(666e) Multiscale Modelling of a Typical Industrial Oxidation Reactor for Terephthalic Acid Production Via a Hybrid Multizonal-CFD Approach

Detailed models of multiphase reactive systems are known to give realistic representation of a ?well-mixed? reactor, but face difficulty in description of multiple scales systems like industrial scale multiphase reactors. The governing physical and chemical phenomena act on different time and space scale and modelling these systems involve tight interactions between several simultaneous phenomena taking place like reaction kinetics, mass transfer, heat transfer and mixing. As a consequence, the characterisation of the complex interactions between the fluid dynamics and the other phenomena should take into account the double-scale nature of these interactions.

In this work, we propose a multiscale approach, the Multizonal/CFD (Computational Fluid Dynamics) approach that captures both the hydrodynamics and complex physical/chemical phenomena occurring in a process (e.g., population balances). A fine spatial resolution is preserved in the CFD model to describe the hydrodynamics and the geometry of the system; on the other hand, a coarser grid is adopted by collecting a set of CFD cells to form a ?well-mixed? and homogeneous compartment (or zone), within which a detailed set of modelling equations can be solved by a highly accurate discretisation scheme (e.g., Bezzo et al., 2004; Laakkonen et al., 2007). The Multizonal (MZ) model, which describes the whole process except hydrodynamics, is constituted by a small number of interconnected compartments, each of them representing a spatial region of the process equipment. Each zone exchanges information (mass and /or energy flows) with the adjacent zones, all containing the same set of equations. Therefore, this multi-scale approach yields to two different grids and two different subproblems, each solved by means of a specialised tool. The exchange of information between the two scales is obtained through aggregation and disaggregation procedures.

The capabilities of this general purpose MZ/CFD framework is demonstrated here by applying the technique to analyse performance of a continuous, baffled, three phase, agitated reactor for production of terephthalic acid from p-xylene. Oxidation of p-xylene is a highly exothermic reaction carried out in the presence of acetic acid. The reaction takes place in the liquid phase and is typically mass transfer limited. As terephthalic acid is sparingly soluble in the reaction medium, it crystallises out and the product is drained out of the reactor in the form of slurry. In addition to the desired reaction, undesirable intermediates and combustion products are also produced.

A zone model capturing all the above stated phenomena is implemented in the general purpose modelling tool gPROMS^® (Process Systems Enterprise Ltd.), describing the whole process through a set of equations, which considers the solid, liquid and gas phase. Mass transfer from the gas-phase to the liquid phase is modelled in a rigorous way by taking into account the effect of bubble sizes. Accordingly, a full bubble population balance is considered and solved. The crystal production and precipitation is directly derived from the reaction kinetics. The vapour-liquid equilibrium, rigorous energy balance and heat transfer are also implemented. Some control loops complete the system description.

The CFD model, implemented in the FLUENT^® (by Ansys, Inc.) environment, describes the hydrodynamics of the vessel solving only the momentum and total mass balances. An Eulerian-Eulerian approach is adopted to represent the gas and slurry phases. The slurry is described as a pseudo-homogeneous phase comprising both the liquid and solid. The gas phase is here represented in a simplified way assuming an average size diameter for the air bubbles. Turbulence is defined in terms of a standard k-ε model.

As this Multizonal modelling approach is computationally very efficient, the technique can be exploited to carry out process optimization studies like identification of the optimal reactor internal configuration to maximise process objective. The framework is applicable to other systems with strong interactions among several diverse phenomena, conserving high modularity, flexibility and efficiency.

References:

Bezzo, F., Macchietto, S. and Pantelides, C.C., 2004. A general methodology for hybrid multizonal/CFD models. Part I. Theoretical framework. Comput Chem Eng, 28:501-511.

Laakkonen M., Moilanen P., Alopaeus V. and Aittamaa J., 2007. Modelling of local bubble size distributions in agitated vessels. Chem Eng Sci, 62:721-740.