Method for purifying lactams
A method and a plant are disclosed for purifying lactams, particularly lactams obtained by cyclizing hydrolysis of aminonitrile. The purification of ε-caprolactam obtained by cyclizing hydrolysis of aminocapronitrile is described which includes eliminating the ammonia from the reaction medium of the hydrolysis, then recovering the lactam from said medium in purified form. The recovery is carried out by performing at least a distillation of the lactam in the presence of a base producing optionally a fronts fraction comprising compounds more volatile than the lactam, a fraction comprising the lactam to be recovered to the degree of desired purity and a distillation tails comprising the lactam and compounds less volatile than the lactam. The distillation tails are treated by various processes such as evaporation in thin layers to recover the major part of the caprolactam and recycling the latter in the purification process. The method and plant enables a high rate of recovery of the caprolactam contained in the hydrolysis medium, while observing required criteria of purity.
1. A plant for the purification of lactams by distillation, comprising:
three distillation columns mounted in series, the first being a dehydration column, the second column and the third column acting to separate either the compounds which are less volatile than the lactam as a bottom product of either the second or the third column, or the compounds which are more volatile,
means for adding an acidic compound to the lactam before it is fed to the dehydration column,
means for adding a basic compound to the lactam before it is fed to the third column, and
means for withdrawing the purified lactam from the third column, and
means for recovering the lactam present in the bottom product of either the second or third column containing compounds less volatile than the lactam, and for recycling recovered lactam for distillation.
2. The plant according to claim 1 , further comprising:
means for removing the compounds which are more volatile than the lactam as a top fraction from the second column.
3. The plant according to claim 1 , further comprising:
means for separating the compounds which are less volatile than lactams as distillation bottom product from the third column.
4. The plant according to claim 1 , further comprising:
means for treating the distillation bottom product from the third column,
means for recovering the lactam present, and
means for recycling the recovered lactam for distillation.
5. The plant according to claim 4 , wherein the means for treating is selected from the group consisting of a means for evaporating by thin-film or thin-layer evaporation, a means for crystallizing, and a means for washing with water.
6. The plant according to claim 1 , further comprising:
means for removing the compounds which are more volatile than the lactam as a top fraction from the third column, and
means for removing the compounds which are less volatile than the lactam as distillation bottom product from the second column.
7. The plant according to claim 6 , further comprising:
means for treating the distillation bottom product from the second column,
means for recovering the lactam present, and
means for recycling the recovered lactam for distillation.
8. The plant according to claim 1 , wherein:
the means for adding a basic compound to the lactam comprises means for adding the basic compound before it is fed to the dehydration column.
9. The plant according to claim 1 , wherein the basic compound comprises alkali metal hydroxides.
10. The plant according to claim 1 , wherein the amount of the basic compound is between 0.05 g and 2 g per 1 kg of lactam.
11. The plant according to claim 1 , wherein the acidic compound is sulphuric acid.
12. The plant according to claim 1 , wherein the amount of the acidic compound is between 0.5 and 1 mol per mole of free bases present in the lactam to be purified.
13. The plant according to claim 1 , further comprising:
means for obtaining the lactam to be purified from a medium resulting from the hydrolysis of an aminonitrile after removing ammonia and treating with an ion-exchange resin.
14. The plant according to claim 13 , further comprises:
means for hydrogenating the medium resulting from the hydrolysis of the aminonitrile, and hydrogenating before removing the ammonia.
15. The plant according to claim 13 , further comprising:
means for hydrolyzing the aminonitrile in the vapor phase or in the liquid phase.
16. The plant according to claim 15 , further comprising:
means for cyclized hydrolyzing in the liquid phase, in the presence of a solvent.
17. The plant according to claim 1 , wherein the lactam is caprolactam.
18. A plant according to claim 1 , wherein:
the means for adding the basic compound comprises means for adding the basic compound to the dehydration column.