Process Plant Technology has designed and supplied over 200 evaporators and crystallisers since 1974. The reference list includes almost every sector of industry, from food and chemicals, to fertilisers, pulp and paper, mining and general effluents. Process Plant Technology offers innovative, custom-designed falling film, flash or forced circulation evaporators to meet customers’ needs.
With the increasing cost of energy, thermal efficiency is a primary consideration in evaporator design. Each application is carefully evaluated, taking into consideration the available energy sources and their relative costs, versus the capital cost of the plant itself. The cost of site construction is continuously reduced through modularisation and off-site plant construction.
The diverse nature of the products handled, has necessitated the design and construction of evaporators in a number of materials, including rubber lined steel, GRP, graphite, various stainless steel grades, high Ni/Cr alloys, duplex stainless steels, nickel and titanium.
An evaporator is used to evaporate a volatile solvent, usually water, from a solution. Its purpose is to concentrate non-volatile solutes such as organic compounds, inorganic salts, acids or bases. Typical solutes include phosphoric acid, caustic soda, sodium chloride, sodium sulphate, gelatine, syrups, Kraft liquors and urea.
In many applications, evaporation results in the precipitation of solutes in the form of crystals, which are usually separated from the solution with cyclones, settlers, wash columns, elutriating legs, filters or centrifuges. Examples of precipitates are sodium chloride, sodium sulphate, sodium carbonate and calcium sulphate. The desired product can be the concentrated solution, the precipitated solids, or both.
In some applications the evaporator is used primarily to recover a solvent, such as potable water from saline water. In any case, the relatively pure condensed water vapour from many evaporators is recovered for boiler feed makeup, salt washing, salt dissolving, pump seals, instrument purges, equipment and pipeline washing and other uses.
Process Plant Technology designs and manufactures four general types of evaporators. The first two are used for concentrating solutions where no precipitates are formed. The other two are used for salting or scaling applications.
- The long-tube vertical (LTV) rising-film evaporator is commonly used to concentrate many non-salting liquors. For liquors requiring evaporation at extremely low ΔT’s, and for critically heat-sensitive liquors, the long-tube vertical (LTV) falling-film evaporator is preferred. LTV rising-film and LTV falling-film evaporators generally provide maximum evaporative performance for the least capital investment.
- For applications where precipitates are formed during evaporation, a forced-circulation evaporator, usually with a submerged inlet, is the most desirable choice.
- The Calandria, or short tube evaporator is less popular today than in the past, although some companies still prefer this type of evaporator over other designs. The Calandria can handle both salting and scaling-type applications as well as those where no precipitates are formed.
All of these evaporators require a vapour-liquid separation device and a (shell & tube) heat exchanger.
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