Operational Conditions

The following are typically key operational parameters for an extraction process:

Operating Temperature

Operating Pressure

Residence Time

Feed Flow Rate

Composition

Temperature of entering stream

Pressure of entering stream

As in many separation processes, the pressure and temperature play a large role in the effectiveness of the separation.  In order for a good split of the feed stream, vaporization must be prohibited.  The process will be adversely affected if one or more of the components are allowed to vaporize.  In addition, the temperature should be high enough that the components are all soluble with one another.  Moderate temperatures can facilitate the selection of a suitable solvent.  With these rules in mind, most extraction processes can proceed at or near ambient temperature and pressure which is a key advantage of extraction.

Temperature can also be used as a variable to alter selectivity.  Elevated temperatures are sometimes used to keep viscosity low minimizing mass-transfer resistances.  Other parameters to be considered are selectivity, mutual solubility, precipitation of solids, and vapor pressure.

In many applications, a “gently” separation process is needed as an high temperatures could destroy the desired product (pharmaceutical industry).  For these applications, extraction is ideal since the only temperature requirement is that dictated by the solubility. The challenge in these cases is to find a suitable solvent for the extraction.

Operating pressure has a negligible affect on extraction performance and therefore most extractions take place at atmospheric pressure unless governed by vapor pressure considerations.

Residence time is an important parameter in reactive extraction processes (e.g., metals separations, formaldehyde extraction from aqueous streams) and in processes involving sensitive components (e.g., antibiotics & vitamins).

Selecting a Solvent

Any liquid to be used as a solvent will not posses all the properties considered desirable for extraction and hence compromise is usually necessary for selecting the best solvent.  While selecting a solvent for extraction, qualities such as selectivity, recoverability, distribution coefficients, and density should be given careful consideration.

• Selectivity – The ratio of the concentration of solute to feed solvent in the extract phase to that in raffinate phase is called the selectivity or separation factor.  It is a measure of the effectiveness of the extraction solvent for separating the constituents of feed.  Selectivity should be greater than 1 for all extraction operations.  If it is equal to 1, then separation by extraction is not possible.
• Recoverability - As the solvent must be recovered for reuse (usually by distillation), the formation of an azeotrope with the extracted solute is not desirable.  To minimize recovery costs, the relative volatility should be high.  The latent heat of vaporization of the solvent should be as small as possible whenever the solvent is to be vaporized.
• Density – A difference in the densities of saturated liquid phases should be as large as possible for physical separation of the phases.
• Interfacial Tension – The interfacial tension should be higher for the coalescence of droplets to occur more readily.
• The solvent should be inexpensive, non-toxic, and non-flammable if possible.  The solvent should have a low viscosity, freezing point, and vapor pressure, for ease in handling and storage.  The solvent should be chemically stable and inert to the other components.

Rohit Ramesh Rewagad and Piyush Khatavkar
Bachelor of Technology in Chemical Eng.
Laxminarayan Institute of Technology, Nagpur India
Email: piyushkhatavkar ”at” gmail.com