Pressure effects in adsorbers and adsorptive reactors

Large transients and oscillations in pressure and flow rate were predicted by an adiabatic equilibrium-staged model when a concentrated strongly adsorbed propane feed was loaded onto zeolite 5A adsorbent initially presaturated with a nonadsorbed component. Oscillations were directly linked to the concentration front of the adsorbable component. When compared to published isothermal results, the adiabatic model produced transients of decreased magnitude in pressure and flow rate, and oscillations were damped. For the adiabatic case, initial breakthrough occurred at slightly earlier times. A parametric study confirmed that a strong finite sink for the adsorbable component and a highly concentrated amount of adsorbable component must be present for oscillations to occur within a staged model. Previous experimental work showed oscillations for columns connected in series with each column behaving as a stage. Adsorbate heat capacity, assumed to be a saturated liquid phase, was shown to be an important energy balance parameter storing approximately 12% of adsorption and compression energy. This term should be included in nonisothermal dynamic models. Kinetic energy and compression effects were determined to be negligible compared to other energy terms. The model was extended to study adiabatic adsorptive reactors. For the reverse water-gas shift reaction, enhanced conversion was confirmed. Pressure transients and oscillations did not occur with strong adsorption since a low concentration of adsorbable species was produced. When the feed contained large amounts of adsorbable component, water, oscillations in pressure and flow rate occurred which produced oscillations in product concentrations and reaction rates. This simulation represents unusual conditions for this reaction, but certainly feasible operating conditions for other reactions. Increasing the reaction equilibrium rate constant, which represented new reaction conditions, resulted in a very high conversion with a high purity nonadsorbable product. Enhanced conversion and separation at equivalent energy consumption was confirmed for adsorptive reaction under some but not all conditions.