Evaporator forced-circulation evaporators

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

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). 

Major evaporator designs include forced-circulation, long-tube vertical (both rising and falling film), and calandria-type evaporators. The economics of a particular process will dictate the evaporator style and model best suited to a particular application. Forced-circulation and calandria evaporators are required for processes where crystals are formed. These evaporators are designed to keep crystals suspended in solution to prevent scaling of the equipment. Long-tube vertical evaporators are used to concentrate a liquid that does not have solids present. 

A variety of forced-circulation evaporator designs are available, with and without elutriating legs or cyclones. Different inlets and outlets are used on the vapor body, tailored for the particular application. For some applications, such as super-... 

Figure 9.18 Continuous crystallizers, a) draft-tube and baffle (DTB), (b) single effect forced-circulation evaporative, (c) Oslo or Krystal type after Rohani, 2001)...

Natural circulation Forced circulation Short-tube evaporators 200-600 400-2,000 hio = (16.1/dJ [Bik(cjx/k)f ... 

This represents a special case of high-level turbulence at a surface by the formation of steam and the possibility of the concentration of ions as water evaporates into the steam bubbles . For those metals and alloys in a particular environment that allow diffusion-controlled corrosion processes, rates will be very high except in the case where dissolved gases such as oxygen are the main cathodic reactant. Under these circumstances gases will be expelled into the steam and are not available for reaction. However, under conditions of sub-cooled forced circulation, when cool solution is continually approaching the hot metal surface, the dissolved oxygen... 

Long tube vertical evaporators with either natural or forced circulation are most popular. Tubes are 19-63 mm dia and 12-30 ft long. 

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