N-Methyl-2-pyrrolidone (NMP) DISTILLATION

Process description:
NMP is recovered from diluted aqueous solution by distillation, taking advantage of the relative volatility difference between NMP and water. The feed stream is first concentrated in a section using multiple effects technology, concentrated NMP is then distilled and purified in a separated section. Tails containing high boiling are treated in a squeezing unit to improve NMP recovery yield.
Hereunder an example of a multiple plant suitable for low acqueos solution concentration.
NMP Concentration:
This unit consists of 3 columns (or effects), one operating at atmospheric pressure and two operating under increasing vacuum, to allow heat integral recovery from one column to the next one. From the final concentration Column bottom a NMP concentrated stream is drawn and fed to the NMP purification unit.
NMP Purification :
This column works under high vacuum to let the bottom temperature stay below 120°C, avoiding NMP degradation, and consumes live steam from B.L..
The reboiler is a falling film evaporator using forced circulation in order to reduce as much as possible the residence time at high temperatures and have capability to handle potential high viscosities caused by high boiling (PVP) accumulation.
NMP vapours are drawn in vapour phase, condensed and collected : part of distilled NMP is sent back as reflux and the remainder is sent O.S.B.L.
Water vapours from the top of the Column , which may have a residual NMP content according to the feed concentration, are condensed by chilled water : part is refluxed back to top of the Column and the remainder could be sent back to NMP Concentration columns in case NMP residual content is high.
NMP Purges squeezing:

From the bottom of the purification column a stream, containing NMP plus the high boiling (which are mostly PVP), is taken and fed to the thin film evaporator.
This unit works under high vacuum to keep temperatures below 120°C and distill the most of the NMP, which is condensed , collected and recycled .
High boiling are discharged at higher concentration drum and delivered O.S.B.L. for disposal as special waste.
This units is very efficient and especially designed to handle high viscosities and improve recovery yield.