Yield plug flow reactor , ideal

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. 

If a tubular-flow reactor is equipped with a recycle arrangement, as shown in Fig. 7, the mixing pattern is somewhere between the two ideal limits of plug flow and ideal back-mixing. Such a system can be useful for controlling product distribution from a complex reaction. Consider the simultaneous occurrence of reactions (17) and (105) where reaction (105) is second-order and B is the desired product. The discussion above would suggest that plug flow would enhance the relative yield of B but back-... 

Plug flow reactors are characterized by a unique residence time for all molecules and can be operated at t = It is evident that any residence time distribution (RTD) in the reactor will diminish the yield of the intermediate. Therefore, the highest yield is always obtained for a uniform residence time corresponding to an ideal plug flow reactor [7]. Evidently, the yield increases with decreasing ratios of the two rate constants /c. 

The isothermal plug-flow mass balance for first-order irreversible chemical kinetics in an ideal packed catalytic tubular reactor with significant external mass transfer resistance yields the following functional form for the conversion of reactant A at high mass transfer Peclet numbers ... 

More complicated approaches are required when the reaction yield is sensitive to the mode of mixing and the process is carried out in a reactor which is neither in ideal plug flow, nor perfectly mixed, but somewhere between. The mixing is then described by a model of the flow pattern, whose equations must be solved together with those describing the reaction kinetics. The solution of the equations is iterative and generally requires a substantial computer programme. 

The ability to approach an ideal case is not only valuable in terms of reducing the risk of scaleup. For many cases the ideal plug-flow or stirred tank reactor is also an optimal reactor (44), and we often improve the yield by approaching plug-flow more closely. 

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