Periodic behavior of pressure swing adsorption cycles and coadsorption of light and heavy n-alkanes on activated carbon.

摘要

Physical adsorption is used industrially to separate and purify gas mixtures. An adsorption bed is fed cyclically with a gas mixture and partly regenerated to remove the components that accumulate on the adsorbent. The correct design of an adsorption cycle requires that data on the thermodynamic equilibrium of the mixture between the adsorbed phase and the gas phase be known.; In the first part of this work, the coadsorption of low molecular weight n-alkanes of different sizes on BPL activated carbon is studied experimentally. These mixtures exhibit a deviation from ideality that is much more significant than expected based on their corresponding vapor-liquid equilibria. The data are compared with the predictions of the ideal adsorbed solution theory (IAST), which is a benchmark method for the prediction of multicomponent adsorption based on pure component adsorption data. IAST overestimates the partial pressures at all loadings. This means that the mixtures considered exhibit negative deviation from ideality, which can be attributed to adsorbent heterogeneity and steric factors. Three models are used to correlate the data. The rationale behind the choice of models is that they all treat pure component data separately from mixture data. These models show the correct deviation from ideality and in most cases provide an excellent fit to the data.; The second part of this work is an extensive study of the a priori prediction of the periodic behavior of several different pressure swing adsorption (PSA) cycles for gas separation and purification. The goal is to determine directly what asymptotic behavior the PSA cycle will have when used to separate or purify a gas mixture, given the parameters that characterize the interaction of the gas mixture with an adsorbent. Our feed mixture consists of one, two, three or more adsorbable components present in trace amounts in an inert gas, and the bed is regenerated incompletely with pure inert gas. The PSA cycles consist of a high pressure feed and a low pressure purge. The bed contains one or more layers of adsorbent. The assumption is made that equilibrium exists between the adsorbed phase and the gas phase at each cross section in the bed and that the bed is isothermal. Equilibrium between the gas phase and the adsorbed phase is described with Langmuir isotherms. The method of characteristics is used to solve the material balances for each adsorbate. The development shows how material balances on adsorbates can be used to predict the periodic behavior of cycles. Knowledge of the asymptotic dynamics of PSA cycles is used to perform optimization analyses to determine the best operating conditions for separation and purification processes.