Series reactions CSTRs

As can be seen, the principal differences of this process from the earlier German process includes a) reflux cooling of the first reaction zone, b) the possible division of the first reaction zone into a series of CSTR s and c) the incorporation of a devolatilization step. 

Numerical calculations are the easiest way to determine the performance of CSTRs in series. Simply analyze them one at a time, beginning at the inlet. However, there is a neat analytical solution for the special case of first-order reactions. The outlet concentration from the nth reactor in the series of CSTRs is... 

Thus the respective rate expressions depend upon the particular concentration and temperature levels, that exist within reactor, n. The rate of production of heat by reaction, rg, was defined in Sec. 1.2.5 and includes all occurring reactions. Simulation examples pertaining to stirred tanks in series are CSTR, CASCSEQ and COOL. 

For series reactions in steady-state operation of a CSTR, it is a matter of determining the stationary-state concentrations of species (product distribution) in or leaving each stage. 

Table 18.1 Comparison of PFR and CSTR for series-reaction network A -4 B -+ C (isothermal, constant-density system K = kz/ki)...

Figure 4-S Ca, Cb, and Cc versus k x for series reactions B — C in a CSTR. Plots shown are for kij k = 0.1, 1.0, and 10. The yield to form B exhibits a maximum at a particular residence time r, ax, but this maximum is lower than in a PFTR.

We can now begin to formulate the general principles of the choice of PFTR and CSTR for optimum yield of an intermediate in irreversible series reactions. 

For series reactions with an intermediate desired, there is always an optimum T for maximum yield, and the PFTR gives a higher maximum yield if both reactions have positive order, while the CSTR gives a higher maximum yield if the reactions are negative order (a rather rare occurrence). For series reactions with the final product desired, the PFTR requires the shorter time and gives less intermediate for positive-order kinetics. 

In continuous emulsion polymerization of styrene in a series of CSTR s, it was clarified that almost all the particles formed in the first reactor (.2/2) Since the rate of polymerization is, under normal reaction conditions, proportional to the number of polymer particles present, the number of succeeding reactors after the first can be decreased if the number of polymer particles produced in the first stage reactor is increased. This can be realized by increasing emulsifier and initiator concentrations in the feed stream and by lowering the temperature of the first reactor where particle formation is taking place (2) The former choice is not desirable because production cost and impurities which may be involved in the polymers will increase. The latter practice could be employed in parallel with the technique given in this paper. 

If series reactions are conducted in a CSTR, the concentrations in the reactor can be adjusted to influence selectivity and conversion. Because the production of the undesirable product C depends on the concentration of the desired product B, this concentration should be kept small. The reactor can be operated with low conversion (small concentration of B). 

The reaction is conducted at 140-180°C and 0.80-2 MPa in a series of CSTRs or in a single tower oxidizer. The reaction selectivity depends highly on the catalyst. For example, a cobalt-based soluble catalyst gives a ratio ketone/ alcohol of about 3.5. In order to maximize the yield the conversion is kept low. Note that a more selective process based on boric acid was developed, but the oxidation agent is expensive and the technology rather complicated. 

Another example cited in Luyben and Luyben (1997) is when a large jacketed CSTR is replaced by several smaller CSTRs in series. For most reactions, a series of CSTRs has a lower total volume than a single CSTR for the same production rate and operating temperature. This smaller total reactor volume produces a smaller surface area and a larger AT, resulting in poor temperature control, particularly in the first reactor. 

Consider the following series of CSTRs accomplishing a first-order reaction (reactors of equal size). 

To demonstrate these ideas, let us consider three different schemes of reactora in series two CSTRs, two PFRs, and a PER connected to a CSTR. To size these reactors we shall use laboratory data that give the reaction rate at different conversions. The reactors will operate at the same temperature and pressure as were used in obtaining the laboratory data. 

Series reactions take place in a CSTR with heat effects. [1st Ed. P9-23]... 

Time Varying Input to a CSTR with a Series Reaction... 

Consider a continuously stirred tank reactor (CSTRs) sustaining the series reaction (A—-—>B —-—>C ).[1], [9] The material balance equations are as follows ... 

Consider the irreversible series reaction A—taking place in N CSTRs in series. The governing equation for the concentration of A in a particular tank is given by ... 

With the availability of various computer packages, it has become very easy for chemists to perform in-depth analysis of chemical systems. The present models can be adapted to other systems via proper rate expressions, physical properties and heats of reaction. Work is under progress to expand this model to a series of CSTRs. 

P8-28- This problem concerns series reaction with heat effects in a CSTR. The instruction could extend the problem statement in tlie text to ask the students to carry out a parameter sensitivity. E.g. increase/decrease/ j and 2- This problem could be alternated with P8-31, which involves parallel reactions. 

If the reaction rate is neither strictly increasing nor strictly decreasing, the reactor configuration with the smallest volume becomes a series of CSTRs and PFRs. Such an example is shown in Chapter 8, Example 8,4. Levenspiel provides further discussion of these interesting cases [11, pp. 128-1561. 

Consider a series of CSTRs and PFRs with a single nth-order reaction taking place in which... 

The CSTR is used extensively in situations where intense agitation is required, such as the addition of a gaseous reactant to a liquid by transfer between the bubbles and the continuous liquid, and the suspension of a solid or second liquid within a continuous liquid phase. Polymerization reactions are sometimes conducted in CSTRs. It is common to employ a cascade or series of CSTRs in which the effluent from the first reactor is used as feed to the second and so forth down the cascade (Figure 1.4). The cascade permits one to realize high conversion of reactant, while minimizing total reactor volume. 

We can begin by computing the selectivities and yields for the series network in the CSTR versus the PFR first. Consider the simplest series reaction network ... 

A question arises how to reduce this difference The solution consists of using a series of CSTRs instead a single PFR. Since the reaction rate is higher in each intermediate volume, finally a much higher productivity than in a single equivalent reactor is obtained (Fig. 8.6). At limit, an infinite series of CSTRs behaves as a single PFR of the same volume. 

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