kilomentor | 04 February, 2007 10:29
As one scales up a chemical process development step, one of the potential problems that I have found it difficult to foresee is emulsion formation. In about 40 years of experience the single most frequent cause for a call in the night from the pilot plant production management is an unpredicted emulsion, which interferes with a separation of two liquid phases. When one of these emergencies occurs, it is a blessing to have whatever you know about solving the problem assembled in a single place, because quite a few people are waiting for your instructions.
Emulsions are caused it is said by the repulsion of same charged particles which are residing in the droplets of one of the liquid phases. According to this principle the emulsion should be destabilized by ions of the opposite charge. Higher charge increases the efficiency of discharging the offending ions. Thus K+< Ca2+< Al3+ and similarly Cl- <SO42-. According to this thinking aluminum sulfate should be a great agent to dissolve in water for breaking an emulsion. This is theory, mind you, I have never used more than ordinary salt. The addition of salt to break an emulsion would not be the first thing to try because it is irreversible; that is once the salt has been added it cannot be taken out again if you change your mind.
Raising the temperature of the mixture of liquid phases very often leads to breakup of the emulsified layer and a clear separation of phases. The logic is that the separation of phases is a kinetic difficult; the phases are inherently essentially immiscible all that is needed is to speed up the rate. Whether you can do this must take into account the stability of the desired product but this is rarely a problem for the warming that probably is needed. The advantage of the warming solution is that it is reversible. If it fails just go to plan B.
If one examines closely a portion of the emulsion, one can sometimes get a useful clue to the action that will work. Sometimes the more vigorous stirring in the plant setting has suspended small gas bubbles in the droplets of one phase causing them to float rather than settle. These gas droplets can also be associated with some sediment that is suspended therein as well. Application of vacuum to a gently stirred mixture of the emulsion and separated phases can cause these bubbles to break followed by a separation of the phases. This attempt is particularly easy to try in the lab on a 500 ml sample of emulsion from the plant. Just place the filled flask on the rotovap; rotate gently and apply a water aspirator vacuum. Gentle warming is also easy to try out in this configuration. A note of caution should be registered here. You may see a clearing of the emulsion and there is a temptation to take the clarified two-phase mixture and for added safety filter it under vacuum through a pad of Celite. This filtration can undo all the good you have done. Sucking the last of the solvent through the Celite can put gas right back into the phases!
Stirring the emulsion composition with Celite and then filtering can often break solid stabilized emulsions. When the small particles of solid, which often carry those pesky charges are removed, the phases may separate easily. One should be on guard during this that an emulsion caused by the gas bubbles sucked off the Celite does not replace the solid stabilized emulsion. Pressure filtration appears to solve this completely, which is fine since in the pilot plant the transfer is by pressure not vacuum.
Finally, an emulsion may be caused by a small amount of a surface-active ingredient. Treatment with a material like activated charcoal that itself has a large surface area may be effective if the surfactant is taken out on the solid when it is filtered.
Addition of small amounts of an alcohol have also been known to help but remember these additions are irreversible and may affect other aspects of your planned isolation.