Reactors, continuous backmix tubular

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. 

Continuous homogeneous reactors Longitudinal tubular reactor (no backmixing)... 

If either the batch or the tubular type of reactor is chosen, the reactor size and product distribution can be calculated by using the batch or longitudinal-flow equations. For a stirred-tank continuous reactor, the backmixing equations can be used. If a packed or baffled tower is used, then the calculations must be made for both the longitudinal and back-mixing cases. Proper extrapolation must then be made from empirical data or previous experience. 

Continuous flow stirred-tank reactors are normally just what the name implies tanks into which reactants flow and from which a product stream is removed on a continuous basis. CFSTRs, CSTRs, C-star reactors, and backmix reactors are only a few of the names applied to the idealized stirred-tank flow reactor model. We will use the letters CSTR in this book. The virtues of a stirred-tank reactor lie in its simplicity of construction and the relative ease with which it may be controlled. These reactors are used primarily for carrying out liquid phase reactions in the organic chemicals industry, particularly for systems that are characterized by relatively slow reaction rates. If it is imperative that a gas phase reaction be carried out under efficient mixing conditions similar to those found in a stirred-tank reactor, one may employ a tubular reactor containing a recycle loop. At sufficiently high recycle rates, such systems approximate the behavior of stirred tanks. In this section we are concerned with the development of design equations that are appropriate for use with the idealized stirred-tank reactor model. 

However, it is very difficult to suspend solid particles effectively and still maintain plug flow. An agitated or pulsed column reactor may be applicable (see section 43,1,5), but then the effects of the residence time distribution have to be taken into account. For large scale operations a series of CSTR s is often more practical, even if the residence time distribution is greater, since it offers better possibilities for effective suspension of coarse particles, and also for heat transfer to the vessel wall. However, one should be aware of the fact that in a continuous reactor with backmixing the conversion of solid particles can never be complete. Because of the residence time distribution, a fraction of the solid particles, those with an individual residence time shorter than the dissolution time, will leave the reactor. For obtaining a complete conversion of the solid, a tubular reactor, that guarantees a certain minimum residence time, will have to be installed after the last CSTR. An alternative is to separate the unconverted solid and return it to the first reactor. 

In this chapter, we describe several ideal types of reactors based on two modes of operation (batch and continuous), and ideal flow patterns (backmix and tubular) for the continuous mode. From a kinetics point of view, these reactor types illustrate different ways in which rate of reaction can be measured experimentally and interpreted operationally. From a reactor point of view, the treatment also serves to introduce important concepts and terminology of CRE (developed further in Chapters 12 to 18). Such ideal reactor models serve as points of departure or first approximations for actual reactors. For illustration at this stage, we use only simple systems. 

When a monomer split-feed operation based on the experimental result shown in Fig. 32 was applied, for example, to a continuous tubular pre-reactor with some backmixing, the number of polymer particles increased by about 30% at Mpi=0.02 g/cm -water, compared to the number produced in a batch reactor, as shown in Fig. 33. 

Fluidized beds give relatively higher performance, but within a narrow operating window. Another type of reactors, the slurry reactor, effectively utilizes the catalyst because of their small particle size in the micrometer range. However, catalyst separation is difficult and a filtration step is required to separate fine particles from the product. Moreover, when applied in the continuous mode, backmixing lowers the conversion and usually the selectivity [2]. Conventional continuous tubular reactors are used as falling film or wall reactor with catalyst coated on the wall however, supply/removal of heat and often broad residence time distribution because of large reactor diameters are two main drawbacks commonly encountered with such reactors. 

Example 9.11 Which type of isothermal reactor would produce the narrowest possible distribution of chain lengths in a free-radical addition polymerization continuous stirred tank reactor (CSTR, or backmix), batch (assume perfect stirring in each of the previous), plug-flow tubular, or laminar-flow tubular ... 

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