GB979963A - Improved method and apparatus for water purification by crystallization - Google Patents

Abstract

979,963. Production of fresh water from saline water by freezing. J. W. PIKE. Aug. 22, 1961, No. 30320/61. Heading F4H. In the production of potable water from a saline solution such as sea water, a super cooled saline solution at a temperature within the metastable condition is mixed with a saline mother liquor having ice crystals suspended therein. Fresh water from the solution in the metastable condition deposits on and expands the size of the ice crystals which can be removed for washing and melting. Saline feed water enters through a line 1 and is cooled in a heat exchanger 4 to substantially the freezing point temperature. It is then mixed with a circulating mother liquor comprising dissolved solids, ice and water in a line 5 and pumped to a higher pressure in order to discharge into an evaporator 7. A volatile liquid hydrocarbon refrigerant such as isobutane is introduced into the mixture in line 5 through a line 14, reducing the temperature of the mixture into the metastable condition, that is 0.05‹ to 1.5‹ F. below the initial freezing point of the mixture. The refrigerant vapour leaves the evaporator by a line 11 and flows to a compressor 12 and a condenser 29 for redelivery through the line 14. The super cooled saline mixture or solution flows through a line 8 into a crystallizer 9 through a centre pipe 10, entering near the top of the crystallizer. The mother liquor in the crystallizer contains a suspension of seed crystals. The fresh water content of the super cooled saline mixture tends to solidify on the crystals present, building these into larger crystals which tend to rise, whilst new crystals are also formed. The super cooled saline mixture on flowing down the crystallizer reacts against the upwardly flowing ice crystals. By maintaining a downward flow in this manner, the crystals may be maintained in suspension whilst they are growing. Finally, large crystals rise to the top of the crystallizer and seed crystals are forced to the bottom to be recirculated with the mother liquor for a repetition of the cooling cycle. Large crystals and mother liquor are continuously removed from the top of the crystallizer by a pump 16 through a line 15 and discharged into a refrigerantremoving vacuum chamber 17. The slurry underflow discharges through a line 21 into a gravity separator 22 where the ice crystals are decanted off through a line 24 to a filter 25. The filtered ice crystals are washed with fresh water delivered through a pipe 30 and are then melted in the refrigerant condenser 29. Residual mother liquor from the filter 25 and mother liquor from the separator 22 pass through the cooling side of the heat exchanger 4. Saline solution is defined as encompassing solutions described as brackish and also sea water. Such solutions may contain salts such as sodium chloride, calcium chloride, magnesium chloride and similar salts formed from alkali earth metals and the halogens. Suitable refrigerators mentioned include certain paraffins and olefinic hydrocarbons which have the required vapour pressure characteristics Among these are butane, propane, propylene, ethylene and ethane.