Hollow fiber technology properties

Technological complexity of ELM and problems with the stability of SLM has led researchers in the recent years to look for alternative BLM or organic (water-immiscible) LM. A big number of OHLM (or BLM) separation systems are described in the literature. They can be divided according to type of membrane walls (barriers) used planar, spiral wound and hollow fiber, and to hydrophobic/hydrophilic properties of the membrane. The hydrophobic membranes are immobilized by LM organic solutions and hydrophilic or ion-exchange membranes are immobilized by aqueous feed and strip solutions. 

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. 

One unique application area for PSF is in membrane separation uses. Asymmetric PSF membranes are used in ultrafiltration, reverse osmosis, and ambulatory hemodialysis (artificial kidney) units. Gas-separation membrane technology was developed in the 1970s based on polysulfone hollow-fiber membranes. The PRISM (Monsanto) gas-separation system based on this concept has been a significant breakthrough in gas-separation technology (see Membrane Technology). Additional details are available on the use of polysulfone in membrane separations (45), as well as gas transport properties of polysnlfone and polyethersulfone (46-50). 

Membranes can be thought of as special types of films that provide specific end use characteristics. Membrane technology has replaced some conventional techniques for separation, concentration or purification [78]. Applications include desalination, dialysis, blood oxygenators, controlled release drug delivery systems and gas separation. Processing of polymer films and membranes is well known to affect the morphology, which in turn affects the physical and mechanical properties. As is true for all films, membrane separation properties are based on both the chemical composition and the structure resulting from the process. Membranes are produced in two major forms, as flat films and as porous hollow fibers, both of which will be discussed in this section. 

The technology of membrane separations is a new and growing field where the polymer membrane contributes unique separation properties based on its structure and, to some extent, on its chemical composition. Various manufacturing processes are used to create special structures in forms such as flat films and hollow fibers. Lonsdale [143] provides a review of the history and current status of separation media and their applications, and a text [144] provides a discussion of the materials science of synthetic membranes. 

Polymer matrix selection determines minimum membrane performance while molecular sieve addition can only improve membrane selectivity in the absence of defects. Intrinsically, the matrix polymer selected must provide industrially acceptable performance. For example, a mixed matrix membrane using silicone rubber could exhibit properties similar to intrinsic silicone rubber properties, O2 permeability of 933 Baiters and O2/N2 permselectivity of 2.1 (8). The resulting mixed matrix membrane properties would lie substantially below the upper boimd trade-off curve for gas permeability and selectivity. In contrast, a polymer exhibiting economically acceptable permeability and selectivity is a likely candidate for a successful polymer matrix. A glassy polymer such as Matrimid polyimide (PI) is an example of such a material because it exhibits acceptable properties and current technology exists for formation of asymmetric hollow fibers for gas separation (10). 

There is also an interest in application of synthetic fibers. Two directions are common surface modification and development of fibers with special morphology. The controlled composition of synthetic fibers gives opportunities to regulate their surface properties to meet specific requirements giving the product formulator new tools to make product improvement. Synthetic fibers can be produced in variety of shapes and sizes which can be tailored to specific applications in new products. Ultra small fibers, some hollow, with a wide variety of surface morphologies can be produced economically to meet specific requirements of a wide variety of high technology products. 

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