Double effect evaporators

A forward-feed double-effect evaporator, having 10 m2 of heating surface in each effect, is used to concentrate 0.4 kg/s of caustic soda solution from 10 to 50 per cent by mass. During a particular run, when the feed is at 328 K, the pressures in the two calandrias are 375 and 180 kN/m2 respectively, and the condenser operates at 15 kN/m2. For these conditions, calculate ... 

Reavell(17) has given a comparison of costs for the concentration of a feed of a heat-sensitive protein liquor at 1.70 kg/s from 10 per cent to 50 per cent solids, on the basis of a 288 ks (160 hour) week. These data are shown in Table 14.3. It may be noted that, when using the double-effect evaporation with vapour compression, a lower temperature can be used in the first effect than when a triple-effect unit is used. In determining these figures no account has been taken of depreciation, although if this is 15 per cent of the capital costs it does not make a significant difference to the comparison. 

A liquor containing 15 per cent solids is concentrated to 55 per cent solids in a double-effect evaporator, operating at a pressure in the second effect of 18 kN/m2. No crystals are formed. The flowrate of feed is 2.5 kg/s at 375 K with a specific heat capacity of 3.75 kJ/kg K. The boiling-point rise of the concentrated liquor is 6 deg K and the steam fed to the first effect is at 240 kN/m2. The overall heat transfer coefficients in the first and second effects are 1.8 and 0.63 kW/m2 K, respectively. If the heat transfer area is to be the same in each effect, what areas should be specified ... 

Figure 12.18 Simplified flow sheet of a double-effect evaporation unit.

A double effect evaporator (two evaporators in series) is used to produce fresh water from seawater containing 3.5 wt% dissolved salts. A flowchart for the process is shown here. 

Figure 3. Double-effect evaporator with thermocompressor flowsheet. Courtesy of Crown Iron...

Figure 4. Double-effect evaporation with forced circulation and external heating. Courtesy of Crown Iron Works Co. (9).

Sodium Phosphate.—Solutions are concentrated from 6 to 24 per cent in a horizontal-tube double-effect evaporator with a steam pressure of 5 lb. and a vacuum of 26 in., at the rate of IK gal. per square foot. Evaporators are of cast iron or steel, with tubes of steel. 

Milk.—For the manufacture of condensed or evaporated milk, skimmed milk is usually concentrated to M or K of its original volume, and this is done in single-effect evaporators of the film or coil type. The capacity is from 2 to 3 gal. per square foot, with low-pressure steam, and up to 4 gal. per square foot with high-pressure steam, at a vacuum of from 26 to 28 in. So as not to cook the milk, the temperature must not exceed 60°C., and in Europe skimmed milk has been concentrated in double-effect evaporators of the rapid-circulation type with good success. 

For smaller installations, the concentration of the crude pyroligneous acid and acetate of lime is done in a double-effect evaporator, with the acid in the second effect. 

Zinc Chloride.—This material is recovered by systematic washing of hard fibre. The diluted liquor has a density of from 16 to 20°B6., and is concentrated to from GS to 72°B6. in a double-effect evaporator which has a capacity of from 1.3 to 1.8 gal. per square foot, with a steam pressure of from 5 to 20 lb. and a vacuum of 27... 

You can have any number of evaporators connnected together in this way. Each evaporator in this sequence is called an effect, and the entire sequence is called a multiple-effect evaporator. In the figure above, we show a double-effect evaporator. In order to maintain a driving force for heat transfer in each effect, we must have... 

The milk powder was obtained from a mixture of a full cream milk (125 kg) containing a moderate level of PCBs, a full cream milk (190 kg) with very low levels of PCBs and a skimmed milk (140 kg), also with very low levels of PCBs. All the milk was obtained from mid-southern Germany. The mixed milk was heat-treated at 85 C for 30-60 seconds in a plate heat exchanger and then condensed at 45-62 C to 38% dry mass in a double effect evaporator. The condensed milk was spray dried at 170-195 C in a pilot dryer with concurrent flow. Ten batches of 3-4 kg powder were sealed into polyethylene (PE) bags. 

Solvent (hexane) extraction of soybeans is a diffusion process achieved by immersing solid in solvent or by percolating solvent through a bed of solids. Rotary (deep-bed), horizontal belt, and continuous loop extractors are used for soybeans (Woerfel 1995). Solvent is recovered from the mixture of solvent and extracted oil (miscella) by double-effect evaporator and steam stripping and from flake by a desolventizer-toaster, and is recycled. 

From the surge tank, the bisulfite solution flows steadily into a forced-circulation evaporator—crystallizer, which is the heart of the regeneration system. Heat can be provided by the low-pressure exhaust steam that normally would be discharged into the atmosphere. Operating plants are currently using steam at pressures as low as 15 lb/sq in., but even lower pressures can be used. In large plants such as power plants, the regeneration system should be operated as a double-effect evaporator which would reduce steam consumption by 40-45%. 

Cui, W.K. and Mujumdar, A.S., 1984, A novel steam jet and double-effect evaporation dryer part I—Mathematical model, in Drying 84 (A.S. Mujumdar, Ed.), Hemisphere, New York, pp. 468-473. 

Double-effect evaporators tend to produce steam economies of 1.6-1.85 in backward-flow systems. Triple-effect steam economies are about 2.5, and flow arrangements are backward or mixed. Steam economy is variable and as a primary measure of performance may be unreliable. For example, the extent of use of steam condensate for heat interchange varies. This practice retains heat within the evaporator system and always improves the measured steam economy. The overall heat economy, however, may not improve. Recycling colder condensate to the boilers may simply transfer some of the load from the process to the utility area. 

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