Page 153 - Process Modelling and Simulation With Finite Element Methods

140 Process Modelling and Simulation with Finite Element Methods

recycled for reactor integration. Thus, “lumped variables” of unit operations
become distributed constraints for others. That FEMLAB can be called by
Simulink for greater detailed modelling of some unit operations is a feature that
allows better plant simulation. The commercial plant simulation packages, such
as AspenPlus and HYSYS, have implemented links to computational fluid
dynamics packages to improve detailed simulation of selected unit operations.
This trend will be come a flood, as it is less expensive and safer to simulate
“what if’ scenarios than to implement them on real plant. Examples of extended
multiphysics will make the concept clearer. We will start with a 1-D convection-
diffusion-heterogeneous reaction model for a fixed bed supported catalyst
system.

4.2 Heterogeneous Reaction in a Fixed Bed with Premixed Feed

Recently Mchedlov et al. [3] proposed a general lumped parameter model for
heterogeneous reaction in a dispersed phase. The model focuses on situations
where mass transfer is asymmetric, i.e. some species have greater mass
transfer coefficients with the dispersed phase than others. Any number of
physicochemical interactions could lead to this situation, but invariably it is in
only slow flows, as through porous media, where kinetic asymmetry can survive.
Turbulence usually leads to equal mass transfer coefficients for each species.
Consider the reaction

u+v+w (4.2)
which only occurs in the dispersed phase. The lumped parameter model gives
three convection-diffusion-mass transfer equations in the bulk phase, which for
steady operation read as:

(4.3)

Figure 4.2 Schematic of a fixed bed with reaction largely localized.