Yield plug flow reactor

Du Pont uses a Hquid-phase hydrogenation process that employs a palladium —platinum-on-carbon catalyst. The process uses a plug-flow reactor that achieves essentially quantitative yields, and the product exiting the reactor is virtually free of nitroben2ene. 

Consecutive reactions, isothermal reactor cmi < cw2, otai = asi = 0. The course of reaction is shown in Fig. 5.4-71. Regardless the mode of operation, the final product after infinite time is always the undesired product S. Maximum yields of the desired product exist for non-complete conversion. A batch reactor or a plug-flow reactor performs better than a CSTR Ysbr.wux = 0.63, Ycstriiuix = 0.445 for kt/ki = 4). If continuous operation and intense mixing are needed (e.g. because a large inteifacial surface area or a high rate of heat transfer are required) a cascade of CSTRs is recommended. 

For this last stage, the one-at-a-time procedure may be a very poor choice. At Union Carbide, use of the one-at-a-time method increased the yield in one plant from 80 to 83% in 3 years. When one of the techniques, to be discussed later, was used in just 15 runs the yield was increased to 94%. To see why this might happen, consider a plug flow reactor where the only variables that can be manipulated are temperature and pressure. A possible response surface for this reactor is given in Figure 14-1. The response is the yield, which is also the objective function. It is plotted as a function of the two independent variables, temperature and pressure. The designer does not know the response surface. Often all he knows is the yield at point A. He wants to determine the optimum yield. The only way he usually has to obtain more information is to pick some combinations of temperature and pressure and then have a laboratory or pilot plant experimentally determine the yields at those conditions. 

One problem with this or any other method using gradients is that the best path obtained is dependent on the units used. If different units are used a different path will be indicated. To illustrate this, suppose it is desired to improve the yield (y) of a plug flow reactor when the feed rates and compositions are constant. At the usual operating conditions of 50 psia and 500°K a yield of 60 lb/hr is obtained. In what order should the pressure (P) and the temperature (T) be changed To reduce costs, it is desirable to minimize the number of experiments performed, hence the method of steepest ascent is to be used.When a test is performed at 50 psia and 510°K, the yield is found to be 60 lb/hr. When another experiment is run at 60 psia and 500°K, the yield is again 60 lb/hr. If the surface is linearized it can be expressed as ... 

For an example, let us return to the plug flow reactor for which it was desired to obtain the optimum yield by varying the temperature and pressure. An initial step... 

Hence the area under the curve of y versus CA multiplied by the ratio of stoichiometric coefficients represents the overall change in valuable product concentration between the inlet and outlet streams in a plug flow reactor or in a batch reactor. For the case of a CSTR the instantaneous yield is evaluated at the effluent composition, and the corresponding equation is... 

Neither B nor the undesirable products are present in the feedstream. Determine the maximum yields of B that can be obtained in the limit where the conversion level approaches 100% for both a plug flow reactor and a continuous flow stirred tank reactor. 

Equation 9.1.11 is appropriate for determining the overall yield in a plug flow reactor. 

Comparison of maximum yields for series reactions in stirred tank and plug flow reactors. 

Comparison of the fractional yields of V in mixed and plug flow reactors for the consecutive first-order reactions. A A- V W. (Adapted from Chemical Reaction Engineering, Second Edition, by O. Levenspiel. Copyright 1972. Reprinted by permission of John Wiley and Sons, Inc.)... 

D-Pantolactone and L-pantolactone are used as chiral intermediates in chemical synthesis, whereas pantoic acid is used as a vitamin B2 complex. All can be obtained from racemic mixtures by consecutive enzymatic hydrolysis and extraction. Subsequently, the desired hydrolysed enantiomer is lactonized, extracted and crystallized (Figure 4.6). The nondesired enantiomer is reracemized and recycled into the plug-flow reactor [33,34]. Herewith, a conversion of 90-95% is reached, meaning that the resolution of racemic mixtures is an alternative to a possible chiral synthesis. The applied y-lactonase from Fusarium oxysporum in the form of resting whole cells immobilized in calcium alginate beads retains more than 90% of its initial activity even after 180 days of continuous use. The biotransformation yielding D-pantolactone in a fixed-bed reactor skips several steps here that are necessary in the chemical resolution. Hence, the illustrated process carried out by Fuji Chemical Industries Co., Ltd is an elegant way for resolution of racemic mixtures. 

Tan et al. [29] demonstrated the use of a plug flow reactor of immobilized Lactobacillus kefiri cells for the synthesis of the intermediate (5I )-hydroxyhexane-2-one. This immobilized-cell reactor operated at a maximum conversion yield of 100% and a selectivity of 95%. The production of (5/ )-hydroxyhexane-2-one was extended to an operation time of 6 days. During this time (91 residence times), a space-time yield of 87gL xday 1 and a productivity of 07 8 gwet cell weight 1 were obtained. 

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. 

What reactor holding time will yield a product in which Cr = 0.9 mol/ liter (a) in a plug flow reactor, (b) in a mixed flow reactor, and (c) in a minimum-size setup without recycle ... 

The over-all fractional yields from mixed and plug flow reactors processing A from Cao to C / are related by... 

Figure 8.6 Comparison of the fractional yields of R in mixed flow and plug flow reactors for the unimolecular-type reactions...

Again, as with the two-reaction set, we find that a plug flow reactor yields a higher maximum concentration of any intermediate than does a mixed flow reactor. 

In all cases studied, the membrane reactor offered a lower yield of formaldehyde than a plug flow reactor if all species were constrained to Knudsen diffusivities. Thus the conclusion reached by Agarwalla and Lund for a series reaction network appears to be true for series-parallel networks, too. That is, the membrane reactor will outperform a plug flow reactor only when the membrane offers enhanced permeability of the desired intermediate product. Therefore, the relative permeability of HCHO was varied to determine how much enhancement of permeability is needed. From Figure 2 it is evident that a large permselectivity is not needed, usually on the order of two to four times as permeable as the methane. An asymptotically approached upper limit of... 

In order for a membrane reactor to produce yields of HCHO greater than in a plug flow reactor, the membrane must be permselective for this species. The more permselective the membrane is to formaldehyde the better the membrane reactor performs until the formaldehyde is approximately one thousand times more permeable than methane. At this limit, the concentration of HCHO is essentially equal on both sides of the membrane at all times. No further improvement is possible by increasing the diffusivity of the formaldehyde further because there is... 

Partial oxidation of methane in the membrane reactor configuration shown in Figure 1 will not lead to higher yields of desired products than a plug flow reactor unless the diffusivity of the intermediate product, formaldehyde, is approximately four times that of methane. Presently available membranes that can withstand partial oxidation temperatures do not satisfy this criterion. 

Assess the potential of a thermal treatment of the off-gas, assuming that the reaction takes place in a plug-flow reactor. Determine the optimum reactor conditions for destroying N2O, while optimizing the yield of NO. The off-gas stream can be assumed to have the following composition (volume-based) 30% N20,0.7% NO, 300 ppm CO, 3% H20,4% O2, balance N2. 

Reactor type For the highest relative yield of P a batch or tubular plug-flow reactor should be chosen. If a continuous stirred-tank system is adopted on other grounds, several tanks should be used in series so that the behaviour may approach that of a plug-flow tubular reactor. 

Equation 5-354 determines the value of the recycle ratio for a given conversion, the residence time, and the rate constant in a plug flow reactor. Alternatively, an increase in R will lower the conversion since it produces backmixing in the reactor as it mixes with the feed entrance in the plug flow reactor. As R it yields... 

Yield-Based Reactor Fractional Conversion Reactor Combined Specification Model Well-Stirred Reactor Model Plug Flow Reactor Model Two Phase Chemical Equilibrium General Phase and Chemical Equilibrium... 

It is worthwhile to compare the conversion obtained in an isothermal plug flow reactor with that obtained in a CSTR for given reaction kinetics. A fair comparison is given in Fig. 7.3 for irreversible first-order kinetics by showing the conversion obtained in both reactors as a function of To- The conversion of A obtained in a plug flow reactor is higher than that obtained in a CSTR. This holds for every positive partial reaction order with respect to A. For multiple reactions selectivities and yield enter into the picture. 

For positive reaction orders with respect to the reactants the intermediate yields obtained in a CSTR are lower than those obtained in a batch or a plug flow reactor. 

The primary reason for choosing a particular reactor type is the influence of mixing on the reaction rates. Since the rates affect conversion, yield, and selectivity we can select a reactor that optimizes the steady-state economics of the process. For example, the plug-flow reactor has a smaller volume than the CSTR for the same production rate under isothermal conditions and kinetics dominated by the reactant concentrations. The opposite may be true for adiabatic operation or autocata-lytic reactions. For those situations, the CSTR would have the smaller volume since it could operate at the exit conditions of a plug-flow reactor and thus achieve a higher overall rate of reaction. 

Note that the first reaction is autocatalytic in component B. Reactant A is the limiting component. Component B is the desired product while C and D are by-products. Both reactions have moderate heats of reaction and relatively low activation energies. To maximize the yield of 23, a plug-flow reactor was selected. The economic objectives are throughput and yield. What is the appropriate partial control scheme for this reactor ... 

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