Abstract:
A system and a method is provided for removing carbon dioxide from a gas stream. One aspect of the method includes introducing a carbon dioxide-containing gas stream to an absorber. The gas stream is contacted with an ammonia-containing solvent for absorbing, with the ammonia-containing solvent, the carbon dioxide from the gas stream, thereby removing the carbon dioxide from the gas stream.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims benefit under 35 U.S.C. §119(e) of co-pending, U.S. Provisional Patent Application, Ser. No. 61/617,720, entitled “METHOD AND SYSTEM FOR CARBON DIOXIDE REMOVAL,” filed March 30, 2012, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure is generally directed to the removal of carbon dioxide from a gas stream. More particularly, the present disclosure is directed to a system and method for precipitating ammonium salts as a means to separate a carbon dioxide rich phase from a carbon dioxide semi-lean phase and thus reduce the energy requirements during regeneration. 
       BACKGROUND 
       [0003]    Combustion of fossil fuels typically produces an exhaust gas stream (commonly referred to as a “flue gas stream”) that contains contaminants, such as carbon dioxide (CO 2 ), sulfur oxides (SOx), nitrogen oxides (NOx), mercury, and carbon containing species, as well as particulate matter such as dust or fly ash. To meet requirements established under certain laws and protocols, plants that burn fossil fuels subject the resultant flue gas stream to various processes and systems to reduce or eliminate the amount of contaminants present in the flue gas stream prior to releasing the flue gas stream to the atmosphere. 
         [0004]    In one example, carbon dioxide is removed from a gas stream by introducing the gas stream to an absorber column (“absorber”). In one embodiment, the gas stream contacts a solvent in a counter-current flow within the absorber. Contact between the solvent and the gas stream allows the solvent to absorb and thus remove the CO 2 from the gas stream. The gas stream that is substantially free of the CO 2  is typically sent to an exhaust stack while the CO 2  rich solvent is processed in a regenerator to remove the CO 2 . The solvent from the regenerator is then cycled back to the absorber for further use. 
         [0005]    When using reactive solvents, such as ammonia, for CO 2 removal, the solution reactions between the solvent(s) and the carbon dioxide may form a precipitate such as, for example, ammonium salts. When utilizing a chilled ammonia absorption process (often referred to as “CAP”), the process operates at a low temperature (typically below 20° C.) and high CO 2  loadings to minimize ammonia volatility. High ammonia molarity and high recirculation rate (of the ammonia) around the absorber may be needed to achieve the desired CO 2  removal from the flue gas stream. 
         [0006]    In addition to achieving the desired amount of CO 2  removal from the flue gas stream, it is also desirable to reduce ammonia loss. Reduction of ammonia loss involves low temperatures and low free ammonia molarity. Also, an ammonia solvent has the tendency to precipitate ammonium bicarbonate at high free ammonia molarity, low temperature, and high CO 2  loading, thus limiting the cyclic capacity of the solvent between the absorber and the regenerator. The consequence of this is that higher circulation rates are required between the absorber and regenerator to achieve the desired CO 2  capture rates. 
       SUMMARY 
       [0007]    According to aspects illustrated herein, there is provided a method for removing carbon dioxide from a gas comprising introducing a carbon dioxide-containing gas stream to an absorber; contacting the gas stream with an ammonia-containing solvent, the ammonia-containing solvent having a molarity between about 0.5 molar and about 13 molar; and absorbing the carbon dioxide from the gas stream with the ammonia-containing solvent, thereby removing the carbon dioxide from the gas stream and forming a CO 2 -rich stream. 
         [0008]    According to further aspects illustrated herein, there is provided a method for removing carbon dioxide from a carbon dioxide-containing gas stream comprising introducing a carbon dioxide-containing gas stream to an absorber having a temperature of about 45° C. or less; contacting the carbon dioxide-containing gas stream with an ammonia-containing solvent having a molarity between about 0.5 molar and about 13 molar; absorbing carbon dioxide from the gas stream; and forming a precipitate between the carbon dioxide and the ammonia-containing solvent. 
         [0009]    In yet a further aspect illustrated herein, there is provided a system for removing carbon dioxide from a gas stream comprising an absorber configured to receive a carbon dioxide-containing gas stream and an ammonia-containing solvent, the ammonia-containing solvent having a molarity between about 0.5 molar and about 13 molar, the carbon dioxide-containing gas stream and the ammonia-containing solvent being contacted to remove carbon dioxide from the gas and form a carbon dioxide-rich stream; and a regenerator fluidly coupled to the absorber, wherein the regenerator is configured to receive at least a portion of the carbon dioxide-rich stream and to remove carbon dioxide from the carbon dioxide-rich stream to form a regenerated solvent to be introduced to the absorber for further absorption and removal of carbon dioxide. 
         [0010]    The above described and other features are exemplified by the following figures and in the detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Referring now to the figures which are exemplary embodiments and wherein like elements are numbered alike: 
           [0012]      FIG. 1  is a schematic depiction of a system for removal of carbon dioxide from a gas stream; 
           [0013]      FIG. 2  is a graph illustrating carbon dioxide capture rate in a system according to an embodiment described herein; 
           [0014]      FIG. 3  is a graph illustrating reboiler temperature and lean loading. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  illustrates a system  100  for removal of carbon dioxide (CO 2 ) from a gas stream. The system  100  includes a columnar absorber  110 , wherein a CO 2 -containing gas stream  112 , such as, for example, a flue gas stream, is introduced and contacted with a solvent, such as CO 2 -lean stream  114   a  and/or CO 2  semi-lean stream  114   b.  The CO 2 -containing gas stream  112  may be contacted with the solvent  114   a,    114   b  in a counter-current manner; however, it is contemplated that the CO 2 -containing gas stream  112  may be contacted with the solvent  114   a,    114   b  in any manner or direction that is desired in the system  100 . 
         [0016]    The CO 2 -lean stream  114   a  and/or CO 2  semi-lean stream  114   b  are both ammonia-containing solvents that absorb CO 2  from the gas stream  112 . The solvent  114   a,    114   b  includes a low molarity ammonia having a molarity between about 0.5 molar to about 13 molar. In one embodiment, the molarity of the ammonia-containing solvent  114   a,    114   b  is between about 0.5 molar to about 6 molar in the absorber  110 . 
         [0017]    Using a solvent having low ammonia molarity reduces ammonia loss and the saturation concentration for CO 2  is increased and therefore, the ammonia-containing solvent can absorb more moles of CO 2  per mole of ammonia as compared to solvents having a higher molarity. In one embodiment, the ammonia-containing solvent  114   a  comprises a CO 2 -lean stream and the ammonia-containing solvent  114   b  comprises a CO 2  semi-lean stream. As further described below with respect to absorption of CO 2  from the flue gas stream and regeneration of the ammonia-containing solvent, the CO 2 -lean stream  114   a  comprises a regenerated solvent  149  that is cycled back to the absorber  110  after exiting a regenerator  144  wherein the regenerated solvent  149  is stripped of carbon dioxide. As further described below with reference to  FIG. 3 , the regenerated solvent  149 , and thus the CO 2 -lean stream  114   a,  exhibits lean CO 2  loading ranging from 0.00 to 0.45 moles of CO 2 /moles of ammonia. As further described below with reference to passing the flue gas stream to a precipitating means  132  and subsequently to a separation device  138 , the CO 2  semi-lean stream  114   b  comprises a stream  120   a  that is extracted from the separation device  138  and is cycled back to the absorber  110  without passing through the regenerator  144 . 
         [0018]    In one embodiment, the ammonia-containing solvent, CO 2 -lean stream  114   a  and/or CO 2  semi-lean stream  114   b,  also includes a catalyst in the form of an enzyme. In one embodiment, the enzyme is a metalloenzyme, such as, for example, carbonic anhydrase. In another embodiment, the catalyst is added to the ammonia-containing solvent, CO 2 -lean stream  114   a  and/or CO 2  semi-lean stream  114   b,  to increase the total loading of the solution and/or favor the formation of a bicarbonate salt precipitate. In one embodiment, the absorber  110  is operated at a temperature of about 45° C. or less. In another embodiment, the absorber  110  is operated in accordance with CAP, as described above, such that the temperature of the absorber  110  is about 20° C. It is contemplated that a top section  110   a  of the absorber  110  could be operated at a temperature of about 10° C. or less. 
         [0019]    As shown in  FIG. 1 , the gas stream  112  enters a bottom portion  116  of the absorber  110  and travels up a length L of the absorber  110  where it is contacted with the CO 2  semi-lean stream  114   b  in a first absorption section  118 . The contact between the CO 2  semi-lean stream  114   b  and the gas stream  112  forms a stream  120  that is rich in CO 2  and ammonia (NH 3 ), and a stream  122  containing reduced CO 2 . The stream  120  is removed from the absorber  110  and a portion  120   a  of the stream  120  is recycled to the absorber  110  via a feedback loop  124  and introduced to the absorber  110  as the CO 2  semi-lean stream  114   b,  while the remaining portion of the stream  120  is provided as CO 2 -enriched phase stream  140  to a regeneration system  126 . 
         [0020]    Meanwhile, in the absorber  110 , the reduced CO 2  containing gas stream  122  continues to a second absorption section  128 , where the reduced CO 2  containing gas stream  122  is contacted with CO 2 -lean stream  114   a.  In the second absorption section  128 , more CO 2  is absorbed from the gas stream to form a stream  129  that is substantially reduced in carbon dioxide content. In other embodiments, it is contemplated that the absorber  110  may have more than two absorption sections as illustrated. In yet another embodiment, the sections of the absorber  110  comprise more than one separate column or unit. 
         [0021]    The stream  129  may be processed through one or more wash sections  130  in the absorber  110  prior to being emitted from the absorber  110  at an outlet  119 . The molarity of ammonia present in the wash section  130  is between about 0 molar to about 3 molar. The stream  129  having a substantially reduced carbon dioxide content may be subjected to further processing in another portion of the system  100  or may be released to an environment. 
         [0022]    The stream  120  is rich in CO 2  and NH 3  as a result of the reaction between the CO 2  in the gas stream  112  and the CO 2 -lean stream  114   a  and/or CO 2  semi-lean stream  114   b  in the absorber  110 . In one embodiment, prior to being provided to the regeneration system  126 , a first pressure of the stream  120  is elevated via a first pump  121 A and subsequently the stream  120 , having an elevated pressure or a second pressure, is cooled via a chiller  123  and provided to a precipitating means  132 . In one embodiment, the precipitating means  132  is a crystallizer, which forms a precipitate  134 . The precipitate  134  may be, for example, an ammonium salt. This speciation results in an enthalpy of regeneration that is about 15% lower according to equation 1 and 2: 
         [0000]      CO 2 (g)+NH 3 (aq)+H 2 O=NH 4 HCO 3 (aq) ΔH=−64 kJ/mol CO 2    (1)
 
         [0000]      CO 2 (g)+2NH 3 +NH 2 CO 2 NH 4 (aq) ΔH=−74 kJ/mol CO 2    (2)
 
         [0023]    As noted, the precipitate  134  may be, for example, an ammonium salt, and more particularly ammonium bicarbonate, carbamate and/or carbonate. The speciation of the three ammonium salts, bicarbonate, ammonium carbamate and carbonate, in an ammonia solution depends on several variables. It has previously been demonstrated that in an ammonia solution with constant molarity, such as, for example, 1.34 mol/L, the speciation of bicarbonate will increase as the total of carbon content of the solution increases. A low molarity, highly-loaded ammonia solution favors the formation of ammonium bicarbonates over carbamates. It has further been demonstrated that at constant temperature, bicarbonate speciation decreases as a percent of total carbon anions in solution as ammonia molarity increases. Thus, at low ammonia molarity, a larger proportion of the ammonium bicarbonate salt precipitate  134  exists in the solution. Additionally, at low ammonia molarity, the desired capture rate of CO 2  can be achieved with appropriate sizing of the absorber  110 . 
         [0024]      FIG. 2  illustrates that CO 2  capture rates of 85% or above are achieved having an absorber length of 30 meters, shown generally by a first plot  410 , when the absorber  110  is run at a temperature of about 20° C. A 90% capture rate of CO 2  may be achieved upon the regulation of the temperature of the absorber  110  via a control system  136  in communication with the absorber  110  and the regeneration system  126  ( FIG. 1 ). In one embodiment and as further shown in  FIG. 1 , the control system  136  comprises a controller  137  in communication with a plurality of devices  90 - 99  for measuring and selectively adjusting a plurality of operating parameters such as, for example, temperature, pressure, volumetric flow rate, molarity and mass concentrations of each stream of system  100 . The devices  90 - 99  include, for example, sensors or other measurement devices, flow control valves, pumps and other flow control means. Such devices  90 - 99  are configured to transmit to, and receive from, the controller  137  one or more signals for operation of such devices, and the controller  137  is configured to receive and transmit multiple signals simultaneously, at elevated temperature ranges, and having a resistance to vibration, impact and electrical noise. While the control apparatus  136  is shown and described as comprising a controller  137 , the present invention is not limited in this regard as the control apparatus  136  may comprise, for example, a Programmable Logic Controller (“PLC”), a distributed control systems (“DCS”), a computer or any type of microprocessor or like programmable control device having software installed therein, a server connected to one or more programmable devices, or any like controller without departing from the broader aspects of the invention. As used herein, the term “computer” encompasses desktops, laptops, tablets, handheld mobile devices, mobile phones and the like. 
         [0025]    Referring to  FIG. 2 , the first plot  410  and a second plot  411  display the CO 2  capture rate in ammonia at 1.5M as a function of a length “L” of the absorber  110 . The X axis  400  shows the absorber height L in meters. The first Y axis  420  shows % CO 2  capture. A second Y axis  440  shows CO 2  concentration at the outlet  119 . The graph of  FIG. 2  illustrates the following data points for plots  410  and  411 , as summarized in Tables 1A and 1B below. Table 1A provides the CO 2  Capture rate as a function of packing height; and Table 1B provides the CO 2  outlet concentration as a function of packing height. The first plot  410  shows CO 2  capture approaching 80% at approximately 20m and approaching 90% at approximately 50 m. Conversely, a curve  411  shows the concentration of CO 2  at the outlet of the absorber  110 , which approaches 0.01 molar at approximately 40 m. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1A 
               
               
                   
               
               
                   
                 % CO 2  captured 
                 Absorber length L in meters 
               
               
                   
               
             
             
               
                   
                 50% 
                  2 m 
               
               
                   
                 60% 
                  8 m 
               
               
                   
                 72% 
                 10 m 
               
               
                   
                 80% 
                 15 m 
               
               
                   
                 88% 
                 40 m 
               
               
                   
                 90% 
                 78 m 
               
               
                   
                 90% 
                 100 m  
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1B 
               
               
                   
               
               
                   
                 CO 2  Outlet Concentration - Molar 
                 Absorber length L in meters 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0.07 
                  5 m 
               
               
                   
                 0.10 
                 10 m 
               
               
                   
                 0.11 
                 20 m 
               
               
                   
                 0.125 
                 40 m 
               
               
                   
                 0.125 
                 80 m 
               
               
                   
                 0.125 
                 100 m  
               
               
                   
               
             
          
         
       
     
         [0026]    Turning back to  FIG. 1 , after the precipitate  134  is formed, a stream  120   b  containing the precipitate  134  is provided to a separation device  138 . The separation device  138  may be any type or kind of device that is capable of separating solids (e.g., precipitate) from liquid (e.g. stream  120   b ), including but not limited to the separation device  138  being a cyclone. 
         [0027]    The separation device  138  separates the portion  120   a  from the stream  120   b  containing the precipitate  134 . The portion  120   a  of the stream  120  is recycled to the absorber  110  via the feedback loop  124 . The separation device  138  provides a CO 2 -enriched phase or stream  140  to the regeneration system  126 . In one embodiment, prior to being provided to the regeneration system  126 , a first pressure of the stream  140  is elevated via a second pump  121 B and subsequently the stream  140 , having an elevated pressure or a second pressure, is passed to the regeneration system  126 . The CO 2 -enriched phase, stream  140 , including the precipitate, is provided to the regeneration system  126 . In one embodiment as described above, the ammonia-containing solvent, CO 2 -lean stream  114   a and/or CO   2  semi-lean stream  114   b,  includes a catalyst in the form of an enzyme. As a result, the stream  120  that is removed from the absorber  110 , cooled via the chiller  123 , provided to the precipitating means  132 , and subsequently provided to the separation device  138  as the stream  120   b  containing the precipitate  134  correspondingly includes the catalyst. The separation device  138  separates the portion  120   a,  including the catalyst, from the stream  120   b  containing the precipitate  134 , and the portion  120   a,  including the catalyst, is recycled to the absorber  110  via the feedback loop  124 . 
         [0028]    As shown in  FIG. 1 , the CO 2 -enriched phase stream  140  is provided to a heat exchanger  142  prior to being provided to a regenerator  144 . In the regenerator  144 , the CO 2 -enriched phase stream  140  is stripped of carbon dioxide by breaking the chemical bond between the carbon dioxide and the solvent. The carbon dioxide is removed from the solvent by the introduction of heat to the regenerator  144 . A reboiler  150  is provided to further process a regenerated solvent  149  exiting the regenerator  144 . In one embodiment, the regenerator  144  is operated at a temperature of about 110° C. In another embodiment, the regenerator  144  operates at less than 100° C. It is contemplated that the temperature of the regenerator  144  and the reboiler  150  may be controlled via the control system  136 . 
         [0029]    Very high CO 2  loading is achievable when the ammonia-containing solvent  114   a,    114   b  has a molarity between about 0.5 molar and 6 molar and the precipitate  134  is formed. This results in a high partial pressure of CO 2  in the stream  140 , which enables regeneration to be conducted at a temperature as low as 115° C. In  FIG. 3 , a CO 2  rich loading of 0.8 molar has been reached and for a regeneration pressure of 2 bars, simulation shows that for a 90% CO 2  removal, temperatures as low as 115° C. are suitable to regenerate the solvent. As the temperature of the regenerator  144  decreases, it is understood that CO 2  removal will be improved during the regeneration process. 
         [0030]    Referring to  FIG. 3 , the plot shows the reboiler  150  temperature and lean solution CO 2  loading as a function of regenerator temperature when the ammonia concentration is at 1.5M and the rich loading is 0.8. The X axis  500  shows the pressure in the regenerator  144  ranging from 0 bars to 10 bars. The first Y axis  520  shows the reboiler  150  temperature in ° C. ranging from 100° C. to 140° C. The second Y axis  540  shows lean CO 2  loading in moles of CO 2 /moles of ammonia ranging from 0.00 to 0.45 [mole CO 2 /mole NH 3 ]. The graph of  FIG. 3  illustrates the following data points for a third plot  510  and a fourth plot  511 , as summarized in Tables 2A and 2B below. Table 2A provides a reboiler temperature as a function of regenerator temperature; and Table 2B provides a lean CO 2 loading as a function of regenerator temperature The third plot  510  illustrates the reboiler  150  temperature increases as pressure increases, wherein at approximately 2 bars, the reboiler  150  temperature is about 114° C. and at approximately 10 bars the reboiler  150  temperature is about 134° C. The fourth plot  511  illustrates the lean CO 2  loading increases as pressure increases, wherein at approximately 2 bar, the CO 2  loading is about 0.15 [mole CO 2 /mole NH 3 ] and at approximately 10 bars, the CO 2  loading is about 0.40 [mole CO 2 /mole NH 3 ]. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 2A 
               
               
                   
               
               
                   
                 Regenerator pressure (bars) 
                 Reboiler temperature 
               
               
                   
               
             
             
               
                   
                 1.5 bars   
                 102° C. 
               
               
                   
                 2.5 bars   
                 114° C. 
               
               
                   
                 4 bars 
                 124° C. 
               
               
                   
                 6 bars 
                 129° C. 
               
               
                   
                 9 bars 
                 133° C. 
               
               
                   
                 10 bars  
                 134° C. 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 2B 
               
               
                   
               
               
                   
                 Regenerator pressure (bars) 
                 Lean CO 2  loading 
               
               
                   
               
             
             
               
                   
                 1.5 bars   
                 0.13 mole CO 2 /mole NH 3   
               
               
                   
                 2.5 bars   
                 0.16 mole CO 2 /mole NH 3   
               
               
                   
                 4 bars 
                 0.23 mole CO 2 /mole NH 3   
               
               
                   
                 6 bars 
                 0.30 mole CO 2 /mole NH 3   
               
               
                   
                 9 bars 
                 0.37 mole CO 2 /mole NH 3   
               
               
                   
                 10 bars  
                 0.40 mole CO 2 /mole NH 3   
               
               
                   
               
             
          
         
       
     
         [0031]    Referring back to  FIG. 1 , after being stripped from the solvent, the carbon dioxide is released from the regenerator  144  as a stream of carbon dioxide  146 . In one embodiment, the stream of carbon dioxide  146  is sent to another section of the system  100  for further processing, storage or use, while the regenerated solvent  149  leaves the regenerator bottom via line  148 . At least a portion of the regenerated solvent is passed to the reboiler  150  via the line  148 . While not shown in  FIG. 1 , it is contemplated that the system  100  may include one or more pumps that facilitate the movement of the regenerated solvent  149  throughout the system. 
         [0032]    In the reboiler  150 , the regenerated solvent  149  is boiled to generate vapor  152 , which is returned to the regenerator  144  to drive separation of carbon dioxide from the solvent. In addition, reboiling of the regenerated solvent  149  may provide further carbon dioxide removal from the regenerated solvent  149 . 
         [0033]    The regenerated solvent  149  is passed to the heat exchanger  142  for heat-exchanging with the CO 2 -enriched phase stream  140 . Heat-exchanging allows for heat transfer between the solutions, resulting in a cooled regenerated solvent  149   a  and the heated CO 2 -enriched phase stream  140 . The regenerated solvent  149   a  is thereafter cycled to the next round of absorption in the absorber as the CO 2 -lean stream  114   a.  It is contemplated that the regenerated solvent  149   a  may be cooled via one or more chillers  141  prior to being introduced to the absorber  110 . In one embodiment, the regenerated solvent  149   a  is referred to as a first regenerated solvent  149   a;  and a second regenerated solvent  149   b  is extracted from the first regenerated solvent  149   a.  Thereafter, the second regenerated solvent  149   b  is cycled together with the portion  120   a  of the stream  120  to the absorber  110  via the feedback loop  124  as the CO 2  semi-lean stream  114   b.  In one embodiment, the controller  137  is configured to measure and selectively adjust the flow rate of at least one of the regenerated solvent  149   a  and the second regenerated solvent  149   b  to respectively adjust the CO 2 -content of CO 2 -lean stream  114   a and/or the CO   2  semi-lean stream  114   b.    
         [0034]    The foregoing system and method provides increased efficiency of carbon dioxide capture and lower ammonia emission. Utilization of a lower molarity ammonia-containing solvent permits increased carbon dioxide loading, which reduces the number of wash sections  130  utilized in the absorber  110 . Additionally, the utilization of a lower molarity solution will permit the regenerator  144  to operate at a lower pressure and lower temperature, which may contribute to efficiency and cost savings. These advantages, as well as others, are highlighted in the Examples included below: 
       EXAMPLES 
     Example 1 
     Comparison of Carbon Capture Utilizing Ammonia-Containing Solvents Having Different Molarities 
       [0035]    At low ammonia molarity, a larger proportion of bicarbonate is formed and more of the bicarbonate can be dissolved in the solution. Data from a comparison of two cases having 90% CO 2  capture with high and low molarity ammonia solvent is provided in Table 3 below. The loss in cyclic capacity associated with the lower concentration of ammonia is partially compensated by the higher CO 2  loading as shown in Table 3. In such a case, the ammonia concentration is reduced by a factor of 5.48, yet the circulation rate in the absorber correspondingly increased by a factor of 3.64. The low molarity and relatively higher lean loading of CO 2  led to a reduction of ammonia loss in the absorber by a factor of 6.3. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Case 1 
                 Case 2 
                 Ratio 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 NH 3  concentration (% wt) 
                 13.7 
                 2.5 
                 0.18 
               
               
                   
                 Circulation rate (kg/hr) 
                 2.20E+06 
                 8.00E+06 
                 3.64 
               
               
                   
                 Lean loading (mol/mol) 
                 0.3 
                 0.39 
                 1.30 
               
               
                   
                 Rich loading (mol/mol) 
                 0.57 
                 0.78 
                 1.37 
               
               
                   
                 Free ammonia (mol/L) 
                 5.6 
                 0.9 
                 0.16 
               
               
                   
               
             
          
         
       
     
       Example 2 
     Precipitation Formation 
       [0036]    At a higher ammonia concentration solution, the high CO 2  loaded solution starts precipitating earlier as compared to lower molarity solution. An Ammonium Bicarbonate Solubility Index (“SI”) at different NH 3  Molarity is provided in Table 4 below. The data in Table 4 confirms precipitation can occur at 0.46 loading at 10M solutions at 4.4° C. whereas 0.66 loading is needed for 4M solutions to precipitate at 4.4° C. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Temperature, C. 
                 NH 3  Molarity (M) 
                 CO 2  loading 
                 SI 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                   
                 4.4 
                 4 
                 0.66 
                 1 
               
               
                   
                   
                   
                 10 
                 0.46 
                 1 
               
               
                   
                   
               
             
          
         
       
     
       Example 3 
     Ammonia Emissions from an Absorber 
       [0037]    Operating the system  100  at a lower ammonia molarity may reduce ammonia emissions from an absorption section  118 ,  128  to a wash section  130 . In one embodiment, the absorber  110  may have three absorption sections. It has been demonstrated that as lean solution CO 2  loading increases, for example from about 0.23 to about 0.47, ammonia emissions decrease, for example approximately 35%. 
       Example 4 
     Energy Consumption 
       [0038]    The lower molarity ammonia-containing solution does not increase the amount of energy required to regenerate the solution  149  in the regenerator  144 , and reduces the energy consumption in the absorber  110  because of the lower ammonia emissions from the absorption section to the wash section. 
       Example 5 
     Overall Carbon Dioxide Capture Efficiency 
       [0039]    The carbon dioxide removal efficiency may be reduced when using lower molarity ammonia-containing solvents at the same operating conditions at the same absorber bed height as compared to systems utilizing higher molarity solvents. However, low molarity solvents permit the system to operate at a higher temperature. Operation of the system at a higher temperature may improve kinetics, which in turn increases carbon dioxide capture efficiency. Operation at a higher temperature will also reduce the load needed to cool the absorber  110 . 
         [0040]    The various embodiments of the present invention described herein above provide a system and method for precipitating ammonium salts as a means to separate a CO 2 -rich phase from a CO 2 -semi-lean phase and thus reduce the energy requirements during regeneration. Such a system and method uses lower molarity ammonia (0.5-6 molar) with minimized ammonia losses to the gas overheads (up to an order of magnitude lower) and provides significant cyclic capacity while maintaining adequate adsorption kinetics in the absorber. The process takes advantage of precipitating ammonium salts post absorber and in a controlled manner by cooling the CO 2 -rich solvent, therefore separating a CO 2 -and-ammonia-rich phase, which is sent to the regenerator for an energy-efficient regeneration. The semi-lean phase is recirculated to the absorber to be enriched again above its low-temperature saturation point. Ammonia plays a dual role of an accelerator of the CO 2  absorption reaction in the liquid phase as well as a precipitating agent for the separation of the CO 2 -rich phase from the CO 2 -semi-lean phase. Since both ammonia and CO 2  are concentrated in the CO 2 -rich phase, their partial pressures are higher and upon heating of the solution, regeneration of CO 2  at higher pressure can take place. Since ammonia is more volatile than water, significant regeneration can take place at relatively lower temperature (100° C. or below) and higher pressure. Therefore, the process allows for lower temperature, higher pressure CO 2  regeneration with the ammonia as a main stripping agent. The ammonia loss to the gas phase is proportional to the liquid phase free ammonia (non-reacted ammonia) concentration. At lower molarity, the saturation concentration for CO 2  is significantly increased and much higher CO 2 -rich loading can be reached. 
         [0041]    While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or matter to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.