Reversible gas adsorption in coated wall flow tube reactors. Model simulations for Langmuir kinetics

A kinetic model has been developed to simulate reversible gas adsorption in coated wall flow tube reactors (CWFTs). The motivation of this work is to provide the theoretical framework for modelling studies in support of the results obtained from CWFT studies on the adsorption and desorption behaviour of atmospheric trace gases on ice surfaces at temperatures relevant to the upper troposphere/lower stratosphere (190-230K). The model consists of an axial sequence of individual flow tube sections of equal volumes in which the gas phase is homogeneously mixed and interactions with the coated wall occur by adsoption and desorption exclusively. The adsorption rate is assumed to be kinetically controlled and not to be transport limited. Moreover, chemical reactions are not considered. Simulations have been performed for the temporal behaviour of the gas phase concentration at the exit of the flow tube as a function of laboratory time for typical operation procedures of CWFTs with moveable injectors i.e. (i) instantaneous injection of the gas, (ii) instantaneous termination of the gas flow and (iii) movements of the injector with constant velocities. It is found that the temporal profiles show complex behaviours due to the overlap of adsorption and desorption upon successive exposure of the gas to different length of the ice surface. The validity of the model is demonstrated for the adsorption of acetone on an ice surface at 200 K as a case study.