Brine multiple-effect evaporation

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. 

For a detergent grade product, brines may be ponded for natural evaporation or may be put through multiple-effect evaporators for concentration, and then chilled to 0°C or below to induce crystallization of the decahydrate, Glauber s salt. More frequently, however, by-product sodium sulfate, which is frequently recovered from process liquors by a crystallization step, is produced on a sufficient scale and purity (up to 99.77% Na2S04) to supply the small detergent grade market at 100-139/ tonne in 1995 [27]. 

Evaporation is energy-intensive. The latent heat associated with separation of a ton of dry NaCl from saturated solution is about 6.5 GJ. With KCl, the equivalent figure depends more on the temperature assumed for the brine as produced but is in the range of 3.5-4 GJ. Energy economization is therefore a major consideration in evaporation process design. Two different approaches are in widespread use, multiple-effect evaporation and vapor recompression. 

FIGURE 7.9. Allocation of temperature potential in multiple-effect evaporation. (Evapwation of NaO brine forward feed.)... 

Multiple-Effect Evaporation. The discussion of brine evaporation in Section 7.1.5.2A showed how installing a number of effects and using process vapor as a source of heat reduces the amount of energy that must be added to the process. The boiling point rise (BPR) of brine solutions makes some of the available temperature potential useless as a driving force for heat transfer. This limits the number of effects and the achievable steam economy. Similar limitations exist in caustic evaporation but are much more severe. 

Parallel-feed multiple-effect evaporators. Parallel feed in multiple-effect evaporators involves the adding of fresh feed and the withdrawal of concentrated product from each effect. The vapor from each effect is still used to heat the next effect. This method of operation is mainly used when the feed is almost saturated and solid crystals are the product, as in the evaporation of brine to make salt. 

Maintenance requirements could be listed in the following order, from lowest to highest maintenance thermocompression once-through multistage flash multistage flash with brine recirculation reverse osmosis multiple-effect evaporation mechanical vapor compression. Several factors are important ... 

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. 

Water is then removed from the treated brine by multiple-effect vacuum evaporation with low-pressure steam providing the heat for the first effect (Fig. 6.2). 

Forced Circulation. The fourth is a process which was proposed by Dodge and Eshaya and extensively studied from an engineering and economic standpoint under Office of Saline Water auspices (15). A pilot plant was proposed (14), but no steps were taken to implement the proposal. The method involves heating of brine under nonboiling conditions at linear flow velocities of the order of 8 to 10 feet per second, followed by flash evaporation and compression of the flashed vapor for use as the heating medium. Drop wise condensation of the vapor was another feature of the process. A somewhat similar process has been selected for the government demonstration plant at Roswell, N. M. This plant will use vapor compression in conjunction with multiple-effect (two or three effects) evaporation, and as far as we are aware this is the first time that this combination has ever been used. 

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