Amino Acid Salts Promoted Aqueous Ammonia Solutions for Post-Combustion Capture of CO_2

摘要

Aqueous ammonia (NH_3) is regarded as one of the most cost-effective solvents for carbon dioxide (CO_2) separation processes but suffers from low CO_2 absorption rates and high NH_3 vapour losses. It has been proved that amino acid salts are outstanding additives which can improve CO_2 absorption rate in NH_3 solution due to their high reaction rates with CO_2. In this study, we measured the overall mass transfer coefficients (K_G) of CO_2 in potassium salts of sarcosine, proline and taurine alone solutions and K_G of CO_2 and NH_3 losses in NH_3 solutions containing the above three amino acid salts using a wetted-wall column contactor at 288 K and CO_2 loading from 0.0 to 0.5 (mol CO_2/mol total amine). To understand the effect of added amino acid salts on the K_G of CO_2 and NH3 loss, we used the reaction schemes developed for the proline-NH_3-CO_2-water (PRO~--NH_3-CO_2-H_2O) system and sarcosine-NH_3-CO_2-water (SAR~--NH_3-CO_2-H_2O) to predict species distribution in CO_2-loaded NH_3, NH_3/PRO~- and NH3/SAR~- blended solutions and to explain our experimental observations. Addition of potassium salts of sarcosine, proline and taurine results in improvement of K_G of CO_2 absorption in NH_3 solutions, but at expense of an increase in NH_3 vapour losses. The improvement in mass transfer upon addition of amino acid salts is due to the faster reaction of CO_2 with amino acid salts than with NH3. A case study of SAR~- addition to aqueous NH_3 suggests that the greater loss of NH_3 upon addition of SAR~- can be ascribed to the decrease of solubility of CO_2 and NH_3 in the NH_3/SAR~- blended solution. The investigation of K_G of CO_2 and NH_3 vapour losses in NH_3 and other amines (PZ, 1-MPZ, DEA and MEA) blended solutions also proves the competition for CO_2 is one of the reasons for the increasing of NH_3 vapour losses.