Hollow-fiber system

Membrane extraetion ean easily be set up in flow systems and ean be eonneeted in an automated way on-line to various analytieal instmments. Also, offline eheap hollow-fiber systems are available. High degrees of elean up and enriehment ai e obtained, whieh is very important for reliable and sensitive ehemieal analysis. Reeent applieations of automated detenuination of PCB and phenols will be presented. 

Membrane systems consist of membrane elements or modules. For potable water treatment, NF and RO membrane modules are commonly fabricated in a spiral configuration. An important consideration of spiral elements is the design of the feed spacer, which promotes turbulence to reduce fouling. MF and UF membranes often use a hollow fiber geometry. This geometry does not require extensive pretreatment because the fibers can be periodically backwashed. Flow in these hollow fiber systems can be either from the inner lumen of the membrane fiber to the outside (inside-out flow) or from the outside to the inside of the fibers (outside-in flow). Tubular NF membranes are now just entering the marketplace. 

The main disadvantage of all these systems is the Hmitation of scale-up. Monoclonal antibodies are produced by multiplying the hollow fiber systems and stirred tank reactors with membrane aeration are known up to 100 liter. Small quantities of product can be produced by these systems but they are not suitable for real industrial scale-up. 

Takeuchi, K., Koike, K. and Ito, S. (1990) Production of ris-unsaturated hydrocarbons by a strain of Rhodococcus in repeated batch culture with a phase-inversion, hollow fiber system. Journal of Biotechnology, 14, 179-186. 

The hollow-fiber systems for gas separation or the tubular microfiltration systems have to be pyrolyzed before mounting in the membrane housing, because of the large shrinkage during pyrolysis. That is the most critical step in the fabrication of a separation system. 

Ultrafiltration hollow-fiber modules are usually made with a shell and tube configuration. The fibers are potted at both ends of the module with the fiber lumen open for recirculation of the process stream (Figure 21). Naturally, strainers or prefilters must be utilized to eliminate plugging of the fibers. At Nude-pore, it has been shown that larger diameter hollow fibers, 1.5 to 3mm in i.d., are much less prone to fouling. Fortunately, all UF hollow fiber systems can be back-washed and are amenable to a number of cleaning techniques. 

Kolf s first tubular dialyzer, shown in Figure 12.2, required several liters of blood to prime the system, a major operational problem. In the 1950s, tubular dialyzers were replaced with coil (spiral) devices, also developed by Kolf and coworkers. This coil system was the basis for the first disposable dialyzer produced commercially in the early 1960s. The blood volume required to prime the device was still excessive, however, and during the 1960s the plate-and-frame and hollow fiber devices shown in Figure 12.3 were developed. In the US in 1975, about 65 % of all dialyzers were coil, 20 % hollow fiber systems and 15 % plate-and-frame. Within 10 years the coil dialyzer had essentially disappeared, and the market was divided two-thirds hollow fibers and one-third plate-and-frame. By 1996, hollow fiber dialyzers had more than 95 % of the market. 

In gas permeation, a gas species is separated based mainly on its permeability in hollow fiber and spiral wound membranes. The hollow fiber systems can have an inside diameter up to 200 xm and hence very large surface-to-volume ratios, but high pressure drops inside the tubes. The basic flow equation for a species / is... 

In hollow fiber systems, the shells are 0.10 toU20m in diameter and 3 to 6 m long. The inside and outside ameters of each of the diffuser elements range from 0.4 to 0.8 mm. In sheet systems, which normally have six membrane windings in series alternating with flow systems, the shells are approximately 10,20 and 25 cm in diameter. 

Several delivery systems have been described in which the primary role of the polymer is to serve as a convenient carrier for the agent rather then serve any specific rate controlling function. Ashare et al, have described a hollow fiber system in whidi the agent is contained within the bore and release is effected by its evaporation and subsequent diffusion through the air above it within the bore. One of its intriguing uses is for release of pheromones, i.e. sex attractants, for control of insects. An alternate concept is release of such agents held within the pore of a suitably constructed open celled polymeric foarn. ... 

Examples of the model considerations, presented below, can be regarded as simplified examples only. The processes have been studied using analytical [1] and numerical [27] solutions for both the local steady-state [1,2,69,70] and dynamic nonstationary [27,71,72] conditions, respectively. Both models, HLM [1] and MHS [27], with some modifications may be used for theoretical analysis of all OHLM systems. Considerations for hollow-fiber systems are presented also in a short. 

Due to the increased application of MoAbs in diagnostics and therapeutics, considerable effort has been made to develop technology for the large-scale production of MoAbs. Examples of the currently employed culture systems are hollow-fiber systems, suspensions, solid-phase cell immobilization, perfusion reactor, and encapsulation... 

The greater success with the Dorr Oliver system is probably due to the cellulosic nature of these membranes, as the largomycin F-II supernatant did contain polypropylene-derived antifoaming agents which are known to suppress flux rates across polysulfone membranes. The flux rates for all sheet systems were superior to those observed with the hollow fiber system. 

In hollow fiber systems (Figure 22.49c), a bundle of hollow fibers are contained within a shell and the suspension flows inside the hollow fiber and the permeate is collected in the shell side. Hollow fibers are best suitable for applications without solid particles. In addition to the above three types, there are also rotating discs, annual gap, and vibrating disc systems for membrane cross-flow filtration. 

Thymosin fraction 5 (TF5) is a partially purified mixture of polypeptides prepared from calf thymus glands as starting material (Hooper et al., 1975). The crude thymus extract is purified by a heat step, acetone precipitation, and fractionation with ammonium sulfate. The 25%-50% ammonium sulfate precipitate is further subjected to ultrafiltration using an Amicon DC-2 hollow fiber system to yield fraction 5 that is lyophilized. Fraction 5 consists of 10 major and at least 30 minor polypeptides on analytical isoelectric gel focusing (Fig. 8), with molecular weights ranging from 1000 to 15,000, and is... 

Researchers at Oregon State University have demonstrated the advantages of microchannel architecture in improving the hemodialysis process. Using microchannel architectare, they were able to show 70-80% reductions in the necessary transfer area relative to commercial hollow fiber systems for the clearance of creatinin (Fig. 7.23) and urea (Fig. 7.24) from a simulated blood stream [285]. The microchannel advantage, as has been seen in other applications, comes in the form of well-defined and narrow channels that facilitate rapid mass transfer into and out of the fluid media. This approach is expected to change the current paradigm in hemodialysis from clinical treatment to at-home use, and may allow for the creation of a wearable hemodialyzer [286]. 

For reaching the same oxygen production, single-hole tubes of 10 mm diameter would increase the amount of units to 41, comparable to multi-channel monoliths (39 units). The extreme restriction in membrane length of hollow fiber systems would result in 1,800 modules. The difference in expansion behavior of vessel wall and membrane tubes can be compensated by mounting the tubes in a... 

A major problem associated with a cofactor-requiring enzyme in a hollow fiber system is retention of the relatively small cofactor within the dlalysate side of the hollow fiber dlalyzer... 

Glucuronides have been synthesized batch-wise or in a hollow fiber system using microsomal or soluble enzyme preparations (5-5). Furthermore, they have been prepared with enzymes immobilized to polymeric supports (6). Here we describe the continuous synthesis of glucuronide conjugates in a 10-mL membrane reactor (7). 

A new development in this field is the use of fluidized-bed systems instead of a packed bed. For this purpose, steam reforming of methane has been used as a model reaction [88]. From experimental and theoretical work it can be concluded that fluidized-bed membrane reactors potentially represent a promising system as problems of heat transfer and equilibrium limitations can be addressed simultaneously. As one of the major problems encountered is to provide sufficient membrane area per volume, possible solutions are the use of hollow-fiber systems [13] or membranes based on microsystem technology. In Fig. 5.7 an indication can be obtained for the potential of this approach to enlarge the effective membrane area versus the superficial area of the wafers used [89]. 

The effect of the wetting characteristics of the membrane in the hydrolytic activity of immobilized lipase was evaluated by Bouwer and co-workers [113]. These authors suggested that the larger thickness of the reaction layer in hydrophilic membranes, as compared to hydrophobic membranes, allowed the retention of the full enzyme activity. A lipase adsorbed onto polypropylene membranes in a hollow-fiber system was used for the selective hydrolysis of menhaden oil [112]. The experimental set-up led to the release of about 88% of the fatty acid residues and retention of more than 90% of the aimed eicosapen-taenoic and docosahexaenoic acids in a space-time of 3.5 h, while the half-life of the enzyme, under ideal operational conditions was 170 h. The enzymatic synthesis of an aspartame precursor in an organic-aqueous membrane-assisted two-liquid phase system has been continuously studied by Isono and co-workers [121, 149,156,201]. Productivity was increased from 6.6 to 8.4kgm d [121]. 

Gas permeation systems typically use hollow-fiber or spiral-wound membranes, although hollow-fiber systems are more common tBaker. 2004k Cellulose acetate membranes are used for carbon dioxide recovery, polysulfone coated with silicone rubber is used for hydrogen purification, and conposite membranes are used for air separation. The feed gas is forced into the membrane module under pressure. Retentate, which does not go through the membrane, will become concentrated in the less permeable gas. Retentate exits at a pressure that will be close to the input pressure. The more permeable species will be concentrated in permeate. Permeate, which has passed through the membrane, exits at low pressure. The operating cost for a gas permeator is the cost of conpression of the feed gas and the irreversible pressure difference that occurs for the gas that permeates the membrane. A typical hollow-fiber unit will contain 5000 m membrane area per m at a cost of approximately 200/m. ... 

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