Parallel reactions in a PFR

We will now give three examplc.s of multiple reactions with heat effects Example 12-5 discusses parallel reactions. Example 12-6 discusses series reactions, and Example 12-7 discusses complex reactions. 

Pure A is fed at a rate of 100 mol/s, a temperature of 150°C, and a concentration of 0.1 mol/dm. Determine the temperature and molar flow rate profiles down the reactor. 

Steady-State Nonisothermai Reactor Design—Flow Reactors with Heat Exchange Chapter 12 

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Example. 8-11 Parallel Reaction in a PFR with Heat. Effects... 

Example 8-10 Parallel Reactions in a PFR with Heal Effects... 

Figure 18.4 Vessel configuration for parallel reactions in Example 18-6 (a) single PER (b) three PFRs of same total volume in series with FBo split evenly among them...

In a parallel reaction network of first-order reactions, the selectivity does not depend upon reaction time or residence time, since both products are formed by the same reactant and with the same concentration. The concentration of one of the two products will be higher, but their ratio will be the same during reaction in a batch reactor or at any position in a PFR. The most important parameters for a parallel reaction system are the reaction conditions, such as concentrations and temperature, as well as reactor type. An example is given in the following section. 

In this section we give the energy balance for multiple reactions that are in parallel and/or in series. The energy balance for a single reaction taking place in a PFR was given by Equation (8-60)... 

In biocatalysis, selectivity is also of great importance. However, in biocatalysis competition often takes place between parallel reactions A —> P and A —> Q (which is also important in homogeneous enantioselective catalysis), or A P and B Q (kinetic resolution, A and B are enantiomers and so are P and Q). With the latter type of competition, the selectivity in a CSTR is reduced as compared to a PFR, just as in the consecutive reactions of the example. On the other hand, for the parallel reactions of A, the selectivity in a CSTR is the same as in a PFR. 

Find the conversion for a first-order reaction in a composite system that consists of a perfect mixer and a PFR in parallel. 

Rule 4. Complex reactions can be analysed by means of simple series and parallel reactions. In the case of series-parallel reactions of first-order, the behaviour as series reactions dominates. A PFR reactor is more advantageous for the production of the intermediate component. 

EP.IO The parallel reactions are irreversible and the A- 2R and A- S product rates are of second order with respect to reactants. Reaction was performed in a PFR reactor. Feed composition was 50% for A and 50% for Inert. The molecular weights are 40 and 20, respectively. The exit flow rate of product R is 6 mol/h. The total flow rate is 1000 mL/min at 10 atm and constant temperature 400° C. Assume ideal gases. The reactor volume is 2 L and the selectivity relative to R is 85%. Calculate constants ki and k2-... 

Equation (19-22) indicates that, for a nominal 90 percent conversion, an ideal CSTR will need nearly 4 times the residence time (or volume) of a PFR. This result is also worth bearing in mind when batch reactor experiments are converted to a battery of ideal CSTRs in series in the field. The performance of a completely mixed batch reactor and a steady-state PFR having the same residence time is the same [Eqs. (19-5) and (19-19)]. At a given residence time, if a batch reactor provides a nominal 90 percent conversion for a first-order reaction, a single ideal CSTR will only provide a conversion of 70 percent. The above discussion addresses conversion. Product selectivity in complex reaction networks may be profoundly affected by dispersion. This aspect has been addressed from the standpoint of parallel and consecutive reaction networks in Sec. 7. 

Figure 14-18(a) describes a real PFR or PER with channeling that is modeled as two PFRs/PBRs in parallel. The two parameters are the fraction of flow 10 the reactors [i.e., (3 and (1 - p)] and the fractional volume [i.e.. a and (1 - Qf] of each reactor. Figure 14-18(b) describes a real PFR/PBR that has a backmix region and is modeled as a PFR/PBR in parallel with a CSTR. Figures H-19(a) and (b) show a real CSTR modeled as two CSTRs with interchange. In one case, the fluid exits from the top CSTR (a) and in the other case the fluid exits from the bottom CSTR. The parameter p represents the interchange volumetric flow rate and a the fractional volume of the top reactor, where the fluid exits the reaction system. We note that the reactor in model 14-19(b) was found to describe extremely well a real reactor used in the production of terephthalic acid. A number of other combinations of ideal reactions can be found in Levenspiel. ... 

This much said, let us now examine the behavior of a PFR in this predicament. The classical example was provided by Froment and Bischolf [G.F. Froment and K.B. Bischoff, Chem. Eng. Set, 10, 189 17, 105 (1962)] for isothermal conditions, with catalyst deactivation by either parallel or series reaction steps (see Chapter 3), and our favorite imaginary reaction A B. Thus we deal with overall sequences such as (XXVI) or (XXVII) of Chapter 3. 

In reactions where the product does not react further (i.e., parallel reactions), yields and selectivities can be easily calculated from the ratios of the rates. Where a product reacts further, no such simple analysis is possible, and resort to numerical solution is often necessary. As a general rule, however, whenever an intermediate product is the desired product, PFR is the preferred reactor. 

SE.17 A reaction in the gas phase A—+ S is processed in two reactors, PFR operating in parallel, the first one operates isothermally at 2 atm and 200°C and the other adiabatically. The reactant A (pure) at a rate of lOmol/min is introduced separately, with... 

After extracting the kinetic parameters, selected results for CO oxidation over were used to analyze the effect of non-uniform temperature and velocity distributions on the conversion of CO. In order to determine the optimum number of multiple CSTR s to capture the behavior of a PFR, the rate law of Oh and Carpenter (14) for the NO+CO reaction was used to model a monolith channel as a CSTR in series. The results indicated that it was sufficient to use 5 reactors in series to capture the performance of the PFR behavior in the NO+CO reaction The cells of a monolith reactor were taken as independent parallel reactors ignoring the mass transfer and diffusion through the ceramic pores. The axial and radial temperature and velocity profiles collected from the literature(4,5) are used to calculate the... 

The ideal plug flow reactor PFR is a simplified picture of the motion of a fluid in a tubular reactor as it is assumed that all fluid elements move with a uniform velocity along parallel streamlines and thus have a fixed residence time r. Strictly speaking, this assumption breaks the hydrodynamic rule that the velocity is zero at the wall (no slip condition. Figure 3.2.22). The steady-state mass balance of a PFR for a constant volume reaction can be deduced from the one-dimensional mass balance for a differential small element with thickness Az in direction of flow ... 

The following simple model system may illustrate this situation. Let us assume we have two ideal PFRs, each with the same volume. Both reactors are run in parallel. If we assume a first-order reaction with a rate constant of 1 s and a residence time in each reactor of 1 s we obtain a conversion of 63.2% in each reactor [Eq. (4.10.25)]. But if we divide the total volume rate unequally, the residence times... 

Undesirable parallel reactions can thus force us to select a CSTR, although the low production capacity requires a considerably larger reactor volume than in the case of a PFR or a BR. 

Mr. P (MSc) suggests that a CSTR should be selected, since backmixing in parallel reactions always favors the reaction of the lowest order, that is, A B. Mr. Q (MSc), however, claims that when dealing with reactions of the type A B C, one should always select a PFR, because a PFR favors the formation of the intermediate product, B. Mr. Q backs up his argument with the tables below, which illustrate cb/cqa as a function of the conversion level of A. The tables (calculated by Mr. Q) are, however, valid for PFRs only. To resolve the conflict, the boss Mr. H (PhD) delegated to Miss S (a student) the task of finding out the possibilities for utilizing a CSTR. Let us assume that you are Miss S ... 

An important ramification of eq. (19) and eq. (20) is that a plug flow reactor is the optimal reactor for first-order reactions. For a single reaction that is easy to see. If we have a PFR of mean residence time t and a reactor of an arbitrary RTD of the same mean residence time, then the PFR will yield the lowest exit reactant concentration. The reactor with the arbitrary RTD can be viewed as a set of PFRs of different residence times in parallel. Some of these with residence times lower than t have higher than the desired value, those with residence times higher than t have lower However, the concentration of the mixture lies on... 

A whole new set of questions now arises. Should the new reactor be a CSTR or a PFR Should it be in series or in parallel with the original reactor If in series, should the new reactor precede or follow the original The answers to these questions will depend on the kinetics of the reaction, and on whether the original reactor was a CSTR or a PFR. Mechanical considerations might also enter into the decision. 

Suppose you have two identical PFRs and you want to use them to make as much product as possible. The reaction is pseudo-first-order and the product recovery system requires a minimum conversion of 93.75%. Assume constant density. Do you install the reactors in series or parallel Would it affect your decision if the minimum conversion could be lowered ... 

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