Cellulose acetate membrane performance

Two common types of membrane materials used are cellulose acetate and aromatic polyamide membranes. Cellulose acetate membrane performance is particularly susceptible to annealing temperature, with lower flux and higher rejection rates at higher temperatures. Such membranes are prone to hydrolysis at extreme pH, are subject to compaction at operating pressures, and are sensitive to free chlorine above 1.0 ppm. These membranes generally have a useful life of 2 to 3 years. Aromatic polyamide membranes are prone to compaction. These fibers are more resistant to hydrolysis than are cellulose acetate membranes. 

E. W. Funk, S. S. Kulkami, A. X. Swamikannu, Effect of impurities on cellulose acetate membrane performance, in Recent Advances in Separation Techniques - III, 82, AIChE Symposium Series No. 250 (1986). 

PERFORMANCE OF HOMOGENEOUS AND ASYMMETRIC CELLULOSE ACETATE MEMBRANES... 

This presentation will discuss the membrane performance and its physical and chemical changes under unfavourable conditions. This kind of studies will give us information on trouble-shooting counter-measures for unexpected membrane deteriorations, and on the durability of a cellulose acetate membrane under adverse conditions. 

Because such behaviour would affect the performance of cellulose acetate membranes in desalination, it was studied twelve years ago by one of the authors using methods similar to those described here. The results were not widely publicized at that time (3). The work has now been repeated in greater detail and the earlier findings confirmed. This new work is reported here. 

Chlorine is the oldest and most widespread method of water disinfection. In reverse osmosis systems, chlorine may be added to feedwater for control of micro-organisms and, in addition, to prevent membrane fouling by microbiological growth. According to Vos et al. [i,2], chlorine will attack cellulose diacetate membranes at concentrations above 50 ppm. Membranes were found to show a sharp increase in salt permeability and a decrease in strength after one week of continuous exposure. Under milder conditions (10 ppm chlorine for 15 days) no detectable change in performance was observed. Spatz and Friedlander [3] have also found cellulose acetate membranes to be resistant to chlorine when exposed to 1.5 ppm for three weeks. 

Initially all membranes were exposed to 3 ppm chlorine in buffer solutions at pH levels of 3.0, 5.8, and 8.6 for three weeks. Both cellulose acetate type membranes C-2 and V-1 were unaffected by chlorine under these conditions. Continued exposure at higher chlorine levels did not alter baseline membrane performance. For example, membrane C-2 exposed to 125 ppm chlorine for 10 days at pH 3 continued to perform at baseline levels. In subsequent work, cellulose acetate membranes were also found to be unresponsive to bromine, iodine, and chlorine dioxide. It can be generally concluded that cellulose acetate type membranes are halogen resistant. 

Figure 7. Comparative performances of PA-, MPC-, and F-based cellulose acetate membranes at medium and high operating pressures...

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... 

Cellulose acetate membrane was studied because of its past use in concentrate preparation and the need to better define its performance for specific organic recovery. Cellulose acetate continues to be widely used for a variety of industrial and commercial water purification applications. Cellulose acetate was not expected to perform at the level of the more highly cross-linked and inert thin-film composite membrane. 

As expected, compounds demonstrating consistent negative rejections with cellulose acetate membranes (dichlorophenol, biphenyl, furfural, chloroform) were not recovered to any extent. Compounds with the best rejections (>90%) were the better recovered substances. The FT-30 composite membrane clearly demonstrated superior performance to the cellulose acetate membrane for organic rejection, concentration, and recovery. Sodium chloride rejection was no indicator of potential organic rejection. 

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. 

Cellulose acetate was the first high-performance reverse osmosis membrane material discovered. The flux and rejection of cellulose acetate membranes have now been surpassed by interfacial composite membranes. However, cellulose acetate membranes still maintain a small fraction of the market because they are easy to make, mechanically tough, and resistant to degradation by chlorine and other oxidants, a problem with interfacial composite membranes. Cellulose acetate membranes can tolerate up to 1 ppm chlorine, so chlorination can be used to sterilize the feed water, a major advantage with feed streams having significant bacterial loading. 

FIGURE 20.12 Effect of crossflow velocity on the 6 h permeate flux in CMF of beer performed at various TMP values, a process temperature of 0°C, and cellulose acetate membranes of 0.45 p,m pore size. (From Moram, C.I., Optimization and membrane processes with applications in the food industry Beer microfiltration. PhD thesis. University Dunarea de Jos Galati, Romania, 1999.)... 

Lajimi, R.H. et al., Change of the performance properties of nanofiltration cellulose acetate membranes by surface adsorption of polyelectrolyte multilayers, Desalination, 163, 193, 2004. 

Cellulose acetate is an ester of cellulose and acetic acid. Hence, hydrolysis takes place when the pH of the solution with which a cellulose acetate membrane is in contact is too high or too low, lowering the degree of acetylation, defined as the number of hydroxyl groups (total of three in one D-glucopyranose unit) that can be acetylated. Because a degree of acetylation above 2.5 is required for satisfactory salt rejection in seawater desalination, excessive hydrolysis results in poor membrane performance. The pH values between 5 and 7 should be maintained when cellulose acetate membranes are used. ... 

In pilot plant experiments we have used a 7 m DDS-module type 40 with tight cellulose acetate membranes type DDS-990. Concentration has been performed at pH = 4.0-4.5 in a batch system at ambient temperature using 30 kp/m2 delivered by a Rannie piston pump. 

Reverse osmosis separations of 12 alkali meteil halides in methanol solutions have been studied using cellulose acetate membranes of different surface porosities. Data for surface excess free energy parameters for the ions and ion pairs Involved have been generated for the above mend>rane material-solution systems. These data offer a means of predicting the performance of cellulose acetate membranes in the reverse osmosis treatment of methanol solutions involving the above ions from only a single set of experimental data. 

For HF cellulose acetate membranes are used setting limits for temperature, 85°F, as well as for pH, 2.5 to 8.0. Still with those limitations it is possible to keep the systems at an accept ably high performance level for extensive periods of time, when operating on spent sulphite liquor. 

Both noncellulosic membranes performed as well as the cellulose acetate membrane, yielding solute rejections greater than 95%. 

Experiments at 25 °C were performed to determine salt (NaCl) rejection of a cellulose acetate membrane. 

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. 

A detailed study of SPSF desalination membranes was carried out by Brousse and coworkers43. Sulfonation was effected by chlorosulfonic acid on a commercial material (Polysulfone P 1700, Union Carbide), and the products as well as their sodium salts were cast from highly polar solvents. Their performance was compared to that of noncharged cellulose-acetate membranes, largely being used for desalination of brackish water. 

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