Patent GB-841374-A (GB)
Title: Purification of saline water by crystallization with a volatile refrigerant
IPC: C02F1/22
Filed: 1957-09-04
Abstract: 841,374. Purifying water by freezing. ESSO RESEARCH & ENGINEERING CO. Sept. 4, 1957 [Sept. 11, 1956], No. 27922/57. Class 29 A method of purifying brine by freezing includes the steps of contacting the brine in a freezing zone with a liquid refrigerant having a higher vapour pressure than water thus vaporizing the refrigerant and forming a slurry of ice crystals in more concentrated brine, removing the vaporized refrigerant from the freezing zone and compressing and cooling it, removing a slurry of ice and concentrated brine from the freezing zone, separating the ice from the brine and washing the ice with water of a lower salt content than the original brine, removing the ice from the water, bringing the ice into contact with the cooled compressed refrigerant to condense the refrigerant and melt the ice, and stripping the melted ice of refrigerant and recirculating the refrigerant to the freezing zone. Brine is pumped through a precooler 3 to a freezing vessel 4, containing a stirrer 5, and liquid refrigerant in droplet form is bubbled through the brine from pipe 8. The resultant ice slurry and some concentrated brine is removed continuously or in batches to a settling vessel 111 from where the ice is passed by line 110 to a filter 11 where the ice is washed and separated from the brine, or by line 114 to a slurry pump 34 from which it is led to a washer 21 where the ice crystals are freed of interstitial brine. Salt free ice water is passed to the upper part of the washer 21 by line 37 to act as the suspension medium for the rising ice crystals. The layers of salt free water and brine are kept distinct by slowly extracting through pipe 32 liquid at the boundary of the two layers. The slurry is led by line 22 to the condensing vessel 16 where it is brought into direct contact with compressed gaseous refrigerant which has been drawn from the freezing vessel 4 by compressor 12 and passed through a cooler 14 The refrigerant in condensing melts the ice crystals and is preferably immiscible with water so that it stratifies at the top of the condensing vessel and is drawn off through line 18. Fresh water is drawn from the bottom of the vessel through line 23 and, to displace the refrigerant, is led to a stripper 24 in a form of a fractionating column into which steam or warm inert gas is fed by line 25. Volatile refrigerant is vaporized and drawn off by pipe 26 and cooled at 27. Water condensate may be refluxed to the stripper 24. Uncondensed vapour and gas is led to a receiver 129 where the refrigerant stratifies on the fresh water and is drawn off through line 131. Fresh water is drawn from the bottom of the stripper. If only the filter 11 is used to separate ice and brine, the washer 21 is omitted and the ice crystals are washed in the filter and then reslurried with water of low salt content taken from the bottom of the condensing vessel before the crystals are passed to the condensing vessel. Two freezing vessels may be used in series in the brine circuit, with separate compressors having the same discharge pressure and with the second vessel maintained at a lower pressure than the first. The refrigerant after extraction from the condensing vessel, stripper and receiver 129 is returned to the freezing zone through expansion -valve 19. Propane is the preferred refrigerant.