A Process Dynamic Modeling and Control Framework for Performance Characterization and Enhancement of Pd-Based Membrane Reactors

The syngas produced by coal gasification processes can be utilized in Pd-based water gas shift membrane reactors for the production of pure H₂.  Pd/Alloy composite membrane reactors exhibit comparative advantages over traditional packed bed reactors such as simultaneous reaction/separation in one compact unit and increased reaction yields. Within the above context, the present research introduces a comprehensive dynamic process modeling and analysis framework, as well as various process control strategies to further enhance membrane reactor performance resulting in a substantially smaller and functional, safer, environmentally friendlier and more energy efficient process.

In particular, a dynamic modeling framework has been developed mathematically realized by a system of unsteady state material balance equations [1] for both reaction and permeation sides of the membrane reactor in order to analyze and characterize the behavior of a catalytic high temperature water-gas shift Pd-based membrane reactor. In the proposed modeling approach, the original partial differential equations describing the various mass balances were discretized by partitioning the process spatial domain into N sections, for which a well-mixed continuous-stirred tank reactor model is postulated comprised of a system of ordinary differential equations appropriately written as mass balance equations for each component/species [2]. The above modeling approach is particularly suitable and necessary to ensure and facilitate the physical realizability of the proposed process control system. Indeed, proportional integral (PI) controllers were designed and properly tuned in order to induce desirable performance characteristics in the operation of the membrane reactor.  Detailed simulation results showed that the membrane reactor reached steady state in 10 s with a CO conversion of 97% exceeding the traditional packed bed conversion levels by ˜18%. It should be noted that the reactor's exit stream consisted of 1.9% CO, 72.6% CO₂ and 25.5% H₂ (dry basis) after the implementation of the proper process control strategy.

Two types of feedback control strategies, the servo and regulator, were evaluated [3]. In the servo case, the desired response of the process to a set-point change was enforced by the controller when the CO fraction at the retentate side decreased by 50%. On the other hand, in the regulator case, the total process pressure viewed as a disturbance decreased from 15 to 10 atm unexpectedly. The adverse effect of the decreased total pressure on the overall CO conversion was eliminated with the aid of a PI controller while the CO conversion remained at 97.1 %. Finally, the possibilities of attaining not only extra pure H₂ production objectives, but also CO₂ under high pressure, thus making it amenable to sequestration, were also demonstrated in the context of the present study.

[1] Marcano, J. G. S and Tsotsis, T. T. , “Catalytic Membranes and Membrane Reactors”, Weinheim : Wiley-VCH, c2002.

[2] Reyes F. and Luyben W. L., Ind. Eng. Chem. Res., 39 (2000) 3335 – 3346.

[3]  Stephanopoulous G., Chemical Process Control, Prentice Hall, Inc, 1984.