Loeb-Sourirajan technique

Phase separation membranes. This category includes membranes made by the Loeb-Sourirajan technique involving precipitation of a casting solution by immersion in a nonsolvent (water) bath. Also covered are a variety of related techniques such as precipitation by solvent evaporation, precipitation by absorption of water from the vapor phase, and precipitation by cooling. 

The Loeb-Sourirajan technique is now recognized as a special case of a more general class of membrane preparation process, best called the phase separation... 

The first phase separation membrane was developed at UCLA from 1958 to 1960 by Sidney Loeb, then working on his Master s degree, and Srinivasa Sourirajan, then a post-doctoral researcher. In their process, now called the Loeb-Sourirajan technique, precipitation is induced by immersing the cast film of polymer solution... 

Since the discovery of the Loeb-Sourirajan technique in the 1960s, development of the technology has proceeded on two fronts. Industrial users of the technology have generally taken an empirical approach, making improvements in the technique based on trial and error experience. Concurrently, theories of membrane formation based on fundamental studies of the precipitation process have been developed. These theories originated with the early industrial developers of membranes at Amicon [19,22,24] and were then taken up at a number of academic centers. Unfortunately, much of the recent academic work is so complex that many industrial producers of phase separation membranes no longer follow this literature. 

The cellulose acetate membranes used were batch 316(0/25) -type membranes (24) made by the general Loeb-Sourirajan technique... 

Cellulose acetate Loeb-Sourirajan reverse osmosis membranes were introduced commercially in the 1960s. Since then, many other polymers have been made into asymmetric membranes in attempts to improve membrane properties. In the reverse osmosis area, these attempts have had limited success, the only significant example being Du Font s polyamide membrane. For gas separation and ultrafUtration, a number of membranes with useful properties have been made. However, the early work on asymmetric membranes has spawned numerous other techniques in which a microporous membrane is used as a support to carry another thin, dense separating layer. 

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 ... 

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. 

Figure 8.8 The technique devised by Henis and Tripodi [23] to seal defects in their selective polysulfone Loeb-Sourirajan membrane...

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 first breakthrough came in 1959 when Sourirajan and Loeb discovered a method to make a very thin cellulose acetate (CA) membrane using the phase inversion method [4]. This technique produces homogenous membranes with an asymmetric (or anisotropic) structure. The membranes were subsequently found to be skinned when examined under an electron microscope by Riley in 1964 [3]. The membranes consisted of a very thin, porous salt-rejecting barrier of CA, integrally supported by a fine CA porous substrate. Pictures of asymmetric membranes are shown in Figures 1.2 and 1.3. These early Loeb-Sourirajan (L-S) membranes exhibited water fluxes that were lOtimes higher than those observed by Reid, and with comparable salt rejection [5]. The membrane flux was 8—18 1/m /h (knh) with 0.05% NaCl product water from a 5.25% NaCl feedwater... 

The importance of the thickness of the membrane is evident from Equation (1.1) since flux is inversely proportional to thickness. The Loeb—Sourirajan RO membranes produced by the phase-inversion technique have an effective skin thickness of 0.1-0.2 pm that makes it possible to achieve acceptable fluxes (2—20 l/m" /h) at reasonable feed pressures (30—60 bar g) for water desalination. UF membranes with a similar but less tight skin structure (pore size 1-50 nm vs. 0.6 nm for RO membranes) have fluxes on the order of 15—1501/m /h (hnh) at feed pressures of 1—5 bar g. The ability to minimise thickness without introducing defects relies upon controlling the membrane morphology during fabrication. 

There are four main types of polymeric membranes (a) Loeb—Sourirajan phase separation RO, UF and MF membranes, (b) interfacial composite RO and NF membranes, (c) solution-coated composite GS membranes, and (d) other anisotropic membranes such as plasma polymerisation coated. Several methods of manufacturing synthetic membranes are given in Table 1.5. Each method produces different membrane morphology porosity, pore size distribution, and ultrastructure. Membrane formulation techniques are discussed in detail in several texts [8, 16—18]. 

Hollow-fiber fabrication methods can be divided into two classes (62,63). The most common is solution spinning, in which a 20-30% polymer solution is extruded and precipitated into a bath of a nonsolvent, generally water. Solution spinning allows fibers with the asymmetric Loeb-Sourirajan structure to be made. An alternative technique is melt spinning, in which a hot polymer melt is extruded from an appropriate die and is then cooled and solidified in air or a quench tank. Melt-spun fibers are usually relatively dense and have lower fluxes than solution-spim fibers, but, because the fiber can be stretched after it leaves the die, very fine fibers can be made. Melt spinning can also be used with polymers such as poly(trimethylpentene), which are not soluble in convenient solvents and are difficult to form by wet spinning. 

The development of asymmetric membrane technology in the 1960 s was a critical point in the history of gas separations. These asymmetric structures consist of a thin (0.1 utol n) dense skin supported on a coarse open-cell foam stmcture. A mmetric membranes composed of the polyimides discussed above can provide extremely high fluxes throuj the thin dense skin, and still possess the inherently hij separation factors of the basic glassy polymers from which they are made. In the early 1960 s, Loeb and Sourirajan described techniques for producing asymmetric cellulose acetate membranes suitable for separation operations. The processes involved in membrane formation are complex. It is believed that the thin dense skin forms at the... 

Phase inversion is a process in which a polymer is transformed from a liquid to a solid state. There are a number of methods to achieve phase inversion. Among others, the dry-wet phase inversion technique and the temperature induced phase separation (TIPS) are most commonly used in the industrial membrane manufacturing. The dry-wet phase inversion technique was applied by Loeb and Sourirajan in their development... 

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