Study of a six-bed pressure swing adsorption process

A mathematical model of a six-bed, ten-step PSA operation is developed. The process cycle considered resembles an industrial hydrogen recovery process from the refinery fuel gas. Three hydrocarbon impurities (methane, ethane, and propane) are considered in the feed gas. The adsorbent used is activated carbon. The nonisothermal, bulk separation PSA model adopts the linear driving force approximation for particle uptake and the extended Langmuir isotherm to represent adsorption equilibrium. A transient pressure equation is incorporated to account for the dynamics in variable pressure steps. The model was verified using experimental results from a computer-controlled, laboratory-scale PSA unit. Considering the complexity of operation, comparison of various stream flow rates, concentrations, bed pressure and temperature profiles indicates that the model provides a sufficiently accurate prediction of the PSA performance. Parametric studies further show that its product purity declines relatively quickly with increasing cycle time and decreasing high operating pressure due to the breakthrough of a relatively sharp methane front. There is no real advantage in operating the PSA unit beyond a high operating pressure of about 18.0 bar.