Cascading, continuous stirred tank reactors

During the manufacturing process, if the grafting increases during early stages of the reaction, the phase volume will also increase, but the size of the particles will remain constant [146-148]. Furthermore, reactor choice plays a decisive role. If the continuous stirred tank reactor (CSTR) is used, little grafting takes place and the occlusion is poor and, consequently, the rubber efficiency is poor. However, in processes akin to the discontinuous system(e.g., tower/cascade reactors), the dispersed phase contains a large number of big inclusions. 

This section is concerned with batch, semi-batch, continuous stirred tanks and continuous stirred-tank-reactor cascades, as represented in Fig. 3.1 Tubular chemical reactor systems are discussed in Chapter 4. 

For steady-state operation of a continuous stirred-tank reactor or continuous stirred-tank reactor cascade, there is no change in conditions with respect to time, and therefore the accumulation term is zero. Under transient conditions, the full form of the equation, involving all four terms, must be employed. 

For any continuous stirred-tank reactor, n, in a cascade of reactors (Fig. 3.13) the reactor n receives the discharge from the preceding reactor, n-1, as its feed and discharges its effluent into reactor n+1, as feed to that reactor. 

Figure 3.13. Cascade of continuous stirred-tank reactors.

CONTINUOUS STIRRED TANK REACTOR CASCADE (CSTR)... 

A cascade of three continuous stirred-tank reactors arranged in series, is used to carry out an exothermic, first-order chemical reaction. The reactors are jacketed for cooling water, and the flow of water through the cooling jackets is countercurrent to that of the reaction. A variety of control schemes can be employed and are of great importance, since the reactor scheme shows a multiplicity of possible stable operating points. This example is taken from the paper of Mukesh and Rao (1977). 

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. 

In Section 11.1.3.2 we considered a model of reactor performance in which the actual reactor is simulated by a cascade of equal-sized continuous stirred tank reactors operating in series. We indicated how the residence time distribution function can be used to determine the number of tanks that best model the tracer measurement data. Once this parameter has been determined, the techniques discussed in Section 8.3.2 can be used to determine the effluent conversion level. 

Figure 4.13 Enzymatic by-product removal synthesis of dinitrodibenzyl from nitrotoluene applying a cascade of continuous stirred-tank reactors while degassing with nitrogen...

The system mostly applied in practice for supply of ozone is the bubble column and the stirred tank reactor. With these reactor systems it is always possible to set up the complete reactor modification as a plug flow reactor, a continuous flow single stirred tank reactor or a cascade of stirred tank reactors. 

Several continuous stirred tank reactors are often operated in series or cascade as shown in Fig. 13. In this way, the disadvantages of the relatively low reactant concentration on the one hand, and by-passing on the other, may be partially off-set. As the number of tanks in series increases, the performance of the complete system approaches that of a plug-flow reactor and, in the limit of an infinite number of tanks, becomes equal to it. 

Sulfonation of p-nitrotoluene (PNT) is performed in a cascade of Continuous Stirred Tank Reactors (CSTR). The process is started by placing a quantity of converted mass in the first stage of the cascade, a 400-liter reactor, and heating to 85 °C with jacket steam (150°C). PNT melt and Oleum are then dosed in simultaneously (exothermal reaction). When 110°C is reached, cooling is switched on automatically. On the day of the accident, a rapid increase in pressure took place at 102 °C. The lid of the reactor burst open and the reaction mass, which was decomposing, flowed out like lava, causing considerable damage. 

Classical chemical reaction engineering provides mathematical concepts to describe the ideal (and real) mass balances and reaction kinetics of commonly used reactor types that include discontinuous batch, mixed flow, plug flow, batch recirculation systems and staged or cascade reactor configurations (Levenspiel, 1996). Mixed flow reactors are sometimes referred to as continuously stirred tank reactors (CSTRs). The different reactor types are shown schematically in Fig. 8-1. All these reactor types and configurations are amenable to photochemical reaction engineering. 

The Hoechst slurry process was improved over the years and has evolved into what is now called the Hostalen process. Hostalen is a slurry-cascade process that is capable of producing a wide range of molecular weight distributions of HOPE. The modern Hostalen process employs 2 continuous stirred tank reactors that can be run in series or in parallel to produce unimodal and bimodal HOPE (11). 

Slurry processes in hydrocarbon diluent are used in the production of HDPE, including bimodal polymers produced in the cascade process in which different hydrogen concentrations are applied in two or more reactors in series. Liquid loop reactors are generally used with a light hydrocarbon diluent such as isobutane, whereas heavier hydrocarbon diluents are typically used in continuous stirred tank reactors. 

Calculate the reactor size requirements for one continuously stirred tank reactor (CSTR). Also calculate the volume requirements for a cascade composed of two identical CSTRs. Assume isothermal operation at 25°C where the reaction rate constant is equal to 9.92m /(kgmol ks). Reactant concentrations in the feed are each equal to 0.08kgmol/m, and the liquid feed rate is equal to 0.278 m /ks. The desired degree of conversion is 87.5%. 

Figure 1.2 Continuous reactors (a) tubular reactor, (b) continuous stirred-tank reactor (CSTR), and (c) cascade of CSTRs.

This can be regarded as the link between the ideal continuous stirred-tank reactor (tank number N = 1) and the ideal tubular reactor (N oo). For a first-order reaction and a cascade of N identical tanks, Equation (2.2-21) is obtained ... 

Hatton and coworkers have also analyzed processes involving ELMs. Using their advancing-front model as a basis, they have studied staged operations (10 1), continuous stirred tank reactors (105), and mixer cascades (106). One interesting aspect of their analysis is the effect of emulsion recycle. They analyzed the effect on extraction rate of recycling used emulsion and combining this with new emulsion. 

Figure 8.2 Types of continuous-flow stirred-tank reactors (a) three-stage cascade of stirred-tank reactors (b) vertically staged cascade of three stirred tanks. Compartmented versions of a battery of stirred tanks in a single horizontal shell may also be employed. (Adapted from J. R. Couper, W. R. Penney, J. R. Fair, and S. M. Walas. Chemical Process Equipment Selection and Design. Copyright 2010. Used with permission of Elsevier.)...

S.3.2.3 Size Comparisons Between Cascades of Ideal Continuous Stirred-Tank Reactors and Plug Flow Reactors... 

A continuous cascade of stirred tank reactors consists of N tanks in series as shown below. 

In Section 3.4.2, it was shown that the RTD in real reactors can be described with a series of ideally continuous stirred tank reactors. The scheme of such a cascade of continuous stirred tanks is shown in Figure 3.21. The total volume is divided in N equal sized stirred vessels. 

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