Stirred tank reactor , practical

Interesting and practically important idea of separation of processes of olefins and dienes (co)polymerization in the presence of complex Ziegler-Natta catalysts into fast (formation of active sites and reaction mixture) and slow ((co)polymerization itself) stages was proposed. In practical aspect it determines advisability of use of tubular turbulent pre-reactor before stirred tank reactor with large reagents residence time in reaction zone for realization of the main process ((co)polymerization). 

Gradientless differential reactors allow evaluation of kinetic data practically free of distortion by heat/temperature effects. Depending on the flow, a distinction is made between reactors with outer and inner circulation (recycle reactor, continuous stirred tank reactor. Figure 4.11.1). Evaluation of kinetic measurements by means of the differential method is straightforward as the algebraic balance equation for a stirred tank reactor can be applied (prerequisite high recycle ratio R). In practice it is found that recycle ratios of more than 10 are sufficient to achieve practically ideal... 

In fact, it is often possible with stirred-tank reactors to come close to the idealized well-stirred model in practice, providing the fluid phase is not too viscous. Such reactors should be avoided for some types of parallel reaction systems (see Fig. 2.2) and for all systems in which byproduct formation is via series reactions. 

Continuous-Flow Stirred-Tank Reactor. In a continuous-flow stirred-tank reactor (CSTR), reactants and products are continuously added and withdrawn. In practice, mechanical or hydrauHc agitation is required to achieve uniform composition and temperature, a choice strongly influenced by process considerations, ie, multiple specialty product requirements and mechanical seal pressure limitations. The CSTR is the idealized opposite of the weU-stirred batch and tubular plug-flow reactors. Analysis of selected combinations of these reactor types can be useful in quantitatively evaluating more complex gas-, Hquid-, and soHd-flow behaviors. 

Biocatalysts in nature tend to be optimized to perform best in aqueous environments, at neutral pH, temperatures below 40 °C, and at low osmotic pressure. These conditions are sometimes in conflict with the need of the chemist or process engineer to optimize a reaction with respect to space-time yield or high product concentration in order to facilitate downstream processing. Furthermore, enzymes and whole cells are often inhibited by products or substrates. This might be overcome by the use of continuously operated stirred tank reactors, fed-batch reactors, or reactors with in situ product removal [14, 15]. The addition of organic solvents to increase the solubility of substrates and/or products is a common practice [16]. 

It is common practice to use geometric similarity in the scaleup of stirred tanks (but not tubular reactors). This means that the production-scale reactor will have the same shape as the pilot-scale reactor. All linear dimensions such as reactor diameter, impeller diameter, and liquid height will change by the same factor, Surface areas will scale as Now, what happens to tmix upon scaleup ... 

This chapter develops the techniques needed to analyze multiple and complex reactions in stirred tank reactors. Physical properties may be variable. Also treated is the common industrial practice of using reactor combinations, such as a stirred tank in series with a tubular reactor, to accomplish the overall reaction. 

This chapter treats the effects of temperature on the three types of ideal reactors batch, piston flow, and continuous-flow stirred tank. Three major questions in reactor design are addressed. What is the optimal temperature for a reaction How can this temperature be achieved or at least approximated in practice How can results from the laboratory or pilot plant be scaled up ... 

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. 

For practical purposes it is often beneficial to use a heterogeneous system with the enzyme as a solid preparation which easily can be separated from the product in the liquid phase. Solid enzyme preparatiorrs can conveniently be used in packed bed and stirred tank reactors. As in other cases with heterogeneous catalysis, mass trarrsfer limitations can reduce the overall reaction rate, but usually this is no major problem. 

The perfectly stirred reactor (PSR) or continuously stirred tank reactor (CSTR) is an idealization that proves useful in describing laboratory experiments and can often be used in the modeling of practical situations. As illustrated in Fig. 16.4, gases enter the reactor with a mass-flow rate of m, a temperature of T, and a mass-fraction composition of Y . Once inside the reactor, the gases are presumed to mix instantaneously and perfectly with the gases already resident in the reactor. Thus the temperature and composition within the reactor are perfectly uniform. 

All chemical reactions are accompanied by some heat effects so that the temperature will tend to change, a serious result in view of the sensitivity of most reaction rates to temperature. Factors of equipment size, controllability, and possibly unfavorable product distribution of complex reactions often necessitate provision of means of heat transfer to keep the temperature within bounds. In practical operation of nonflow or tubular flow reactors, truly isothermal conditions are not feasible even if they were desirable. Individual continuous stirred tanks, however, do maintain substantially uniform temperatures at steady state when the mixing is intense enough the level is determined by the heat of reaction as well as the rate of heat transfer provided. 

Almost innumerable instances of such reactions are practiced. Single-batch stirred tanks, CSTR batteries, and tubular flow reactors are all used. Many examples are given in Table 17.1. As already pointed out, the size of equipment for a given purpose depends on its type. A comparison has been made of the production of ethyl acetate from a mixture initially with 23% acid and 46% ethanol these sizes were found for 35% conversion of the acid (Westerterp, 1984, pp. 41-58) ... 

There is one further point of comparison. Interpretation of results from a stirred-tank reactor depends on the assumption that the contents of the tank are well mixed. Interpretation of results from a tubular reactor rests on the assumption of plug flow unless the flow is laminar and is treated as such. Which of these two assumptions can be met most satisfactorily in practical experiments Unless the viscosity of the reaction mixture is high or the reaction extremely fast, a high speed stirrer is very effective in maintaining the contents of a stirred tank uniform. On the other hand, a tubular reactor may have to be very carefully designed if back-mixing is to be completely eliminated, and in most practical situations there is an element of uncertainty about whether the plug flow assumption is valid. 

One of the simplest practical examples is the homogeneous nonisothermal and adiabatic continuous stirred tank reactor (CSTR), whose steady state is described by nonlinear transcendental equations and whose unsteady state is described by nonlinear ordinary differential equations. 

In Equation 2.21, the index i refers to all compounds of the reaction mass and to the reactor itself. However, in practice, for stirred tank reactors, the heat capacity of the reactor is often negligible compared to that of the reaction mass. In order to simplify the expressions, the heat capacity of the equipment can be ignored. This is justified by the following example. For a 10 m3 reactor, the heat capacity of the reaction mass is in the order of magnitude of 20000kJ K 1 whereas the metal mass in contact with the reaction medium is about 400 kg, representing a heat capacity of about 200 kj K"1, that is, ca. 1% of the overall heat capacity. Further, the error leads to a more critical assessment of the situation, which is a good practice... 

---