Plug flow reactor Recycle

Another view is given in Figure 3.1.2 (Berty 1979), to understand the inner workings of recycle reactors. Here the recycle reactor is represented as an ideal, isothermal, plug-flow, tubular reactor with external recycle. This view justifies the frequently used name loop reactor. As is customary for the calculation of performance for tubular reactors, the rate equations are integrated from initial to final conditions within the inner balance limit. This calculation represents an implicit problem since the initial conditions depend on the result because of the recycle stream. Therefore, repeated trial and error calculations are needed for recycle... 

Figure 6 shows typical results obtained with the plug-flow quartz reactor containing 0.5 g of Sr(lwt%)/La203 catalyst operated in the continuous flow recycle mode. The inlet CH partial pressure was 20 kPa (20% CH in He) at inlet flowrates of 7.1 and 14.3 cm STP/min. A 20% O2 in He mixture was supplied directly, at a flowrate Fog, in the recycle loop via a needle valve placed after the reactor (Fig. 1). The methane conversion was controlled by adjusting Fog, which was kept at appropriately low levels so that the oxygen conversion...

The crude MNB is washed to remove residual acid and the impurities formed during the nitration reaction. The product is then distilled and residual benzene is recovered and recycled. Purified MNB is fed, together with hydrogen, into a liquid phase plug-flow hydrogenation reactor that contains a DuPont proprietary catalyst. The supported noble metal catalyst has a high selectivity and the MNB conversion per pass is 100%. 

For elucidation of mechanisms, rate data at very low conversions may be highly desirable. They can be obtained more easily from a batch reactor than from a CSTR or plug-flow tubular reactor. A standard CSTR would have to be operated at very high flow rates apt to cause fluid-dynamic and control problems. The same is true for a standard tubular reactor unless equipped with a sampling port near its inlet, a mechanical complication apt to perturb the flow pattern. If the problem of confining the reaction to a very small flow reactor can be solved—as is possible, for example, for radiation-induced reactions—a differential reactor operated once-through or with recycle may be the best choice. 

Plug-flow membrane reactors arc not faced with potentially unstable states since no back mixing of mass or heat is involved. However, when the product is recycled, heat is exchanged between the product and the feed streams or dispersive backmixing exists, multiple steady slates can occur and membrane reactor stability needs to be considered. 

The simplest flow-sheet for the reaction Aj o Aj is the RD column sequence with an external recycling loop shown in Fig. 5.1. The system as a whole is fed with pure Aj. According to the assumed relative volatility of the two components a > 1, the reaction product A2 is enriched in the column distillate product whereas the bottom product contains non-converted reactant Aj, which is recycled back to the reactor (continuous stirred tank reactor, CSTR, or plug flow tube reactor, PFTR). The process has two important operational variables the recycling ratio cp = B/F, that is the ratio of recycling flow B to feed flow rate F, and the reflux ratio of the distillation column R = L/D. At steady-state conditions, D = F since the total number of moles is assumed to be constant for the reaction Aj A2. As principal design variables, the Damkohler number. 

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

Fig. 8. Combined flow reactor models (a) parallel flow reactors with longitudinal diffusion (diffusivities can differ), (b) internal recycle—cross-flow reactor (the recycle can be in either direction), comprising two countercurrent plug-flow reactors with intercormecting distributed flows, (c) plug-flow and weU-mixed reactors in series, and (d) 2ero-interniixing model, in which plug-flow reactors are parallel and a distribution of residence times dupHcates that...

The smaller reactor approaches plug-flow behavior and exhibits a large temperature gradient. In this case, external recycle provides the same degree of back-mixing as is provided by internal circulation in the larger diameter reactor. 

Bubble columns in series have been used to establish the same effective mix of plug-flow and back-mixing behavior required for Hquid-phase oxidation of cyclohexane, as obtained with staged reactors in series. WeU-mixed behavior has been established with both Hquid and air recycle. The choice of one bubble column reactor was motivated by the need to minimize sticky by-products that accumulated on the walls (93). Here, high air rate also increased conversion by eliminating reaction water from the reactor, thus illustrating that the choice of a reactor system need not always be based on compromise, and solutions to production and maintenance problems are complementary. Unlike the Hquid in most bubble columns, Hquid in this reactor was intentionally weU mixed. 

Recycling increases the size of the reactor and degrades the plug flow charac teristics, so there must be practical compensation by adjustment of the temperature or composition. 

Plug flow reactors with recycle exhibit some of the characteristics of CSTRs, including the possibility of multiple steady states. This topic is explored by Penmutter Stah dity of (%emical Reactors, Prentice-Hall, 1972). 

The volumetric flowrate into the plug flow is Uq and the feed concentration of A is C g- portion of A exiting from the reactor is fed back through a pump and mixed with the feed stream, referred to as R (i.e., the recycle ratio). The volumetric flowrate at the entrance of the reactor is u = UgCl + R). A balance at the mixing point M gives... 

We wish to compare the performance of two reactor types plug flow versus CSTR with a substrate concentration of Csf = 60g-m 3 and a biomass yield of Y = 0.1. In a plug flow bioreactor with volume of 1 m3 and volumetric flow rate of 2.5 m -li what would be the recycle ratio for maximum qx compared with corresponding results and rate models proposed for the chemostat ... 

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. 

In plug flow reactor the value for Cs is reduced from 12 to 3 g-m 3, then the retention time and rate model with recycle ratio in a plug flow reactor can be written as ... 

Reaction takes place only within the plug flow element of the recycle reactor, and the gross product stream from this element is divided into two portions one becomes the net product and the second is mixed with fresh feed. The mixture of the fresh feed and recycle stream is then fed to the plug flow element. By varying the relative quantities of the net product and recycle streams, one is able to obtain widely varying performance characteristics. At... 

The basic design equation for a plug flow reactor (equation 8.2.7) may be used to describe the steady-state conversion achieved in the plug flow element of the recycle reactor ... 

Examination of the limiting forms of equation 8.3.72 for R = 0 and R = 00 indicates that the recycle reactor can approach either plug flow or CSTR behavior. For intermediate values of the recycle ratio this equation can be integrated if the form of the reaction rate expression is known. 

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. 

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