Abstract:
A process for the recovery of CO 2  from a flue gas is provided. This process includes compressing a flue gas to a first pressure, cooling the flue gas to a first temperature, and drying flue gas by a drying means. This process includes adsorbing CO 2  in a first adsorbent bed, wherein the first adsorbent bed is isothermally maintained by a first cooling means. The process includes pressurizing the first adsorbent bed to a second pressure with CO 2  at a second temperature, wherein the second pressure is greater than the initial pressure, wherein the second temperature is greater than the initial temperature.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/093,756, filed Sep. 3, 2008, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     CO 2  removal from flue gas has been done on limited scale for long time. Currently, there is an increased emphasis on CO 2  capture to reduce green house emissions. 
     The use of an amine wash has been the most common technology for CO 2  removal. It is energy intensive due to the large amount of heat required for the regeneration of amine solution. The CO 2  product is always at low pressure of around 5 to 10 psig. There are also issues about the degradation of amine solution due to the presence of O 2 , NO x  and SO 2  in the flue gas. 
     CO 2  removal by the use of PSA technology with various solid adsorbents, such as carbon and zeolites has also been studied. Refer to the paper “Flue-Gas Carbon Capture on Carbonaceous Sorbents: Toward a Low-Cost Multifunctional Carbon Filter for Green Energy Producers” by Maciej Radoz, Xudong Hu, Kaspars Krutkramelis, and Young Shen” Ind. Eng. Chem. Res. 2008, 47, 3783-3794. This paper describes a PSA cycle for CO 2  recovery using activated carbon. It teaches that carbon is superior to zeolite for this application. The regeneration of the PSA is done with vacuum or with direct-steam or hot CO 2 . 
     Vacuum regeneration increases the power required to compress the CO 2  to a typical pressure of about 2000 psig. The thermal regeneration with steam or hot CO 2  occurs at ambient pressure. However the compression power for CO 2  remains high. 
     Carbon as a media for adsorption has several disadvantages. Its capacity for CO 2  is low compared to the zeolites. It also has the disadvantage that it co-adsorbs N 2 . CO 2 /N2 selectivity is under 10. CO 2  product is thus contaminated with significant amount of NO 2 , on the order of 10% or more. 
     SUMMARY 
     The present invention is a process for the recovery of CO 2  from a flue gas. This invention includes compressing a flue gas to a first pressure, cooling the flue gas to a first temperature, and optionally drying flue gas by a drying means. The present invention also involves adsorbing CO 2  in a first adsorbent bed, wherein said first adsorbent bed is at said first temperature and said first pressure, wherein said first adsorbent bed is isothermally maintained by a first cooling means. The present invention also includes pressurizing said first adsorbent bed to a second pressure with CO 2  at a second temperature, wherein said second pressure is greater than said first pressure, wherein said second temperature is greater than said first temperature. The present invention also includes heating said first adsorbent bed to a third temperature with a first heating means, thereby increasing said first adsorbent bed to a third pressure, wherein said third temperature is greater than said second temperature, wherein said third pressure is greater than said second pressure, thereby releasing a first portion of adsorbed CO 2 . The present invention also includes lowering the pressure of said first adsorbent bed in at least one more stage, while said first adsorbent bed is isothermally maintained at said third temperature by a second heating means, thereby releasing at least a second portion of adsorbed. The present invention also includes cooling the released CO 2  with at least a second cooling means, sending said cooled CO 2  to one or more surge drums, and purging said first adsorption bed with a N2 stream, simultaneously cooling said first adsorption bed with a third cooling means. The present invention includes compressing said cooled CO 2  from said surge drums in a multi-stage compressor, wherein the pressure of the CO 2  from each surge drum is approximately equal to the inter-stage pressure into which it is admitted, and regenerating said first adsorption bed with a N2 stream, wherein said regeneration stream is heated by a third heating means. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present process uses zeolite as the medium for separation. The zeolite chosen has small capacity for water, and its CO2 loading capacity is not greatly affected by the presence of moisture. Optionally moisture is removed by passing the gas over another adsorbent such as alumina. CO 2  is adsorbed at a low pressure of abut 1 to 2 bara. However the regeneration is done at elevated temperature of about 300-400° C. and at an elevated pressure of about 2-25 bara. 
     The following data is for a H 2  SMR application. The flue gas from the SMR furnace is at ambient pressure and 120-150° C. It contains approximately 20% CO 2  with the balance being N 2  and Ar. The CO 2  loadings on zeolite 13X are as shown below 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 Partial Pressure 
                 Temperature 
                 Loading 
               
               
                 Bara 
                 ° C. 
                 wt % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0.2 
                 20 
                 18 
               
               
                 0.4 
                 20 
                 19.6 
               
               
                 1 
                 300 
                 0.8 
               
               
                 2 
                 300 
                 1.5 
               
               
                 10 
                 300 
                 5.4 
               
               
                 20 
                 300 
                 8.5 
               
               
                   
               
             
          
         
       
     
     This illustrates that the zeolite bed will pick up 18% of its weight when it is in equilibrium with 0.2 bar CO 2  partial pressure (1 bara total pressure, 20% CO 2 ) in the flue gas. If the CO 2  loaded bed is heated to about 300° C. its equilibrium pressure will exceed 20 bar. Equilibrium loading at 300° C. and 20 bar is 8.5%. This means a significant part of CO 2  that was adsorbed on the bed (from 18% to 8.5% loading) can be withdrawn from the bed at 300° C. and 20 bar. When the bed pressure is reduced at about 300° C. below 20 bar more CO2 is released. If the bed pressure is reduced to 1 bar, the residual CO 2  loading is only 0.8%. In this cycle the bed loading changed from 18% to 0.8%. The bed can be considered regenerated. It is cooled and ready for next cycle. 
     The co-adsorption of N 2  is small as shown in the data below for 5° C. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 Partial Pressure of 
                 Partial Pressure of 
                   
               
               
                 N 2   
                 CO 2   
                 Loading, wt % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0.5 bar 
                 0 
                 1.1 
               
               
                 0.5 bar 
                 0.5 bar 
                 0.01 
               
               
                   
               
             
          
         
       
     
     The adsorption of CO 2  is exothermic (30 kJ/g mol CO 2 ). It is desirable to cool the bed during adsorption, to keep it isothermal and maximize the CO 2  loading. The bed design also provides for quick heating to minimize the total cycle time. The total cycle time determines the inventory of the adsorbent and the bed size. A radial bed with heat transfer coil provides low pressure drop and quick heating and cooling cycle. Structured adsorbent can be formed into bed with channels that can pass heating or cooling media. 
     The N 2  stream out of the adsorption bed is dry and mostly free of CO 2 . It can be used as regeneration gas. 
     Heat integration is important for the thermal efficiency of the process. The heat in flue gas at about 120-150° C. is a significant source of heat. If the CO 2  product is further compressed to about 150-200 bar, the heat of compression can also be used as a source of heat. Heat exchange between the bed being heated and the bed being cooled is also important. 
     The heat in the syngas can be utilized to heat the glycol to the desired temperature. Normally heat in syngas below about 120-150° C. is rejected to air or cooling water. Part of this heat can also be utilized for heating the adsorbent bed. 
     A glycol solution or other heat transfer media such as Dowtherm is proposed to recover heat from flue gas, syngas, and compressor intercooler. A similar heat transfer media is proposed for heating and cooling the zeolite beds. 
     The heat in the flue gas or the heat of compression may also be used to produce refrigeration by absorption refrigeration cycle (such as ammonia absorption refrigeration or lithium bromide refrigeration). The refrigeration can be used to chill the flue gas to about 5° C. This will reduce the amount of moisture in the flue gas. It will also increase CO 2  loading on the zeolite by about 10%. 
     Due to great affinity of most zeolites for water, all the moisture present in the flue gas will be removed. The zeolite 13X picks up about 20-25 times its weight in water. It is preferred that a separate bed upstream be provided to remove moisture. This bed is regenerated by the N 2  stream exiting the CO 2  adsorption bed. The N2 stream is heated by heat exchange with hot glycol. 
     In one embodiment of the proposed process, the flue gas is compressed in a blower to about 0.3-0.5 barg. This flue gas is cooled to about 40 C by indirect heat exchange with glycol and cooling water or air. It may be further cooled to 5 C by heat exchange against a refrigerated media. The condensed moisture is then removed from the cooled flue gas. The cooled flue gas is sent to a drier bed. The dry flue gas is sent to a CO 2  adsorber that removes CO 2  and lets N 2  and Ar pass through. The adsorber bed is provided with a heat transfer coil. The heat of adsorption is removed by circulating cold glycol in the bed. When one CO 2  adsorber bed is consumed, the next bed is brought online for adsorption. The consumed bed is heated for regeneration and thermo-compression of CO 2 . 
     The regeneration of the bed is done at an elevated pressure of about 20-25 barg. The consumed bed is pressurized by product CO 2  stored at about 10-15 bar. The CO 2  is preheated to 300 C before sent for pressurization. Hot glycol at about 300-370 C is circulated in the coils in the bed. As the temperature of the bed rises, the pressure in the bed also increases corresponding to equilibrium pressure of CO 2  on zeolite. The back pressure control valve on the bed opens at 20 bar and hot CO 2  at about 300 C is cooled by glycol and sent to 20 bar Surge Drum. CO 2  is further compressed by a centrifugal compressor from 20 bar to the use pressure of about 150-200 bar. 
     When the bed being regenerated comes in equilibrium at about 300 C, no more CO 2  is released at 20 bar. Its pressure is slowly dropped to about 15 bar to release more CO 2 , maintaining the bed temperature at 300 C. The CO 2  released between 20 and 15 bar is stored in a surge drum at 15 bar. It is cooled against a glycol solution before sent to surge drum. One part of the CO 2  stored at 15 bar is used for repressurization of the bed to be regenerated, remaining being compressed in the centrifugal compressor for end use. 
     This step is repeated at several pressure levels, for example at 10, 5 and 1 bar, while maintaining the bed temperature at 300 C, to fully regenerate the bed. The regenerated bed is cooled by flowing cold N 2  stream from the top of bed on CO 2  adsorption. Cold glycol is also circulated in the bed to recover the heat from the bed. 
     The number of pressure-stages for regeneration are optimized with respect to compression power, and the compressor selection is then matched accordingly. Obviously, the greater the number of stages, the lower will be the compression power. However, there is a practical limit. A typical CO 2  compressor may require between 8-9 stages for compression from about 1 bar to 200 bar. With interstage levels at probably 2, 4, 7, 10, 20, 35, 50, 90, 140, 200 bar. 
     During the repressurization with pure CO 2 , the bed loads up with more CO 2 . For example, the loading of CO 2  at the end of the CO 2  adsorption is 18%. When this bed is repressurized to 15 bar using pure CO 2  from a surge drum, its loading increases to about 25%. This increased CO 2  will be available at 20 bar when the bed temperature is increased. 
     Various process steps are described as follow:
         Compression of flue gas to 0.3 barg   Cooling and chilling the flue gas to 5 C.   Drying flue gas over a adsorbent bed.   CO 2  adsorption at 5 C and 0.2 barg (0.1 bar pressure drop in the cooling/chilling). Cooling the bed with glycol to achieve isothermal conditions.   Pressurization of consumed bed with hot CO 2  to about 15 bar in 3 or 4 stages, using CO 2  stored in surge drums.   Simultaneously heating the bed with hot glycol to 300 C. Release CO 2  from the bed at 20 bar. 20 bar CO 2  is cooled with glycol and sent for further compression, if desired.   Lowering the bed pressure in several stages (15, 10, 7, 2, 1 bar), keeping the bed temperature at 300 C. Recovered CO 2  is cooled with glycol and sent to their respective surge drums.   Purging the bed with N 2  stream from the bed on CO 2  adsorption. Simultaneously cooling it with glycol.   Regeneration of drying bed with N 2  from CO 2  adsorption bed. N 2  is heated by heat exchange with hot glycol.   A glycol system that recovers heat from flue gas, process syngas, compressor interstages, and supplies heat to various users. Additional heat required can be supplied by steam or any hot stream in the SMR.       

     In one embodiment of the proposed apparatus, the adsorption reactors are designed to provide low pressure drop, quick heating and cooling. The adsorption reactors also provide for cooling during the adsorption step. 
     Glycol can flow from one bed to another, or glycol can be stored in surge drums at various temperature levels. The flow of glycol into or out of these surge drums is matched by heating and cooling requirements of the beds. 
     For partial CO2 recovery, CO2 released at lower pressures can be vented. 
     Radial beds with imbedded coils can provide these features. 
     Internal insulation of the beds is important for quick heating and cooling cycle. The vessel can be insulated with refractory and a metal liner, or it can be insulated by having thin gap (one or more) filled with gas like Argon. 
     A multi-stream heat exchanger for heat integration between various CO 2  and glycol streams is proposed for this application. These exchangers provide close temperature approach and also provide flexibility in heat transfer.