Abstract:
A method for recovering C 2  and higher weight hydrocarbons, or alternatively C 3  and higher weight hydrocarbons, from off gas, such as refinery off gas, wherein the method avoids the need to significantly compress contaminated off gas in most cases, and is robust in response to pressure and temperature variations in the off gas feed.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention concerns the efficient processing of off gas to recover ethane, ethylene, and higher hydrocarbons. 
       BACKGROUND OF THE INVENTION 
       [0002]    Off gas, for example that produced by a refinery (refinery off gas) or an olefins plant, is generally composed of methane, hydrogen, ethane, ethylene, propane, propene, and heavier hydrocarbons. If recovered, the hydrocarbons are valuable product which otherwise would be lost with the off gas in the plant&#39;s fuel gas system. 
         [0003]    Refinery off-gas usually contains H 2 , CO, CO 2 , O 2 , CH 4 , C 2 H 4 , C 2 H 6 , C 3 H 8 , C 3 H 6  together with some trace impurities such as such as oxygen, ammonia, nitriles, acetylenes, sulfur compounds, butadiene, chlorides, arsenic, mercury, and water in addition to acid gases H 2 S, CO 2 , and COS. These off-gases are produced from refinery units that manufacture conversion products such as hydrotreaters, alkylation units, fluid catalytic cracking units, platformers, etc. Valuable products including hydrogen, olefins, natural gas liquids (NGL) and higher Btu fuel gas can be recovered from the off-gas if an off gas processing unit is installed. 
         [0004]    Similar to refinery off gas, the off gas from olefins plants can also be processed to recover valuable products. The off gas from olefins plants typically is richer in ethylene or propylene and the off gas has different species of trace impurities from those in the refinery off gas. 
         [0005]    Other plants, as well, may produce off gas with C 2  and higher hydrocarbons, for which the method of the present invention may be useful in providing cost effective recovery of valuable C 2  and heavier hydrocarbons. 
         [0006]    Currently, these valuable hydrocarbons may be recovered from the off gas by at least two different methods. A circulating lean oil process may be used to absorb propylene and heavier components from refinery off gases. Although the absorption process provides a reasonable recovery of propylene and heavier components, it is energy intensive and requires several pieces of operating equipment. The amount of equipment needed generally leads to an increased quantity of control loops and the need for expensive plot space. 
         [0007]    Cryogenic expander based technologies are increasingly used in preference to the lean oil absorption methods, because these technologies provide higher ethylene and ethane recoveries. A typical cryogenic expander based process involves a series of progressive cool-down steps in plate fin heat exchangers and vapor-liquid separation steps, followed by demethanization. 
         [0008]    Currently, turbo expanders are used in combination with external refrigeration to increase the thermodynamic efficiency of the process, thus achieving higher percentages of natural gas liquids (“NGL”) recovery. The requirement of external direct refrigeration requires more equipment, controls, and instrumentation, as well as storage and handling of the refrigerant that is used. The storage of refrigerant also raises additional safety considerations due to these extra hydrocarbons being stored at the plant site. 
         [0009]    Off gas is usually available at a relatively low pressure of about eighty psia. To achieve higher NGL recoveries, the cryogenic expander based units require feed gas compression. The compression of dirty off gas is troublesome during operations. The off gas composition is a mix of waste gas coming out of various units. These units may operate at different capacities, and any one or more of them may not be operating at any particular time. Thus, an off gas stream will vary appreciably in composition and flow rate depending on the source and the types of units operating at a particular time. 
         [0010]    Generally, the compressors can be designed for a range of composition for the feed gas. However, it is difficult to predict the range of composition and flow fluctuation for the off gas. Any change in composition outside the design range will result in reduced capacity or loss of recovery of NGL. Similar problems are faced in turbo expander operations. Moreover, if the content of heavier hydrocarbons increases in the off gases then condensation of these hydrocarbons takes place at higher pressure in the upstream section, resulting in loss of valuable NGL. 
         [0011]    Various contaminants that appear in off gas also cause mechanical problems for rotating machinery, resulting in sometimes frequent maintenance downtime and a resulting significant loss of revenue. The variations in off gas feed stream mol weights and flow characteristics also cause problems for turbo expanders used in off gas processing, again often resulting in significant maintenance downtime. Similarly, unsteady operating conditions can result in leakages in heat exchanger cores. The fluctuations in composition of the off gas also affects refrigeration requirements, thereby affecting the external direct refrigeration system. 
         [0012]    In an attempt to circumvent at least some of these problems, less efficient reciprocating compressors are often used to compress the off gas feed stream. However, it would be more desirable to process the feed gas without compression. 
         [0013]    Thus, it is desirable to provide an efficient process for off gas processing that has good adaptability to the feed composition variation. 
         [0014]    Another challenge for this recovery process is to keep the operating temperatures above certain levels to reduce the risk of blue oil formation. 
         [0015]    It is also an object of the invention to recover the valuable hydrocarbons (C 2 +) from off gas without, or with minimal, compression of the feed gas. 
         [0016]    It is a further object of this invention to extract the valuable hydrocarbons from off gas by using as part of the apparatus a turbo expander for which the refrigerant is product, feed gas, reflux formed during an intermediate part of the process, or a mixture of two or more of these. Using these sources for the refrigerant eliminates the need for storage of a specific refrigerant type. Further, use of a turbo expander in the refrigerant loop also helps to startup the plant at reduced capacity, allowing the plant to generate the required refrigerant needed to attain the full capacity of the plant. 
         [0017]    It is yet another object of the invention to efficiently recover ethane and ethylene from the off gas in a cost effective process. 
       SUMMARY OF THE INVENTION 
       [0018]    The method of the present invention alleviates many of the concerns discussed above. Utilizing this method, no feed gas compression is required at the inlet for most cases. In some cases, feed gas may be available at a lower pressure than usual, for example, approximately fifty psia. In such cases, it may be desirable to compress the feed gas to about eighty psia, but even in such circumstances, the amount of compression needed is minimal compared to the prior art, the expected ranges more predictable and easier to design for, and the expected stress on (and resultant maintenance needs of) compression equipment will be significantly lowered. 
         [0019]    Feed gas is chilled and the heavier hydrocarbons are separated in the low pressure separator. This low pressure operating point maintains the feed gas far away from the phase envelope, almost eliminating the possibility of hydrocarbon condensation upstream of the dehydrators. 
         [0020]    Additionally, the method of this invention minimizes the effects of changes in operating temperature due to process operating conditions changes upstream, or due to ambient heat loss. No temperature controls are required for feed gas, eliminating expensive control systems. 
         [0021]    Further, there is no turbo expander in the feed gas stream, eliminating the exposure of turbo expanders to dirty feed gas. A turbo expander is utilized on the refrigerant side, and is thus exposed only to clean refrigerant, thus reducing expected turbo expander maintenance downtime. Because the refrigerant used is a mixture of partially processed feed gas and product streams, no storage of refrigerant is required. In fact, the plant may be started up on feed gas without the refrigerant present, allowing production of refrigerant “on the fly.” This method also allows higher recovery of ethane and ethylene without lowering the operating temperature below certain cryogenic temperatures, thereby avoiding formation of blue oil. 
         [0022]    In one embodiment of the invention, dehydrated off gas arrives as feed gas at a temperature of approximately 100° F. and a pressure of approximately 80 psia. This feed gas is cooled to approximately −80° F. in a first heat exchanger (preferably a brazed aluminum plate fin exchanger), yielding condensed hydrocarbon as part of the feed. The condensed hydrocarbon is separated in a low pressure separator and pumped back through the heat exchanger, where it aids in cooling the feed gas, then to a distillation column. The condensed hydrocarbon is warmed to approximately 90° F., and preferably arrives at the distillation column at a pressure of approximately 355 psia. 
         [0023]    The vapor separated from the low pressure separator is routed through the first heat exchanger and also aids in cooling the feed gas, and is then compressed to approximately 580 psia in a two stage centrifugal compressor. The inlet feed to the compressor is preferably at approximately 68° F. This compressed gas is then cooled in steps, first in a second heat exchanger (preferably an air cooler or cooling water heat exchanger), and next in the first heat exchanger to about −108° F. The hydrocarbon liquid formed as a result of cooling to a very low temperature is separated in a high pressure separator and is fed to the distillation column, preferably on the top most tray section. The separated vapor from the high pressure separator is heated in the first heat exchanger and is sent out as lean gas at about 85° F. 
         [0024]    The distillation column preferably operates at approximately 350 psia at the bottom and approximately 340 psia at the top. The distillation column overhead is cooled in the first heat exchanger to create reflux. Condensed liquid from the reflux is separated out in a reflux drum and is then is fed to the column top tray section. The vapor from the reflux drum is combined with the vapor from the high pressure separator and leaves as lean gas after being heated in the first heat exchanger. 
         [0025]    C 2 + product is recovered from the distillation column bottom. The column bottom temperature is preferably maintained at about 88° F. This temperature makes it possible to utilize a reboiler at the distillation column bottom that exchanges heat with, and cools, the refrigerant after the final stage of refrigerant compression. 
         [0026]    Refrigeration is provided by means of a closed loop turbo expander cycle. The refrigerant is made by mixing part of the vapor exiting the reflux drum with part of the bottom product from the distillation column. The refrigerant is compressed in a refrigerant compressor to a pressure of about 700-750 psia and cooled in steps, first in a third heat exchanger (preferably an aircooler or a cooling water heat exchanger), then in the distillation column reboiler, and finally in the first heat exchanger. After passing through the first heat exchanger, the refrigerant is at approximately −52° F. 
         [0027]    The refrigerant is flashed in a first refrigerant separator at about 500 psia. The flashed gas is further expanded in a turbo expander to a pressure of approximately 170 psia. The pressure of the separated liquid from the refrigerant separator is let down by a control valve to the same pressure (approximately 170 psia). This liquid is then mixed together with the output gas from the turbo expander, and then enters the first heat exchanger to provide additional refrigeration. The refrigerant exits the first heat exchanger at about 70° F. and passes to a second refrigerant separator. 
         [0028]    Gas output from the second refrigerant separator is fed to a turbo compressor associated with the turbo expander. The partially compressed gas from the turbo compressor is cooled in a fourth heat exchanger, then is fed to a third refrigerant separator. Gas output from the third refrigerant separator returns to the refrigerant compressor to complete the closed loop refrigerant system. Liquid remaining in the second and third refrigerant separators may be removed as needed via first and second control valves, respectively. If continuous condensation is observed, pumps may be added to the system to relieve this condition. 
         [0029]    In an alternative embodiment of the invention, dehydrated off gas arrives as feed gas at a temperature of approximately 100° F. and a pressure of approximately 85 psia. This feed gas is cooled to approximately −82° F. in a first heat exchanger (preferably a brazed aluminum plate fin exchanger), yielding condensed hydrocarbon as part of the feed. The condensed hydrocarbon is separated in a low pressure separator and pumped back through the heat exchanger, where it aids in cooling the feed gas, then to a distillation column. The condensed hydrocarbon is warmed to approximately 90° F., and preferably arrives at the distillation column at a pressure of approximately 355 psia. 
         [0030]    The vapor separated from the low pressure separator is routed through the first heat exchanger and also aids in cooling the feed gas, and is then compressed to approximately 475 psia in a two stage centrifugal compressor. The inlet feed to the compressor is preferably at approximately 68° F. This compressed gas is then cooled in steps, first in a second heat exchanger (preferably an air cooler or cooling water heat exchanger), and next in the first heat exchanger to about −118° F. The hydrocarbon liquid formed as a result of cooling to a very low temperature is separated in a high pressure separator and is fed to the distillation column on the top most tray section. The separated vapor from the high pressure separator is heated in the first heat exchanger and is sent out as lean gas at about 95° F. 
         [0031]    The distillation column preferably operates at approximately 330 psia at the bottom and approximately 320 psia at the top. The distillation column overhead is cooled in the first heat exchanger to create reflux. Condensed liquid from the reflux is separated out in a reflux drum and is then is fed preferably to the column top tray section. The vapor from the reflux drum is combined with the vapor from the high pressure separator and leaves as lean gas after being heated in the first heat exchanger. 
         [0032]    C 2 + product is recovered from the distillation column bottom. The column bottom temperature is preferably maintained at about 82° F. This temperature makes it possible to utilize a reboiler at the distillation column bottom that exchanges heat with, and cools, the refrigerant after the final stage of refrigerant compression. Refrigeration is provided by means of a closed loop turbo expander cycle. The refrigerant is made by mixing part of the vapor exiting the reflux drum with part of the bottom product from the distillation column. The refrigerant is compressed in a refrigerant compressor to a pressure of about 310-330 psia and cooled in steps, first in a third heat exchanger (preferably an aircooler or a cooling water heat exchanger) and then in the distillation column reboiler. 
         [0033]    At this stage, the refrigerant is partially condensed. The partially condensed refrigerant is separated in a first separator (preferably an expander suction drum separator). The vapor exiting the first separator is fed to an expander, reducing the pressure to about 135 psia. This expanded refrigerant is then further cooled in the first heat exchanger to about −110° F., then is flashed in a second separator. The vapor feed and the liquid feed from the second separator are further flashed, respectively, by first and second control valves to about 50 psia, then the vapor and liquid feeds are remixed to form a mixed stream. 
         [0034]    The liquid separated from the first separator is also further cooled in the first heat exchanger to about −110° F., and is then flashed by a third control valve to about 50 psia. The flashed liquid stream is mixed with the mixed stream to provide refrigerant to the first heat exchanger. The refrigerant exits the first heat exchanger at about 45° F. and passes to a second refrigerant separator. 
         [0035]    Gas output from the second refrigerant separator is fed to a turbo compressor associated with the turbo expander. The partially compressed gas from the turbo compressor is cooled in a fourth heat exchanger, then is fed to a third refrigerant separator. Gas output from the third refrigerant separator returns to the refrigerant compressor to complete the closed loop refrigerant system. Liquid remaining in the second and third refrigerant separators may be removed as needed via first and second control valves, respectively. If continuous condensation is observed, pumps may be added to the system to relieve this condition. 
         [0036]    In another alternative embodiment of the invention, dehydrated off gas arrives as feed gas at a temperature of approximately 100° F. and a pressure of approximately 85 psia. This feed gas is cooled to approximately −65° F. in a first heat exchanger (preferably a brazed aluminum plate fin exchanger), yielding condensed hydrocarbon as part of the feed. The condensed hydrocarbon is separated in a low pressure separator and pumped back through the heat exchanger, where it aids in cooling the feed gas, then to a distillation column. The condensed hydrocarbon is warmed to approximately 42° F., and preferably arrives at the distillation column at a pressure of approximately 110 psia. 
         [0037]    The vapor separated from the low pressure separator is routed through the first heat exchanger and also aids in cooling the feed gas, and is then compressed to approximately 110 psia in a centrifugal compressor. The inlet feed to the compressor is preferably at approximately −65° F. This compressed gas is fed to a distillation column. 
         [0038]    The distillation column preferably operates at approximately 110 psia at the bottom and approximately 100 psia at the top. The distillation column overhead is cooled in the first heat exchanger to create reflux. Condensed liquid from the reflux is separated out in a reflux drum and is then is fed to the column top tray section. The vapor from the reflux drum is combined with the vapor from the high pressure separator and leaves as lean gas after being heated in the first heat exchanger. C 3 + product is recovered from the distillation column bottom. The column bottom temperature is preferably maintained at about 77° F. This temperature makes it possible to utilize a reboiler at the distillation column bottom that exchanges heat with, and cools, the refrigerant after the final stage of refrigerant compression. 
         [0039]    Refrigeration is provided by means of a closed loop turbo expander cycle. The refrigerant is made by mixing part of the vapor exiting the reflux drum with part of the bottom product from the distillation column. The refrigerant is compressed in a refrigerant compressor to a pressure of about 700-800 psia and cooled in steps, first in a third heat exchanger (preferably an aircooler or a cooling water heat exchanger), then in the distillation column reboiler, and finally in the first heat exchanger. After passing through the first heat exchanger, the refrigerant is at approximately −2° F. 
         [0040]    The refrigerant is flashed in a first refrigerant separator at about 760 psia. The flashed gas is further expanded in a turbo expander to a pressure of approximately 210 psia. The pressure of the separated liquid from the refrigerant separator is let down by a control valve to the same pressure (approximately 210 psia). This liquid is then mixed together with the output gas from the turbo expander, and then enters the first heat exchanger to provide additional refrigeration. The refrigerant exits the first heat exchanger at about 70° F. and passes to a second refrigerant separator. 
         [0041]    Gas output from the second refrigerant separator is fed to a turbo compressor associated with the turbo expander. The partially compressed gas from the turbo compressor is cooled in a fourth heat exchanger, then is fed to a third refrigerant separator. Gas output from the third refrigerant separator returns to the refrigerant compressor to complete the closed loop refrigerant system. Liquid remaining in the second and third refrigerant separators may be removed as needed via first and second control valves, respectively. If continuous condensation is observed, pumps may be added to the system to relieve this condition. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]      FIG. 1  is a schematic representation of one embodiment of the present invention. 
           [0043]      FIG. 2  is a schematic representation of an alternative embodiment of the present invention. 
           [0044]      FIG. 3  is a schematic representation of another alternative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
     Example 1 
       [0045]    In one embodiment of the invention, dehydrated off gas arrives as feed gas  10  at a temperature of approximately 100° F. and a pressure of approximately 85 psia. Feed gas  10  is cooled to approximately −80° F. in a first heat exchanger  12  (preferably a brazed aluminum plate fin exchanger), yielding partially condensed hydrocarbon  11  as part of the feed. The condensed hydrocarbon  13  is separated in a low pressure separator  14  and pumped by first pump  16  back through the first heat exchanger  12 , where it aids in cooling the feed gas  10 , then to a distillation column  18 . The condensed hydrocarbon  13  is warmed in the first heat exchanger  12  to approximately 90° F., and preferably arrives at the distillation column  18  at a pressure of approximately 355 psia. 
         [0046]    The vapor feed  20  separated from the low pressure separator  14  is routed through the first heat exchanger  12  and also aids in cooling the feed gas  10 , and is then compressed to approximately 580 psia in a two stage centrifugal compressor  22 . The inlet feed  23  to the two stage centrifugal compressor  22  is preferably at approximately 68° F. The compressed gas  25  is then cooled in steps, first in a second heat exchanger  24  (preferably an air cooler or cooling water heat exchanger), and next in the first heat exchanger  12  to about −108° F. The hydrocarbon liquid feed  26  formed as a result of cooling to a very low temperature is separated in a high pressure separator  28  and is fed to the distillation column  18 , preferably on the distillation column top tray section  29 . The separated vapor  30  from the high pressure separator  28  is heated in the first heat exchanger  12  and is sent out as lean gas  32  at about 85° F. 
         [0047]    The distillation column  18  preferably operates at approximately 350 psia at the distillation column bottom  34  and approximately 340 psia at the distillation column top  36 . The distillation column overhead  38  is cooled in the first heat exchanger  12  to create reflux  40 . Reflux condensed liquid  42  from the reflux  40  is separated out in a reflux drum  44  and is then pumped by second pump  46  to the distillation column top tray section  29 . The reflux vapor  48  from the reflux drum  44  is combined with the separated vapor  30  from the high pressure separator  28  and leaves as lean gas  32  after being heated in the first heat exchanger  12 . First pressure control valve  50  regulates the pressure of the lean gas  32 . 
         [0048]    C 2 + bottom product  52  is recovered from the distillation column bottom  34 . The distillation column bottom  34  temperature is preferably maintained at about 88° F. This temperature makes it possible to utilize a reboiler  54  at the distillation column bottom  34  that exchanges heat with, and cools, the refrigerant stream  56  after the final stage of refrigerant compression. 
         [0049]    Refrigeration is provided by means of a closed loop turbo expander cycle. The refrigerant stream  56  is made by mixing part of the reflux vapor  48  exiting the reflux drum  44  with part of the bottom product  52  from the distillation column  18 . (Piping omitted for clarity). The refrigerant stream  56  is compressed in a refrigerant compressor  58  to a pressure of about 700-750 psia and cooled in steps, first in a third heat exchanger  60  (preferably an aircooler or a cooling water heat exchanger), then in the distillation column reboiler  54 , and finally in the first heat exchanger  12 . After passing through the first heat exchanger  12 , the refrigerant stream  56  is at approximately −52° F. 
         [0050]    The refrigerant stream  56  is flashed in a first refrigerant separator  62  at about 500 psia. The flashed refrigerant gas  64  is further expanded in a turbo expander  66  to a pressure of approximately 170 psia. The pressure of the separated refrigerant liquid  68  from the first refrigerant separator  62  is let down by second pressure control valve  70  to the same pressure (approximately 170 psia). The separated refrigerant liquid  68  is then mixed together with the first gas output  72  from the turbo expander  66 , and then enters the first heat exchanger  12  to provide additional refrigeration. The warmed refrigerant stream  74  exits the first heat exchanger  12  at about 70° F. and passes to a second refrigerant separator  76 . 
         [0051]    Second gas output  78  from the second refrigerant separator  76  is fed to a turbo compressor  80  associated with the turbo expander  66 . The partially compressed gas  82  from the turbo compressor  80  is cooled in a fourth heat exchanger  84 , then is fed to a third refrigerant separator  86 . Third gas output  88  from the third refrigerant separator  86  returns to the refrigerant compressor  58  to complete the closed loop refrigerant system. Liquid remaining in the second and third refrigerant separators  76 ,  86  may be removed as needed via first and second control valves  90 , 92 , respectively. If continuous condensation is observed, pumps (not shown) may be added to the system to relieve this condition. 
       Example 2 
       [0052]    In an alternative embodiment of the invention, dehydrated off gas arrives as feed gas  210  at a temperature of approximately 100° F. and a pressure of approximately 85 psia. This feed gas  210  is cooled to approximately −82° F. in a first heat exchanger  212  (preferably a brazed aluminum plate fin exchanger), yielding partially condensed hydrocarbon  211  as part of the feed. The condensed hydrocarbon  213  is separated in a low pressure separator  214  and pumped by first pump  216  back through the first heat exchanger  212 , where it aids in cooling the feed gas  210 , then to a distillation column  218 . The condensed hydrocarbon  213  is warmed in the first heat exchanger  212  to approximately 90° F., and preferably arrives at the distillation column  218  at a pressure of approximately 355 psia. 
         [0053]    The vapor feed  220  separated from the low pressure separator  214  is routed through the first heat exchanger  212  and also aids in cooling the feed gas  210 , and is then compressed to approximately 475 psia in a two stage centrifugal compressor  222 . The inlet feed  223  to the compressor is preferably at approximately 68° F. The compressed gas  225  is then cooled in steps, first in a second heat exchanger  224  (preferably an air cooler or cooling water heat exchanger), and next in the first heat exchanger  212  to about −118° F. The hydrocarbon liquid feed  226  formed as a result of cooling to a very low temperature is separated in a high pressure separator  228  and is fed to the distillation column  218 , preferably on the distillation column top tray section  229 . The separated vapor  230  from the high pressure separator  228  is heated in the first heat exchanger  212  and is sent out as lean gas  232  at about 95° F. 
         [0054]    The distillation column  218  preferably operates at approximately 330 psia at the bottom and approximately 320 psia at the top. The distillation column overhead  238  is cooled in the first heat exchanger  212  to create reflux  240 . Reflux condensed liquid  242  from the reflux  240  is separated out in a reflux drum  244  and is then is fed to the distillation column top tray section  229 . The reflux vapor  248  from the reflux drum  244  is combined with the separated vapor  230  from the high pressure separator  228  and leaves as lean gas  232  after being heated in the first heat exchanger  212 . First pressure control valve  250  regulates the pressure of the lean gas  232 . 
         [0055]    C 2 + bottom product  252  is recovered from the distillation column bottom  234 . The distillation column bottom  234  temperature is preferably maintained at about 82° F. This temperature makes it possible to utilize a reboiler  254  at the distillation column bottom  234  that exchanges heat with, and cools, the refrigerant stream  256  after the final stage of refrigerant compression. 
         [0056]    Refrigeration is provided by means of a closed loop turbo expander cycle. The refrigerant stream  256  is made by mixing part of the reflux vapor  248  exiting the reflux drum  244  with part of the bottom product  252  from the distillation column  218 . (Piping omitted for clarity). The refrigerant stream  256  is compressed in a refrigerant compressor  258  to a pressure of about 310-330 psia and cooled in steps, first in a third heat exchanger  260  (preferably an aircooler or a cooling water heat exchanger) and then in the distillation column reboiler  254 . 
         [0057]    At this stage, the refrigerant stream  256  is partially condensed. The partially condensed refrigerant stream  257  is separated in a first refrigerant separator  262  (preferably an expander suction drum separator). The vapor  264  exiting the first refrigerant separator  262  is fed to a turbo expander  266 , reducing the pressure to about 135 psia. The first gas output  272  from the turbo expander  266  is then further cooled in the first heat exchanger  212  to about −110° F., then is flashed in an intermediate refrigerant separator  267 . The intermediate vapor feed  269  from the intermediate refrigerant separator  267  is further flashed by first control valve  273  to about 50 psia. The intermediate liquid feed  271  is regulated by second control valve  275 , and the intermediate vapor feed  269  and the intermediate liquid feed  271  are remixed to form a mixed stream  277 . 
         [0058]    The separated refrigerant liquid  268  from the first refrigerant separator  262  is also further cooled in the first heat exchanger  212  to about −110° F., and is then flashed by a third control valve  279  to about 50 psia. The flashed liquid stream  281  is mixed with the mixed stream  277  to provide refrigerant  283  to the first heat exchanger  212 . The refrigerant  283  exits the first heat exchanger  212  at about 45° F. and passes to a second refrigerant separator  276 . 
         [0059]    Second gas output  278  from the second refrigerant separator  276  is fed to a turbo compressor  280  associated with the turbo expander  266 . The partially compressed gas  282  from the turbo compressor  280  is cooled in a fourth heat exchanger  284 , then is fed to a third refrigerant separator  286 . Gas output from the third refrigerant separator  286  returns to the refrigerant compressor  258  to complete the closed loop refrigerant system. Liquid remaining in the second and third refrigerant separators  276 ,  286  may be removed as needed via first and second control valves  290 ,  292 , respectively. If continuous condensation is observed, pumps (not shown) may be added to the system to relieve this condition. 
       Example 3 
       [0060]    In another alternative embodiment of the invention, dehydrated off gas arrives as feed gas  310  at a temperature of approximately 100° F. and a pressure of approximately 85 psia. Feed gas  310  is cooled to approximately −65° F. in a first heat exchanger  312  (preferably a brazed aluminum plate fin exchanger), yielding partially condensed hydrocarbon  311  as part of the feed. The condensed hydrocarbon  313  is separated in a low pressure separator  314  and pumped by first pump  316  back through the first heat exchanger  312 , where it aids in cooling the feed gas  310 , then to a distillation column  318 . The condensed hydrocarbon  313  is warmed in the first heat exchanger  312  to approximately 42° F., and preferably arrives at the distillation column  318  at a pressure of approximately 110 psia. 
         [0061]    The vapor feed  320  separated from the low pressure separator  314  is compressed to approximately 110 psia in a centrifugal compressor  322 . The inlet feed  323  to the centrifugal compressor  322  is preferably at approximately −65° F. The compressed gas  325  is fed to distillation column  318 . 
         [0062]    The distillation column  318  preferably operates at approximately 110 psia at the distillation column bottom  334  and approximately 100 psia at the distillation column top  336 . The distillation column overhead  338  is cooled in the first heat exchanger  312  to create reflux  340 . Reflux condensed liquid  342  from the reflux  340  is separated out in a reflux drum  344  and is then pumped by second pump  346  to the distillation column top tray section  329 . The reflux vapor  348  from the reflux drum  344  leaves as lean gas  332  after being heated in the first heat exchanger  312 . First pressure control valve  350  regulates the pressure of the lean gas  332 . 
         [0063]    C 3 + bottom product  353  is recovered from the distillation column bottom  334 . The distillation column bottom  334  temperature is preferably maintained at about 77° F. This temperature makes it possible to utilize a reboiler  354  at the distillation column bottom  334  that exchanges heat with, and cools, the refrigerant stream  356  after the final stage of refrigerant compression. 
         [0064]    Refrigeration is provided by means of a closed loop turbo expander cycle. The refrigerant stream  356  is made by mixing part of the reflux vapor  348  exiting the reflux drum  344  with part of the bottom product  353  from the distillation column  318 . (Piping omitted for clarity). The refrigerant stream  356  is compressed in a refrigerant compressor  358  to a pressure of about 700-800 psia and cooled in steps, first in a third heat exchanger  360  (preferably an aircooler or a cooling water heat exchanger), then in the distillation column reboiler  354 , and finally in the first heat exchanger  312 . After passing through the first heat exchanger  312 , the refrigerant stream  356  is at approximately −2° F. 
         [0065]    The refrigerant stream  356  is flashed in a first refrigerant separator  362  at about 760 psia. The flashed refrigerant gas  364  is further expanded in a turbo expander  366  to a pressure of approximately 210 psia. The pressure of the separated refrigerant liquid  368  from the first refrigerant separator  362  is let down by second pressure control valve  370  to the same pressure (approximately 210 psia). The separated refrigerant liquid  368  is then mixed together with the first gas output  372  from the turbo expander  366 , and then enters the first heat exchanger  312  to provide additional refrigeration. The warmed refrigerant stream  374  exits the first heat exchanger  312  at about 70° F. and passes to a second refrigerant separator  376 . 
         [0066]    Second gas output  378  from the second refrigerant separator  376  is fed to a turbo compressor  380  associated with the turbo expander  366 . The partially compressed gas  382  from the turbo compressor  380  is cooled in a fourth heat exchanger  384 , then is fed to a third refrigerant separator  386 . Third gas output  388  from the third refrigerant separator  386  returns to the refrigerant compressor  358  to complete the closed loop refrigerant system. Liquid remaining in the second and third refrigerant separators  376 ,  386  may be removed as needed via first and second control valves  390 ,  392 , respectively. If continuous condensation is observed, pumps (not shown) may be added to the system to relieve this condition. 
         [0067]    Those of skill in the art will understand that the above descriptions and operating parameters are provided by way of example only, and do not limit the scope of the invention as described in the following claims.