The recycle reactor

The most general form of the recycle flow mixing system is illustrated in Fig. 13. 

The flow-mixing transfer functions Gi(s) and G2(s) can be chosen to represent any desired parallel or series combinations of PFRs and CSTRs or other more complex elements. Making a mass balance at the entry flowmixing point 

Rearranging eqn. (59) and replacing q with RQ, where i is the recycle ratio 

Gibilaro [49] has considered a recycle model of the form of eqn. (60) where Gj (s) and G2(s) are general series combinations of PFR and equal size CSTR reactors and he gives sixteen references to published work involving more restricted forms of Gj (s) and G2 is). With an infinite choice over the forms of G (s) and G2(s) and the magnitude of R, the recycle model is seen to be the most flexible of all flow-mixing models. The performance of each specific form of Gj (s) as a potential reactor must be investigated individually in practice, the model is often reduced to a pure PFR element 

Imagine a first-order reaction taking place in such a system under conditions where rk, i.e. VkjQ, is 10 and R is 5. Using the technique previously adopted in Sect. 5.1 and outlined in Appendix 2, we can readily calculate that this system would achieve 96.3% conversion of reactant. Under these conditions, the recycle reactor volume turns out to be 3.03 times that of an ideal PFR required for the same duty. This type of calculation allows Fig. 14 to be constructed this is similar in form to Fig. 12, but lines of constant for the tanks-in-series model have been replaced by lines of constant recycle ratio for the recycle model. From a size consideration alone, the choice of a PFR recycle reactor is not particularly 

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Figure 1.3.2 gives another perspective for scale-down to recycle reactor studies. In this actual case, after preliminary studies in a recycle reactor, a 5-stage adiabatic reactor was envisioned (Betty 1979.) Scaling down the proposed commercial reactor, a 3 diameter tube was designed with elaborate temperature compensation (heating and insulation) for pilot-plant studies (Betty 1968, 1969.) Small squares in the proposed reactor represent side views of cylindrical catalyst cutouts for the recycle reactor... 

The differential reactor is the second from the left. To the right, various ways are shown to prepare feed for the differential reactor. These feeding methods finally lead to the recycle reactor concept. A basic misunderstanding about the differential reactor is widespread. This is the belief that a differential reactor is a short reactor fed with various large quantities of feed to generate various small conversions. In reality, such a system is a short integral reactor used to extrapolate to initial rates. This method is similar to that used in batch reactor experiments to estimate... 

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... 

If a catalyst is coking up or falling apart in a short time in the recycle reactor then idow will decrease and becomes unknown after a time. In this case is best to improve the life time or the mechanical properties of the catalyst before making tests in the recycle reactor. 

Test for the Recycle Reactor Flowsheet for the experimental unit... 

The most important conclusions of their work were that the inter-particle effects are reasonably small for the particular recycle reactor design, and that kinetic constants determined in the recycle reactor were trustworthy. 

An important improvement would be the significant reduction of the empty volume in the recycle reactor. This calls for a special insert to block out most of the empty space without choking the flow. A practical solution of this type is on the drawing board. 

Recycle reactors at that time were called Backmix Reactors. They were correctly considered the worst choice for the production of a reactive intermediate, yet the best for kinetic studies. The aim of the kinetic study for ethylene oxidation was to maximize the quality of the information, leaving the optimization of production units for a later stage in engineering studies. The recycle reactors could provide the most precise results at well defined conditions even if at somewhat low selectivity to the desired product. 

Example 4.5 Suppose the recycle reactor in Figure 4.2 is used to evaluate a catalyst for the manufacture of sulfuric acid. The catal5Tic step is the gas-phase oxidation of sulfur dioxide ... 

When the mass transfer resistances are eliminated, the various gas-phase concentrations become equal a/(/, r, z) = j(r, z) = a(r, z). The very small particle size means that heat transfer resistances are minimized so that the catalyst particles are isothermal. The recycle reactor of Figure 4.2 is an excellent means for measuring the intrinsic kinetics of a finely ground catalyst. At high recycle rates, the system behaves as a CSTR. It is sometimes called a gradientless reactor since there are no composition and temperature gradients in the catalyst bed or in a catalyst particle. 

Example 10.11 The piping in the recycle reactor of Example 10.9 has been revised to lower the recycle line and pump volume to 100 cm. What eflbct will this have on the exit concentration of component A if all other conditions are held constant ... 

Simulation studies are also conducted for a dispersed PFR and a recycle reactor at 260 °C, 500 psig and feed with DCPD=0.32 mol/min, CPD=0.96mol/min and ethylene=3.2mol/min. Peclet number (Pe) or the recycle ratio is selected as a variable parameter for the dispersed PFR or for the recycle reactor, respectively. Conversion approaches to that of PFR over Pe=50 as can be seen in Fig.4. It is also worth mentioning that the reactor performance is improved with recycle if the residence time is low. 

The recycle reactor is shown schematically in Figure 1. It consists of a catalytic or electrocatalytic reactor unit with a bypass loop, a recycle pump and a molecular sieve trap unit. The latter comprises one or two packed bed columns in parallel each containing 2-10 g of Linde 5A molecular sieve pellets. On line gas chromatography (Shimadzu 14A) was used for the analysis of CH4, O2, CO, CO2, C2H4 and C2H6 in the reactants and products. 

It follows from the equation above that c, if Fv.rec Fvj This means that for a recycle stream much larger than the feed stream, the catalyst bed operates as a differential reactor, while the whole system gives an outlet concentration differing significantly from that of the feed. This significantly simplifies problems of chemical analysis. In practice, the recycle reactor operates differentially if the recycle ratio Fv.ret/Fv.f is larger than 25. The rate is then given by the overall rate ... 

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. 

Plug flow reactor (PFR) with recycle. The recycle reactor is characterized by a non-zero value of R, that is the ratio between the mass flow rate of the recycled stream and the feeding rate Q. The material balance reads for this case as... 

Consider the following gas-phase reaction that occurs at 350 K and a constant pressure of 200 kPa (Lynch, 1986) A- B + C, for which the rate law is (-/a) = 0.253cA/(l + 0.429cA)2, where (- rA) has units of mol m-3 s-1 cA has units of mol m-3. Pure A is fed to a reactor at a rate of 8 mol s-1. The desired fractional conversion, fA, is 0.99. A recycle PFR is proposed for the reaction. When the recycle ratio, R, is zero, the recycle reactor is equivalent to a PFR. As R approaches infinity, the system is equivalent to a CSTR. However, it is generally stated that the recycle reactor behavior is close to that of a CSTR once R reaches approximately 10 to 20. Furthermore, it is often stated that, for an equivalent fractional conversion, the volume... 

Combine this with the equation of the recycle reactor derived in problem P4.06.06,... 

In this chapter we deal with single reactions. These are reactions whose progress can be described and followed adequately by using one and only one rate expression coupled with the necessary stoichiometric and equilibrium expressions. For such reactions product distribution is fixed hence, the important factor in comparing designs is the reactor size. We consider in turn the size comparison of various single and multiple ideal reactor systems. Then we introduce the recycle reactor and develop its performance equations. Finally, we treat a rather unique type of reaction, the autocatalytic reaction, and show how to apply our findings to it. 

This recycle ratio can be made to vary from zero to infinity. Reflection suggests that as the recycle ratio is raised the behavior shifts from plug flow R = 0) to mixed flow (R = oo). Thus, recycling provides a means for obtaining various degrees of backmixing with a plug flow reactor. Let us develop the performance equation for the recycle reactor. 

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