Thermal Desalination of Sea Water

Description and Advantages

A new thermal desalination system that uses plate heat exchange technology, developed by the Danish company Alfa Laval, eliminates major problems inherent to the desalination process, such as corrosion and scaling by chemical precipitation of salts from seawater.

The new system, called Pressed Plate, using a Falling Film (PPFF), passes a smooth, even, thin film of water over multiple plate surfaces, resulting in high heat transmission coefficients. Seawater falls down one side of the plate as a thin film and evaporates because of the heat being transferred from condensing water vapour on the other side of the plate. The vapour is collected, condensed and extracted as pure distilled water, while the remaining seawater is extracted as brine and returned to the sea.

The patented multi-plate system is constructed from Titanium and can be dismantled to permit manual cleaning. Its design allows for future capacity expansion - something that has not been possible with any other thermal desalination process before.

Multi-Effect (ME) distillation, using the plate technology, will become a strong alternative to the Multi-Stage Flash (MSF) process that has been used for the past 45 years in large-capacity plants, especially in the Middle East. The new plate-based ME system can match the production capacity of MSF systems, but with lower electric energy requirements and a lower capital cost. These advantages translate into fewer dollars per cubic metre of water.

Compared to other technologies using ME distillation process with a traditional tube-and-shell system, the PPFF configuration achieves greater thermal efficiency. That means installations can be smaller, requiring less land and costing less to deliver and install.

The controlled water film on plates avoids dry spots, which can lead to scaling - a problem with tube-and-shell installations. All plants are designed to avoid scaling, but errors in operation or changes in the chemistry of the raw water can lead to hard scaling.

Tolerance to extreme difficult conditions is a major advantage of PPFF. Once hard scaling has occurred in a tube-and-shell installation, it cannot be removed chemically or mechanically. The plant has to be re-fitted with new tubes at high cost, both in financial capital and in the loss of water production.

It is easier for personnel without special technical skills to operate distillation processes, in particular, ME-based processes on plates. ME systems are robust and preferred by industry because of their proven reliability and greater tolerance of errors.

Source: Water & Wastewater International