Nanofiltration Desal

A summary of results obtained in the total circulation experiments with the 13 different nanofiltration membranes is listed in Table 22.2. The purpose of the nanofiltration treatment is to improve the purity of anserine and carnosine contained in the feed solution by removing impurities such as creatinine and sodium chloride into the permeate (Figure 22.8). Therefore, a membrane that shows higher rejection ability against anserine and carnosine and low rejection ability against creatinine and sodium ion is preferred for this purpose. Furthermore, higher permeate flux value implies that a process with the membrane will require smaller membrane area and lower initial cost. Based on these criteria, four membranes (NFT-50, DRA-4510, Desal DK, and Desal DL) were chosen as suitable membranes for purification and concentration of anserine and carnosine from chicken extract. 

GE-Osmonics (part of GE Water Technologies) commercializes the Desal 5 nanofiltration membranes, used for removal of hardness and other contaminants, alcohol recovery from aqueous solution and removal of salt from salt whey. The membrane has 4 layers, a polyester nonwoven, an asymmetric micro-porous polysulfone and two proprietary thin films, which might be based on sulfonated polysulfone and polypiperazineamide [34]. A comparison between DesaF 5 and NF270 for nanofiltration has been reported by [44] (Tab. 4.3). 

Such membranes have been applied in the separation of heavy metals from acids and highly alkaline solutions, and in the recovery of alkaline solutions used in cleaning processes. Desal nanofiltration membranes commercialized by GE Osmonics can also work at very low pH levels. They have been used to recover heavy metals and clarify 35% sulfuric acid feed streams or 25% phosphoric acid streams. They have also been applied to permeate boric acid and reject radionuclides at a nuclear power station. Somicon/Nitto Denko announces nanofiltration membranes (NTR-7410, 7430 and 7450 HG) for pH range 1-12, temperatures up to 90 °C and a maximum pressure of 50 bar [37]. 

Sulfonated polysulfone seems to also play an important role in nanofiltration and reverse osmosis membranes commercialized by Desal. According to Petersen [34] the Desal-5 membrane appears to consist of three layers a microporous polysulfone, a sulfonated overlay and a top ultrathin layer based on polypipera-zineamide. 

One of the new techniques to scan membranes and determine pore size distributions is positron, annihilation spectroscopy (PAS). With this method also the free spaces in nanofiltration membranes can be determined. For instance in a study by Boussu et al. [9] it was found that some much-studied NF membranes (like Desal-5 DL, NTR7450) have two sizes of spaces, one size about 0.12-0.15 nm and the other between 3nm and 4nm. It could be speculated that diffusive transport would happen through the smaller spaces (the size of a water molecule), depending on the hydrophilicity properties of the membrane, and convective transport through the larger spaces, depending on size and charge conditions. 

Figure 32.10 AFM images describing the roughness of nanofiltration membranes (a) NF-PES-10 and (b) Desal 5 DL. The measurements were performed with NC-AFM (Boussu et al., 2005, with permission from Elsevier).

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