Hollow fibers separation factor

Membranes offer a format for interaction of an analyte with a stationary phase alternative to the familiar column. For certain kinds of separations, particularly preparative separations involving strong adsorption, the membrane format is extremely useful. A 5 x 4 mm hollow-fiber membrane layered with the protein bovine serum albumin was used for the chiral separation of the amino acid tryptophan, with a separation factor of up to 6.6.62 Diethey-laminoethyl-derivatized membrane disks were used for high-speed ion exchange separations of oligonucleotides.63 Sulfonated membranes were used for peptide separations, and reversed-phase separations of peptides, steroids, and aromatic hydrocarbons were accomplished on C18-derivatized membranes. 

Hydrogel membranes are particularly attractive because of high permeability and separation factor [300], and good stability for CO2/N2 separation [299], PVDF hollow fiber membrane modified by alkali was coated by PYA hydrogel on its surface and PVDF-PVA hydrogel membranes show better hydrophilic performance. For carbonate hydrogel (sodium carbonate concentration of 3.7 %) membrane, C02, concentration from 1.3 % to 0.6 % in feed gas could be decreased to 0.9-0.4 % at the outlet at 25 °C. 

Facilitated transport of penicilHn-G in a SLM system using tetrabutyl ammonium hydrogen sulfate and various amines as carriers and dichloromethane, butyl acetate, etc., as the solvents has been reported [57,58]. Tertiary and secondary amines were found to be more efficient carriers in view of their easy accessibility for back extraction, the extraction being faciUtated by co-transport of a proton. The effects of flow rates, carrier concentrations, initial penicilHn-G concentration, and pH of feed and stripping phases on transport rate of penicillin-G was investigated. Under optimized pH conditions, i. e., extraction at pH 6.0-6.5 and re-extraction at pH 7.0, no decomposition of peniciUin-G occurred. The same SLM system has been applied for selective separation of penicilHn-G from a mixture containing phenyl acetic acid with a maximum separation factor of 1.8 under a liquid membrane diffusion controlled mechanism [59]. Tsikas et al. [60] studied the combined extraction of peniciUin-G and enzymatic hydrolysis of 6-aminopenicillanic acid (6-APA) in a hollow fiber carrier (Amberlite LA-2) mediated SLM system. 

On the contrary, silica membranes with pore diameters in the 0.3-0.8 nm on porous glass supports have been claimed to exhibit an H2/CO separation factor as high as 30. This is significantly higher than what Knudsen diffusion alone predicts. It is possible that some molecular sieving takes place in the reportedly small pores in the silica membranes. This postulation appears to be further reinforced by another study using hollow fiber silica... 

As for CO2, VOCs can also be removed by using immobilized liquid membranes. Obuskovic et al. [35] immobilized a thin layer of silicone oil in the microporous of the hollow fiber polypropylene membrane beneath the dense-coated skin. The performance of the system has been proved for toluene, methanol, and acetone removal from N2. With respect to the simple hollow fiber, the presence of the oil layer led to a 2-5 VOC more enriched permeate (due to the reduction of nitrogen flux) with a separation factor of 5-20 times higher (depending on the VOC and the feed gas flowrate). The membrane was stable for 2 years. 

Table 1. Permeability of polymers and ideal separation factors for composite hollow fiber membranes (20 °C).

Figure 14 The effect of counter-Ion on flux and separation factor of a Naflon hollow fiber.

The various modifications of the polyethylene Ion-exchange hollow fiber membranes also proved that here we have a workable concept. From this family of special Interest membranes, results were obtained for anion-exchange hollow fiber membranes. Separation factors of 83, 18 and 3.5 were obtained for feed mixtures of 2-propanol, ethanol and methanol, respectively (each feed mixture was composed of 20 wt.X alcohol and the separation was conducted at 23°C). 

A hollow-fiber liquid membrane was used in a separation of D,L-lactic acid and D,L-alanine resolution [196]. In this case, the enantioselective transport of solutes performed in one module was facihtated by N-3,5-dinitrobenzoyl-L-alanine octylester chiral selector, dissolved in toluene. The maximum D,L-lactic acid separation factor achieved was 2.00 and that for the D,L-alanine was 1.75. In both cases, the D-enantiomer flux was preferred. These values correspond to the enantiomeric excess 33.5% ee and 27.2% ee, respectively, and are not as good as in the first example. However, note that in this case, only one separation step took place and feed phase was circulated in the module. 

Recently, Bao et al. [68] compared the efficiency of facilitated transport of CO2 across a liquid membrane by different carriers (diethanolamine (DEA) and carbonic anhydrase (CA) + bicarbonate (NaHCO3) in a polypropylene HFCLM configuration. The hollow fibers used are made of polypropylene, pore size 0.04 pm. In all the experiments, the measured CO2 permeance and selectivity (CO2/O2) using CA bicarbonate as carrier was higher than in the case of DEA. The separation factor (CO2/O2) using DEA was about 152 which are 65% lower than the selectivity calculated with CA bicarbonate. 

Ethylene has been separated from ethane by a silver nitrate solution passing countercurrent in a hollow fiber poly-sulfone.165 This separation has also been performed with the silver nitrate solution between two sheets of a polysilox-ane.166 A hydrated silver ion-exchanged Nafion film separated 1,5-hexadiene from 1-hexene with separation factors of 50-80.167 Polyethylene, graft-polymerized with acrylic acid, then converted to its silver salt, favored isobutylene over isobutane by a factor of 10. Olefins, such as ethylene, can be separated from paraffins by electroinduced facilitated transport using a Nafion membrane containing copper ions and platinum.168 A carbon molecular sieve made by pyrolysis of a polyimide, followed by enlargement of the pores with water at 400 C selected propylene over propane with an a-valve greater than 100 at 35°C.169... 

The separation factor was determined at various flow rates of the mobile phase ranging from 0.3 to 3 mL/min, i.e., the residence times ranged from 4.6 to 46 s in the BS A-multilayer-adsorbed porous hollow-fiber membrane. As a result, the separation factor was constant... 

Generally, zeolite membranes are synthesized on tubular supports, and used as tubular-type modules. Packing density (i.e. membrane separation area/module volume ratio) of the tubular module was low as it was compared with that of the polymeric membranes. Xu et al. [32] synthesized of NaA zeolite membrane on a ceramic hollow fiber with an outer diameter of 400 p,m, a thickness of 100 p,m and an average pore radius of 0.1 (rm. The quality of the as-synthesized NaA zeolite membrane held a He/N2 separation factor of 3.66 and He permeance of 10.1 X 10 mol/(m. s.Pa). 

An alternative approach to solving stability problems with ILMs is presented by Bhave and Sirkar (114). Aqueous solutions are immobilized in the pore structure of hydophoblc, polypropylene hollow fibers by a solvent exchange procedure. Gas permeation studies are reported at pressures up to 733 kPa with the high pressure feed both on the shell and lumen sides of the laboratory scale hollow fiber permeator. No deformation of the hollow fibers is observed. Immobilizing a 30 weight % KjCO, solution in the hollow fibers greatly improved the separation factor, a(C02/Na). from 35.78 with pure water to 150.9 by a facilitated transport mechanism. Performance comparisons with commercial COj separation membranes are made. 

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