Quadruple-effect evaporators

Fuel oils are used extensively in War Plants, as, for example, a typical WWII TNT manufg plant used them iii sulfuric acid concentrators, in quadruple effect evaporators of TNT, red waste water, in rotating drums for incineration of the so-called "thick liquor" and as foel for heating boilers... 

Batch crystallizers are used primarily for production of fine chemicals and pharmaceuticals at the rate of 1-100 tons/week. The one exception is the sugar industry that still employs batch vacuum crystallization on a very large scale. In that industry, the syrup is concentrated in triple- or quadruple-effect evaporators, and crystallization is completed in batch vacuum pans that may or may not be equipped with stirrers [Fig. 16.11(g)]. 

The cell liquor flowing from the cathode compartment of each cell is collected for evaporation to 50 percent product caustic. The diaphragm cell liquor contains 10-12 percent NaOH, 14-16 percent NaCl, 0.1—0.5 percent Na2S04, 0.O2-O.O4 percent Na2C03, and 0.01-0.2 percent NaC103. The evaporation is carried out in nickel-lined quadruple-effect evaporator systems. The salt content of membrane cell effluent is so low that only a simple flash evaporator is required. This represents a considerable reduction in investment versus diaphragm caustic plants. Because of the high... 

Sugar, Glucose and Maltose.—One ton of sugar cane will give about 150 lb. of sugar in Louisiana, and from 200 to 225 lb. in the tropics. The juice will amount to 200 gal. per ton, with a density of from 5K to 8°Brix. It is concentrated in a triple or quadruple-effect evaporator to about 27°B6., with a steam pressure of 10 lb. and a vacuum of from 27 to 28 in. The capacity is 3 to 4 gal. per square foot, and the Standard, Lillie, Kestner, Sanborn and Swenson evaporators are the usual types. The 27°B6. solution is concentrated to 42° in single effects of the coil or Standard type. The capacity is about 1 gal. of water evaporated per square foot, and either live or exhaust steam is used in these pans. 

Beet Sugar.—One ton of beets will give about IK ton of juice, having a density of from 7 to 8K°B6. The concentration is done in two stages, and triple- or quadruple-effect evaporators with a capacity of from 3 to 4 gal. per square foot are used for the concentration up to 27°B6. The Wellner Yelinek, Swenson and Kilby construction are the usual types. The final concentration up to 42°B. is done in single effects of the coil or Standard type, with a capacity of about 1 gal. per square foot. 

Mercerizing Liquors.—Quantities of liquor will vary considerably with the process of washing used in each plant, and a reasonable average will give 2 gal. of 6°Twaddell waste liquor for each pound of cotton mercerized. These liquors are usually concentrated to from 70 to 80 Twaddell in a triple- or quadruple-effect evaporator of the horizontal-tube or rapid-circulation type. The capacity is from 2H to 3H gal. per square foot, with a steam pressure of from 10 to 15 lb. Special construction and entrainment separators are necessary to prevent losses by foaming. The last effect will frequently show incrustations of sodium carbonate which can be dissolved in hot water or weak hquor. Evaporators are usually built of steel with steel or charcoal-iron tubes. 

FIGURE 6.3 Quadruple effect evaporator-crystallizer. Pressure and boiling temperature decrease from left to right, temperature decreases about 20-30°C across the four effects. Feed brines to each effect may be directly from brine wells, or may be fed cascade fashion from the filtrate to the salt brine slurry of the previous effect, or a combination of these methods. (Adapted from Kirk-Othmer [10], with permission.)... 

The deleterious effects of hot spots are well known, and they can also play a role in the damage of higher alloys, particularly when the base operating temperatures already are high and little margin for error remains. This is the case in most quadruple-effect evaporators, and causes of corrosion problems are not always easy to discover. 

Modern diaphragm cell plants use triple-effect evaporators and, in many cases, quadruple-effect evaporators. 

Each effect is arranged in series and operated so that each succeeding one operates under a higher vacuum (lower pressure). This arrangement allows the juice to be drawn from one vessel to the next and permits it to boil at a low temperature. The concentrated juice (syrup) is removed from the last effect by a pump. Triple-, quadruple-, and quintuple-effect evaporators are used, but the quadruple-effect type is most common. In a four (quadruple )-effect evaporator, one pound of steam evaporates four pounds of water. 

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