Heat pump operated freeze concentration system with tubular heat exchanger for seawater desalination

Abstract

Desalination of seawater can provide an almost inexhaustible source of freshwater if it can be made affordable. Distillation and filtration, the commonly used processes, have high operating and maintenance cost. Low operating temperature and low latent heat of fusion of water makes freezing technology worth considering for further development. Here, water is selectively frozen, in the form of ice from sea water, and melted after isolating it from the concentrated seawater to generate pure water which can be made potable. However, desalination processes based on freezing have not been exploited because of the operational difficulties in ice–water separation, high cost of equipment and high parasitic power requirement. This paper discusses a patented layer freezing based technology which has competitive initial and operating cost and eliminates operational difficulties of conventional freezing system. It is scalable and is coupled with a heat pump which selectively freezes water from seawater in the evaporator and melts the ice in the subsequent phase when it serves as a condenser. The condenser optimally utilises the latent heat of melting of ice to partially condense the refrigerant and the excess heat is rejected to ambient. It avoids the need of ice scraper/separation mechanisms. Use of vented-double-wall tube–tube heat exchanger, TT_HE, enables refrigerant and seawater/potable water to exchange heat without the use of intermediate fluids, while keeping the inclusion low. TT_HE is a reliable heat exchanger which ensures that refrigerant and seawater/water do not mix. Operating COP of the heat pump is in the range of 8 to 12, which results in specific energy consumption in the range of 9 to 11 kWh e/m 3 of water produced. Comparison of features with other freezing desalination processes is also presented.