Membranes asymmetric cellulose acetate

Reverse Osmosis. This was the first membrane-based separation process to be commercialized on a significant scale. The breakthrough discovery that made reverse osmosis (qv) possible was the development of the Loeb-Sourirajan asymmetric cellulose acetate membrane. This membrane made desalination by reverse osmosis practical within a few years commercial plants were installed. The total worldwide market for reverse osmosis membrane modules is about 200 million /yr, spHt approximately between 25% hoUow-ftber and 75% spiral-wound modules. The general trend of the industry is toward spiral-wound modules for this appHcation, and the market share of the hoUow-ftber products is gradually falling (72). 

Astroquartz, fiber reinforcement for ceramic- matrix composite, 5 558t Asymmetric allylboration, 13 669-671 Asymmetric cellulose acetate membranes, 21 633... 

PERFORMANCE OF HOMOGENEOUS AND ASYMMETRIC CELLULOSE ACETATE MEMBRANES... 

In 1968 we started investigations of RO applications for desalting brackish water. In the course of the investigations, we have found the spirally wound module of asymmetric cellulose acetate RO membrane shows excellent durabilities against fouling materials and free chlorine. 

Deterioration of Asymmetric Cellulose Acetate Membranes with NaOCl -------Structural and Chemical Change... 

Analysis of the Mechanisum of Deterlorationof Asymmetric Cellulose Acetate Membrane by Sodium Hypochlorite... 

The development of the Loeb-Sourlrajan asymmetric cellulose acetate membrane (1) has been followed by numerous attempts to obtain a similar membrane configuration from virtually any available polymer. The presumably simplistic structure of this cellulose acetate membrane - a dense, ultrathln skin resting on a porous structure - has been investigated by transmission and scanning electron microscopy since the 1960s (2,3). The discovery of macrovoids ( ), a nodular intermediate layer, and a bottom skin have contributed to the question of the mechanism by which a polymer solution is coagulated to yield an asymmetric membrane. 

The successful development of asymmetric cellulose acetate membranes by Loeb and Sourirajan in the early sixties, at the University of California, Los Angeles, has been primarily responsible for the rapid development of Reverse Osmosis (RO) technology for brack sh/sea water desalination. Reverse Osmosis approaches a reversible process when the pressure barely exceeds the osmotic pressure and hence the energy costs are quite low. Theenergy requirement to purify one litre of water by RO is only O.OO3 KW as against 0,7 KV required just to supply the vaporisation energy to change the phase of one litre of water from liquid to vapour by evaporation. Thus RO has an inherent capability to convert brackish water to potable water at economic cost and thus contribute effectively to the health and prosperity of all humanity. 

The origin of thin-film-composite reverse osmosis membranes began with a newly formed research institute and one of its first employees, Peter S. Francis. North Star Research and Development Institute was formed in Minneapolis during 1963 to fill a need for a nonprofit contract research institute in the Upper Midwest. Francis was given the mission of developing the chemistry division through support, in part, by federal research contracts. At this time the Initial discoveries by Reid and Breton ( ) on the desalination capability of dense cellulose acetate membranes and by Loeb and Sourlrajan (,2) on asymmetric cellulose acetate membranes had recently been published. Francis speculated that improved membrane performance could be achieved, if the ultrathin, dense barrier layer and the porous substructure of the asymmetric... 

In 1966, Cadotte developed a method for casting mlcroporous support film from polysulfone, polycarbonate, and polyphenylene oxide plastics ( ). Of these, polysulfone (Union Carbide Corporation, Udel P-3500) proved to have the best combination of compaction resistance and surface microporosity. Use of the mlcroporous sheet as a support for ultrathin cellulose acetate membranes produced fluxes of 10 to 15 gfd, an increase of about five-fold over that of the original mlcroporous asymmetric cellulose acetate support. Since that time, mlcroporous polysulfone has been widely adopted as the material of choice for the support film in composite membranes, while finding use itself in many ultrafiltration processes. 

A great deal has been written about the mechanism involved in the formation of asymmetric cellulose acetate membranes (29). 

In an early application, an enzyme electrode system was reported for the determination of creatinine and creatine, using a combination of creatinine amidohy-drolase, creatine amidinohydrolase and sarcosine oxidase, co-immobilized on an asymmetric cellulose acetate membrane. Thus, the hydrogen peroxide produced was detected to give a quantitative measure of creatine and creatinine in biological fluids [70]. 

Asymmetric cellulose acetate membranes were developed in the early 1960s by Loeb and Sourirajan (2). For more than a decade, cellulose acetate and its blends were the only commercially available RO membranes. Improved membranes (with respect to operating pH, biodegradation, compaction, and organic compound rejection) were developed in the early 1970s (3). These membranes used aromatic... 

Since Loeb and Sourirajan 8) found how to cast asymmetric cellulose acetate membranes, which consist of a very thin surface layer, supported by a more porous thick layer in 1962, many workers have investigated the preparation and performance of cellulose acetate membranes. 

Asymmetric cellulose acetate membrane developed Loeb and Sourirajan -1962... 

A similar application is the processing of fuel gas, whose major components are hydrogen (about 80%) and methane (about 20%). Asymmetric cellulose acetate membranes have been used successfully to extract the more valuable hydrogen at high purity. New membrane materials more resistant to harsh conditions will accelerate the application of other H2 recovery schemes for... 

A decade after Dr. Hassler s efforts, Sidney Loeb and Srinivasa Sourirajan at UCLA attempted an approach to osmosis and reverse osmosis that differed from that of Dr. Hassler. Their approach consisted of pressurizing a solution directly against a flat, plastic film.3 Their work led to the development of the first asymmetric cellulose acetate membrane in 1960 (see Chapter 4.2.1).2 This membrane made RO a commercial viability due to the significantly... 

E. 1960 - Loeb and Sourirajan develop asymmetric cellulose acetate membrane at UCLA... 

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... 

H. Bokhorst, F. W. Altena and C. A. Smolders, Formation of asymmetric cellulose acetate membranes, Proc. Inst. Cong. Desalination and Water Re-use, Manama (1981), pp. 349-360. 

Schulz and Asumnaa (48), based on their SEM observation, assumed that the selective layer of an asymmetric cellulose acetate membrane for reverse osmosis consists of closely packed spherical nodules with a diameter of 18.8 nm. Water flows through the void spaces between the nodules. Calculate the water flux by Eq. (30) assuming circular pores, the cross-sectional area of which is equal to the area of the triangular void surrounded by three circles with a diameter of 18.8 nm (as shown in Eig. 8). 

Figure 4 is a micrograph of the skin structure of an asymmetric polyamide and Figure 5 is a micrograph of the skin structure of an asymmetric cellulose acetate membrane. 

Hon-celluloslc Membranes. Despite an Intensive search for more favorable membrane polymers, cellulose acetate remained the best material for reverse osmosis until 1969 when the first B-9 permeator for brackish water desalination was Introduced by Du Font. Richter and Hoehn ( ) Invented aromatic polyamide asymmetric hollow-fiber... 

These types of cellulose acetate composite membranes are of historical interest only. During the period when this research was done the composite membranes made using very thin cellulose acetate barrier layers (under 100 nm) looked attractive for their high flux properties. However, later optimization of the asymmetric cellulose acetate membrane process improved flux and, in general, outdistanced composite CA types for practical, low cost membrane manufacture. 

Table III illustrated this phenomenon, wherein a single test specimen (made with the piperazine trimesamide homopolymer) was sequentially exposed to feed solutions of sodium chloride, magnesium chloride, sodium sulfate and magnesium sulfate. The chloride salts were both poorly retained while retention of the sulfate salts was excellent. Thus, salt retention in the carboxylate-rich NS-300 membrane was controlled by the anion size and charge. This membrane could not distinguish between the univalent sodium ion and the divalent magnesium ion, which is the opposite of the behavior observed for asymmetric cellulose acetate membranes. Salt passage through the NS-300 membrane may be described as anion-controlled.

In order to exclude disturbing substances, Newman (1976), Tsuchida and Yoda (1981), and Palleschi et al. (1986) covered the platinum electrode by a H2O2 selective asymmetric cellulose acetate membrane. The membrane (thickness, 15.3 pm) was prepared from acetyl cellulose... 

In the lactate analyzer HER-100 (Omron Tateisi, Japan) an asymmetric cellulose acetate membrane is used, bearing LOD covalently bound by -y-aminopropyl triethoxysilane and crosslinked by glutaralde-hyde (Tsuchida et al., 1985). The membrane is highly selective for hydrogen peroxide. The analyzer has been employed for lactate assay in... 

FIGURE 20.5-1 Demonstration of typical permeability creep behavjor observed for asymmetric membranes. Both cellulose acetate and polysulfone display this behavior, which is believed to he related to deasification of the dense separating layer or compaction of the support foum structure shown in Fig. 20. J-... 

The superior flux and rejection capabilities of the thin film composite membrane has been demonstrated at the municipal wastewater reclamation facility of the Orange County Water District in California. Both asymmetric cellulose acetate and thin film composite membranes were tested on lime clarified secondary effluent. The pilot plants were operated at 85% recovery and the rejections reported in Table 4.5 are the percent rejection of the constituents in the feed-water and not the rejection of the average concentration of the specific constituents in the feed/reject stream. Use of the average concentration would give a higher rejection in both cases. 

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. 

During the period of 1965 to 1972, the best data on flux and salt rejection for cellulose acetate membranes were exhibited by the composite membranes. However, these membranes never reached commercial viability efforts on them died out completely by 1975. Reasons for this appear to be threefold. First, composite cellulose acetate membranes were technically difficult to scale up. Second, the advent of noncellulosic composite membranes in 1972 (the NS-100 membrane) offered much more promise for high performance (salt rejection and water flux), especially for seawater desalination. Third, continual improvements in asymmetric cellulose acetate membrane casting technology (such as the development of swelling agents and of blend membranes) brought the performance of asymmetric membranes to full equality with composite cellulose acetate membranes. 

---