High temperature carbon dioxide chemisorbents: Applications, characterization, and study of the chemical nature of chemisorbent surfaces

The separation of gas mixtures by adsorption on micro- and meso-porous solids is a key unit operation found on many plants in the chemical, petrochemical, environmental, pharmaceutical, and electronic gas industries. The capture and storage of CO2 from gas streams has received considerable attention in recent years due to concerns about the effect of increasing CO2 concentration in the atmosphere.;Three target areas where CO2 could be separated from industrial gas streams by using adsorbents have been identified and include (1) the capture of CO2 from H2 streams produced by the Water Gas Shifting (WGS) of synthesis gas produced by coal gasification, (2) the capture of CO 2 from H2 streams produced by Steam Methane Reforming (SMR), and (3) the capture of CO2 from industrial flue gases from power plants. In the first two cases, a CO2 adsorbent material is mixed with an appropriate reaction catalyst to invoke Le Chatelier's principle and shift the overall equilibrium of either WGS or SMR by removal of the CO 2 product from the reaction zone. An essentially pure stream of H 2 (dry basis) is produced at feed gas pressure. Thermal Swing Sorption Enhanced Reaction (TSSER) processes have been developed for this purpose and entails using a shell and tube style sorber reactor where the tubes are packed with the sorbent-catalyst mixture. The sorbed CO2 is periodically regenerated by counter currently purging the reactor with super heated steam at feed gas pressure but at a temperature higher than that of the feed step. The reactor is heated by cross-flow of a heating fluid in the shell side of the vessel.;The research carried out in our laboratories for the last five years has been focused on the characterization and practical application of two different chemisorbent materials that chemisorb CO2 at high temperatures. The materials are Na2O-promoted Al2O3 (150-450°C) and K2CO3-promoted hydrotalcite (400-590°C). The chemisorption process is reversible on these materials allowing for repeated cycling of the material. They both display the previously described requirements for SER applications as revealed by extensive characterization and have been used to propose processes for (1) direct production of fuel cell grade H 2 and compressed CO2 by-product by SE-WGS of synthesis gas, (2) direct production of COx-free H2 for supplying fuel to a Polymer Electrolyte Membrane for decentralized electricity production from pipeline natural gas, and (3) recovery of a pure, compressed CO 2 stream from the flue gas of industrial power plants.;An experimental sorption apparatus was constructed to measure the characteristics of CO2 sorption on these two materials using column breakthrough experiments. The sorption equilibrium was found to deviate from the simple Langmuir isotherm equation in the high pressure region, and a new analytical sorption model accounting for both Langmuirian chemisorption and a surface complexation reaction was developed. The new model was found to provide an excellent fit for the measured sorption isotherm data.;DRIFTS and Raman were used to compare the industrially produced sorbents with their unsupported counterparts, and samples of the Na2O Al 2O3 were synthesized to see how the surface changed with promotion. It was found that the industrially produced Na2O Al 2O3 was just short of monolayer coverage and had carboxylate groups on the surface that reacted with gas phase CO2 to form both bidentate and monodentate structures. The K2CO3 hydrotalcite was found to have past monolayer coverage of the promoter as evidenced by Raman bands indicative of crystalline K2CO3. In addition, this material had other surface bound K2CO3 groups that reacted with gas phase CO2 to form two similar but energetically different bidentate structures on the surface. (Abstract shortened by UMI.)...