Design and optimization of binary membrane-based separations

Rugved Prakash Pathare, Purdue University

Abstract

Membrane based gas separation processes have become increasingly popular in the last three decades with applications including air separation and natural gas purification. However, for most current applications, commercially available membranes have modest values of permselectivity. In such cases, there is an unfavorable trade-off between purity and recovery for single-stage separation and cascading is required. In this dissertation, the design and optimization of membrane cascades was systematically studied for binary gas separations. Existing concepts in cascade design theory were extended, and four main methods for cascade design were identified. These are: (1) The constant separation factor method (CSF), (2) The variable separation factor method (VSF) with mixing loss allowed, (3) The variable separation factor method with mixing loss not allowed and (4) Design using the Intermediate stage cascade arrangement. Through enumeration and simulation of all feasible cascade configurations of interest, we identified the globally optimal configuration with the minimum energy usage to produce a unit quantity of product for a given separation problem. Interestingly, the same configuration was identified by all four methods as the globally optimal solution. Moreover, the globally optimal solution is such that all of the stages in that configuration have similar operating parameters, regardless of which method is used. An important finding in this regard is that mixing losses due to recycle streams were also inherently absent in the global optimum. It is only when the number of stages are lowered below the optimal number that the inefficiency from mixing losses due to recycle streams becomes significant. In such cases cascades with intermediate stages tend to have the lowest specific energy demand. We also identified that no single configuration is always optimal over a wide range of separation requirements, and the optimal configuration depends on the level of desired purity and recovery. Moreover, we find that there are smooth transition points with respect to purity and recovery at which one configuration becomes more energy efficient than another. Through our comprehensive investigation into the principles behind cascade design, we provide a systematic approach for the design of the most energy-efficient membrane cascade for binary gas separations.

Degree

Ph.D.

Advisors

Agrawal, Purdue University.

Subject Area

Chemical engineering

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