Batch-mass processes

Classification of Processes and Reactors. Most styrene polymers are produced by batch suspension or continuous mass processes. Some are produced by batch mass processes. Mass in this sense includes bulk polymerization of the polymer... 

Figure 1. Batch-mass process flow diagram (1)...

Access to Practice. Publications and patents on the batch mass process are limited. Bishop s book CD contains the most detailed description of the polymerization press and mass-suspension processes for PS and HIPS. Fong (16) presents an economic analysis of the press process based on Bishop s description. Patent references are few for the batch-mass process the 1939 Bakelite patent on transfer of prepoly syrup to chambers or containers is of historical interest (17). 

Batch Mass Reactors. The batch-mass reactors used in these processes are of two types low conversion agitated kettles and high conversion static reactors with extended cooling surfaces. 

Process flow for a typical batch-mass polystyrene process(1) is shown in Figure 1. Styrene monomer is charged to the low conversion prepolymerization reactor with catalyst and other additives, and the temperature is increased stepwise until the desired conversion is reached. It is then transferred into the press. Polycycles are 6 to 14 hours in the low conversion reactor, and 16 to 24 hours in the press. At completion, the cakes are then cooled with water and removed from the press to be ground and then (usually) extruded into pellets. 

The second large-scale process was the batch mass suspension process. Monsanto did the pioneer work on this (41). In this process, prepolymerization is carried out in bulk and main polymerization in suspension the latter is taken to conversions of over 99%. In contrast to the continuous mass process, peroxide starters are used in order to achieve a high conversion at tolerable reaction times. Figure 3 shows a basic flow diagram of such a plant. A detailed discussion of advantages and disadvantages of the two processes can be found in R. Bishop s monograph published in 1971 (42), and it is continued in a paper by Simon and Chappelear in 1979 (43). It was a decisive factor for the economic success of impact polystyrene that these processes had been completely developed and mastered in theory and practice. 

Grewer, T., DECHEMA Monograph, 1980, 88(1818-1835), 21-30 One of the principal causes of batch chemical processes becoming unstable is the combination of a high reaction exotherm and a low reaction rate (or rates if there is more than one component reaction in the overall process). A secondary cause of reaction delay leading to instability is too little mass transfer. To permit safe operation of such reaction systems, the reaction energy (which may be expressed as a dimensionless reaction number) and the reaction rate(s) must be known. The possibility of there being a lower safe limit as well an upper safe limit to reaction temperature is discussed. 

Simple sublimation is a batch-wise process in which the solid material is vaporised and then diffuses towards a condenser under the action of a driving force attributable to difference in partial pressures at the vaporising and condensing surfaces. The vapour path between the vaporiser and the condenser should be as short as possible in order to reduce mass-transfer resistance. Simple sublimation has been used for centuries, often in very crude equipment, for the commercial production of ammonium chloride, iodine, and flowers of sulphur. 

Sample Size - Because of their mass processing nature, comnercial food systems machinery tend to utilize large sample sizes. When novel products are tested, the experimental samples are sometimes too small to be used in such a manner. Model Systems employing smaller batch sizes provide the answers. 

The example is concerned with a batch chlorination process. At the beginning, the fresh toluene charged to the reactor will contain no dissolved chlorine. After bubbling of the chlorine has commenced, a period of time will need to elapse before the concentration of the dissolved chlorine rises to a level that just matches the rate at which it is being removed from the solution by reaction. To avoid such a complication in this example, calculations are carried out for the stage when, after chlorine bubbling has continued at a steady rate, the fractional conversion of the toluene has reached a value of 0.10. It is then assumed that, at any instant in time, the rate of mass transfer of chlorine from the gas phase is just equal to the rate at which it reacts in the bulk of the liquid, i.e. the rate is given by equation 4.17. 

While vinyl acetate is normally polymerized in batch or continuous stirred tank reactors, continuous reactors offer the possibility of better heat transfer and more uniform quality. Tubular reactors have been used to produce polystyrene by a mass process (1, 2), and to produce emulsion polymers from styrene and styrene-butadiene (3 -6). The use of mixed emulsifiers to produce mono-disperse latexes has been applied to polyvinyl toluene (5). Dunn and Taylor have proposed that nucleation in seeded vinyl acetate emulsion is prevented by entrapment of oligomeric radicals by the seed particles (6j. Because of the solubility of vinyl acetate in water, Smith -Ewart kinetics (case 2) does not seem to apply, but the kinetic models developed by Ugelstad (7J and Friis (8 ) seem to be more appropriate. 

The general mode of operation of a cyclic batch adsorption process is illustrated in Fig. 8. In its simplest form such a process employs two adsorbent beds, each of which is alternately saturated and regenerated. During the saturation or adsorption cycle, adsorption is continued until the mass transfer zone has almost reached the bed outlet. At this point the beds are switched so that the spent bed is replaced by a freshly regenerated bed, while the more strongly absorbed species is removed from the spent bed in... 

Equations 1 to 3 represent the generic mass balances for cells, substrate and product in a batch cultivation process ... 

The definition of, and the necessary conditions for, the existence of an arheotrope are based on the material balances of a batch distillation process in the presence of a stagnant or flowing sweep gas (Fig. 4.13). Let us first consider a nonreactive liquid mixture. In this case, the component and total mass balances are given by ... 

This gives rise to a warning In feasibility studies for an open batch distillation process certain assumptions are made as to the heating policy (see e.g. Ref. [7]). Since the ratio of the evaporation velocity to the liquid phase mass transfer coefficient uiiq/knq also depends on the heating policy , one must ensure that this ratio is sufficiently low otherwise the composition of the reactive arheotrope will also depend on the heating policy. ... 

The aim of this section is to show that in an open batch distillation process both gas-phase and liquid-phase mass transfer resistances have a remarkable effect on the compositions at which nonreactive arheotropes and reactive arheotropes appear. 

Reactor and Process Types. It is interesting to subdivide previous work not covered by reactor and process type (Table II). This could be considered a particular two-dimensional projection of the multidimensional space along the coordinates of reactor type and process type. The number of references in each subsection of the field is shown. Obviously, the greatest amount of work has been done in batch, mass/solution polymerization. This is undoubtedly because of the relative simplicity of the mathematical formulation as much as the commercial importance of this case. The same comment applies to the extension of mass/solution... 

At the same end-point, no matter how defined, fhe rheological and dimensional properfies of fhe granules are similar. As we will see from fhe examples described below, that means that the density and dynamic viscosity of the wet mass are constant, and the only variables that are left are the process variables, namely batch mass, impeller diameter and speed, and the geometry of the vessel. 

On one side of the membrane is the solvent, and on the other is the solution to be separated. The particles will pass from the solution side to the solvent side, in the direction of decreasing solute concentration. In a batch dialysis process, the mass transfer of solute passing through the membrane at a given time is ... 

Edible oil deodorizing high vacuum. For <0.6 kg/s, irregular production, use batch. Requires processing time = 4 h/batch, low heat recovery. For >0.6 kg/s, continuous with processing time = 1 h. Keep liquid films thin to promote mass transfer of volatiles and use astute distribution of sparge steam. 

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