Plug flow, in reactors

The effectiveness of a fluidized bed as a ehemical reactor depends to a large extent on the amount of convective and diffusive transfer between bubble gas and emulsion phase, since reaction usually occurs only when gas and solids are in contact. Often gas in the bubble cloud complex passes through the reactor in plug flow with little back mixing, while the solids are assumed to be well mixed. Actual reactor models depend greatly on kinetics and fluidization characteristics and become too complex to treat here. 

The model is a reactor in plug flow with inlet flows on the side programmed to result in a particular RTD. The relation between the distribution functions is... 

Solids and Gas Both in Plug Flow, When solids and gas pass through the reactor in plug flow, their compositions change during passage. In addition, such operations are usually nonisothermal. 

Piston flow signifies that some of the liquid passes through the reactor in plug flow. This liquid, unlike that involved in short-circuiting, has a certain residence time in the reactor. In certain operations, it is essential that the flow approach as close as possible some ideal situation, usually plug flow (e.g., in continuous, large-scale chromatographic separations). 

We start by considering the steady state in a cylindrical irregularity, extending downward into the bed from near its upper surface. Outside the disturbed region, the reaction can be described by the usual heat and material balance equations for a reactor in plug flow,... 

Another advantage of spherical reactors is that they are the most aco-nonucal shape for high pressures. As a first approximation we will as.sume that the fluid moves down through the reactor in plug flow. Consequently, because... 

Another way of looking at the segregation model for a continuous-flow system is the PFR shown in Figures 13-15(a) and (b). Because the fluid flows down the reactor in plug flow, each exit stream corresponds to a specific residence time in the reactor. Batches of molecules are removed from the reactor at different locations along the reactor in such a manner so as to duplicate the RTD function, (/). The molecules removed near the entrance to the reactor... 

In the past these transport phenomena usually have been treated assuming that the fluid passes through the reactor in plug flow. This assumption then has been revised after careful studies have revealed, that there is not only a microscopic but also a macroscopic void fraction distribution. Consequently,there is a certain... 

Because the characteristic of tubular reactors approximates plug-flow, they are used if careful control of residence time is important, as in the case where there are multiple reactions in series. High surface area to volume ratios are possible, which is an advantage if high rates of heat transfer are required. It is sometimes possible to approach isothermal conditions or a predetermined temperature profile by careful design of the heat transfer arrangements. 

The smaller reactor approaches plug-flow behavior and exhibits a large temperature gradient. In this case, external recycle provides the same degree of back-mixing as is provided by internal circulation in the larger diameter reactor. 

Often, complete mixing cannot be approached for economic reasons. Inactive or dead zones, bypassing, and limitations of energy input are common causes. Packed beds are usually predominantly used in plug flow reactors, but they may also have small mixing zones... 

Real reactors deviate more or less from these ideal behaviors. Deviations may be detected with re.sidence time distributions (RTD) obtained with the aid of tracer tests. In other cases a mechanism may be postulated and its parameters checked against test data. The commonest models are combinations of CSTRs and PFRs in series and/or parallel. Thus, a stirred tank may be assumed completely mixed in the vicinity of the impeller and in plug flow near the outlet. 

We wish to compare the performance of two reactor types plug flow versus CSTR with a substrate concentration of Csf = 60g-m 3 and a biomass yield of Y = 0.1. In a plug flow bioreactor with volume of 1 m3 and volumetric flow rate of 2.5 m -li what would be the recycle ratio for maximum qx compared with corresponding results and rate models proposed for the chemostat ... 

In plug flow reactor the value for Cs is reduced from 12 to 3 g-m 3, then the retention time and rate model with recycle ratio in a plug flow reactor can be written as ... 

In principle, TPD can also be applied to high-surface area catalysts in plug-flow reactors. Often, however, the curves are seriously broadened by mass-transport phenomena. Hence, the use of single crystals or particles on planar supports offers great advantages for these investigations. 

When choosing between different types of reactors, both continuous and batch reactors were considered from the point of view of the performance of the reactor (continuous plug-flow and ideal batch being equivalent in terms of residence time). If a batch reactor is chosen, it will often lead to a choice of separator for the reactor effluent that also operates in batch mode, although this is not always the case as intermediate storage can be used to overcome the variations with time. Batch separations will be dealt with in Chapter 14. 

As with continuous processes, the heart of a batch chemical process is its reactor. Idealized reactor models were considered in Chapter 5. In an ideal-batch reactor, all fluid elements have the same residence time. There is thus an analogy between ideal-batch reactors and plug-flow reactors. There are four major factors that effect batch reactor performance ... 

Size Comparisons Between Cascades of Ideal Continuous Stirred Tank Reactors and Plug Flow Reactors. In this section the size requirements for CSTR cascades containing different numbers of identical reactors are compared with that for a plug flow reactor used to effect the same change in composition. 

At 25 °C the reaction is first-order in each reactant with a rate constant of 9.92 m3/ kmole-ksec. A feed stream containing equimolal quantities of B and C (0.1 kmole/m3) is to be processed at a rate of 0.1111 m3/ksec. A tubular reactor (assume plug flow) with an effective volume of 2.20 m3 is to be employed in the processing operation. 

For a few highly idealized systems, the residence time distribution function can be determined a priori without the need for experimental work. These systems include our two idealized flow reactors—the plug flow reactor and the continuous stirred tank reactor—and the tubular laminar flow reactor. The F(t) and response curves for each of these three types of well-characterized flow patterns will be developed in turn. 

The choice of a reactor is usually based on several factors such as the desired production rate, the chemical and physical characteristics of the chemical process, and the risk of hazards for each type of reactor. In general, small production requirements suggest batch or semi-batch reactors, while large production rates are better accommodated in continuous reactors, either plug flow or continuous stirred tank reactors (CSTR). The chemical and physical features that determine the optimum reactor are treated in books on reaction engineering and thus are not considered here. 

It is not practical to stir all reaction systems, for example, bulk polymerizations, postpolymerization reactions, fixed-bed catalytic reactors, and plug-flow reactors. Although multipoint temperature sensing is often used as a key solution to determine a runaway in nonagitated vessels, the occurrence of hot spots may not always be detected. 

A fluidized bed reactor has a substantial free space above the main level of the catalyst for purpose of disengaging entrainment. In this region plug flow may be assumed to prevail. An overall appropriate model accordingly will consist of well mixed and bypass zones in parallel followed by a plug flow zone. The fraction of flow in bypass is 1-a and the fraction of vessel volume in plug flow is 2. Find the transfer function and equations for the responses to step and impulse inputs of tracer. 

A reaction A = B => C is to be conducted in this vessel with Cb0 - 0. In plug flow at the residence time in this vessel conversion of A "would be 70%. What is the concentration of B at the outlet of this reactor in segregated flow Pertinent functions of the tracer response are... 

An enzyme reaction has the M-M rate equation with rm = 13 mol/liter.min and K, = 0.03 mol/liter. Starting concentration is Cs = 10 mol/liter and the flow rate is 10 liter/hr. Find conversions in plug flow and stirred tank reactors. 

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