In multiple effect evaporation

Calcium Chloride. Distiller waste Hquor from synthetic plants can be evaporated in multiple effect evaporators, precipitating residual sodium chloride. The resulting mother Hquor is then further evaporated to a molar ratio of lCaCl2 2H20 and cooled to produce flakes that are dried in rotary or... 

The vegetable-tanning materials are commercially extracted using hot water. The extraction is normally done in countercurrent extractors that permit the final removal of the extracts with fresh water. The dilute extracts are then evaporated to the desired concentration in multiple effect evaporators. Some extracts may be further dried by spray drying or any other means that proves effective without overheating the extract. Extract preparation depends on the type of extract, the si2e of the operation, and the desired concentration of the final product. 

Electrodialysis. Electro dialytic membrane process technology is used extensively in Japan to produce granulated—evaporated salt. Filtered seawater is concentrated by membrane electro dialysis and evaporated in multiple-effect evaporators. Seawater can be concentrated to a product brine concentration of 200 g/L at a power consumption of 150 kWh/1 of NaCl (8). Improvements in membrane technology have reduced the power consumption and energy costs so that a high value-added product such as table salt can be produced economically by electro dialysis. However, industrial-grade salt produced in this manner caimot compete economically with the large quantities of low cost solar salt imported into Japan from Austraha and Mexico. 

Stewart, G. and BEVERIDGE, G. S. G. Computers and Chemical Engineering 1 (1977). 3. Steady-state cascade simulation in multiple effect evaporation. 

The spent black liquor from the kraft pulping of pines contains the less volatile products of the wood resin in the form of sodium salts or soaps. The liquor first is concentrated in multiple-effect evaporators, and then the concentrate is sent to settling tanks. The soaps rise to the surface, are skimmed off, and then are acidified with sulfurous or sulfuric acid. The crude tall oil rises to the top and is mechanically separated. Crude tall oil from southern pines contains 40-60 percent resin acids and 40-55 percent fatty acids with 5-10 percent neutral substances. These components are separated by fractional distillation under vacuum. 

The dark-colored clarified liquor is presure-filtered and concentrated to 60 percent solids in multiple-effect evaporators. The concentrated liquor is decolorized with granular carbon in columns 12 ft in diameter and 30 ft high in a countercurrent manner that is, liquor flows upward in the columns, while a portion of the carbon is removed from the bottom periodically. Carbon is used at a rate of 2.5 percent of dry solid processed, and approximately 5 percent of carbon is lost during revivification.91 In some cases, granular carbon has been replaced by synthetic polymeric adsorbents to decolorize the syrup. Low ash syrups usually are deionized with ion-exchange resins. The processed liquor is evaporated to a final solids content of 75-85 percent in a single-effect evaporator. 

The filtrate is then concentrated to 60% of dry substance in multiple-effect evaporators economical in energy consumption ( 1 lb of water evaporated by 1/n lb of steam, where n is the number of effects). However, the temperature to which the liquor is exposed in the multiple-effect evaporators is much higher than in the single-effect, vacuum evaporators that are used in the final evaporation of refined liquor to afford finished-syrup concentration this exposure to high temperature causes, in the syrup liquor, formation of color and off-fla-... 

An alternative scheme is to split the evaporation process into stages (in different vessels), commonly referred to as effects, where the flow of liquor may be in the same direction, the backward direction, or normal to the flow of vapor. In such a scheme, the vapor generated in a given effect is used to boil the liquid in a different effect where a proper temperature driving force exists. In multiple-effect evaporation, one exchanges savings in steam costs for increased capital investment in equipment. 

Steam-heated evaporators are available in two major types single-effect and multiple-effect evaporators. Single-effect evaporators are subdivided into short-tube, long-tube, and agitated-film evaporators. Here will be discussed the common, vertical, short-tube evaporator, which also constitutes the basic repeated unit in multiple-effect evaporators. 

In multiple-effect evaporating system, a number of evaporators are connected in series so that the vapor produced by the steam in the first unit is introduced into the steam chest of the second effect the vapor of this evaporator is again passed into the steam chest of the third effect, and so on as shown in Fig. 2. 

Potassium Hydroxide.—Usually concentrated from 10 to 46 per cent in multiple-effect evaporators of vertical-tube construction, with a steam pressure of from 5 to 20 lb. and a vacuum of from 26 to 28 in., at a capacity of about 1 gal. per square foot. Potassium carbonate or chloride are recovered in salt filters. Evaporator shells are of cast iron and tubes of charcoal iron. 

Sulphuric Acid.— Weak acid solutions are successfully concentrated in multiple-effect evaporators of the vertical-tube or horizontal-tube type, having cast-iron shells and lead lining, with lead tubes in some cases the whole apparatus is built of lead. Capacity, steam pressure and vacuum depend on special conditions. 

Black Liquor from Soda-pulp Mills.—One ton of pulp will produce about 3,300 gal. of 5 deg. liquor, and this is concentrated to 35 deg. in multiple-effect evaporators of the horizontal and film type, with a capacity of from 2 to 2K gal. per square foot, according to the steam pressure, which will be from 10 to 25 lb., with a vacuum of from 27 to 28 in. Special construction and separators are necessary on account of the excessive foaming. Evaporators must have cast-iron or steel shells with wrought-iron or steel tubes, as the liquors are strongly alkaline. 

Sulphite Waste Liquor.—One ton of pulp will produce from 2,500 to 5,000 gal. of waste liquors, the density of which will vary from 6 to 10 per cent. It is usually concentrated after neutrahzation to about 30°B6. (50 per cent solids) in multiple-effect evaporators of the vertical or rapid-circulation type, and the capacity is from 2 4 to 3J gal. per square foot, with a steam pressure of 5 lb. and a vacuum of from 27 to 28 in. The tubes will foul very quickly, on account of the large amount of calcium sulphate contained in the weak solution, and therefore evaporators are to be constructed so that tubes can readily be cleaned mechanically. 

Tank Water from Packing Houses.—Sheep and calves will produce from 1)4 to 2 gal., hogs from 2) i to 3 gal. and beeves from 7 to 10 gal. of tank water, with a density of about 4°B6., and the quantity will be larger if the percentage of sohds is lower. In most cases, 3°B6. hquor is concentrated to about 27° in multiple-effect evaporators of the horizontal type having a capacity of 1.8 gal. per square foot, with a steam pressure of from 3 to 5 lb. and a vacuum of from 27 to 28 in. There is not much tendency to foam, but sometimes tank water will form a heavy scale on the tubes which has to be removed by boiling with alkaline solutions. Waste waters are sometimes slightly acid, and evaporators must have cast-iron shells and copper or brass tubes. 

Figure 17. Patterns ofliquid flow in multiple-effect evaporators Forward feed, Backward feed, fcj Mixed Feed.

Energy economy and evaporative capacity are the major measures of evaporator performance. When evaporating water with steam, the economy is nearly always less than 1.0 for single-effect evaporators, but in multiple-effect evaporators it is considerably greater. Other performance variables to be considered include product quality, product losses, and decrease in performance as scaling, salting, or fouling occurs. 

Patterns of liquor flow in multiple..effect evaporators (rz) forward feed b) backward feed (c) mixed feed d) parallel feed, (omn) Liquor streams. (-) Steam and vapor condensate streams. 

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