Vacuum desalination for water purification using waste heat

Abstract

Vacuum desalination is a process by which water is vaporized at a lower temperature when subjected to vacuum pressure. The heat energy requirement for desalination using a distillation process can be brought down by reducing the boiling temperature. The boiling temperature of seawater can be lowered to as low as 400°C by the creation of a vacuum. Using waste heat from a steam turbine, a pilot study was conducted in the laboratory to investigate the feasibility of using a vacuum desalination process for water supply. The treatment system is designed to minimize the heat requirement. Initially, the seawater is converted to a wet steam in a saturated vapour heater. This vapour heater receives heat from the superheated steam which is due for condensation. The dry component of the wet steam is heated to superheated condition by a superheated vapour heater. This superheated vapour heater receives heat from the waste heat from a steam turbine. Superheated vapour is used to covert incoming seawater to wet steam and then be condensed as the product of the system for potable use. The investigation shows that a vacuum desalination system using turbine exhaust waste steam is possible and has several advantages. Seawater can be boiled at a lower temperature and thus save in energy consumption. Based on experimental results, water boils at 40–90°C at the corresponding vacuum pressure of 0.1–0.7 bar, respectively. The consumption of heat from the waste steam is minimized as the superheated vapour is used to heat the influent seawater. Most of the heat from the waste heat is required to compensate for the heat losses in the system. The energy efficiency of the system depends very much on decreasing the heat losses from the system. It is possible to use nonconventional energy sources such as lower pressure waste steam for vacuum desalination. Hard scale formation in the system can be minimized as scale forming is the major problem in a high temperature distillation process. The qualities of distillates are excellent.