Single-phase flows reacting

The present chapter focuses on single-phase flow, but extensions to multi-phase flow or monitoring other variables, such as the coke content and / or activity level of catalyst pellets, concentrations within coalescing droplets, changing pore sizes in a reacting solid, biochemical properties of growing cells, sizes of growing crystals, are possible. Some of these extensions are dealt with in Chapters 13 and 14. 

Specific examples of single-phase turbulent reacting flows in a tubular jet reactor are discussed in Section 9.10.4.1. We select representative industrial problems of engineering interest for the two fundamental classes of polymerization reactions, namely addition and condensation polymerization. For each example, we present a general overview of the problem and detailed reacting flow analyses, followed by useful process design and operational information. 

Computational fluid dynamics (CFD) is rapidly becoming a standard tool for the analysis of chemically reacting flows. For single-phase reactors, such as stirred tanks and empty tubes, it is already well-established. For multiphase reactors such as fixed beds, bubble columns, trickle beds and fluidized beds, its use is relatively new, and methods are still under development. The aim of this chapter is to present the application of CFD to the simulation of three-dimensional interstitial flow in packed tubes, with and without catalytic reaction. Although the use of... 

A similar route to Bi2Sr2CaCu/)8 tf developed by Beltran et al. (41) involves the reaction of Bi2Cu04, CaCOs, SrCOs, and CuO in appropriate ratios. These materials are well mixed and heated to 860°C in an alumina boat under flowing 02 for 16 hours. The reacted powder is ground, compressed into pellets, and sintered at 880°C for 20 hours and furnace-cooled to room temperature. This technique reportedly also led to single-phase samples. 

Some simplified analyses have been performed for the combustion (1-9) and pure vaporization (10,11,12) of sprays. Recently more elaborate numerical codes on the unsteady, two-dimensional, two-phase, chemically reacting flows are also being developed (13), In all these studies the spray is frequently assumed to be sufficiently dilute such that the mutual interferences between the motion and the vaporization processes of the individual droplets are either completely neglected or are manifested only through their collective modifications of the state of the bulk gas. (The term vaporization is used here to imply both combustion and pure vaporization.) Hence the vaporization and kinematic behavior of a single, isolated droplet in an infinite expanse of gas serve as fundamental inputs to the spray analysis. These single-droplet phenomena are discussed in this chapter. 

The relative importance of the specific physical phenomena mentioned strongly depends on the type of flow under consideration. In this section, the discussion is limited to single-phase, constant density flows under isothermal conditions with constant viscosity and equal diffusivities. The emphasis is placed on the modeling of turbulent mixing and on the interactions between turbulent mixing and chemical reactions in non-premixed turbulent reacting flows. 

Let us consider then what can be done to characterize the heterogeneous systems in a practical way. From a theoretical standpoint it seems that we should determine the RTD of reactants in the reacting environment. Then, by analogy to homogeneous systems, we should be able to predict reactor performance in case of first-order reactions and possibly bound it for n-th order reactions. From the practical standpoint it is clear that we can perform tracer experiments using tracers that distribute themselves differently between various environments. In the limit we can use some that cannot enter at all a particular environment and hence trace the flow of a single phase only. We will now examine whether such tracer studies could yield the information of interest. 

Stirred tanks are modeled assuming that both phases are well mixed. Tray columns are usually modeled as well mixed on each tray so that the overall column is modeled as a series of two-phase, stirred tanks. (Distillation trays with tray efficiencies greater than 100% have some progressive flow within a tray.) When reaction is confined to a single, well-mixed phase, the flow regime for the other phase makes little difference but when the reacting phase approximates piston flow, the flow regime in the other phase must be considered. The important cases are where both phases approximate piston flow, either countercurrent or cocurrent. 

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