Membranes aromatic polyamides

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. 

Polybenzimidazolone membrane 21 developed by Teijin Ltd. had the following permeative characteristics Water permeation, 840 1/m2 - day salt rejection, 99.5% (1% NaCl aqueous solution, 80 kg/cm2)69). The membrane was less sensitive to plasticization with water than cellulose acetate and aromatic polyamide membranes... 

Permeability of aromatic polyamide membranes have been improved by modification of aromatic rings with pendant polar groups, for examples sulfonic, carboxylic, carboxamide, and sulfonamide groups, in addition to the before-mentioned methoxy group. 

Nonetheless a few commercially successful noncellulosic membrane materials were developed. Polyamide membranes in particular were developed by several groups. Aliphatic polyamides have low rejections and modest fluxes, but aromatic polyamide membranes were successfully developed by Toray [25], Chemstrad (Monsanto) [26] and Permasep (Du Pont) [27], all in hollow fiber form. These membranes have good seawater salt rejections of up to 99.5 %, but the fluxes are low, in the 1 to 3 gal/ft2 day range. The Permasep membrane, in hollow fine fiber form to overcome the low water permeability problems, was produced under the names B-10 and B-15 for seawater desalination plants until the year 2000. The structure of the Permasep B-15 polymer is shown in Figure 5.7. Polyamide membranes, like interfacial composite membranes, are susceptible to degradation by chlorine because of their amide bonds. 

R. McKinney and J.H. Rhodes, Aromatic Polyamide Membranes for Reverse Osmosis Separations, Macromolecules 4, 633 (1971). 

K. 1971 - Richter-Hoehn at DuPont patents aromatic polyamide membrane (see Chapter 4.2.2)... 

Cellulose acetate and linear aromatic polyamide membranes were the industry standard until 1972, when John Cadotte, then at North Star Research, prepared the first interfacial composite polyamide membrane.8 This new membrane exhibited both higher throughput and rejection of solutes at lower operating pressure than the here-to-date cellulose acetate and linear aromatic polyamide membranes. Later, Cadotte developed a fully aromatic interfacial composite membrane based on the reaction of phenylene diamine and trimesoyl chloride. This membrane became the new industry standard and is known today as FT30, and it is the basis for the majority... 

Aromatic polyamide membranes were developed by a few companies, including Toray, Monsanto and DuPont. DuPont developed a linear aromatic polyamide (nylon) membrane with pendant sulfonic acid groups, which they commercialized as the Permasep B-9 and B-10 membranes and is shown in Figure 4.7 (Permasep is a registered trademark of E. I. Du Pont De Nemours Company, Inc. Wilmington, DE). Just as CA membranes were created out of... 

Figure 4.7 Aromatic polyamide membrane developed by DuPont.

As discussed in Chapter 4.2.2, DuPont introduced linear aromatic polyamide membranes in hollow fine fiber form as the B-9 (brackish water) and B-10 (seawater) Permeators. These Permeators were available in 4-, 8- and 10-inch diameter models. The 4-, 8-, and 10-inch B-9 Permeators were capable of producing 4,200, 16,000, and 25,000 gallon per day of permeate, respectively, at 75% recovery (standard test conditions 1,500 ppm NaCl at 400 psig and 25°C).28 Permeators ranged from about 47 inches to 53 inches in length. DuPont discontinued these modules in 2001. 

Chemical modification of a membrane surface can be used in combination with spacers and periodic applications of bioacids [70]. The paper by Redondo, however, is short on specifics (e.g., details of chemical modification of aromatic polyamides membrane surface), and therefore not very useful to those looking for insights into membrane fouling. 

Aromatic polyamide, another polymeric material used for seawater desalination, can tolerate a wider pH range from 5 to 9. However, aromatic polyamide membranes are known to be susceptible to chlorination in the presence of chlorine in water. 

Hoehn, H.H. Aromatic polyamide membrane. In Materials Science of Synthetic Membranes, ACS Symposium Series 26 Lloyd, D.R., Ed. American Chemical Society Washington, DC, 1985 81-98. 

For a variety of reasons, a comprehensive review of the history, current developments and recent results in Du Pont research on aromatic polyamide membranes is not possible. In fact, this paper will be limited to Du Pont research and limited further to some key polyamides and derivatives that illustrate the relationship between structure-level and membrane properties. 

In discussing the architecture and properties of aromatic polyamide membranes, it is convenient to refer to four levels of structure. Broadly speaking, these levels of structure are useful for understanding the properties of any synthetic membrane, irrespective of what type of polymer is used to make the membrane or whether the membrane is intended for RO, gas separation or ultrafiltration. The levels of structure as used in this paper are defined in Table II. 

In order to achieve the desired flux levels in membranes, it is generally necessary to reduce the thickness of the diffusion layer to low levels. One way to do this is to cast the membrane with asynmetric morphology. This was done with the aromatic polyamide membranes intended for water desalination. 

In addition to DMAC, the solvents DMF, DMSO and UMP can be used for casting aromatic polyamide membranes. Lyotropic salts that give good polyamide membranes are those with lithium, calcium and magnesium as the cation and chloride, bromide, iodide, nitrate, thiocyanate and perchlorate as the anion. 

The nature of asymmetry in aromatic polyamide membranes has by Panar, Hoehn and Hebert (18). Electron freeze-cleaved aromatic polyamide membranes show considerable substructure as part of Structure Level III character. Details of the morphology are apparent in the various electron micrographs shown in this paper. 

The role of phase inversion processes in the production of micro-porous aromatic polyamide membranes is discussed by Strathmann (25). Acknowledgments... 

T. Matsuura. Sourirajan s group at the Divison of Chemistry, National Research Council of Canada have published extensively on their studies with aromatic polyamide membranes (26-46), Others that the author wishes to acknowledge for their citations and discussions of material presented in this paper are V. T. Stannett and H. B. Hopfenberg at North Carolina State University W. J. Koros, D. R. Paul and D. R. Lloyd at the University of Texas at Austin J. M. S. Henis at Monsanto, and H. R. Lonsdale at Bend Research (47). 

These organic rejections, while greatly exceeding the capabilities of cellulose acetate membranes, are not appreciably different than rejection levels exhibited by aromatic polyamide membranes (FT-30, Permasep B-9). Nor do they match the organic rejection characteristics of the PEC-1000 membrane. The Solrox membrane is not resistant to chlorine, and its water flux is somewhat low (about 25 gfd at 650 psi net driving pressure). Consequently, it has not become a significant competitive membrane in the world marketplace. 

In reverse osmosis membranes, we tried to introduce high amide linkage into polyamide membrane to realize better salt rejection and better water flux. Consequently, crosslinked fully aromatic polyamide membrane from 1,3,5-Triaminobenzene has found to have excellent separation performance and durability. Moreover, based on "UTC-70", fully aromatic polyamide membrane from 1,3,5-Triaminobenzene commercialized by Toray, various types of membrane have developed to satisfy different requirements in wide ranges of application. In such membranes, controlling membrane performance is accomplished through composition of membrane materials, control of polycondensation reaction, physical treatment and chemical treatment, which are closely related to chemical and physical structures of membranes. 

The incorporation of positive charges has decreased the fouling susceptibility of membranes even more effectively. This is the principle of the aromatic polyamide membrane series commercialized by Hydranautics as low fouling composite membranes (LFC). Cationic charge-modified nylon membranes are also commercially available from CUNO 3M, under the trademark Zeta Plus . Pall Corp. sells cationic charge-modified nylon membranes under the trademark Ngg Posidyne. There are different ways to make the membrane positively charged. A patent from Millipore [148] describes the surface modification of hydrophobic membranes by contacting them with a solution of polyamine epichlorohydrin... 

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