Perstraction

Introduction

Perstraction is the separation technique developed from biphase extraction. In this technique, streams of aqueous and organic phases which are immiscible are separated by a membrane. Specifically the organic solvent is extracted through the membrane, leaving the aquous phase behind.

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.

Membrane perstraction is not the only type of perstraction technique. Capsular perstraction is fast saving energy and solvents. Unfortunately capsules ca not be produced large amounts in the cheap way and the alginate capsule is labile mechanically. Enzymes can be immobilized to the capsule membrane.

Perstraction in butanol fermentation

Perstraction is used in the butanol fermentation. Toxic butanol is a problem in high initial sugar concentrations. The research of product removal started in the end of the 1980s. Butanol has been tried to remove during fermentation by many techniques like adsorption, gas stripping and liquid–liquid extraction. The latter has been reported to be one of the economical techniques. Nevertheless it includes problems. Extractants can be toxic to cells, they can accumulate to wrong places like cells, loss of them phase separation may be bad etc.

A solution is perstraction to these problems. A butanol permeable membrane is placed in between the fermentation broth and extractant solvents. Fermentation products diffuse through the membrane by the vapour pressure difference. The technique enhances cell growth, improves productivity and recovers butanol economically. Nevertheless, it should be developed. Butanol diffuses through the connecting tube which means loss of product. Secondly the butanol flux could be higher through a membrane.

Amino acids separation through the charged membrane

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.

The clean groundwater

Pollutants can be deleted from groundwater by perstraction. Different techniques have been patented. The oldest one has published in 1990 and the youngest one in 1998. In the 2000s has been done few patent applications but no granted patents.

Organic compounds through a membrane has been concentrated from groundwater. The concentration factor is from 1 000 to 10 000 bringing 0.1 ppb concentrations to between 0.1 and 1.0 ppm. Besides the concentration of a contaminant has been analyzed in real-time. The membrane is polymer like polysulphane. The hole diameter is 300 µm and thickness is 30 µm.

Removal of pharmaceuticals from water

The pharmaceuticals pass sewage treatment plants. They like estrogen conjugates may cause problems. Drugs of the research were common, present in the aquatic environment and inability to be adequately removed by sewage treatment plants. There were seven different drugs in the research. Dibutyl sebacate and oleic acid formed liquid cores in capsules because they do not diffuse away from capsules and have affinity for drugs. Capsule external diameters were 740 µm and 680 µm and internal diameters were 570 µm and 500 µm. Agitation was 300 rpm. Equilibrium times were 30, 50 and 90 minutes.

Since dibutyl sebacate and oleic acid were different affinity for drugs, they were used concurrently. Four drugs were extracted effectively for 40–50 minutes (at least 50% removed). Extraction rates did not change significantly above 150 rpm. Membrane thickness did not affected significantly. On the contrary the capsule size was remarkable for mass transfer.

Hydrophobic gelganamycin separated from aqueous media

An antibiotic called geldanamycin was separated from media by the capsular perstraction. Geldanamycin is hydrophobic. Outer particle diameter varied from than less 500 to 750 µm. Alginate formed the shell of capsule and its thickness varied from 30 to 90 µm. Dibutyl sebacate or oleic acid as liquid core extracted geldanamycin well. The bigger agitation and thinner capsule membrane were, the faster transfer rate was.

Geldanamycin was back-extracted from capsules. Dibutyl sebacate capsules were disposable because liquid core came out from capsules in the back-extraction. On the contrary oleic acid remained in capsules during the back-extraction when an extractant was saturated with oleic acid. Nevertheless the presence of oleic acid in the back-extraction solution demanded more purification steps (precipitation, centrifugation and filtration). Oleic acid was removed because it prevents crystallization of geldanamycin. Therefore geldanamycin was crystallized and the end product was highly purified.

Enzymes can be immobilized to the capsule membrane. In this case the capsule external diameter was 500 µm and internal diameter 300 µm. The product of enzyme-catalyzed reaction can be concentrated to capsules and the end-product inhibition is low. Enzyme recycling could be performed by back-extracting the product. The technique has been applied to the hydrolysis of penicillin G.