Non-isothermal numerical simulations of dual reflux pressure swing adsorption cycles for separating N2 + CH4

Abstract

A non-isothermal model of dual reflux pressure swing adsorption (DR-PSA) was developed using a commercially available software package to numerically solve the dynamics of the unit operation. Importantly the model includes a full energy balance, which is a feature not reported previously in the literature for simulations of DR-PSA cycles even though bed temperature swings of 10–20K have been observed in experimental studies. The simulation allowed solution of the pressure-flow network for cycles in the PL-A configuration, where feed gas enters the middle of the low pressure column and where pressure inversion is conducted by transferring gas rich in the more adsorbed component between beds. At cyclic steady state the simulations contained material balance errors comparable in magnitude to those reported previously for isothermal DR-PSA simulations; however, these were accounted for using a robust correction scheme. Predictions of the pressure, flow and temperature profiles within the beds for a range N2+CH4 mixtures being separated using activated carbon were in good agreement with the corresponding results of 24 DR-PSA experiments recently reported by Saleman et al. (2015). The root mean square deviation of the predicted methane mole fractions from the experimental values were 0.003 and 0.024 for the light (N2-rich) and heavy (CH4-rich) product streams, respectively. Parametric studies conducted with the model show how the cycle design can be optimised with respect to reflux flow and the feed-purge step duration, and also illustrate the need for reliable values of the sorption mass transfer coefficient.