Plug flow reactor combinations with CSTR

A tubular bioreactor design with operational may lead to a CSTR, having sufficient recycle ratio for plug flow that behave like chemostat. The recirculation plug flow reactor is better than a chemostat, with maximum productivity at C, 3 g-m 3. Combination of plug flow with CSTR which behave like chemostat was obtained from the illustration minimised curve with maximum rate at CSf = 3 g-m-3. 

The CRE approach for modeling chemical reactors is based on mole and energy balances, chemical rate laws, and idealized flow models.2 The latter are usually constructed (Wen and Fan 1975) using some combination of plug-flow reactors (PFRs) and continuous-stirred-tank reactors (CSTRs). (We review both types of reactors below.) The CRE approach thus avoids solving a detailed flow model based on the momentum balance equation. However, this simplification comes at the cost of introducing unknown model parameters to describe the flow rates between various sub-regions inside the reactor. The choice of a particular model is far from unique,3 but can result in very different predictions for product yields with complex chemistry. 

As the main responsible for the changes in the material balance, the chemical reactor must be modelled accurately from this point of view. Basic flowsheeting reactors are the plug flow reactor (PFR) and continuous stirred tank reactor (CSTR), as shown in Fig. 3.17. The ideal models are not sufficient to describe the complexity of industrial reactors. A practical alternative is the combination of ideal flow models with stoichiometric reactors, or with some user programming. In this way the flow reactors can take into account the influence of recycles on conversion, while the stoichiometric types can serve to describe realistically selectivity effects, namely the formation of impurities, important for separations. Some standard models are described below. 

The second reactor is to be a plug flow reactor with an effective volume of 1600L. Oxygen is injected at several points along the length of the reactor to maintain a constant concentration of this species over the entire length of the PFR. This concentration is such that the aforementioned rate expression and rate constant remain valid. What fraction of the C that enters the CSTR is converted to P in the indicated combination of reactors ... 

Real reactors deviate more or less from these ideal behaviors. Deviations may be detected with re.sidence time distributions (RTD) obtained with the aid of tracer tests. In other cases a mechanism may be postulated and its parameters checked against test data. The commonest models are combinations of CSTRs and PFRs in series and/or parallel. Thus, a stirred tank may be assumed completely mixed in the vicinity of the impeller and in plug flow near the outlet. 

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. 

So far we have been considering temperature ramping only, with flow rates held constant. In this section we consider the possibility of varying the flow rate during a run. The TS-CSTR turns out to be very simple to deal with, with marvelous possibilities for interactive control. The TS-PFR, as usual, requires much more careful consideration. In both the plug flow and CSTR reactors, flow scanning can be used alone or in combination with temperature scanning. 

The basic reactor choice is between a packed bed, stirred tank, or combinations thereof (with or without possibilities for enzyme retention by membranes or immobilization) and compartmentalization. The packed bed reactor can only handle immobilized enzyme(s) and operates in plug-flow mode (mixing only in the radial, rather than axial direction). For many enzymes their kinetics are such that operation in plug-flow hydrodynamic mode is favored. Hence, in cases where mixing is required (e.g., for pH control via addition of a neutralizing acid or alkali or addition of an inhibitory substrate), then the use of multiple CSTRs can be used (Figure 20.3). 

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