Vacuum Evaporator Design and Operation
Vacuum evaporation can of course be used for the evaporation of any liquids as a means of lowering the temperature of the boiling point and in many cases the distillate is the required product. Brine evaporators like the earlier sugar evaporators are appropriately known as “salting” or “crystallising” evaporators and these are applied in the production of a wide range of crystal products where water is usually the distillate.
Designers of an evaporator had to meet two basic requirements. One was the method of continuously removing the crystals from the vessels when these are under vacuum and another was the efficient transfer of the steam heat to the brine without the formation of excessive scale on the transfer surface. The latter is of course not peculiar to brine as anyone boiling milk on a hot plate will realise. As with boiling milk it is necessary to “stir” the brine and the velocity of the brine along the heat transfer surface is an important parameter in a vacuum plant. In early multi-effect evaporators the central portion of each vessel was a multi tube heat exchanger known as a calandria, with steam in the jacket and brine flowing through the tube. The rate of flow of the brine was partly by convection and partly aided by a rotating stirrer. In later vacuum plants the internal calandria has been superceded by the use of external calandrias or heat exchangers with forced circulation thereby increasing the velocity of the brine along the calandria tubes before being tangentially expelled into the evaporator vessel.
The change from internal calandria to evaporators with external calandrias with forced circulation has brought a change to the crystal form of the salt. Whereas the slow circulating internal calandrias yielded a salt with perfect cube crystals, the modern evaporators produce salt crystals with rounded corners and the different types of evaporator can almost be “fingerprinted”.
The main impurities in natural brine are calcium, magnesium and sulphate. In open pans these produce a thick scale on the pan bottom which, with fine pans, must be removed weekly when the mother liquor from the week’s salt making is run to waste and replaced with fresh brine. With a vacuum plant it is necessary to remove the scale forming impurities from the brine in a purification process. Calcium is precipitated as carbonate and magnesium as hydroxide. In some processes the sulphate can be reduced as barium sulphate but usually the evaporator is operated in such a way that the sulphate level in the mother liquor is not allowed to build up to a concentration that would permit the crystallization of hydrated sodium sulphate with the salt.
In the early days of the vacuum process it was necessary to discharge the mother liquor and any build up of solid impurities in the vessels by “boiling out” which was done every few days. This reduced the time for salt making and, furthermore, led to a considerable discharge of brine to the adjacent canal or river. With modern plants, improved methods of brine purification and evaporator operation have enabled the plant to operate for as much as six months between boil-out and to operate with minimal loss of brine to the environment.