Abstract:
A process for producing carbon dioxide from a gas stream containing same pretreats the incoming raw gas to remove contaminants and particularly to protect degradation of the membranes utilized for separation. The temperatures of the various gas streams are carefully controlled to reduce water from the stream. By-product and other gas streams of the process are recycled in order to increase efficiency by utilizing the heating or cooling properties of the streams. In addition, streams containing minor portions of carbon dioxide are returned to the system for recovery.

Description:
TECHNICAL FIELD 
     The present invention relates to the separation of carbon dioxide from a raw gas stream containing same. 
     BACKGROUND 
     High purity carbon dioxide is an important chemical used extensively by the food and beverage industry, and, amongst other things, for PH control in wastewater treatment and as a carbon source in chemical manufacturing. Carbon dioxide is used to carbonate soft drinks and liquid carbon dioxide is expanded to freeze fresh foods. The primary sources of carbon dioxide are by-product streams from chemical processes, natural underground formations and combustion gases. Combustion gases are the oldest and most widely available source. 
     Heretofore, attempts to recover carbon dioxide from exhaust gases were by processes using a chemical solvent to scrub the gas and thereafter distilling to recover the carbon dioxide. Such methods are costly and meet with problems regarding corrosion and solvent degradation. Examples of such processes and methods to alleviate the corrosion problems are found in U.S. Pat. No. 2,065,112 which issued in 1936, U.S. Pat. No. 2,399,142 which issued in 1946, U.S. Pat. No. 2,377,966 which issued in 1945, U.S. Pat. No. 4.477.419 which issued in 1948, U.S. Pat. No. 3,137,654 which issued in 1964. The alkanolamine process described in some of these patents is still widely used today. 
     More recently, commercial membrane technology has been developed for separating acid gases like carbon dioxide and hydrogen sulfide from light hydrocarbon gases. Examples of such art are found in U.S. Pat. No. 4,130,403 which issued Dec. 19, 1978 to T. E. Cooley et.al., U.S. Pat. No, 4,639,257 which issued Jan. 27, 1987 to Melvyn Duckett et.al., and U.S. Pat. No. 5,233,837 which issued Aug. 10, 1993 to Richard Callahan. The features of these patents, particularly the use of membrane separation, was a definite step forward in the carbon dioxide separation art. However, the prior art in this field continues to suffer from overall efficiency of the process and undesirable membrane life. These problems have been significantly mitigated by the apparatus and process of this invention. 
     SUMMARY OF THE INVENTION 
     The invention encompasses a system for recovering or at least increasing the relative concentration of a target gas from or in a mixture of gases in which the target gas is entrained. In one aspect of the invention, a target gas, specified as carbon dioxide, is separated from a raw gas stream which contains carbon dioxide in an amount, on a dry basis, in the range of about 10 percent to about 85 percent. 
     In the process, contaminants are removed from the gas stream by direct contact cooling and passing the gas through heat exchangers and water knockouts. Contaminants are further removed by an absorption filter and a coalescing filter. The gas is thereby conditioned for efficient membrane separation. 
     The resultant treated gas is thereafter progressively passed through first and second membranes with nonpermeate gas recycling from the second membrane to the inlet of the first membrane. The gas is further filtered as it passes between the first and second membranes. Permeate gas passing through the second membrane is pre-conditioned and water is removed prior to passing the gas through a desiccant filter from which desiccated gas is thereafter cryogenically distilled and liquid carbon dioxide is recovered. 
     Effluent gas from the distilling column is recycled to the cooling apparatus immediately upstream of the desiccant filter, for improved efficiency. 
     Further, efficiencies are realized by capturing vapors from carbon dioxide storage and returning them to the system for reprocessing. 
     In another aspect of the invention, the process of the invention remains basically the same, but greater detail is claimed with regard to pretreating the process streams and recycling specific streams for purposes of increased efficiency of the process, improved quality of carbon dioxide from the process, and increased carbon dioxide recovery: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention and its advantages will be apparent from the detailed description taken in conjunction with the accompanying single drawing in which: 
     FIG. 1 is a schematic view of the apparatus of the process of this invention. 
    
    
     DETAILED DESCRIPTION 
     The preferred embodiments of the present invention and its advantages are best understood by referring to FIG. 1 of the drawings. 
     Referring to FIG. 1, the raw feed gas enters the gas processing apparatus 10 of this invention via line 12 and, where particulate matter is heavily laden in the raw feed gas stream, the gas stream preferably enters the system through a venturi scrubber 14. 
     The venturi scrubber 14 is positioned ahead of a direct contact cooler 16 which contains a water scrubbing fluid that is recirculated through the direct contact cooler 16 by a pump 18 and cooled by a recycle heat exchanger 20. Excess water in the direct contact cooler 16, from condensation of water vapor in the raw gas stream, is discharged and constant water level is maintained in the direct contact cooler 16. 
     Where sulfur dioxide is present in the raw gas stream entering the direct contact cooler 16, it is preferred that sodium hydroxide be added to the water scrubbing fluid to remove the sulfur dioxide. The recycle heat exchanger 20 is preferably a gas-gas recycle heat exchanger 20 that is cooled by air, but can be a water cooled heat exchanger without departing from this invention. 
     The gas stream exiting the direct contact cooler 16 passes through a recycle gas-gas heat exchanger 22 which cools the gas stream to about 90 degrees F. by heat exchange with a hereafter more fully described refrigerant heat exchanger 26 to progressively lower the temperature of the gas stream entering a first water knockout 28. 
     In the refrigerant heat exchanger 26, the gas stream has been cooled sufficiently that the dewatered gas stream exiting the top of the first water knockout 28 and entering a compressor 30 is substantially free of water which would condense in the compressor 30 and cause damage. The first water knockout 28 has sufficient static head to dump condensate water to atmosphere even when the gas stream is under slight vacuum. 
     The pressure of the gas stream is increased as it passes through the compressor 30 and is thereafter cooled by heat exchanger 32 and refrigerant heat exchanger 34 and water is removed from the gas stream by another second knockout 36. The pressure of the gas stream exiting the top of the second knockout 36 is sufficient to subsequently pass through the heat exchanger 22 and subsequently through a coalescing filter 37, adsorption filter 38, and a particulate filter 40 and exit the particulate filter 40 at a pressure designated as the optimum operating pressure for the first membrane 42 immediately downstream therefrom. Alternatively, if an oil free compressor is used in place of compressor 30, such as a centrifugal compressor, coalescing filter 37, adsorption filter 38, and particulate 40 should not be necessary and could be omitted. 
     The gas stream exiting the compressor 30 is cooled by the water or air cooled heat exchanger 32 to a temperature of about 100 degrees F. and immediately thereafter is deep cooled by the refrigerant heat exchanger 34 to a temperature sufficient to achieve about 40 per cent relative humidity at the first membrane feed temperature and pressure. It should be noted that in passing through the second water knockout 36, and particularly through the gas-gas heat exchanger 22, the gas stream is heated and subsequently contacts the first membrane at a temperature of about 100 degrees F. 
     As the gas passes through the coalescing filter 37 and the adsorption filter 38, traces of compressor oil, sulfur compounds and other particulates are removed. The gas stream then passes through the particulate filter 40 which functions to remove particulate contaminants passing from the adsorption filter 38. Prior to the first membrane 42, a recycle gas stream 44 is mixed with the gas stream entering at the point in the filters best able to remove the contaminates, optimally after particulate filter 40, if the compressor 58 is oil free. 
     The pressure drop across the first membrane 42, between the feed stream and the non-permeate stream, is maintained in the range of about 5-30 psig., preferably at about 5 psig. operating at about 100 degrees F. and a 90 psig. feed pressure. At the first membrane 42, the gas stream is separated into a nonpermeate stream and a permeate stream. The nonpermeate stream is removed from the system through a line 43. The permeate gaseous stream remaining in the system is discharged from and exits the first membrane 42 for further treatment by a second membrane 48. 
     The permeate stream discharged or exiting from the first membrane 42 is thereafter preferably pressurized by an oil-lubricated screw compressor 50 to about 90 psig, cooled by a water or air heat exchanger 52 to about 100 degrees F., and thereafter filtered through a coalescing filter 53, adsorption filter 54, and particulate filter 56 to remove any trace of compressor oil. Alternatively, if an oil-free compressor is used in place of compressor 50, such as a centrifugal compressor, coalescing filter 53, adsorption filter 54 and particulate filter 56 should not be necessary and could be omitted. The gas stream exiting from the coalescing filter 56 is then passed into contact with the second membrane 48. 
     The second membrane 48 separates the incoming gas stream previously discharged as permeate from the first membrane 42 into a nonpermeate recycle gas stream 44 and a discharged permeate stream having a relatively higher concentration of the carbon dioxide target gas relative to the recycle gas stream. The nonpermeate recycle gas stream 44, as described above, is recycled and enters the inlet of the first membrane 42 via a compressor 58 and a heat exchanger 60. The recycle gas stream 44 is pressurized and cooled to about the pressure and temperature of the gas entering the inlet of the first membrane 42. The reason for recycling the nonpermeate gas stream from the second membrane is that it generally contains a greater volume of carbon dioxide than the feed stream entering the first membrane 42. 
     The first membrane 42 and second membrane 48 of this invention can be any of the well known operable membranes, but it is preferred to use those wherein the carbon dioxide permeability is at least 10 times that of the gas or gases from which it is to be separated under the chosen separation conditions. Examples of suitable membranes are those formed from polysulfone, polyimide, polyamide, glassy polymers or cellulose acetate. 
     It should be understood that it is desired that the permeate stream of the first membrane 42 be of optimum carbon dioxide recovery and that the permeate stream of the hereafter second membrane 48 be of optimum carbon dioxide purity. It should also be understood that it is preferred that the membrane material forming each of the membranes be the same. 
     The pressure differential between the permeate gaseous stream previously discharged from the first membrane 42, measured at the point of entry of the second membrane 48 and the discharged non-permeate gaseous stream 44 exiting the second membrane 48, is preferably maintained at a higher value than the pressure differential between the corresponding incoming stream 43 and exiting non-permeate stream of the first membrane 42. Preferably, the pressure differential between the incoming stream and exiting non-permeate stream associated with the second membrane 48 should be substantially maintained at about 30 psig, while the pressure differential between the incoming stream and non-permeate stream associated with the first membrane 42 should be substantially maintained at about 5 psig. 
     The permeate gas stream discharged from the second membrane 48 is cooled by a heat exchanger 62 to a temperature of about 50 degrees F. and immediately thereafter a recycle stream 64 is added to the cooled stream exiting the heat exchanger 62. The recycle stream 64 is a stream which comprises vapors that are recovered from a carbon dioxide insulated storage tank 66 of the gas processing apparatus 10 of this invention. The gas stream resulting from apparatus 10 has been tested to be in excess of 90 per cent by volume carbon dioxide at about 100 degrees F. and atmospheric pressure. Pre-cooling the gas stream results in lowering the horsepower required to boost the pressure of the stream to the liquefaction pressure and assure no water will condense during compression. 
     The resultant gas stream is thereafter pressurized by compressor 68 to a pressure of about 300 psig, cooled by heat exchangers 70, 72, and passed to a water knockout 74. The first entered heat exchanger 70 is a water or air heat exchanger which functions to cool, to a temperature of about 100 degrees F., the gas stream exiting from the compressor 68. The cooled gas stream is thereafter passed through a refrigerant heat exchanger 72, deep cooled to a temperature of about 50 degrees and delivered to the water knockout 74 where additional water may be removed. 
     It should be understood that the compressors 30, 50, and 68 used in this invention can be of various types and makes such as centrifugal, reciprocating, or screw type, without departing from this invention. However, an oil lubricated screw type compressor is preferred. 
     The gas stream is passed from the water knockout 74, into and through a desiccant filter system 76. Preferably, the desiccate filter system 76 has first desiccant filter bed 78 and second desiccant filter bed 80 connected in parallel and controllable to selectively pass the gas stream through a selected one of the first desiccant filter bed 78 and second desiccant filter bed 80. By this construction, when one of the first desiccant filter bed 78 and second desiccant filter bed 80 is drying the gas received from the water knockout 74, the other desiccant filter bed can be regenerated. Regeneration of one of the first desiccant filter bed 78 and second desiccant filter bed 80 generally requires about eight hours of heating by passing desiccated carbon dioxide or distillation column vent gas through the bed at a temperature of about 450 degrees F., after which the bed must be cooled for about four hours. 
     The preferred desiccant filter media is one of activated alumina or molecular sieves, however it would be understood that other media can be used without departing from this invention. Vent gas passing from the desiccant filter system via line 82 is not generally recirculated into the system since it would undesirably add water to the gas processing system. 
     A carbon dioxide gas stream is discharged from the desiccate filter system 76, cooled by first and second gas-gas heat exchangers 84, 86 and thereafter passed to an upper portion of a distillation column 88 via a primary condenser 90. 
     In the distillation column 88, fluid from the primary condenser 90 passes downwardly in countercurrent flow to stripping vapors generated by reboiling the liquid carbon dioxide at cryogenic temperatures in the range of about -10 degrees F. to about 10 degrees F. 
     Noncondensible gases and some carbon dioxide gas, hereafter referred to as effluent gas, leaves the top of the distillation column 88 and pass through an overhead condenser 92 where a significant portion of the carbon dioxide is recovered from a sump 94 of the overhead condenser 92, or simply falls back into the column where the overhead condenser 92 is situated directly above the distillation column 88, and returned to an upper portion of the distillation column 88 as reflux. Both the primary condenser 90 and the overhead condenser 92 are chilled with refrigerant. The preferred refrigerant is liquid ammonia, although other refrigerants may be used, such as propane, fluorocarbons, or carbon dioxide. 
     An effluent gas stream from an upper portion of the sump 94, or from the overhead condenser 92 itself if mounted atop the distillation column 88, is recycled back into the gas processing system 10. The effluent gas stream 96 is first passed in heat exchange relationship through the second gas-gas heat exchanger 86 and then in heat exchange relationship through the first gas-gas heat exchanger 84. By so prechilling the feed to the distillation column with this effluent gas stream 96, the overall load on the primary condenser is reduced, thereby representing the saving of power. 
     There is a pressure let down valve between gas-gas heat exchangers 84, 86 to reduce the temperature of the gas in effluent gas stream 96 after it is heated in gas-gas heat exchanger 86. The effluent gas stream is thereafter split into two separate streams. One of the streams 98 of the split is heated by passing through an electric heater then passed to the desiccant dryer for operation of regeneration and the other stream 100 of the split is recycled into the inlet of the first membrane 42 (as described in U.S. Pat. No. 4,639,257 &#34;Recovery of Carbon Dioxide From Gas Mixture&#34; which issued to Melvyn Duckett,et.al on Jan. 27, 1987 and which is hereby incorporated by reference), or vented to atmosphere. 
     A solution reboiler 102 receives carbon dioxide discharging from the bottom of the distillation column 88. The solution reboiler 102 generates the stripping vapors which are passed through line 104 into a bottom portion of the distillation column 88. The solution reboiler 102 has liquid carbon dioxide on the shell side of the solution reboiler 102 and distillation column 88 feed, prior to the primary condenser 90, on the tube side of the solution reboiler 102. Any stream warmer than the column bottom fluid can be used in the reboiler. The preferred warm vapor used to reboil the solution is a slip stream of compressed and dehydrated carbon dioxide, which further reduces the refrigeration load on the primary condenser 90 and avoids waste of power. 
     Liquid carbon dioxide passes from the bottom of the distillation column 88 via line 106 at a pressure of about 250 psig and a temperature in the range of about -10 degrees F. to about 10 degrees F. This liquid carbon dioxide stream passes through a refrigerant cooled subcooler 108 where the liquid is cooled below the bubble point by several degrees and then passed to the aforementioned insulated storage tank 66 where overhead vapor is recycled to the intake of compressor 68. By so returning tank vapors, the efficiency of carbon dioxide recovery is improved. The following are example operating conditions and recoveries of the process of this invention: 
     
         ______________________________________EXAMPLE             % Vol.STREAM  DESCRIPTION SCFM    CO2  N2   02   H20______________________________________12      Raw feed gas               7,200   18.0 69.3 2.7  10.0   M-1 NP vent 5,090   2.1  96.5 1.4  044      M-2 recycle 3,055   26.3 66.4 7.3  trace   Column feed 1,392   85.6 5.5  8.9  096      Column effluent                 500   60.0 15.3 24.7 098      Regen gas feed                 49    60.0 15.3 24.7 0100     Column vent   451   60.0 15.3 24.7 0______________________________________EQUIPMENT   DESCRIPTION   TEMP.F.  PSID______________________________________20          Heat exchanger                     106.0    522          Gas-Gas HTX   10.0     2                     (approach)26          Heat exchanger                     45.0     232          Heat exchanger                     100.0    234          Heat exchanger                     45.0     252          Heat exchanger                     101.2    260          Heat exchanger                     100.0    262          Heat exchanger                     45.0     270          Heat exchanger                     100.0    272          Heat exchanger                     45.0     284          gas-gas HTX   10.0     2                     (approach)86          gas-gas HTX   10.0     2                     (approach)90          Primary cond. -30.0    292          Vent cond.    -31.0    2108         Subcooler     -15.0    2       Heater        450.0    2       Refrig. Pkg.  100.0    2       Refrig. Pkg.  bubble   2                     point102         Reboiler      4.8      2       (bottom)______________________________________EQUIPMENT DESCRIPTION  PSIG    STAGES  BHP______________________________________18        Wash water pump                  90.0    1       1330        1st.stage comp.                  111.0   2       1,59550        2nd.stage comp.                  107.0   2       92958        Recyc.Comp.  92.0    1       10268        Prod. Comp.  343.0   2       460     Refrig. Comp.                  200.0   1       218     Refrig. Comp.                  58.5    1       120______________________________________                  NON-PERM.EQUIPMENT  DESCRIPTION PSID       MODULES______________________________________42         1st stage   5          581      membrane48         2nd stage   30         46      membrane______________________________________ 
    
     The overall process efficiency and, specifically, membrane life is markedly improved by the novel techniques employed in the process. Pretreating the raw feed gas removes trace contaminants such as sulfur dioxide, nitrogen oxides, and particulate. Chilling the raw feed gas stream upstream of the first membrane 42 and second membrane 48 improves removal of the bulk of the water and thereby extends membrane life, reduces compressor horsepower, and reduces downstream desiccant refrigeration heat. The first membrane 42 and the second membrane 48 are protected by the coalescing filter 37, adsorption filter 38, particulate filter 40, and heat exchanger 52, coalescing filter 53, adsorption filter 54, and particulate filter 56 to remove all traces of sulfur compounds, residual oxides of nitrogen, and particulate. Recovery of carbon dioxide from the first membrane 42 is maximized and purity of carbon dioxide from the second membrane 48 is maximized by recycling the recycle gas stream 44 of the second membrane 48 back to the first membrane 42. 
     Recycling vent streams from distillation and storage improves efficiency of the system and avoids the waste of power. Further efficiencies are realized by the process of this invention by prechilling distillation column feed with distillation column vent gas using an interstage flash to recover additional chilling after the first of such exchanger, expanding the non-permeate by-product stream from the first membrane 42, and using the cooling affect to partially condense refrigerant used to liquefy the carbon dioxide vapor. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.