Reactors recycle

There is of course no point in adding recycle to a CSTR because reactants and products are assumed to be already mixed instantly, and recycle would not change the performance at all except for adding unnecessary pipes and pumps. 

The volumetric flow rate into the reactor system (tubular reactor and associated pipes for recycle) is Vg and the feed concentration is C q as before. However, a portion of the exit stream from the reactor is fed back and mixed with the feed stream. We call this portion R the recycle ratio. (Note that in this section we use R as recycle ratio while everywhere else R means tube radius.) The volumetric flow rate into the reactor is now 

This indicates that the feed into the PFTR is therefore diluted with the product stream. A balance on the flow of A at the mixing point yields 

The residence time in the plug-flow reactor is shortened by the addition of recycle and is given by 

Thus in the RTR the PFTR section sees (1) a diluted feed, (2) a higher flow rate, and (3) a shorter residence time if the recycle ratio R is greater than zero. However, the mass balance on the PFTR itself is unchanged. 

In this section, we present the two-mode models for loop and recycle reactors. In a loop reactor (Fig. 10) of loop length L, a flow rate of qm with an average velocity of u-m) enters and leaves the reactor at points a = 0 and x — l, respectively (where a is the length coordinate along the loop), and the total flow rate in the loop is Q + qin between points a = 0 and x — l, and is Q between points x — l and x — L, due to a recycle rate of Q. The recycle ratio A is the ratio of the volume of fluid returned to the reactor entrance per unit time to the volume of fluid leaving the system per unit time, and is given by A = Q/q-m. 

Neglecting axial diffusion (or assuming L/a 1), the three-dimensional CDR equation of a laminar flow loop reactor is given by 

Using the spatial averaging procedure illustrated in Section II, we average Eqs. (167) (171) in the transverse direction to obtain the two-mode model for a loop reactor, which is given by 

For the special case when no reaction occurs between x — l and x — L, i.e. Cjjnfx — 1) — Cj,m(x — L), the loop reactor reduces to a recycle reactor of length /, the two-mode model for which is given by 

The two-mode loop and recycle reactor models are two-parameter two-mode models. Here, the two parameters are the recycle ratio A, and the dimensionless 

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The per pass ethylene conversion in the primary reactors is maintained at 20—30% in order to ensure catalyst selectivities of 70—80%. Vapor-phase oxidation inhibitors such as ethylene dichloride or vinyl chloride or other halogenated compounds are added to the inlet of the reactors in ppm concentrations to retard carbon dioxide formation (107,120,121). The process stream exiting the reactor may contain 1—3 mol % ethylene oxide. This hot effluent gas is then cooled ia a shell-and-tube heat exchanger to around 35—40°C by usiag the cold recycle reactor feed stream gas from the primary absorber. The cooled cmde product gas is then compressed ia a centrifugal blower before entering the primary absorber. 

Cropley, J.B., Systematic Errors in Recycle Reactor Kinetic Studies, Chemical Engineeiing Piogiess, February 1987, 46-51. (Model building, experimental design)... 

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... 

For conditions in industrial production reactors and in corresponding recycle reactors, the mass transfer coefficients of Gamson et al (1943) will be used. These are approximately correct and simple to use. There may be better correlations for specific cases and especially for larger molecules, where diffiisivity is low and Schmidt number is high. In such cases literature referring to given conditions should be consulted. 

Figure 2.3.2 (Kraemer and deLasa 1988) shows this reactor. DeLasa suggested for Riser Simulator a Fluidized Recycle reactor that is essentially an upside down Berty reactor. Kraemer and DeLasa (1988) also described a method to simulate the riser of a fluid catalyst cracking unit in this reactor.

Figure 2.3.2 The fluidized recycle reactor of Kraemer and deLasa.

Actually, the very first homemade recycle reactors in 1965 at Union Carbide Corp. were of this type. In an ordinary one gallon top-agitated... 

In a differential reactor the product stream differs from the feed only very slightly, so the addition of products to the feed stream can be avoided if most of the product stream is recycled. The feed can be made up mostly from the recycle stream with just enough starting materials added to replace that which was converted in the reaction and blown off in the discharge stream. This is the basis of loop or recycle reactors, as will be explained later. 

Jankowski et al (1978) discuss in detail the great variety of gradientless reactors proposed by several authors with a pictorial overview in their paper. All of these reactors can be placed in a few general categories (1) moving catalyst basket reactors, (2) external recycle reactors, and (3) internal recycle reactors. 

In summary, external recycle reactors are expensive and their usefulness is limited. They can be practical for simple chemical systems where no condensation can occur and neither high pressure nor high temperature is needed. For example Carberry et al (1980) preferred an external recycle reactor over a spinning basket reactor for the study of CO oxidation in dry air at atmospheric pressure. 

The older internal recycle reactors of Berty et al (1969), and Berty (1974) are shown on Figures 2.4.3 a, b. The reactor of Romer and Luft (1974) uses no mechanical moving parts. The recirculation is generated by the feed gas as it expands through a nozzle. A major disadvantage of using a jet is that feed rate and recirculation rate are not independent. Due to the low efficiency of jet pumps, recycle rates are quite low. 

The operational characteristics of the older Berty reactors are described in Berty (1974), and their use in catalyst testing in Berty (1979). Typical uses for ethylene oxide catalyst testing are described in Bhasin (1980). Internal recycle reactors are easy to run with minimum control or automation. 

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... 

Peclet number independent of Reynolds number also means that turbulent diffusion or dispersion is directly proportional to the fluid velocity. In general, reactors that are simple in construction, (tubular reactors and adiabatic reactors) approach their ideal condition much better in commercial size then on laboratory scale. On small scale and corresponding low flows, they are handicapped by significant temperature and concentration gradients that are not even well defined. In contrast, recycle reactors and CSTRs come much closer to their ideal state in laboratory sizes than in large equipment. The energy requirement for recycle reaci ors grows with the square of the volume. This limits increases in size or applicable recycle ratios. 

The most reliable recycle reactors are those with a centrifugal pump, a fixed bed of catalyst, and a well-defined and forced flow path through the catalyst bed. Some of those shown on the two bottom rows in Jankowski s papers are of this type. From these, large diameter and/or high speed blowers are needed to generate high pressure increase and only small gaps can be tolerated between catalyst basket and blower, to minimize internal back flow. 

The original recycle reactor developed at Union Carbide Corporation in 1962 (Berty et al 1968) was modified or adapted by several people to different projects. Many recycle reactors were also designed by others for... 

In Chapter 1, Figure 1.4.1 (Berty et al, 1969) shows the actual measurement results of the older 5 diameter recycle reactor performance, using two different types of equipment. 

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. 

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