Case 1 – Distillation on solvent recovery plant regenerated with inert gas
The solvent recovered in plants regenerated with inert gas (nitrogen), for example in the flexible packaging printing sector, is often mixed (acetate, alcohol) with traces of water or other impurities.
For high solvent flow rate, often continuous distillation with 2 or 3 columns is chosen:
– The first column allows you to remove impurities (water, high boiling components, acetic acid) from the bottom, together with a reduced amount of solvent; the top fraction is transferred to the second column.
– The second column recovers pure acetate from the bottom, with an acetate azeotrope + alcohol at the top.
– The third column, when present, removes part of the acetate (to be recycled upstream) to enhance the purity of the recovered alcohol.
Some heat recovery exchangers increase the overall energy efficiency, preheating the supply with outgoing hot products.
It is also possible to use a single heat exchanger as a condenser of the first column and reboiler of the second, further increasing the energy recovery and simplifying the plant layout.
Whereas with low incoming solvent flow rates, the solution of a single batch column does the work of the first two columns, reducing the investment cost.
Case 2 – Distillation on solvent recovery plant regenerated with steam
For solvents coming from recovery plants regenerated with steam, its decanter separates the aqueous phase from the organic phase.
A distillation column could be necessary to treat the organic phase, if several solvents which cannot be mixed with water are fed simultaneously, for example toluene and hexane in the adhesive tape production process. A single column can separate the two solvents, with excellent performance and low consumption.
Solvents which can be mixed in water treated in steam systems represent a more complex situation.
Several columns in series are necessary to achieve effective separation of the organic compounds from the water and to increase its purity.