Hollow fiber membrane aromatic polyamide

The membranes currently used for seawater desalination are aromatic polyamide membrane (hollow fiber, Du Pont), cellulose acetate membranes (hollow fiber, Toyobo), thin-flim composite membranes (spiral-wound, UOP). thin-film composite membranes (spiral-wound. Dow), and thln-fllm composite membranes (spiral-wound. Toray). 

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... 

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. 

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. 

Parallel to the development of sheet membranes based on CA and CTA, the development of hollow fine fibers spun from nylon was made. With the development by Du Pont of the B-9 and B-10 perme-ators based on aromatic polyamides, the hollow fine fibers were a realistic alternative to CA membranes. 

Another important membrane useful for seawater, wastewater, nickel-plating rinse solutions, and other solutes is the synthetic aromatic polyamide membrane Permasep, made in the form of very fine hollow fibers (LI, P3). This type of membrane used industrially withstands continued operation at pH values of 10 to 11 (S4). Many other anisotropic membranes have also been synthesized of synthetic polymers, some of which can be used in organic solvents, at higher temperatures, and at high or low pH (M2, Rl). 

Aromatic polyamide (aramid) membranes are a copolymer of 1-3 diaminobenzene with 1-3 and 1-4 benzenedicarboxylic acid chlorides. They are usually made into fine hollow fibers, 93 [Lm outer diameter by 43 [Lm inner diameter. Some flat sheet is made for spirals. These membranes are widely used for seawater desalination and to some extent for other process applications. The hollow fibers are capable of veiy high-pressure operation and have considerably greater hydrolytic resistance than does CA. Their packing density in hoUow-fiber form makes them veiy susceptible to colloidal fouling (a permeator 8 inches in diameter contains 3 M fibers), and they have essentially no resistance to chlorine. 

As Figure 5.12 shows, Toray s PEC-1000 crosslinked furfuryl alcohol membrane has by far the best sodium chloride rejection combined with good fluxes. This explains the sustained interest in this membrane despite its extreme sensitivity to dissolved chlorine and oxygen in the feed water. Hollow fine fiber membranes made from cellulose triacetate by Toyobo or aromatic polyamides by Permasep (Du Pont) are also comfortably in the one-stage seawater desalination performance range, but the water fluxes of these membranes are low. However, because large-surface-area, hollow fine fiber reverse osmosis modules can be... 

Hollow fine fiber modules made from cellulose triacetate or aromatic polyamides were produced in the past for seawater desalination. These modules incorporated the membrane around a central tube, and feed solution flowed rapidly outward to the shell. Because the fibers were extremely tightly packed inside the pressure vessel, flow of the feed solution was quite slow. As much as 40-50 % of the feed could be removed as permeate in a single pass through the module. However, the low flow and many constrictions meant that extremely good pretreatment of the feed solution was required to prevent membrane fouling from scale or particulates. A schematic illustration of such a hollow fiber module is shown in Figure 3.47. 

In 1971, E. I. Du Pont De Nemours Company, Inc. (DuPont) patented a linear aromatic polyamide with pendant sulfonic acid groups, which they commercialized as the Permasep B-9 and B-10 membranes (Permasep is a registered trademark of DuPont Company, Inc. Wilmington, DE). These membranes exhibited higher water flux at slightly lower operating pressures than cellulose acetate membranes. The membranes were cast as unique hollow fine fibers rather than in flat sheets or a tubes (see Chapter 4.3.4). 

Explorations with homogeneous membranes quickly showed that the flux-selectivity requirements for water desalination membranes would demand more than a simple melt-spun hollow fiber. In fact, it has been necessary to work out structure-property relationships on all levels of structure to bring RO membrane technology involving aromatic polyamides to its current status. 

A membrane cell recycle reactor with continuous ethanol extraction by dibutyl phthalate increased the productivity fourfold with increased conversion of glucose from 45 to 91%.249 The ethanol was then removed from the dibutyl phthalate with water. It would be better to do this second step with a membrane. In another process, microencapsulated yeast converted glucose to ethanol, which was removed by an oleic acid phase containing a lipase that formed ethyl oleate.250 This could be used as biodiesel fuel. Continuous ultrafiltration has been used to separate the propionic acid produced from glycerol by a Propionibacterium.251 Whey proteins have been hydrolyzed enzymatically and continuously in an ultrafiltration reactor, with improved yields, productivity, and elimination of peptide coproducts.252 Continuous hydrolysis of a starch slurry has been carried out with a-amylase immobilized in a hollow fiber reactor.253 Oils have been hydrolyzed by a lipase immobilized on an aromatic polyamide ultrafiltration membrane with continuous separation of one product through the membrane to shift the equilibrium toward the desired products.254 Such a process could supplant the current energy-intensive industrial one that takes 3-24 h at 150-260X. Lipases have also been used to prepare esters. A lipase-surfactant complex in hexane was used to prepare a wax ester found in whale oil, by the esterification of 1 hexadecanol with palmitic acid in a membrane reactor.255 After 1 h, the yield was 96%. The current industrial process runs at 250°C for up to 20 h. 

During the 1960 s, the DuPont Company screened numerous polymers to determine the suitability of materials other than cellulose acetate for use in reverse osmosis desalination. The results of this work indicated that aromatic polyamides were the "choice as the best polymer type for use in the DuPont commercial permeators".7 The company was most successful in developing an asymmetric aromatic polyamide reverse osmosis membrane in a hollow fine fiber configuration which successfully competed with cellulose acetate in the market place. 

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. 

To overcome the problems of cellulose acetate membranes, many synthetic polymeric materials for reverse osmosis were proposed, but except for one material, none of them proved successful. The only one material, which could remain on the market, was the linear aromatic polyamide with pendant sulfonic acid groups, as shown in Figure 1.2. This material was proposed by DuPont, which fabricated very fine hollow fiber membranes the modules of this membrane were designated B-9 and B-10. They have a high rejection performance, which can be used for single-stage seawater desalination. They were widely used for mainly seawater or brackish water desalination and recovery of valuable materials such as electric deposition paints, until DuPont withdrew them from the market in 2001. 

The separation of water from soluble dissolved species in water using membrane processes is often referred to as reverse osmosis, as pressiue must be applied to overcome the osmotic pressiue. This is particularly relevant for water desalination but is also important for itrilk and whey concentration, fruit juice concentration, maple syrup concentration, and suaose and dextrose concentration. The membranes employed for these processes are similar, and the membrane module con-straction is usually in the spiral woimd configuration. Early desalination membranes were based on cellulose acetate as an asymmetric membrane produced by the phase inversion process. Another early reverse osmosis membrane involved extraded thin hollow fibers of polyamides (both aliphatic and aromatic). The rntetfadal polymerization of polyamides on a pororrs polysirlfone support developed in the 1970s (FihnTec) became one of the preferred membranes and... 

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