Assessment of the New Kinetically Limited LDF Model for Diffusion-Limited Separations by PSA

The new kinetically limited linear driving force (KLLDF) model was assessed against the traditional LDF model for predicting the behavior of diffusion-limited separation by pressure swing adsorption (PSA). Carbon molecular sieve MSC 3K-172 was utilized to kinetically separate 85 vol % methane (CH4) from 15 vol % carbon dioxide (CO2) using two different 1-bed 6-step PSA cycle schedules, A and B. They were identical except that A utilized an idle (I) step, while B instead utilized a light reflux (LR) step, with these steps having the same durations to keep the total cycle times the same. The cycle step sequences were feed pressurization (FP), feed (F), equalization down (EqD), countercurrent depressurization (CnD), LR (B), Eq up (EqU), and I (A). The Eq steps were bed-to-tank-to-bed. The LDF and KLLDF models and the mixed gas dual process Langmuir (DPL) model were executed in the dynamic adsorption process simulator (DAPS). The LDF mass transfer coefficients were measured independently, with the same ones used in both kinetic models, making the assessment predictive and fair. Eleven PSA experiments were carried out (5 with the LR cycle) at 295 K, and 345 and 101 kPa as the high and low pressures. Two cycle times (48 and 192 s) and six feed flow rates (2-15 SLPM) were evaluated, with corresponding changes in the FP and F step times, while the other steps shared two different step times. The PSA process performance results from the 11 PSA runs unequivocally showed that the KLLDF model provided nearly quantitative predictions of the experimental purities and recoveries of CH4 and CO2, respectively, in the light and heavy products. The LDF model provided only qualitative predictions of the trends with extremely poor performance predictions. The weakness of the LDF model was due to its driving force being based on the partial pressure of each component in the gas mixture. In contrast, the LDF driving force of the KLLDF model depends on the actual adsorbed phase loading of each component inside the pore structure instead of its partial pressure outside of it.