04 20130101; F25J 2205/04
20130101; F25J 3/0233 20130101; F25J 2210/12 20130101 |
Class at
Publication: |
62/621 ;
62/623 |
International
Class: |
F25J 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 7, 2000 |
GB |
0000327.7 |
Claims
We claim:
1. A process for the separation of a heavier hydrocarbon fraction
from a gaseous feed comprising a mixture of hydrocarbons, which
process comprises: (a) cooling the gaseous feed to produce a
partially condensed stream, (b) separating the partially condensed
stream to form a first liquid stream and a first gaseous stream,
(c) subcooling at least a portion of the first liquid stream, (d)
expanding said subcooled stream, (e) passing at least a portion of
the expanded stream from (d) as a liquid feed to a fractionation
column, (f) producing a cooled stream by cooling a separated lights
fraction of the feed, and (g) recovering said heavier hydrocarbon
fraction as a bottoms fraction from said column, characterised in
that (i) at least a part of the subcooling required in subcooling
step (c) is provided by transfer of heat to the expanded stream
from (d), (ii) at least a portion of the cold required to cool the
separated lights fraction to produce said cooled stream (f) is
provided by transfer of heat to the expanded stream from (d), and
(iii) at least a portion of said cooled stream (f) is introduced to
an upper part of the column.
2. A process according to claim 1 in which at least a part of the
subcooling required in subcooling step (c) is provided by transfer
of heat to overhead vapour from said fractionation column.
3. A process according to claim 1 wherein overhead vapour from the
fractionation column is heated in heat exchange with the first
liquid stream from step (b) prior to expansion.
4. A process according to claim 3 wherein overhead vapour from the
fractionation column and the expanded stream from step (d) are
heated in heat exchange with the first liquid stream from step (b)
prior to expansion in a single heat exchanger.
5. A process according to claim 4 wherein net refrigeration
available within the heat exchanger is used to cool said separated
lights fraction in step (f).
6. A process according to claim 5 wherein said separated lights
fraction comprises a recycle stream derived from overheads from the
fractionation column.
7. A process of claims 1 wherein overhead vapour from the
fractionation column is rewarmed, compressed and cooled to provide
a recycle stream and a residue gas product.
8. A process according to claim 7 wherein overhead vapour from the
fractionation column and the expanded stream from step (d) are
heated in heat exchange with the subcooled first liquid stream from
step (b) prior to expansion in a single heat exchanger.
9. A process according to claim 8 wherein net refrigeration
available within the heat exchanger is used to cool said separated
lights fraction in step (f).
10. A process according to claim 9 wherein said separated lights
fraction comprises a recycle stream derived from overheads from the
fractionation column.
11. A process according to claim 10 wherein at least a portion of
said recycle stream is condensed/subcooled and returned to an upper
part of the fractionation column.
12. A process according to claim 1 wherein said first liquid stream
is expanded and separated to give a further gaseous stream and at
least a portion of the remainder is subjected to the processes of
steps (c) and (d).
13. A process according to claim 12 wherein at least a part of said
further gaseous stream is cooled to form at least a portion of
cooled stream (f).
14. A process according to claim 13 wherein said further gaseous
stream or a portion thereof is combined with a recycled stream
derived from column overheads to form said separated lights
fraction, which is introduced into the fractionation column at a
point above the liquid feed.
15. A process according to claim 13 wherein said cooled stream (f)
or a portion thereof is expanded prior to its introduction into the
fractionation column.
16. A process according to claims 13 wherein said further gaseous
stream or a portion thereof is combined with the whole of said
recycle stream prior to being transferred to the fractionation
column.
17. A process according to claim 1 wherein said cooled stream (f)
or a portion thereof is expanded to produce an at least partially
condensed stream which is introduced into the fractionation
column.
18. A process according to claim 17 wherein said first gaseous
stream or a portion thereof is introduced into the fractionation
column at a point above the liquid feed.
19. A process according to claims 1 wherein said first gaseous
stream is work expanded and separated to give a further gaseous
stream and a further liquid stream, said further gaseous stream is
partially condensed in a heat exchanger and then fed to the
fractionation column.
20. A process according to claim 19 wherein said further gaseous
stream is introduced into the fractionation column at a point above
the liquid feed.
21. A process according to claim 19 wherein said further liquid
stream is also introduced into the fractionation column.
22. A process according to claim 19 wherein said further gaseous
stream is partially condensed in heat exchange with said expanded
stream from step (d).
23. A process according to claim 22 wherein overhead vapour from
the fractionation column and the expanded stream from step (d) are
heated in heat exchange with said first liquid stream from step (b)
prior to expansion.
24. A process according to claim 1 wherein one of said first
gaseous stream and said first liquid stream is expanded and
separated to give a further gaseous stream and a further liquid
stream.
25. A process according to claim 1 wherein both of said first
gaseous stream and said first liquid stream are expanded, combined
and separated to give a further gaseous stream and a further liquid
stream.
26. A process according to claim 1 wherein said first gaseous
stream is work expanded prior to separation.
27. A process according to claims 1 wherein said first gaseous
stream and said first liquid stream are fed to a reequilibration
device which produces a first output which is richer in heavier
hydrocarbons than the first liquid stream, and a second output
which is leaner in heavier hydrocarbons than the first gaseous
stream, wherein said second output is cooled and partially
condensed and directly or indirectly fed to an upper part of the
fractionation column as said cooled stream (f) and wherein said
first output is subjected the processes of steps (c) and (d).
28. A process according to claim 27 wherein said first gaseous
stream is work expanded before being fed to the reequilibration
device.
29. A process according to claim 27 wherein said reequilibration
device is a high pressure wash column.
30. A process according to claim 1 wherein said second output is
partially condensed in heat exchange with said expanded stream from
step (d).
31. A process according to claim 30 wherein said second output is
partially condensed and at least a portion of the first liquid
stream from step (b) is subcooled in heat exchange with said
overhead vapour from said fractionation column and said expanded
and at least partially evaporated subcooled stream from step
(d).
32. A process according to claim 30 wherein said partially
condensed output is fed to a separator, which produces an
additional gaseous stream and an additional liquid stream, wherein
at least a portion of said additional liquid stream is fed to an
upper part of the fractionation column.
33. A process according to claim 32 wherein at least a portion of
said additional liquid stream is fed to the reequilibration
device.
34. A process according to claim 32 wherein said additional gaseous
stream is warmed to produce a residual gas product.
35. A process according to claim 32 wherein said additional gaseous
stream is warmed in heat exchange with at least a portion of said
first liquid stream from step (b) prior to expansion.
36. A process according to claim 35 wherein said additional gaseous
stream and said overhead vapour from said fractionation column are
warmed in heat exchange with at least a portion of said first
liquid stream from step (b) prior to expansion, and the second
output from the reequilibration device.
37. A process according to claims 27 wherein at least a portion of
said first output is subcooled, expanded and at least partially
evaporated, and at least a portion of the expanded and at least
partially evaporated subcooled stream is used as a liquid feed for
the fractionation column.
38. A process according to claim 1 wherein an overhead fraction
from said fractionation column is warmed, and successively
compressed and cooled to produce a residue gas product.
39. A process according to claim 38 wherein a portion of said
residue gas product is cooled and recycled to the top of the
fractionation column as said cooled stream (f).
40. Apparatus for the separation of a heavier hydrocarbon fraction
from a gaseous feed comprising a mixture of hydrocarbons, wherein
said mixture is cooled, partially condensed, separated into a first
liquid stream and a first gaseous stream and at least a portion of
each of the first liquid stream and the first gaseous stream are
passed to a fractionation column in which said separation is
carried out, which apparatus comprises: (i) conduit means for
transferring at least a portion of the first liquid stream to a
heat exchanger in which said portion is subcooled, (ii) means for
expanding said subcooled stream, (iii) conduit means for
transferring at least a portion of said expanded stream to the heat
exchanger in which the subcooling is effected, whereby at least a
part of the subcooling is provided by transfer of heat to the
expanded stream, and (iv) conduit means for introducing a cooled
stream produced by cooling a separated lights fraction of the feed
to an upper part of the column, at least a portion of the cold
required to cool the separated lights fraction being provided by
transfer of heat to said expanded stream.
41. Apparatus according to claim 40 wherein said cooled, partially
condensed mixture is separated into a first gaseous stream and a
first liquid stream in a first separator, and said apparatus
further comprises means for expanding said first liquid stream and
means for separating said first expanded liquid stream into a
further gaseous stream and a further liquid stream, wherein said
further liquid stream is transferred to the heat exchanger.
42. Apparatus according to claim 40 further comprising means for
partially condensing the first gaseous stream, separation means for
separating at least a portion of said partially condensed stream
into a further gaseous stream and a further liquid stream, and
means for transferring at least a portion said further gaseous
stream to the fractionation column.
43. Apparatus according to claim 40 wherein said cooled, partially
condensed mixture is separated into a first gaseous stream and a
first liquid stream in a first separator, and said apparatus
includes (a) means for work expanding said first gaseous stream,
(b) means for expanding said first liquid stream, (c) a
reequilibration device which is arranged to receive said expanded
first gaseous stream and said expanded first liquid stream, and
produces a first output which is richer in heavier hydrocarbons
than said first expanded liquid stream, and a second output which
is leaner in heavier hydrocarbons than said first gaseous stream,
(d) conduit means for transferring at least a portion of said
second output to the fractionation column, and (e) conduit means
for transferring said first output to the heat exchanger stream
into a further gaseous stream and a further liquid stream, wherein
said further liquid stream is transferred to the heat exchanger.
Description
INTRODUCTION AND BACKGROUND
[0001] This invention relates to a process, and the apparatus for
effecting such a process, for the cryogenic fractionation of
gaseous hydrocarbon feeds to extract and recover the valuable
heavier components thereof. The invention is particularly concerned
with a process for high recovery of ethane and heavier components
from a natural gas feed. The process is not limited to the recovery
of paraffinic compounds such as ethane found in natural gas, but
also, for example, to olefins such as ethylene often found in gases
associated with petroleum refining or petrochemicals
manufacture.
[0002] Conventional processes to effect very high recovery of
ethane and heavier components from natural gas typically utilise a
combination of heat exchange, turbo-expansion, phase separation and
fractionation steps. The use of turbo-expansion produces work,
which can be used to drive a compressor to supplement residual gas
compression, and by removing energy from the feed gas produces low
temperature.
DESCRIPTION OF PRIOR ART
[0003] In such conventional processes feed gas is partially
condensed in a heat exchange system, which typically includes
rewarming residual vapour and may include other cold streams such
as refrigerant from a mechanical refrigeration cycle. Partial
condensation results in a liquid stream, enriched in the valuable
heavy components being recovered and a vapour stream, which may
undergo further partial condensation steps. These partial
condensation steps result finally in one or more liquid streams and
a high pressure vapour stream. The liquid streams are expanded and
fed to a demethaniser column, which removes the majority of the
methane and lighter components, to produce a stable liquid stream.
The high pressure vapour stream is work expanded giving a two phase
stream which is fed to the demethaniser at a point above the
expanded liquid streams.
[0004] It is conventional for the demethaniser column to be
refluxed with a stream colder than the expander exhaust. A number
of processes have been proposed, which differ in their selected
demethaniser reflux stream. These processes do however share the
principle of judiciously using heat exchanger surface area to make
good use of the available refrigeration and to thus give lower
process temperatures. Losses of the valuable ethane and heavier
components in the demethaniser overheads can thus be reduced
without decreasing the demethaniser column pressure and therefore
without excessive power requirement.
[0005] These processes give an improvement over traditional
processes, which use the expander exhaust as the top feed to the
demethaniser. Increasing recovery of ethane and heavier components
in these traditional processes requires a reduction in demethaniser
and expander exhaust pressure to reduce temperatures. Very high
ethane recovery can therefore result in uneconomically high power
requirements in either recompression of the residual vapour to
required product pressure, external refrigeration to increase
liquids condensation or in feed gas compression which also
increases liquids condensation.
[0006] It is common for the selected source of demethaniser reflux
to be lean in the components being recovered. A particularly
effective reflux stream is that derived from the demethaniser
overheads, which in a process effecting very high recovery of
ethane from natural gas may be nearly pure methane. A conventional
overhead condenser, condensing overhead vapour at column pressure,
can not usually be utilised due to the absence of process streams
at a lower temperature to provide the necessary refrigeration. In
the process of U.S. Pat. No. 4,839,545, a portion of the
demethaniser overhead vapour is compressed in a standalone
compressor, such that it can be condensed in heat exchange with
other process streams to reflux the demethaniser.
[0007] U.S. Pat. Nos. 4,171,964 and 4,157,904 describe processes in
which streams relatively rich in ethane are sent to the top of the
demethaniser to act as reflux, and thus do not provide very high
recovery of ethane. GB 2,309,072 and WO 98/50742 disclose
hydrocarbon gas processing apparatus wherein a recycle stream is
used to reflux the demethaniser.
[0008] A configuration in which a portion of the residue gas, which
has been rewarmed and compressed to a pressure suitable for export,
is recycled, condensed, subcooled and expanded to reflux the
demethaniser column shown in FIG. 1. This configuration is less
thermodynamically efficient than that of a standalone compressor,
due to the losses inherent in warming and re-cooling the residue
vapour. The process is however simpler as a standalone compressor
is not required.
[0009] The pressure at which the recycle stream is cooled and
condensed will typically be optimised to minimise residual gas
compression power requirement. It is desirable to sub-cool the
recycle stream to within a small approach to the demethaniser
overhead temperature, which is the coldest stream in the process.
This minimises evolution of vapour on expanding the liquid to
column pressure and therefore maximises the liquid available to
reflux the rising vapour in the column. At lower recycle stream
pressures, compression power requirements are reduced, but the
cooling curve becomes less linear and a pinch can occur which
limits the temperature to which the recycle stream can be
cooled.
SUMMARY OF INVENTION
[0010] According to one aspect of the invention there is provided a
process for the separation of a heavier hydrocarbon fraction from a
gaseous feed comprising a mixture of hydrocarbons, which process
comprises:
[0011] (a) cooling the gaseous feed to produce a partially
condensed stream
[0012] (b) separating the partially condensed stream to form a
first liquid stream and a first gaseous stream
[0013] (c) subcooling at least a portion of the first liquid
stream
[0014] (d) expanding said subcooled stream
[0015] (e) passing at least a portion of the expanded stream from
(d) as a liquid feed to a fractionation column
[0016] (f) producing a cooled stream by cooling a separated lights
fraction of the feed
[0017] (g) recovering said heavier hydrocarbon fraction as a
bottoms fraction from said column
[0018] characterised in that (i) at least a part of the subcooling
required in subcooling step (c) is provided by transfer of heat to
the expanded stream from (d), (ii) at least a portion of the cold
required to cool the separated lights fraction to produce said
cooled stream (f) is provided by transfer of heat to the expanded
stream from (d), and (iii) at least a portion of said cooled stream
(f) is introduced to an upper part of the column.
[0019] It will be appreciated that as a result of having been
expanded in step (d) and subjected to a heat transfer operation to
provide at least part of the subcooling required from step (c), the
feed to the fractionation column will be in an at least partially
evaporated state.
[0020] In a preferred manner of operation according to the
invention at least a part of the subcooling required in subcooling
step (c) is provided by transfer of heat to overhead vapour from
said fractionation column. When operated in this manner overhead
vapour from the fractionation column may be heated in heat exchange
with the first liquid stream from step (b) prior to expansion. More
preferably a process is provided wherein overhead vapour from the
fractionation column and the expanded stream from step (d) are
heated in heat exchange with said first liquid stream from step (b)
prior to expansion in a single heat exchanger.
[0021] It will be appreciated that in accordance with the
invention, net refrigeration available within the heat exchanger
may be used to cool said stream (f). Preferably the cooled stream
(f) comprises a recycle stream derived from overheads from the
fractionation column, and is used to reflux the top section of the
fractionation column.
[0022] By heat-exchanging the first liquid stream and the separated
lights fraction with the expanded stream from (d), against the
evaporating stream derived from the subcooled stream, a stream of
lower temperature is produced that ultimately leads to reduced
overheads temperature and increased recovery.
DESCRIPTION OF DRAWINGS
[0023] Embodiments of the invention will now be described in more
detail with particular reference to the accompanying drawings of
which:
[0024] FIG. 1 describes a prior art process for the separation of
heavier hydrocarbons from a gaseous hydrocarbon feed.
[0025] FIG. 2 describes a first embodiment of the present invention
wherein a heavier hydrocarbon fraction may be separated from a
gaseous hydrocarbon feed. The feed is partially condensed and then
separated into the first gaseous stream and the first liquid
stream. The first liquid stream is then subcooled in the heat
exchanger, expanded, at least partially evaporated, and then passed
to the fractionation column.
[0026] FIG. 3 displays a variation of the first embodiment wherein
the first liquid stream is separated into a further liquid stream
and a further gaseous stream, and the further liquid stream is then
transferred to the fractionation column via the heat exchanger, as
in FIG. 2.
[0027] FIG. 4 displays another variation of the first embodiment
wherein the first gaseous stream is separated into a further
gaseous stream and a further liquid stream, and then both streams
are transferred to the fractionation column.
[0028] FIG. 5 describes a second embodiment of the invention for
separating a heavier hydrocarbon from a gaseous hydrocarbon feed
wherein the first liquid stream and the first gaseous stream are
transferred to a reequilibration device which produces a first
output and a second output. The first output, which is richer in
heavier hydrocarbons, is then transferred to the fractionation
column via the heat exchanger as in FIG. 2.
[0029] In a first embodiment of the invention a process is provided
wherein overhead vapour from the fractionation column is rewarmed,
compressed and cooled to provide a recycle stream and a residue gas
product. Preferably the overhead vapour from the fractionation
column and the expanded stream from step (d) are heated in heat
exchange with the subcooled first liquid stream from step (b) prior
to expansion in a single heat exchanger. This enables small
temperature differences to be achieved between cooling and warming
streams and gives improved use of available refrigeration.
[0030] The process of the present invention according to this
embodiment, as shown in FIG. 2, offers an improvement over the
conventional residue gas recycle process, by enabling subcooling of
the recycle stream to a close approach to the demethaniser
overheads temperature at lower recycle pressures than is
conventionally possible. Alternatively for a fixed recycle
pressure, the recycle stream can be subcooled to a lower
temperature. This can result in reduced process power requirement,
increased recovery of the desired heavy hydrocarbons or a
realisation of both of these effects.
[0031] A particular advantage of the process of the invention
according to this embodiment is that liquid from the first
separator may be subcooled and expanded, providing refrigeration at
a temperature level such that it can contribute to the
condensation/subcooling of the recycle stream and subcooling of
liquid from the first separator. This makes improved use of
available refrigeration to remove the restrictive temperature
`pinch` such that the recycle stream can be subcooled to a smaller
temperature approach to the demethaniser residue gas. This enables
high recovery of the desirable components to be achieved with lower
recycle pressures and lower compression power requirements.
[0032] Thus in a preferred aspect, this embodiment may also include
process elements whereby net refrigeration available with in the
heat exchanger is used to cool a further process stream comprising
a separated lights fraction of the feed. Preferably this further
process stream comprises a recycle stream derived from overheads
from the fractionation column. More preferably at least a portion
of this recycle stream is condensed/subcooled and returned to an
upper part of the fractionation column.
[0033] The liquid from the first separator having been subcooled,
expanded and evaporated may be fed to the demethaniser column at a
mid-stage, as a two phase stream. The location of the feed point
may be optimised to maximise process efficiency. In addition to the
subcooling, expanding and evaporation of liquid from the first
separator, the conventional residual gas recycle process can be
further improved by the addition of other process features. These
features give improved demethaniser rectification, and for a given
recovery of ethane, the required recycle flow of residue gas to
reflux the demethaniser is reduced and therefore overall power
requirement is reduced.
[0034] In one variation of this first embodiment a process is
provided wherein the first liquid stream is expanded and separated
to give a further gaseous stream and at least a portion of the
remainder is subjected the processes of steps (c) and (d).
[0035] This feature of expanding the liquid from the first
separator to an intermediate pressure to flash off the lighter
components is shown in FIG. 3. The methane rich flash vapour may be
separated from the liquid, which may subsequently be subcooled,
expanded and evaporated. The flash vapour may be combined with the
residual gas recycle stream. For a given reflux flow, the recycle
flow is reduced thereby reducing compression power.
[0036] Thus the invention also provides a process wherein at least
a part of the further gaseous stream is transferred to the
fractionation column. Preferably this further gaseous stream or a
portion thereof is introduced into the fractionation column at a
point above the liquid feed. More preferably the further gaseous
stream or a portion thereof is cooled, and optionally expanded
prior to its introduction into the fractionation column.
[0037] As described above, it is further within the scope of the
invention to provide a process wherein the further gaseous stream
or a portion thereof is combined with the recycle stream prior to
being transferred to the fractionation column. Preferably the
further gaseous stream or a portion thereof and the recycle stream
are cooled and expanded to produce an at least partially condensed
(e.g. liquid) stream which is introduced into the fractionation
column. Most preferably the first gaseous stream or a portion
thereof and the recycle stream are introduced into the
fractionation column at a point above the liquid feed.
[0038] In another variation of this first embodiment a process is
provided wherein the first gaseous stream is work expanded and
separated to give a further gaseous stream and a further liquid
stream, said further gaseous stream is partially condensed in a
heat exchanger and then fed to the fractionation column. It is
preferred that in the process the further gaseous stream is
introduced into the fractionation column at a point above the
liquid feed. Preferably the further liquid stream is also
introduced into the fractionation column.
[0039] It is also within the scope of this variation to provide a
process wherein the further gaseous stream is partially condensed
in heat exchange with the expanded stream from step (d). Preferably
overhead vapour from the fractionation column and the expanded
stream from step (d) are heated in heat exchange with the first
liquid stream from step (b) prior to expansion.
[0040] The process of the invention may also be operated in a
manner wherein one of said first gaseous stream and said first
liquid stream is expanded and separated to give a further gaseous
stream and a further liquid stream, or wherein both of said first
gaseous stream and said first liquid stream are expanded, combined
and separated to give a further gaseous stream and a further liquid
stream.
[0041] In a preferred aspect a process is provided wherein the
first gaseous stream may be work expanded prior to separation.
[0042] The particular feature described above of separating the two
phases of the expander exhaust stream, and subsequently partially
condensing the vapour phase in heat exchange with other process
streams is shown in FIG. 4. The partially condensed vapour phase
may be fed to the column at a separate point above that at which
the liquid phase is fed.
[0043] Contacting the vapour phase of a partially condensed feed
gas with a light hydrocarbon stream can selectively remove the
heavier components from that vapour. A process utilising a wash
column operating at high pressure to separate the heavier
components from a gaseous hydrocarbon feed is described in our
copending UK patent application no. 9826999.6. This procedure may
be incorporated in the process of the present invention whereby a
residual vapour is produced at high pressure from the wash column.
Liquid from the wash column is passed to a low pressure
fractionation column which produces a stabilised liquid product and
a lower pressure residual vapour stream.
[0044] In the application of high propane recovery from natural
gas, vapour from the top tray of a deethaniser column can be
partially condensed to provide reflux to both the deethaniser and
the high-pressure wash column. Whilst sufficient reflux can be
generated for high recovery of propane and heavier components from
natural gas, in most cases, sufficient reflux cannot be generated
economically by this route to effect high recovery of ethane and
heavier components. For economical high ethane recovery from
natural gas, an alternative reflux stream for the high-pressure
wash column must therefore be found.
[0045] In a second embodiment of the invention, the process of
subcooling, expanding and evaporating liquid to provide
refrigeration at a low temperature level can be applied to a
flowsheet using a two column process such that it is suitable for
high recovery of ethane and heavier components, as shown in FIG.
5.
[0046] In general terms, in the process of this invention according
to this embodiment, high pressure vapour from the first separator
undergoes work expansion to a medium pressure and is fed to a
mid-stage of a wash column. The liquid from the first separator may
be expanded to medium pressure and fed to the bottom of the wash
column.
[0047] The wash column overheads, at a medium pressure, may be
partially condensed in a well integrated heat exchange operation. A
portion of the liquid may be used to reflux the wash column and the
remaining portion expanded and used to reflux the demethaniser.
Liquid from the bottom of the wash column may then be subcooled,
expanded and rewarmed prior to being fed to the demethaniser
column. By this arrangement refrigeration is provided at a
temperature level such that it can contribute to the partial
condensation of the wash column overheads.
[0048] Thus in the second embodiment of this invention there is
provided a process wherein the first gaseous stream and the first
liquid stream are fed to a reequilibration device which produces a
first output, richer in heavier hydrocarbons than the first liquid
stream, and a second output, leaner in heavier hydrocarbons than
the first gaseous stream, wherein the second output is cooled and
partially condensed and directly or indirectly fed to an upper part
of the fractionation column as said cooled stream (f) and wherein
the first output (which comprises at least a portion of the first
liquid stream from step (b)) is subjected to the processes of steps
(c) and (d). The reequilibration device may preferably be a high
pressure wash column.
[0049] In a preferred manner of operation the first gaseous stream
is work expanded before being fed to the reequilibration
device.
[0050] It is within the scope of this embodiment to provide a
process wherein the second output is partially condensed in heat
exchange with the expanded stream from step (d). Preferably the
second output is partially condensed and at least a portion of the
first liquid stream from step (b) is subcooled in heat exchange
with both the overhead vapour from the fractionation column and the
expanded stream from step (d).
[0051] The invention also provides a process according to this
second embodiment wherein the partially condensed second output is
fed to a separator, which produces an additional gaseous stream and
an additional liquid stream, wherein at least a portion of the
additional liquid stream is fed to an upper part of the
fractionation column. Preferably at least a portion of said
additional liquid stream is also fed to the reequilibration
device.
[0052] In a further aspect of this embodiment, the additional
gaseous stream may be warmed to produce a residual gas product.
Preferably the additional gaseous stream is warmed in heat exchange
with at least a portion of the first liquid stream from step (b)
prior to expansion. More preferably the additional gaseous stream
and the overhead vapour from the fractionation column are warmed in
heat exchange with the first liquid stream from step (b) prior to
expansion, and the second output from the reequilibration
device.
[0053] In both embodiments described above, an overhead fraction
from the fractionation column may be warmed, and successively
compressed and cooled to produce a residue gas product. In the
first embodiment of the invention, a portion of this residue gas
product is recycled back to reflux the top section of the
fractionation column.
[0054] The invention further provides apparatus for the separation
of a heavier hydrocarbon fraction from a gaseous feed comprising a
mixture of hydrocarbons, wherein the mixture is cooled, partially
condensed, separated into a first liquid stream and a first gaseous
stream and at least a portion of each of the first liquid stream
and the first gaseous stream are passed to a fractionation column
in which said separation is carried out, which apparatus
comprises:
[0055] (i) conduit means for transferring at least a portion of the
first liquid stream to a heat exchanger in which said portion is
subcooled,
[0056] (ii) means for expanding said subcooled stream,
[0057] (iii) conduit means for transferring at least a portion of
said expanded stream to the heat exchanger in which the subcooling
is effected, whereby at least a part of the subcooling is provided
by transfer of heat to the expanded stream.
[0058] (iv) conduit means for introducing a cooled stream produced
by cooling a separated lights fraction of the feed to an upper part
of the column, at least a portion of the cold required to cool the
separated lights fraction being provided by transfer of heat to
said expanded stream
[0059] The invention further provides apparatus as described above
wherein the cooled, partially condensed mixture is separated into a
first gaseous stream and a first liquid stream in a first
separator, and said apparatus further comprises means for expanding
the first liquid stream and means for separating the first expanded
liquid stream into a further gaseous stream and a further liquid
stream, wherein the further liquid stream is transferred to the
heat exchanger.
[0060] The above-defined apparatus may further comprise means (e.g.
an expansion turbine) for partially condensing the first gaseous
stream, separation means for separating at least a portion of the
partially condensed stream into a further gaseous stream and a
further liquid stream, and means for transferring at least a
portion of the further gaseous stream to the fractionation
column.
[0061] The apparatus of the invention may be adapted for carrying
out the second process embodiment of the invention, wherein the
cooled, partially condensed mixture is separated into a first
gaseous stream and a first liquid stream in a first separator. In
this case the apparatus additionally includes
[0062] (a) means for work expanding said first gaseous stream,
[0063] (b) means for expanding said first liquid stream,
[0064] (c) a reequilibration device which is arranged to receive
said expanded first gaseous stream and said expanded first liquid
stream, and produces a first output which is richer in heavier
hydrocarbons than said first expanded liquid stream, and a second
output which is leaner in heavier hydrocarbons than said first
gaseous stream,
[0065] (d) conduit means for transferring at least a portion of
said second output to the fractionation column, and
[0066] (e) conduit means for transferring said first output to the
heat exchanger.
DESCRIPTION OF SPECIFIC EMBODIMENTS
Embodiment 1
[0067] The invention is described below in terms of a process for
high ethane recovery. The description should be read in conjunction
with the flow diagram in FIG. 2.
[0068] A feed gas at an elevated pressure 2 is passed through heat
exchange system 4 where it is cooled and partially condensed. The
liquid phase 18 is separated from the uncondensed vapour phase 10
in vapour/liquid separator 8. A first gaseous stream comprising
vapour 10 is work expanded in turbo-expander 12 to give a two phase
stream 14 which is fed to the upper portion of a fractionation
column in the form of demethaniser 16. A first liquid stream
comprising liquid 18 is cooled in heat exchange system 20 to give
sub-cooled liquid 22 which is expanded across valve 24 to give a
stream 26 which may be liquid or two phase. Stream 26 is partially
evaporated in heat exchange system 20 to give a two phase stream 28
which is fed to a mid-stage of the demethaniser 16.
[0069] Refrigeration for feed gas cooling is supplemented by
evaporating liquid refrigerant stream 86 in heat exchange system 4
giving refrigerant vapour stream 88. Use of refrigerant from such a
mechanical refrigeration cycle is dependent on feed gas
composition, required recovery levels and economic factors, and is
not an essential feature of the invention.
[0070] The residual vapour from the demethaniser 16 is rewarmed and
compressed in the following manner. Vapour 30 from demethaniser 16
is warmed in heat exchange system 20 giving gas 32 which is further
warmed in heat exchange system 4 to give gas 34. Gas 34 from heat
exchange system 4 is compressed in expander brake 36 giving gas 38
which is cooled in cooler 40 giving gas 42. Gas 42 is compressed in
1st stage compressor 44 giving gas 46 which is subsequently cooled
in cooler 48 giving gas 50. Gas 52, which is a portion of gas 50 is
removed from the compression train, leaving Gas 54. Gas 54 is
compressed in 2nd stage compressor 78 to give gas 80 which is
cooled in cooler 82 to give a residue gas product 84.
[0071] Reflux to the demethaniser is provided in the following
manner. Gas 52 from the 1st stage compressor 44 discharge is cooled
in heat exchange system 4 giving gas 56 and is condensed and
subcooled in heat exchange system 20 giving a subcooled liquid 58.
Subcooled liquid 58 is expanded across valve 60 to the demethaniser
pressure, giving a two phase stream 62 which is fed to the top of
the demethaniser 16.
[0072] Demethaniser reboil is provided in the following manner.
Liquid 64 is drawn from a tray part way up the demethaniser and
thermosyphoned through heat exchange system 4 where it is partially
vaporised to give a two phase stream 66 which is fed back to the
demethaniser. Similarly, liquid 68 is drawn from a tray lower down
the demethaniser than that from which liquid 64 is drawn, and is
thermosyphoned through heat exchange system 4 where it is partially
vaporised to give a two phase stream 70 which is fed back to the
demethaniser. Additionally, liquid 72 is thermosyphoned from the
bottom tray and is fed to heat exchange system 4 where it is
partially vaporised to give a two phase stream 74 which is fed back
to the demethaniser. A stabilised liquid product 76 is drawn from
the bottom of the demethaniser.
[0073] Operation of the separation apparatus depicted in FIG. 2 is
further illustrated by the data in Table 1.
1TABLE 1 Material Balance - to be read in conjunction with FIG. 2
Stream 2 10 14 22 26 28 30 34 Description Demeth- LP Feed Expander
Expander exhaust Subcooled Expanded anises Residue Gas Inlet feed
to demethanises liquid liquid Demethanises lower feed overheads gas
Vapour (molar) 1.0000 1.0000 0.8551 0.8551 0.1449 0.0000 0.0000
0.4679 0.4679 0.5321 1.0000 1.0000 Fraction Tem- (.degree. C.) 30.0
-41.1 -82.4 -82.4 -82.4 -90.1 -89.0 -67.8 -67.8 -67.8 -99.8 15.3
perature Pressure (kPa(a)) 6996 6955 2480 2480 2480 6934 2501 2480
2480 2480 2466 2425 Mass (kg/h) 296046 203372 203372 162015 41357
92674 92674 92674 28448 64226 272281 272281 Flow Molar Flow
Nitrogen (kgmole/ 91 83 83 81 2 9 9 9 8 1 122 122 h) Methane
(kgmole/ 12495 10205 10205 9287 918 2291 2291 2291 1574 717 16588
16588 h) Ethane (kgmole/ 1372 790 790 326 464 582 582 582 83 499 92
92 h) Propane (kgmole/ 610 223 223 20 202 387 387 387 10 377 0 0 h)
i-Butane (kgmole/ 76 18 18 0 18 58 58 58 0 57 0 0 h) n-Butane
(kgmole/ 152 30 30 0 30 122 122 122 0 122 0 0 h) i-Pentane (kgmole/
46 6 6 0 6 40 40 40 0 40 0 0 h) n-Pentane (kgmole/ 46 5 5 0 5 41 41
41 0 41 0 0 h) n-Hexane (kgmole/ 30 2 2 0 2 29 29 29 0 29 0 0 h)
n-Heptane (kgmole/ 15 0 0 0 0 15 15 15 0 15 0 0 h) n-Octane
(kgmole/ 7 0 0 0 0 7 7 7 0 7 0 0 h) Total: (kgmole/ 14942 11362
11362 9715 1646 3581 3581 3581 1675 1905 16801 16801 h) Stream 52
58 62 84 76 86 88 Description Subcooled Refrigerant Recycle gas
recycle Demethanises reflex Residue gas Liquid product Refrigerant
in out Vapour Fraction (molar) 1.0000 0.0000 0.0683 0.0683 0.9317
1.0000 0.0000 0.0000 1.0000 Temperature (.degree. C.) 30.0 -96.8
-102.2 -102.2 -102.2 30.0 24.3 -40.0 -40.0 Pressure (kPa(a)) 4652
4610 2480 2480 2480 6996 2480 111 111 Mass Flow (kg/h) 67560 67565
67565 4639 62926 204721 91330 20903 20903 Molar Flow Nitrogen
(kgmole/h) 30 30 30 6 24 91 0 0 0 Methane (kgmole/h) 4116 4116 4116
278 3837 12472 23 0 0 Ethane (kgmole/h) 23 23 23 0 23 69 1304 0 0
Propane (kgmole/h) 0 0 0 0 0 0 610 474 474 i-Butane (kgmole/h) 0 0
0 0 0 0 76 0 0 n-Butane (kgmole/h) 0 0 0 0 0 0 152 0 0 i-Pentane
(kgmole/h) 0 0 0 0 0 0 46 0 0 n-Pentane (kgmole/h) 0 0 0 0 0 0 46 0
0 n-Hexane (kgmole/h) 0 0 0 0 0 0 30 0 0 n-Heptane (kgmole/h) 0 0 0
0 0 0 15 0 0 n-Octane (kgmole/h) 0 0 0 0 0 0 7 0 0 Total:
(kgmole/h) 4169 4169 4169 285 3884 12632 2310 474 474 Summary
Ethane Recovery 95.0% Propane Recovery 100.0% Residual Gas
Compressor Power 11925 kW @ 75% efficiency Expander Power Output
2750 kW @ 83% efficiency Mechanical Refrigeration Compression Power
1415 kW based on single stage @ 75% efficiency Total power input =
13340 kW
[0074] The above process may be varied in a number of ways. For
example one or more turbo-expansion steps may be utilized, or one
or more steps of partial condensation and phase separation may be
employed.
[0075] Another alternative to the process described above is to use
the turbo-expander 12 to drive other rotating equipment, rather
than drive a compressor used to supplement the residual gas
compressors 44 and 78.
[0076] A further process option is to provide or supplement the
refrigeration requirement by warming or evaporating the liquid
product 76 from the demethaniser, either at the demethaniser
pressure or at an elevated or reduced pressure appropriate to
downstream processing.
[0077] The required refrigeration may also be provided or
supplemented by one or more components of a refrigerant fluid being
compressed, condensed/subcooled and expanded to one or more
pressures enabling evaporation at one or more temperature levels.
One or more liquid streams and a first residual vapour stream may
result from heat exchange of the feed gas with this mechanical
refrigeration cycle.
[0078] Yet another possible process improvement is to combine heat
exchange systems 4 and 20, or to change heat exchange system 4 into
one or more heat exchanger operations. Feed gas 2 may be split to
optimise and improve practicability of heat integration within the
aforementioned heat exchanger operations, and subsequently
recombined to give stream 6.
[0079] Also, integration of the feed gas cooling and the
demethaniser of the process can be optimised by utilizing more than
two or less than two side exchangers.
[0080] The first embodiment described above may be varied as shown
in FIG. 3. The first liquid stream comprising liquid 18 from the
first separator is expanded to an intermediate pressure and the
resultant two phase stream 92 is separated in a second
vapour/liquid separator 94. The further liquid stream comprising
liquid 98 is subcooled in heat exchanger system 20, expanded and
partially evaporated in heat exchanger system 20. The further
gaseous stream comprising vapour 96 is combined with the residual
gas recycle stream 56 and passed to heat exchanger system 20.
[0081] A second variation to the first embodiment is shown in FIG.
4. The two phases of stream 14, the expander exhaust, are separated
in a second vapour/liquid separator 90. The further liquid stream
comprising liquid 96 is fed to the demethaniser. The further
gaseous stream comprising vapour 92 is partially condensed in heat
exchange system 20 and subsequently fed at a point in the
demethaniser above the liquid feed.
Embodiment 2
[0082] The second embodiment of the invention is described below in
terms of a process for high ethane recovery. The description should
be read in conjunction with the flow diagram in FIG. 5.
[0083] A feed gas at an elevated pressure 2 is passed through heat
exchange system 4 where it is cooled and partially condensed to
give a two phase stream 6. The liquid phase 48 is separated from
the uncondensed vapour phase 10 in vapour/liquid separator 8. The
first gaseous stream comprising vapour 10 is work expanded in
turbo-expander 12 to give a two phase stream 14 which is fed to a
mid point of a reequilibration device in the form of high pressure
wash column 16. A first liquid stream comprising liquid 48 is
expanded to a medium pressure across valve 50 to give a two phase
stream 52 which is fed to the bottom of the high pressure wash
column 16.
[0084] Refrigeration for feed gas cooling is supplemented by
evaporating liquid refrigerant stream 108 in heat exchange system 4
giving refrigerant vapour stream 110. Use of refrigerant from such
a mechanical refrigeration cycle is dependent on feed gas
composition, required recovery levels and economic factors, and is
not an essential feature of the invention.
[0085] A first output from the high pressure wash column comprising
liquid 36 is subcooled in heat exchange system 20 to give subcooled
liquid 38. Liquid 38 is expanded across valve 40 to give stream 42,
which may be liquid or two phase. Stream 42 is partially evaporated
in heat exchange system 20 to give a two phase stream 46 which is
fed to a mid stage of the demethaniser 54.
[0086] A second output from the high pressure wash column
comprising vapour 18 is partially condensed in heat exchanger
system 20 to give a two phase stream 22. An additional liquid
stream comprising liquid 26 is separated from an uncondensed
additional gaseous stream comprising vapour 62 in vapour/liquid
separator 24. The liquid 26 is split, with a portion 28 being fed
to reflux the high pressure wash column 16. The remaining portion
30 is expanded across valve 32 to give a two phase stream 34 which
is fed to an upper part of the demethaniser 54, in order to provide
the necessary reflux.
[0087] The residual vapour from the demethaniser is rewarmed and
compressed in the following manner. Vapour 56 from demethaniser is
warmed in heat exchange system 20 giving gas 58 which is further
warmed in heat exchange system 4 to give gas 60. Gas 60 from heat
exchange system 4 is compressed in the expander brake 68 giving gas
70 which is subsequently cooled in cooler 72 giving gas 74. Gas 74
is compressed in 1st stage compressor 76 to give gas 78 which is
cooled in cooler 80 to give gas 82. Gas 82 is mixed with gas 66 to
give gas 84. Gas 84 is compressed in 2nd stage compressor 100 to
give gas 102 which is cooled in cooler 104 to give a residue gas
product 106.
[0088] The additional gaseous stream from the vapour/liquid
separator 24 comprising residual vapour 62 is warmed in heat
exchange system 20 giving gas 64 which is further warmed in heat
exchange system 4 to give gas 66. Gas 66 from heat exchange system
4 is mixed with gas 82 to give gas 84.
[0089] Demethaniser reboil is provided in the following manner.
Liquid 86 is drawn from a tray part way up the demethaniser and
thermosyphoned through heat exchange system 4 where it is partially
vaporised to give a two phase stream 88 which is fed back to the
demethaniser. Similarly, Liquid 90 is drawn from a tray lower down
the demethaniser than that from which liquid 86 is drawn, and is
thermosyphoned through heat exchange system 4 where it is partially
vaporised to give a two phase stream 92 which is fed back to the
demethaniser. Additionally, liquid 94 is thermosyphoned from the
bottom tray and is fed to heat exchange system 4 where it is
partially vaporised to give a two phase stream 96 which is fed back
to the demethaniser. A stabilised liquid product 98 is drawn from
the bottom of the demethaniser.
[0090] Operation of the separation apparatus depicted in FIG. 5 is
further illustrated by the data in Table 2.
2TABLE 2 Material Balance - to be read in conjunction with FIG. 5
Stream 2 10 14 52 18 62 Description Expander exhaust feed HP wash
column Wash column HP Residue Feed Gas Expander Inlet to HP wash
column lower feed vapour vapour Vapour Fraction (molar) 1.0000
1.0000 0.7867 Vapour Liquid 0.4308 Vapour Liquid 1.0000 1.0000
Temperature (.degree. C.) 30.0 -51.1 -78.5 -78.5 -78.5 -73.4 -73.4
-73.4 -86.4 -89.8 Pressure (kPa(a)) 6996 6955 3549 3549 3549 3549
3549 3549 3528 3528 Mass Flow (kg/h) 296046 128859 128859 95357
33501 167187 54988 112199 174123 106294 Molar Flow Nitrogen
(kgmole/h) 91 61 61 57 3 31 25 5 92 74 Methane (kgmole/h) 12495
6637 6637 5503 1134 5859 3156 2702 10334 6409 Ethane (kgmole/h)
1372 436 436 161 276 936 102 834 191 47 Propane (kgmole/h) 610 120
120 13 107 490 11 478 0 0 i-Butane (kgmole/h) 76 10 10 0 10 66 0 65
0 0 n-Butane (kgmole/h) 152 18 18 0 17 135 1 134 0 0 i-Pentane
(kgmole/h) 46 4 4 0 4 42 0 42 0 0 n-Pentane (kgmole/h) 46 3 3 0 3
43 0 43 0 0 n-Hexane (kgmole/h) 30 1 1 0 1 29 0 29 0 0 n-Heptane
(kgmole/h) 15 0 0 0 0 15 0 15 0 0 n-Octane (kgmole/h) 7 0 0 0 0 7 0
7 0 0 Total: (kgmole/h) 14942 7290 7290 5735 1555 7652 3296 4355
10618 6530 Stream 28 56 34 38 42 Description Wash column reflux
Demethanises overheads Demethanises column reflux Subcooled liquid
Expanded liquid Vapour Fraction (molar) 1.0000 1.0000 0.3062 Vapour
Liquid 0.0000 0.0000 Temperature (.degree. C.) -89.8 -109.2 -109.6
-109.6 -109.6 -100.2 -99.7 Pressure (kPa(a)) 3528 1735 1756 1756
1756 3528 1777 Mass Flow (kg/h) 106294 98435 33058 9887 23172
156695 156695 Molar Flow Nitrogen (kgmole/h) 10 18 9 6 3 9 9
Methane (kgmole/h) 2012 6063 1913 602 1311 4173 4173 Ethane
(kgmole/h) 74 22 70 2 68 1255 1255 Propane (kgmole/h) 0 0 0 0 0 610
610 i-Butane (kgmole/h) 0 0 0 0 0 76 76 n-Butane (kgmole/h) 0 0 0 0
0 152 152 i-Pentane (kgmole/h) 0 0 0 0 0 46 46 n-Pentane (kgmole/h)
0 0 0 0 0 46 46 n-Hexane (kgmole/h) 0 0 0 0 0 30 30 n-Heptane
(kgmole/h) 0 0 0 0 0 15 15 n-Octane (kgmole/h) 0 0 0 0 0 7 7 Total:
(kgmole/h) 2096 6103 1992 610 1382 6420 6420 Stream 46 60 66 84 98
100 102 Description HP Residue Refrigerant Refrigerant Demethansis
lower feed Recycle gas gas Residue gas Liquid product in out Vapour
Fraction (molar) 0.4454 Vapour Liquid 1.0000 1.0000 1.0000 0.0000
0.0000 1.0000 Temperature (.degree. C.) -84.4 -84.4 -84.4 24.1 24.1
30.0 6.8 -40.0 -40.0 Pressure (kPa(a)) 1756 1756 1756 1694 3487
6996 1756 111 111 Mass Flow (kg/h) 156695 47674 109021 98436 106294
204730 91317 26437 26437 Molar Flow Nitrogen (kgmole/h) 9 8 1 18 74
91 0 0 0 Methane (kgmole/h) 4173 2738 1436 6063 6409 12473 23 0 0
Ethane (kgmole/h) 1255 107 1148 22 47 69 1303 0 0 Propane
(kgmole/h) 610 6 603 0 0 0 610 600 600 i-Butane (kgmole/h) 76 0 76
0 0 0 76 0 0 n-Butane (kgmole/h) 152 0 152 0 0 0 152 0 0 i-Pentane
(kgmole/h) 46 0 46 0 0 0 46 0 0 n-Pentane (kgmole/h) 46 0 46 0 0 0
46 0 0 n-Hexane (kgmole/h) 30 0 30 0 0 0 30 0 0 n-Heptane
(kgmole/h) 15 0 15 0 0 0 15 0 0 n-Octane (kgmole/h) 7 0 7 0 0 0 7 0
0 Total: (kgmole/h) 6420 2860 3561 6103 6530 12633 2309 600 600
Summary Ethane Recovery 95.0% Propane Recovery 100.0% Residual Gas
Compressor Power 11528 kW @ 75% efficiency Expander Power Output
934 kW @ 83% efficiency Mechanical Refrigeration Compression Power
1790 kW based on single stage @ 75% efficiency Total power input =
13318 kW
[0091] The above process may be varied in a number of ways. For
example, one or more turbo-expansion steps may be utilized, or one
or more steps of partial condensation and phase separation may be
employed.
[0092] Another alternative to the process described above is to use
the turbo-expander 12 to drive other rotating equipment, rather
than drive a compressor used to supplement the residual gas
compressors 76 and 100.
[0093] A further alternative is for one or both of (a) the portion
of the additional liquid stream comprising liquid 26 which is fed
to reflux the high pressure wash column and (b) the portion of
liquid 26 which is fed to an upper part of the demethaniser 54, may
be subcooled in heat exchange system 20 prior to passing to either
the high pressure wash column 16 or valve 32 respectively.
[0094] Yet another possible process improvement is to combine heat
exchange systems 4 and 20, or to change heat exchange system 4 into
one or more heat exchanger operations. Feed gas 2 may be split to
optimise and improve practicability of heat integration within the
aforementioned heat exchanger operations, and subsequently
recombined to give stream 6.
[0095] A further process option is to provide or supplement the
refrigeration requirement by warming or evaporating the liquid
product 98 from the demethaniser, either at the demethaniser
pressure or at an elevated or reduced pressure appropriate to
downstream processing. Alternatively, the refrigeration requirement
may be provided or supplemented by expanding the residual vapours
from the high pressure wash column.
[0096] The required refrigeration may also be provided or
supplemented by one or more components of a refrigerant fluid being
compressed, condensed/subcooled and expanded to one or more
pressures enabling evaporation at one or more temperature levels.
One or more liquid streams and a first residual vapour stream may
result from heat exchange of the feed gas with this mechanical
refrigeration cycle.
[0097] Integration of the feed gas cooling and the demethaniser of
the process can be further optimised by utilizing more than two or
less than two side exchangers.
[0098] It may also be advantageous within the above process to
reboil the high pressure wash column, for example in heat exchange
with a portion of feed gas, to reduce the load on the
demethaniser.
[0099] As a further improvement, reflux to the high pressure wash
column may be provided by totally condensing a portion of the high
pressure wash column second output, comprising overhead vapour 18,
rather than partially condensing the whole of vapour 18.
[0100] Also, the vapour and liquid phases of the expander exhaust
stream 14 may be separated in a vapour/liquid separator, with the
liquid phase being fed to a mid-stage of the high pressure wash
column. The vapour phase may be partially condensed in heat
exchange system 20 with the resultant two phase stream being fed at
a higher point in the high pressure wash column to provide all or
part of the required reflux.
* * * * *