Batch evaporation

The heat requirements in batch evaporation are the same as those in continuous evaporation except that the temperature (and sometimes pressure) of the vapor changes during the course of the cycle. Since the enthalpy of water vapor changes but little relative to temperature, the difference between continuous and batch heat requirements is almost always negligible. More important usually is the effect of variation of fluid properties, such as viscosity and boiling-point rise, on heat transfer. These can only be estimated by a step-by-step calculation. 

Virtanen, J. 1984. Automatic control of batch evaporative crystallization. In Industrial Crystallization 84. The Hague September 1984. Eds. S.J. Jancic and E.J. de Jong. Elsevier Science. 

The aqueous waste stream (2AW) containing the 241 was concentrated and stripped of acid using two batch evaporators in the Low Activity Waste (LAW) system. The first concentration step was performed in the LAW batch evaporator. Acid stripping with water and additional evaporation was performed in the second LAW batch evaporator. The average concentration of the Am entering this two-step evaporation process was 3.4 x 10 g/L after the first step, the Am concentration was 0.08 to 0.15 g/L after the second step, the Am concentration was 0.2 to 0.3 g/L, and the nitric acid concentration was 2.0 to 2.5M. 

Reduction. To convert plutonium to inextractable Pu(III) and neptunium to still extractable Np(IV), 0.5 M U(IV) reductant is added to the aqueous stream from the HC unit. It is necessary to hold the reacting mixture for half an hour or more to obtain nearly complete reduction of neptunium. This is best done batchwise in a set of reactors, some of which would be reducing while others are receiving feed to be reduced. Reduced fuel is then concentrated to 1.172 Af uranium in a set of batch evaporators. 

A batch evaporative crystallizer (Figure 10.2) was used by Baliga (1970) to study the crystallization kinetics of potassium sulfate crystals. The crystallizer was equipped with a reflux condenser and a controlled distillate splitter so that the net solvent removal rate could be controlled closely. Heating of the crystallizer... 

Figure 10.2 Laboratory batch evaporative crystallizer equipment. (Reproduced with permission from Baliga 1970.)...

Figure 10.5 Industrial calandria batch evaporative crystallizer. (Reproduced by permission of the American Institute of Chemical Engineers 1984 AIChE from Advances in Industrial Crystallization Techniques, Bennett, R.C. AIChE Symposium Series, vol. 80, no. 240, pp. 45-54 (1984).)...

The type and area of the energy-exchange surface as well as the process control must be adequate to accommodate both low evaporation rates at the beginning of the batch evaporation and elevated evaporation rates in the final stage of the run. These rates may differ by two orders of magnitude. 

Figure 9.16. A pressure controlled batch evaporating crystallizer A, 2 C crystallization vessel, B, baffle, C, stirrer, D, motor, E, variable speed controller, F, heating mantle, G, H, condensers, I, condensate receiver, J, drying tower, K, vacuum pump, L, contact manometer, M, electromagnetic valve, N, control box, P, electronic thermometer, R, vacuum gauge. After Mullin and Broul, 1978)...

Table 7.2 Alternative reaction schemes for nylon 6,6 oligomerization in a batch evaporator...

A more frequent method of operation is semibatch in which feed is continuously added to maintain a constant liquid level until the entire charge reaches the final concentration. Continuous-batch evaporators usually have a continuous feed, and over at least part of the cycle, a continuous discharge. One method of operation is to circulate from a storage tank to the evaporator and back until the entire tank is at a specified concentration and then finish the evaporation in batches. 

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