Effect of Energy Balance Approximations on Simulation of Fixed-Bed Adsorption

The sensitivity of fixed-bed simulations to common energy balance approximations is examined for an adiabatic adsorption process. Local equilibrium theory is used to determine temperature rise and breakthrough behavior for water vapor adsorbed in a fixed bed of 4A zeolite molecular sieve. Cases are considered in which the adsorbed-phase heat capacity is varied between gas- and liquid-phase values, and the isosteric heat is allowed to vary with temperature and loading or is assigned a constant value. The differences in plateau temperature, partial pressure, and loading for the different approximations are small, but the impact on breakthrough behavior can be large. The most dramatic differences from the base case occur when the adsorbed-phase heat capacity is approximated by that of a liquid and the isosteric heat is a constant; these approximations lead to a largely overestimated breakthrough time.