Vapor Recompression

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In a conventional evaporator, the vapor stream is condensed, so that its valuable energy content in form of condensation heat is lost. Vapor recompression evaporation is based on the heat-pumping technique. A portion or the total vapor flow is compressed to a higher pressure and subsequently condensed in the same evaporator. In many cases, only multistage evaporator systems with three to five stages achieve the same specific efficiencies [15].

For the recompression of solvent vapors, steam jet ejectors operated by high pressure steam can be applied. Alternatively, this can be done mechanically with centrifugal compressors or with positive displacement machines. However, the energy consumption of the compressor increases with increasing temperature difference between the condensing compressed vapors and the boiling liquid. Therefore, vapor recompression is not economical for solutions with high boiling point elevation [15] .

The operating principle of a steam jet ejector is the same as that of a steam jet pump. Since it is simple and has no moving parts, its operation is free of breakdowns and it is relatively cheap. However, the efficiency is low and usually decreases in operation due to nozzle erosion, which is caused by water droplets in the drive steam. Steam jet ejectors are more favorable for vapor recompression in the area of lower working pressures from the perspective of energy efficiency. Higher flow rates, above 5000 Nm3h-1, are the field ofapplication for mechanical compressors [15] .

Mechanical vapor compressors use all the vapor compared with steam jet compressors, which only compress a part of the vapor. Therefore, compared with multistage and thermal vapor recompression the wasted condensation heat is significantly lower in mechanical vapor recompression. As a result, evaporation plants with mechanical vapor recompression require particularly low amounts of energy [15] .

Single stage centrifugal fans are used in evaporation plants because they are simple and maintenance friendly [15]. For high pressure increases, multiple-stage compressors can be used. Frequently, the energy of the vapor condensate is used to preheat the feed.

Economically, mechanical vapor recompression is worth considering only when the temperature difference between the condensing compressed vapor and the boiling solution is small, and a single-stage compressor is sufficient. In practice, the temperature difference must be less than 20 °C, which is the case only for solutions having low boiling point elevation [15].

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