Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1315188
Gas recovery process
This invention relates to the processing of gas
streams for the recovery of ethane, propane and
substantially all of the heavy hydrocarbon content from
methane-rich process gas streams. It is a divisional of
Canadian Application Serial No. 555,112, filed
December 22, 1987.
Gas streams containing hydrocarbons and other gases
which may be processed according to the present invention
include natural gas, synthetic gas streams obtained from
other hydrocarbon materials such as coal, crude oil,
naphtha, oil shale, tar sands and lignite. Available
processes for separating these materials include those
based upon cooling and refrigeration of gas, oil
absorption, refrigerated oil absorption, and the more
recent cryogenic process utilizing the principle of gas
expansion through a mechanical device to produce power
while simultaneously extracting heat from the system.
Depending upon the pressure of the gas source, the
richness (ethane and heavier hydrocarbons content) of the
gas and the desired end products, each of these prior
processes or a combination thereof may be employed.
The cryogenic expansion type process is now
generally preferred for ethane recovery, especially when
the gas stream is available at higher pressure, because
it provides maximum simplicity with ease of start up,
operating flexibility, good efficiency, safety, and good
reliability. U.S. Patents Nos. 3,360,944, 3,292,380 and
*
- 2 - 1 315 1 88
3,292,381 describe typical processes o~ the above type.
The cryogenic expansion process can generally be
described as condensation of liquids in excess of the
recovered liquid products from the feed gas followed
by elimination of the undesired light components, i.e.,
methane. In a typical cryogenic expansion recovery pro-
cess, a feed gas stream under pressure is chilled and
partially condensed by heat exchange with other streams
of the process and/or supplementary external sources
of cooling, such as propane compression-refrigeration
system. The condensed liquid is separated from the gas
stream in one or more separators. The condensed liquid
contains most of the desired C2 components. It is then
expanded to a lower pressure. The vapourization occuring
during expansion of the liquid results in further cooling
of the remaining liquid. The cooled stream, comprising
a mixture of liquid and vapour, is demethanized in a
demethanizer column. The demethanizer is a fractionat-
ing column in which the expansion - cooled stream is
fractionated to separate residual methane, nitrogen and
other volatile gases as overhead vapour from the desired
products of ethane, propane and heavier components as
bottom products.
If the feed stream is not totally condensed, the
vapour remaining from this partial condensation is passed
through a turbo-expander, or expansion valve, to the
demethanizer operating pressure. Additional liquids are
condensed as a result of the further cooling of the stream
by expansion. Liquid thus obtained is also supplied as a
feed to the demethanizer. Typically the remaining vapour
and the demethanizer overhead vapour are combined as the
residue gas. It is normally required to recompress the
residue gas to the original feed gas pressure level.
In the ideal operation of such separation process,
the overhead vapours leaving the process will contain
131~188
-- 3 --
I
substantially all of the methane found in the feed gas
to the recovery plant, and substantially no hydrocar-
bons equivalent to the ethane or heavier components.
The bottoms fraction leaving the demethanizer will con-
tain substantially no methane. In practice, however,
this ideal situation is not achieved mainly because the
conventional demethanizer is operated primarily as a
stripping column. The methane, therefore, typically
comprises vapour not subjected to any rectification step.
Substantial losses of ethane occur because the vapours
remaining from low temperature separation steps contain
ethane and heavier components which could be recovered
if those vapours could be brought to lower temperatures
or if they were brought in contact with a significant
quantity of relatively heavy hydrocarbons, for example,
C3 and heavier, capable of absorbing the ethane. Over-
all recovery of the ethane can be further increased by
altering the temperature distribution in the demethanizer
column so as to decrease the temperature at the upper
stages of the column by removing heat from one or more
of the feeds thereto.
During the past decade, significant efforts have been
devoted to improving the cryogenic expansion recovery pro-
cess for higher recovery of the desire components. Among
~5 such processes there may be mentioned the gas-subcooled
process described in Campbell, et al., Canadian Patent
1,041,003 and the liquid-subcooled process described in
Campbell, et al., Canadian Patent 1,048,397.
It is the object of the above Campbell, et al. pro-
cesses to maximize the quantity of liquids recovered per
unit of energy input. The conventional single stage
expansion process is normally limited to ethane recovery
of about 80 percent without disproportionate horsepower
requirements. Above 80 percent recovery, larger expan-
sion ratios are required with a resulting rapid increase
1315188
-- 4
in recompression horsepower. The above-mentioned gas-
subcooled process and liquid-subcooled process have the
ability to provide high ethane recoveries of above 85
percent while requiring no additional external horsepower
and in some instances less horsepower.
Although the ethane recovery can be improved by the
above processes, the overall propane and heavier
components losses tend to increase. This is because the
~ content in the top liquid feed to the demethanizer is
highsr than is the case for conventional processes.
These heavier components tend to absorb the ethane in the
rising vapour while a slight portion of them is stripped
by the vapour and appears as part of the residue gas.
It is the object of the present invention to modify
the above gas-subcooled process and liquid-subcooled
process to further increase the ethane recovery and
reduce the C3 losses.
Since the residue gas is at a high pressure, it can
be condensed at a higher temperature. It is then sub-
cooled and expanded before returning to the demethanizer
as reflux. It has been found that for a conventional
single stage expansion cryogenic process, a very high
recycle reflux rate is required of about 30 to 40 percent
by volume of the residue gas rate at zero recycle,
depending on the feed gas composition, in order to
achieve 95 % + ethane recovery because of the high C2 in
the vapour from the stripping section. This, of course,
drastically increases the compression load. Since the
gas-subcooled process and the liquid-subcooled process of
the prior patents already reduce a large portion of the C2
in the stripped vapour, the required reflux rate and
hence additional compression requirements are
significantly reduced. It has been found with the
process of the present invention that a reflux rate of
about 10 to 20 percent will increase the recovery of
ethane to over 95 percent with minimum increase in
compression load. This is accomplished by the particular
manner of utilizing the rectifying. In addition to the
1315188
higher ethane recovery at lower compression loads, the
present invention permits processing of natural gases
wit]h higher C02 concentration than could be tolerated by
the prior art processes.
For a better understanding of the present invention,
reference is made to the following examples and drawings:
Figure 1 is a flow diagram of a prior art li~uid-
subcooled processing plant incorporating a set of
conditions for a typical rich natural gas stream;
Figure 2 is a flow diagram showing the present
invention processing the rich natural gas stream with two
rectifying sections with high upper reflux rates:
Figure 3 is a flow diagram showing the present
invention processing the rich natural gas stream with two
rectifying sections with low upper reflux rates;
Figure 4 is a graph of ethane recovery versus reflux
rate for the present invention when processing the rich
natural gas stream;
Figure 5 is a flow diagram of a prior art gas-
subcooled processing plant incorporating a set of
conditions for a typical lean natural gas stream;
Figure 6 is a flow diagram showing the present
invention processing the lean natural gas stream; and
Figure 7 is a flow diagram showing a conventional
single-stage expander plant completed with a recycle
reflux loop.
In the following explanation of the above figures,
tables are provided summarizing flow rates, calculated
for the representative processing conditions. In the
tables, the values for the flow rates (in pound moles per
hour) have been rounded to the nearest whole number
~ 6 - 1315188
for convenience. Temperatures indicated on the figures
are approximate values rounded to the nearest degree.
Example 1 tPrior Art)
Referring to Figure 1, plant inlet gas which has been
dehydrated at an earlier stage enters the process at 1000
psia and 100F as stream 50. It is divided into two
parallel streams and is cooled to 27F by heat exchange
with cool residue gas at 18F in exchanger 10, with pro-
duct liquid stream 57 in exchanger 11 and with demethan-
ger liquid at 20F in demethanizer reboiler 12. From
these exchar.gers, the streams are combined and is furtherchilled to 19F in exchanger 13 by heat exchange with the
residue gas at -9F. If desired, exchangers 10, 11, 12,
and 13 can physically be in a singe exchanger unit. From
exchanger 13 the inlet gas enters a gas chiller, exchanger
14, where it is chilled to -4F with propane refrigerant
at -14F. The cooled stream is again divided into two
parallel streams, and further chilled to -45F by heat
exchange with the cold residue gas at -129F in exchanger
15 and with demethanizer liquids at -57F in the demetha-
nizer side reboiler 16. The streams are recombined and
the recombined stream is pressure reduced through valve
17 to 925 psia and -48F, becoming stream 51. It then
enters the high pressure separator 18.
The chilled gas stream 52 from the high pressure
separator flows through the expander 20 where it is ex-
panded from 925 psia to 300 psia. The expansion chills
the gas to -123F. Expander 20 is preferably a turbo-
expander, having a compressor 25 mounted on the expander
unit. From the expander outlet, the two phase stream
flows to demethanizer 23 as its middle feed.
The cooled liquid stream 53 from the high pressure
separator 18 flows through exchanger 19 where it is sub-
cooled to -127F by heat exchange with the cold stream
from expansion valve 21. The sub-cooled liquid is then
divided into two portions. The first portion (stream
- - 131~188
55) flows through expansion valve 21 where it undergoes
e~pansion and flash vapourization as the pressure is
reduc:ed from about 920 psia to 310 psia. It then flows
through exchanger 19 to subcool the liquids from sepa-
rator 18. From exchanger 19, the stream is heated to-58F and then flows to demethanizer 23 as its lowest-
feed. The second portion (stream 54) from exchanger 19,
still at high pressure, is expanded through expansion
valve 22. The flash vapourization, as the pressure is
reduced from about 920 psia to 300 psia, chills this
stream down to -136F. The cold stream from expansion
valve 22 then proceeds to the demethanizer at the top
tray as its top feed.
~he residue gas (stream 56), from the demethanizer
overhead, is used to chill the feed gas in exchanger
lQ, 13 and 15. Exiting from exchanger 13 at 76F, it
is then compressed to 344 psia in compressor 25 driven
by expander 20 and directly coupled thereto. The gas
then enters compressor 26 and cooler 27 where it is
compressed and cooled to a final discharge pressure
and temperature of 815 psia and 105F (stream 58).
Major stream component flow rates, outlet liquid
recoveries, compression and refrigeration load require-
ments for this process shown in Figure 1 are given in
the following table.
Table 1
Stream Flow Rate Summary (lbmoles/hr)
Comp./Str. No.50 52 53 56 57 58
N2 167 120 47 167 0 167
C02 118 43 75 53 65 53
Cl 54922940 2552 5462 30 5462
C2 1208 294 914 140 1068 140
C3 429 48 381 6 423 6
C4~ 271 11 260 0 271 0
Total 76853456 4229 5828 1057 5828
- s
1315188
Recoveries
Ethane = 88.4~
Propane = 98.6%
Compression and Refrigeration Load
Compression (compressor 26) : 3280 hp
Refrigeration Load : 4.22 MMBtu/hr of propane refrigeration -
at -14F
Example 2 (Prior Art)
In Figure 5 a lean natural gas stream is processed in
a Campbell, et al. Gas Sub-cooled processing unit. The
feed gas, which has been partially chilled and dehydrated
in previous process units, enters the process at 950 psia
and 15F as stream 50. It is divided into two parallel
streams. The first stream is chilled to -27F in exchan-
ger 10 with residue gas at -51F. The second stream is
chilled to -19F in exchanger 11 by heat exchange with the
demethanizer liquids at -30F. The two streams recombine
and flows to the high pressure separator 12 at 940 psia
and -23F.
The separator vapour (stream 51) is divided into
two streams. The first stream flows through the turbo-
expander 16 where it is expanded from 940 psia to about
300 psia. The expansion chills the gas down to -103F.
The two phase stream, from the expander outlet, flows to
the demethanizer 17 as its lowest feed. The second vapour
stream combines with the separator liquid (stream 52 and
is chilled to -58F in exchanger 13 by the residue gas at
-109F. It is then heat exchanged to -87F with demetha-
nizer liquids at -102F in exchanger 14. It is finally
condensed and subcooled to -138F in exchanger 15 by heat
exchanger with the residue gas at -149F from the demetha-
nizer overhead. This condensed stream flows through valve
22 and expands to the demethanizer operating pressure of
about 300 psia.
9 131 ~1~8
Flash vapourization further chills the stream to
-150F prior entering the demethanizer as its top feed.
The demethanizer 17 is reboiled by two side reboilers
and a main reboiler 21 heated by a heating medium. From
S the dennethanizer overhead, the residue gas chills the feed
gas in exchangers 10, 13 and 15. It exits the gas/gas ex- j
changers at 10F and flows to other process units as a
cooling medium. It returns to the compression facilities
at 250 psia and 109F. It is compressed to 1275 psia by
compressors 18 and 19. Compressor 18 is driven by the
expander 16. The residue gas is finally cooled to 120F
(stream 55) in cooler 20.
Major stream component flow rates, outlet liquid
recoveries, compression requirements for this process
shown in Figure 5 are given in the following table.
Table 2
Stream Flow Rate Summary (lbmoles/hr)
Comp./Str. ~o.50 51 52 53 54 55
N2 58 56 2 58 0 58
Cl 8214 7649 565 8181 33 8181
C2 1129 898 231 78 1051 78
C3 435 265 170 4 431 4
C4+ 133 52 81 0 133 0
Total 9969 8920 1049 8321 1648 8321
~ecoveries
Ethane = 93.1
Propane = 9`9.1%
Compression Requirements
Compression (compressor 19) : 6870 hp
- ln - 131~18~
Example 3 ~Prior Art)
Figure 7 shows a conventional single-stage expander
plant processing the lean natural gas stream. A recycle
reflux loop is added to increase the recoveries.
Feed gas at 950 psia and 15F enters the process as
stream S0. It is divided into two streams and are chilled
to -42F. The first stream is cooled by residue gas at
-63F in exchanger 10; the second stream is chilled by heat
exchanger with demethanizer liquids at -32F and -94F in
exchanger 11 and 12 respectively. From these exchangers
the two streams combine and is expanded to 850 psia through
valve 13. The temperature drops to -50F after the ex-
pansion. The two phase stream then flows to the high
pressure separator 14.
The separated vapour (stream 51) from the separator
is expanded through expander 15 into the demethanizer 17
as its middle feed. Expander 15 is a turbo-expander unit.
The work expansion from 850 psia to about 300 psia drops
the stream temperature to -116F. The separated liquids
(stream 52) from the separator is pressure reduced through
valve 16 into the demethanizer as its lowest feed. The
flash vapourization chills the stream temperature down
to -98F.
From the demethanizer overhead, the residue gas
(stream 53) at -155F flows to exchanger 18 to condense
and subcool the recycle reflux stream. Its temperature
is raised to -63F and flows to exchanger 10 to chill the
feed gas. It is heated up to 9F in exchanger 10 and
is then sent to other process units as a cooling medium.
It returns to the compression facilities at 2S0 psia and
109F. It is then compressed and cooled to 1275 psia and
120F by compressor 21 and 22 and cooler 23. Compressor
21 is driven by the turbo-expander.
A portion of the high pressure residue gas (stream
56) is recycled as a reflux stream to the demethanizer.
From cooler 23 outlet, the recycle reflux stream is first
131~188
cooled to 38F by reboiling the demethanizer in exchanger
19. It is then condensed and subcooled by heat exchange
with ~he cold residue gas at -155F in exchanger 18 to
-144F. It is finally expanded through valve 20 and
flows to the top demethanizer tray. The expansion drops
the temperature to -161F.
Results for this case are summarized in the following
table.
Table 3
Stream Flow Rate Summary (lbmoles/hr)
Comp.~Str. No. 50 51 52 53 54 55 56
N2 58 54 4 79 0 58 21
Cl 8214 6814 140011142 34 8180 2962
C2 1129 610 519 63 1083 46 17
C3 435 128 307 0 435 0 0
C4+ 133 18 115 0 133 0 0
Total 9969 7624 234511284 1685 8284 3000
Recycle Reflux Rate
Reflux/ Residue Gas at zero recycle = .361 t36.1%)
note: residue gas rate at zero recycle is approximately
8500 lbmoles/hr based on 80% ethane recovery
Recoveries
Ethane = 95.9%
Propane = 100.0%
5 Compression Re~uir~ments
Compression ~compressor 22) : 10080 hp
131~188
~xample 4
The present invention is shown in Figure 2. The basic
process configuration is similar to that of Figure 1, but
with an addition of a rectifying section to the
demethanizer and a recycle reflux loop.
Four to six trays are added to the top section of the
demethanizer of Figure 1 as a second rectifying section.
It is here where the cold lean recycle reflux stream
- contacts the rising vapour from the stripping section
below. A majority of C2+ components in the vapour is
condensed and returned to the stripping section.
A portion of the high pressure residue gas from the
cooler 27 outlet, at 815 psia and 105F, is recycled as
reflux to the demethanizer. The recycle stream is first
lS chilled to -103F in exchanger 28 by heat exchange with a
slip residue gas stream at -129F. It is then condensed
and subcooled to -150F in exc'nanger 29 by heat exchange
wth the total residue gas stream from the demethanizer
overhead at -155F. The liquid reflux stream is then
pressure reduced to the demethanizer operating pressure
through expansion valve 30. The flash vapourization
process further chills the reflux stream to -165F prior
entering the demethanizer top tray.
From the demethanizer overhead, the residue gas stream
is first used to condense and subcool the reflux in
exchanger 29. It is then divided into two parallel
streams. The first portion is used to chill the reflux
stream in exchanger 28. The second portion chills the
feed gas in exchangers 10, 13 and 15. The two streams
recombine at the inlet to the expander driven compressor
25. It is then compressed by compressors 25 and 26 and
then cooled by cooler 27 to 815 psia and 105F.
- 13 - 1315188
Other changes to the ~rocess of Figure 1 are as
follows:
1) There is a shift in duty between exchanger 10 and
11; exchanger 10 decreases in duty whilst exchanger
11 increases;
2) There is a shift in duty between exchanger 15 and
16; exchan~er 15 decreases on duty whilst exchanger
16 increases; and
3) Pump 24, compressor 26 and cooler 27 require an in-
crease in capacity.
Major stream component flow rates, recycle reflux
rate, outlet liquid recoveries, compression and refrigera-
tion load requirements for this invention shown in Figure
2 are given in the following table.
Table 4
Stream Flow Rate Summary _lbmoles/hr)
Comp./Str. No.50 52 53 56 57 58 59
N2 167120 47 202 0 167 35
C02 118 43 75 45 81 37 8
Cl 549229402552 6610 32 5460 1150
C2 1208294 914 40 1175 33 7
C3 429 48 381 0 429 0 0
C4+ 271 11 260 0 271 0 0
Total 768534564229 6897 1988 5697 1200
Recycle Reflux Rate
Reflux/Residue Gas at zero recycle = .211 (21.1%)
Recoveries
Ethane = 97.3%
Propane - 100.0%
- l~ - 131~1~8
Compression and Refrigeration Load
Compression (compressor 26) : 4100 hp
Refrigeration Load : 4.22 MMBtu/hr of propane refrigeration
at -14F
Figure 3 shows the operating conditions of the present
invention at a reduced recycle reflux rate. The results
are summari~ed in the following table.
Table 5
Stream Flow Rate Summary (lbmoles/hr)
Comp.!Str. No. 50 52 53 56 57 58 59
N2 167 120 47 184 0 167 17
C02 118 43 75 53 70 48 5
Cl 5492 2940 2552 6029 30 5462 567
C2 1208 294 914 114 1105 103 11
C3 429 48 381 0 429 0 0
C4+ 271 11 260 0 271 0 0
Total 7685 3456 4229 6380 1905 5780 600
Recycle Reflux Rate
Reflux/Residue Gas at zero recycle = .103 (10.3~)
Recoveries
Ethane = 91.5%
Propane = 100.0%
Compression and Refrigeration Load
Compression (compressor 26) : 3770 hp5 Refrigeration Load : 4.22 MMBtu/hr of propane refrigeration
at -14F
- 15 - 1 3 1 ~ 188
Figure 4 is a plot of the ethane recovery versus the
reflux rate for the present invention processing the rich
natural gas stream. It can be seen that when the reflux
rate is below 7~ and above 25~, the rate of increase in
ethane recovery is appreciably less than the rate of in-
crease in the reflux rate. Therefore, for this natural
gas stream, the optimum reflux rate range is between 7%
to 25%.
Example 5
Figure 6 depicts the present invention processing the
said lean natural gas stream. The deviations from the
Campbell, et al. process of Example 2 are summarized as
follows:
1) Four to six trays are added to the top section of the
demethanizer as a second rectifying section;
2) A portion of the high pressure residue gas from the
cooler 20 outlet, at 1275 psia and 120F, is recycled
as reflux to the demethanizer. The recycle stream
(stream 56) is first cooled to 35F by reboiling the
demethanizer in exchanger 23. This will reduce the
reboiling load in exchanger 21. It is then let down
to 940 psia through valve 24. This step is to reduce
the design pressure for the downstream exchanger units.
The gas is cooled to 21F as resulted from the expan-
sion. It is then chilled to -49F in exchanger 25
by heat exchange with residue gas at -103F. It is
further chilled to -94F in exchanger 26 by the cold
demethanizer liquids at -108F. It is finally con-
densed and subcooled to -148F in exchanger 27 by the
cold residue gas at -161F. It is then expanded to
300 psia through valve 28 and enters the demethanizer
as top feed. The expansion drops the reflux stream
temperature to -164F;
~ l6 ~ 1 31~ 188
3) The residue gas from the demethanizer overhead
(stream 53) is divided into two streams; the first
portion chills the reflux stream in exchangers 25
and 27, the other stream chills the feed gas in
exchangers 13 and 15. The two streams recombine
at the inlet to exchanger 10;
4) An additional demethanizer top side reboiler (ex-
changer 25) is included to chill the reflux stream.
The draw-off is from the same tray as for the side
reboiler ~lA, and
5) Compressor 19 and cooler 20 require an increase in
capacity.
Data for this case are given in the following table.
Table 6
Stream Flow Rate Summary (lbmoles/hr)
Comp./Str. No.S0 51 52 53 54 55 56
N2 58 56 2 66 0 58 8
Cl 8214 7649 565 9271 34 8180 1091
C2 1129 898 231 22 1110 19 3
C3 435 265 170 0 435 0 0
C4+ 133 52 81 0 133 0 0
Total 9969 8920 1049 9359 1712 8257 1102
2ecycle Reflux Rate
Reflux/Residue Gas at zero recycle = .132 (13.2%)
Recoveries
Ethane = 98.3%
Propane = 100.0%
Compression Requirements
Compression (compressor 19) : 7910 hp
