Segregated stirred tank reactor

The concept of a well-stirred segregated reactor which also has an exponential residence time distribution function was introduced by Dankwerts (16, 17) and was elaborated upon by Zweitering (18). In a totally segregated, stirred tank reactor, the feed stream is envisioned to enter the reactor in the form of macro-molecular capsules which do not exchange their contents with other capsules in the feed stream or in the reactor volume. The capsules act as batch reactors with reaction times equal to their residence time in the reactor. The reactor product is thus found by calculating the weighted sum of a series of batch reactor products with reaction times from zero to infinity. The weighting factor is determined by the residence time distribution function of the constant flow stirred tank reactor. 

We have just described a completely segregated stirred tank reactor. It is one of the ideal flow reactors discussed in Section 1.4. It has an exponential distribution of residence times but a reaction environment that is very different from that within a perfectly mixed stirred tank. 

Figure 15.11 Extremes of micromixing in a stirred tank reactor (a) ping-pong balls circulating in a agitated vessel, the completely segregated stirred tank reactor, (b) molecular homogeneity, the perfectly mixed CSTR.

The completely segregated, continuous-how stirred tank reactor... 

The completely segregated stirred tank can be modeled as a set of piston flow reactors in parallel, with the lengths of the individual piston flow elements being distributed exponentially. Any residence time distribution can be modeled as piston flow elements in parallel. Simply divide the flow evenly between the elements and then cut the tubes so that they match the shape of the washout function. See Figure 15.12. A reactor modeled in this way is said to be completely segregated. Its outlet concentration is found by averaging the concentrations of the individual PFRs ... 

A continuous bulk polymerization process with three reaction zones in series has been developed. The degree of polymerization increases from the first reactor to the third reactor. Examples of suitable reactors include continuous stirred tank reactors, stirred tower reactors, axially segregated horizontal reactors, and pipe reactors with static mixers. The continuous stirred tank reactor type is advantageous, because it allows for precise independent control of the residence time in a given reactor by adjusting the level in a given reactor. Thus, the residence time of the polymer mixtures can be independently adjusted and optimized in each of the reactors in series (8). 

When, however, an extraction, or an extraction combined with a chemical reaction, is carried out between two phases in a continuous stirred tank reactor in which there is no interaction occurring between the dispersed particles (complete segregation), the dispersed particles will have different concentrations because of the spread in residence time. Any kind of interaction between the dispersed particles (e.g., by diffusion or by continuous coalescing and redispersion) then tends to eliminate these concentration differences. 

In this chapter some effects of segregation on the kinetics of a chemical reaction between two liquid phases carried out in a continuous stirred tank reactor (CSTR) will be discussed. In the derivations of these effects it will be assumed that during the reaction the dispersed phase is maintained (e.g., in the case of extraction combined with chemical reaction) and that all dispersed drops have the same size. This means that when there is segregation it is only the age distribution which causes a concentration distribution in the dispersed phase. 

In a continuous stirred tank reactor in which the dispersed phase is segregated, a spread in the drop size distribution is present, and there is mass transfer limitation, the spread in the concentration distribution will... 

Batch stirred tank reactor. Low-volume products flexibility to produce numerous grades (batch-to-batch variability) and homogenous (narrow MWD) or segregated (broad MWD). 

Fig. 8.14 Conversion versus residence lime for an exothermic reaction in (1) a tubular reactor, (2) a completely mixed stirred tank, and (3) a completely segregated stirred tank.

We shall consider three methods of estimating deviations from ideal reactor performance. The first method is to determine the actual RTD from experimental response data and then calculate the conversion by assuming the flow to be wholly segregated (Sec. 6-8). This model should be a good approximation, for example, for a tubular-flow reactor, where the flow is streamline. It would not describe a nearly ideal stirred-tank reactor, for here the fluid is nearly completely mixed when it enters the reactor. In this case no error is introduced by an approximation of the RTD, since the actual... 

To illustrate the use of Eq. (6-39) suppose we use the RTD for a stirred-tank reactor [Eq. (6-12)] but assume segregated flow. We wish to calculate the conversion for an irreversible first-order reaction. The function x(6) is, from the second entry of Table 2-5,... 

In contrast, suppose complete micromixing is assumed. Then, for the same RTD, an ideal stirred-tank reactor results. The conversion for a first-order reaction in this case is given by Eq. (4-7), which is identical to the above expression for segregated flow. This verifies the conclusion of Eq. (6-36) that the extent of micromixing does not affect conversion for first-order kinetics (as long as the correct RTD is used). The same develop-... 

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