The Plug Flow Reactor

A particular differential segment of material within the PFR, when observed on its own, is assumed to be perfectly uniform in concentration. In this way, a change 

VppR is the volume of the PFR and Q is the volumetric flow rate at the exit of the PFR. t then has units of (time). 

The given description of residence time is the most natural choice for many reactor problems, although, this may not be the sole choice available. Take, for example, if reaction rate is expressed with respect to catalyst mass. An appropriate set of units for the rate might then incorporate the mass of the catalyst contained within the reactor body over reactor volume. In Chapter 9, we will define r in terms of mass fractions for variable density systems. 

The reactor is represented as a collection of beakers traveling on the conveyor at a constant speed S and a fixed length L. At the left side of the belt, a beaker containing the feed concentration is placed on the belt and allowed to traverse along its length. When the beaker reaches the end of the belt, it is removed. It is assumed that each beaker that 

At a fixed position At each point along the length of the belt, we may simply stop and observe the concentration at a fixed location on the belt. From this perspective, the concentration of the mixture is fixed for a specific position along the conveyor belt. Furthermore, since reaction rate is a function of concentration, reaction rate will also be constant for a fixed location on the conveyor belt. 

The concept of the plug flow for piston flow) reactor denotes an ideal tubular reactor, in which all fluid elements travel in one direction with exactly the same speed. They move as a plug would, or as a liquid does when propelled by a piston. That means that radial velocity gradients are negligible, and that there is no axial mixing. Even axial diffusion is neglected. 

This ideal model may be approximated in three different practical situations  

In each of these three cases there will be a thin layer of fluid flowing with a lower than average speed close to the solid wall (or solid particles), and in the case of turbulent flow there will be a certain rate of forward and backward mixing. However, in many practical cases these effects are relatively small. 

It is not always simple to determine a priori whether a given reactor behaves as an 

Let us now consider a chemical reaction taking place in a plug flow reactor. The mass balance reads as follows (for constant density) 

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The first distinction to be drawn, as far as heat transfer is concerned, is between the plug-flow and continuous well-mixed reactor. In the plug-flow reactor shown in Fig. 13.1, the heat transfer can take place over a range of temperatures. The shape of the profile depends on... 

The computer results from Table 5-13 show the calculated compositions of benzene, diphenyl, triphenyl, and hydrogen. At a fixed feedrate, increasing V/F values correspond to movement through the plug flow reactor (i.e., increasing reactor volume). Thus, these results illustrate how the composition varies with position in the reactor. Here, the mole fraction of benzene decreases steadily as the reaction mixture progresses in the reactor, while the composition of diphenyl increases and reaches a maximum between 1,684 and 1,723 hr and thereafter decreases. This is often typical of an intermediate in consecutive reactions. 

The inlet concentration of component A at the entrance of the plug flow reactor is... 

Equation 8-155 shows that the conversion in the dispersion reactor will always be less than that of the plug flow reactor (C >... 

Continuous Stirred Tank and the Plug Flow Reactors... 

The plug flow reactor, which has the dimension of r = 2 mm is now loaded with 1.0 g of catalyst. Will this set-up fulfill the requirements formulated above ... 

Estimate the space velocity needed to ensure that the conversion does not exceed say a 5% conversion at the exit of the plug flow reactor. 

Two template examples based on a capillary geometry are the plug flow ideal reactor and the non-ideal Poiseuille flow reactor [3]. Because in the plug flow reactor there is a single velocity, v0, with a velocity probability distribution P(v) = v0 16 (v - Vo) the residence time distribution for capillary of length L is the normalized delta function RTD(t) = T 1S(t-1), where x = I/v0. The non-ideal reactor with the para-... 

Plug-flow reactor. Consider now the plug-flow reactor in Figure 5.3a, in which Component i is reacting. A material balance can be carried out per unit time across the incremental volume dV in Figure 5.3a ... 

Alternatively, the residence time in the plug-flow reactor could be calculated from the batch equations given in Table 5.10. This... 

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... 

The Plug Flow Reactor (PFR)—Basic Assumptions and Design Equations... 

Consider the plug flow reactor used for the pyrolysis of methyl acetoxypropionate in Illustration 8.4. 

For isothermal operation at 500 °C and 5 atrp, it was shown that the space time required to achieve 90% conversion was 29.9 sec. Compare this value with the mean residence time of the material in the plug flow reactor. 

This equation differs from that for the plug flow reactor (8.2.9) in that for a CSTR the rate is evaluated at effluent conditions and thus appears outside the integral. 

As Levenspiel points out in his discussion of this same reaction network, it is relatively easy to extend the use of figures like Figure 9.9 (PFR) to casis in which intermediates may be present in the feed to the plug flow reactor either by virtue of their presence in a recycle stream or in the raw feedstream. The progress of the reaction... 

If A has significant economic value then it should be separated from the reactor effluent stream and recycled for subsequent use. Since the conversion level is higher in the plug flow reactor, the recycle rate will be much smaller and the demands on the separation equipment for reclaiming species A will also be somewhat smaller. Even when species A is of relatively little economic value, there may be circumstances when the costs associated with meeting the pollution control requirements for the process effluent will dictate separation and recycle of this reactant as the most economic alternative. 

These properties are those of the stream entering the plug flow reactor. The design equation for this reactor is... 

Equations B, D, and F may now be solved simultaneously to determine the required space time in the plug flow reactor. 

The relationship between the temperature and the fraction conversion at a particular point in the plug flow reactor can be obtained by setting Sa2 = Sa and TieavingPFR equal to T. Thus,... 

For a few highly idealized systems, the residence time distribution function can be determined a priori without the need for experimental work. These systems include our two idealized flow reactors—the plug flow reactor and the continuous stirred tank reactor—and the tubular laminar flow reactor. The F(t) and response curves for each of these three types of well-characterized flow patterns will be developed in turn. 

The plug flow reactor has a flat velocity profile and no longitudinal mixing. These idealizations imply that all fluid elements leaving the reactor have the same age (T). The F(t) function for this system must then be... 

Let xp and xc represent the space times of the plug flow reactor and the continuous stirred tank reactor respectively. Consider the following reactor combination... 

They determined the ratio of the dispersion reactor volume to the plug flow reactor volume necessary to accomplish the same degree of conversion for several values of the dimensionless dispersion parameter S)l/uL. Figure 11.10 summarizes their results. It is evident that for high... 

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. 

Continuous reactor a reactor characterized by a continuous flow of reactants into and a continuous flow of products from the reaction system examples are the plug flow reactor (PFR) and the continuous stirred tank reactor (CSTR). 

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