Multiphase reactors CSTRs

This situation describes an emulsion reactor in which reacting drops (such as oil drops in water or water drops in oil) flow through the CSTR with stirring to make the residence time of each drop obey the CSTR equation. A spray tower (liquid drops in vapor) or bubble column or sparger (vapor bubbles in a continuous liquid phase) are also segregated-flow situations, but these are not always mixed. We wiU consider these and other multiphase reactors in Chapter 12. 

Sohds reactors are multiphase reactors so we must consider both the conversions of sohd Xs and of fluid phases X to specify these reactors. In this chapter we have not considered the reactor equations that must be solved to predict the overall conversion of reactants in the reactor. As might be expected, we assume PFTR and CSTR reactors, and solve for conversion X in the reactor. 

Let us consider a simple example of multiphase reactors where the reaction A B occurs in a CSTR, but now A enters the reactor in phase a but does not react until it enters phase P, where the homogeneous rate is itC. As an example A could be an ester in an oil phase O, which will hydrolyze into an alcohol and an acid when it comes in contact with a water... 

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

Multiphase Reactors Reactions between gas-liquid, liquid-liquid, and gas-liquid-solid phases are often tested in CSTRs. Other laboratory types are suggested by the commercial units depicted in appropriate sketches in Sec. 19 and in Fig. 7-17 [Charpentier, Mass Transfer Rates in Gas-Liquid Absorbers and Reactors, in Drew et al. (eds.), Advances in Chemical Engineering, vol. 11, Academic Press, 1981]. Liquids can be reacted with gases of low solubilities in stirred vessels, with the liquid charged first and the gas fed continuously at the rate of reaction or dissolution. Some of these reactors are designed to have known interfacial areas. Most equipment for gas absorption without reaction is adaptable to absorption with reaction. The many types of equipment for liquid-liquid extraction also are adaptable to reactions of immiscible liquid phases. 

Different types of reactors are applied in practice (Figure 1.14). Stirred tank reactors (STR), very often applied for homogeneous, enzymatic and multiphase heterogeneous catalytic reactions, can be operated batchwise (batch reactor, BR), semi-batchwise (semibatch reactor, SBR) or continuously (continuous strirred tank reactor, CSTR)... 

It is important to remember that a simulation results are only as good as the mathematical models on which those are based. However, there are still many items that are neglected or treated insufficiently in mathematical models, such as ageing and deactivation of the catalyst and the stability of the products under actual production conditions. Many commercial flowsheeting programs still rely totally on idealistic behaviour. Many programs have only very limited number of reactor types like tube and CSTR. Common multiphase reactors where mass transfer phenomena also plays important role are missing. Also idealized separation models are common. 

Why are the CSTRs worth considering at all They are more expensive per unit volume and less efficient as chemical reactors (except for autocatalysis). In fact, CSTRs are useful for some multiphase reactions, but that is not the situation here. Their potential justification in this example is temperature control. BoiUng (autorefrigerated) reactors can be kept precisely at the desired temperature. The shell-and-tube reactors cost less but offer less effective temperature control. Adiabatic reactors have no control at all, except that can be set. 

Our treatment of Chemical Reaction Engineering begins in Chapters 1 and 2 and continues in Chapters 11-24. After an introduction (Chapter 11) surveying the field, the next five Chapters (12-16) are devoted to performance and design characteristics of four ideal reactor models (batch, CSTR, plug-flow, and laminar-flow), and to the characteristics of various types of ideal flow involved in continuous-flow reactors. Chapter 17 deals with comparisons and combinations of ideal reactors. Chapter 18 deals with ideal reactors for complex (multireaction) systems. Chapters 19 and 20 treat nonideal flow and reactor considerations taking this into account. Chapters 21-24 provide an introduction to reactors for multiphase systems, including fixed-bed catalytic reactors, fluidized-bed reactors, and reactors for gas-solid and gas-liquid reactions. 

Example 4.2 applied the method of false transients to a CSTR to find the steady-state output. A set of algebraic equations was converted to a set of ODEs. Chapter 16 shows how the method can be applied to PDEs by converting them to sets of ODEs. The method of false transients can also be used to find the equilibrium concentrations resulting from a set of batch chemical reactions. Formulate the ODEs for a batch reactor and integrate until the concentrations stop changing. Irreversible reactions go to completion. Reversible reactions reach equilibrium concentrations. This is illustrated in Problem 4.6(b). Section 11.1.1 shows how the method of false transients can be used to determine physical or chemical equilibria in multiphase systems. 

Membrane reactors were classically grouped according to the hydrodynam-ics/configuration of the system in CSTR and PFR types [106]. However, this proved vmable to comprise some commonly used types in UF, such as flat membranes or dead-end operated modules and multiphase bioreactors. A classification based on the contact mechanisms that bring together substrate and biocatalyst was thus proposed [110]. Thus, membrane reactors could be divided into direct contact, diffusion contact, and interfacial contact reactors. 

Advantages (i) this method allows one to point out promising flowsheets (ii) the reaction-separation vector has the same geometric properties as simple reactor models, allowing the application of the theory derived for PFRs and CSTRs and (in) this method can be easily extended to multiphase systems. 

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