Abstract:
A system for cleaning a carbon dioxide gas stream of water soluble contaminants with improved recovery of carbon dioxide wherein the water soluble contaminants are removed into countercurrently flowing water which undergoes at least one pressure reduction step releasing absorbed carbon dioxide for recapture and recycle into the carbon dioxide gas stream.

Description:
TECHNICAL FIELD 
     This invention relates generally to the production of carbon dioxide. 
     BACKGROUND ART 
     Carbon dioxide has a large number of uses. For example, carbon dioxide is used to carbonate beverages, to chill, freeze and package seafood, meat, poultry, baked goods, fruits and vegetables, and to extend the shelf-life of dairy products. It is an important environmental component in industrial waste and process water treatment as a replacement for sulfuric acid to control pH levels. Other uses included drinking water treatment, an environmentally-friendly pesticide and an atmosphere additive in greenhouses to improve the growth of vegetables. 
     Generally carbon dioxide is produced by purifying a waste stream which is a by-product of an organic or inorganic chemical process. The waste stream comprises carbon dioxide and water soluble contaminants from the chemical process, and the carbon dioxide must be cleaned of these contaminants prior to recovery. The cleaning results in the loss of some of the carbon dioxide. 
     As the demand for carbon dioxide continues to increase, more effective and efficient carbon dioxide cleaning systems are desirable in order to cost effectively improve the recovery of the carbon dioxide. 
     Accordingly it is an object of this invention to provide a system which can effectively process a crude carbon dioxide feed stream which contains water soluble contaminants in a more efficient manner than that possible with conventional carbon dioxide processing systems thereby improving the recovery of the carbon dioxide. 
     SUMMARY OF THE INVENTION 
     The above and other objects, which will become apparent to one skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is: 
     A method for recovering carbon dioxide from a gas stream comprising: 
     (A) compressing a gas stream comprising carbon dioxide, water vapor and water soluble contaminants to produce a compressed gas stream; 
     (B) cooling the compressed gas stream to condense at least some of the water vapor, and separating the resulting two-phase fluid into a carbon dioxide-richer gas stream containing water soluble contaminants, and into a remaining liquid; 
     (C) passing the carbon dioxide-richer gas stream into and up an absorption column, passing water into and down the absorption column, and passing water soluble contaminants and some carbon dioxide from the upflowing carbon dioxide-richer gas stream into the downflowing water within the absorption column to produce cleaned carbon dioxide and contaminant-bearing water containing carbon dioxide gas absorbed therein; 
     (D) withdrawing and recovering cleaned carbon dioxide from the absorption column; and 
     (E) withdrawing contaminant-bearing water from the absorption column, reducing the pressure of the contaminant-bearing water, releasing absorbed carbon dioxide gas from the contaminant-bearing water, and passing the released carbon dioxide gas into the carbon dioxide-richer gas stream for passage into the absorption column. 
     Another aspect of this invention is: 
     Apparatus for cleaning carbon dioxide comprising: 
     (A) a compressor and means passing a feed stream comprising carbon dioxide, water vapor and water soluble contaminants to the compressor; 
     (B) an aftercooler, a phase separator, means for passing fluid from the compressor to the aftercooler, and means for passing fluid from the aftercooler to the phase separator; 
     (C) an absorption column, means for passing fluid from the phase separator into the lower portion of the absorption column, and means for passing water into the upper portion of the absorption column; 
     (D) means for recovering cleaned carbon dioxide from the upper portion of the absorption column; and 
     (E) a pressure reducing device, means for passing fluid from the lower portion of the absorption column to the pressure reducing device, and means for passing fluid from the pressure reducing device to the phase separator. 
     As used herein, the term “absorption column” means a vessel wherein a gas and liquid are contacted to transfer one or more components from the gas to the liquid. Typically, the contact will be with upward flow of gas and downward flow of liquid on mass transfer elements such as random or structured packing or trays. 
     As used herein, the term “phase separator” means a vessel wherein a two phase feed can be separated into its separate gas and liquid fractions. Typically, the phase separator will be a vessel with sufficient cross-sectional area so that the gas and liquid will be disengaged by gravity, with liquid removal at the bottom and vapor removal at the top of the vessel. 
     As used herein, the term “water soluble contaminants” means any gaseous contaminant that is appreciably soluable in water such as methanol or ethanol. 
     As used herein, the terms “upper portion” and “lower portion” mean those sections of a column respectively above and below the mid point of the column. 
     As used herein, the term “indirect heat exchange” means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other. 
     As used herein, the term “aftercooler” means an indirect heat exchanger wherein a compressed gas stream comprising water vapor is cooled and at least some of the water vapor is condensed. 
     As used herein, the term “light contaminants” means one or more species having a vapor pressure greater than that of carbon dioxide. Examples of light contaminants include nitrogen, oxygen, argon, hydrogen, carbon monoxide and methane. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole FIGURE is schematic representation of one particularly preferred embodiment of the invention wherein the feed stream additionally comprises light contaminants and multiple compression and pressure reduction stages are employed. 
    
    
     DETAILED DESCRIPTION 
     The invention will be described in detail with reference to the Drawing. Referring now to the FIGURE, gas stream  20  comprises generally from about 90 to 99 mole percent carbon dioxide and up to about 1.0 mole percent water soluble contaminants along with water vapor. In the embodiment of the invention illustrated in the FIGURE, gas stream  20  also comprises light contaminants, typically in a concentration up to about 5 mole percent. 
     Crude carbon dioxide gas stream  20  is cooled in heat exchanger  1  by indirect heat exchange with a suitable fluid such as forced air, cooling water or a liquid refrigerant, and then passed in stream  21  into phase separator  2  wherein it is separated into a gas portion and a liquid water portion. The gas portion is passed as feed or gas stream  22  to compressor  5  wherein it is compressed to a pressure generally within the range of from about 4 to 6 atmospheres to produce compressed gas stream  23 . 
     Compressed gas stream  23  comprising carbon dioxide, water vapor and water soluble contaminants as well as light contaminants is cooled in aftercooler  6  by indirect heat exchange with a suitable fluid such as forced air, cooling water or a liquid refrigerant. The resulting two-phase fluid is passed in stream  24  into phase separator  7  wherein it is separated into a carbon dioxide-richer gas stream  25 , which also contains water soluble contaminants and light contaminants, and also into remaining liquid which is primarily water. 
     Carbon dioxide-richer gas stream  25  is passed to compressor  9  wherein it is further compressed to a pressure generally within the range of from about 15 to 25 atmospheres to produce further compressed gas stream  26 . Further compressed gas stream  26  comprising carbon dioxide and water soluble contaminants as well as light contaminants is cooled in aftercooler  10  by indirect heat exchange with a suitable fluid such as forced air, cooling water or a liquid refrigerant. The resulting two-phase fluid is passed in stream  27  into phase separator  11  wherein it is separated into further carbon dioxide-richer gas stream  28 , which also contains water soluble contaminants and light contaminants, and also into further remaining liquid which is primarily water. 
     Carbon dioxide-richer gas stream  28  is passed into the lower portion of absorption column  13  and flows up the absorption column. Water  29  is pressurized by passage through mechanical pump  15  to a pressure sufficient for introduction into the top of absorption column  13 , generally within the range of from 250 to 370 pounds per square inch absolute (psia). Pressurized water  30  is directed through flow/pressure control valve  14  and then as stream  31  is passed into the upper portion of absorption column  13  and down the absorption column. 
     Absorption column  13  is a vertically oriented vessel generally having random or structured packing as the column internals, i.e. vapor/liquid contact elements, although the column internals may also comprise trays. The column internals serve to facilitate contact between the descending water and the rising gas. As the water descends within absorption column  13  in countercurrent flow to the ascending gas, water soluble contaminants are passed from the rising gas into solution within the descending water so that the descending water is progressively enriched in the water soluble contaminants and the rising gas is progressively depleted of the water soluble contaminants. In addition, some carbon dioxide from the rising gas is absorbed into the downflowing water. This results in the production of cleaned carbon dioxide at the top of column  13  and contaminant-bearing water which also contains some carbon dioxide gas absorbed therein at the bottom of column  13 . 
     The cleaned carbon dioxide is withdrawn from the upper portion of column  13  in stream  32  and, if desired, may be recovered directly. The embodiment of the invention illustrated in the FIGURE is a particularly preferred embodiment wherein the cleaned carbon dioxide undergoes additional processing steps prior to recovery. In the embodiment illustrated in the Figure, gas stream  32  is further cleaned of any remaining water soluble contaminants as well as of water vapor picked up in the countercurrent upflow within column  13  by passage through adsorber  16  which may comprise one or more beds of activated carbon and/or silica. If desired, as illustrated in the Figure, the resulting further cleaned and dried gas  33  may be passed from adsorber  16  to refrigerative means  18  wherein it is cooled and any remaining water is condensed and removed. 
     As mentioned earlier, in the embodiment of the invention illustrated in the Figure the feed gas also contains light contaminants. In this case the cleaned and dried carbon dioxide gas stream may be passed as stream  34  into a distillation column, illustrated in representational fashion in the Figure as element  19 , wherein it is separated by rectification into purified carbon dioxide  35  and light contaminant top vapor  36 . The purified carbon dioxide  35  is recovered as a fluid comprising from about 99.9 to 99.999 mole percent carbon dioxide. 
     The contaminant-bearing water is withdrawn from the lower portion of column  13  in stream  37 . Further remaining liquid is withdrawn from phase separator  11  in stream  38 , passed through valve  12  and as stream  39  combined with stream  37  to form combined stream  40 . Stream  40  is passed through pressure reducing device  17  which in the embodiment illustrated in the Figure is an expansion valve. Pressure reducing device  17  could be other pressure reducing means such as a turboexpander. As contaminant-bearing water stream  40  passes through pressure reducing device  17 , its pressure is reduced, generally to be within the range of from 4 to 6 atmospheres, and in the process carbon dioxide gas, which had been absorbed into the contaminant-bearing water, is released resulting in the generation of two phase stream  41  comprising contaminant-bearing water and gaseous carbon dioxide. Two-phase stream  41  is passed into phase separator  7  wherein the released gaseous carbon dioxide in stream  41  is passed into the carbon dioxide-richer stream for eventual passage into absorption column  13  and ultimately for recovery. In this way, some carbon dioxide in the original gas stream  20 , which would otherwise have been lost as a consequence of the cleaning process, is recaptured and recovered thus improving the recovery of carbon dioxide in the cleaning system. 
     The contaminant-bearing water combines with remaining liquid in phase separator  7  and is passed out from phase separator  7  in stream  42  and through pressure reducing device  8 , such as an expansion valve or turboexpander, wherein its pressure is reduced generally to be within the range of from 1 to 2 atmospheres. In the process remaining carbon dioxide gas still absorbed within the contaminant-bearing water is released resulting in the generation of two phase stream  43  comprising contaminant-bearing water and gaseous carbon dioxide. Two phase stream  43  is passed into phase separator  2  wherein the released gaseous carbon dioxide in stream  43  is passed into the gas forming gas stream  22  for eventual passage into absorption column  13  and ultimate recovery, thus further improving the carbon dioxide recovery. The residual contaminant-bearing water is combined with the liquid portion of stream  21  in phase separator  2 , withdrawn from phase separator  2  in stream  44 , passed through valve  3  and as stream  45  passed to capture means, e.g. chemical sewer,  4  for ultimate disposal. 
     With the practice of this invention recovery of carbon dioxide from any given crude carbon dioxide feed stream can be increased by from 0.5 to 1.0 percent over conventional systems which clean a crude carbon dioxide gas stream of water soluble contaminants. 
     Although the invention has been described in detail with reference to a certain particularly preferred embodiment, those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims.