Abstract:
The present invention provides for a method and apparatus for purifying carbon dioxide. Sulfur species are efficiently and effectively removed from the carbon dioxide by a series of steps which include heater/heat exchange means, impurity adsorption means and cooling means.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention provides a method for removing impurities from a gas. In particular, this invention provides a method for removing sulfur species from carbon dioxide.  
       BACKGROUND OF THE INVENTION  
       [0002]     Carbon dioxide is used in a number of industrial and domestic applications, many of which require the carbon dioxide to be free from various impurities. Unfortunately, carbon dioxide obtained from natural sources such as gas wells, chemical processes, fermentation processes or produced in industry, particularly carbon dioxide produced by the combustion of hydrocarbon products, often contains impurity levels of sulfur compounds such as carbonyl sulfide (COS) and hydrogen sulfide (H 2 S). When the carbon dioxide is intended for use in an application that requires the carbon dioxide to be of high purity, such as in the manufacture and cleaning of foodstuffs and beverage carbonation, medical products and electronic devices, the sulfur compounds contained in the gas stream must be removed to very low levels prior to use. The level of impurity removal required varies according to the application of carbon dioxide. For example, for beverage application the total sulfur level in carbon dioxide (CO 2 ) ideally should be below 0.1 ppm. Removal to similar levels is required for electronic cleaning applications.  
         [0003]     Various methods for removing sulfur compounds and hydrocarbon impurities from gases such as carbon dioxide are known. For example, U.S. Pat. No. 4,332,781, issued to Lieder et al., discloses the removal of COS and H 2 S from a gas stream by first removing the H 2 S from the hydrocarbon gas stream by contacting the gas stream with an aqueous solution of a regenerable oxidizing reactant, which may be a polyvalent metallic ion, such as iron, vanadium, copper, etc., to produce a COS-containing gas stream and an aqueous mixture containing sulfur and reduced reactant. The COS in the gas stream is subsequently hydrolyzed to CO 2  and H 2 S by contacting the gas stream with water and a suitable hydrolysis catalyst, such as nickel, platinum, palladium, etc., after which the H 2 S and, if desired, the CO 2  are removed. This step can be accomplished by the earlier described H 2 S removal step or by absorption. The above-described process involves the use of cumbersome and costly equipment and liquid-based systems which require considerable attention and may result in the introduction of undesirable compounds, such as water vapor, into the carbon dioxide product.  
         [0004]     U.S. Pat. Nos. 5,858,068 and 6,099,619 describe the use of a silver exchanged faujasite and an MFI-type molecular sieve for the removal of sulfur, oxygen and other impurities from carbon dioxide intended for food-related use. U.S. Pat. No. 5,674,463 describes the use of hydrolysis and reaction with metal oxides such as ferric oxide for the removal of carbonyl sulfide and hydrogen sulfide impurities from carbon dioxide.  
         [0005]     It is known to directly remove sulfur compounds, such as H 2 S from a gas stream by contacting the gas stream with metal oxides, such as copper oxide, zinc oxide or mixtures of these. It is also known to remove sulfur impurities such as COS by first hydrolyzing COS to H 2 S over a hydrolysis catalyst and then removing H 2 S by reaction with metal oxides. Removal of H 2 S by reaction with metal oxides can become expensive, since the catalyst is non-regenerable and expensive, when impurities such as COS and H 2 S are present in more than trace amounts. Lower cost materials for the removal of COS and H 2 S and other sulfur impurities such as mercaptans and dimethyl sulfide are desired to reduce CO 2  purification cost.  
         [0006]     Since many end users of carbon dioxide require the carbon dioxide they use to be substantially free of sulfur compounds, and because natural sources of carbon dioxide and industrially manufactured carbon dioxide often contain sulfur compounds, economic and efficient methods for effecting substantially complete removal of sulfur compounds from carbon dioxide gas streams, without concomitantly introducing other impurities into the carbon dioxide, are continuously sought. The present invention provides a simple and efficient methods for achieving these objectives.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides for a method of purifying a gas comprising the steps of heating an impure gas stream to above ambient temperature and feeding the impure gas stream into a sulfur removal unit; optionally further heating the gas from sulfur removal unit and feeding the gas to a reactor bed to remove impurities by oxidation; cooling the carbon dioxide stream exiting the reactor or the sulfur removal bed; optionally removing the moisture and other impurities; and optionally feeding the purified gas to a process requiring purified gas.  
         [0008]     In an embodiment, the gas comprises carbon dioxide, and the impurities comprise sulfur species.  
         [0009]     In another embodiment of the present invention, there is provided a method of removing sulfur species from a carbon dioxide gas stream in a carbon dioxide production plant comprising the steps of heating a low pressure impure carbon dioxide stream to above ambient temperature and feeding it into a sulfur removal unit; optionally further heating the carbon dioxide from sulfur removal unit and feeding said carbon dioxide to a reactor bed to remove impurities by oxidation; cooling the carbon dioxide stream exiting the reactor or the sulfur removal bed; compressing the resulting carbon dioxide and further purifying/liquefying it; and optionally feeding the purified carbon dioxide to a process requiring purified carbon dioxide.  
         [0010]     In yet another embodiment of the present invention, there is provided a method of removing sulfur species from a carbon dioxide gas stream in a carbon dioxide production plant comprising the steps of compressing the carbon dioxide stream from a carbon dioxide source; optionally further heating the carbon dioxide exiting the last compressor stage, optionally adding oxygen to it and feeding it into a sulfur removal unit; optionally further heating the carbon dioxide from sulfur removal unit and feeding said carbon dioxide to a reactor bed to remove impurities by oxidation; cooling the carbon dioxide stream exiting the reactor or the sulfur removal bed; and further purifying the resulting carbon dioxide and optionally feeding the purified carbon dioxide to a process requiring purified carbon dioxide. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     While the specification concludes with claims distinctly pointing the subject matter that Applicants regard as their invention, the invention would be better understood when taken in connection with the accompanying drawings in which:  
         [0012]      FIG. 1  is a schematic description of the overall process for removing sulfur impurities from carbon dioxide;  
         [0013]      FIG. 2  is a schematic description of purifying carbon dioxide in a carbon dioxide production plant; and  
         [0014]      FIG. 3  is another schematic description of purifying carbon dioxide in a carbon dioxide production plant. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     The carbon dioxide that is typically produced for industrial operations has a number of impurities present in it. These impurities will often be a concern for many uses of the carbon dioxide, but in the production of products intended for human consumption such as carbonated beverages, and electronic manufacturing the purity of the carbon dioxide is paramount and can influence the taste, quality, and legal compliance of the finished product.  
         [0016]     The impure carbon dioxide which can be obtained from any available source of carbon dioxide will typically contain as impurities sulfur compounds such as carbonyl sulfide, hydrogen sulfide, dimethyl sulfide, sulfur dioxide and mercaptans. This invention describes novel and low cost methods for the removal of various sulfur impurities. The impurity removal can be used in various ways depending on whether the carbon dioxide is purified at a production plant, or at the point of use. Various point of use applications of carbon dioxide include a beverage filling plant, a food freezing plant, an electronics manufacturing plant and a fountain type carbon dioxide dispensing location.  
         [0017]     For the purposes of this invention at least some of the sulfur impurities such as hydrogen sulfide and carbonyl sulfide are removed at an elevated temperature, a temperature of 50° to 150° C. In a point of use application this temperature can be obtained by using a combination of heater and heat exchange means. In a production plant this temperature may be obtained during the compression of feed carbon dioxide after the final compression stage but before the aftercooler. In a production plant, for the feed containing high levels of sulfur compounds (&gt;10 to several hundred ppms), it is highly desirable to remove sulfur compounds prior to compression and the temperature for sulfur removal is obtained by heater and heat exchanger means. The impure carbon dioxide gas stream having been raised to the proper temperature is directed to a sulfur reactor bed. This bed is typically a vessel that will contain certain catalyst and adsorbent materials which will either react with or adsorb the sulfur compounds.  
         [0018]     Preferably the catalyst materials are those that will cause the H 2 S and COS to convert to elemental sulfur which is retained on the purification media or react with the sulfur impurities to form metal oxides. The sulfur impurities such as mercaptans can simply be adsorbed on the purification media. Some of the materials may require oxygen to convert sulfur compounds such as hydrogen sulfide to sulfur and both oxygen and water to convert carbonyl sulfide to hydrogen sulfide and then to sulfur. The sulfur purification materials according to this invention include carbonates and hydroxides such as sodium and potassium hydroxides or carbonates on activated carbon; metal oxides such as copper, zinc, chromium or iron oxide either alone or supported on a microporous adsorbent such as activated alumina, activated carbon or silica gel. Other materials such as a CuY zeolite are effective for the removal of carbonyl sulfide, dimethyl sulfide and sulfur dioxide impurities through reaction. Mercaptans are typically removed by adsorption on support such as activated carbon. Supported hydroxides and carbonates are preferred materials because of high capacities and significantly lower cost compared to oxide type materials. These materials typically cost less than $2.0-4.0 per kg compared to $10-20 per kg for metal oxides such as zinc, zinc/copper oxide.  
         [0019]     For materials containing hydroxide and carbonates on activated carbon or activated alumina hydrogen sulfide reacts with oxygen to form elemental sulfur which is then retained on activated carbon. For these materials carbonyl sulfide reacts with water in the feed to form hydrogen sulfide which is then converted to sulfur in presence of oxygen and retained on activated carbon. Use of elevated temperatures for sulfur removal significantly improves removal capacity for both hydrogen sulfide and carbonyl sulfide compared to operation near ambient temperatures. Inventors have unexpected discovered that these materials have little carbonyl sulfide removal capability and reasonable hydrogen sulfide capacity at ambient temperature. However, at elevated temperatures carbonyl sulfide removal capacity increases by a factor of 10-50 and the sulfur dioxide removal capacity increases by over a factor of 2. Impurities such as mercaptans are also removed by these materials. For materials containing metal oxides and zeolites the sulfur impurities are removed by reaction to form metal sulfides. Combinations of oxides/zeolites and hydroxide/carbonate materials can be used for optimum removal of sulfur impurities. While most sulfur impurities are removed by these materials, some of other sulfur impurities such as dimethyl sulfide are not removed to a significant extent and may need to be removed by other means.  
         [0020]     The stream exiting the sulfur removal bed can optionally be further heated and sent to a catalytic reactor for oxidation of various hydrocarbon impurities. The stream exiting the reactor beds or the sulfur removal beds is cooled to close to ambient temperatures in heat exchange means.  FIG. 1  is an overview of the carbon dioxide purification process according to this invention. Depending on impurities in the feed some components of this process can be eliminated. Carbon dioxide containing impurities is directed from source  2  and line  5  to a first heat exchanger  20 . Oxygen is added to this stream via line  4  for use in the sulfur removal bed and in the catalytic reactor. The first heat exchanger  20  will raise the temperature of the impure carbon dioxide from about ambient to between 40 and 120° C. The heated impure carbon dioxide leaves the first heat exchanger through line  7  to a heater  30  where its temperature is further increased to between 50 and 150° C. For certain situations the heat exchanger  20  may be eliminated and only heater  30  may be used to increase the temperature of the stream. The impure carbon dioxide leaves the heater through line  9  and enters the sulfur removal bed  40 . The sulfur removal bed contains various materials such as supported carbonates, hydroxides and oxides for the removal of various sulfur impurities such as hydrogen sulfide, COS and mercaptans.  
         [0021]     The impure carbon dioxide which is now essentially free of most sulfur impurities is optionally directed through line  11  to a second heat exchanger  50  where its temperature is raised to over 150° C. The impure carbon dioxide exits the second heat exchanger through line  13  and is further heated to a temperature between 150 and 450° C. in a heater not shown. The heated carbon dioxide enters an optional catalyst reactor  60  containing a pelleted or a monolith catalyst. Various impurities such as benzene and aldehydes in the feed react with oxygen in the catalytic reactor and are converted to carbon dioxide and water. Some of the remaining sulfur impurities in the feed may be converted to sulfur dioxide in this reactor.  
         [0022]     The now essentially purified carbon dioxide gas stream leaves the catalytic reactor bed through line  15  where it returns to the second heat exchanger  50 .  
         [0023]     The purified carbon dioxide gas stream leaves the second heat exchanger through line  17  and is directed into the first heat exchanger  20  where its temperature is reduced to less than 40° C. The cooled purified carbon dioxide gas steam can be sent to downstream processing equipment  70  through line  19  where it is further purified and optionally liquefied. It can also be sent to a CO 2  use process, unit  80 , via line  21 .  
         [0024]     Purification of carbon dioxide in a carbon dioxide production plant is shown in  FIGS. 2 and 3 .  
         [0025]     In  FIG. 2 , carbon dioxide from a source  100  is sent to an optional purification unit  105 . This unit may consist of one or more purification processes chosen from adsorption, water wash column, electrostatic precipitator or a filtration unit. The carbon dioxide exiting unit  105  is sent to a heat exchanger  110  to raise its temperature to between 40 and 120° C. and oxygen is added to this stream at line  115 . The stream exiting unit  110  is sent to an optional heater unit  120  to further increase its temperature to around between 50 and 150° C. and is then sent to the sulfur removal unit  125  where sulfur impurities such as hydrogen sulfide, carbonyl sulfide, and mercaptans are removed by reaction with metal oxides, hydroxides or carbonates, or copper exchanged zeolites. Some of the reaction products such as sulfur may also be adsorbed on supports such as activated carbons and activated alumina.  
         [0026]     The stream exiting the sulfur removal unit  125  is further heated in an optional heat exchanger  130  and optional heater  135  and enters the optional catalytic reactor  140 . The catalytic reactor contains supported noble metal catalysts such as palladium or platinum in pelleted or monolith forms. The catalytic reactor operates at a temperature between 150 and 450° C. depending on the impurities in the feed stream. The hydrocarbon impurities are oxidized to water and carbon dioxide in this reactor. The stream exiting reactor  140  is cooled in heat exchanger  130  and heat exchanger  110 . If reactor  140  is not used stream exiting the sulfur bed  125  is cooled in heat exchanger  110 . The stream exiting heat exchanger  110  is compressed in a compressor  145  to pressures between 10 and 20 bara and is cooled in an aftercooler  150  to a temperature close to ambient. The cooled, purified carbon dioxide gas steam can optionally be sent to downstream processing equipment  155  where it is further purified and optionally liquefied. It can also be sent to a CO 2  use process, unit  160 .  
         [0027]     The embodiment in  FIG. 2  is particularly advantageous for feeds containing high levels of sulfurs, from 10 ppm to several hundred ppms. If this feed was sent directly to compressor  145 , expensive materials of construction such as stainless steel may be needed to minimize the corrosion in the compressor. However, efficient high temperature removal of sulfurs in unit  125  obviates this need and a compressor made of carbon steel can be used, reducing the capital cost for the compressor by a factor  2  to  3 .  
         [0028]     In  FIG. 3 , carbon dioxide from source  200  is sent to an optional purification unit  205 . This unit may consist of one or more purification processes chosen from adsorption, water wash column, electrostatic precipitator or a filtration unit. The carbon dioxide exiting unit  205  is sent to a compressor  210  to raise its pressure to between 10 and 20 bara and oxygen is added to the compressed stream at line  215 . The stream exiting the final compression stage will be at a temperature between 70° and 95° C. and is sent to an optional heater unit  220  to further increase its temperature to between 80 and 150° C. and is then sent to the sulfur removal unit  225  where sulfur impurities such as hydrogen sulfide, carbonyl sulfide, and mercaptans are removed by reaction with metal oxides, hydroxides or carbonates, or copper exchanged zeolites. Some of the reaction products such as sulfur may also be adsorbed on supports such as activated carbons and activated alumina.  
         [0029]     The stream exiting the optional sulfur removal unit  225  is further heated in an optional heat exchanger  230  and optional heater  235  and enters the optional catalytic reactor  240 . The catalytic reactor contains supported noble metal catalysts such as palladium or platinum in pelleted or monolith forms. The catalytic reactor operates at a temperature between 150 and 450° C. depending on the impurities in the feed stream. The hydrocarbon impurities are oxidized to water and carbon dioxide in this reactor. The stream exiting reactor  240  is cooled in heat exchanger  230  and is further cooled in an aftercooler  245  to a temperature close to ambient. The cooled, purified carbon dioxide gas steam can optionally be sent to downstream processing equipment  250  where it is further purified and optionally liquefied. It can also be sent to a CO 2  use process, unit  260 .  
       EXAMPLE 1  
       [0030]     A feed containing 9 ppm COS in carbon dioxide at a pressure of 14.6 bara and a temperature of 100° C. was passed through a bed containing 0.12 kgs of activated carbon containing 20 wt % potassium carbonate at a flow rate of 19.8 std liters/min. About 100 ppm of oxygen was added to the feed. An equilibrium COS capacity of 5.15 wt % was obtained at this temperature. The same feed was passed through the same bed at a temperature of 25° C. and an equilibrium COS capacity of &lt;0.1 wt % was obtained.  
       EXAMPLE 2  
       [0031]     The same feed now containing 50 ppm H 2 S in carbon dioxide at a pressure of 14.6 bara and a temperature of 100° C. was passed through a bed containing 0.154 kgs of activated carbon containing 20 wt % potassium carbonate at a flow rate of 15.6 std liters per min. About 100 ppm oxygen was added to the feed. An equilibrium H 2 S capacity of 18 wt % was obtained. The same feed was passed through the same bed at a temperature of 25° C. and an equilibrium H 2 S capacity of around 10 wt % was obtained.  
         [0032]     Both these experiments indicate that a significant improvement in the removal capacity for COS and H 2 S is possible by operating at an elevated temperature.  
       EXAMPLE 3  
       [0033]     Testing was performed using a purification skid containing 17.1 kgs of activated carbon impregnated with 20 wt % potassium carbonate. Carbon dioxide at a pressure of 17 bara, at a temperature of 85° C., and at a flow rate 109.7 std m 3 /hr was passed through the bed. The feed contained 25-100 ppb of ethyl and methyl mercaptans. No mercaptans were seen at the bed outlet during a test period of about one week.  
         [0034]     While the present invention has been described with reference to several embodiments and examples, numerous changes, additions and omissions, as will occur to those skilled in the art, may be made without departing from the spirit and scope of the present invention.