Plug flow reactor recycle operation

A plug flow reactor is operated with partial recycle of unconverted... 

A plug flow reactor is operated with a ratio 3 = R /V of recycle flow to fresh flow. The reaction is first order. Find the ratio Cr/Cf of the effluent and feed concentrations. 

A plug flow reactor is operated with partial recycle of unconverted material. The reaction is second order liquid phase, 2A => B. Concentration of... 

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. 

Experimental plug flow reactors may be small diameter tubes or packed beds with a larger ratio of diameter to length. The argument in favor of their employment is that they may simulate commercial units more closely. Rate data from pilot plant or commercial units also may need to be analyzed. A short packed bed may be operated with a high recycle ratio and will thus achieve substantially isothermal behavior and may have appreciable change in conversion between the net input and output streams. 

These findings differ from ordinary nth-order reactions (n > 0) where the plug flow reactor is always more efficient than the mixed flow reactor. In addition, we should note that a plug flow reactor will not operate at all with a feed of pure reactant. In such a situation the feed must be continually primed with product, an ideal opportunity for using a recycle reactor. 

At present conversion is 2/3 for our elementary second-order liquid reaction 2A 2R when operating in an isothermal plug flow reactor with a recycle ratio of unity. What will be the conversion if the recycle stream is shut off ... 

The recycle reactor is used to reach an operating condition between the theoretical boundaries predicted by the continuous stirred tank reactor and the plug flow reactor. 

Finally, some remarks on the operation of mechanically agitated gas-liquid reactors are worth mentioning. The mode of operation (i.e., batch, semibatch, continuous, periodic, etc.) depends on the specific need of the system. For example, the level of liquid-phase backmixing can be controlled to any desired level by operating the gas-liquid reactor in a periodic or semibatch manner. This provides an alternative to the tanks in series or plug flow with recycle system and provides a potential method of increasing the yield of the desired intermediate in complex reaction schemes. In some cases of industrial importance, the mode of operation needs to be such that the concentration of the solute gas (such as Cl2, H2S, etc.) as the reactor outlet is kept at a specific value. As shown by Joshi et al. (1982), this can be achieved by a number of different operational and control strategies. 

The H-Oil reactor (Fig. 21) is rather unique and is called an ebullated bed catalytic reactor. A recycle pump, located either internally or externally, circulates the reactor fluids down through a central downcomer and then upward through a distributor plate and into the ebullated catalyst bed. The reactor is usually well insulated and operated adiabatically. Frequently, the reactor-mixing pattern is defined as backmixed, but this is not strictly true. A better description of the flow pattern is dispersed plug flow with recycle. Thus, the reactor equations for the axial dispersion model are modified appropriately to account for recycle conditions. 

Another model, which will not be analyzed, is the plug-flow reactor with recycle shown in Fig. 6-1. The reactor itself behaves as an ideal tubular type, but mixing is introduced by the recycle stream. When the recycle rate becomes very large, ideal stirred-tank performance is obtained, and when the recycle is zero, plug-flow operation, results. The response data on the actual reactor are used to evaluate the recycle rate and then the conversion is estimated for a plug-flow reactor with this recycle rate. 

In this chapter, the analysis of chemical reactors is expanded to additional reactor configurations that are commonly used to improve the yield and selectivity of the desirable products. In Section 9.1, we analyze semibatch reactors. Section 9.2 covers the operation of plug-flow reactors with continuous injection along their length. In Section 9.3, we examine the operation of one-stage distillation reactors, and Section 9.4 covers the operation of recycle reactors. In each section, we first derive the design equations, convert them to dimensionless forms, and then derive the auxiliary relations to express the species concentrations and the energy balance equation. 

A recycle reactor is a mathematical model describing a steady plug-flow reactor where a portion of the outlet is recycled to the Met, as shown schematically in Figure 9.5. Although this reactor configuration is rarely used in practice, the recycle reactor model enables us to examine the effect of mixing on the operations of continuous reactors. In some cases, the recycle reactor is one element of a complex reactor model. Below, we analyze the operation of a recycle reactor wifii multiple chemical reactions, derive its design equations, and discuss how to solve fiiem. 

For comparison, for adiabatic plug-flow reactor (R = 0) of the same volume, the outlet extent is Zout — 0.742, and 9out =1.135. The production rate of product B is 1,131 mol/min. For adiabatic CSTR R = oo) of the same volume, the outlet extent is Zout = 0.841, and 9out= 1.132. The production rate of product B is 1260 mol/min. Note that for both isothermal and adiabatic operation, a recycle reactor provides a higher production rate of product B than a corresponding plug-flow reactor and a CSTR. 

The flows in PFR and MFR can be precisely deflned by simple mathematical eqnations, and the batch reactor is simply the batch version of the PFR. A reactor is now considered where the flow is between plug and fnlly mixed, i.e., a nonideal reactor. Two common examples of such partially mixed reactors are the recycle reactor and the tanks-in-series reactor. In the recycle reactor, part of the outlet from a reactor is recycled at the inlet, thns establishing some mixing between the downstream and the upstream fluids. In the tanks-in-series reactor, several mixed-flow reactors are operated in series. A single MFR is fully mixed, whereas an inflnite nnmber of MFRs (or a... 

V decreases with increasing conversion of A. Thus, if it is possible to remove small amounts of V cheaply from large volumes of the reaction mixture, the optimum reactor configuration and mode of operation would involve the use of a plug flow reactor with low conversions of A per pass coupled with a separator to remove the product V and to recycle unconverted reactants. The exact conversion level to be employed will depend on an economic analysis of the combined reactor-separator system. 

In cases where unconverted reactants can readily be separated from the product stream, it may be preferable to use only the CSTR operating at the maximum rate, regardless of the conversion level desired, because the reactants separated can be recycled. In this case one must determine the relative costs of operating in the separation and recycle mode vis-a-vis the costs of utilizing the second-stage plug flow reactor and the attendant separation costs necessary to obtain the final product. 

For autocatalytic reactions it is often appropriate to employ a reactor configuration in which a CSTR operating at the maximum rate is followed by a plug flow reactor. Another alternative would be to employ a recycle reactor in which a portion of the effluent from a PFR is recycled to the reactor inlet. 

Although the system could have been operated in a continuous mode, it was decided to operate it in a batch mode. In the batch high-recycle mode the reactor mass balance is that for a stlrred-tank batch reactor and thus over a period of time is analogous to the mass balance for a plug flow reactor over a distance along... 

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...

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