Simulation And Optimization Of Elevated Temperature Pressure Swing Adsorption System For Pre-combustion CO₂Capture

The elevated temperature pressure swing adsorption (PSA) based on potassiumpromoted hydrotalcites solid sorbent is a promising technology for pre-combustion CO2capture in Integrated Gasification Combined Cycle (IGCC) power plant by taking theadvantages of high partial pressure of CO2in the syngas after water-gas shift reaction.And the sensible heat can be saved without cooling the gas down to ambienttemperature which is critical for traditional pressure swing adsorption units. As animportant technology route for pre-combustion CO2capture, the optimal design of PSAbed and system is also significantly crucial for improving the gas separation efficiencyand for lowering system costs based on high CO2adsorption capacity sorbent. Sinceexperimental studies on PSA unit are expensive, time-consuming and labor-intensive,quantitative mechanism models for the reactions and transport within the PSA beds anddynamic system simulation are essential for elevated temperature PSA technologydevelopment.An elementary reaction kinetic model consisting of three reversible reactionsrelated with Elovich equation is developed to predict the CO2adsorption kineticbehavior for potassium promoted hydrotalcites. A quasi two dimensional PSA bedmodule is developed by considering comprehensive coupling effects from mass,momentum and energy transport mechanisms, and mass transportation within pores,integrated with dynamic boundary condition. Thermo gravimetric analysis (TGA) and ahigh pressure adsorption apparatus are respectively used below atmospheric pressureand above atmospheric pressure and the results indicate that the modeling results agreewell the experimental results.Dynamic adsorption process of pilot-scale test is studied by PSA bed module. Theeffects of flow rate, pressure and temperature on breakthrough curves and CO2distribution are simulated for optimization of PSA system.Then the elevated temperature PSA system modeling framework isdeveloped by coupling some PSA bed modules according to certain operatingprocedures. Using this modeling framework, the effects of adsorption time, residence time, purge to feed ratio, working pressure and adsorbents capacity on PSA cycle gasseparation performance are studied based on a four-bed two-pressure-equalization PSAprocess for designing the pilot-scale test setup.To achieve over90%CO2capture ratio and high H2recovery, heavy CO2reflux isadded to the PSA process. When heavy CO2reflux ratio is0.4, both CO2capture ratioand H2recovery ratio reaches90%. Furthermore, pressure equalization is also veryimportant for high H2recovery ratio and CO2capture ratio. The effects of adsorptionbed number and pressure-equalization step in a cycle are studied based on a five-bedthree-pressure-equalization PSA process as comparison with the four-bedtwo-pressure-equalization PSA process. The results show that higher H2recovery ratiowith the same CO2capture ratio can be achieved by optimizing CO2reflux ratio orpurge time for the five-bed three-pressure-equalization PSA process. However morepressure equalization steps will limit the flexibility of operation and more beds willresult in complicated procedure, higher capital and operation cost. Both efficiency andcost should be taken into accout when designing the PSA system.