Processes Loeb-Sourirajan

The seminal discovery that transformed membrane separation from a laboratory to an industrial process was the development, in the early 1960s, of the Loeb-Sourirajan process for making defect-free, high flux, asymmetric reverse osmosis membranes (5). These membranes consist of an ultrathin, selective surface film on a microporous support, which provides the mechanical strength. The flux of the first Loeb-Sourirajan reverse osmosis membrane was 10 times higher than that of any membrane then avaUable and made reverse osmosis practical. The work of Loeb and Sourirajan, and the timely infusion of large sums of research doUars from the U.S. Department of Interior, Office of Saline Water (OSW), resulted in the commercialization of reverse osmosis (qv) and was a primary factor in the development of ultrafiltration (qv) and microfiltration. The development of electro dialysis was also aided by OSW funding. 

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. 

Membranes made by the Loeb-Sourirajan process consist of a single membrane material, but the porosity and pore size change in different layers of the membrane. Anisotropic membranes made by other techniques and used on a large scale often consist of layers of different materials which serve different functions. Important examples are membranes made by the interfacial polymerization process discovered by Cadotte [15] and the solution-coating processes developed by Ward [16], Francis [17] and Riley [18], The following sections cover four types of anisotropic membranes ... 

Water precipitation (the Loeb-Sourirajan process) The cast polymer solution is immersed in a nonsolvent bath (typically water). Absorption of water and loss of solvent cause the film to rapidly precipitate from the top surface down... 

Polymer Precipitation by Water (the Loeb-Sourirajan Process)... 

The technology to fabricate ultrathin high-performance membranes into high-surface-area membrane modules has steadily improved during the modem membrane era. As a result the inflation-adjusted cost of membrane separation processes has decreased dramatically over the years. The first anisotropic membranes made by Loeb-Sourirajan processes had an effective thickness of 0.2-0.4 xm. Currently, various techniques are used to produce commercial membranes with a thickness of 0.1 i m or less. The permeability and selectivity of membrane materials have also increased two to three fold during the same period. As a result, today s membranes have 5 to 10 times the flux and better selectivity than membranes available 30 years ago. These trends are continuing. Membranes with an effective thickness of less than 0.05 xm have been made in the laboratory using advanced composite membrane preparation techniques or surface treatment methods. 

Most of today s ultrafiltration membranes are made by variations of the Loeb-Sourirajan process. A limited number of materials are used, primarily polyacrylonitrile, poly(vinyl chloride)-polyacrylonitrile copolymers, polysulfone, poly(ether sulfone), poly(vinylidene fluoride), some aromatic polyamides, and cellulose acetate. In general, the more hydrophilic membranes are more fouling-resistant than the completely hydrophobic materials. For this reason water-soluble... 

The Loeb-Sourirajan process often is referred to as diffusion induced phase separation (DIPS) to reflect the role of diffusion in forming the asymmetric structure. Liquid-liquid phase separation and the resulting asymmetric structure arise from diffusion of a solvent (acetone) out of the film and diffusion of a nonsolvent (water) into the film. This physical interpretation provided the basis for the development of asymmetric membrane manufacturing processes for other polymer - solvent - non-solvent systems. 

The seminal discovery that transformed membrane separation fi-om a laboratory to an industrial process was the development in the 1960s of the Loeb-Sourirajan process to make defect-free ultrathin cellulose acetate membranes [1]. Loeb and Sourirajan were trying to use membranes to desalt water by reverse osmosis (RO). The concept of using a membrane permeable to water and impermeable to salt to remove salt from water had been known for a long time, but the fluxes of aU the membranes then available were far too low for a practical process. The Loeb-Sourirajan breakthrough was the development of an anisotropic membrane. The membrane consisted of a thin, dense polymer skin 0.2-0.5 pm thick sup-... 

The development of the sol-gel process in the beginning of the eighties can be considered as the breakthrough in inorganic membranes comparable to the Loeb-Sourirajan process... 

Despite a long history, it was not until 150 years later that gas separations using polymer membranes became a reality in the gas separation industry. Until the 1970s, the dense polymer membranes available were too thick to obtain the high permeation flux required for practical applications. The development of the Loeb-Sourirajan process for making defect-free, high-flux, ultrathin asymmetric membranes for RO was a milestone in membrane history (Loeb and Sourirajan, 1963). Since that time, many polymer membranes based upon asymmetric membranes and their modules have been developed and evaluated in pilot-scale for MF, UF, and RO applications (Baker, 2000). 

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