Recycle flow pattern

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

Jet Mixers Continuous recycle of the contents of a tank through an external pump so arranged that the pump discharge stream appropriately reenters the vessel can result in a flow pattern in the tank which will produce a slow mixing aciion [Fossett, Trans. Jnst. Chem. Eng., 29,322 (1951)]. 

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

We particularly like these three flow or reacting patterns because they are easy to treat (it is simple to find their performance equations) and because one of them often is the best pattern possible (it will give the most of whatever it is we want). Later we will consider recycle reactors, staged reactors, and other flow pattern combinations, as well as deviations of real reactors from these ideals. 

Deviation from the two ideal flow patterns can be caused by channeling of fluid, by recycling of fluid, or by creation of stagnant regions in the vessel. Figure 11.1 shows this behavior. In all types of process equipment, such as heat exchangers, packed columns, and reactors, this type of flow should be avoided since it always lowers the performance of the unit. 

Experiments at 300°C in a packed bed reactor with very large recycle stream give the results shown below for the first order catalytic decomposition A - R —S. Under the best possible conditions (always at 300°C) what Cr max/ Ao may we expect, and how do you suggest we get this (what flow pattern and particle size, large or small) ... 

Aiba (A3), Fox and Gex (F8), Kramers, Baars and Knoll (K15), Metzner and Taylor (MIO), Norwood and Metzner (N3), Van de Vusse (V5) and Wood et al. (W12) have studied flow patterns and mixing times. In addition, Brothman et al. (B22), Gutoff (G9), Sinclair (S16) and Weber (W3) analyzed flow in a stirred tank in terms of the recycle flow model of Fig. 23F. This model corresponds to the draft-tube reactor, and with sufficiently large recycle rate the performance prediction of this model approximates backmix flow. 

In addition to using different catalyst flow patterns in packed and slurry reactors, the flow can be varied to attain different catalyst contacting patterns. As shown in Figure 7-27, many flow patterns such as radial flow and fluid recirculation can be used. These allow variation of the flow velocity u for a given reactor size and residence time x. These recirculation flow patterns approach the flow of recycle reactors so the reactor performance approaches that of a CSTR at high recirculation. 

For any more complex flow pattern we must solve the fluid mechanics to describe the fluid flow in each phase, along with the mass balances. The cases where we can still attempt to find descriptions are the nonideal reactor models considered previously in Chapter 8, where laminar flow, a series of CSTRs, a recycle TR, and dispersion in a TR allow us to modify the ideal mass-balance equations. 

Typical Results Recycle flow ratio and flow pattern... 

Figure 1.187 Simulated flow pattern of the first element of the recycle-flow micro mixer (Re = 14 150 pm) [56] (by courtesy of Transducer Research Foundation).

The residence time distribution of the recycle reactor was determined by tracer experiments. This permitted the interpretation of the flow patterns in the reactor, so that the degree of mixing could be quantified. 

This concept was used for the study of the deactivation of n-hexane catalytic cracking on a US Y zeolite catalyst. The interpretation of the flow patterns in the recycle reactor, necessary for the quantification of the degree of mixing, was based upon tracer experiments. 

Knowledge of the flow patterns for RO systems is fundamental to the understanding of how an RO system functions. Arrays, passes, recycle, and multiple trains are terms used to describe an RO system that are discussed in this chapter. 

Simulation models are needed to evaluate commercial unit performance because of the strong emphasis on recycle within the process, as is shown in Figures 1 and 2. Analysis of such flow patterns require tedious, time-consuming, trial-and-error calculations that are best done by computer. 

The construction of a combined model starts with one image (created, supposed or seeded) where it is accepted that the flow into the device is composed of distinct zones which are coupled in series or parallel and where we have various patterns of flow flow zones with perfect mixing, flow zones with plug flow, zones with stagnant fluid (dead flow). We can complete this flow image by showing that we can have some by-pass connections, some recycled flow and some slip flow situations in the device. 

For elucidation of mechanisms, rate data at very low conversions may be highly desirable. They can be obtained more easily from a batch reactor than from a CSTR or plug-flow tubular reactor. A standard CSTR would have to be operated at very high flow rates apt to cause fluid-dynamic and control problems. The same is true for a standard tubular reactor unless equipped with a sampling port near its inlet, a mechanical complication apt to perturb the flow pattern. If the problem of confining the reaction to a very small flow reactor can be solved—as is possible, for example, for radiation-induced reactions—a differential reactor operated once-through or with recycle may be the best choice. 

Same as above. Effect of cocurrent or counter-current flow patterns studied. Effect of recycle streams or intermediate feeds studied. 

Many conventional wastewater treatment processes that have long been in use are now considered impractical because they require a large amount of space, a large number of unit operations, and are affected by problems associated with odor and other emissions. Recent years have seen an increasing trend toward process intensification, which has led to the development of advanced membrane processes that are simple to construct and operate, have well-defined flow patterns, better dispersion effects, relatively low power consumption, lower emissions, and high mass-transfer performance, which are compact and recyclable. 

Several sophisticated techniques and data analysis methodologies have been developed to measure the RTD of industrial reactors (see, for example, Shinnar, 1987). Various different types of models have been developed to interpret RTD data and to use it further to predict the influence of non-ideal behavior on reactor performance (Wen and Fan, 1975). Most of these models use ideal reactors as the building blocks (except the axial dispersion model). Combinations of these ideal reactors with or without by-pass and recycle are used to simulate observed RTD data. To select an appropriate model for a reactor, the actual flow pattern and its dependence on reactor hardware and operating protocol must be known. In the absence of detailed quantitative models to predict the flow patterns, selection of a model is often carried out based on a qualitative understanding of flow patterns and an analysis of observed RTD data. It must be remembered that more than one model may fit the observed RTD data. A general philosophy is to select the simplest model which adequately represents the physical phenomena occurring in the actual reactor. 

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. 

Modification of flow pattern (e.g., the use of radial flow reactors in catalytic reforming and ammonia synthesis) to reduce the pressure drop and thus enhance the recycle compressor capacity. 

Calculate the effect of recycle on the conversion in tubular reactors with plug flow. NONIDEAL FLOW PATTERNS AND POPULATION BALANCE MODELS 655... 

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