Solvents for Extraction

30 April 2003

I see many people using ethyl acetate for extraction but this can be a poor choice of solvent for scale up. Why?

1. Water has a relatively high solubility in ethyl acetate

This causes three problems. A saturated solution of ethyl acetate in water can be an excellent hydrolysis medium for your product (e.g. if it is an ester). In the lab, you may not notice a problem, but if you scale up the process, the extended time you spend on the plant will cause much more hydrolysis.

The second problem is that the water (if slightly acidic or basic) may cause hydrolysis of the ethyl acetate itself. Since this yields ethanol and acetic acid, this will allow more water to become dissolved in the solvent. On scale up, of course, when the extraction takes more time, the situation is exacerbated. Washing of the solvent may also be a problem. The third problem is that the water has at some stage of the process to be removed from the solvent by drying. In the lab this means that more drying agent is needed. In the plant where, I hope, that drying agents will not be used, this means azeotropic removal of solvent in the evaporation stage. Of course this may also cause further hydrolysis of the desired compound.

2. Ethyl acetate has an appreciable solubility in water

This has a number of effects, including the partition coefficient which will vary dependent on whether the aqueous layer becomes saturated with ethyl acetate (salts in the water, will reduce the solubility, of course). If the aqueous layer in the extraction is the undesired layer, i.e. it will be disposed of, then a fraction of the ethyl acetate charged to the reaction (possibly 5-10%) may be lost in the aqueous layers. This is not only costly, but may affect the easy of disposal of the aqueous layer to effluent. It is not very environmentally friendly!

3. What is the alternative?

Higher esters such as isopropyl acetate or butyl acetates will give better layer separation, leave less organic solvent in the aqueous layer, and the organic layer will have a lower water content and be easier to dry. The higher cost of these solvents may be outweighed by these convenience and environmental factors. If solvent recovery is practiced, more of the higher boiling solvents will be recovered than is possible with ethyl acetate, where the removal of water can be quite difficult.

For more information on solvent recovery and partition coefficients see I Smallwood “Solvent Recovery Handbook” published in McGraw-Hill Education, ISBN 0070 584354.