Abstract:
A cryogenic process for separating methane from ethane and heavier hydrocarbons in which a high pressure gas feed is divided into two gas streams. The gas is cooled either before or after it is divided and this step may include some condensation in which case the condensate is separated from the gas. One of the divided gas streams is expanded through a work expansion machine down to the pressure of the fractionation column. Any separated condensate is also expanded to the column pressure. The second divided gas stream is further cooled by heat exchange and then expanded down to an intermediate pressure whereby a portion is condensed. This condensate is separated from the remaining gas and then expanded to the column pressure. The remaining gas is further cooled and expanded and fed to the column as the top feed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an improved cryogenic gas separation process. 
     Mixtures of gases are frequently separated into the component gases by cryogenic techniques. An example is the separation of ethane (and heavier hydrocarbons) from methane. Recent increases in the market for ethane, propane and heavier hydrocarbons have created the need for processes yielding higher recovery of these products. 
     Several variations of prior art cryogenic separation processes are described in U.S. Pat. No. 4,278,457 issued July 14, 1981 and the present invention will be compared to the processes disclosed in that U.S. patent. 
     U.S. Pat. No. 4,278,457 deals primarily with the problem of increasing ethane recovery while at the same time reducing the danger of CO 2  icing. This is accomplished by splitting the vapor stream to the demethanizer column into two portions. This was found to reduce the risk of CO 2  icing without increasing column overhead temperature so that the ethane recovery was not adversely effected. The vapors can be split either before or after the preliminary cooling stages. The first portion of the vapor is cooled to substantial condensation, expanded to the column operating pressure and supplied as a column feed usually at the top of the column. The second portion of the vapor is expanded through a work expansion machine. This stream is cooled sufficiently prior to expansion so that the column top temperature can be controlled by the column top feed. The column refrigeration is provided by the combined cooling effect of the first and second portions of the split vapor feed. Any condensed liquids that result can be expanded and supplied as a lower mid-column feed. 
     SUMMARY OF THE INVENTION 
     In the present invention the feed gas is also split into two portions with one portion being expanded in the normal manner through a work expansion machine and then fed to the column as a mid-point feed. The other portion of the feed gas is cooled and then expanded in a low pressure cold separator to partially condense liquid and separate the liquid from the remaining gas. The liquid from the low pressure cold separator is fed to the column at a mid-point. The vapor from the low pressure cold separator is cooled and expanded and fed to the column as a top feed. The use of this technique of cooling and flashing the one portion of the vapor stream to partially condense and then separating provides a relatively pure methane stream for the top column feed. This superior reflux stream greatly increases the efficiency of the process as compared to prior technology for ethane recovery. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow diagram of a typical prior art cryogenic natural gas processing plant incorporating a split vapor feed. 
     FIG. 2 is a flow diagram of a cryogenic natural gas processing plant in accordance with the present invention. 
     FIG. 3 is a flow diagram of a variation of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, inlet gas enters the process at 120° F. and 910 psia as stream 10. This inlet gas stream has been pretreated as necessary to remove any concentration of sulfur compounds and water. This feed stream 10 is split into streams 12 and 14 with the stream 12 being cooled in heat exchanger 16 and stream 14 being cooled in heat exchangers 18 and 20. Heat exchanger 16 is cooled by the residue gas stream 22a. Heat exchangers 18 and 20 are the reboiler and side reboiler for the demethanizer respectively. These feed gas streams 12 and 14 after cooling are then recombined to form a partially condensed, cooled feed gas stream 24 at -30° F. and a pressure of about 910 psia. The vapor and liquid phases in this partially condensed feed gas stream 24 are then separated in the separator 26. The liquid recovered in separator 26 is flash expanded in expansion valve 28 to form the stream 30 at -71° F. and supplied to demethanizer 32 as a lower mid-column feed. 
     The vapors 33 from separator 26 are divided into two branches 34 and 36. The branch 34 is cooled by residual gas stream 22 in heat exchanger 38 to -120° F. which results in the condensation of substantially all of the stream at 34a. The cooled stream 34a after the heat exchanger 38 is then flash expanded through valve 40 to form the demethanizer top feed stream 34b at a temperature of -164° F. 
     The branch 36 of the vapor from the cold separator 26 is expanded through the expansion engine 42 to form stream 36a at -125° F. and supplied as the upper mid-column feed to the demethanizer 32. The work from the expansion engine 42 is employed to recompress the residue gas stream 22 after it has passed through the heat exchanger 16. The bottom product from the demethanizer is withdrawn as stream 44. The overhead residual gas stream is recompressed to 900 psia in recompressor 46. 
     FIG. 2 which illustrates one embodiment of the present invention will now be referred to and compared to the prior art shown in FIG. 1. Like reference numerals refer to like process steps or equipment or process streams. 
     In the FIG. 2 embodiment of the present invention, the vapors 33 from the high pressure cold separator 26 are again split into two streams 34 and 36. Stream 36 is expanded in the usual fashion through the expansion engine 42 to form stream 36a which is supplied to the demethanizer 32 as an upper mid-column feed. The liquid from the high pressure cold separator 26 is also treated just as in the prior art and is expanded through valve 28 and then fed as stream 30 to the demethanizer 32 as the lower mid-column feed. 
     The remaining vapor stream 34 from the high pressure cold separator 26 which has a temperature of about -42° F. is partially condensed in the heat exchanger 48 by heat exchange contact with the residue gas stream 22 from the top of the demethanizer 32. This partially condensed vapor stream 35, which is typically 60 to 70% liquid on a molar basis, is then expanded to an intermediate pressure through the expansion valve 50 resulting in about 30 to 60% liquid on a molar basis. The liquid is separated from the vapor in the low pressure cold separator 52. The liquid from the low pressure cold separator 52 is expanded through the expansion valve 54 to form stream 56 which is supplied to the demethanizer 32 as a lower top-column feed. The vapor from the low pressure cold separator 52 is fed as stream 58 through the heat exchanger 60 in heat exchange contact with the residue gas stream 22 from the top of the demethanizer 32 to form a stream 58a at a temperature of about -130° F. This stream 58a is then expanded through valve 62 forming stream 58b at a temperature of -169° F. which is then supplied as the upper top-column feed to the demethanizer 32. The product from the bottom of the demethanizer 32 is pumped through heat exchanger 63 to provide additional cooling for stream 14. Typical compositions for the feeds to the column for the FIG. 2 embodiment in mole % would be as follows: 
     
                       TABLE I______________________________________        Stream      Stream  StreamComponent    36          58      56______________________________________Nitrogen     1.19        1.67    0.55Carbon Dioxide        0.71        0.50    1.01Methane      91.87       96.06   86.18Ethane       4.56        1.62    8.53Propane      1.18        0.13    2.60i-Butane     0.22        0.01    0.50i-Pentane    0.23        0.01    0.54Pentane Plus 0.04        --      0.09TOTAL        100.00      100.00  100.00______________________________________ 
    
     One variation of the present invention which is shown in FIG. 2 is that the liquid stream 30 which is withdrawn from the bottom of the high pressure cold separator 26 through the expansion valve 28 may be combined with stream 36a through valve 64 (which would otherwise be closed) to form one mid-column feed rather than the two separate upper and lower mid-column feeds. 
     A stream flow summary comparing the prior art process of FIG. 1 with the processes of FIG. 2 (both the 3 stream feed and the 4 stream feed to the demethanizer) is set forth in Table II which follows. In the table, compositions expressed as flow rates are given in pound moles per hour. The following assumptions and criteria were used in the computer simulation to develop the stream flow summary comparison. 
     1. The inlet gas contains 19 pound moles per hour of nitrogen. 
     2. The split between iso and normal butane and hexanes was assumed. 
     3. The expansion engine efficiency was assumed to be 78%. 
     4. The expansion engine compressor efficiency was assumed to be 72%. 
     5. The expander engine bearing loss was assumed to be 2%. 
     6. The demethanizer was assumed to have 14 theoretical stages. 
     7. The recompressor efficiency was assumed to be 75%. 
     8. The physical property data used was SRK K-values and RICE enthalpies. 
     
                       TABLE II______________________________________         FIG. 1  FIG. 2         (Prior Art)                 3 Feeds   4 Feeds______________________________________Inlet GasComposition, Mol/HrNitrogen        19        19        19Carbon Dioxide  12        12        12Methane         1,447     1,447     1,447Ethane          90        90        90Propane         36        36        36i-Butane        11        11        11n-Butane        15        15        15Hexanes         17        17        17TOTAL           1,647     1,647     1,647Bottom ProductComposition, Mol/HrCarbon Dioxide  2.0       2.0       2.0Methane         2.6       2.7       3.0Ethane          76.5      77.3      81.5Propane         35.8      35.9+     35.9+i-Butane        11.0      11.0      11.0n-Butane        15.0      15.0      15.0Hexanes Plus    17.0      17.0      17.0TOTAL           159.9     160.9     165.4Horsepower Required           1,165     1,181     1,178Product Recovery, %Ethane          85.0      85.9      90.6Propane         99.4      99.9+     99.9+Pressures, psiaStream 10       910       910       910Stream 58       --        565       565Stream 22       250       250       250Temperatures, °F.Stream 10       120       120       120Stream 34, 36   -30       -42       -42Stream 34a, 58a -120      -130      -130Stream 34b,58b  -164      -169      -169Stream 36a      -124      -134      -134Stream 22       -156      -159      -161Stream 22a      -85       -97       -101Heat Balance, MMBTU/HR           +0.09     +0.04     +0.04______________________________________ 
    
     It can be seen from Table II that the same inlet gas composition was used for each case, i.e., the prior art system depicted in FIG. 1 and the present invention depicted in FIG. 2 including both the 3 feed and the 4 feed variations. From the bottom product composition it can be seen that the total amount of bottom product is increased slightly in the case of 3 feeds and significantly increased in the case of 4 feeds over the prior art. Also, the total quantity of propane in the bottom product composition is increased slightly while the total quantity of ethane is increased significantly. The data for the percentage of product recovery shows that the amount of the total propane that is recovered in the bottom product is increased slightly while the amount of total ethane that is recovered in the bottom product is increased significantly, from 85.0% in the case of the prior art up to 85.9% in the case of 3 feeds and 90.6% in the case of 4 feeds in the present invention. Also, it can be seen that this increased product recovery is accomplished with very little change in the horsepower requirements. 
     FIG. 3 illustrates the present invention as applied to a system in which the feed gas is not partially condensed and which does not utilize the high pressure cold separator 26. In this embodiment, the inlet gas 10 is again divided into streams 12 and 14 with the stream 12 being cooled in heat exchanger 16 and the stream 14 being cooled in the heat exchangers 18 and 20. As in the FIG. 2 embodiment, the streams 12 and 14 after cooling are then recombined to form stream 24. This stream 24, which in this case is still all in the vapor phase, is then split into stream 34 and 36 just as was done with the vapor from the high pressure cold separator in the FIG. 2 embodiment. The stream 36 is supplied to the expansion motor 42 and then supplied as stream 36a to the demethanizer 32 as a lower column feed. 
     The stream 34 is handled just as in FIG. 2 by passing it through the heat exchanger 48 to form the stream 35 which is then passed through the expansion valve 50 into the low pressure separator 52. The liquid from the low pressure separator is passed through the expansion valve 54 to form the steam 56 which is fed to the demethanizer 32 as a mid-column feed. The vapor from the low pressure cold separator 52 is cooled in the heat exchanger 60 and then expanded through the valve 62 to form the stream 58b which is the upper column feed. Typical compositions for the feeds to the column for the FIG. 3 embodiment in mole % would be as follows: 
     
                       TABLE III______________________________________        Stream      Stream  StreamComponent    36          58      56______________________________________Nitrogen     0.59        0.85    0.34Carbon Dioxide        0.59        0.40    0.76Methane      93.82       97.44   90.66Ethane       3.16        1.14    4.92Propane      1.06        0.13    1.87i-Butane     0.39        0.02    0.72n-Butane     0.39        0.02    0.73Pentane Plus --          --      --TOTAL        100.00      100.00  100.00______________________________________ 
    
     A stream flow summary comparing the process of FIG. 3 with similar prior art processes which do not incorporate the present invention such as, for example, the prior art processes depicted in FIGS. 3, 5 and 6 in the previously mentioned U.S. Pat. No. 4,278,457 is set forth in Table IV which follows. The same assumptions and criteria were used for this comparision except that the inlet gas was assumed to contain 38 pound moles per hour of nitrogen. The ranges given in Table IV for the prior art represent the range of values obtained in the computer simulation of the 3 processes depicted in the previously mentioned figures of U.S. Pat. No. 4,278,457. 
     
                       TABLE IV______________________________________            Prior Art FIG. 3______________________________________Inlet GasComposition, Mol/HrNitrogen            38         38Carbon Dioxide      39         39Methane             6,181      6,181Ethane              208        208Propane             70         70i-Butane            26         26n-Butane            26         26TOTAL               6,588      6,588Bottom ProductComposition, Mol/HrCarbon Dioxide      3.6-6.7    13.4Methane             5.3-5.4    9.8Ethane              178.3-181.9                          196.8Propane             68.8-68.9  69.8i-Butane            25.9       26.0n-Butane            25.9       26.0TOTAL               311.9-332.0                          341.9Horsepower Required 3,090-3,155                          3,224Product Recovery, %Ethane              85.7-87.5  94.6Propane             98.3-98.4  99.8Pressures, psiaStream 10           910        910Stream 58           --         600Stream 22           360        360Heat Balance, MMBTU/HR               +0.09      +0.49______________________________________ 
    
     It can be seen from Table IV that the percentage of total propane removed in the bottom product is increased from about 98.4 up to 99.8% while the percentage of total ethane recovered is increased from about 87.5 up to 94.6%. From the bottom product composition it can be seen that the total amount of bottom product is increased slightly and that the total quantity of ethane in the bottom product is increased significantly. Although it is not shown in the Table, the temperature of the residue gas leaving the top of the demethanizer in the prior art is at -145° F. whereas in the present invention shown in FIG. 3 it is at -149° F.