Vapor Compression Evaporation

Figure 8.6.5 is the flowsheet (Browning 1970) of the 1 million gpd OSW Demonstration Plant at Roswell, New

FIG. 8.6.5 Forced circulation vapor compression plant. Flowsheet of 1 million GPD OSW demonstration plant at Roswell, New Mexico.

Mexico. Alkaline brackish water from an artesian well is the feed. This water contains 15,000 ppm of mixed salt and is richer than seawater in scale formers, especially calcium salts. For scale prevention the water is first fed through an ion-exchange system, removing about 87% of calcium. It is then preheated to 145°F and treated with sulfuric acid for reaction of the remaining calcium and magnesium salts. Gases such as carbon dioxide, nitrogen, and oxygen are removed in a vacuum degasifier; caustic is then added to neutralize the pH.

The treated water is pumped to 130 psia, heated to 214°F and fed to the first-effect through the suction side of the brine circulation pump. In the forced circulation evaporator, the saline water is heated by the compressed second-stage steam, which condenses to fresh water. The saline water flashes at the top of the tube at the rate of 1 lb of steam per 250 lb of water circulated. The vapor from the dome of the first effect heats the saline water in the second effect and condenses to give fresh water. The vapor from the dome of the second effect is compressed to heat the first effect. The compressor operating across the two effects is a five-stage axial-flow machine driven by a 2,000-hp electric motor.

The high circulation rate of water in the forced circulation evaporator is intended to obtain a heat transfer coefficient as high as possible. Thus, the necessary heat transfer is satisfied without excessively large surface areas, while the required temperature difference is minimized. The relationship between this temperature difference and the power consumption by the compressor and the circulation pump has an impact on plant and water costs (Othmer 1969; Gilliland 1955).

The sludge recycle system (Figure 8.6.5) serves as a backup for the ion-exchange system for scale prevention. This system is devised to achieve a 1% slurry in the first effect so that scaling material will precipitate on the seed crystals and not on the heating surfaces. As compared to the multi-effect evaporation plants, the VC plant occupies less space and uses mainly electrical energy. The energy used is about 60 kwh per 1,000 gal of fresh water produced. For these and other reasons, the vapor compression unit is applicable where demand is small and a compact, efficient unit is required. It is used to supply potable water on ships and in areas where inexpensive power is available.

The force-circulation evaporator with its high velocity of circulation makes this process useful for processing brackish water containing large amounts of scale-forming salts. High reliability of mechanical components, such as the compressor, is essential. Also, the practical size of this compressor limits the size of the vapor compressor evaporator. The high performance heat-transfer surfaces developed for the VTE system may also be helpful. Likewise, the MSF process may recover heat from brine blowdown in large-scale operations.

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