Reactors in series and parallel

The definitions of the three ideal reactors, and the fundamentals of ideal reactor sizing and analysis are covered in Chapters 3 and 4. Graphical interpretation of the design equations (the Levenspiel plot ) is used to compare the behavior of the two ideal continuous reactors, the plug flow and continuous stirred-tank reactors. This follows the pattern of earlier texts. However, in this book, graphical interpretation is also used extensively in the discussion of ideal reactors in series and parallel, and its use leads to new insights into the behavior of systems of reactors. 

A single reactor is by no means the only possible configuration employed patents disclose configurations including multiple reactors in series and parallel, with one or both types within the same production line. Multiple reactors permit the production of resins with a wider variety of properties than that available from a single reactor. 

Fig. 8. Combined flow reactor models (a) parallel flow reactors with longitudinal diffusion (diffusivities can differ), (b) internal recycle—cross-flow reactor (the recycle can be in either direction), comprising two countercurrent plug-flow reactors with intercormecting distributed flows, (c) plug-flow and weU-mixed reactors in series, and (d) 2ero-interniixing model, in which plug-flow reactors are parallel and a distribution of residence times dupHcates that...

Suppose you have two identical PFRs and you want to use them to make as much product as possible. The reaction is pseudo-first-order and the product recovery system requires a minimum conversion of 93.75%. Assume constant density. Do you install the reactors in series or parallel Would it affect your decision if the minimum conversion could be lowered ... 

The Total Connectivity Model The connectivity model is a reactor superstructure formulation that attempts to approximate different reactor types using a network of small CSTRs. Combination of CSTRs in series and parallel allows for the approximation of different fundamental reactor types, specificdly ... 

Figure 17.1 shows that it is possible to combine reactors and vary their residence times or volumes so that the final volume is equivalent or equal to the volume of a single reactor. The main advantage of combining reactors in series or parallel is to utilize less volume to yield the same efficiency, yield, selectivity, and final conversion as much as larger reactor yields. Two reactors in series are represented in Figure 17.1. If one assumes a PFR model for both reactors, the shaded areas under the kinetic curve ABEA and BCDE represent the two PFRs in series. The conversion at the outlet of the first and second reactors is X i and Xa2, respectively. Note that the area is proportional to the volume of each reactor, and therefore the total volume is the sum of the volumes Vi-Fy2=VppR.

As mentioned before, the snm of the volnmes of tanks (CSTR) in series is smaller than the volume of a single CSTR (see Fignre 17.1). Besides, when the reactor volume is constant, the average residence time is equal to the space time fcsTR = tcsTR for each reactor in series, bnt it is different for a single reactor. The reaction kinetics must be known and it is valid for any reactor in series or parallel. 

The main goal is nsnally determining the required number of reactors in series or parallel to achieve the maximnm desired conversion or productivity and to minimize the reactor volnme. 

For the same production capacity, the oxygen-based process requires fewer reactors, all of which operate in parallel and are exposed to reaction gas of the same composition. However, the use of purge reactors in series for an air-based process in conjunction with the associated energy recovery system increases the overall complexity of the unit. Given the same degree of automation, the operation of an oxygen-based unit is simpler and easier if the air-separation plant is outside the battery limits of the ethylene oxide process (97). 

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 have considered two types of ideal flow reactor the piston flow reactor and the perfectly mixed CSTR. These two ideal types can be connected together in a variety of series and parallel arrangements to give composite reactors that are... 

A parallel reactor system has an extra degree of freedom compared with a series system. The total volume and flow rate can be arbitrarily divided between the parallel elements. For reactors in series, only the volume can be divided since the two reactors must operate at the same flow rate. Despite this extra variable, there are no performance advantages compared with a single reactor that has the same total V and Q, provided the parallel reactors are at the same temperature. When significant amounts of heat must be transferred to or from the reactants, identical small reactors in parallel may be preferred because the desired operating temperature is easier to achieve. 

The temperature counterpart of Q>aVR ccj-F/R and if ccj-F/R is low enough, then the reactor will be adiabatic. Since aj 3>a, the situation of an adiabatic, laminar flow reactor is rare. Should it occur, then T i, will be the same in the small and large reactors, and blind scaleup is possible. More commonly, ari/R wiU be so large that radial diffusion of heat will be significant in the small reactor. The extent of radial diffusion will lessen upon scaleup, leading to the possibility of thermal runaway. If model-based scaleup predicts a reasonable outcome, go for it. Otherwise, consider scaling in series or parallel. 

This section indicates a few useful generalizations that are pertinent in considerations of isothermal series and parallel combinations of ideal plug flow and stirred tank reactors. 

Comparison may be made of the reactor performance of tanks in series and tanks in parallel, in light of the different RTDs shown in Figure 17.4. If the two tanks shown in Figure 17.3 are arranged in parallel, and the flow, q, is split evenly between them, they act together... 

Extension of the Kunii-Levenspiel bubbling-bed model for first-order reactions to complex systems is of practical significance, since most of the processes conducted in fluidized-bed reactors involve such systems. Thus, the yield or selectivity to a desired product is a primary design issue which should be considered. As described in Chapter 5, reactions may occur in series or parallel, or a combination of both. Specific examples include the production of acrylonitrile from propylene, in which other nitriles may be formed, oxidation of butadiene and butene to produce maleic anhydride and other oxidation products, and the production of phthalic anhydride from naphthalene, in which phthalic anhydride may undergo further oxidation. 

Syncrude s Sulphur Recovery Units are two parallel trains of the conventional Claus plant consisting of thermal conversion and three catalytic reactors in series. Air is supplied to the furnace to oxidize sufficient H2S to SO2 such that an H2S S02 ratio of 2.1 1 is effected (1). The sulphur recovery efficiency is designed for approximately 95% (2) however, experience to date, at near design capacity, has indicated 98% H2S conversions and 97% sulphur recoveries (1). 

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