Water regeneration, membrane separation

A more promising approach for the synthesis of hydrophobic substances with ADHs is published by Kruse et al. [159, 238], They use a continuously operating reactor where the enzyme containing water phase is separated from the hydrophobic substrate-containing organic phase by a membrane. The hydrophobic product is extracted continuously via a hydrophobic membrane into an hexane phase, whereas the coenzyme is regenerated in a separate cycle, that consists of a hydrophilic buffer system. This method decouples advantageously the residence time of the cofactor from the residence time of the substrate. Several hydrophobic alcohols were prepared in this way with (S)-ADH from Rhodococcus erythropolis (Table 16). 

L.F. Liu, P.H. Zhang, F.L. Yang, Adsorptive removal of 2,4-DCP from water by fresh or regenerated chitosan/ACF/Ti02 membrane . Separation and Purification Technology, 70, 354-361, (2010). 

Forward osmosis (FO) is a membrane-separation process that uses osmotic pressure difference between a concentrated draw solution and a feed stream to drive water across a semipermeable membrane [63]. The basis of FO is osmosis, a natural and spontaneously occurring process. It is strictly direct osmosis across an RO membrane. A draw solute of high osmotic pressure, e.g., ammonium carbonate passes across one side of the FO membrane, and a high salinity solution, e.g., seawater flows across the other side of the membrane, as shown in Figure 1.17. Water transfers from the seawater to the draw solute side due to osmotic flow. It is then necessary to regenerate the draw solute and recover the water transferred by the FO process, e.g., in a distillation unit. The primary challenge is... 

The RO unit is the pivotal process since water production by membrane separation declines with time mainly due to fording and other factors discussed in Chapter 2. The RO system must be run under conditions that minimise decline in flux while maintaining high product water quality. The pre-treatment system must be stable and reliable to ensure the RO unit operates continuously without frequent shutdowns for cleaning to restore flux and rejection. A stable RO membrane performance, in turn, is required to ensure the pohshing system produces water that meets the product water specifications without frequent shutdowns for regeneration of ion-exchange resins. 

A third important area for gas separation is the removal of water vapor from air or from hydrocarbons. This separation is easily accomplished using adsorption, but the adsorbent beds require periodic regeneration. Membranes, which do not require this regeneration, show selectivities over air and hydrocarbons of thousand to one. Hydrocarbon losses, which are significant now, should improve as the membranes evolve. 

Treatment of separately collected and reconcentrated baths that initially contain dyestuff concentrations of approximately 1 g/L and are reconcentrated to approximately 10-20 g/L dyestuff by membrane filtration. Such techniques yield considerable amounts of recyclable water, but care has to be taken to avoid any disturbing effect during reuse caused by salt and alkali content in the regenerate. The concentrated dyestuff solution can be treated with similar methods as concentrated dye solutions from fillings of padder. 

Figure 4.8—Membrane and electrochemically regenerated suppressors. Two types of membrane exist those that allow the permeation of cations (H+ and Na+) and those that allow the permeation of anions (OH and X ). a) The microporous cationic membrane model is adapted to the elution of an anion. Only cations can migrate through the membrane (corresponding to a polyanionic wall that repulses the anion in the solution) b) Anionic membrane suppressor placed after a cationic column and in which ions are regenerated by the electrolysis of water. Note in both cases the counter-current movement between the eluted phase and the solution of the suppressor c) Separation of cations illustrating situation b).

Liquid membranes of the water-in-oil emulsion type have been extensively investigated for their applications in separation and purification procedures [6.38]. They could also allow extraction of toxic species from biological fluids and regeneration of dialysates or ultrafiltrates, as required for artificial kidneys. The substrates would diffuse through the liquid membrane and be trapped in the dispersed aqueous phase of the emulsion. Thus, the selective elimination of phosphate ions in the presence of chloride was achieved using a bis-quaternary ammonium carrier dissolved in the membrane phase of an emulsion whose internal aqueous phase contained calcium chloride leading to phosphate-chloride exchange and internal precipitation of calcium phosphate [6.1]. 

The two water desalination applications described above represent the majority of the market for electrodialysis separation systems. A small application exists in softening water, and recently a market has grown in the food industry to desalt whey and to remove tannic acid from wine and citric acid from fruit juice. A number of other applications exist in wastewater treatment, particularly regeneration of waste acids used in metal pickling operations and removal of heavy metals from electroplating rinse waters [11]. These applications rely on the ability of electrodialysis membranes to separate electrolytes from nonelectrolytes and to separate multivalent from univalent ions. 

In 1996, a paper was published which was dedicated to selecting suitable membranes for separations in organic solvents [466]. Membranes tested in an asymmetrical channel included polysulfone MWCO 20,000 g/mol, regenerated cellulose MWCO 20,000 g/mol, PTFE pore size 0.02 mm, polyaramide MWCO 50,000 g/mol, poly(vinylidene fluoride) MWCO 50,000 g/mol, poly(phenylene oxide) MWCO 20,000 g/mol and a DDS fluoro polymer MWCO 30,000 g/mol. The first membrane was tested with water, the others with THF or a THF/ace-tonitrile mixture. Numerous problems occurred with the different membranes. The best membrane for THF was found to be the DDS fluoro polymer membrane. 

The membrane permeation was tested for separation of HTO from water at INCT [114]. Membranes made from regenerated cellulose, polysulphone, and polytetrafluoroethylene were used. Separation factors anio/HTO obtained in membrane process were not higher than 1.04 for cellulose and 1.02 for polysulphone, while for PTFE membranes were as high as 1.06-1.22. 

Polymeric membranes for separation of hydrogen and oxygen isotopes were studied at INCT, Warsaw [92-95,140-141]. Both hydrophUic barriers, such as regenerated cellulose and hydrophobic PTFE membranes, were tested. The regenerated cellulose appeared to be a very good system to get high separation factors and to consider membrane permeation as possible and competitive method for enrichment deuterium and 0 in natural water. 

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