Multi-effect distillation plants: state of the art

The multi-effect distillation (MED) process is the oldest process in desalination. References and patents have existed since 1840, more than 150 years ago. Vertical tubes, horizontal tubes and different types of submerged tubes have been commercialized and were used until 1960 when multi- stage flash (MSF) dominated the desalination market. MSF plants are presently the most widely used and are considered as reliable sources for the production of fresh water from the sea in Middle East countries, in general, and in the Gulf region, in particular. Development of MED in the last few years has brought this process to the point of competing technically and economically with the MSF process. Major features of the MED process are low primary energy consumption, low heat transfer area and high gain ratio. This paper discusses the general features of existing commercial MED plants and associated technical aspects related to steam, condensers, evaporators, pumps and capacity. It also discusses gain ratio, operating temperature, materials of construction, operation and maintenance of these plants, associated problems, and other available information. Attempts to identify areas of development are also presented.

1. According to the latest technology used in the ME system, there are two ways to increase the GOR: increase the number of effects, which are limited to the maximum brine temperature due to risk of scaling; or couple the ME system to a heat pump. There are three types of heat pumps that can be coupled to the ME system: a Li-bromide/water absorption heat pump, an adsorption pump and a thermal vapor compression pump. 2. Combination of ME with a solar power plant is one of the most promising techniques, especially in Arabian Gulf countries where this source of energy is abundant. 3. The LT plant had reduced scaling and corrosion rates at acceptable levels, overcoming the main problems plaguing conventional HT distillation plants. 4. In designing a water plant to meet modern conditions, higher efficiency will be a major goal; this means efficiency not only in energy consumption, but also in operation costs, reliability and maintenance.