Continuous stirred tank reactor operating points

A cascade of three continuous stirred-tank reactors arranged in series, is used to carry out an exothermic, first-order chemical reaction. The reactors are jacketed for cooling water, and the flow of water through the cooling jackets is countercurrent to that of the reaction. A variety of control schemes can be employed and are of great importance, since the reactor scheme shows a multiplicity of possible stable operating points. This example is taken from the paper of Mukesh and Rao (1977). 

Chemical reactors intended for use in different processes differ in size, geometry and design. Nevertheless, a number of common features allows to classify them in a systematic way [3], [4], [9]. Aspects such as, flow pattern of the reaction mixture, conditions of heat transfer in the reactor, mode of operation, variation in the process variables with time and constructional features, can be considered. This work deals with the classification according to the flow pattern of the reaction mixture, the conditions of heat transfer and the mode of operation. The main purpose is to show the utility of a Continuous Stirred Tank Reactor (CSTR) both from the point of view of control design and the study of nonlinear phenomena. 

Property 1. Consider an exothermic continuous stirred-tank reactor with temperature dependence Arrenhius-type, there is a stable equilibrium point such that, under the isothermic operation (i.e., as reactor temperature X2 is constant). 

One of the advantages of the continuous stirred-tank reactor is the fact that it is ideally suited to autothermal operation. Feed-back of the reaction heat from products to reactants is indeed a feature inherent in the operation of a continuous stirred-tank reactor consisting of a single tank only, because fresh reactants are mixed directly into the products. An important, but less obvious, point about autothermal operation is the existence of two possible stable operating conditions. 

The operation and description of a temperature scanning continuously stirred tank reactor (TS-CSTR) is, in principle, much simpler than for the TS-PFR. It turns out that rates can be calculated from each individual point in each run, and that flow rates and temperature ramping do not need the same careful control as the TS-PFR. Nevertheless, the operation of die reactor should approach the perfectly mixed condition very closely. Although in practice it may be difficult to make the necessary physical arrangements for complete and instantaneous mixing within the reactor, as with other TS reactor types there are verification procedures that will reveal if proper operating conditions are not being met. 

Multiple steady-state behavior is a classic chemical engineering phenomenon in the analysis of nonisothermal continuous-stirred tank reactors. Inlet temperatures and flow rates of the reactive and cooling fluids represent key design parameters that determine the number of operating points allowed when coupled heat and mass transfer are addressed, and the chemical reaction is exothermic. One steady-state operating point is most common in CSTRs, and two steady states occur most infrequently. Three stationary states are also possible, and their analysis is most interesting because two of them are stable whereas the other operating point is unstable. 

The 2,5-dihydrofurane (bp = 66 °C) and the crotonaldehyde (bp = 104 C) can then be separated from the reaction mixture by distillation due to their low boiling points, while the higher boiling oligomers remain in the IL-catalyst phase. This phase is fruther treated with an extractant solvent such as naphtha, to recover the oligomers and recycle the IL-catalyst. The plant is operated with three continuous, stirred-tank reactors, a wiped-film evaporator, a distillation train, and a continuous, counter-current, liquid-liquid extractor for recovery of the catalysts. The plant is now idle, because the market for the product has declined. 

When stirred-tank reactors are operated in the batch mode, all ingredients are added at or near the beginning of the reaction cycle, the reaction is allowed to proceed to a desired end point, and the product latex is removed for further processing. Strict batch operation has a number of disadvantages. First, the heat load on the cooling system can be very nonuniform. The production rates from such reactors can be limited by the capability of the heat removal system during the peak in the exotherm. The use of mixed initiator systems (fast and slow) and the continuous addition of a fast initiator are two ways of trying to deal with this problem. 

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