Perstraction

Perstraction is a membrane extraction process, where two liquid phases are contacted across a membrane. The desired species in the feed, selectively crosses the membrane into the extracting solution. Perstraction was originially developed to overcome the downsides of liquid-liquid extraction, for example extractant toxicity and emulsion formation. Perstraction, or membrane extraction, has been applied to many fields including fermentation, waste water treatment and alcohol-free beverage production.

Perstraction is the separation technique developed from liquid-liquid extraction. Due to the presence of the membrane a wider selection of extractants can be used, this can include the use of miscible solutions, for example the recovery of ammonia from waste water using sulphuric acid.

This process is analogous to pervaporation in some ways. But the permeate is in liquid phase. Perstraction technique eliminates the problem of phase dispersion and separation altogether.

A basic perstraction is called the single perstraction or membrane perstraction. An advantage is minimizing toxic damage to microorganisms or enzymes. Nevertheless perstraction includes problems like expensive membranes, clogging and fouling of membranes.

Perstraction has been combined with the ABE (acetone butanol ethanol) fermentation for butanol production. Butanol is toxic to the fermentation, therefore perstraction can be applied to remove the butanol from the vicinity of the bacteria as soon as it is produced. Liquid-liquid extraction (LLE) was combined with the ABE fermentation for in situ product recovery, but the extractants with the highest affinity for butanol tend to be toxic to the bacteria. The application of LLE would also require the extractant to be sterilised prior to contact with the fermentation broth. Perstraction can overcome these problems due to a membrane separating the fermentation broth from the extractant. As an in situ product recovery technique for the ABE fermentation perstraction is still in its development stages.

A membrane brings many new elements for the separation. Amino acids has been separated by perstraction. Membranes did not only separate extractants and the primary solution but also were selective for amino acids. Charged membranes were used. So they selected amino acids by pKa. Besides the selectivity of a membrane is affected by its thickness, pore diameter and charge potential. The bigger pore is, the better amino acids permeate the membrane. The higher charge potential is, the bigger electrostatic rejection effects are. The thinner membrane, the less it is selective.

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