Consecutive reactions, batch reactor

Consecutive reactions, isothermal reactor cmi < cw2, otai = asi = 0. The course of reaction is shown in Fig. 5.4-71. Regardless the mode of operation, the final product after infinite time is always the undesired product S. Maximum yields of the desired product exist for non-complete conversion. A batch reactor or a plug-flow reactor performs better than a CSTR Ysbr.wux = 0.63, Ycstriiuix = 0.445 for kt/ki = 4). If continuous operation and intense mixing are needed (e.g. because a large inteifacial surface area or a high rate of heat transfer are required) a cascade of CSTRs is recommended. 

For consecutive reactions batch and plug flow reactors will always give a higher selectvity than well mixed continuous reactors. For competitive-consecutive reactions the best selectivity can be found in semi-batch reactors. However, this type of reactions can in some cases be carried out with an acceptable selectivity in a continuous well mixed reactor, when sufficient excess of the "other reactant (that does not cause undesired reactions) is applied. 

Selectivity A significant respect in which CSTRs may differ from batch (or PFR) reaclors is in the product distribution of complex reactions. However, each particular set of reactions must be treated individually to find the superiority. For the consecutive reactions A B C, Fig. 7-5b shows that a higher peak value of B is reached in batch reactors than in CSTRs as the number of stages increases the batch performance is approached. 

At a fixed temperature, a single, reversible reaction has no interior optimum with respect to reaction time. If the inlet product concentration is less than the equilibrium concentration, a very large flow reactor or a very long batch reaction is best since it will give a close approach to equilibrium. If the inlet product concentration is above the equilibrium concentration, no reaction is desired so the optimal time is zero. In contrast, there will always be an interior optimum with respect to reaction time at a fixed temperature when an intermediate product in a set of consecutive reactions is desired. (Ignore the trivial exception where the feed concentration of the desired product is already so high that any reaction would lower it.) For the normal case of bin i , a very small reactor forms no B and a very large reactor destroys whatever B is formed. Thus, there will be an interior optimum with respect to reaction time. 

Example 7.5 Suppose the consecutive reactions 2A B C are elementary. Determine the rate constants from the following experimental data obtained with an isothermal, constant-volume batch reactor ... 

Figure 2.6. The hydrogenation of substituted nitro arenes to amines in a batch reactor involves two consecutive reactions when a standard platinum catalyst is used.

Cybulski (1990) simulated the behaviour of a batch (BSTR) and semibatch (SBSTR) reactor in which consecutive reactions take place ... 

David, R., Muhr, H. and Villermaux, J., The Yield of a Consecutive-Competitive Reaction in a Double Jet Semi-Batch Reactor Comparison between Experiments and a Multizone Mixing Model, Chem. Eng. Sci. 1992, 47 (9-11), 2841-2846. 

TEMPERATURE OPTIMIZATION OF BATCH REACTOR CONSECUTIVE AND PARALLEL REACTION SEQUENCE... 

The first-order consecutive exothermic reaction sequence, A —> B —> C, is carried out in a thick-walled, jacketed batch reactor, provided with both jacketed heating and cooling, as shown below. 

Let us consider a batch reactor where the following consecutive reactions take place (Smith, 1981)... 

As an example for precise parameter estimation of dynamic systems we consider the simple consecutive chemical reactions in a batch reactor used by Hosten and Emig (1975) and Kalogerakis and Luus (1984) for the evaluation of sequential experimental design procedures of dynamic systems. The reactions are... 

It should be noted that there are cases in which some selectivity will be lost in choosing a semi-batch mode over a simple batch reactor. If the desired product decomposes by a consecutive reaction, the yield will be higher in the batch reactor [177]. If, on the other hand, the reactants are producing by-products by a parallel reaction, the semi-batch process will give the higher yield. In any case, if the heat production rate per unit mass is very high, the reaction can then be run safely under control only in a semi-batch reactor. 

Let us consider the batch reactor modeled in Sec. 3.9 (Fig. 3.9). Steam is initially fed into the jacket to heat up the system to temperatures at which the consecutive reactions begin. Then cooling water must be used in the jacket to remove the exothermic heats of the reactions. 

Two consecutive, first-order reactions take place in a perfectly mixed, isothermal batch reactor. 

Jl An isothermal perfectly mixed batch reactor has consecutive first-order reactions... 

The catalyst/substrate ratio is 1.5 mol% for the supported ionic liquid phase (SILP) catalyst, 3 mol% for the impregnated catalyst and 2 mol% for the homogeneous reaction aRuns 1 -4 are consecutive experiments with the same catalyst in a stirred batch reactor. bDimeric Cr (salen) catalyst impregnated on silica cHomogeneous reaction at 0-2 OC optimized for product selectivity dHomogeneous reaction at room temperature optimized for product selectivity... 

With consecutive—parallel systems in which the reaction steps are not first order, analytical expressions for species concentrations as functions of time (which would apply to batch reactors) are sometimes unobtainable. Numerical procedures can be used. However, analytical procedures can still be used to obtain some indication of relative yields of reaction... 

Industrially relevant consecutive-competitive reaction schemes on metal catalysts were considered hydrogenation of citral, xylose and lactose. The first case study is relevant for perfumery industry, while the latter ones are used for the production of sweeteners. The catalysts deactivate during the process. The yields of the desired products are steered by mass transfer conditions and the concentration fronts move inside the particles due to catalyst deactivation. The reaction-deactivation-diffusion model was solved and the model was used to predict the behaviours of semi-batch reactors. Depending on the hydrogen concentration level on the catalyst surface, the product distribution can be steered towards isomerization or hydrogenation products. The tool developed in this work can be used for simulation and optimization of stirred tanks in laboratory and industrial scale. 

For the consecutive reactions, A B C, the specific rates are equal and B0 = 0, Find the maximum value of B/Aq in (a) Batch reactor (b) Two stage CSTR. 

If consecutive reactions are conducted in a batch reactor, the optimization of the process includes finding the optimum time to stop the batch and determining the optimum temperature. To illustrate the issues, we take the simple reactions... 

Figure 4.44 Fed-batch reactor with consecutive reactions F ramped from 2 to 1 L/min.

Consider the consecutive reactions, A -A B -A C, with rate constants of kj = 1015exp(— 10,000/r) and kn = 10s exp(—5000/T). Find the temperature that maximizes bout for a CSTR with 1=2 and for a batch reactor with a reaction time of 2 h. Assume constant density with = cin = 0. 

Equation (19-22) indicates that, for a nominal 90 percent conversion, an ideal CSTR will need nearly 4 times the residence time (or volume) of a PFR. This result is also worth bearing in mind when batch reactor experiments are converted to a battery of ideal CSTRs in series in the field. The performance of a completely mixed batch reactor and a steady-state PFR having the same residence time is the same [Eqs. (19-5) and (19-19)]. At a given residence time, if a batch reactor provides a nominal 90 percent conversion for a first-order reaction, a single ideal CSTR will only provide a conversion of 70 percent. The above discussion addresses conversion. Product selectivity in complex reaction networks may be profoundly affected by dispersion. This aspect has been addressed from the standpoint of parallel and consecutive reaction networks in Sec. 7. 

The major advantage of the use of CuHY as a catalyst for this reaction is the ease with which it can be recovered from the reaction mixture by simple filtration if used in. a batch reactor (alternatively it can be used in a continuous flow fixed bed reactor). We have carried out the heterogeneous asymmetric aziridination of styrene until completion, filtered and washed the zeolite then added fresh styrene, PhI=NTs and solvent, without further addition of chiral bis(oxazoline), for several consecutive experiments. The yield and the enantioselectivity decline slightly on reuse we have found that adsorbed water can build up within the pores of the zeolite on continued use and we believe that this is the cause of loss of activity and enantioselection. However, full enantioselectivity and yield can be recovered if the catalyst is simply dried in air prior to reuse, or alternatively the catalyst can be recalcined and fresh oxazoline ligand added. 

For the consecutive reactions A => B => C, a higher yield of intermediate B is obtained in batch reactors or PFRs than in CSTRs. 

In Section 6.1 we saw. that the undesired product could be minimized by adjusting the reaction conditions (e.g., concentration) and by choosing the proper reactor. For series of consecutive reactions, the most important variable is time space-time for a flow reactor and real-timE for a batch reactor. To illustrate the importance of the time factor, we consider the sequence... 

If the first reaction is slow and the second reaction is fast, it will be extremely difficult to produce species B. If the first reaction (formation of B) is fast and the reaction to form C is slow, a large yield of B can be achieved. However, if the reaction is allowed to proceed for a long time in a batch reactor, or if the tubular flow reactor is too long, the desired product B will be converted to C. In no other type of reaction is exactness in the calculation of the time needed to carry out the reaction more important than in consecutive reactions. 

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