Membrane degasification

An integrated membrane approach in UPW systems consists of four major membrane-based water treatment components ultrafiltration (UF), reverse osmosis (RO), electrodeionization (EDI), and membrane degasification. Each process is unique and contributes particular advantages to the system design. As the need increases and the costs become more acceptable, these technologies will become lynchpins of UPW systems. 

If high overall water treatment system recovery is desired, it is feasible to recycle some or aU of the concentrate bleed flow. In many situations, concentrate bleed is purer than the raw water. However, when the EDI feed stream contains high levels of CO2, its buildup may prevent recycling without de-carbonation of the EDI feed stream. De-carbonation of the EDI feed water using membrane degasification units shows a lot of promise. Alternatively, pH of the upstream RO feed water is increased by the addition of a small amount of sodium hydroxide, which converts carbon dioxide to sodium bicarbonate, which can be rejected by conventional polyamide RO membranes as follows ... 

Membrane degasification units are devices that can be used to permit mass transfer between a gaseous phase and a liquid phase without dispersing one phase into another. The gas layer is stabilized within the pores of a hydrophobic microporous filter in membrane degasification units. Solutes that are volatile can pass across these membranes, but nonvolatile solutes and aqueous liquids such as electrolytes are completely retained. High flux occurs when the solute is volatile and when it is relatively insoluble in water. 

The membrane degasification technology has three major advantages and one potential disadvantage over conventional equipment based on packed towers. The advantages are (Reed et al., 1995) ... 

Figure 13.14 Typical membrane degasification unit installation.

Membrane degasification units generally use hydrophobic hollow-fiber polypropylene or polytetrafluoroethylene (PTFE) (Teflon) microporous membrane (Wiesler, 1996). Hollow-fiber modules are made by potting the desired number of fibers into an external shell. The potting compound may be polyurethane, epoxy, polyolefin, or fluorinated polymers. Since the membranes are hydrophobic and have small pores (Fig. 13.15), water will not easily pass through the pores. 

Figure 13.15 Membrane morphology in membrane degasification unit.

Extensive research both in industry and academia has resulted in the innovation of porous membranes, which are gas fiUed, with much smaller mass transfer resistance. Parallel research on microporous membranes has adjusted the pore size and membrane hydrophobicity, again yielding a much smaller mass transfer resistance. However, modules with different geometries perform differentiy. Flow outside of, but perpendicular to, the fiber bundle offers reasonably fast mass transfer. Not surprisingly, this geometry is chosen in most of the commercial membrane degasification units. 

Mass Transfer in Membrane Degasification Commercial membrane degasification units involve mass transfer in hollow-fiber modules where the vacuum and sweep gases are applied inside the fibers, and the water flows outside the flbers in cross flow perpendicular to the fiber axis. The mass transfer involves three sequential steps. First, dissolved gas diffuses out of the water to the membrane surface. Second, it diffuses into vapor pores in the walls of the hydrophobic hollow fibers. Third, when the dissolved gas reaches the other wall of the fibers, it diffuses into the surrounding nitrogen sweep gas in a high vacuum condition. 

Figure 13.16 Membrane degasification module. (Courtesy of Membrana.)...

The above form of the mass transfer equation is adopted in the design of membrane degasification units. 

Dey and Thomas 2003 studied the performance of membrane degasification units in terms of organic removal, including THMs. The effect of variation in flow rate and applied vacuum level on the removal rate of TOC and chloroform are, respectively, given in Tables 13.6 and 13.7. 

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