Sequences of stirred tank reactors

This kind of oscillation is often very undesirable and it is important to be aware of the possibility of its arising. Douglas has shown that there are some cases in which it can be exploited as for instance when the mean value of the production rate is greater than that of the steady state about which the limit cycle is oscillating. Though it is beyond our scope to try to describe this in detail, it is vital for the student to realize that this kind of behavior is possible. 

Exercise 7,6 J, Show that if the controller acts on deviations of composition and it is not always possible to get control of an 

Up to this point, we have considered only the design and behavior of a single reactor. We now have to see how one of the principal defects of the stirred tank reactor may be at least partially removed. This defect is the large holding time that is needed because the reaction is being conducted at product conditions where the reaction rate is low because the extent is high. 

For example, for an irreversible first order reaction with r(f, T) we have a holding time 

Another way of showing that the holding time requirement of the stirred tank must always be greater than that of the batch time at the same temperature is to compare the two formulae 

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Figure 4.4 Stereospecific esterification synthesis of (/t)-/V-(fe/V-hutoxycarbonyl)-3-hydroxyinethyl-piperidine from (7 ,5)-A,-(/< r7-butoxycarhonyl)-3-hydroxy methyl piperidine applying a sequence of stirred-tank reactor, extraction module, second stirred-tank reactor and second extraction module... 

Figure 4.14 Chemical adducts for by-product removal synthesis of isopropyl-cis-Ag-hexadecanoate from isopropylpalmitate applying a repetitive batch process using a sequence of stirred-tank reactor, extraction module, filtration step and chromatographic downstream processing...

Studies in optimization-III The optimum operating conditions in sequences of stirred tank reactors. Chem. Eng. ScL 13, 75-81 (1960). 

Optimal bypass rates for sequences of stirred tank reactors Can. J. Chem. Eng. 39, 121— 126 (1961). 

If a stage of a reactor is insulated as far as possible from direct transfer of heat it is called adiabatic. Thus a sequence of stirred tanks 40... 

Fig. 3.3. Comparison of concentration and temperature profiles for a tubular reactor and sequence of stirred tanks. (Courtesy of N. R. Amundson.)...

More complicated problems for sequences of stirred tanks can be devised, but they follow the pattern of multibed adiabatic tubular reactors to which we now turn. 

The monomer feed is converted into Polyamide-6 by polycondensation and polyaddition reactions [930]. This reaction step can be realized by a complex reactor which can be modeled as a sequence of stirred tank and plug-flow reactors. An exemplary model flowsheet comprising two reactors (CSTR) with an intermediate water separation (Split) is shown in Fig. 5.20. Such a model of the reaction section can be analyzed by means of Polymers Plus, an extension of Aspen Plus for handling polymer materials [513]. 

The Ballestra system consists of a number of stirred-tank reactors, arranged in a cascade sequence or train. Each reactor is equipped with a high-speed turbine impeller to disperse the gas and to mix and circulate the organic phase within the reactor. Cooling is facilitated by cooling coils located within the body of the reactor and a cooling jacket around the reactor. 

The second important environment in which coupled reactions can occur is that of a batch reactor. We will assume that our batch reactor behaves as a well stirred tank reactor, such that all participants are well mixed and concentration gradients do not occur. We will also assume that our coupled reactions proceed only in the forward direction, such that our sequence of elementary steps is reduced to... 

In a stirred-tank reactor, the reactants are diluted immediately on entering the tank in many cases this favours the desired reaction and suppresses the formation of byproducts. Because fresh reactants are rapidly mixed into a large volume, the temperature of the tank is readily controlled, and hot spots are much less likely to occur than in tubular reactors. Moreover, if a series of stirred tanks is used, it is relatively easy to hold each tank at a different temperature so that an optimum temperature sequence can be attained. 

The reactor system may consist of a number of reactors which can be continuous stirred tank reactors, plug flow reactors, or any representation between the two above extremes, and they may operate isothermally, adiabatically or nonisothermally. The separation system depending on the reactor system effluent may involve only liquid separation, only vapor separation or both liquid and vapor separation schemes. The liquid separation scheme may include flash units, distillation columns or trains of distillation columns, extraction units, or crystallization units. If distillation is employed, then we may have simple sharp columns, nonsharp columns, or even single complex distillation columns and complex column sequences. Also, depending on the reactor effluent characteristics, extractive distillation, azeotropic distillation, or reactive distillation may be employed. The vapor separation scheme may involve absorption columns, adsorption units,... 

L. T. Fan, L. E. Erickson, R. W. Sucher, and G. S. Mathad. Optimal design of a sequence of continuous-flow stirred-tank reactors with product recycle. I EC, 4 431, 1965. 

Stochastic modeling is used when a measurable output is available but the inputs or causes are unknown or cannot be described in a simple fashion. The black-box approach is used. The model is determined from past input and output data. An example is the description of incomplete mixing in a stirred tank reactor, which is done in terms of contributions of dead zones and short circuiting. In these cases, a sequence of output called a time series is known, but the inputs or causes are numerous and not known in addition, they may be unobservable. Though the causes for the response of the system are unknown, the development of a model is important to gain understanding of the process, which may be used for future planning. 

Kim, S. H., Han, S. K., and Shin, H. S. 2005. Performance comparison of a continuous-flow stirred-tank reactor and an anaerobic sequencing batch reactor for fermentative hydrogen production depending on substrate concentration. Water Sci. Technol., 52 (10-11), 23-29. 

In reactions with parallel steps of different reaction orders or with sequential steps, selectivities depend on the reactor type. Batch and plug-flow tubular reactors give higher selectivities to the product formed by the parallel step of higher order, or to the first product in a step sequence, than do continuous stirred-tank reactors. 

Sketch the RTD curves for the sequence of plug flow and continuous stirred tank reactors given in Figure 8.3.1. 

Consider a sequence of identical stirred tank reactors, all at the same temperature, in which a first order irreversible reaction A B takes place. If the feed concentration of A is 1 and the extent of reaction o -, the concentration of A at any time is c = 1 — we will take this as the concentration variable. Then... 

This gives us considerable confidence in this model and shows the importance of considering the optimal design of a sequence of identical stirred tank reactors. 

Deans and Lapidus (1960) made use of the analogy between a tubular reactor and a sequence of continuous flow stirred tank reactors for the modelling of heterogeneous reactors. In cell models the reactor is subdivided into small finite elements, the height of each being usually equal to one pellet diameter and each of these elements is considered to be a small CSTR in which the fluid phase is perfectly mixed. In the simple cell model the temperature and concentration conditions of any given cell are dependent only on those of the previous row of cells. 

The simplest flow-sheet for the reaction Aj o Aj is the RD column sequence with an external recycling loop shown in Fig. 5.1. The system as a whole is fed with pure Aj. According to the assumed relative volatility of the two components a > 1, the reaction product A2 is enriched in the column distillate product whereas the bottom product contains non-converted reactant Aj, which is recycled back to the reactor (continuous stirred tank reactor, CSTR, or plug flow tube reactor, PFTR). The process has two important operational variables the recycling ratio cp = B/F, that is the ratio of recycling flow B to feed flow rate F, and the reflux ratio of the distillation column R = L/D. At steady-state conditions, D = F since the total number of moles is assumed to be constant for the reaction Aj A2. As principal design variables, the Damkohler number. 

A cascade consisting of two identical stirred-tank reactors is to be used to manufacture the intermediate product B, which is formed and consumed in the following sequence of hquid-phase reactions ... 

The work-up of batch processes, run in stirred vessels, had often faced the challenge to efficiently separate and recover the enzyme used. Meanwhile, there is abundant know-how available to immobilise enzymes on different carriers, though some issues need always to be considered maintained activity of the enzyme, its stability towards solvents and the operating temperature used in a reaction. Enzyme immobilisation allows for continuous reactions carried out in columns or in a sequence of continuous stirred-tank reactors. Certain advantages are offered by Degussa s enzyme-membrane-reactor (EMR), where the enzyme is surrounded by a hoUow-fibre membrane, that is permeable to substrate and product. 

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