IDE Technologies

Review of med fundamentals and costing

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Courtesy of IDE Technologies

Introduction
Low Temperature Multi Effect Distillation (LT-MED) is one of the most efficient thermal desalination processes currently in use. It incorporates technological advances which have resulted in reliable, durable and economical desalination plants producing high purity product water.

These advances are as follows:

  1. Development of a unique design of a falling film horizontal tube evaporator/condenser with high heat transfer coefficient, utilizing only latent-heat transfer, avoiding sensible heat pick-up.
  2. Superior thermodynamic efficiency (compared to MSF) and very low pressure drops at high volumetric vapor flows, as prevailing in low temperature operation. This enabled the optimization of the process for operation at a maximum brine temperature of 70 °C.
  3. The low temperature operation has made possible the utilization of economical and durable materials of construction such as aluminum alloy for heat transfer tubes, plastic process piping and epoxy-painted carbon steel shells which show a better resistance when matched with aluminum alloy or titanium.
  4. The economy of using aluminum tubes for heat transfer as compared with copper alloy tubes, which are essential for higher temperature plants (used by other distillation manufacturers), enables the increase of the heat transfer area per ton of water produced in the desalination plant for the same investment costs.
  5. The significant increase in heat transfer area, in addition to the thermodynamic superiority of MED over the MSF process, results in a very low temperature drop per effect (1.5 - 2.5 °C), enabling the incorporation of a large number of effects (10 to 16) even with a maximum brine temperature as low as 70 °C, consequently resulting in very high economy ratios (product to steam).
  6. Possibility of using low-cost/low-grade heat available through cogeneration schsmes to minimize the energy cost component.
  7. Minimal requirements for intake and pre-treatment systems.

The practical experience with commercial plants using the above mentioned advances has shown the remarkable stability, flexibility and reliability of the low temperature process in comparison with others. Continuous research and development improved further the advantages of the low temperature process by increasing the unit's capacities; decreasing the energy consumption; and lowering levels of scaling and corrosion, both being significantly reduced due the low temperature operation.

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