Recycle-flow reactor

There are situations where a portion of the produet stream in a plug flow reaetor is returned to the entranee of the feed stream by means 

Introduction to Reactor Design Fundamentals for Ideal Systems 411 

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 

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

For a first order irreversible reaetion (-r ) = kC, whieh gives 

For a first order irreversible reaction (-rA) = kCA, which gives 

The recycle ratio in terms of the fractional conversion XA for the first order irreversible reaction is 

Equation 5-354 determines the value of the recycle ratio for a given conversion, the residence time, and the rate constant in a plug flow reactor. Alternatively, an increase in R will lower the conversion since it produces backmixing in the reactor as it mixes with the feed entrance in the plug flow reactor. As R it yields 

Equation 5-356 is the familiar CFSTR for a first order reaction. As the recycle of the fluid increases, the reactor becomes completely 

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Fig. 18. Flow recycle reactor (Berty type). (Berty, 1974 also from Chaudhari el al.. 1986, by courtesy of Marcel Dekker, Inc.)...

Fig. 1-3 Typical reactors (a) tubular-flow recycle reactor, (b) multi tube-flow reactor, (c) radial-flow catalytic reactor, (d) stirred-tank reactor with internal cooling, (e) loop reactor, (f) reactor with intercoolers (opposite)...

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. 

In a recycle reactor (RR), the conservation statement is that everything produced per pass must be removed, and everything consumed per pass must be supplied by the recycle flow. (See Figure 1.6.1.) The measured rate is r , whatever causes it to be the given value. Here C is the concentration of the reactant and thus, the stoichiometric coefficient a=-l. 

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. 

These requirements can be derived from the above conditions. On the left hand side, the temperature derivative of the heat removal rate can be calculated if the flow over the catalyst is known. This is possible in recycle reactors. On the right hand side, the inequalities represent the two stability criteria, which contain three derivatives ... 

In this work we present results obtained both with batch and continuous flow operation of the gas-recycle reactor-separator utilizing Ag and Ag-Sm203 electrocatalysts and Sr(lwt%) La203 catalysts, in conjunction with Linde molecular sieve 5A as the trapping material, and discuss the synergy between the catalytic and adsorption units in view of the OCM reaction network. 

Appropriate setting of two on-off valves (Fig. 1) allows the system to be operated either as a batch recycle reactor or as a continuous-flow steady-state recycle reactor. 

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...

To obtain a sample, the reactor was manually pressurized with nitrogen to 10 to 12 psi, and the four-port switching valve was activated to divert the flow of reactor solvent to recycle. The pressure forced about 4 ml of sample through the 5-pl slot of the tantalum valve. The valve was then switched to transfer the sample to the HPLC located about 15 feet away. The solvent for transfer and dynamic dilution was ethoxyethanol, with a boiling point of... 

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. 

You have available in your pilot plant a small recycle reactor whose flow pattern may be represented schematically by the following diagram. 

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... 

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