Abstract:
A process for recovering carbon dioxide from a gas containing both carbon dioxide and oxygen wherein a carbon dioxide and oxygen containing gas is first contacted with an alkanolamine solution to produce an alkanolamine loaded with carbon dioxide and oxygen, the oxygen is separated from the absorption solution, such as by stripping with a scavenging gas, such as the carbon dioxide containing fluid obtained from the separation of carbon dioxide from the alkanolamine solution or nitrogen, prior to any heating of the absorption solution, and carbon dioxide is steam stripped from the absorption solution and recovered. By stripping the oxygen out of the alkanolamine solution, oxidative degradation of the alkanolamine can be reduced.

Description:
TECHNICAL FIELD 
     This invention relates generally to the recovery of carbon dioxide and, more particularly, to the recovery of carbon dioxide from a feed mixture which also contains oxygen. 
     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 include 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, which comprises a high concentration of carbon dioxide, is condensed and purified in multiple stages and then distilled to produce the product grade carbon dioxide. 
     As the demand for carbon dioxide continues to increase, alternate sources of carbon dioxide are being used to supply the crude carbon dioxide feed to the purification system. Such alternate feeds have a much lower concentration of carbon dioxide and thus need to be upgraded, i.e. the concentration of the carbon dioxide must be increased, before product grade carbon dioxide can be effectively produced. These alternate feeds with much lower carbon dioxide concentrations are referred to as lean feeds. An example of such a lean feed is flue gas from, for example, a combustion source such as a boiler, internal combustion engine, gas turbine or lime kiln. 
     Upgrading of the carbon dioxide concentration in a feed can be carried out in a number of ways. One particularly preferred method is the chemical absorption of carbon dioxide from the crude carbon dioxide feed into an alkanolamine based absorbent. The resulting carbon dioxide loaded absorbent then undergoes separation into carbon dioxide product for recovery and into alkanolamine containing absorbent which is may be recycled for reuse within the recovery system. 
     Often the crude carbon dioxide feed contains significant levels of oxygen which can cause degradation of the alkanolamines reducing their utility in the recovery system and also causing corrosion problems in the system. Those skilled in the art have addressed this problem in one of two ways. In one method, chemical inhibitors are added to the absorber fluid to protect against degradation by inhibiting the oxidation of the alkanolamines. In another method, a combustible fuel is added to the crude carbon dioxide feed for combustion with the oxygen in a catalytic combustion reaction. While both methods are effective they are both characterized by high capital costs and, moreover, are complicated to operate. 
     Accordingly, it is an object of this invention to provide a system which can more effectively recover carbon dioxide or other absorbate from an oxygen containing feed using an alkanolamine based absorbent to upgrade the feed. 
     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 comprising: 
     (A) passing a feed mixture comprising oxygen and carbon dioxide in countercurrent mass transfer contact with absorbent comprising at least one alkanolamine, and passing oxygen and carbon dioxide from the feed mixture into the absorbent to obtain carbon dioxide loaded absorbent containing dissolved oxygen; 
     (B) separating oxygen from the carbon dioxide loaded absorbent to obtain oxygen depleted carbon dioxide loaded absorbent; 
     (C) heating the oxygen depleted carbon dioxide loaded absorbent to obtain heated carbon dioxide loaded absorbent; 
     (D) separating carbon dioxide from the absorbent to obtain a carbon dioxide rich fluid; and 
     (E) recovering carbon dioxide rich fluid. 
     Another aspect of the invention is: 
     Apparatus for recovering absorbate from an oxygen-containing feed mixture comprising: 
     (A) an absorption column, means for passing a feed mixture comprising oxygen and absorbate into the lower portion of the absorption column, and means for passing absorbent comprising at least one alkanolamine into the upper portion of the absorption column; 
     (B) an oxygen separator and means for passing fluid from the lower portion of the absorption column into the oxygen separator; 
     (C) a heat exchanger and means for passing fluid from the oxygen separator to the heat exchanger; 
     (D) a stripping column and means for passing fluid from the heat exchanger to the upper portion of the stripping column; and 
     (E) means for recovering absorbate from the upper portion of the stripping column. 
     As used herein, the term “absorption column” means a mass transfer device that enables a suitable solvent, i.e. absorbent, to selectively absorb the absorbate from a fluid containing one or more other components. 
     As used herein, the term “stripping column” means a mass transfer device wherein a component such as absorbate is separated from absorbent, generally through the application of energy. 
     As used herein, the term “inhibitor” means a chemical or mixture of chemicals that inhibits or reduces the rate of a reaction. For example, copper carbonate in combination with one or more of dihydroxyethylglycine, alkali metal permanganate, alkali metal thiocyanate, nickel or bismuth oxides with or without alkali metal carbonate inhibits oxidative degradation of an alkanolamine. 
     As used herein the term “oxygen scavenging gas” means a gas that has an oxygen concentration less than 2 mole percent, preferably less than 0.5 mole percent, and which can be used to strip dissolved oxygen from a liquid. 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The sole FIGURE is a schematic representation of one particularly preferred embodiment of the invention wherein the oxygen separator comprises an oxygen stripping column. 
    
    
     DETAILED DESCRIPTION 
     The invention will be described in greater detail with reference to the Drawing. Referring now to the FIGURE, feed gas mixture  1 , which typically has been cooled and treated for the reduction of particulates and other impurities such as sulfur oxides (SOx) and nitrogen oxides (NOx), is passed to compressor or blower  2  wherein it is compressed to a pressure generally within the range of from 14.7 to 30 pounds per square inch absolute (psia). Feed gas mixture  1  generally contains from 2 to 50 mole percent carbon dioxide as the absorbate, and typically has a carbon dioxide concentration within the range of from 3 to 25 mole percent. Feed gas mixture  1  also contains oxygen in a concentration generally within the range of from less than 1 to about 18 mole percent. Feed gas mixture  1  may also contain one or more other components such as trace hydrocarbons, nitrogen, carbon monoxide, water vapor, sulfur oxides, nitrogen oxides and particulates. 
     Compressed feed gas mixture  3  is passed from blower  2  into the lower portion of absorption column  4  which is operating at a temperature generally within the range of from 40 to 45° C. at the top of the column and at a temperature generally within the range of from 50 to 60° C. at the bottom of the column. Absorbent  6  is passed into the upper portion of absorption column  4 . Absorbent  6  comprises at least one alkanolamine species. Examples of alkanolamines which may be employed in absorber fluid  6  in the practice of this invention are monoethanolamine, diethanolamine, diisopropanolamine, methyldiethanolamine and triethanolamine. Generally the alkanolamines are employed as an aqueous solution. The concentration of the alkanolamine(s) in absorbent  6  will be within the range of from 5 to 80 weight percent, and preferably from 10 to 50 weight percent. A preferred primary alkanolamine for use in the absorbent fluid in the practice of this invention is monoethanolamine, preferably in a concentration within the range of from 5 to 25 weight percent, more preferably in a concentration within the range of from 10 to 15 weight percent. Preferred secondary alkanolamines for use in the absorbent fluid in the practice of this invention are diethanolamine and diisopropanolamine. 
     Within absorption column  4  the feed gas mixture rises in countercurrent flow against downflowing absorbent. Absorption column  4  contains column internals or mass transfer elements such as trays or random or structured packing. As the feed gas rises, most of the carbon dioxide within the feed gas, oxygen, and small amounts of other species such as nitrogen, are absorbed into the downflowing absorber liquid resulting in carbon dioxide depleted top vapor at the top of column  4 , and into carbon dioxide loaded absorbent containing dissolved oxygen at the bottom of column  4 . The top vapor is withdrawn from the upper portion of column  4  in stream  5  and the carbon dioxide loaded absorbent is withdrawn from the lower portion of column  4  in stream  7 . 
     Dissolved oxygen eventually causes degradation of alkanolamines thereby leading to corrosion and other operating difficulties. The level of the dissolved oxygen in the carbon dioxide loaded absorbent is reduced such as by contacting the absorbent with oxygen scavenging gas in a mass transfer device such as the oxygen stripping column illustrated in the FIGURE. 
     The carbon dioxide loaded absorbent containing dissolved oxygen in stream  7  is passed from the lower portion of absorption column  4  into the upper portion of additional stripping column  151 . It is an important aspect of this invention that the fluid comprising stream  7  does not undergo any heating from its withdrawal from absorption column  4  to its passage into oxygen stripping column  151 . Oxygen scavenging gas is passed into the lower portion of stripping column  151  in stream  152 . One source of oxygen scavenging gas is an oxygen free carbon dioxide stream. Examples of such a stream include carbon dioxide rich vapor stream  16 , shown in the FIGURE as stream  71 , carbon dioxide from a storage tank, or carbon dioxide from a further downstream process. Other oxygen free gases such as nitrogen can also be used. 
     Within stripping column  151  the oxygen scavenging gas rises in countercurrent flow against downflowing carbon dioxide loaded absorbent. Stripping column  151  contains column internals or mass transfer elements such as trays or random or structural packing. As the oxygen scavenging gas rises, oxygen within the absorbent is stripped from the downflowing absorbent into the upflowing scavenging gas resulting in oxygen containing scavenging gas at the top of stripping column  151 , and into oxygen depleted carbon dioxide loaded absorbent at the bottom of stripping column  151 . The oxygen containing scavenging gas is withdrawn from the upper portion of column  151  in stream  150 . Stream  150  will typically contain some carbon dioxide in addition to oxygen and other species. This stream can be vented to the atmosphere, used as is, or mixed with the final product carbon dioxide in stream  16 , as shown in the FIGURE as stream  72 . The oxygen depleted carbon dioxide loaded absorbent, typically containing less than 2 ppm oxygen and preferably less than 0.5 ppm is withdrawn from the lower portion of column  151  in stream  153 , passed to liquid pump  8  and from there in stream  9  to and through heat exchanger  10  wherein it is heated by indirect heat exchange to a temperature generally within the range of from 90 to 120° C., preferably from 100 to 110° C. 
     The heated carbon dioxide loaded absorbent is passed from heat exchanger  10  in stream  11  into the upper portion of second or main stripping column  12  which is operating at a temperature typically within the range of from 100 to 110° C. at the top of the column and at a temperature typically within the range of from 119 to 125° C. at the bottom of the column. As the heated carbon dioxide loaded absorbent flows down through stripping column  12  over mass transfer elements which can be trays or random or structured packing, carbon dioxide within the absorbent is stripped from the absorbent into upflowing vapor, which is generally steam, to produce carbon dioxide rich top vapor and remaining absorbent. The carbon dioxide rich fluid is withdrawn from the upper portion of stripping column  12  in top vapor stream  13  and passed through reflux condenser  47  wherein it is partially condensed. Resulting two phase stream  14  is passed to reflux drum or phase separator  15  wherein it is separated into carbon dioxide rich gas and into condensate. The carbon dioxide rich gas is removed from phase separator  15  in stream  16  and recovered as carbon dioxide product fluid having a carbon dioxide concentration generally within the range of from 95 to 99.9 mole percent on a dry basis. By “recovered” as used herein it is meant recovered as ultimate product or separated for any reason such as disposal, further use, further processing or sequestration. The condensate, which comprises primarily water and alkanolamines, is withdrawn from phase separator  15  in stream  17 , passed through liquid pump  18  and as stream  19  into the upper portion of stripping column  12 . 
     Remaining alkanolamine-containing absorbent which also contains water is withdrawn from the lower portion of stripping column  12  in stream  20  and passed to reboiler  21  wherein it is heated by indirect heat exchange to a temperature typically within the range of from 119 to 125° C. In the embodiment of the invention illustrated in the FIGURE, reboiler  21  is driven by saturated steam  48  at a pressure of 28 pounds per square inch gauge (psig) or higher, which is withdrawn from reboiler  21  in stream  49 . The heating of the alkanolamine-containing absorbent in reboiler  21  drives off some water which is passed as steam in stream  22  from reboiler  21  into the lower portion of stripping column  12  wherein it serves as the aforesaid upflowing vapor. The resulting alkanolamine-containing absorbent is withdrawn from reboiler  21  in liquid stream  23 . A portion  24  of stream  23  is fed to reclaimer  25  where this liquid is vaporized. Addition of soda ash or caustic soda to the reclaimer facilitates precipitation of any degradation byproducts and heat stable amine salts. Stream  27  depicts the disposal of any degradation byproducts and heat stable amine salts. The vaporized amine solution  26  can be reintroduced into stripping column  12  as shown in the FIGURE. It can also be cooled and directly mixed with stream  6  entering the top of absorption column  4 . Also, instead of the reclaimer  25  shown in the FIGURE, other purification methods such as ion-exchange or electrodialysis could be employed. 
     The remaining portion  148  of heated alkanolamine-containing absorbent  23  is passed to solvent pump  35  and from there in stream  29  to and through heat exchanger  10  wherein it serves to carry out the aforesaid heating of the carbon dioxide loaded absorbent and from which it emerges as cooled alkanolamine-containing absorbent  34 . 
     Stream  34  is cooled by passage through cooler  37  to a temperature of about 40° C. to form cooled absorbent  38 . A portion  40  of stream  38  is passed through mechanical filter  41 , from there as stream  42  through carbon bed filter  43 , and from there as stream  44  through mechanical filter  45  for the removal of impurities, solids, degradation byproducts and heat stable amine salts. Resulting purified stream  46  is recombined with stream  39  which is the remainder of stream  38  to form stream  55 . Storage tank  30  contains additional alkanolamine for makeup. Alkanolamine absorbent is withdrawn from storage tank  30  in stream  31  and pumped by liquid pump  32  as stream  33  into stream  55 . Storage tank  50  contains makeup water. Water is withdrawn from storage tank  50  in stream  51  and pumped by liquid pump  52  as stream  53  into stream  55 . Streams  33  and  53  together with stream  55  form combined absorbent stream  6  for passage into the upper portion of absorber column  4  as was previously described. 
     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. For example the invention may be used for separating other compounds other than or in addition to carbon dioxide, such as hydrogen sulfide. A rigorous definition of such generalized recovery process is: 
     A method for recovering absorbate comprising: 
     (A) passing a feed mixture comprising oxygen and absorbate in countercurrent mass transfer contact with absorbent comprising at least one alkanolamine, and passing oxygen and absorbate from the feed mixture into the absorbent to obtain absorbate loaded absorbent containing dissolved oxygen; 
     (B) separating oxygen from the absorbate loaded absorbent to obtain oxygen depleted absorbate loaded absorbent; 
     (C) heating the oxygen depleted absorbate loaded absorbent to obtain heated absorbate loaded absorber fluid; and 
     (D) separating absorbate from the absorber fluid to obtain an absorbate rich fluid.