Reactors semibatch

Semibatch copolymerization is most often done in an attempt to maintain a reasonably constant copolymer composition when the comonomers are of widely varying reactivities [1]. Semibatching of initiator is often done to maintain temperature control in a heat transfer-limited kettle, and semibatch addition of initiator or chain transfer agent may be used to maintain a desired MWD. Quantitative strategies for semibatching may be developed through empirical experimentation at the bench or pilot scale, or, if accurate mathematical models are available, classical trajectory optimization techniques may be used [2,3]. 

Semibatch reactors (SBRs) are very common in industrial organic synthesis in general. The basic principle is that a reactant is placed in the reactor and the same or a second reactant, usually the latter, is added continuously. The product 

Rgure 10.7 Representative modes of semibatch operation (SBO) for different reaction schemes. Bold underlined letters indicate components already present in the reactor. 

First consider the general case of a simple irreversible second-order reaction 

The differential equations for these reactions and analytical solutions wherever possible are included in Table 10.2. The semibatch mode of operation of the Van de Vusse scheme gives results similar to those of the recycle reactor. Thus higher yields and selectivities for product R can be realized than in a PFR or an MFR when 3[/4]q 2  

Part 1. Matrix for identification of terms for constructing design equations 

One of the best reasons to use semibatch reactors is to enhance selectivity in liquid-phase reactions. For example, consider the following two simultaneous reactions. One reaction produces the desired product D 

Three variables can be used to formulate and solve semibatch reactor problems the concentrations, Cj, the number of moles, Nj, and the conversion. X. 

3 Writing the Semibatch Reactor Equations in Terms of Concentrations 

Recalling that the number of moles of A, is just the product of concentration of A. Ca, and the volume, V, we can rewrite Equation (4-51) as 

We note that since the reactor is being filled, the volume. V. varies with time. The reactor volume at any lime ( can be found from an overall mass balance 

In this chapter, the analysis of chemical reactors is expanded to additional reactor configurations that are commonly used to improve the yield and selectivity of the desirable products. In Section 9.1, we analyze semibatch reactors. Section 9.2 covers the operation of plug-flow reactors with continuous injection along their length. In Section 9.3, we examine the operation of one-stage distillation reactors, and Section 9.4 covers the operation of recycle reactors. In each section, we first derive the design equations, convert them to dimensionless forms, and then derive the auxiliary relations to express the species concentrations and the energy balance equation. 

Principles of Chemical Reactor Analysis and Design, Second Edition. By Uzi Mann Copyright 2009 John Wiley Sons, Inc. 

To derive the design equation of a semibatch reactor, we write a species balanee for any speeies, say species j, that is not fed continuously into the reactor. Its molar balance equation is 

Consider first a semibatch reactor with liquid-phase reactions. To derive a relation for the change in the volume of the reactor, we write an overall mass balance over the reactor  

Since the volumetric injection flow rate finj(r) may vary with time, we have to solve Eq. 9.1.4 simultaneously with Eq. 9.1.2. 

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Specific reactor characteristics depend on the particular use of the reactor as a laboratory, pilot plant, or industrial unit. AH reactors have in common selected characteristics of four basic reactor types the weH-stirred batch reactor, the semibatch reactor, the continuous-flow stirred-tank reactor, and the tubular reactor (Fig. 1). A reactor may be represented by or modeled after one or a combination of these. SuitabHity of a model depends on the extent to which the impacts of the reactions, and thermal and transport processes, are predicted for conditions outside of the database used in developing the model (1-4). 

Semibatch Reactors. Semibatch reactors are the most versatile of reactor types. Thermoplastic injection molds are semibatch reactors in which shaped plastic articles are produced from melts. In mol ding thermoplastics, large clamping forces of up to 5000 metric tons are needed to keep molds together, while highly viscous polymers are forced into their cavities. Heat transfer is critical. If the molds are too cold, polymers soHdify before filling is completed if they are too hot, the time required for cooling delays production. 

When the desirable product of a complex reaction is favored by a high concentration of some reactant, batch or semibatch reactors can Be made superior to CSTRs. 

The semibatch reactor where the incoming and outgoing mass flows are not equal to each other, and the total mass of the reacting mixture is not constant. 

Steensma, M., and K. R. Westererp (1988). Thermally Safe Operation of a Cooled Semibatch Reactor. Slow Liquid-Liquid Reactions. Chemical Engineering Science 43, 8, 2125-32. 

The model is able to predict the influence of mixing on particle properties and kinetic rates on different scales for a continuously operated reactor and a semibatch reactor with different types of impellers and under a wide range of operational conditions. From laboratory-scale experiments, the precipitation kinetics for nucleation, growth, agglomeration and disruption have to be determined (Zauner and Jones, 2000a). The fluid dynamic parameters, i.e. the local specific energy dissipation around the feed point, can be obtained either from CFD or from FDA measurements. In the compartmental SFM, the population balance is solved and the particle properties of the final product are predicted. As the model contains only physical and no phenomenological parameters, it can be used for scale-up. 

Villemiaux, J., 1989. A simple model for partial segregation in a semibatch reactor. American Institute of Chemical Engineers Annual Meeting, San Francisco, Paper 114a. 

Semibatch Model "GASPP". The kinetics for a semibatch reactor are the simpler to model, in spite of the experimental challenges of operating a semibatch gas phase polymerization. Monomer is added continuously as needed to maintain a constant operating pressure, but nothing is removed from the reactor. All catalyst particles have the same age. Equations 3-11 are solved algebraically to supply the variables in equation 5, at the desired operating conditions. The polymerization flux, N, is summed over three-minute intervals from the startup to the desired residence time, t, in hours ... 

For all likely operating conditions, (ie., for t < X), the appropriate values of the concentration and the polymerization rate constant are the values calculated at t = t ( 2). To prove this, the exit age distribution function for a backmix reactor was used to weight the functions for Cg and kj and the product was integrated over all exit ages (6). It is enlightening at this point to compare equation 18 with one that describes the yield attainable in a typical laboratory semibatch reactor at comparable conditions. ... 

The yield that can be attained by a semibatch process is generally higher because the semibatch run starts from scratch, with maximum values of both variables Cg (o) = Cg and k] (o) = k . However, the yield from a continuous run in which t equals the batch time is governed by the product of Cg (t) and kj (t), so > and k (t) = k °. Because neither of these conditions is likely to be fulfilled completely, a continuous polymerization in a backmix reactor will probably always fail to attain the Y attainable by a semibatch reactor at the same t. However, several backmix reactors in series will approach the behavior of a plug flow continuous reactor, which is equivalent to a semibatch reactor. 

This section is divided into three parts. The first is a comparison between the experimental data reported by Wisseroth (].)for semibatch polymerization and the calculations of the kinetic model GASPP. The comparisons are largely graphical, with data shown as point symbols and model calculations as solid curves. The second part is a comparison between some semibatch reactor results and the calculations of the continuous model C0NGAS. Finally, the third part discusses the effects of certain important process variables on catalyst yields and production rates, based on the models. 

The effects of diffusion and catalyst decay cause yields from a continuous backmix reactor to be 25 to 30% lower than from a semibatch reactor at the same residence time. This yield penalty can be reduced by staging backmix reactors in series. 

As expected, heat exchanged per unit of volume in the Shimtec reactor is better than the one in batch reactors (15-200 times higher) and operation periods are much smaller than in a semibatch reactor. These characteristics allow the implementation of exo- or endothermic reactions at extreme operating temperatures or concentrations while reducing needs in purifying and separating processes and thus in raw materials. Indeed, since supply or removal of heat is enhanced, semibatch mode or dilutions become useless and therefore, there is an increase in selectivity and yield. 

From the process safety point of view, the evaluation of the intrinsic safe character of HEX compared to batch or semibatch reactors has been investigated [33, 37]. Two points clearly show the interest in the HEX reactors ... 

Factors re.sponsible for the occurrence of scale-up effects can be either material factors or size/shape factors. In addition, differences in the mode of operation (batch or semibatch reactor in the laboratory and continuous reactor on the full scale), or the type of equipment (e.g. stirred-tank reactor in the laboratory and packed- or plate- column reactor in commercial unit) can be causes of unexpected scale-up effects. A simple misuse of available tools and information also can lead to wrong effects. 

Example 5.3.1.5. Selectivity versus mode of operation in semibatch reactors... 

In order to illustrate how the mode of operation can positively modify selectivity for a large reactor of poor heat-transfer characteristics, simulations of the reactions specified in Example 5.3.1.4 carried out in a semibatch reactor were performed. The reaction data and process conditions are essentially the same as those for the batch reactor, except that the initial concentration of A was decreased to cao = 0.46 mol litre, and the remaining amount of A is dosed (1) either for the whole reaction time of 1.5 h with a rate of 0.1 mol m s", or (2) starting after 0.5 h with a rate of 0.15 mol m " s". It is assumed that the volume of the reaction mixture and its physical properties do not change during dosing. The results of these simulations are shown in Fig. 5.3-15. The results of calculation for reactors of both types are summarized in Table 5.3-3. 

It is clear from the presented data that the yield and selectivity in a large semibatch reactor can be improved compared to those in a small batch reactor that has much better heat-transfer capability. This has been achieved by decreasing the rate of heat evolution, which has been obtained by lowering the instantaneous concentration of reactant A. The results also indicate that the dosing policy can have a very significant influence on reactor performance. 

Figure 5.3-15. Concentration versus time in a semibatch reactor.

Single phase Gas-liquid or liquid-liquid (semibatch reactors) Catalytic (three-phase) (semibatch or continuous) ... 

Semibatch reactors are operated in two different modes (1) Some components of the reaction mixture are loaded into the reactor. After the operating conditions have reached the required level, the other components are dosed continuously or portion-wise, whereby temperature and pressure are kept as close as possible to profiles determined as optimum ones. This mode of... 

Semibatch reactors are often used to mn highly exothermic reactions isothermally, to run gas-liquid(-solid) processes isobarically, and to prevent dangerous accumulation of some reactants in the reaction mixture. Contrary to batch of)eration, temperature and pressure in semibatch reactors can be varied independently. The liquid reaction mixture can be considered as ideally mixed, while it is assumed that the introduced gas flows up like a piston (certainly this is not entirely true). Kinetic modelling of semibatch experiments is as difficult as that of batch, non-isotherma experiments. 

Increase of semibatch reactor productivity for Ciba-Geigy Stoessel (1994)... 

Example 5.4.5.1. Application of the E-model for simulation of the coupling of I-naphthol with diazotized sulphanilic acid in a semibatch reactor (after Baidyga and Bourne, 1989b). 

S.4. Guidelines for scale-up of semibatch reactors for fast homogeneous reactions in the absence of data on chemical kinetics and on the distribution of energy dissipation in the reaction zone... 

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