Membrane modules purity

Membrane modules have found extensive commercial appHcation in areas where medium purity hydrogen is required, as in ammonia purge streams (191). The first polymer membrane system was developed by Du Pont in the early 1970s. The membranes are typically made of aromatic polyaramide, polyimide, polysulfone, and cellulose acetate supported as spiral-wound hoUow-ftber modules (see Hollow-FIBERMEMBRANEs). 

To demonstrate the potential of the process in obtaining both enantiomers at a high purity, experiments were performed using racemic norephedrine as the compound to be separated. Two columns of seven small membrane modules were used. The enantiomer ratios in the outflows during start-up are shown in Fig. 5-15. It can be concluded that the system reaches equilibrium within approximately 24 h, and that both enantiomers are recovered at 99.3-99.8 % purity. 

Cabral and coworkers [253] have investigated the batch mode synthesis of a dipeptide acetyl phenylalanine leucinamide (AcPhe-Leu-NH2) catalyzed by a-chymotrypsin in a ceramic ultrafiltration membrane reactor using a TTAB/oc-tanol/heptane reverse micellar system. Separation of the dipeptide was achieved by selective precipitation. Later on the same group successfully synthesized the same dipeptide in the same reactor system in a continuous mode [254] with high yields (70-80%) and recovery (75-90%). The volumetric production was as high as 4.3 mmol peptide/l/day with a purity of 92%. The reactor was operated for seven days continuously without any loss of enzyme activity. Hakoda et al. [255] proposed an electro-ultrafiltration bioreactor for separation of RMs containing enzyme from the product stream. A ceramic membrane module was used to separate AOT-RMs containing lipase from isooctane. Application of an electric field enhanced the ultrafiltration efficiency (flux) and it further improved when the anode and cathode were placed in the permeate and the reten-tate side respectively. 

Another example of the use ofHF-SLM separation concerns the resolution of racemic ofloxacin [200]. This important drug, a fluoroquinolone antibiotic with one chiral center, was separated in chiral systems by hoUow-fiber liquid-supported membrane technology combining with countercurrent fractional extraction. The two chiral solutions contained L-dibenzoyltartaric acid and D-dibenzoyltartaric acid in 1-octanol, and flowed through the lumen side and the sheU side of fibers, respectively. The solution which Uowed through the lumen side of fibers also contained racemic ofloxacin. The waU of hoUow fibers was fiUed with an aqueous of 0.1 mol/l Na2[ ll O4/[ fd O4 buffer solution of pH 6.86 containing 2 mmol/1 of cetyltrimethylammonium bromide for 48 h. The obtained optical purity for ofloxacin enantiomers was up to 90% when 11 hoUow-fiber membrane modules of 22 cm in length in series were used. 

Packaging of membranes in suitable element or modules that began in the 1960s resulted in several membrane modules by the 1980s spiral-wound modules with CA and TFC polyamide membranes, PA and CTA hoUow-fibre membranes, and UF polysulphone hoUow-fibre membranes. These membranes and modules found their own niches in the marketplace in the 1980s broadly speaking, CA spiral-wound membranes for industrial and municipal water treatment, TFC spiral-wound RO membranes for water desalination and high-purity water production, hoUow-fibre RO membranes for seawater desalination, and hoUow-fibre UF and MF membranes for industrial... 

Scheme 14). More specifically, a commercially available Uquid—Hquid membrane module (Zaiput) was used. This device allowed an efficient separation of the aqueous and organic phase and the desired oxazolines 60 were obtained in good to excellent yields (60—98%) and in high purity. 

The membrane material and all other parts of the membrane module which come in contact with the gases should be stable and chemically compatible not only with the gas mixture to be separated but also with the trace impurities present therein. All materials used in the membrane module should also have sufficient mechanical properties to resist any creep at operating temperature and pressure. The membrane should offer high gas ffux and high selectivity for compact design, higher gas purity and lower gas losses. 

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