Schalk Cloete

An Effective Reaction Rate Model for Gas-Solid Reactions with High Intra-Particle Diffusion Resistance

Abstract An approximate analytical expression for estimating the effectiveness factors of non-catalytic gas-solid reactions is proposed. The new expression is derived from the analytical solution for simple first order reactions (Ishida and Wen 1968. Comparison of kinetic and diffusional models for solid-gas reactions. AIChE Journal 14, 311–317. http://dx.doi.org/10.1002/aic.690140218). The scaled Thiele modulus concept is introduced to account for the variations of the reaction rate form that differs from the first order. The validity of the new expression is demonstrated for the reactions of different orders and of different forms via comparisons against a complete particle-reactor model using the collocation method for solving heat and mass fluxes inside the particles. In addition, the proposed approach is applied to redox reactions of ferric oxide where non-isothermal condition, net consumption of gaseous reactant, and parallel reactions are encountered. The results show that the effectiveness factor method compared well with the orthogonal collocation method over a wide range of Thiele moduli, reaction orders and reaction forms. Therefore, the proposed expression can serve as a generic replacement for more complex and computationally expensive combined particle-reactor modelling which is often employed in reactor systems with significant intra-particle diffusion resistances.

Design strategy for a chemical looping combustion system using process simulation and computational fluid dynamics

A strategy for design and optimisation of chemical processes involving multiple fluidised bed reactors is presented through a combination of standard design calculations, process simulation and Computational Fluid Dynamics (CFD). The strategy is demonstrated in designing a Chemical Looping Combustion (CLC) process that generates 12.5 kW of heat in the air reactor. The resulting design strategy will allow for very economical investigations into various design and optimisation considerations. It also offers a platform from which to conduct virtual prototyping investigations for new process concepts, which will lead to significant economic benefits when compared with a traditional experimental process development strategy.

Numerical Investigations to Quantify the Effect of Horizontal Membranes on the Performance of a Fluidized Bed Reactor

Abstract Reactive simulations of gas-solid flows occurring in fluidized beds with horizontal membrane insertion were carried out using computational fluid dynamics based on the kinetic theory of granular flows. The effect of altering the membrane arrangement on the overall reactor performance (degree of conversion achieved) was investigated by means of fractional factorial designs. When membranes served only as hydrodynamic modifiers to the flow, it was found that significant improvements could be obtained by optimising the membrane arrangement. A slightly larger improvement could be attained by injecting some of the reacting gas through the membranes. When compared to the improvements that can be attained by simply scaling up the reactor height and especially the reactor width, however, these improvements were of less significance. The use of membranes solely for altering reactor hydrodynamics by serving as obstructions and gas injection points can therefore not be merited. Further optimisation studies into membrane arrangement are therefore only recommended for specific processes in which the membranes play a central role by extracting some of the process gasses.

Closure Development for Multi-Scale Fluidized Bed Reactor Models: A Case Study

Abstract Chemical looping reforming (CLR) processes offer textbook examples for challenges in chemical engineering with respect to transport limitations. Phenomena that potentially need to be considered in a rigorous reactor model include (i) diffusion in solids and nanometer-scale pores, (ii) heat and mass transfer between suspended particles and the ambient gas, (iii) meso-scale phenomena such as clustering, as well as (iv) large-scale phenomena such as particle and gas-phase dispersion in the reactor’s axial direction. Here we summarize key scientific advances made in the “NanoSim” project, which established a computational platform that enables modelling a large range of these phenomena. Specifically, we show that already at the particle scale significant uncertainties are introduced when modelling chemical reactors in very detail. This is due to the nature of gas-particle flow, i.e., the spontaneous formation of heterogeneities (i.e., so-called meso-scale structures) that impact flow, species transport and reactions. The key finding is that these heterogeneities must be accounted for in typical CLR applications to correctly predict reaction rates in an industrial-scale reactor.