Evaporators forced circulation

It is convenient to classify evaporators into natural circulation evaporators, forced-circulation evaporators, and film evaporators. 

Evaporators in which circulation is maintained, independent of the evaporation rate or heating temperature, through the heating element are known as forced circulation evaporators. Forced circulation systems are illustrated in Figs. i(a) and S(b). Forced circulation systems are more expensive than comparable natural circulation evaporators and are, therefore, used only when necessary. 

The parallel product flow configuration for a two-effect evaporative forced-circulation crystallizer is illustrated in Figure 9.16. The feed rate is maintained at a constant rate by feedback flow control. The level is controlled by varying the mother liquor return from downstream solid-liquid separation to the suction of a centrifugal pump. 

Much of the equipment used in the recovery system is identical with or closely related to equipment used in other chemical industries. This includes multiple-effect evaporators, and forced-circulation concentrators, causticizing equipment, and lime kiln. The function and nature of equipment essentially unique to the kraft recovery system are discussed herein. 

Water is evaporated from purified brine using multiple-effect or vapor recompression evaporators (Figs. 3 and 4). Multiple-effect systems typically contain three or four forced-circulation evaporating vessels (Fig. 4) connected together in series. Steam from boilers suppHes the heat and is fed from one evaporator to the next to increase energy efficiency in the multiple-effect system. 

Fig. 2. Forced circulation evaporators (a) submerged-tube, shown as circulating magma crystallizer (b) submerged-tube, shown as suspension type...

Forced-Circulation Evaporators In evaporators of this type in which hydrostatic head prevents boiling at the neating surface, heat-... 

FIG. 11-122 Evaporator types, a) Forced circulation, (h) Siibmerged-tiihe forced circulation, (c) Oslo-type crystallizer, (d) Short-tube vertical, (e) Propeller calandria. (f) Long-tube vertical, (g) Recirculating long-tube vertical, (h) Falling film, (ij) Horizontal-tube evaporators. G = condensate F = feed G = vent P = product S = steam V = vapor ENT T = separated entrainment outlet. 

Frequent difficulties with forced-circulation evaporators ... 

The type of evaporator to be used and the materials of construc tion are generally selected on the basis of past experience with the material to be concentrated. The method of feeding can usually be decided on the basis of known feed temperature and the properties of feed and produc t. However, few of the listed variables are completely independent. For instance, if a large number of effects is to be used, with a consequent low temperature drop per effect, it is impractical to use a natural-circiilation evaporator. It expensive materials of construction are desirable, it may be found that the forced-circulation evaporator is the cheapest and that only a few effec ts are justifiable. 

FK . 18-64 Forced -circulation (evaporative) crystallizer, (Swenson Process Equipment, Inc.)... 

The external heating boxes for the caustic soda evaporator with forced circulation must be anodically protected separately. Ring-shaped impressed current electrodes of carbon steel are mounted and insulated on supporting brackets (see Fig, 21-12). 

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