Piecewise linear model predictive control of a rapid pressure swing adsorption system

Abstract

Rapid pressure swing adsorption (RPSA) is a gas separation technology used in the small-scale oxygen concentrator devices. These devices are commonly used to produce high purity (~90%) oxygen from air for oxygen rehabilitation therapy, but can also produce a much wider range of oxygen purities for other applications. RPSA is a complex, cyclic, nonlinear switched logic process resulting from the coupling of gas adsorption, heat transfer, flow reversal effects, and process logic switches. For RPSA devices to operate satisfactorily, feedback control is critical but challenging due to their inherent complexity. In this article, we present a piecewise linear model predictive control framework for operation and control of a single-bed RPSA system. A set of coupled, nonlinear partial differential equations with flow switching conditions is used as a plant model for the RPSA process. Subspace system identification with pseudo-random binary sequence signals applied to this plant model at multiple operating points is used to generate a family of piecewise linear models for use in the model predictive controller algorithm. Detailed descriptions of the RPSA plant model, the multiple linear model identification procedure, the controller formulation and model switching logic are presented. The closed-loop system is evaluated in simulation using several realistic set point tracking and disturbance rejection cases.

Correlation Analysis between Working Conditions and the Performance of a Small-Scale Pressure Swing Adsorption Oxygen Production Process

“…A Skarstorm-like PSA cycle consisting of four steps is employed, which is simple but widely used for small-scale air separation processes. − The cycle consists of (i) co-current pressurization (PR) with the feed gas, (ii) selective adsorption (AD) of N 2 from a compressed air stream to produce oxygen-enriched gas, (iii) counter-current blowdown (BD) of the bed to atmospheric pressure, and (iv) purge (PU) with the product gas near the atmospheric pressure level. The O 2 -enriched product gases from the AD step (ii) are collected, and the N 2 -enriched effluent gases from the BD and PU steps are discarded.…”

“…Pressure swing adsorption (PSA) has become a widely used unit of operation for gas separation and purification, especially in carbon dioxide capture − and air separation. − One of the commercially successful applications of PSA air separation technology is direct ∼93% O 2 production from compressed air for medical use. − A common practice is to use small-diameter adsorbent particles and optimize working conditions to enhance the oxygen production performance of a PSA system. − Working conditions determine the efficiency of a PSA system employing a given adsorbent. An outstanding example of the significance of appropriate working conditions is that the reduction of cycle time could significantly decrease the bed size factor (BSF, kg of adsorbent in the adsorption unit per ton of product per day production rate (TPD)), , which is another expression of adsorbent utilization and is inversely proportional to the productivity.…”