Membrane barrier layers

A fundamental difference exists between the assumptions of the homogeneous and porous membrane models. For the homogeneous models, it is assumed that the membrane is nonporous, that is, transport takes place between the interstitial spaces of the polymer chains or polymer nodules, usually by diffusion. For the porous models, it is assumed that transport takes place through pores that mn the length of the membrane barrier layer. As a result, transport can occur by both diffusion and convection through the pores. Whereas both conceptual models have had some success in predicting RO separations, the question of whether an RO membrane is truly homogeneous, ie, has no pores, or is porous, is still a point of debate. No available technique can definitively answer this question. Two models, one nonporous and diffusion-based, the other pore-based, are discussed herein. 

Equation 7 shows that as AP — oo, P — 1. The principal advantage of the solution—diffusion (SD) model is that only two parameters are needed to characterize the membrane system. As a result, this model has been widely appHed to both inorganic salt and organic solute systems. However, it has been indicated (26) that the SD model is limited to membranes having low water content. Also, for many RO membranes and solutes, particularly organics, the SD model does not adequately describe water or solute flux (27). Possible causes for these deviations include imperfections in the membrane barrier layer, pore flow (convection effects), and solute—solvent—membrane interactions. 

The permeable membrane (barrier layer) plays a crucial role for the ALILE process. Usually, the barrier layer is formed by exposure of the Al-coated glass substrate to air. The thickness of this barrier layer (A1 oxide), which is on a nanometer scale, can be influenced by the variation of the exposure time. With increasing exposure time, the thickness of the barrier layer increases. The thicker the barrier layer, the lower is the nucleation density and the longer is the process time necessary to form a continuous poly-Si film [19]. By X-ray photoelectron spectroscopy (XPS) measurements it was shown that the barrier layer stays in place during the whole ALILE process. 

Salt rejections by the NS-300 membrane toward synthetic seawater improved as the isophthalamide content of the barrier layer increased. Surprisingly, membrane flux peaked rather than simply declining as a function of increasing isophthalamide content. This is illustrated by the data in Table II. Maximum water permeability characteristics were found at an approximate copolymer ratio of 67 percent isophthalic and 33 percent trimesic groups. The differences in the magnesium sulfate versus sodium chloride rejection appear to be due to the anionically charged nature of the membrane barrier layer, which is rich in carboxylate groups. 

Shortly after the concept of an asymmetric membrane was established, composite membrane research was initiated. A composite membrane is also asymmetric but it consists of two polymer layers which are the membrane barrier layer and the porous support layer (Figure 4.6). The porous support is formed separately, by conventional membrane casting techniques, from one polymer. The porous support has a thickness of between 75 and 100 micrometers and its porosity is due to numerous small perforations through the support. The membrane barrier layer is a dense thin film of another polymer that is formed or deposited in a subsequent operation on the porous support. The membrane barrier layer varies in thickness from 400 to 1,000 angstroms. 

Several polymers have been used as porous supports. One of the earliest composite membrane systems was a porous support formed from cellulose nitrate-cellulose acetate with a membrane barrier layer of cellulose triacetate. While this membrane successfully desalted seawater, it was fragile and expensive. To a large degree, present day commercially available composite membranes use a polysulfone porous support. 

Membrane barrier layers have been formed on porous supports in the following manner 8... 

The solution coating technique was used in the preparation of the cellulose triacetate membrane discussed above. A solution of cellulose triacetate in chloroform was deposited on the porous support and the solvent was then evaporated leaving a thin film on the porous support. Thin film polymerization was used to prepare a polyfuran membrane barrier layer on polysulfone. In this case, the monomer furfuryl alcohol is polymerized in situ by adjustment of pH and temperature. This membrane proved to be highly susceptible to oxidizing agents and is of limited value. By far the most valuable technique in the formation of membrane barrier layers is interfacial polycondensation. In this method, a polymer is formed on the porous support surface at the interface of organic and aqueous phases by reaction of specific molecules dissolved in each phase. It is by this method that a number of polyamides and polyurea membrane barrier layers have been formed on polysulfone. Elements containing these membranes are available commercially. 

The thin film composite membrane exhibited superior overall rejection performance in these tests, with ammonia and nitrate rejection showing an outstanding improvement. It has also been reported that silica rejection by the thin film composite membranes is superior to that of cellulose acetate. While the above data indicates a marginal improvement in the rejection of chemical oxygen demand (COD), which is an indication of organic content, other tests conducted by membrane manufacturers show that the polyurea and polyamide membrane barrier layers exhibit an organic rejection that is clearly superior to that of cellulose acetate. Reverse osmosis element manufacturers should be contacted for rejection data on specific organic compounds. ... 

Reverse osmosis membrane is produced in sheet form-up to 60 inches wide and lengths up to 1,500 feet-and as a hollow fine fiber. The asymmetric cellulose acetate was originally produced as a sheet and later as a hollow fine fiber. The asymmetric aromatic polyamide was originally produced as a hollow fine fiber and later in sheet form. The composite membranes with polyamide or polyurea membrane barrier layers are produced in sheet form as of the end of 1987, but research has been and will continue to be done to produce the composite reverse osmosis membranes as a hollow fine fiber. 

Chlorine has been added to the feedwater upstream of reverse osmosis pretreatment. However, since chlorine will depolymerize the polyurea membrane barrier layer in the spiral wound element, with subsequent loss of desalination properties, the chlorine is removed in the pretreatment system dechlorination basin. This removal is chemically accomplished by the addition of sodium bisulfite. The chlorine level in the influent and effluent to the dechlorination basin is continuously monitored. The feedwater is then transferred from the dechlorination basin to the cartridge filter feed pumping station by gravity flow and it is then pumped to the cartridge filters. 

Figure 5.12 Topside and underside of the FT-30 composite reverse osmosis membrane (a) topside showing well-developed ridge-and-valley structure, and also an area of membrane barrier layer folded over upon itself (b) underside of the barrier layer (foldover zone) showing the network of micropores inside the ridge-and-valley structure.

An RO membrane acts as a barrier to flow, allowing selective passage of a particular species (solvent) while other species (solutes) are retained partially or completely. Solute separation and permeate solvent (water in most cases) flux depend on the material selection, the preparation procedures, and the structure of the membrane barrier layer [5,15]. Cellulose acetate (CA) is the material for the first generation reverse osmosis membrane. The announcement of CA membranes for sea water desalination by Loeb and Sourirajan in 1960 triggered the applications of membrane separation processes in many industrial sectors. CA membranes are prepared by the dry-wet phase inversion technique. Another polymeric material for RO is aromatic polyamide [16]. 

MF symmetric membrane barrier layer thickness = 10-150 )lm MF asymmetric membrane barrier layer thickness = 1 )lm UF (asymmetric) membrane barrier layer thickness = 0.1-1.0 )lm RO/NP (asymmetric) membrane barrier layer thickness = 0.1-1.0 )lm [17]. 

Equation 9.15 describes the transport of substrate glucose through the concen-trated boundary layer. An equation similar to Equation 9.15 should be applicable to the ultrafiltration membrane (barrier layer b) and... 

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