U.S. patent number 3,780,534 [Application Number 04/843,427] was granted by the patent office on 1973-12-25 for liquefaction of natural gas with product used as absorber purge.
This patent grant is currently assigned to Airco, Inc.. Invention is credited to Domenick R. Biava, Antony Lofredo.
United States Patent |
3,780,534 |
Lofredo , et al. |
December 25, 1973 |
LIQUEFACTION OF NATURAL GAS WITH PRODUCT USED AS ABSORBER PURGE
Abstract
This invention relates to a process for purifying and liquefying
a natural gas stream comprising the steps of passing said stream
through absorption units to remove impurities, compressing said
stream, liquefying said stream by heat exchange with a low
temperature nitrogen refrigeration cycle, stripping the nitrogen
from said liquefied stream, utilizing said nitrogen in said
refrigeration cycle, storing a portion of said liquefied stream,
and condensing the boil-off from said storage with low temperature
nitrogen from said cycle.
Inventors: |
Lofredo; Antony (Springfield,
NJ), Biava; Domenick R. (Somerset, NJ) |
Assignee: |
Airco, Inc. (New York,
NY)
|
Family
ID: |
25289943 |
Appl.
No.: |
04/843,427 |
Filed: |
July 22, 1969 |
Current U.S.
Class: |
62/623; 62/927;
62/47.1 |
Current CPC
Class: |
C07C
7/12 (20130101); C07C 7/005 (20130101); F25J
1/0268 (20130101); F25J 1/0052 (20130101); F25J
3/0233 (20130101); F25J 1/0265 (20130101); F25J
1/0072 (20130101); F25J 1/0022 (20130101); F25J
1/0283 (20130101); F25J 1/0205 (20130101); F25J
3/029 (20130101); F25J 1/0045 (20130101); F25J
1/0288 (20130101); F25J 3/0257 (20130101); F25J
1/0231 (20130101); F25J 1/005 (20130101); F25J
1/025 (20130101); F25J 3/0209 (20130101); C07C
7/005 (20130101); C07C 9/04 (20130101); C07C
7/12 (20130101); C07C 9/04 (20130101); F25J
2290/62 (20130101); F25J 2200/74 (20130101); F25J
2245/42 (20130101); F25J 2205/04 (20130101); F25J
2270/90 (20130101); F25J 2270/16 (20130101); Y02C
20/40 (20200801); F25J 2245/90 (20130101); F25J
2270/42 (20130101); F25J 2220/66 (20130101); Y10S
62/927 (20130101); F25J 2270/12 (20130101); F25J
2215/04 (20130101); F25J 2200/72 (20130101); Y10S
62/908 (20130101); F25J 2245/02 (20130101); F25J
2270/04 (20130101); F25J 2220/64 (20130101); Y02C
10/12 (20130101); F25J 2240/60 (20130101); F25J
2200/04 (20130101) |
Current International
Class: |
C07C
7/00 (20060101); F25J 1/00 (20060101); F25J
3/02 (20060101); F25J 1/02 (20060101); F25j
001/02 (); F25j 003/02 () |
Field of
Search: |
;62/9,11,17,18,23,24,26,27,28,30,39,40,50,54 ;55/62,73,74,80 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yudkoff; Norman
Assistant Examiner: Purcell; Arthur F.
Claims
What is claimed is:
1. A process for the production of substantially pure methane and
liquefied natural gas from a feed stream of natural gas comprising
the steps of:
a. purifying said feed streams to remove moisture and carbon
dioxide, said purification step being at least partially
accomplished by passing the said natural gas stream through
adsorption units, regenerating said units using a recirculating
stream of warm natural gas, and cooling said units following
regeneration with a recirculating stream of cool natural gas, while
continuously purging said warm and cool streams by providing to
said streams a bleed stream of purified, dry, natural gas taken
from said feed stream beyond said adsorber units, to prevent an
excessive buildup of contaminants;
b. removing heavy hydrocarbons from said feed stream;
c. liquefying said feed stream by heat exchange with a closed loop
nitrogen refrigeration cycle;
d. rectifying said liquefied feed stream to strip nitrogen and part
of the methane therefrom;
e. separately, further rectifying said stripped nitrogen and
methane to produce substantially pure methane;
f. supplying said further rectified nitrogen to the refrigeration
cycle to replace the losses, and accumulating an inventory of
excess nitrogen for use in replenishing said losses in said cycle,
and directing the substantially pure methane to a storage facility;
and
g. subcooling the remaining liquefied natural gas feed stream and
directing the same to a storage facility.
2. A process according to claim 1, wherein step (b) is accomplished
by liquefying said heavy hydrocarbons and separating said liquefied
hydrocarbons from said stream.
3. A process according to claim 2, further comprising cooling at
least a portion of the high pressure nitrogen feed stream in said
refrigeration cycle with said hydrocarbons.
4. A process according to claim 2, wherein said hydrocarbons are
used as fuel to drive a compressor in said refrigeration cycle.
5. A process according to claim 1, including venting said bleed
stream following purging into a fuel gas stream for driving a
compressor in said refrigeration cycle.
6. A process according to claim 1 including withdrawing the
boil-off from the LNG storage facility and condensing the same by
heat exchange with liquid nitrogen from the refrigeration cycle,
returning the condensed boil-off to the storage facility.
7. A process according to claim 6 wherein the condensed boil-off is
joined by means of an eductor with the liquefied natural gas being
directed to the storage facility.
8. A process according to claim 1 including sweetening the
liquefied natural gas in the storage facility by introducing
substantially pure methane into said facility.
9. A process for the production of substantially pure methane and
liquefied natural gas from a feed stream of natural gas comprising
the steps of:
a. removing moisture and carbon dioxide from said feed stream;
b. liquefying at least part of the hydrocarbons heavier than
methane in a first cooling step;
c. separating said at least part from said feed stream;
d. liquefying said feed stream in a second cooling step;
e. rectifying said liquefied feed stream to strip nitrogen and at
least part of the methane therefrom;
f. further rectifying said stripped nitrogen and methane to produce
substantially pure nitrogen and substantially pure methane;
g. further cooling the remainder of the liquefied feed stream in a
first subcooling step and in a second subcooling step;
h. expanding the subcooled feed stream and storing the same as
LNG;
i. compressing a refrigerant gas in a plurality of steps;
j. splitting said compressed refrigerant gas into a major stream
and a minor stream;
k. cooling said major stream in said first cooling step, said
second cooling step, and said first subcooling step;
l. dividing said major stream into two further streams, liquefying
one of said further streams in said second subcooling step, work
expanding the other of said streams and passing the resulting cold
gas through said second and first subcooling steps and returning
said gas to be compressed;
m. cooling at least part of said minor stream by heat exchange with
the liquefied hydrocarbons produced in steps (b) and (c);
n. expanding said minor stream, combining it with the substantially
pure nitrogen from step (f) and passing the combined streams
through said second cooling step to provide at least part of the
refrigeration therefore; and
o. further expanding the warmed minor stream, combining the
expanded minor stream with the other of said streams of step (l)
and passing the so-combined streams through said second cooling
step and first cooling step to provide at least part of the
refrigeration therefore.
10. A process according to claim 9, including cooling at least part
of said minor stream with a Freon refrigeration cycle, combining
said at least part with the part cooled in step (m).
11. A process according to claim 9, including subcooling the
substantially pure methane produced in step (f) in the first and
second subcooling steps and directing the same to a methane storage
facility.
12. A process according to claim 1, including sweetening the stored
LNG referred to in step (h) by injecting substantially pure methane
into said LNG.
13. A process according to claim 9 including condensing the
boil-off from the stored LNG by heat exchange with the liquefied
further stream referred to in step (l), returning the condensed
boil-off to the stored LNG.
Description
In recent years the usage of natural gas as a fuel in both
commercial and household applications has increased tremendously
and a network of natural gas pipelines has developed throughout the
country. Due to the fact that the supply of natural gas into the
distribution network is relatively constant the gas distribution
system is inherently unable to deliver to users more gas than is
supplied to the distribution system. In order to anticipate
seasonal loads (winter months), unexpected loads, and to insure
continuity of service it has been found necessary to store the
natural gas at various locations throughout the country. One method
of doing this is of course to store natural gas in its gaseous form
in containers or tanks which are capable of satisfying peak loads.
This method of storage is not, however, attractive due to the fact
that large tanks which are capable of storing gas at high pressures
are both expensive and unsightly.
The utilities and other companies which use natural gas have
therefore begun to store the natural gas in liquid form. In recent
years a number of natural gas liquefaction plants have been
developed throughout the world. The majority of these plants
liquefy the natural gas which comes from a gas distribution system
and store the resulting liquid in large insulated vessels. When a
peak demand occurs, the liquid storage is drawn down and vaporized
and supplied to the user to supplement the gas which is normally
supplied from the gas distribution system. LNG plants have also
been installed at points of shipment (i.e., seaports, etc.) since
the gas is usually shipped in liquid form.
The liquefaction cycles which were employed in the prior art plants
have important disadvantages and limitations. Most of the prior art
cycles employ cascade or modified cascade refrigeration cycles.
These cycles are extremely complicated in view of the fact that a
plurality of refrigerants are used and these require separate
sources of supply and different expansion and compression
equipment. The prior art systems furthermore generally utilize
expensive compressors and heat exchangers, etc. A plant built
according to the present invention utilizes centrifugal machinery
at high efficiencies and eliminates the need for cold blowers and
cold pumps for the transfer of LNG to storage. The subject design
also provides for the control of composition of the LNG in storage
and eliminates the high boilers in storage. In addition natural gas
is used in an efficient manner to regenerate the absorbers and then
to drive compressors.
It is the principal object of the present invention to utilize a
non-hydrocarbon gas in the refrigerant cycle for liquefying natural
gas. Since the refrigerant is a non-hydrocarbon and its properties
are well known and well defined, its thermodynamic and physical
properties in the refrigeration cycle can be accurately
predicted.
It is a further object of the present invention to utilize natural
gas to regenerate absorbers in an economical fashion. It is a
further object of the present invention to utilize natural gas as a
fuel to drive the gas turbine which drives the main compressor for
the refrigeration cycle.
It is another object of the present invention to utilize a
refrigerant fluid which can be obtained from the natural gas which
is being liquefied. This will allow the refrigeration cycle to be
self sufficient so that deliveries of refrigerant fluid from
outside the area of the liquefaction plant will not be
necessary.
It is another object of the present invention to liquefy at least
part of the refrigerant fluid and to then use the refrigerant
liquid to maintain a constant LNG composition in the storage tank
by liquefying the apors which are generated in the storage
tank.
It is another object of the present invention to purify the input
natural gas and to obtain therefrom essentially pure methane.
These and other objects of the present invention will become
apparent from the following description and the accompanying
drawing in which:
FIGS. 1 and 2 when taken together show a flow diagram of a
liquefied natural gas plant according to one embodiment of the
present invention.
Referring to FIG. 1, natural gas enters the process through conduit
11. The feed gas generally comes from a distribution network or it
may come directly from a natural gas well head or other source. The
inlet temperature, pressure and flow of the feed gas remain
relatively constant and the particular system is designed
accordingly. In a typical instance the infeed gas is at
approximately 200 psig and 85.degree.F and flows at the rate of
about 10 MM SCFD. The system would of course be readily adapted to
accommodate other inlet pressures, temperatures and flows.
A typical natural gas feed stream contains methane, nitrogen,
ethane and other hydrocarbons down to pentanes in varying
proportions. The particular embodiment of the invention which will
now be described for purposes of illustration is especially suited
for treating and liquefying natural gas having the following major
constituents in mol percents: N.sub.2 - 3 percent, methane - 88
percent, ethane - 6 percent, propane - 1 percent, CO.sub.2, 1
percent and remaining impurities.
The feed stream passes through separator 12 which removes the
entrained liquid odorants, sulphurs, etc. The feed gas is then
dried to approximately -100.degree.F dew point in the desiccant
drier (molecular sieve) drier 13. This drier is one of a pair of
adsorption driers, 13, 13A, which serves to further remove moisture
and odorants, sulphurs and sulphur compounds from the feed gas
stream. As shown in FIG. 1 adsorber 13 is on stream while adsorber
13A is being regenerated. The regeneration process will be
described in detail hereinbelow. The feed stream then proceeds
through a filter 14 which removes any adsorber particles which may
have been entrained in the gas stream as it passes through the
adsorber 13. Carbon dioxide and remaining odorant, sulphur and
sulphur compounds are removed in adsorption unit 15. As shown in
FIG. 1, unit 15 is on stream while unit 15A is being regenerated
and while unit 15B is being cooled. The adsorption units 15 may
take the form of ambient temperature molecular sieve adsorption
units. The main purpose of these units is to remove the CO.sub.2
and the abovementioned contaminants from the feed gas stream.
After the feed gas passes through adsorber 15 it proceeds through a
further adsorber particle filter 16. The feed gas is then
compressed to an optimum pressure of between 600-700 psia by feed
gas compressor 20. The necessity for this compressor of course
depends upon the pressure of the feed gas entering the system.
After being compressed in compressor 20 the feed gas passes through
an after cooler 21 wherein it is cooled with water in the
conventional manner. The compressed and cooled feed gas then enters
the liquefaction portion of the process.
Before proceeding with the description of the liquefaction cycle, a
more detailed description of the abovedescribed purification
equipment will now be given in view of the fact that this forms an
important portion of the present invention.
A portion (approximately 14 percent) of the incoming feed gas is
diverted through conduit 22 to a cooling cycle for adsorber 15b.
This cycle includes a cool-down blower 23 which pulls a suction on
a cool-down exchanger 24 which is water cooled. The cool-down
blower 23 circulates cool natural gas through the adsorber 15b to
cool the molecular sieve adsorption unit after regeneration to a
desirable temperature. While this cooling is taking place adsorber
15a is being regenerated with hot natural gas which comes from
regeneration heater 26. The heater 26 is warmed up by using the
waste heat from the gas turbine which drives the compressor 39. The
natural gas coming from the adsorber 15a passes through a cooler 27
wherein it is cooled with water in an indirect manner and a portion
of this cooled natural gas is then utilized to regenerate the
adsorption unit 13a. The regeneration of adsorber unit 13a occurs
by first heating the unit to drive off the accumulated water and by
then cooling the unit to the desired temperature. During the time
when the unit is being heated the regeneration gas is heated in
heater 26 by passing it through conduits 28 and 29 and then
upwardly through unit 13a. When sufficient heating of the unit has
taken place the unit is cooled by directing the regenerative gas
through cooling unit 30 which is cooled by water and the flow
through conduits 28 and 29 is stopped by a suitable valve
arrangement. The cooled natural gas after flowing through heat
exchanger 30 then passes up through adsorption unit 13a to cool the
same to the desired temperature. The natural gas which leaves the
unit 13a passes through suitable conduit 31, joins with the flow
from conduit 31a, and then passes to the gas turbine 39a wherein it
is used as the fuel which drives the turbine. It is not necessary
that all of the gas flowing through conduit 36 pass through the
adsorber being regenerated or cooled and therefore a portion of the
flow is bypassed through conduit 31a. A flow regulating valve is
placed in conduit 31a to control the flow. Although a turbine is
preferred as the driving force for the compressor 39, any suitable
engine may be used. Hereinafter the term "engine" is used in a
generic sense to describe turbines, reciprocating engines, etc. As
mentioned above, the exhaust gases from this engine or turbine are
used in the regenerative heater 26.
A portion of the cooled natural gas stream coming from cooling unit
27 is drawn into regeneration blower 35 which forces this portion
through the regenerative heater 26 and then into the adsorption
unit which is being regenerated 15a.
The unique flow arrangement of this semi-open system requires the
use of only a minimum amount of regeneration gas, because of the
recirculation feature. A regeneration flow is circulated by blower
35, heated to approximately 550.degree.F by the gas turbine exhaust
gas in heat exchanger unit 26, and then passes through the
molecular sieve absorption unit being regenerated and in the case
of FIG. 1 this would be unit 15a. The gas is then cooled in cooling
unit 27 to an allowable blower inlet temperature and recirculated
by means of the blower 35. Note that a similar loop is provided
utilizing blower 23 and heat exchanger 24 for cooling the molecular
sieve adsorption unit after it has been regenerated. The three
molecular sieve adsorption units are arranged so that at a given
period of time, one unit will be in active service, one will be in
regeneration and the third will be in cool-down. The flows for the
three operations are sequenced by an automatic timer controlling
pneumatic valves in a manifolded system. In a typical operation a 2
hour cycle is utilized. This depends on CO.sub.2 content and the
equipment used. After adsorption unit 15 has been on-stream for 2
hours, the timing device will be actuated and direct the feed gas
flow to unit 15b which has just undergone cooling. The regeneration
stream will be directed to unit 15 to regenerate the same and the
cooling stream will be directed to unit 15a to cool the same. When
unit 15b has been on stream for a 2 hour cycle, the feed gas will
be directed to unit 15a, etc. The details of the automatic timer
controlling circuit with its associated valves is not described in
detail in this application in view of the fact that this type of
control apparatus is well known in the art and may take many forms.
For example the valves may be either pneumatically, electrically or
hydraulically operated.
The regeneration and cooling recycle streams are continuously
purged with a bleed stream (22) of purified, dry natural gas to
prevent an excessive buildup of contaminants in the streams. As
shown the bleed stream flow enters the cool-down reycle stream and
is in turn vented into the regeneration recycle stream.
The bleed stream is removed through conduit 36 from the
regeneration cycle stream at the suction side of blower 35. The
bleed stream is then alternately heated by gas turbine exhaust gas
in exchanger 26 or cooled in exchanger 30, respectively, to
regenerate and cool the adsorption driers 13, 13a.
The adsorption driers 13, 13a are arranged and controlled similarly
to the molecular sieve adsorption units 15, 15a, 15b. During the
regeneration cycle each adsorption drier is first heated by the
purge gas which has passed through the heater 26 and is then cooled
by the purged gas which has passed through the cooling unit 30. A
suitable timing device coupled with responsive control valves
control the flow of the purge gas through the adsorption drier
being regenerated. After an adsorption drier has been on stream for
a predetermined period of time, for example 8 hours, a timing
circuit will direct the incoming feed gas to the adsorption drier
which has just undergone regeneration and will place the drier
which has been on stream in the above-described regeneration cycle.
Cycle time depends on water content and equipment.
After the bleed or purge stream has passed through the adsorption
drier undergoing regeneration it is subsequently vented into the
fuel gas system for the gas turbine driver through conduit 31. This
fuel stream joins with stream 31a and with fuel stream 37 and goes
to the gas turbine 39a. The hot exhaust from the turbine flows
through a suitable conduit 37a and flows through heat exchanger 26
to warm the same. Thus our invention provides a unique process for
efficiently regenerating the adsorption units and for providing
fuel gas to the refrigeration compressor.
The compressed and cooled main feed stream enters the liquefier
portion of the process through conduit 17, the main flow stream
being indicated in heavier lines in FIG. 1. The natural gas which
is delivered to the liquefier portion is essentially dry, purified,
and of a suitable pressure. A suitable pressure for the instant
embodiment was found to be approximately 650 psia at ambient
temperature. This stream is cooled by nitrogen in exchanger 40. The
temperature of the natural gas stream is lowered in this exchanger
to approximately -75.degree.F. At this temperature and pressure
approximately 1 percent of the feed gas will separate as liquid in
the separator 41. Substantially all heavy hydrocarbons (C.sub.5 +)
will be removed in the separator 41 and will be found in the liquid
phase. The liquid formed in the separator passes through conduit 42
into exchanger 43 wherein its refrigeration capacity is given up to
the refrigerant stream. An expansion valve 42a is provided in line
37 to reduce the pressure of the stream to the fuel gas system
pressure. After the liquid has been boiled off in exchanger 43 it
proceeds through conduit 37 to the fuel gas supply system to the
engine 39a. The amount of refrigerant nitrogen passing through
exchanger 43 is regulated so that all the available refrigeration
of liquid stream 37 is recovered.
The vapor from separator 41, which is approximately 90 percent
methane, passes through conduit 46 into balancing exchanger 47 and
then into liquefier 48 wherein the stream is liquefied. The
liquefied stream passes through conduit 49 into a nitrogen
stripping column 50 which is operated at approximately 600 psia.
The stream 49 is expanded by means of a suitable pressure
controller 50a to the desired column pressure.
Methane, free of heavy hydrocarbons, but including all the nitrogen
in the feed gas, is taken off the top of the stripping column
through conduit 55 and fed into the nitrogen-methane column 56. In
the nitrogen-methane column essentially pure methane is made and
nitrogen which can be used in the refrigeration cycle is also
separated out. The details of this column will be described below.
Liquefied natural gas from the bottom of the nitrogen stripping
column 50 passes through conduit 70 through exchangers 71 and 72
wherein the natural gas is subcooled. The subcooled stream passes
through eductor 74 and then into the LNG storage tank 75 with
essentially no flash.
As mentioned above, same methane which includes all of the nitrogen
in the feed gas is taken off of the nitrogen stripping column 50
and is fed through conduit 55 into nitrogen methane column 56.
Essentially pure (99+percent) methane is removed from the lower
portion of this column and passed through conduit 57 into
exchangers 71 and 72 wherein it is subcooled and it then passes
through an expansion device 59 without flash into a methane tank 58
wherein it is stored in liquid form at a pressure above ambient.
The methane that is stored can be used to "sweeten" the LNG in the
main tank or distributed elsewhere as a pure product through piping
system 153. Methane is transferred to the main tank or loading
station by storage pressure differential through line 153a.
The nitrogen is removed from the top of the column 56 through
conduit 60 and passed through exchangers 72 and 71 to remove the
refrigeration therefrom and this stream then couples with the
nitrogen refrigeration stream 61. An automatic vent arrangement
100a is provided on condenser 100 to vent helium and other low
boilers that may be present in the feed gas. The combined stream
enters liquefier 48. Thus the nitrogen which is in the natural gas
feed stream is removed therefrom in the instant
purification-liquefaction process and is utilized in the
refrigeration cycle. Since the amount of nitrogen returned to the
suction of the nitrogen compressor is greater than the high
pressure nitrogen flow to the liquefier by the amount of nitrogen
stripped from the natural gas, an inventory or nitrogen is
accumulated in the storage tank 62, which also serves as a surge
tank. When a surplus of nitrogen is available, the high pressure
nitrogen head pressure increases and the surplus nitrogen is put
into storage tank 62 by pressure control valve 63. When additional
nitrogen is required by the cycle, the high pressure nitrogen head
pressure decreases and the additional nitrogen is taken from the
storage tank 62 by pressure controller 64 and flows into the cycle
nitrogen compressor suction.
A description of the refrigeration cycle which may be described as
a closed loop nitrogen cycle providing all the refrigeration
required to liquefy the natural feed gas will now be given.
Nitrogen is compressed to approximately 400 psia in cycle nitrogen
compressor 39 and is then after-cooled in after-cooler 80. This
pressure is optimum for the conditions of the present cycle but
could be varied to suit other conditions. The nitrogen is then
further compressed in compressors 81, 82 and 83 to approximately
650 psia. After leaving compressor 83 the nitrogen is sent through
a conventional after cooler 84 to remove the heat of compression.
The nitrogen then proceeds through conduit 85 and is split into two
streams 86 and 87. Most (75 percent) of the high pressure feed
proceeds through conduit 86 into heat exchanger 40 wherein it is
cooled. The cooled stream 88 which leaves the exchanger 40 then
proceeds to reboiler 90 wherein it is further cooled by boiling the
liquid obtained from the lowest portion of the nitrogen stripping
column. The cooled nitrogen stream is further cooled as it proceeds
through conduit 91 and passes through liquefier 48 and subcooling
heat exchangers 71 and 72. The nitrogen is substantially liquefied
in exchanger 72 and then passes through conduit 92 into a chamber
92a with an automatic vent 926 to purge the system of
noncondensible helium and other low boilers. It is then split into
two streams, one of which proceeds through conduit 93 and the other
which proceeds through conduit 94 to expansion valve 95. The
nitrogen liquid which proceeds through conduit 93 is used in
condenser 100 after it is expanded through valve 101 to a suitable
pressure (approximately 160 psia). The liquid serves to condense
the nitrogen obtained from the top of the methane column to produce
reflux for the column. The nitrogen which is boiled off combines
93a with the nitrogen and low boilers which are leaving the column
through conduit 60 and they then flow via conduit 96 to the
exchangers. The portion of the liquid which proceeds through
conduit 94 is expanded through valve 95 into liquid nitrogen
separator 105. The pressure in the separator is dependent upon the
selected operating pressure of the cycle compressor 39. The liquid
nitrogen passes through conduit 106 into boil-off condenser 107
wherein the liquid nitrogen is itself boiled off and returns via
conduit 109, while condensing the boil-off from LNG tank 75. The
nitrogen which is thus boiled off escapes through conduit 97 and
joins with the stream 96. The natural gas boil-off is drawn through
conduit 108 into the boil-off condenser 107 by means of the
thermosiphon action of the cold condenser. The condenser boil-off
is returned to the LNG tank by means of eductor 74. By condensing
the boil-off and returning it to the LNG tank the composition of
the liquid natural gas which is stored in the tank in liquid form
is maintained constant. If no provision were made for recondensing
the boil-off the composition of the liquid remaining in the tank
would change over a period of time due to the fact that the high
boilers would boil off first leaving the lower boilers in liquid
form.
The portion (24 percent) of the nitrogen refrigerant which passes
through conduit 87 is cooled in a freon refrigeration unit 110 to
approximately -20.degree.F. Other types of refrigeration systems
could be used in place of a freon system and it has been found that
under certain circumstances there is no need for additional
refrigeration at this point. This stream is then joined with the
portion of the nitrogen stream which has been allowed to come
through exchanger 43. The combined stream then passes through
conduit 44 to high level expander 111 wherein the nitrogen is
expanded to a lower pressure and temperature (160 psi,
-145.degree.F) and passed through liquefier 48 in countercurrent
relationship with the product stream which is being cooled. Note
that the stream 61 combines with stream 112 which is made up of the
boil-off from the liquid nitrogen separator 105, the boil-off from
the condenser 100 and the nitrogen and low boilers obtained via
conduit 60 from the upper portion of the methane column 56. These
combined streams serve to assist in cooling and liquefying the
natural gas passing through liquefier 48. The combined and warmed
streams then pass through conduit 113 into reheat expander 114
wherein the gases are expanded to a lower temperature
(-150.degree.F) and pressure (85 psia). The gases then pass through
conduit 115 and combine with gases from conduit 116 and flow
through liquefier 48, heat exchanger 47, and then through conduit
117 into heat exchanger 40 and then return through conduit 118 to
the suction side of the compressor 39.
Shaft seal leakage from cycle compressor 39 and the
expander/compressors is trapped out of the oil systems of these
units, recompressed by a seal gas compressor 121, cooled in
after-cooler 120 and delivered to the suction side of the
compressor 39. Oil vapor is removed in a suitable adsorption filter
122. Thus the seal gas leakage is returned to the cycle nitrogen
compressor section for reuse.
During periods when the liquefier is inoperative, the nitrogen
refrigerant gas is stored at ambient conditions in storage tank 62
which was described above and which can be sealed off. The storage
tank 62 is sized to accommodate a full refrigerant gas charge, thus
allowing stop-restart operations typical of peak shaving plants
with no refrigerant inventory problems.
The majority (90 percent) of the main refrigerant gas stream which
passes into liquefier 48 from conduit 91 is split from the main
stream after it has passed through heat exchanger 71. This is
directed through conduit 130 and proceeds to nitrogen stripping
column condenser 131 wherein it serves to condense the vapor coming
from the top of the nitrogen stripping column to provide reflux
therefor. The nitrogen vapor coming from the condenser 131 passes
through conduit 132 and is expanded in low level expander 133 to a
lower temperature (-265.degree.F) and pressure (90 psia) and then
it is passed through heat exchanger 72 and 71 and into conduit 116
which then joins the stream in conduit 115 for passing through the
liquefier 48.
Part of the reflux for the nitrogen stripping column comes from
nitrogen methane rectification column reboiler 140. This reboiler
boils the liquid which is obtained from the lower portion of the
column 56 and at the same time condenses a portion of the vapor
which passes from the top of column 50 through conduit 141. Note
that the vapor coming from conduit 141 is split to pass through
both the condenser 131 and the reboiler 140. This has been found to
be a most economical way to produce reflux for the column 50 and to
boil off the liquid collected in the bottom of the column 56.
It is readily apparent that the process described in detail above
is especially suited not only for situations wherein the total
amount of feed gas introduced is liquefied and stored for use at a
future date, but also for situations wherein only a selected
portion of the feed gas is liquefied. The LNG which is stored in
the storage tank 75 may be withdrawn in liquid form. To ship
liquefied natural gas by truck, etc., to remote locations it is
only necessary that the liquid be withdrawn through conduit 141 and
transferred into the transport vehicle. In a typical system the
head of liquid in the tank will serve to transfer the liquid into a
transport vehicle. During times when the liquefier is not in
operation, the vapor which is in the tank above the liquid is
withdrawn through conduit 142. To bring the gas up to proper
pressure for entrance into the distribution network, it is passed
through a heater 144 and then the distribution network, it is
passed through a heater 144 and then compressed by suitable
compressor 143 and then forwarded to the distribution network. For
peak showing requirements, liquid may be withdrawn through conduit
145 and pumped in a suitable liquid pump 146 and then forwarded to
a vaporizing unit 147. The vaporized product may be then forwarded
to the distribution network as desired. The details regarding the
construction of the LNG tank 75 are not described in detail due to
the fact that this may be a conventional tank design to store
cryogenic fluids. The tank is normally operated at 0 psig. The
methane tank 58 stores essentially pure liquid methane which may be
used to sweeten the product in the LNG tank if this is found
necessary.
The operation of the liquefier and distribution columns and
associated equipment can be controlled to vary the methane
concentration of the LNG sent to storage tank 75 which provides the
advantage of producing LNG within acceptable calorific limits.
Whereas this invention has been described primarily with respect to
the liquefaction of natural gas and is especially suited therefor,
it may also be used for the liquefaction of other gases. The
temperatures, pressures, flow rates, etc., have been cited for
purposes of illustration and to facilitate an understanding of the
invention. These may be varied to satisfy required conditions.
The preferred embodiment of the present invention has been
described in detail, however it is apparent that modifications to
the process and apparatus may be made without departing from the
spirit and scope of the invention.
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