Polyamide and Composite Membranes

Polyamide membranes were developed in an effort to improve upon the performance of CA membranes. In particular, the higher operating pressure and relatively low salt rejection of CA membranes were holding back RO technology from becoming more commercially acceptable. 

and competed directly with CA membranes. DuPont withdrew their polyamide membranes from the market in 1991. DuPont sold their B15, A15, and X20 membranes under development to TriSep Corporation, who kept them for a few years before they licensed the FT30 chemistry from FilmTec. 

PA membrane surface Polymeric support Fabric backing 

As discussed above, the charge on the polyamide membrane is negative, thereby attracting cationic polymer should it break through the pretreatment equipment. 

Unlike CA membranes, polyamide membranes cannot tolerate free chlorine or any other oxidizers. Some manufacturers quote 200-1,000 ppm-hrs of exposure until the membrane rejection is lost. This means after 200-1,000 hours of exposure to 1 ppm free chlorine, the membrane rejection will have approximately double the salt passage. Chlorine attack is faster at alkaline pH than at neutral or acidic pH. Polyamide membranes can tolerate higher operating temperatures than CA membranes can. Polyamide membranes 

---

Polyether urea (PEU) is another type of thin-film membrane. This membrane differs from polyamide membranes in the surface charge and morphology. Polyether urea membranes have a slightly positive charge to them. Further, the surface of a PEU membrane is smooth, similar to a CA membrane, thereby minimizing the potential for fouling. Elydranautics CPA membrane is an example of a polyamide/polyurea composite membrane. 

Two polyamide/polysulfone composite membranes, a commercial membrane (C-PA) and an experimental one (PAO) are studied in this Chapter. The polysulfone support of sample C-PA is the porous P-PS membrane previously studied. The roughness of both polyamide membranes is higher than that of other studied membranes, being the experimental composite membrane more than three times rougher than the commercial one (Ra(C-PA) = 25.0 nm and Ra(PAO) = 81.7 nm obtained from 15 jm AFM images), as can be observed in Figme 9 AFM images. 

Acid activated composite membranes were experimentally prepared in the same way than the experimental PAO polyamide/polysulfone composite membrane. Different concentrations of di-(2-ethyl hexyl)dithiophosphoric acid (DTPA) were added to interfacial-polymerization monomer solutions. Molecular structure of this organic acid is shown in Scheme 4. This activating agent is expected to be the carrier for heavy metallic ions, such as thallium, cadmium, zinc or uranium, between the media at both membrane sides [8-9, 63-65]. In this chapter, two activated membranes are studied DT50 and DT200 fabricated from 50 and 200 mM acid solutions, respectively. 

In order to correlate chemieal analysis of aetivated membrane surface with other characteristic parameters, tangential streaming potential measurements at a constant NaCl concentration (5x10 " M) but different pHs were carried out with the DT200 sample and analysed using the local dissociation model [68-69], which allows the determination of the pKa and the munber of acid sites accessible on the membrane surface (N ), and their comparison with the same parameter for PAO polyamide/polysulfone composite membrane (the un-modified base membrane). The obtained values and surface roughness are ... 

RO membranes are divided into three categories, i.e., cellulose acetate based membranes, polyamide based membranes and composite membranes. All of these three kinds of membranes are discussed below. 

Cellulose acetate, the earhest reverse osmosis membrane, is still widely used. Asymmetric polyamide and thin-film composites of polyamide and several other polymers have also made gains in recent years whereas polysulfone is the most practical membrane material in ultrafiltration appHcations. 

NOTE The above applies to composite polyamide and polyvinyl derivative membranes. Do NOT use chlorine, which is suitable only for cellulose acetate membranes. 

A third relevant factor is the chemical composition of the membranes, with cellulose acetate, polyamide and a number of composite membranes sharing the seawater installed capacity. It is my estimate that polyamide membranes have at least 90% of the market. 

Limited testing on chlorine sensitivity of poly(ether/amidel and poly(ether/urea) thin film composite membranes have been reported by Fluid Systems Division of UOP [4]. Poly(ether/amide] membrane (PA-300] exposed to 1 ppm chlorine in feedwater for 24 hours showed a significant decline in salt rejection. Additional experiments at Fluid Systems were directed toward improvement of membrane resistance to chlorine. Different amide polymers and fabrication techniques were attempted but these variations had little effect on chlorine resistance [5]. Chlorine sensitivity of polyamide membranes was also demonstrated by Spatz and Fried-lander [3]. It is generally concluded that polyamide type membranes deteriorate rapidly when exposed to low chlorine concentrations in water solution. 

By contrast, membranes U-1, A-2 and X-2 are all chlorine sensitive, each responding in a unique manner. U-1 is a thin film composite membrane, the active layer consisting of cross-linked poly(ether/urea) polymer. A-2 is a homogeneous aromatic polyamide containing certain polyelectrolyte groups. X-2 is a thin film composite membrane of proprietary composition. 

Fig. 1. Water flux and NaCl rejection of several membrane types (10), where (D) represents seawater membranes, which operate at 5.5 MPa and 25°C ( ), brackish water membranes, which operate at 1500 mg/L NaCl feed, 1.5 MPa, and 25°C and (SSI) nanofiltration membranes, which operate at 500 mg/L NaCl feed, 0.74 MPa, and 25°C. A represents cellulose acetate—cellulose triacetate B, linear aromatic polyamide C, cross-linked polyether D, cross-linked fully aromatic polyamide E, other thin-film composite membranes F, asymmetric membranes G, BW-30 (FilmTec) H, SU-700 (Toray) I, A-15 (Du Pont) J, NTR-739HF (Nitto-Denko) K, NTR-729HF (Nitto-Denko) L, NTR-7250 (Nitto-Denko) M, NF40 (FilmTec) N, NF40HF (FilmTec) O, UTC-40HF (Toray) P, NF70 (FilmTec) Q, UTC-60 (Toray) R, UTC-20HF (Toray) and S, NF50 (FilmTec). To convert MPa to psi,...

Nanofiltration membranes usually have good rejections of organic compounds having molecular weights above 200—500 (114,115). NF provides the possibility of selective separation of certain organics from concentrated monovalent salt solutions such as NaCl. The most important nanofiltration membranes are composite membranes made by interfacial polymerization. Polyamides made from piperazine and aromatic acyl chlorides are examples of widely used nanofiltration membrane. Nanofiltration has been used in several commercial applications, among which are demineralization, oiganic removal, heavy-metal removal, and color removal (116). 

Kawaguchi et al.105) in Teijin Ltd. prepared a similar polyamide composite membrane from piperazine, trimesoyl chloride, and isophthaloyl chloride on a polysulfone support. The membrane exhibited high chlorine-resistance and excellent pressure-resistance. When used for reverse osmosis of an aqueous solution of 0.5% NaCl and NaOCl (available Cl 4 5 ppm) at pH 6.5 7.0, 25 °C, and 42,5 kg/cm2, the water permeation was 1400 and 13301/m2 - day and desalination was 93.4% and 95.7% after 2 and 100 hr, respectively. 

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. 

P. Eriksson, Water and Salt Transport Through Two Types of Polyamide Composite Membranes, J. Membr. Sci. 36, 297 (1988). 

Salts rejected by the membrane stay in the concentrating stream but are continuously disposed from the membrane module by fresh feed to maintain the separation. Continuous removal of the permeate product enables the production of freshwater. RO membrane-building materials are usually polymers, such as cellulose acetates, polyamides or polyimides. The membranes are semipermeable, made of thin 30-200 nanometer thick layers adhering to a thicker porous support layer. Several types exist, such as symmetric, asymmetric, and thin-film composite membranes, depending on the membrane structure. They are usually built as envelopes made of pairs of long sheets separated by spacers, and are spirally wound around the product tube. In some cases, tubular, capillary, and even hollow-fiber membranes are used. 

Reverse osmosis membranes can be divided into subclasses according to their solute/water selectivity and operating pressure regimes. Figure 30 shows a number of commercial membranes developed for seawater and brackish desalination, and for nanofiltration. These include cellulose ester and polyamide asymmetric membranes available since the 1960s, and high-performance composite membranes developed in the 1970s. Collectively, they make it possible to produce potable water from virtually all saline water sources. 

The membranes under study are thin-film composite membranes composed of two layers as illustrated in Fig. 3 a thin polyamide film as active layer and a large mesoporous polysulphone as the support layer. The three studied membranes are 2 NF membranes, noted NF90, NF270 and a low-polarization reverse osmosis (LPRO) membrane, noted BW30. All membranes were purchased from Filmtec (DOW, USA) the specifications of the membranes are given in Table 2. The chemical structures of the support and active layer materials are reported in Fig. 4 [86], Polyamide material is the more used but some authors have reported results... 

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... 

Since the late 1970 s, researchers in the US, Japan, Korea, and other locations have been making an effort to develop chlorine-tolerant RO membranes that exhibit high flux and high rejection. Most work, such as that by Riley and Ridgway et.al., focuses on modifications in the preparation of polyamide composite membranes (see Chapter 4.2.2).11 Other work by Freeman (University of Texas at Austin) and others involves the development of chlorine-tolerant membrane materials other than polyamide. To date, no chlorine-resistant polyamide composite membranes are commercially available for large-scale application. 

Rejection is a property of the specific feed water component and the membrane of interest. Table 3.2 lists the general rejection ability of the most common polyamide composite RO membranes. Note that ionic charge of the component of interest plays a role its rejection by an RO membrane the rejection of multi-valent ions is generally greater than for mono-valent ions. 

---