Cascade of three reactors

CASCSEQ - Cascade of Three Reactors with Sequential Reactions... 

Cascade of three Reactors with Sequential Reactions 276... 

Readers may wish to compare and contrast the methods of analysis and results of this illustrative example with the corresponding aspects of Illustration 8.7. This example employed a graphical approach to analyze the behavior of a cascade of three reactors employed to conduct a reaction whose rate increased continuously with reactant concentration. 

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). 

Obviously this approach is not easily extended to cascades containing more than three reactors and, in those cases, an alternative trial and error procedure is preferable. One chooses a reactor volume and then determines the overall fraction conversion that would be obtained in a cascade of N reactors. When one s choice of individual reactor size meets the specified overall degree of conversion, the choice may be regarded as the desired solution. This latter approach is readily amenable to iterative programming techniques using a digital computer. 

The synthesis is carried out in a cascade of three consequtive reactors 9 of the same type. The parent substances are continuously sent through collector 8 into first reactor 9 from there, the products of the reaction flow into the second and the third reactors. The given level in the first two reactors is supported by pouring the products through the piping for the third, a pressure regulator is used with a gate located on the outlet pipe. Strict observance of the level is necessary to keep the calculated synthesis time. 

Other chemical companies have also designed their own continuous process to produce high-impact polystyrene (HIPS), such as the Dow process, which consists of three elongated reactors in series (US Patent 2727 884, 1955) the BASF process, which consists of a prepolymerization CSTR followed by cascade of three CSTRs (US Patent 3 658 946, 1972) the Shell process, which consists of three CSTRs followed by a plug flow reactor (US Patent 4011 284, 1977) and the Monsanto process, which consists of a CSTR followed by a horizontal plug flow reactor (US Patent 3 903 202, 1975). 

An optimized cascade of three CSTRs 144.17 L A plug-flow reactor (from Example 7.1) 100.08 L... 

Figure 8.2 Types of continuous-flow stirred-tank reactors (a) three-stage cascade of stirred-tank reactors (b) vertically staged cascade of three stirred tanks. Compartmented versions of a battery of stirred tanks in a single horizontal shell may also be employed. (Adapted from J. R. Couper, W. R. Penney, J. R. Fair, and S. M. Walas. Chemical Process Equipment Selection and Design. Copyright 2010. Used with permission of Elsevier.)...

Figure 8.6 (a) Reaction rate versus reactant concentration plot for typical reactions single idetil CSTR reactor, (b) Reaction rate versus reactant concentration plot for typical reactions cascade of three CSTR reactors. 

An exothermic reaction that obeys a second-order rate expression (r = kC ) is to be accomplished in a cascade of three identical stirred tanks operating in series. To (approximately) balance the heat loads on the various reactors, each of the reactors will be operated at a different temperature. These temperatures are to be selected in a manner such that the rates of reaction are to be the same in each reactor. To minimize losses of the organic solvent during operation, it will be necessary to operate the third reactor at 140°C. At this temperature the reaction rate constant is equal to 500 L/(mol h). If the effluent from the third reactor corresponds to 99% conversion and if the volumetric flow rate to the cascade is equal to 1.8 m /h when the concentration of A in the feed stream is 1.5 M, how large must each of the reactors be If the activation energy for the reaction is 20 kcal/g-mol, at what temperatures should the first and second reactors be operated ... 

Figure 9.3 Instantaneous yield versus reactant concentration curves and their relation to overall product concentration changes (a) plug flow or batch reactor, (b) single CSTR (c) cascade of three arbitrary-sized CSTRs in series. (Adapted from O. Levenspiel...

Determine the space time corresponding to the maximum rate of consumption of CgHjCN. Plot the rate versus the concentration of CgHjCN. On this plot indicate the operating line for the single CSTR. Then use the plot to determine the concentrations of benzonitrile that would be present in the effluent from each reactor in a cascade of three identical reactors, the first of which operates at the maximum rate of destruction of the nitrile. Use the plots of species concentration versus time to ascertain whether there is a space time that maximizes the concentration of the amide in the effluent from the CSTR. Comment. 

Case 2 Cascade of three CSTRs. If we denote the composition leaving the nth reactor by C , it is readily shown that... 

Consider a cascade of three CSTBRs that may differ in volume from one another. Within any individual tank, the volume of the aqueous phase remains constant as the biochemical reactions proceed. The corresponding volumes of the aqueous phase are Vjj,, and. Each reactor is supplied... 

We can refine this analysis comparing different cases with cascades of three model reactors, namely CSTRs, plug-flow reactors (PFRs), and TAP reactors (including thin-zone TAP reactors), for which the models are presented in Chapters 3 and 5. All results of our analysis are given in Table 8.1. 

ILLUSTRATIVE EXAMPLE 9.17 Refer once again to Illustrative Example 9.14. Determine the effect of using a cascade of three CSTRs that differ in size on the volume requirements for the reactor network. If the same feed composition and flow are employed and if the reactors are operated isothermally at 25°C, determine the minimum total volume required and the manner in which the volume should be distributed between the three reactors. An overall conversion of0.875 is to be achieved again. For this problem, first outline the method of solution. Then solve the problem. 

Let US now consider a cascade of three stirred tank reactors of volumes of Vi,V2, Vs and with current concentrations of Ci, C2, C3, respectively. The scheme of the reaction mixture flows for the each reactor is represented in Fig. 2.13. 

If the reaction occurs in the liquid phase at 25 °C, determine the reactor volume requirements for cascades of one and three identical CSTR s. The rate at which liquid feed is supplied is 0.278 m3/ksec. Use the graphical approach outlined previously. The following constraints are applicable. 

I. G. Farbenindustrie in Germany implemented such a concept to produce polystyrene commercially in the 1930s. Two CSTRs in parallel followed by a plug flow reactor were used in their process. During World War II, Union Carbide applied for a patent (US Patent 2496653, 1950) for a continuous polystyrene process. Their process consisted of three cascade CSTR reactors followed by a plug flow reactor. The temperature in the three CSTR reactors is 100, 115-120 and 140 °C, respectively. The conversion at the outflow of the third CSTR reactor is around 85 %. The temperature in the plug flow reactor is between 210 and 215 °C. The final conversion at the plug flow reactor was claimed to be 97 %. 

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