Membrane hollow-fiber

Hollow fiber spinning is usually based on the dry-wet phase inversion process that involves the following four steps  

For polymeric hollow fiber membranes, spinning parameters are crucial factors that must be controlled during the preparation of membranes. These parameters include the amount and type of polymers, solvents, additives mixed into the spinning dope solution, the dope and bore fluid rate, the kind of bore fluid, the fiber take-up velocity, the air-gap distance (unless wet spinning is used), and the coagulant bath temperature and the kind of coagulant bath. 

Hollow fiber membranes are treated in this section with the discussion of porous materials. 

Hollow fiber membranes are generally solvent spun and as such can be considered a sub-group of solvent spinning. Major polymers used in these processes are polysulfone, Polyvinylidene diflouride(PVDF) and polyether-sulfone. The major applications are hemodialysis, water purification, and air separation. This is a huge business that has been growing at over 10% per annum for many years. Although the product usage in annual tons is very small, the end-use revenue is in billions (Anon., 2011). 

An example of a PE hollow fiber membrane was prepared for SEM by fracturing in liquid nitrogen to show the bulk cross sectional structure (Fig. 5.53A), and a cold razor blade was used to fracture the fiber for the longitudinal view (Fig. 5.53B). Combination of both views shows the dimensions and the porous structure. 

Hollow liber membranes are numerous small hollow fibers with semi-permeable walls, and are assembled within a cylindrical shell/jacket to function as a bioreactor. One of the clinical applications of hoUow fiber bioreactors is the hemodialyzer. These hollow fiber membranes are produced by solution-based processing method by solvent phase separation. This process has been used to produce filtration membranes in the past [11], and is now being used to produce tissue engineering scaffolds [12-14]. 

In most cases, hoUow fibers are used as cylindrical membranes that permit selective exchange of materials across their waUs. However, they can also be used as containers to effect the controUed release of a specific material (2), or as reactors to chemically modify a permeate as it diffuses through a chemically activated hoUow-fiber waU, eg, loaded with immobilized enzyme (see Enzyme applications). 

HoUow-fiber membranes, therefore, may be divided into two categories (/) open hoUow fibers (Eigs. 2a and 2b) where a gas or Hquid permeates across the fiber waU, while flow of the lumen medium gas or Hquid is not restricted, and (2) loaded fibers (Eig. 2c) where the lumen is flUed with an immobilized soHd, Hquid, or gas. The open hoUow fiber has two basic geometries the first is a loop of fiber or a closed bundle contained ia a pressurized vessel. Gas or Hquid passes through the smaU diameter fiber waU and exits via the open fiber ends. In the second type, fibers are open at both ends. The feed fluid can be circulated on the inside or outside of the relatively large diameter fibers. These so-caUed large capiUary (spaghetti) fibers are used in microfUtration, ultrafUtration (qv), pervaporation, and some low pressure ( 1035 kPa = 10 atm) gas appHcations. 

(a) Thick-waUed hoUow fiber for high pressure desalination (b) thin-waUed acryHc hoUow fiber (c) sorbent-tiUed fiber. Courtesy of 1. Cabasso. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

In open fibers the fiber wall may be a permselective membrane, and uses include dialysis, ultrafiltration, reverse osmosis, Dorman exchange (dialysis), osmotic pumping, pervaporation, gaseous separation, and stream filtration. Alternatively, the fiber wall may act as a catalytic reactor and immobilization of catalyst and enzyme in the wall entity may occur. Loaded fibers are used as sorbents, and in ion exchange and controlled release. Special uses of hoUow fibers include tissue-culture growth, heat exchangers, and others. 

Hollow fibers have been used since the 1960s in many applications such as reverse osmosis, ultrafiltration, membrane gas separation, artificial organs, and other medical purposes. There are several advantages to hollow fibers over the flat sheet membranes the most important is their high surface-to-volume ratio. The use of hollow fibers has become popular in many industrial sectors since Mahon first patented the hollow fiber membranes [56]. The morphology and performance of hollow fibers are complex functions of many parameters involved in their manufacturing. McKelvey summarized the effect of spinning parameters on the macroscopic dimensions of hollow fibers [57]. 

Earher attempts in the study of hollow fiber morphology are based on cross-sectional pictmes taken by SEM, by which the asynmietric structure of the fiber membranes was clearly seen. In contrast to the SEM, morphological studies of hollow fibers by AFM are mostly based on the image of the fiber surface, either on the inside or the outside. A cross-sectional pictme has seldom been taken. 

Chimg et al. demonstrated the effect of the shear stress working from the spinneret wall to the outermost surface of the spinning dope [58]. The hollow fibers were spun with no air gap so that the surface morphology could be frozen in the coagulation bath immediately after the fibers extruded from the spinneret. Then, the AFM image of the outer surface of polysulfone (PSf) hollow fibers was obtained. 

Fiber ID Shear rate (s Ba (nm) ) Bq (nm) Bz (nm) Dimension of nodules in x-direction (nm) Dimension of nodules (fiber) in extrusion direction (nm) 

Nodular abrogates are assembled to a number of string-like structures and aligned in one row. The similarity between Fig. 4.36a and b most likely means that the alignment of nodular aggregates under the shear force is completed when the air 

Amphiphilic Pluronic triblock copolymers of two blocks of poly-(ethylene oxide) (PEO) and poly(propylene oxide) in between have worth as both the surface modifier and pore former in the fabrication of membranes (77). The effect of Pluronics with different molecular architectures and contents as a pore forming additive for the fabrication of poly(ethersulfone) ultrafiltration hollow fibers has been investigated. 

The spim hoUow fibers were characterized with regard to cross-sectional membrane morphology, membrane surface chemistry, mechanical properties, water permeation, molecular weight cut-off, and pore size distribution. It was observed that the water permeation and molecular weight cut-off of the as-spim hoUow fibers are dependent on the structure of the additives. 

Among all the membranes spun with 5% additives, the hollow fibers spun using Pluronic F127 and F108 as the additives possess the highest water permeation, the lowest molecular weight cut-off, and the narrowest pore size distribution. 

It is suspected that the PEO brush layer formed on the internal pore surface by Pluronic F127 and F108 might reduce the appar- 

A comparison between Pluronic and PEG as additives confirmed the importance of the presence of poly(propylene oxide) chain in Pluronic in the formation of high performance membranes. When Pluronic F127 concentration was 10%, the as-spun hollow fiber exhibited the highest water permeation of 113.8 lm h bar and the lowest molecular weight cut-off of 9,000 Dalton (77). 

32 A three dimensional model of Celgard 2500 (trademark of Celanese Corp., New York) is shown composed of sections cut along, across and in the plane of the machine direction. The surface is shown by an SEI micrograph. (From Sarada et al. [131] reproduced by permission.) 

---

Hollow-fiber membranes may be run with shell-side or tube-side feed, cocurrent, countercurrent or in the case of shell-side feed and two end permeate collection, co- and countercurrent. Not shown is the scheme for feed inside the fiber, common practice in lower-pressure separations such as air. 

Nonselective membranes can assist enantioselective processes, providing essential nonchiral separation characteristics and thus making a chiral separation based on enantioselectivity outside the membrane technically and economically feasible. For this purpose several configurations can be applied (i) liquid-liquid extraction based on hollow-fiber membrane fractionation (ii) liquid- membrane fractionation and (iii) micellar-enhanced ultrafiltration (MEUF). 

Fig. 5-10. Schematic representation of hollow-fiber membrane extraction.

Fig. 5-12. Separation of d,1-leucine in hollow-fiber membrane extraction using a Al- -dodecyl-l-hydrox-yproline solution in octanol as the enantioselective extraction liquid. The modules used were 32 cm long and contained 96 Celgard X-20 polypropylene fibers [57].

These gas transfer membranes or membrane contactors employ microporous polypropylene hollow fiber membranes arranged in a modular design. Oxygenated water flows on the shell side of the hollow fibers, and a strip gas (such as nitrogen) or a vacuum is applied to the inside (lumenside), with the hollow fibers acting as a support medium for intimate contact between the water and gas phases. 

In the present study, we fabricated hollow fiber membrane modules and performed experiments at several conditions. The energy consumption of this process is compared to those of conventional gas absorption processes and membrane gas separation processes. 

Nonbiological methods for removal of trichloroethylene from water are also being studied. These include the use of a hollow fiber membrane contactor (Dr. A.K. Zander, Clarkson University), photocatalysis by solar or artificially irradiated semiconductor powders (Dr. G. Cooper, Photo-catalytics, Inc.), and micellar-enhanced ultrafiltration (Dr. B.L. Roberts, Surfactant Associates, Inc.). 

A bench top polysulfone hollow fiber membrane (0.0325m ) with molecular weight cutoff (MWCO) of 30K (A/G Technology Corp., Needham MA) was used (24). UF was run in a total recycle mode at a rate of 1.2 L/min (flow speed of 0.73 m/sec), cross membrane pressure of 25 PSIG and 10 + 1°C. PE permeability is expressed as the fraction of PEU/mL in the permeate to PEU/mL in the retentate. Data presented are representative of at least duplicate replications. 

Cross-flow ultrafdtration equipment.—The device used is shown in Figure 1. It included a glass reactor (R) with temperature, pH and stirring control, a Minitan pump (P) (Millipore, Bedford, USA), a Harp hollow fiber membrane cartridge (M) (Romicon-Supelco, Bellefonte, USA) with a cut-off of 2000 daltons, and a permeate exit (f) for fraction collection. The retentate (r) was returned to the reactor. 

Hollow-fiber membrane reactor Hydrolysis of sunflower oil Lipase from Rhizopus sp. 122... 

Membranes offer a format for interaction of an analyte with a stationary phase alternative to the familiar column. For certain kinds of separations, particularly preparative separations involving strong adsorption, the membrane format is extremely useful. A 5 x 4 mm hollow-fiber membrane layered with the protein bovine serum albumin was used for the chiral separation of the amino acid tryptophan, with a separation factor of up to 6.6.62 Diethey-laminoethyl-derivatized membrane disks were used for high-speed ion exchange separations of oligonucleotides.63 Sulfonated membranes were used for peptide separations, and reversed-phase separations of peptides, steroids, and aromatic hydrocarbons were accomplished on C18-derivatized membranes. 

Nakamura, M., Kiyohara, S., Saito, K. Sugita, K., and Sugo, T., High resolution of DL-tryptophan at high flow rates using a bovine serum albumin-multilayered porous hollow-fiber membrane, Anal. Chem., 71, 1323, 1999. 

---