Thermodynamic Analysis for the Absorption-Enhanced-Water-Gas-Shift Reaction (Aewgs) for the Production of Hydrogen

Miguel Escobedo Bretado, Virginia Collins-Martínez, and Alejandro Lopez-Ortiz. Chemistry of Materials, CIMAV, Miguel de Cervantes 120, Chihuahua, Mexico

A novel process concept for the production of high purity hydrogen in a single step is introduced consisting of a combination of the water gas shift (WGS) and CO2 solid capture reactions (Absorption Enhanced Water Gas Shift, AEWGS). Thermodynamic analysis for the chemical equilibrium of this reaction system was performed using HSC Chemistry thermodynamic software (Gibbs free energy minimization technique) to select proper CO2 acceptor to be used in such a process. Three different CO2 acceptors were examined for this purpose; calcined dolomite (CaO*MgO), lithium orthosilicate (Li4SiO4) and sodium zirconate (Na2ZrO3). Calculated dry-basis hydrogen concentrations were: 98, 81 and 95%, respectively. However, the combination of high regeneration temperature (900ºC) and moderate number of carbonation/regeneration cycles results in dolomite progressive deactivation. Recent studies in our laboratory indicate that Na2ZrO3 is able to operate with higher thermal stability during multicycle operation. Therefore, the present study was concentrated in the use Na2ZrO3 as a preferred absorbent for the proposed AEWGS process. Variation of the CO/Na2ZrO3/H2O feed molar ratio produced a maximum hydrogen concentration of 97% mol using 1/1/2 and 500ºC at atmospheric conditions. The use of a catalyst for such process may not be needed, since the high temperature at which these reactions proceed may insure homogeneous non-catalyzed reactions. However, if reaction kinetics of the combination of WGS and carbonation reactions are not fast enough to reach equilibrium, a new non-conventional WGS catalyst may be the needed.