Continuous stirred tank reactor tubular

A useful classification of lands of reaclors is in terms of their concentration distributions. The concentration profiles of certain limiting cases are illustrated in Fig. 7-3 namely, of batch reactors, continuously stirred tanks, and tubular flow reactors. Basic types of flow reactors are illustrated in Fig. 7-4. Many others, employing granular catalysts and for multiphase reactions, are illustratea throughout Sec. 23. The present material deals with the sizes, performances and heat effects of these ideal types. They afford standards of comparison. 

In previous studies, the main tool for process improvement was the tubular reactor. This small version of an industrial reactor tube had to be operated at less severe conditions than the industrial-size reactor. Even then, isothermal conditions could never be achieved and kinetic interpretation was ambiguous. Obviously, better tools and techniques were needed for every part of the project. In particular, a better experimental reactor had to be developed that could produce more precise results at well defined conditions. By that time many home-built recycle reactors (RRs), spinning basket reactors and other laboratory continuous stirred tank reactors (CSTRs) were in use and the subject of publications. Most of these served the original author and his reaction well but few could generate the mass velocities used in actual production units. 

There are a variety of ways of accomplishing a particular unit operation. Alternative types of process equipment have different inherently safer characteristics such as inventory, operating conditions, operating techniques, mechanical complexity, and forgiveness (i.e., the process/unit operation is inclined to move itself toward a safe region, rather than unsafe). For example, to complete a reaction step, the designer could select a continuous stirred tank reactor (CSTR), a small tubular reactor, or a distillation tower to process the reaction. 

Various reactor combinations are used. For example, the product from a relatively low solids batch-mass reactor may be transferred to a suspension reactor (for HIPS), press (for PS), or unagitated batch tower (for PS) for finishing. In a similar fashion, the effluent from a continuous stirred tank reactor (CSTR) may be transferred to a tubular reactor or an unagitated or agitated tower for further polymerization before devolatilization. 

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 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. 

Chapter 1 reviews the concepts necessary for treating the problems associated with the design of industrial reactions. These include the essentials of kinetics, thermodynamics, and basic mass, heat and momentum transfer. Ideal reactor types are treated in Chapter 2 and the most important of these are the batch reactor, the tubular reactor and the continuous stirred tank. Reactor stability is considered. Chapter 3 describes the effect of complex homogeneous kinetics on reactor performance. The special case of gas—solid reactions is discussed in Chapter 4 and Chapter 5 deals with other heterogeneous systems namely those involving gas—liquid, liquid—solid and liquid—liquid interfaces. Finally, Chapter 6 considers how real reactors may differ from the ideal reactors considered in earlier chapters. 

The chemical reactor is the unif in which chemical reactions occur. Reactors can be operated in batch (no mass flow into or out of the reactor) or flow modes. Flow reactors operate between hmits of completely unmixed contents (the plug-flow tubular reactor or PFTR) and completely mixed contents (the continuous stirred tank reactor or CSTR). A flow reactor may be operated in steady state (no variables vary with time) or transient modes. The properties of continuous flow reactors wiU be the main subject of this course, and an alternate title of this book could be Continuous Chemical Reactors. The next two chapters will deal with the characteristics of these reactors operated isothermaUy. We can categorize chemical reactors as shown in Figure 2-8. 

Figure 11.9 Different arrangements and modes of operation for membrane bioreactors Continuous Stirred Tank Reactor (CSTR) with recirculation arrangement (a), dead-end cell (b), tubular with entrapped enzyme (c).

If the points lie close to a straight line, this is taken as confirmation that a second-order equation satisfactorily describes the kinetics, and the value of the rate constant k2 is found by fitting the best straight line to the points by linear regression. Experiments using tubular and continuous stirred-tank reactors to determine kinetic constants are discussed in the sections describing these reactors (Sections 1.7.4 and 1.8.S). 

If the compositions vary with position in the reactor, which is the case with a tubular reactor, a differential element of volume SV, must be used, and the equation integrated at a later stage. Otherwise, if the compositions are uniform, e.g. a well-mixed batch reactor or a continuous stirred-tank reactor, then the size of the volume element is immaterial it may conveniently be unit volume (1 m3) or it may be the whole reactor. Similarly, if the compositions are changing with time as in a batch reactor, the material balance must be made over a differential element of time. Otherwise for a tubular or a continuous stirred-tank reactor operating in a steady state, where compositions do not vary with time, the time interval used is immaterial and may conveniently be unit time (1 s). Bearing in mind these considerations the general material balance may be written ... 

The mean residence time for a continuous stirred-tank reactor of volume Vc may be defined as Vc/v in just the same way as for a tubular reactor. However, in a homogeneous reaction mixture, it is not possible to identify particular elements of fluid as having any particular residence time, because there is complete mixing on a molecular scale. If the feed consists of a suspension of particles, it may be shown that, although there is a distribution of residence times among the individual particles, the mean residence time does correspond to Vc v if the system is ideally mixed. 

These show that a single continuous stirred-tank reactor must always be larger than a batch or tubular plug-flow reactor for the same duty, and for high conversions the stirred tank must be very much larger indeed. If two tanks are used, however, the total volume is less than that of a single tank. Although the detailed calculations... 

Fig. 1.22. Comparison of size and cost of continuous stirred-tank reactors with a batch or a tubular plug-flow reactor first-order reaction, conversion 0.9 ...

If two reactants are involved in a reaction, high concentrations of both, at least initially, may be obtained in a batch or tubular plug-flow reactor, and low concentrations of both in a single continuous stirred-tank reactor. In some circumstances, however, a high concentration of reactant A coupled with a low concentration of reactant B may be desirable. This may be achieved in a number of ways ... 

Fig. 1.28. Reactions in series—comparison between batch or tubular plug-flow reactor and a single continuous stirred-tank reactor. Consecutive first-order reactions,...

This expression enhances the fact that the heat release rate is a function of the conversion and will therefore vary with time in discontinuous reactors or during storage. In a batch reaction, there is no steady state. It is constant in the Continuous Stirred Tank Reactor (CSTR) and is a function of the location in the tubular reactor (see Chapter 8). The heat release rate is... 

Although the early literature described the application of a tubular reactor for the production of SBR latexes(1), the standard continuous emulsion polymerization processes for SBR polymerization still consist of continuous stirred tank reactors(CSTR s) and all of the recipe ingredients are normally fed into the first reactor and a latex is removed from the last one, as shown in Figure 1. However, it is doubtful whether this conventional reactor combination and operation method is the most efficient in continuous emulsion polymerization. As is well known, the kinetic behavior of continuous emulsion polymerization differs very much according to the kind of monomers. In this paper, therefore, the discussion about the present subject will be advanced using the... 

While vinyl acetate is normally polymerized in batch or continuous stirred tank reactors, continuous reactors offer the possibility of better heat transfer and more uniform quality. Tubular reactors have been used to produce polystyrene by a mass process (1, 2), and to produce emulsion polymers from styrene and styrene-butadiene (3 -6). The use of mixed emulsifiers to produce mono-disperse latexes has been applied to polyvinyl toluene (5). Dunn and Taylor have proposed that nucleation in seeded vinyl acetate emulsion is prevented by entrapment of oligomeric radicals by the seed particles (6j. Because of the solubility of vinyl acetate in water, Smith -Ewart kinetics (case 2) does not seem to apply, but the kinetic models developed by Ugelstad (7J and Friis (8 ) seem to be more appropriate. 

Continuous-stirred tank reactors lie somewhere between tubular and batch reactors. Mixing and heat transfer problems are similar to those of batch reactors. However, many of the stirred-tank reactors have benefits of the tubular flow reactors. These include isolation of intermediates, automatic control, and low labor costs. 

Backmix flow reactor or continuously stirred tank reactor. The conversion rate is lower than for plug-flow reactors because the reagent is immediately diluted on being introduced into the reactor. Many flow reactors, e.g. tubular reactors, and especially in the turbulent regime are in this class. 

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