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MEE is used when a large amount of water has to be evaporated and steam is inexpensive. The first effect is steam-heated, and vapour from the first evaporator heats the following (second) effect. The second effect vapour heats the third effect, and so on. By utilizing MEE, the specific energy consumption falls significantly.

The number of effects ranges from 2 to 10, and is typically limited by the temperature difference between the heating and cooling media on the one hand and the optimum between investment cost vs. energy savings on the other.

In TVR systems, part of the vapour generated in the evaporator is combined with high-pressure steam in the Thermal Compressor (“booster”). The resulting vapour stream is the evaporator heating medium. TVR can be combined with MEE, to increase the operation’s energy efficiency.

Theoretically, a TVR single effect unit has, approximately, the energy efficiency of a double effect evaporator, at significantly lower CAPEX. However, TVR contaminates the boiler steam it uses, and the steam condensate is usually not returned to the boiler.

Recrystallisation is a purification process, in which impure crystals are dissolved in a clean solvent, and then recrystallized, to produce crystals that are significantly purer (10X or better) than those used for the feedstock.

The actual method of crystallisation can be that of evaporation or cooling, depending upon the solubility of the product; usually the recrystallisation operation is a repetition of the first crystallisation unit. Recrystallisation is typically used for products that require high levels of purity (usually >99.9%), such as pharmaceutical grade or reagent grade components.

The FC is the workhorse of the crystallisation industry and is used for operations that require large amounts of evaporation. A pump is used to mix the slurry in the FC. While an FC provides little latitude in crystal size control, poor design can reduce the crystal size produced. Similarly, optimization of the operating parameters of the FC can minimize scale formation and substantially increase its on-stream time.

Occasionally, FC’s are used for non-crystallizing, but severely scaling evaporation applications, in an effort to prolong the unit’s operating cycle.

The FFE is the most common type of non-precipitating evaporator. It is named from the fact that the liquid to be concentrated cascades down the inside of the heater tube walls as a film. It usually has a pump to circulate process liquid to the top of the vertical heating element, and ensure wetting of the tube walls (in some cases, recirculation is not necessary). FFE’s are suited for MEE applications, as they work well with low steam-to-liquor dT’s. This, and the low residence time of the concentrate, make this evaporator a good choice for thermos-sensitive materials.

SEP has extensive experience in designs that minimize scale formation on tube walls, and extend the operating cycle of the machine.

Reaction crystallisation requires that the feedstocks be mixed with the slurry in the crystallizer. The slurry consists of various potassium salt crystals and mother liquor. The content of the crystallizer is continuously mixed by the internal circulator (the agitator in the draft tube) to enhance the required reaction.

MVR technology utilises compression of the vapours generated in the evaporator to a higher pressure/temperature level, and are subsequently reused as the heating medium. High-speed centrifugal compressors, or a number of fans (operating in series at significantly lower speed than compressors) provide the compression.

Flash evaporation or crystallisation is used to cool a hot brine (and precipitate a compound) without heat exchangers coming into contact with the process liquid or slurry. It provides longer operation cycles for the evaporator or crystalliser, as the scale formation in equipment is reduced substantially.

Feedstock is exposed to low pressure, and flashes (boils) under controlled conditions. The flash generates vapour is condensed separately. The removal of water concentrates and cools the feed and leads to precipitation of product crystals. Forced Circulation or Draft Tube Crystallisers are used for this operation as well as Flash vessels.

The DTB crystalliser is used when larger particles are desired. It combines a clarifier and a mixed tank in a single vessel. The mixed-tank part (utilizing a draft tube) provides improved crystal growth conditions. The clarifier section (Baffle) allows selective removal or destruction of smaller crystals, thus increasing the average product size. Smaller particles are destroyed by heating or by diluting the brine containing them. DTB’s are also used successfully for reaction crystallisation because of the good internal mixing and reactant introduction into a high-turbulence zone. A variation of the DTB, the Draft Tube (DT) crystalliser, contains only the mixed-slurry feature of the DTB (no baffle).

Rising film evaporators, also known as Natural Circulation, or Long-Tube Vertical (LTV) Evaporators, are the oldest and simplest of Evaporators used today. They have no moving parts, and have the advantage of very stable and reliable operation. This type of evaporator is well suited for the concentration of solutions that are not saturated upon concentration, and have low viscosity. However, to obtain natural circulation, higher steam-to-process temperature differences are needed, and this limits their use in some MEE arrangements.

Industrial wastewater (including from fly ash) and shale-produced water can be pre-treated to remove heavy metals, ammonia, magnesium, sulphates, carbonates, and TOC. The Pre-treatment includes pH adjustment, selected metathesis or precipitation reactions (with acids or bases), and activated carbon filtration.

The pre-treatment eases the subsequent evaporative concentration of the brine, and allows for recovery of saleable products, clean condensate, and a greatly reduced plant purge.