Reactor design fundamentals heat transfer

Volumes 1, 2 and 3 form an integrated series with the fundamentals of fluid flow, heat transfer and mass transfer in the first volume, the physical operations of chemical engineering in this, the second volume, and in the third volume, the basis of chemical and biochemical reactor design, some of the physical operations which are now gaining in importance and the underlying theory of both process control and computation. The solutions to the problems listed in Volumes 1 and 2 are now available as Volumes 4 and 5 respectively. Furthermore, an additional volume in the series is in course of preparation and will provide an introduction to chemical engineering design and indicate how the principles enunciated in the earlier volumes can be translated into chemical plant. 

A fundamental aspect in the reactor design is the contribution of different thermal resistances in achieving a highly efficient heat transfer. The overall heat coefficient U is given by the relation ... 

However, each set of factors entering in to the rate expression is also a potential source of scaleup error. For this, and other reasons, a fundamental requirement when scaling a process is that the model and prototype be similar to each other with respect to reactor type and design. For example, a cleaning process model of a continuous-stirred tank reactor (CSTR) cannot be scaled to a prototype with a tubular reactor design. Process conditions such as fluid flow and heat and mass transfer are totally different for the two types of reactors. However, results from rate-of-reaction experiments using a batch reactor can be used to design either a CSTR or a tubular reactor based solely on a function of conversion, -r ... 

Section 2.4.5 will discuss the reason why heat transfer is fundamental in the design and operation of stirred-tank batch reactors. Figure 6 shows some of the batch reactor systems most often encountered in practice. 

Fundamental studies of heat transfer, combined with a basic understanding of the underlying kinetics of the combustion process, will be used to develop a model of the fast-start reactor. The model will then be used to design a fast-start reactor that can meet the prescribed operational targets. 

Gas absorption and any associated chemical reaction is always accompanied by the simultaneous release of heat of solution and heat of reaction. The micro-scale phenomena taking place close to the interface therefore involve the generation and diffusion of heat as well as the diffusion and reaction of material species. In developing a fundamental appreciation of simultaneous mass and heat transfer in gas-liquid reactions it is important for the heat effects to be incorporated into the analysis of diffusion and reaction because the rates and pathways of chemical reactions are usually enormously sensitive to temperature. In particular, for the case of gas-liquid reactor performance, if the heat effects are such that the mass transfer with reaction zone adjacent to the interface is at a temperature significantly different from the bulk, the yield and the selectivity performance will be erroneously interpreted if reaction is assumed to take place at the bulk liquid temperature. In consequence, the basic conceptual design of a commercial gas-liquid reactor could incorporate fallacious reasoning leading to inefficient operation at sub-optimal yield. 

Hydrodynamics, mass, and heat transfer in the commonly used three-phase fixed-bed reactors were briefly outlined. Also, scale-up rules and alternative ways to scale down trickle-bed reactors are discussed. In spite of the extensive studies on the hydrodynamics, mass, and heat transfer in three-phase fixed-bed reactors, clearly, a lot of work remains to be done in providing a fundamentally based description of the effect of pressure on the parameters of importance in three-phase fixed-bed reactors operation, design, and scale-up or scale-down. It is evident that atmospheric data and models/correlations cannot, in general, be extrapolated to operation at elevated pressures. The physics conveyed by the standard two-phase flow models is minimalistic because it insufficiently describes the role and presence of interfaces and their thermodynamic properties. The explicit inclusion of interfaces and interfacial properties is essential because they are known to have a significant role in determining the thermodynamic state of the whole system. 

We shall see in Chapter 6 that energy consumption during industrial synthesis increases in importance with the decrease of the unit price of a particular product. In this section we shall address the fundamental concepts of thermodynamics to the calculation of minimum energy requirements and open-cell voltages. We shall consider energy balances and heat transfer requirements, since both can be an essential part of practical reactor design. 

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