428105 Development and Analysis of System-Wide Model for Heterogeneous Catalytic Reactors: A Hierarchical Approach

Wednesday, November 11, 2015: 4:23 PM
250B (Salt Palace Convention Center)
Moulish Kommu and Niket S. Kaisare, Department of Chemical Engineering, Indian Institute of Technology-Madras, Chennai, India

Heterogeneous catalytic reactors form a large majority of industrial reactors. Catalytic tubular reactors are common in energy and environment applications, where the catalyst is usually coated on the inner surface of the channels. Catalytic monoliths are commonly used as catalytic convertors to reduce hydrocarbon, CO and NOx vehicular emissions, as well as in catalytically stabilized combustion. Catalytic microreactors are being used as thermally integrated reactors for fuel processing and energy applications. With a growing focus on renewable fuels, solid catalyzed systems are being investigated for oil to biodiesel conversion.

Often, the overall system for these applications may consist of multiple reactors in series. The processes are characterized by short time scale, and transients become important. The transport processes include convection of the reacting fluid in the channel, diffusion of the reactants to the catalytic wall, diffusion of the reaction products back to the bulk, heat transfer between the fluid and the wall, and conduction through the solid. The small channel diameters in catalytic monoliths as well as microreactors ensure high catalytic surface area and fast transverse transport. These properties and our extensive knowledge regarding tubular catalytic reactors can be exploited to design efficient catalytic reactors for fuel conversion.

The reactor model is described by partial differential equations in two (or three) spatial dimensions. The complete model is elliptic in spatial domain. Often, computational fluid dynamics (CFD) coupled with appropriate reaction kinetics are used to simulate these systems.  Solving such a model is computationally challenging. Hence, several approximations are made the make the problem computationally tractable. For example, a plug flow reactor assumption may be employed for transient simulation at a system level, wherein we assume complete transvers mixing, no axial dispersion and "homogenized" reaction kinetics at bulk fluid conditions. Between these two examples lie other approximations such as transverse transport using Nusselt and Sherwood numbers, boundary layer approximation, combined 1-D and 2-D models, etc.

The objective of this contribution is two-fold. First, we investigate various approximations of the 2D Navier-Stokes simulations to develop a hierarchy of models of decreasing computational complexity. The second aim is to demonstrate the key features of biofuel convertor, and hence provide efficient design and operation strategies thereof. Significant literature exists on various approximations and analysis of heat and mass transfer in catalytic monolith channels. While catalytic convertor and micro-burners are moderate to high temperature gas phase reactors, a biofuel reactor is an example of low tempreature liquid phase reactor. We extend our earlier work on catalytic microreactors to provide a critical analysis of the various assumptions and features of fuel conversion reactors of practical interest.

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