Modes of reactor

The physical picture of emulsion polymerization is complex due to the presence of multiple phases, multiple monomers, radical species, and other ingredients, an extensive reaction and particle formation mechanism, and the possibility of many modes of reactor operation. 

Steady-state reactors with ideal flow pattern. In an ideal isothermal tubular pZi/g-yZovv reactor (PFR) there is no axial mixing and there are no radial concentration or velocity gradients (see also Section 5.4.3). The tubular PFR can be operated as an integral reactor or as a differential reactor. The terms integral and differential concern the observed conversions and yields. The differential mode of reactor operation can be achieved by using a shallow bed of catalyst particles. The mass-balance equation (see Table 5.4-3) can then be replaced with finite differences ... 

Three modes of reactor operation may be distinguished, batch, semi-batch and continuous. In a batch system all reactants are added to the tank at the given starting time. During the course of reaction, the reactant concentrations decrease continuously with time, and products are formed. On completion of the reaction, the reactor is emptied, cleaned and is made ready for another batch. 

General. In this section, a mathematical dynamic model will be developed for emulsion homopolymerization processes. The model derivation will be general enough to easily apply to several Case I monomer systems (e.g. vinyl acetate, vinyl chloride), i.e. to emulsion systems characterized by significant radical desorption rates, and therefore an average number of radicals per particle much less than 1/2, and to a variety of different modes of reactor operation. 

The next two steps after the development of a mathematical process model and before its implementation to "real life" applications, are to handle the numerical solution of the model s ode s and to estimate some unknown parameters. The computer program which handles the numerical solution of the present model has been written in a very general way. After inputing concentrations, flowrate data and reaction operating conditions, the user has the options to select from a variety of different modes of reactor operation (batch, semi-batch, single continuous, continuous train, CSTR-tube) or reactor startup conditions (seeded, unseeded, full or half-full of water or emulsion recipe and empty). Then, IMSL subroutine DCEAR handles the numerical integration of the ode s. Parameter estimation of the only two unknown parameters e and Dw has been described and is further discussed in (32). 

General. A mathematical model has been developed in the previous section, which can now be employed to describe the dynamic behaviour of latex reactors and processes and to simulate present industrial and novel modes of reactor operation. The model has been developed in a general way, thus being readily expandable to include additional mechanisms (e.g. redox initiation (59)) or to relax any of the underlying assumptions, if necessary. It is very flexible and can cover various reactor types, modes of operation and comonomer systems. It will be shown in the following that a model is not only a useful... 

Equations (1-1) and (1-2) are true in the general case and can be used to study several modes of reactor operation (e.g. batch, semi-batch, continuous, start-up procedures, etc.). If the assumption is made that the reactor is a vessel continuously operating full, i.e. overflow CSTR, then the right hand side (RHS) of equation (1-2) is zero and (1-1) is considerably simplified to yield ... 

Study the system for the kinetic case nAi = nA2 and nB1semi-continuously with slow feeding of B and (ii) as a batch reactor with B charged initially to the reactor and zero feed of B. Compare the results obtained for the two differing modes of reactor operation. 

Figure 7.1 Modes of reactor operation (a) batch reactor, (b) semi-batch reactor, (c) continuous stirred-tank reactor, and (d) continuous plug flow reactor.

Then, a survey of micro reactors for heterogeneous catalyst screening introduces the technological methods used for screening. The description of microstructured reactors will be supplemented by other, conventional small-scale equipment such as mini-batch and fixed-bed reactors and small monoliths. For each of these reactors, exemplary applications will be given in order to demonstrate the properties of small-scale operation. Among a number of examples, methane oxidation as a sample reaction will be considered in detail. In a detailed case study, some intrinsic theoretical aspects of micro devices are discussed with respect to reactor design and experimental evaluation under the transient mode of reactor operation. It will be shown that, as soon as fluid dynamic information is added to the pure experimental data, more complex aspects of catalysis are derivable from overall conversion data, such as the intrinsic reaction kinetics. 

To obtain experimental data suitable for the determination of kinetic parameters, two different operation modes of reactors should be considered, the differential and the integral mode. In the differential mode rates are calculated from small conversions (<10%) within a fixed time span dt. For reactions with two reactants, it is suitable to use excess of one reactant to suppress undesired side reactions. 

In addition to the standard modes of reactor operation, alternative reactor configurations also facilitate... 

Next we derive relationships between the species composition in chemical reactors to the chemical reactions taking place in them. For convenience, we distinguish between two modes of reactor operations batch operation (batch reactors) and steady continuous operation (flow reactors), shown schematically in Figure 2.1. In batch reactors, reactants are charged into the reactor and, after a certain period of time, the products are discharged from the reactor hence, the chemical reactions take place over time. In steady-flow reactors, reactants are continuously fed into the reactor, and products are continuously withdrawn from the reactor outlet hence, the chemical reactions take place over space. 

Figure 2.1 Modes of reactor operations (a) batch reactor and (b) flow reactor.

The contacting pattern is not the only issne that determines the batch, semi-batch, or continnons mode of reactor operation. Sometimes, heat transfer and control of exothermic reactions may reqnire... 

The mode of reactor operation can be classified as batchwise or continuous . Batch reactions are started by filling a reactor with the reaction mixture and stopped after reaching the desired conversion. A steady state is only reached at equilibrium conversion of the reaction. A typical batch reactor is represented by the stirred tank reactor. 

The results obtained from analytical equipment measuring the composition of a reactor output are normally reported in mol fractions. In most cases the mol fractions of components at the outlet are automatically re-normalized so that their sum is one. This is correct as far as mol fractions are concerned, but the atomic balances of the output may well be off due to both analytical error and the process of re-normalization applied to the measured mole fractions. This is an issue that needs to be addressed in each instance, regardless of reactor configuration or the mode of reactor operation. At the same time this is a particularly well conditioned problem to address in the case of the voluminous data from a TSR experiment. 

Catalyst testing under nonisothermal conditions suffers from the fact that activity will change with location inside the reactor corresponding to differences in temperature. These circumstances indicate that as far as it is possible to establish isothermal conditions, such a mode of reactor operation should be realized for the determination of intrinsic activity and selectivity of catalysts. 

Independent of the mode of reactor operation it is of tremendous importance to stop any further addition of fijesh reactants to the system, once a maloperation has occurred. This should be ensured technically, whenever possible. In elderly plants, organizational measures can be sufficient, provided they are of very good quality. The interruption in supplies eitsures that the hazard potential related to the maloperation remains restricted to the substance amoimt present in the reactor at the time of process disturbance. All further elaborations assume this effective interruption in supplies. 

The first step in any consideration of reaction rates is the definition of reaction time. This depends on the mode of reactor operation, batch or continuous (see Figure 4.1). For the batch reactor, the reaction time is the elapsed time whereas for the continuous reactor, it is given by the time the reactant spends in the reactor, called the residence time, that is measured by the ratio of reactor volume to flow rate (volume/volume per unit time with units of time). An equally important consideration is the concept of reaction space (which can have units of volume, surface, or weight), leading to different definitions of the reaction rate. We begin this section by considering different ways of defining the reaction rate based on different definitions of reaction time and space. 

From the discussion in Example 5.2, semibatch operation with continuous addition of DAA to hypochlorite appears to be the preferred mode of reactor operation. Using the same nomenclature as in that example, and employing the equations summarized in Table 10.2 for different variable volume semibatch reactions, the following equation can be written for component i in reaction j ... 

Bioprocessing includes a large variety of metabolic reactions even on the macroscopic level (cf. Fig. 2.16) and at the same time is carried out in different modes of reactor operations. Therefore, complex reaction systems must be treated stoichiometrically, which means that not only complex reactions themselves must be considered but also complex reactor operations. 

Stoichiometry, for example, in the case of nonstationary modes of reactor operation, needs another fundamental equation. For the CSTR the balance of a component is written as... 

Except for the fact that the mode of reactor cooling has been changed from forced circulation to natural circulation (reactor water recirculation pumps have been eliminated), all other sterns and components employed for plant operation are based on the extensive upeiating expel leiice gained fiom the boiling water reactor plants currently in service in Germany as well as on the proven system and component designs implemented in these plants. 

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