Ultrathin 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. 

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. 

The first composite reverse osmosis membrane to be developed and described consisted of an ultrathin film of secondary cellulose acetate deposited onto a porous Loeb-Sourirajan membrane.3 The ultrathin film of cellulose acetate was fabricated by a water surface float-casting technique. This has been described to some extent in the published technical literature,4 5 and in considerable detail in several reports on government-funded research projects.3 6 Figure 5.2 illustrates this process schematically. 

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 adaptation of the principles used in the development of the membranes for the desalination of water via reverse osmosis was all that was needed to develop membranes for gas separation. It was realized that gas separation through dense polymer films was not practicable (due to the low gas flux) unless ultrathin dense films could be obtained. A thin film ensures a high flux even if the intrinsic permeability of the polymer is low. In 1970, using Loeb Sourirajan CA membranes which were dried, Merten and Gantzel obtained a separation factor of approximately 40 for He/N2, with high fluxes equivalent to those of a silicone rubber film 10 pm thick, but which would have a corresponding separation factor of only 1.5. 

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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