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The only methods accepted by the US Pharmacopoeia for producing Water for Injection use distillation. These methods evaporate the feedwater, separate impurities from the vapour, then re-condense the purified water.
Low-velocity separation methods allow the steam-vapour mix to rise through a chamber slowly enough for the droplets with included impurities to drop back under gravity, leaving the vapour dry and free of impurities.
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Thermo-compressor stills need only softened feedwater, not purified. Where water output is needed at ambient temperature, the energy consumption is far lower than for most multi-effect stills.
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Multiple Effect stills operate at high pressures and temperatures so need purified feedwater. Energy consumption improves as the number of effects is increased, ultimately approaching that of thermo-compression stills. Distillate is delivered hot.
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High-velocity separation methods accelerate the steam-vapour mix through spiral baffles, centrifuging out the entrained droplets and impurities.
Condensing pure dry steam to WFI through a simple heat exchanger uses considerable quantities of coolant and steam energy. Multiple-effect stills use a series of evaporator-separator-condenser stages to progressively recover this energy:- the more stages, the better the thermal efficiency.
Thermo-compressors are inherently more efficient again: by evaporating the feedwater at temperatures close to ambient the energy demand is minimised from the outset. As a supplier of both technologies we can offer impartial advice on the different advantages of each. |
