Solvent extraction of wastewater

Chemical engineers do not overlook the organic solvents that might be lost in the spent aqueous phase after the solvent extraction step, but sometimes the concentrations are low enough to send it to the waste treatment step. More often, emulsions would result in too much lost solvent, and it is economic to recover both the solvent in the emulsion and the dissolved solvent. Steam stripping is the usual method, and solvents are selected with suitable vapor pressure for good recovery. Too low a vapor pressure and evaporation will cause air pollution and losses. Too high a vapor pressure means a high temperature to boil off the solvent. Solvent extraction used to cause strong odors, but the process is now ventilated well to protect the operators and the environment. The contaminated air is forced through carbon columns to adsorb the organic compounds; recovery by passing steam through the carbon may work well.

Solvent extraction of wastewater is rare. The parts per million of solvent solubility in water that seem small to chemical engineers appear to be significant B.O.D. to environmental engineers. Recovery of solvent that accompanies the aqueous phase would be costly because of the heat energy required to heat large columes of treated wastewater. The water should be cooled back down before disposal.

There is one very powerful feature of solvent extraction that environmental engineers should find highly appealing— it does not suffer when concentrations are low. All other processes become inefficient as the concentrations approach zero. Adsorption isotherms go through zero. The Monod equation for growth rate of a microorganism goes through zero. Ion exchange is highly concentration dependent. Rates and driving forces drop drastically at low concentrations for all these processes. Solvent extraction is the sole exception. Whatever fraction would go into the solvent phase at higher concentration is just about the same fraction at lower concentration (the correction for going from activity to concentration is almost negligible).

  • Suggestions for future research.
    while on sabbatical leave, ESB, Porto, Portugal July 1996