(696c) 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.