Thermal desalination – MED-TVC – Multi-Effect with Thermal Vapour Compression
WABAG multi-effect distillation desalination plants with thermal vapour compression (MED-TVC) are used for the production of both potable and process water from sea- and brackish water.
The MED-TVC desalination process is normally used for small- to large-scale thermal desalination plants where thermal energy is available in the form of medium-pressure steam (>3 bar). TVC is the most economically advantageous steam heated process for medium-sized plants (5,000 t/d) and it always represents an especially interesting alternative where medium-pressure steam is generated as an industrial by-product, or is available in the course of waste heat utilisation. MED-TVC installations are characterised by low investment and operating costs.
Typical of the WABAG plants of this type are Balikpapan, Indonesia, which provides process water for a refinery complex, and Benghazi North, Libya, where both boiler feed water and drinking water are produced.
In order to enhance efficiency, an MED plant is fitted with thermal steam compressors. The raw water is sprayed onto tube bundles, which are heated on the inside by condensing steam. As a result of the transport of heat via the tube walls, part of the liquid film on the outside of the tube bundle evaporates. The resulting steam is used to heat the next effect (stage), where it again condenses on the inside of the tube bundle. The underpressure steam compressor, which is powered by medium- or low-pressure steam, extracts part of the steam created in the final effect for further compression. This condensed steam is then re-employed as a heat source for the first effect.
As in the MED process, the TVC plant consists of several effects (stages), each of which is equipped with heat exchanger tube bundles. The first effect is heated by steam from the thermo-compressor, where an ejector using medium-pressure, motive steam (>3 bar a) recompresses part of the vapour produced in the last effect. The steam produced in each effect heats up the tubes in the subsequent effect. The condensate from the first effect is recycled to the steam generation system. The condensate from the downstream effects and the final condenser is the source of the desalinated product water (distillate).
The MED-TVC process operates at low temperatures with high thermal efficiency, which results in:
- small to large plant sizes
- low investment costs
- minimised corrosion risk
- reduced scaling risk
- lower thermal energy consumption
- low operating costs
- efficient use of plant volume