Series of 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... 

In contrast, a continuous reactor process is controlled at steady state, thereby ensuring a homogeneous copolymer composition. Therefore, a diblock prepared in a series of CSTRs has precise block junctions and homogeneous compositions of each block. In this case, effective CCTP gives a polymer with precisely two blocks per chain, instead of the statistical multiblock architecture afforded by dual catalyst chain shuttling systems. 

SJi. The initial startup of an adiabatic, gas-phase packed tubular reactor makes a good example of how a distributed system can be lumped into a series of CSTRs in order to study the dynamic response. The reactor is a cylindrical vessel (3 feet ID by 20 feet long) packed with a metal packing. The packing occupies 5 percent of the total volume, provides 50 ft of area per of total volume, weighs 400 ib yft and has a heat capacity of 0.1 Btu/lb °F. The heat transfer coefficient between the packing and the gas is 10 Btu/h It "F. 

Figure 3-9 Total residence time from 1 /r plot for a series of CSTR reactors for 1, 2, 3,4, and n equal-volimie...

For any more complex flow pattern we must solve the fluid mechanics to describe the fluid flow in each phase, along with the mass balances. The cases where we can still attempt to find descriptions are the nonideal reactor models considered previously in Chapter 8, where laminar flow, a series of CSTRs, a recycle TR, and dispersion in a TR allow us to modify the ideal mass-balance equations. 

Table 11.2 gives the total holding times for two values of K, both for a series of CSTRs with minimal total volume and for a series of equal-sized mixed reactors. Total holding times for equal-sized mixed reactors have been calculated using a zero finding routine. The last value in Table 11.2 is the dimensionless holding time for a PFR reactor with Michaelis-Menten kinetics, calculated by means of the following equation ... 

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. 

The objective of this paper is to illustrate, by simulation of the vinyl acetate system, the utility of the analytical predictor algorithm for dead-time compensation to regulatory control of continuous emulsion polymerization in a series of CSTR s utilizing initiator flow rate as the manipulated variable. 

An alternative to the series of CSTRs is to have one reactor followed by a distillation column that recycles reactant back to the reactor. The flowsheet of this process is shown in Figure 2.59. The reactor can be small because the concentration of the reactant in the reactor can be large. The per-pass conversion of reactant is not equal to... 

It is obvious from the table that the conversion of the series of CSTRs increases with the number of CSTRs (total volume is constant), approaching an upper limit that is the value of the conversion of a PFR with the same volume as the total volume of the CSTR system. The PFR itself can be considered as an infinite number of differential homogeneous slices, each one of which behaves like a CSTR. The increase in conversion with the number of CSTRs is at first very rapid, levelling off later and hence, in practice, a number of CSTRs between 10 and 20 has an overall conversion similar to the equivalent total volume PFR. How... 

Multistage CSTR Since tubular reactor performance can be simulated by a series of CSTRs, multistage CSTR tracer models are useful in analyzing data from empty tubular and packed-bed reactors. The solution for a tracer through n CSTRs in series is found by induction from the solution of one stage, two stages, and so on. 

Examples of values of Pe are provided in Fig. 19-8. When Pe is large, n =k Pe/2 and the dispersion model reduces to the PFR model. For small values of Pe, the above equation breaks down since the lower limit on n is n = 1 for a single CSTR. To better represent dispersion behavior, a series of CSTRs with backmixing may be used e.g., see Froment and Rischoff (Chemical Reactor Analysis and Design, Wiley, 1990). A model analogous to the dispersion model may be used when there are velocity profiles across the reactor cross-section (eg., for laminar flow). In this case, the equation above will contain terms associated with the radial position in the reactor. 

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). 

A tubular prereactor, in series with CSTR system, can offer stability advantages, which will he discussed later. A number of other flow alternatives are also possible with a CSTR-series system but these alternates are not widely utilized. An obvious flow alternative for a reactor system consisting of a series of CSTRs would he to introduce some portion of the total recipe at places other than the front end of the reactor train. These intermediate feeds would, in many respects, be analogous to semicootinuous operation of batch reactors. 

Note that the resulting single CSTR volume would have been too large — try to increase the temperature and/or use a series of CSTRs. 

FIG. 7-1 Types of ideal reactors (a) Batch or semibatch. (b) CSTR or series of CSTRs. (c) Plug flow. 

The theoretical RTD responses in Fig. 19-7a are similar in shape to the experimental responses from pilot and commercial reactors shown in Fig. 19-8. The value of n in Fig. 19-8 represents the number of CSTRs in series that provide a similar RTD to that observed commercially. Although not shown in the figure, a commercial reactor having a similar space velocity as a pilot reactor and a longer length typically has a higher n value than a pilot reactor due to greater linear velocity. The variance of the RTD of a series of CSTRs, a, is the inverse of n. 

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. 

A single CSTR is a valuable tool for the study of polymerization kinetics but not for commercial production. As mentioned earlier, systems comprised of a series of CSTRs are most common. Some of the early synthetic rubber processes contain as many as 15 CSTRs. More recent systems are comprised of three to five reactors. Not all reactors need to be the same size. In fact there are substantial reasons for using reactors of different sizes. 

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