Seawater Desalination by Cooling and Freezing Technology

Freeze desalination (FD) separates fresh water from a saline feed by crystallising ice rather than by evaporating water or forcing it through a membrane. Because the latent heat of fusion of water (333.5 kJ kg-1) is only about one-seventh of its latent heat of vaporisation (2 257–2 501 kJ kg-1), and because the ice lattice excludes dissolved ions almost completely, the process has attracted recurrent interest since the 1950s as a low-temperature, low-fouling, chemically undemanding alternative to distillation and reverse osmosis. This paper provides a self-contained engineering treatment of the technology. The thermodynamic basis is developed from the equality of chemical potential at the ice–brine interface, and the freezing-point depression of seawater is quantified with the UNESCO correlation; the least work of separation is compared with the practical energy demand of real machines. Ice-crystal nucleation and growth kinetics, the Burton–Prim–Slichter description of solute entrapment, population balances for crystal size distribution, and counter-current wash-column performance are each reduced to design correlations and presented graphically. Four process families — indirect (surface) freezing, direct-contact secondary-refrigerant freezing, vacuum-freezing vapour compression, and eutectic freeze crystallisation — are described with annotated process flow diagrams and compared on the basis of scalability. A systematic comparison with SWRO, MED-TVC and MSF is then made across nineteen technical and commercial criteria. The central result of this work is that the specific energy consumption of freeze desalination is controlled almost entirely by the crystalliser-to-melter temperature lift: an internal lift of 8.5 K yields 14.0 kWh m-3, a lift of 6.0 K yields 9.7 kWh m-3, and only the import of external waste cold brings the figure to 3.2 kWh m-3, which is competitive with seawater reverse osmosis. Widely quoted claims of 2–4 kWh m-3 for standalone freeze plants are shown to be inconsistent with attainable compressor second-law efficiencies. A detailed case study of a 5 000 m3 d-1 direct-contact plant treating Arabian Gulf seawater at 45 000 mg L-1 is developed to the level of a stream table, an equipment schedule, a pinch analysis of the regenerative exchanger, and a levelised cost of 1.80 USD m-3. It is concluded that freeze desalination is not currently a competitor to SWRO for conventional seawater, but is technically and economically superior for hypersaline feeds above roughly 100 g kg-1, for brine concentration and minimal-liquid-discharge duty, for feeds with severe organic or scaling loads, and wherever waste cold is available at no thermodynamic charge.

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