(579g) Modeling and Nonlinear Operability Analysis of a Membrane Reactor for Direct Methane Aromatization

Authors: 
Carrasco, J. C., West Virginia University
Lima, F. V., West Virginia University
Liu, D., University of Maryland

This presentation addresses the computation of feasible regions for the design and control of the direct methane aromatization (DMA) conversion process of natural gas to fuels and chemicals as an alternative to petroleum. In this case, we focus on the equilibrium-limited production of H2 and C6H6 from CH4 using a catalytic membrane reactor. Membrane reactors (MRs) are emerging systems that enable process intensification by combining two unit operations (reaction and separation) in one process, resulting in higher efficiencies and lower costs. MRs allow higher conversions than conventional packed-bed reactors due to the reaction equilibrium shift towards the products caused by the selective removal of one of these products through the membrane1. Specifically, in the DMA process, the reaction equilibrium is shifted towards products as H2 is removed through a H2-selective membrane (ion transport-based). Restrictions on process target specifications, such as reaction conversions and stream purities, as well as multiple challenges associated with controlling temperatures and compositions, make the problem of designing and controlling MRs for the DMA process a challenging area of research. In this project, the design of MRs employing process operability tools2 enables the mapping of the feasible regions between process input and output variables using a developed MR model. The calculated output constraints can also be used for the design of model predictive controllers3.   

The MR simulation set up considers a shell and tube reactor, in which the CH4 feeds the reaction side (tube packed with catalysts) and the sweep gas (He) flows in the permeation side (shell) assuming cocurrent and countercurrent flow configurations. The H2 produced in the tube permeates to the shell through the membrane layer that is placed on the surface of the tube wall. The produced outlet streams from the tube (retentate) and shell (permeate) are rich in C6H6 and H2, respectively. For the modeling studies, a nonlinear model that characterizes the 2-step reaction mechanism for the non-oxidative conversion of methane is developed, including the calculation of thermodynamics and kinetic properties4. Also, the membrane flux expression assumes a relationship proportional to the transmembrane partial pressure gradient of the species with a ¼ dependence5. Using the developed model, a process operability mapping is then obtained considering the input and output process operating regions. In particular, for the operability calculations, the variables that define the available input set (AIS) for the reactor are the diameter, length, temperature, and pressure; and for the membrane are selectivity and permeance. Based on the defined input specifications, the operability calculations provide an achievable output set (AOS) for the following performance variables: methane conversion, benzene production rate, hydrogen permeation through the membrane, selectivity of benzene/hydrogen, and concentration of the undesired intermediate, ethylene, in the retentate.  The obtained preliminary results demonstrate the capabilities of the nonlinear operability approach to provide guidelines for experimental research. These guidelines include the set of operating conditions for process control and specifications that the design variables associated with the reactor and membrane should have in the laboratory so that desired process output targets can be achieved.


References

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2. Lima F.V., Georgakis C. Input-output operability of control systems: the steady-state case. J. Proc. Cont. 2010, 20, (6), 769-776.

3. Lima F.V., Georgakis C., Smith J.F., Schnelle P.D., Vinson D.R. Operability-Based Determination of Feasible Control Constraints for Several High-Dimensional Nonsquare Industrial Processes. AIChE Journal 2010, 56, (5), 1249-1261.

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5. Li J.L., Yoon H.S., Wachsman E.D. Hydrogen permeation through thin supported SrCe0.7Zr0.2Eu0.1O3-δ membranes; dependence of flux on defect equilibria and operating conditions. J. Membrane Sci. 2011, 381, (1-2), 126-131.