Comparison of Evaporators with Mechanical Steam Compression a nd Evaporators Equipped with a Steam Generator

Abstract

Evaporation is a process through which solvent is partially removed from solutions by subjecting the latter to evaporation. Aqueous solutions of solid materials are generally subjected to evaporation; however, other liquids may also serve as solvents. The solvent being removed is in this case in vapor form and is called secondary vapor. The vapor used for heating and evaporation is called heating steam.

At present, evaporation plants are widely used in purification and concentration of industrial waste waters to obtain demineralized water and sludge, which makes it possible to set up a closed water cycle at a production enterprise, to exclude discharges of hazardous substances into water bodies, and to decrease the amount of discharged waste; in obtaining demineralized water; in desalination of seawater; in evaporation of various process solutions to the required concentration (aluminate solutions, mineral salt solutions, alkalis, acids and so on); in milk and food industries; and in medicine.

As a rule, evaporation is performed by applying the heat of steam produced at a boiler house or a thermal power plant. Steam is an efficient heat carrier owing to its having high heat of condensation. In some cases, when evaporation has to be carried out at high temperature, furnace gases and high-temperature heating agents (diphenyl mixture, superheated water, or oil) are used.

If it is necessary to evaporate liquids sensitive to temperature effects, special equipment for creating deep vacuum (steam-jet and water-jet vacuum pumps, mechanical vacuum pumps, and others) is used. As a result, evaporation can be carried out at 60 °С (in the milk industry) and even at 35°С (in medicine).

In many cases, production sites do not have a steam source, and construction of new boiler houses is economically unprofitable or unfeasible for some or other reason. In such situations, an alternative heat source is selected subject to the operation conditions of a particular plant.

Process circuit versions of evaporation plants with mechanical steam compression and plants using steam generators operating on different kinds of fuel are considered. The possibilities of using one or another version depending on the properties of treated process solutions, process implementation conditions, and the number of shells in the plant (one- or multi-shell evaporation) are studied. The net cost of one ton of steam is evaluated, and the equipment payback period is estimated.