Membrane modules blocked fibers

Figure 4.30 shows a hollow fine fiber membrane module. The fibers are folded in half and the open end of each fiber is "potted" in epoxy "tube sheet," while the folded end is potted in an epoxy, non-porous block. Feed to the module is outside in, which requires less strength on the part of the fiber than inside-out flow would. Also, the pressure drop on the outside of the fibers is much less than would be in the inside of the fiber (which is known as the lumen). 

Hollow fine fiber membranes are extremely fine polymeric tubes 50-200 micrometers in diameter. The selective layer is on the outside surface of the fibers, facing the high-pressure gas. A hollow-fiber membrane module will normally contain tens of thousands of parallel fibers potted at both ends in epoxy tube sheets. Depending on the module design, both tube sheets can be open, or as shown in Figure 8.1, one fiber end can be blocked and one open. The high-pressure feed gas flows past the membrane surface. A portion of the feed gas permeates the membrane and enters the bore of the fiber and is removed from the open end of the tube sheet. Fiber diameters are small because the fibers must support very large pressure differences feed-to-permeate (shell-to-bore). 

Besides non-ideal operation, non-ideal module construction influences the module performance as well. Module performance refers to the recovery, which is the ratio of a product stream to the feed stream of the target compound, for a given product purity. The objective of manufacturing process is to produce uniform, defect free hollow fibers. Unfortunately, real membrane fibers are not uniform since small changes in production conditions as well as manufacturing tolerances result in different fiber properties. Variations in fiber dimensions (diameters, length, and membrane thickness) and fiber properties (permeability) as well as manufacturing defects (blocked fibers, pinholes) have an impact on the module performance. In particular, the variation of fiber diameters, which is one of the major drawbacks in module performance, was well... 

Figure 5.18 Membrane module with blocked fibers at the retentate outlet. The blocked fibers can be assumed as fibers with a recovery of 0.

In order to evaluate if the fibers in a membrane module are blocked or have any defects Feng and Ivory proposed the following procedure. The shell side of the module should be connected to a pure gas supply at a constant pressure where one end of the lumen side is closed (Figure 5.18). Then permeate flow rate measurements are performed. In the second step the first end is closed and the second end should be opened. If all fibers are unblocked then the module performance should not differ in both cases. The whole procedure should be repeated for various pressures in order to obtain a linear dependence of the flow rate from the pressure which indicates that there is no internal leakage caused by defects in the membrane. 

In order to obtain an optimal separation performance various operational as well as design parameters have to be considered carefully. Here the flow through the module, which is usually counter current flow, the location of the feed and active layer are the most important parameters. During module manufacturing various detrimental effects can occur, which reduce the module performance, e.g. variations in fiber dimensions and properties (diameter, length, membrane thickness, permeability) and defects (pinholes, blocked fibers). In order to ensure a proper operation of the membrane module these effects have to be avoided. 

A next-level assay is usually an isolated heart/cardiac tissue preparation. The canine Purkinje fiber assay (GLP) measures several action potential parameters, like resting membrane potential, upstroke velocity, action potential duration and shape, but also if a drug acts reverse-use dependently [72]. Based on changes of the action potential shape it is possible to conclude which ion channels are modulated (e.g., L-type calcium channel block would abolish the plateau phase). The papillary muscle assay (e.g., guinea pigs) determines similar parameters [73]. 

Several classes of polymeric materials are found to perform adequately for blood processing, including cellulose and cellulose esters, polyamides, polysulfone, and some acrylic and polycarbonate copolymers. However, commercial cellulose, used for the first membranes in the late 1940 s, remains the principal material in which hemodialysis membranes are made. Membranes are obtained by casting or spinning a dope mixture of cellulose dissolved in cuprammonium solution or by deacetylating cellulose acetate hollow fibers [121]. However, polycarbonate-polyether (PC-PE) block copolymers, in which the ratio between hydrophobic PC and hydrophilic PE blocks can be varied to modulate the mechanical properties as well as the diffusivity and permeability of the membrane, compete with cellulose in the hemodialysis market. 

KMS—Puron The Koch Membrane Systems (KMS) Puron uses polyether sulfone membranes of 0.05-p,m pore size and fiber diameter of 2.6 mm. A standard modide contains about 30 m. The fibers are vertically aligned and potted only at the base. The top end of each fiber is individually sealed, and this unique arrangement allows a limited range of free movement that avoids blocking of the bundle (see Section 10.5.1.2). The module depicted in Figure 10.5. 

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