Introduction

Pyridine is a solvent which is widely used in the fine chemical and pharmaceutical industries. After use it is frequently rejected from the process in the form of an aqueous solution from which it is economically attractive to recover the pyridine. Unfortunately, conventional distillation cannot achieve recovery of a high-strength pyridine product because it has an azeotrope with water. The simple way around this azeotrope is to use liquid/liquid extraction followed by distillation.

QVF Process Systems has wide experience in liquid/liquid extraction techniques, particularly in applications where the separation of compounds by distillation is either difficult or uneconomic. The recovery of pyridine is a prime example.

This profile is based on an actual process which rejected an aqueous stream containing significant quantities of pyridine. The requirement was to recover the pyridine for re-use as a product of greater than 99% purity. QVF Process Systems was approached to provide the most economic recovery route.

The first need of any liquid/liquid extraction system is to find a solvent which has three characteristics:

1. Good solvent for the material of interest (pyridine, in this case).
2. lmmiscible with the solvent carrying the material of interest (water, in this case).
3. Easy to separate by distillation from the material of interest.

Chlorinated solvents have these properties, but it is absolutely essential in liquid/liquid extraction processes to perform trials to establish the best solvent, the type of equipment to use and the design parameters for full-scale equipment. QVF Process Systems has extensive pilot plant facilities for this purpose.

For this particular application, a solvent could be chosen to make the separation reasonably easy, as few theoretical stages were needed, and following successful trails, two mixer settler units were specified for full scale production. An outline flowsheet is shown above.

Process Description

In the mixer-settlers, the aqueous pyridine solution is intimately contacted with the chlorinated solvent and then the two phases settled out. Because pyridine is much more soluble in the chlorinated solvent than in water, in each mixer-settler progressively more of the pyridine passes into the solvent. The products rejected from the extractors are therefore a chlorinated solvent stream rich in pyridine (the 'Extract') and an aqueous stream stripped of pyridine but containing some dissolved solvent (the 'Raffinate'). Further process steps are provided to treat these streams.

The extract is distilled to produce a high-strength pyridine product (>99.9% of pyridine) along with solvent for re-use. The raffinate is stripped of organics to allow its disposal with the stream guaranteed to contain <0.5% pyridine. The typical product actually contains <0.1% pyridine and also incidentally <10mg/l of the chlorinated solvent.

Associated Processes

The information detailed in this profile is typical of the kind of process that can be handled using liquid/liquid extraction. Other suitable materials include phenol, acetone, acetic acid, ethanol and precious metals.

This Process Profile supersedes all previous issues.

QVF Process Systems pursues a policy of continuous product improvement. We therefore reserve the right to alter any product or process as described and illustrated.