(143e) Design and Control of Integrated Methyl Cyclohexane Dehydrogenation / Nitrobenzene Hydrogenation Plant

Direct coupling of endothermic and exothermic reactions leads to improved thermal efficiency and, for reversible reactions, to increased equilibrium conversion and reaction rate due to equilibrium displacement (Towler and Lynn, 1994). As a result, energy savings and reduced reactor size can be achieved. However, while enhancing reactor performance, coupling endothermic and exothermic reactions in a single unit may require additional separation units and recycles. In practice, energy savings and reduced reactor investments must outweigh the cost of required additional units. Furthermore, it should be remarked that recycle of unconverted reactants can lead to undesired nonlinear phenomena in plantwide systems, with important implications for design and control (Bildea and Dimian, 2003). The research in this field has been mainly focused on the efficient design and analysis of stand-alone reactor units, with the coupling of ethylbenzene dehydrogenation with nitrobenzene hydrogenation as a frequent example demonstrating the effectiveness of the idea (Qin et al, 2003, Abo-Ghander et al, 2008). To assess the feasibility of coupling endothermic and exothermic reactions, operational and control difficulties arising from the more complex behaviour should be taken into account. However, recent studies (Altimari and Bildea, 2008) addressed the problem of integrated design and control of plantwide systems coupling endothermic and exothermic reactions A →P + H (endo) and B + H → R (exo). Hypothetical processes were considered. For several flowsheets and different plantwide control strategies, multiple steady states were invariably detected. Singularity theory was exploited to divide the space of reactor-design parameters into regions characterized by qualitatively-different solution diagrams, implications of the observed behaviour on plant controllability being thoroughly discussed. It was shown that, irrespective of the control structure, state multiplicity cannot be removed if all the reactants (A, H and B) are recycled. In this contribution the theoretical findings are applied to the design and control of a plant coupling methyl cyclohexane (MCH) dehydrogenation and nitrobenzene (NB) hydrogenation for simultaneous production of toluene (T) and aniline (AN), according to the following reactions:

MCH = T + 3 H2 NB + 3 H2 = AN + 2 H2O

In order to prevent catalyst deactivation, a large recycle of hydrogen is required. Therefore, undesired nonlinear phenomena are avoided by achieving high conversion of MCH and thus avoid its recycle. Compared to the classical processes, the main advantages are the reduced sensitivity compared to the stand-alone NB hydrogenation reactor, the reduced energy consumption and the possibility of adiabatic operation. After plant design, a rigorous dynamic model is developed using AspenDynamics. A plantwide control structure is implemented and shown to be able to achieve large production rate changes and to effectively reject various disturbances. In conclusion, this paper illustrates the operational difficulties arising from simultaneously performing exothermic and endothermic reactions, and demonstrates that the integrated plant can be built and safely operated by integrating the design and plantwide control issues.

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Towler G, S. Lynn (1994). Novel applications of reaction coupling: use of carbon dioxide to shift the equilibrium of dehydrogenation reactions. Chem. Eng. Sci., 49, 2585.

Bildea C.S., A.C. Dimian (2003). Fixing flow rates in recycle systems: Luyben's rule revisited. Ind. Eng. Chem. Res., 42, 4578.

Altimari, P., C.S. Bildea (2008), Coupling exothermic and endothermic reactions in plug-flow reactor ? separation ? recycle systems, Ind. Eng. Chem. Res., 47, 6685.

Qin Z, J. Liu, A. Sun, J. Wang (2003). Reaction coupling in the new processes for producing styrene from ethylbenzene, Ind. Eng. Chem. Res., 42, 1329.