Abstract:
A method and apparatus for removing carbon dioxide from a synthesis gas stream containing hydrogen is disclosed. The method includes absorbing the carbon dioxide using a physical solvent under high pressure and then liberating the carbon dioxide in a series of expansion stages where the pressure on the solvent is reduced. The expansion ratio increases with each expansion stage. The apparatus includes expansion stages having throttling devices and expansion tanks operated at increasing expansion ratios. Carbon dioxide is liberated in this manner so as to minimize the energy required compress for transport via a pipe line for sequestration of the gas. Sequestration of the carbon dioxide is preferred to atmospheric venting to curb the release of greenhouse gases.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method and an apparatus for separating gases, for example, removing acid gases from a synthesis gas stream containing hydrogen and derived from a partial oxidation or gasification process. 
     In the production of hydrogen, synthesis gas containing hydrogen as well as other undesired constituents, is derived from various processes such as steam methane reforming, the water gas shift reaction, and the gasification of various solids such as coal, coke, and heavy liquid hydrocarbons present in oil refinery waste products. The undesirable gas components include “acid gases” such as carbon dioxide and hydrogen sulfide. 
     As it is advantageous to “sweeten” the synthesis gas by removing the acid gases before further processing, various types of acid gas removal systems are used. Acid gas removal systems could use either chemical or physical solvents. Acid gas removal systems which use a physical solvent employ solvents such as dimethyl ethers of polyethylene glycol, methanol, or propylene carbonate, which is brought into contact with the synthesis gas under high pressure (e.g., 1,200 psia) wherein the acid gases are preferentially absorbed by the solvent. The solvent is then depressurized in a series of “flash expansions” which liberate the dissolved acid gases from the solvent. The acid gas removal system yields substantially separate gas streams for the hydrogen sulfide and the carbon dioxide. The hydrogen sulfide is directed to a sulfur recovery unit, which most often uses a Claus process to reclaim the sulfur. The carbon dioxide is normally vented to the atmosphere. 
     However, so as not to further contribute to global warming believed to be caused by greenhouse gases such as carbon dioxide, it is advantageous to sequester the carbon dioxide rather than release it to the atmosphere. Considering the volume of gas to be sequestered, it is preferable to use geological formations such as oil wells or underground saline aquifers to store the carbon dioxide. The carbon dioxide may be transported by pipeline and pumped into the well head or aquifer. Sequestration in oil wells confers the added benefit of enhancing oil recovery from operating wells. 
     Sequestration of the carbon dioxide requires that substantial compression and pumping facilities be added to the acid gas removal system in view of the high pressures and large gas volumes which sequestration entails. It is calculated that, for pipeline transport and sequestration of the gases, the carbon dioxide will need to be compressed to pressures as great as 200 bar. Of the various steps involved in the removal of carbon dioxide from a synthesis gas stream including capture of the gas, compression, and transportation to the storage site, compression can account for more than 50% of the cost of the process. It is not surprising, therefore, that efforts have been made to optimize the compression step of the process. 
     In a paper entitled “Shift Reactors and Physical Absorption for Low-CO 2  Emission IGCCs” (Journal of Engineering for Gas Turbines and Power, April 1999, Volume 121, P. 295) authors Chiesa and Consonni describe operation of an acid gas removal system wherein the expansion ratios of the expansion stages of the solvent which liberate the dissolved carbon dioxide are constant for all expansion stages. In this paper, Chiesa and Consonni teach that the power consumption of the separation and compression section of an acid gas removal system does not change appreciably when the pressures of the expansion stages are varied (page 301). 
     In a subsequent paper entitled “Co-production of Hydrogen, Electricity and CO 2  from Coal with Commercially Ready Technology” (International Journal of Hydrogen Energy, 30 (2005) 747-767) authors Chiesa, Consonni et al teach operating an acid gas removal system with “four flash drums to reduce CO 2  compression power” and wherein “the pressures of intermediate flash drums are set to minimize overall CO 2  compression power” (page 753). Table 4 (page 760) from the paper shows that between the second and third expansion stages the expansion ratio is increased. However, a constant expansion ratio is used between the third and fourth stages. 
     In contrast to the aforementioned teachings of the prior art, applicants have found that further significant efficiency improvements can be obtained in the operation of an acid gas removal system that includes compression of the carbon dioxide if at least three pressure reduction stages are used wherein the expansion ratios of each of the stages are increasing. In the method according to the invention, by operating with increasing expansion ratios, more of the carbon dioxide is liberated at elevated pressures, which reduces the work required later for compression. Calculations show that a decrease in compression power consumption as great as 4.5% over the aforementioned prior art (Chiesa et al, 1999) may be obtained by the method of acid gas removal according to the invention. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention concerns a method of removing a gas dissolved in a liquid under pressure. The method comprises reducing the pressure on the liquid at a first expansion ratio. A first portion of the gas is released from the liquid upon the pressure reduction. The gas is then separated from the liquid. The pressure on the liquid is reduced a second time at a second expansion ratio greater than the first expansion ratio, whereupon a second portion of the gas is released from the liquid. The second portion of the gas is separated from the liquid, and the pressure on the liquid is reduced a third time at a third expansion ratio greater than the second expansion ratio, a third portion of the gas being thereby released from the liquid. 
     The method may further include compressing the gas portions released from the liquid, transporting and sequestering the gas portions in a storage facility. 
     In a specific example application, the invention concerns a method of removing carbon dioxide from a gas mixture comprising hydrogen. The method comprises bringing the gas mixture into contact with a liquid physical solvent under pressure. The solvent preferentially absorbs carbon dioxide from the gas mixture. The pressure on the solvent is then reduced in a first pressure reduction stage and a plurality of subsequent pressure reduction stages wherein each subsequent pressure reduction stage occurs at an expansion ratio greater than the pressure reduction stage which preceded it. A portion of the carbon dioxide absorbed by the solvent is released from the solvent at each pressure reduction stage. 
     The method may further include reducing the pressure on the solvent in a product gas recovery stage which precedes the first pressure reduction stage. Carbon dioxide and other gases, including for example hydrogen, are released from the solvent in the product gas recovery stage. The product gas recovery stage is intended to recover hydrogen or other product gases (e.g., carbon monoxide or methane) absorbed by the physical solvent, and may have an expansion ratio greater than the first pressure reduction stage. 
     The method may also include separating the carbon dioxide from the solvent before each subsequent pressure reduction stage, compressing the carbon dioxide and transporting the carbon dioxide for sequestration. 
     The invention also encompasses an apparatus for removing carbon dioxide from a gas mixture. The apparatus uses a liquid physical solvent to preferentially absorb the carbon dioxide and yield a product gas having a lower concentration of carbon dioxide than the gas mixture. The apparatus comprises an absorption vessel adapted to bring the liquid physical solvent into contact with the gas mixture under pressure. The absorption vessel has a solvent inlet for admitting the solvent to the absorption vessel, a gas inlet for admitting the gas mixture to the absorption vessel, a gas outlet for releasing the product gas from the absorption vessel, and a solvent outlet for releasing the solvent from the absorption vessel. For advantageous economic operation a product gas recovery expansion means may be used. The product gas recovery expansion means is adapted to receive the solvent from the absorption vessel. Carbon dioxide and other gases absorbed from the gas mixture are released from the solvent in the product gas recovery expansion means. A compressor having an inlet in fluid communication with the product gas recovery expansion means and an outlet in fluid communication with the gas inlet of the absorption vessel moves gases released from the solvent in the product gas recovery expansion means back to the absorption vessel. The gases released in the product gas recovery expansion means usually have significant product gas in addition to the carbon dioxide, and for this reason it is economical to send the gases released in the product gas recovery expansion means back to the absorption vessel or to other processes so that the product gas absorbed by the solvent can be recovered. Other processes may include, for example, adsorption processes, separation processes, fuel recovery and feed recycle. 
     A first expansion means is adapted to receive the solvent from the product gas recovery expansion means (or from the absorption vessel if no product recovery means is present). Carbon dioxide is released from the solvent in the first expansion means. A second expansion means is adapted to receive the solvent from the first expansion means. Carbon dioxide is released from the solvent in the second expansion means. A third expansion means is adapted to receive the solvent from the second expansion means. Carbon dioxide is released from the solvent in the third expansion means. A compressor facility is in fluid communication with the first, second and third expansion means for receiving the carbon dioxide released from the solvent. The compressor facility compresses the carbon dioxide for transport away therefrom. In the apparatus according to the invention, the second expansion means is configured to reduce the pressure on the solvent at an expansion ratio greater than the expansion ratio of the first expansion means, and the third expansion means is configured to reduce the pressure on the solvent at an expansion ratio greater than the expansion ratio of the second expansion means. Additional expansion means may also be used, each one being configured to reduce the pressure on the solvent at a greater expansion ratio than a preceding expansion means. 
     At least one of the expansion means comprises a throttling means in fluid communication with an expansion tank. The solvent enters the expansion tank through the throttling means which causes carbon dioxide and other gases to be released from the solvent. The gases collect in a gas space above the solvent within the expansion tank. The throttling means may comprise, for example a device such as an orifice, a pipe or a valve. 
     The apparatus may also include a stripping vessel in which residual carbon dioxide is stripped from the solvent by contacting the solvent with a pure gas such as steam or nitrogen. The stripping vessel has a solvent inlet in fluid communication with one of the expansion means for admitting solvent to the stripping vessel. A solvent outlet from the stripping vessel is in fluid communication with the solvent inlet of the absorption vessel. Once the solvent is stripped of residual gas, a pump pumps the solvent from the stripping vessel back to the absorption vessel. A pure gas inlet admits a substantially pure gas to the stripping vessel, and a gas outlet releases the gases from the stripping vessel. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an apparatus for removing a gas dissolved in a liquid according to a method of the invention; and 
         FIG. 2  is a schematic illustration of an apparatus for sweetening synthesis gas according to a method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a schematic representation of an apparatus  3  for removing a gas  5  dissolved in a liquid  7  according to a method of the invention. Apparatus  3  comprises three or more expansion stages  9 ,  11  and  13 . Each expansion stage is comprised of a respective throttling device  15 ,  17  and  19  and a respective expansion tank  21 ,  23  and  25 . Throttling devices  15 ,  17  and  19  could be, for example, an orifice, a valve, a pipe or other device which acts as a constriction to the flow of fluid to each respective expansion tank. As shown, fluid flow to each tank is controlled by a respective throttling device, and the tanks are connected to one another in series. 
     Each tank has a respective gas space  27 ,  29  and  31  where gas  5  which is liberated from the liquid  7  may accumulate and be drawn off by a compressor facility  33 . The compressor facility may conduct the gas to a pipeline for further transport, for example, to a sequestration facility  35 . 
     In operation, liquid  7  containing the dissolved gas  5  is fed through throttling device  15  into expansion tank  21  where the liquid undergoes a reduction in pressure at a first expansion ratio R 1 . (The expansion ratio for a particular expansion stage “R n ” is defined as R n =P n-1 /P n  wherein P n-1  is the absolute pressure of the gas at the preceding stage and P n  is the absolute pressure of a subsequent stage n.) Gas  5 , liberated from the liquid by the pressure reduction, accumulates in the gas space  27  where it is drawn off by the compressor facility  33 . Liquid  7  then passes from expansion tank  21  through throttling device  17  and into expansion tank  23  where there is another reduction in pressure on the liquid, and more gas is liberated. The expansion ratio R 2  for this second expansion stage is greater than the expansion ratio R 1  in the first expansion stage. Gas liberated in the second expansion stage accumulates in gas space  29  and is drawn off by the compressor facility  33 . Liquid  7  then passes from expansion tank  23  through throttling device  19  and into expansion tank  25  where there is another reduction in pressure on the liquid, and more gas is liberated. The expansion ratio R 3  for this third expansion stage is greater than the expansion ratio R 2  in the second expansion stage. Gas liberated in the third expansion stage accumulates in gas space  31  and is drawn off by the compressor facility  33 . Additional expansion stages may be used to further liberate gas at ever increasing expansion ratios according to the invention. The number of expansion stages and the relation of the expansion ratios may be determined to optimize various parameters depending upon the particular application for which the method and apparatus are employed. 
     By way of a practical example of the method and apparatus according to the invention,  FIG. 2  shows a schematic representation of an apparatus  10  for sweetening synthesis gas by the removal of carbon dioxide, it being understood that other applications of the apparatus and method according to the invention are also feasible. 
     Apparatus  10  includes an absorption vessel  12 . The absorption vessel has a solvent inlet  14  for admitting a physical solvent  15  to the vessel, such as dimethyl ethers of polyethylene glycol, methanol, or propylene carbonate, which will preferentially absorb carbon dioxide from a gas mixture. Vessel  12  also has a gas inlet  16  for admitting a synthesis gas  17 . The synthesis gas comprises a mixture containing hydrogen as well as other undesired constituents (such as carbon dioxide) and may be derived from various processes such as steam methane reforming, the water gas shift reaction, and the gasification of various solids such as coal, coke, and heavy liquid hydrocarbons present in oil refinery waste products. Vessel  12  also has a solvent outlet  18  which permits carbon dioxide laden solvent  19  to exit the vessel and a product gas outlet  20  which permits product gas  21  having a low carbon dioxide concentration to leave the vessel for further processing. 
     The absorption vessel  12  may be a high pressure tank which contains structured packings or trays that provide a large surface area to facilitate contact between the synthesis gas  17  and the solvent  15  to promote mass transfer between the gas and the solvent for physical absorption of the carbon dioxide by the solvent. The absorption vessel operates over a pressure range between about 300 psia and 1200 psia and can attain carbon dioxide removal rates above 95%. 
     The solvent outlet  18  is in fluid communication with a product gas recovery expansion means  22 , which includes a throttling means  24  and an expansion tank  26 . The throttling means  24  is a device such as a valve, an orifice, or even a pipe or other device which acts as a constriction to the flow of fluid between the absorption vessel  12  and the expansion tank  26  and causes a throttling process to occur as the solvent  19  moves from the higher pressure absorption vessel to the expansion tank at a lower pressure. The pressure reduction liberates gases absorbed by the solvent, and the gases collect in a gas space  28  in the upper portion of the expansion tank  12 . In this product gas recovery expansion stage of the solvent, significant amounts of product gas (hydrogen) are released from the solvent. To recapture the product gas absorbed by the solvent a compressor  30  draws the gases  23  from the gas space of the expansion tank  26  and pumps them back to the synthesis gas inlet  16  of the absorption vessel  12  where the carbon dioxide constituent may be absorbed again by the solvent and the hydrogen may be entrained and exit the absorption vessel as product gas. A turbine pump could also be used in place of the compressor  30 . 
     As shown in  FIG. 2  a plurality of expansion means  32 ,  34  and  36  are in fluid communication with one another and with the product gas recovery expansion means in a series relationship. Each expansion means comprises a respective throttling means  38 ,  40  and  42  constricting fluid flow to a respective expansion tank  44 ,  46  and  48 . Although three expansion means downstream of the product gas recovery means are shown, it is understood that this is by way of example, and additional expansion means are also feasible. Solvent  19  flows from tank  26  through the various throttling means  38 ,  40  and  42  to each expansion tank  44 ,  46  and  48  in turn, undergoing a reduction in pressure at each expansion stage and releasing carbon dioxide into the gas spaces  50 ,  52  and  54  in each tank. 
     A compressor facility  56  is in fluid communication with the gas spaces  50 ,  52  and  54  of the expansion tanks  44 ,  46  and  48  of the expansion means  32 ,  34  and  36 . The compressor facility comprises multi-stage compressors or pumps which draw the carbon dioxide  25  released from the solvent from the gas spaces and compress the gas to pressures greater than 85 bar so that it may be transported in a pipeline  58  for sequestration in a geological formation such as an oil field or underground saline aquifer. It is in the compressor facility that more than 50% of the cost of operating the system may be incurred, and actions which can be taken to reduce the number and size of the pumps and the power needed to run the pumps may be used to good effect to increase the economic efficiency of the apparatus. 
     The inventors have found that by operating the expansion stages which occur in expansion means  32 ,  34  and  36  at increasing expansion ratios, the power needed to operate the compressor facility may be reduced or minimized. The expansion ratio of an expansion stage is defined as R n =P n-1 /P n  wherein P n-1  is the absolute pressure of stage n−1 and P n  is the absolute pressure of a subsequent stage n. Thus the invention discloses that, to minimize the power consumption of the compressor facility, the relation between the expansion ratios of the expansion stages should be R 1 &lt;R 2 &lt;R 3 &lt; . . . R N  where N is the total number of expansion stages wherein the expansion ratio is increasing. The total number of expansion stages with increasing expansion ratios is preferably not less than 3 for acid gas removal from a synthesis gas stream. Preferably, in a particular expansion stage, P n-1  is greater than or equal to 1.005×P n . 
     The desired expansion ratios in each expansion stage may be obtained by controlling the pressure in each expansion tank  44 ,  46  and  48 . This may be done by using a pressure transducer in each tank which controls a respective variable valve in each of the associated throttling means  38 ,  40  and  42  through respective feed-back loops. Alternately, the throttling means could use fixed orifices appropriately sized at each stage to achieve the desired tank pressures for a given expansion tank size and solvent through-put rate. 
     Solvent  19  leaves the final expansion tank  48  substantially free of carbon dioxide. The solvent could be directly returned to the absorption vessel  12  or it could be sent to a stripping vessel  60 . The stripping vessel removes the last traces of carbon dioxide from the solvent by contacting the solvent with a pure gas  27  such as steam or nitrogen. Stripping vessel  60  comprises structured packing or trays similar to the absorption vessel and includes a pure gas inlet  62  for admitting the steam, nitrogen, or other substantially pure gas which is used to absorb the remaining carbon dioxide from the solvent. The stripping vessel  60  also has a solvent inlet  64  that receives solvent  19  from the final expansion tank  48 . The lean solvent  15 , stripped of carbon dioxide, exits the stripping vessel through a solvent outlet  66  and is returned to the absorption vessel by a pump  68 , either directly as shown or through an intermediate solvent reservoir (not shown). The mixture of once pure gas and stripped carbon dioxide  29  is vented to the atmosphere through a gas outlet  70 . 
     To demonstrate the improved efficiency of the method and apparatus according to the invention over the prior art, calculations were performed simulating a system using increasing expansion ratios as taught by the invention for the apparatus shown in  FIG. 2  having three subsequent expansion stages as described in the 1999 paper of Chiesa et al. For a head to head comparison the same parameters as found in Chiesa et al were used in the simulation. The simulation used methanol solvent at 30° F. The absorption vessel was assumed to operate at 750 psia and 100° F. The operational pressure range of the expansion stages determined by Chiesa et al (300 psia to 16 psia) was used as the boundary conditions to determine the expansion ratios according to the invention. Pressure in the product gas recovery expansion tank  26  was 300 psia at a temperature of 54° F., pressure in the first expansion tank  44  was 150 psia at 25° F., pressure in the second expansion tank  46  was 55 psia at 25° F., and pressure in the third expansion tank  48  was 16 psia at 25° F. Thus the increasing expansion ratios for the simulation of the method according to the invention were R 1 =2.00, R 2 =2.73 and R 3 =3.44. (Note that the expansion ratio between the absorption vessel  12  and the product gas recovery expansion stage is 2.5, but this stage does not produce carbon dioxide gas that is compressed by the compression facility, so it does not figure in the expense or cost saving calculation.) The Expansion ratios in Chiesa et al were a constant for all three expansion stages. The following chart provides a convenient comparison of the process according to the invention with a process having a constant expansion ratio as described in the prior art. The simulation values are shown adjacent to the values obtained with a constant pressure ratio (in parentheses) for each category. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Final Stage Pressure 
                   
                   
               
               
                   
                 (psia) 
               
               
                   
                 starting 
               
               
                 Expansion 
                 pressure 300 psia 
                 Expansion 
               
               
                 Stage 
                 for 1 st  stage 
                 Ratio 
                 % CO2 Released 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 150 
                 (112.5) 
                 2.00 
                 (2.66) 
                 43.7 
                 (60.7) 
               
               
                 2 
                 55 
                  (42.5) 
                 2.73 
                 (2.66) 
                 38.8 
                 (26.1) 
               
               
                 3 
                 16 
                  (16)   
                 3.44 
                 (2.66) 
                 17.4 
                 (13.2) 
               
               
                   
               
             
          
         
       
     
     The chart makes clear that essentially the same total percentage of carbon dioxide is released in both systems, but, in the system using increasing expansion ratios according to the invention, more carbon dioxide is released at a higher pressure for a particular stage (55 psia versus 42.5 psia) between the fixed boundary conditions of 300 psia and 16 psia. Although less carbon dioxide is released in the first stage, the carbon dioxide is released at a higher pressure (e.g., 150 psia versus 112.5 psia) which has a direct effect on compression power. By using increasing expansion ratios, there is an optimization of the trade off between the amount of gas released and the pressure level at which that gas is released, which means that less energy will be required by the compression facility to compress the carbon dioxide  25  released in the system according to the invention. The calculated power required to run the compression facility using increasing expansion ratios according to the invention is 28,000 brake horsepower, considerably less than the 29,000 brake horsepower required when the expansion ratio is held constant across the expansion stages. This is a savings of 4.5%. Smaller but significant savings are predicted in comparison with the results of the 2005 paper by Chiesa et al, which uses a constant expansion ratio for two of the three expansion stages of operation. It should be understood that this simulation is intended to provide an example comparison with the known prior art, and does not fully address the potential for cost savings believed achievable when increasing expansion ratios are used as described and claimed herein.