Batch and Continuous Stirred Tank Reactors

The reactor models considering complete mixing may be subdivided into batch and continuous types. In the continuous stirred tank reactor (CSTR) models, an entering fluid is assumed to be instantaneously mixed with the existing contents of the reactor so that it loses its identity. This type of reactor operates at uniform concentration and temperature levels. For this reason the species mass balances and the temperature equation may be written for the entire reactor volume, not only over a differential volume element. Under steady-state conditions, the species mass and heat balances reduce to algebraic equations. 

For a CSTR which is completely mixed, we start from the averaged species mass (1.301) and heat (1.302) balance equations. No diffusive terms are retained as the reactor volume is assumed to be uniform in composition and temperature, as explained above. The resulting species mass balance yields  

After global integration in z from the inlet to the outlet of the reactor and multipl3dng the resulting relation by the cross section area A, we get  

Let rhslin and msjout represent, respectively, the inlet and outlet mass flow rates of species s, the following relation can then be obtained by use of Green s theorem  

Under steady-state conditions, the simplified species mass balance reduces further to  

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Both batch and continuous stirred tank reactors are suitable for reactions that exhibit pseudo-zero-order kinetics with respect to the substrate concentration. In other words, under operating conditions the rate is more or less independent of the concentration of the substrate. However, for reactions where pseudo-first-order kinetics with respect to the concentrations of the substrates prevail, a batch tank reactor is preferred. Batch tank reactors are also ideally suited when there is a likelihood of the reactant slowly deactivating the catalyst or if there is a possibility of side product formation through a parallel reaction pathway. 

For the simple network 5.26 and a reaction with no fluid-density variation, the magnitude of the effect is easily calculated The cumulative selectivity of conversion to P (moles of A converted to P per mole of A consumed, see definition 1.11) in batch and continuous stirred-tank reactors as a function of fractional conversion,/A, is... 

A fixed-bed adsorption has several advantages over batch and continuous stirred tank reactor (CSTR) because the rates of adsorption depend on the concentration of viruses in solution. This point is especially important for virus removal because of the low concentration of viral contaminants. The design of a fixed-bed adsorption column involves estimation of the shape of the breakthrough curve and the appearance of the breakpoint. Computer simulation studies were done here to demonstrate the performance of a virus adsorber using the surface-bonded QAC beads which have a higher binding affinity for viruses over other proteins. 

Ideal mixing and plug flow. The batch, contlnuous-stirred-tank, and plug-flow reactors are defined by certain idealized assumptions on the fluid flow. The batch and continuous-stirred-tank reactors are assumed to be ideally well mixed, which means that the temperature, pressure and species concentrations are independent of spatial position within the reactor. The plug-flow reactor describes a special type of flow in a itube in which the fluid.is well.mixed in the radial direction and varies... 

In the design of ideal batch and continuous stirred tank reactors, we assume uniform current distribution at the electrodes to simplify the mathematical... 

Tubular reactor Development of a pulsation operation mode to eliminate reactor fouling and plugging comparison between batch and continuous stirred-tank reactors review on tubular reactors 173... 

The two solutions are identical. Hence, for a long time no importance was attributed to the use of a kinetic approach for describing batch polycondensations starting from monomers, and the statistical approach was preferred. Of course, chemical engineers had to deal with semi-batch and continuous stirred tank reactors where the statistical approach, although possible, is cumbersome and error-prone, so a few papers appeared in the 1960s dealing with kinetically controlled linear polycondensations [274—276]. 

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. 

A number of innovative polymerization reactors using loop reactors, plug-flow and static mixer reactors, and continuous stirred-tank reactors have been reported. For example, Wilkinson and Geddes (15) describe a 50-liter reactor that has the same capacity as a 5000-gallon batch reactor. Extruders, thin-film evaporators, and other devices designed to provide intense mixing for viscous or unstable materials have also been used as reactors. 

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. 

FIGURE 1 Selected reactor configurations (a) batch, (b) continuous stirred-tank reactor, (c) plug flow reactor, (d) fluidized bed, (e) packed bed, (f) spray column, and (g) bubble column. 

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

The rational design of a reaction system to produce a desired polymer is more feasible today by virtue of mathematical tools which permit one to predict product distribution as affected by reactor type and conditions. New analytical tools such as gel permeation chromatography are beginning to be used to check technical predictions and to aid in defining molecular parameters as they affect product properties. The vast majority of work concerns bulk or solution polymerization in isothermal batch or continuous stirred tank reactors. There is a clear need to develop techniques to permit fuller application of reaction engineering to realistic nonisothermal systems, emulsion systems, and systems at high conversion found industrially. A mathematical framework is also needed which will start with carefully planned experimental data and efficiently indicate a polymerization mechanism and statistical estimates of kinetic constants rather than vice-versa. 

Reactors are conveniently considered in three idealized categories batch, tubular, and continuous stirred tank reactors (CSTR). The operations of real reactors may be modeled on the basis of one of these types or combination thereof. 

Many types of reactors have entered the field of petroleum refining, but they can be roughly divided into three types 1) batch 2) continuous stirred tank reactor (CSTR) and 3) continuous plug flow. In small-scale studies, the researcher may use a simple pipe reactor, which is operated batchwise. The CSTR reactor is used in small-scale studies for kinetic studies... 

Batch, semibatch, and continuous stirred tank reactors residence time 600 to 15,000 s (10 min to 4 h) heat of reaction primarily exothermic reaction rate slow to moderate. High-pressure autoclaves <100 L. Unique to semibatch phases liquid, gas-liquid, liquid-liquid, gas-liquid catalytic solid. Use where a batch operation is appropriate (Section 16.11.6.24), but one reactant (e.g., gas) needs to be added continuously or if the initial reaction rate is very high. Selectivity is best for parallel reactions. For more details, see CSTR, Section 16.11.6.26. 

Batch, semibatch, and continuous stirred tank reactors residence time 600 to 15,000 s (10 min to 4 h) heat of reaction primarily exothermic reaction rate slow to moderate. High-pressnre... 

There is one significant difference between batch and continuous stirred tanks. The heat balance for a CSTR depends on the inlet temperature, and Tm can be adjusted to achieve a desired steady state. As discussed in Section 5.3.1, this is a very scaleable approach to reactor design. 

As clearly seen from eq. (10.1 )-(l 0.5) they are independent on the reactor type. The characteristic features of different reactors were discussed in Chapter 1. In Table 10.1 we summarize design equations for batch, fixed bed and continuous stirred tank reactors... 

Table 10.1. Design equations for batch, fixed bed and continuous stirred tank reactors...

There are two basic types of ideal reactors, stirred tanks, for reactions in liquids, and tubular or packed-bed reactors, for gas or liquid reactions. Stirred-tank reactors include batch reactors, semibatch reactors, and continuous stirred-tank reactors, or CSTRs. The criterion for ideality in tank reactors is that the liquid be perfectly mixed, which means no gradients in temperature or concentration in the vessel. 

Reactor type and size Continuously stirred tank reactors in series, 10 - 100 Continuously stirred tank reactors in series or batch reactors, 10 -100 m Batch or continuously stirred tank reactors in series, 10 - 50 m ... 

What is the difference between the clock time of a batch reactor (BR) and the space time of the plug-flow reactor (PFR) and continuously stirred tank reactor (CSTR) ... 

Figure 7.5 Residence time distribution function as a function of the dimensioniess reaction time for an ideai batch or continuous stirred tank reactor. CSTR, PSD, and dp represent continuous stirred tank reactor, particie size distribution function, and dimensioniess particie diameter, respectiveiy,...

Copolymers are typically manufactured using weU-mixed continuous-stirred tank reactor (cstr) processes, where the lack of composition drift does not cause loss of transparency. SAN copolymers prepared in batch or continuous plug-flow processes, on the other hand, are typically hazy on account of composition drift. SAN copolymers with as Httle as 4% by wt difference in acrylonitrile composition are immiscible (44). SAN is extremely incompatible with PS as Httle as 50 ppm of PS contamination in SAN causes haze. Copolymers with over 30 wt % acrylonitrile are available and have good barrier properties. If the acrylonitrile content of the copolymer is increased to >40 wt %, the copolymer becomes ductile. These copolymers also constitute the rigid matrix phase of the ABS engineering plastics. 

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