Regeneration of membranes

The regeneration of membrane properties is key to maintaining its selectivity and yield. Without a safe, practical, reproducible, cost-effective, and efficient cleaning procedure, the viability of crossflow filtration is highly questionable [36], The use of the mechanical means discussed above can help extend the membrane performance over a limited period of time, but the complete... 

S. Ebrahim, Cleaning and regeneration of membranes in desalination and wastewater applications state-of-the-art. Desalination 96, 225-238 (1994). 

Another type of membrane is the dynamic membrane, formed by dynamically coating a selective membrane layer on a finely porous support. Advantages for these membranes are high water flux, generation and regeneration in situ abiUty to withstand elevated temperatures and corrosive feeds, and relatively low capital and operating costs. Several membrane materials are available, but most of the work has been done with composites of hydrous zirconium oxide and poly(acryhc acid) on porous stainless steel or ceramic tubes. 

Surface-modified electrodes were used for prevention of high overpotentials with direct oxidation or reduction of the cofactor, electrode fouling, and dimerization of the cofactor [7cj. Membrane electrochemical reactors were designed. The regeneration of the cofactor NADH was ensured electrochemically, using a rhodium complex as electrochemical mediator. A semipermeable membrane (dialysis or ultrafiltration) was integrated in the filter-press electrochemical reactor to confine... 

Table 4 summarizes the efficiency of membrane filtration as preliminary treatment in the hybrid process to obtain regenerated water for industrial reuse. Working with the adequate cleaning cycle to avoid fouling and to keep a constant flux (10 1 min ) important reduction in suspended solids (90%) and turbidity (60%) of the wastewaters is achieved but there is no significant reduction of other chemical or physical parameters, e.g., conductivity, alkalinity or TDS, or inactivation of E. coli. 

Much more simply, the same result can be attained with bipolar membranes, membranes consisting of an anion- and cation-permeable (an anion- and cation-exchange) membrane laminated together. At such a membrane, when mounted between electrodes so that the cation-exchange layer faces the anode, water is split into and OH ions so that the acidic and alkaline solutions required for regeneration as above are produced at the respective surfaces of the bipolar membrane. When such membranes are suitably integrated into the sequence of membranes in the electrodialysis unit above, gas evolution at the electrodes is not needed the acid-base pair is produced with about half the power. 

Table 20-24 compares properties of commonly used polyethersulfone (PES) or regenerated cellulose membranes. Membrane selection is based on experience with vendors, molecular weight rating for high... 

A baseline potential pulse followed each current pulse in order to strip extracted ions from the membrane phase and, therefore, regenerated the membrane, making it ready for the next measurement pulse. This made sure that the potentials are sampled at discrete times within a pulse that correspond to a 6m that is reproducible from pulse to pulse. This made it possible to yield a reproducible sensor on the basis of a chemically irreversible reaction. It was shown that the duration of the stripping period has to be at least ten times longer than the current pulse [53], Moreover the value of the baseline (stripping) potential must be equal to the equilibrium open-circuit potential of the membrane electrode, as demonstrated in [52], This open-circuit potential can be measured prior to the experiment with respect to the reference electrode. 

Mani KN (1988) Proc-APCA Annu Meet 1988, 81st (1) Paper 88/6A 7, Aquateck membrane technology for regeneration of hazardous waste/acid effluent streams Chem Abstr 110 (1989)... 

Once an action potential is initiated at the hillock, it continues in a self-propagating fashion to the end of the axon. Voltage-sensitive ion channels are opened by the depolarization, allowing further influx of Na+ and a spread of the action potential. Thus, the action potential is a self-regenerating spread of membrane depolarization. 

W. Berke, H. J. Schuz, C. Wandrey, M. Morr, G. Denda, and M. R. Kula, Continuous regeneration of ATP in enzyme membrane reactor for enzymatic synthesis, BiotechnoL Bioeng., 32, 130-139 (1988). 

Renal effects have been observed in both male and female rats in a chronic-duration oral study. Male Fischer 344 rats exposed to 1,4-dichlorobenzene at 150 and 300 mg/kg/day for 2 years exhibited nephropathy, epithelial hyperplasia of the renal pelvis, mineralization of the collecting tubules in the renal medulla, and focal hyperplasia of the tubular epithelium. Each of these effects was associated with hyalin droplet formation. There were also increased incidences of nephropathy in female Fischer 344 rats dosed with 1,4-dichlorobenzene at 300 and 600 mg/kg/day. Histopathologically, the nephropathy was characterized by degeneration and regeneration of the tubular epithelium, tubular dilatation with attenuation and atrophy of the epithelium, granular casts in the tubules of the outer stripe of the medulla, thickening of the basement membranes, and minimal accumulation of interstitial collagen (NTP 1987). 

In water, the third-generation (16) and fourth-generation dendrimers (17) induced chirality toward the (S)-enantiomer (50% ee for 16 and 98% ee for 17). In THF high enantiomeric excess was achieved only with the third-generation dendrimer (99% (S) ee for 16 and 3% (S) ee for 17). Dendrimer 16 was recovered from the heterogeneous reaction mixture by nanofiltration on a Millipore microporous membrane system. After regeneration of the catalytic activity by treatment with... 

There are maty other examples of cofactor regeneration reactions and/or of reactions which may be performed in an enzyme membrane reactor. An important example is the regeneration of NADH by formate dehydrogenase (FDH), starting with formate (Wichmaim et al, 1981). The advantage of this reaction is that it is irreversible because carbon dioxide is hberated, while formate is a relatively cheap electron donor. 

In a first reactor, where benzoylformate decarboxylase (BFD) is retained, benz-aldehyde and acetaldehyde are coupled to yield (S)-hydroxy-l-phenylpropanone. This hydroxy ketone is then reduced to the corresponding diol in a second reactor by an alcohol dehydrogenase (ADH). Regeneration of the necessary cofactor is achieved by formate dehydrogenase (FDH) or by other methods. To avoid additional consumption of redox equivalents by unselective reduction of residual starting material from the first reactor, the volatile aldehydes are removed via an inline stripping module between the two membrane reactors. In this setup the diol was produced with high optical purity (ee, de > 90%) at an overall space-time yield of 32 g L d . ... 

The construction of the eye is completely different. The outer layer of this mucosa consists of a tiny tear layer of lipids and water which covers a superficial epithelium closed by double layer lipid membranes of 30-70 nm size interconnected by tight jnnctions. Three to seven layers of epithelial cells cover the stromal structures of conjunctiva or cornea. The conjnnctival surface has interposed cells secreting small amonnts of mucin, the so-called goblet cells, which are typically missing within the corneal epithelinm. The regeneration of epithelial structnres is dne to the limbal stem cells located deep in the Vogt s crypts, for the cornea. 

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