Flux composite membranes, typical

The unusual solubility of gases and vapors in perfluoropolymers has several applications relevant to membrane gas separations. Perfluoropolymers have solubility selectivities that are significantly different from those of hydrocarbon-based polymers. The amorphous perfluoropolymers can be fabricated into thin, high-flux composite membranes, which possess the excellent chemical and thermal stability. Typical reported pure gas permeabilities and selectivities of these fine amorphous perfluoropolymers are shown in Table 16.8 [33]. 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

Since the discovery by Cadotte and his co-workers that high-flux, high-rejection reverse osmosis membranes can be made by interfacial polymerization [7,9,10], this method has become the new industry standard. Interfacial composite membranes have significantly higher salt rejections and fluxes than cellulose acetate membranes. The first membranes made by Cadotte had salt rejections in tests with 3.5 % sodium chloride solutions (synthetic seawater) of greater than 99 % and fluxes of 18 gal/ft2 day at a pressure of 1500 psi. The membranes could also be operated at temperatures above 35 °C, the temperature ceiling for Loeb-Sourirajan cellulose acetate membranes. Today s interfacial composite membranes are significantly better. Typical membranes, tested with 3.5 % sodium chloride solutions,... 

An important problem which arises when quantifying transport processes in membranes is the fact that they typically possess a composite structure. Although one layer is usually the main resistor to transport, effects caused by other layers can generally not be completely neglected (e.g., Ref. [33]). For this reason, integral descriptions of composite membranes should be applied carefully, as they cannot explain observed direction-dependences of fluxes [34]. 

Membranes for pressure and flux testing were mounted, using a brazing process developed at ANL, within four inch long, 0.75 in. O.D., heavy-wall, Iconel 600 tubing which had been machined to form a small seat to accommodate the membrane. Typically, the membrane diameter was 0.69 in. (17.5 mm.). Unmounted membranes of the same composition, as well as membranes of varying Ni content, were also available for characterization studies. Because the pressure tested membranes had to be pre-mounted, the before-and-after characterization studies refer to membranes of the same composition and fabrication, but not the same physical membranes. 

Figure 6. Typical flux behavior with PA-300 polyamide composite membranes. Feed white must previously ultrafiltered through BMR-021006 modulus T = lO C.

Asymmetric/composite membrane This typically consist of a thin (0.5 to 20 microns) fine-pore layer responsible for separation and a support or substrate with single or multiple layers having progressively larger pores which provide the required mechanical strength. This type of structure maximizes the flux by minimizing the overall hydraulic resistance of the permeate (filtrate) flowing across the membrane structure. 

Fabrication of a thin film composite membrane is typically a more expensive route to reverse osmosis membranes because it involves a two-step process versus the one-step nature of the phase inversion film casting method. However, it offers the possibility of each individual layer being tailor-made for maximum performance. The semipermeable coating can be optimized for water flux and solute rejection characteristics. The microporous sublayer can be optimized for porosity, compression resistance and strength. Both layers can be optimized for chemical resistance. In nearly all thin film composite reverse osmosis membranes, the chemical composition of the surface barrier layer is radically different from the chemical composition of the microporous sublayer. This is a common result of the thin film composite approach. 

A typical recipe for an interfacially formed aromatic polyamide composite membrane comprised a 2.0% aqueous solution of the aromatic diamine and a 0.1% nonaqueous solution of trimesoyl chloride. This recipe was extraordinarily simple, and ran quite contrary to experience with piperazine-based membranes. For example, surfactants and acid acceptors in the aromatic diamine solution were generally not beneficial, and in many cases degraded membrane performance by lowering salt rejection. In contrast, surfactants and acid acceptors were almost always beneficial in the NS-300 membrane system. In the nonaqueous phase, use of isophthaloyl chloride as a partial replacement for trimesoyl chloride had relatively little effect on flux, but tended to decrease salt rejection and increase susceptibility to chlorine attack. 

Commercial H2 selective Pd-based membranes are available in the form of relatively thick (20 rm or more) tubes or foils manufactured by cold-working techniques. The H2 flux, being in many cases inversely proportional to the thickness of the membranes, is too low for most applications to give a favorable cost-performance combination. Thus, development of membranes with reduced Pd-alloy layer thickness is necessary. Research in recent years has therefore focused on the development of composite membranes consisting of a thin Pd-based separation layer on a mechanically strong support. The typical state-of-the-art membrane consists of a separation layer of less than about 10 pm thickness on a ceramic or metallic support. Examples of commercial development of composite membranes are given in Section 11.6. 

Homogeneous asymmetric CAs and polyamides made by the phase inversion process and cross-linked TFC polyamides have been the workhorse of RO plants for more than 30 years [21], Both CA and PA membranes possess an economically viable combination of high rejection and water flux [8]. However, TFC membranes now dominate the RO/NF market with CA membranes a distant second. For example, with the exception of Toyobo CTA polymer, all new seawater RO desalination plants deploy interfacial composite membranes of the fuUy aromatic type manufactured by Dow, Hydranautics (Nitto Denko), Tri-Sep and Toray. Along with the ability to remain stable over a greater pH range than cellulose-based membranes, TFC membranes exhibit much higher intrinsic water permeabilities because of their extremely thin ( 100 nm) polyamide-selective layers [21]. A typical spectrum of TFC membranes for various applications is given in Table 1.8. 

Fig. 17.6 (a) Typical SEM cross-sectional image of PVA nanofibrous composite membrane, (b) Relations of permeate flux and solute rejection of the nanofibrous composite membranes with the degree of cross-linking in the PVA hydrogel coating for separation of oil-water emulsion, (c) Schematic illustration of the fabrication process for thin-film nanofibrous composite membianes based on PAN electrospun nanofibrous substrate and cross-linked PVA barrier layer ((a-b) Reprinted with permission from Ref [70]. Copyright 2006, Elsevier, (c) Reprinted with permission from Ref. [74]. Copyright 2010, Elsevier)... 

Figure 3. Membrane fluxes in relation to additive composition at two typical levels of solute rejection...

Yeast rests in fermenting cellars in beer breweries typically have a composition of 90% beer and 10% solids, mainly yeast. The amount of this waste material is 2-3% of the annual output. It can be sold as cattle feed or discharged. In a system with 4 m 0.4 pm ceramic microfiltration membranes, beer recovery amoimts to 42-62% the concentrate contains 23% solid matter [51]. Fluxes in... 

Performance of a specific membrane with a particular feed is typically determined in a laboratory experiment. A heated quantity of feed is run over the membrane in a batch pervaporation test, and samples of feed and permeate are taken periodically and analyzed. Permeate rate is also measured. The temperature of the circulating liquid is thermostatically controlled and the permeate pressure is kept low, perhaps 5-lOmbar. Overall flux rates for permeating and nonpermeating components are determined by mass balance and plotted together with permeate composition against the feed composition. 

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