Studies On Novel Four-Bed Pressure Swing Adsorption Process For Hydrogen Purification

The research progress in hydrogen purification and recovery by pressure swing adsorption (PSA) technology was reviewed. The applications of a novel PSA process under low adsorption pressure were studied experimentally and by computer simulation. A comparison with the conventional PSA process presented.The adsorption isotherms of methane, nitrogen, and hydrogen on ordinary activated carbon (OAC), super activated carbon (SAC), and 5A zeolite (ZMS) were respectively measured by volumetric method in the range of 283K~313K and 0~1.0 MPa. Isosteric heats of adsorption were calculated from these isotherms basing on Clausius-Claperyron equation. The parameters were obtained by fitting the experimental data to a modified Langmuir equation.The experimental setup for adsorption dynamics was established, and the breakthrough experiments were performed under different adsorption pressures and flowrates with activated carbon and 5A zeolite two-sorbent adsorption bed. The breakthrough time of nitrogen and methane in the raw gases could be determined from the breakthrough curves under corresponding conditions. The longer breakthrough time and larger temperature variation in the adsorption bed were attributed to the higher adsorption pressure. When the feed gas flow rates were increased, the breakthrough time of nitrogen and methane were shortened and their difference was reduced. Although the amount of nitrogen and methane adsorbed per gram on SAC is larger than that of OAC, the adsorption capacity of OAC+ZMS and SAC+ZMS layered adsorption bed was quite equivalent, because the density of SAC was smaller than that of OAC.The novel PSA process was used to purify the hydrogen from the mixture of hydrogen, nitrogen, and methane, which was used to simulate the refinery dry gas. In the novel PSA process, two buffer tanks were utilized for depressurization and represssurization, and the operation flexibility of the four-bed PSA process was improved as a consequence.It is found that the novel process could recover hydrogen of purity 99.99% by the moderate recovery under low adsorption pressure. The performance of SAC+ZMS layered adsorption bed was worse than that of the OAC+ZMS layered bed under 1.0 and 0.8 MPa adsorption pressure, but it showed better performance under the adsorption pressure of 0.6 and 0.4 MPa. From the results of two-bed PSA experiments, it is concluded that product purity and recovery could be improved by increasing the volume of the buffer tanks.The breakthrough model comprised a set of algebraic and partial differential equations (PDE's), representing mass, energy, and momentum balances. In the solution scheme, the PDE'swere discretized in the spatial dimension, based on numerical method of lines, to generate a set of time derivative ordinary differential equations (ODE's), and the ODE's were solved using the subroutine DASSL. It was shown that this method was convenient, fast, and stable. The LDF model parameters of hydrogen, nitrogen, and methane on OAC, SAC, and ZMS, were obtained by fitting the experimental data and simulation results, respectively.A numerical simulation model was developed with all the essential features of this novel H2-PSA process considered, which was used to predict the process performance. It is revealed that the product purity, recovery, and the temperature profile along the adsorption bed reached steady state after 30 cycles. The model agreed well with the experimental data. The simulation results indicated that conventional PSA process may have higher product recovery and lower purity than the novel PSA process. Under 1.0 MPa adsorption pressure and the hydrogen purity of 99.999%, the productivity of novel H2-PSA process was 0.43 times higher than the conventional one. It implied that the novel H2-PSA process took great advantage over the common PSA process under lower adsorption pressure, and was suitable for hydrogen recovery from poor-quality and low-pressure gas mixture.