Backmixing, degree

Effect of Different Decrees ofNlixing. Once Eqq is evaluated on the basis of A/, and N- using equation 87, it has to be translated into Ey by considering the degree of mixing on the tray. It is obvious that for a small plate with completely backmixed Hquid,... 

Fig. 28. Relationship between Fqq and at different degrees of Hquid backmixing (4). Curves represent different Peclet numbers. From top to bottom...

A CSTR is a deliberately backmixed reactor and, in principle, its effluent temperature and composition are the same as the reactor contents. With an ideal CSTR, the feed blends instantaneously with the uniform reactor contents. In actual practice, of course, we find that feed blending time may be protracted, and varying degrees of segregation, short circuiting and stagnation exist in the reactor contents. 

Salmi T, Paatero E and Fagerstolt K (1993b) Optimal Degree of Backmixing in Autocatalytic Reactions A Case Study Chlorination of Dodecanoic Acid, Chem Eng Res Des, 71 531-542. 

Open vessel the fluid may have any degree of backmixing at the inlet and outlet that is, the flow at these points may be anywhere between the extremes of PF (no backmixing) and BMF (complete backmixing). 

In a chemical process, the use of recycle, that is, the return of a portion of an outlet stream to an inlet to join with fresh feed, may have the following purposes (1) to conserve feedstock when it is not completely converted to desired products, and/or (2) to improve the performance of a piece of equipment such as a reactor. It is the latter purpose that we consider here for a PFR (the former purpose usually involves a separation process downstream from a reactor). For a CSTR, solution of problem 14-26 shows that recycling alone has no effect on its performance, and hence is not used. However, it provides a clue as to the anticipated effect for a PFR. The recycle serves to back-mix the product stream with the feed stream. The effect of backmixing is to make the performance of a PFR become closer to that of a CSTR. The degree of backmixing, and... 

This diffusive flow must be taken into account in the derivation of the material-balance or continuity equation in terms of A. The result is the axial dispersion or dispersed plug flow (DPF) model for nonideal flow. It is a single-parameter model, the parameter being DL or its equivalent as a dimensionless parameter. It was originally developed to describe relatively small departures from PF in pipes and packed beds, that is, for relatively small amounts of backmixing, but, in principle, can be used for any degree of backmixing. 

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. 

Slurry reactors achieve a similar intimate contacting of oil and catalyst and yet may operate with a lower degree of backmixing than the ebullated or expanded bed. In the slurry design, heavy oil is mixed with finely divided catalyst particles and fed upward, with hydrogen, through an empty reactor vessel. Oil and catalyst flow concurrently and may approach plug-flow behavior. 

Zuiderweg and Nutter [IChemE Symp. Ser. 128, A481 (1992)] explain the loss of capacity/efficiency by a large degree of backmixing and vapor recycle at high flow parameters, promoted by the solid walls... 

The results show that the plant reactor falls between the plug flow and backmixing reactor lines. The distance between lines (2) and (3) in Figure 5-41 measures the degree of backmixing. There is considerable departure from the theoretical plug flow curve. This shows... 

Series-of-stirred-tanks model The series-of-stirred-tanks model (often referred to as the cell model) is perhaps the simplest type of stagewise model for the backmix reactor. In this model, the reactor is represented by a series of perfectly mixed stages. The degree of backmixing is characterized by the number of stages the... 

The modeling and design of a three-phase reactor requires the knowledge of several hydrodynamic (e.g., flow regime, pressure drop, holdups of various phases, etc.) and transport (e.g., degree of backmixing in each phase, gas-liquid, liquid-solid mass transfer, fluid-reactor wall heat transfer, etc.) parameters. During the past decade, extensive research efforts have been made in order to improve our know-how in these areas. Chapters 6 to 8 present a unified review of the reported studies on these aspects for a variety of fixed bed columns (i.e., co-current downflow, co-current upflow, and counter-current flow). Chapter 9 presents a similar survey for three-phase fluidized columns. 

The particle size distribution produced during precipitation is a result of the relative rates of reaction, nudeation, growth, and agglomeration, as well as the degree of backmixing in the precipitator. The kinetics of each of these steps will be discussed next. 

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