SPENT LIQUOR EVAPORATION USING MECHANICAL VAPOUR RECOMPRESSION: A MEANS OF BOOSTING EVAPORATION CAPACITY
Martin, F.
Mechanical Vapour Recompression (MVR) is a very well known technology which has been used for many decades in various Industries where evaporation is required. Its principle is based on the re-use of the vapour produced by evaporation as the heating medium, after having increased its temperature and pressure by mechanical compression. The great advantage of this technology compared to classical evaporation is the use of electrical power instead of live steam, which results in an operating cost reduction when power is available. Cooling water is also saved due to the condensation of the produced vapour after compression. And finally the operating temperature of such plant can be set at lower level than that of a conventional multiflash train or multiple effects evaporators, which results in lower scaling, lower corrosion and higher availability.
The technology involved in Vapour Recompression does not require heavy maintenance since centrifugal fans have replaced the heavy compressors used earlier. These fans operate at low speed and are generally combined with a Frequency Drive for smooth start up and operation.
While MVR evaporation is widely used in heavy industry such as the Pulp and Paper industry, it is very rare to find this technology in the Alumina Industry. However this case study of a recently installed MVR evaporator in an Alumina Refinery in China shall demonstrate the attractiveness of this technical option. As part of the de-bottlenecking of an existing evaporation plant, a spent liquor pre-concentration stage using MVR was installed in an autonomous manner without affecting the existing plant. The evaporation capacity of the line was therefore boosted by 30 t/h with a shut down of only twenty four hours for connections to the existing line.
Taking into account its numerous advantages as well as competitive operating cost, MVR evaporation technology should be seriously considered when upgrading capacity of existing plant.