A cyclic adsorption process simulator was used to determine preliminary bed size and performance characteristics for a sorbent-based atmosphere revitalization (SBAR) system being designed for NASA for the Crew Exploration Vehicle. An initial study of a 2-bed 3-step 3-layer, vacuum swing adsorption cycle, utilizing 50% silica gel, 17% 13X zeolite, and 33% 5A zeolite revealed that a 10 L bed could easily meet the CO2 and H2O removal criteria for a 3 person crew. A parametric study showed that the cycle time, layering percentage of silica gel, and H2O-silica gel mass transfer coefficient were important parameters in the SBAR design. Increasing the cycle time diminished the CO2 and H2O removal performances but resulted in less O2 lost to space. The CO2 and H2O removal performances increased considerably when a silica gel layer was added to the bed, with the above layering percentages being close to optimum. As more silica gel was added to the bed slightly more O2 was lost. Larger values of the H2O-silica gel mass transfer coefficient improved the CO2 and H2O removal performances but at the expense of slightly more O2 lost. Novel features of this layered bed PSA cycle include vacuum regeneration with no purge, which is quite challenging when considering that both CO2 and H2O adsorb very strongly to 13X and 5A zeolites. Also, the design must meet strict requirements on bed volume and oxygen loss while meeting the CO2 and H2O removal criteria. To minimize oxygen loss, an equalization step is being considered, and to minimize volume novel bed configurations that foster isothermal operation are being devised. Results from these studies will be presented, along with any new results obtained for this SBAR system being designed for NASA for the Crew Exploration Vehicle.