Hollow fiber membrane shear rate

Subramanian et al. (2005) pumped the host cells through the lumen of the hollow fibers at shear rates that lead to cell lysis. The adenoviral vectors that were released were concentrated in the retentate. Since adenoviral vectors are nonenveloped icosahedral particles that are about 80 nm in size, the membrane pore size had to be small enough to reject the virus particles. Membranes with pore sizes of 300 kDa (GE Healthcare, Fairfield, CT), 400 kDa, and 0.05 p.m (Spectrum Laboratories, Rancho Dominguez, CA) were tested and found to give good virus rejection. 

Figure 8.4 shows the data [2] of the UF of serum solutions with a hollow fiber-type ultrafilter, with hollow fibers 16 cm in length and 200 pm in i.d., at four shear rates on the inner surface of the hollow-fiber membrane. Slopes of the straight lines, which converge at a point C = Cq on the abscissa, give values at the shear rates given in the figure. 

The shear rate (s ) at the membrane surface is given by = 9ivld (cf. Example 2.2), where v is the average linear velocity (cm s ), d is the fiber inside diameter (cm), and Dp is the molecular diffusion coefficient (cm s ) all other symbols are as in Equation 8.4. Hence, Equation 8.6 is most likely applicable to UF in hollow-fiber membranes in general. 

Here, /p is the filtrate flux (cm min ) averaged over the hollow fiber membrane of length L (cm) and is the shear rate (s ) on the membrane surface, as in Equation 8.6. The volumetric percentage of red blood cells (the hematocrit) was taken as C, and its value on the membrane surface, Cg, was assumed to be 95%. 

With the increase of the concentration of PPESK in the casting solution, the viscosity strongly increases and becomes shear-rate dependent. Then the morphology of the hollow fiber membranes changes from a finger-like structure to sponge-like structure. 

Table 4.8. Effect of shear rate on roughness of the outer surface of hollow fiber membranes ...

Chung, T. S., J. J. Qin, J. Gu, Effect of Shear Rate Within die Spinneret on Morphology, Separation Performance and Mechanical Properties of Ultrafiltration Polyethersulfone Hollow Fiber Membranes. Chemical Engineering Science, 2000, 55(6), 1077-1091. 

The influence of shear rate on membrane structure is illustrated in Figures 31.4 (Ren et al., 2002) and 31.5 (Chung et al., 2002). Figure 31.4 exhibits the cross-section morphology of hollow-fiber membranes spun at shear rates of 812 and 2436 s. Some macrovoids can be observed near the inner skin of fibers spun at a low shear rate (812 s ), while these macrovoids are apparently eliminated or suppressed when the shear rate increases to 2436 s Clearly, high shear rates modify the precipitation path and retard the formation of macrovoids. In addition, with an increase in shear rate, the membrane structure becomes... 

Cao, C., Chung, T. S., Chen, S. B., and Dong, Z. J. (2004). The study of elongation and shear rates in spinning process and its effect on gas separation performance of poly(ether sulfone) (PES) hollow fiber membranes. Chem. Eng. Sci. 59, 1053. 

Wang, K. Y., Matsuura, T., Chung, T. S., and Guo, W. R. (2004h). The effects of flow angle and shear rate within the spinneret on the separation performance of poly(ethersulfone) (PES) ultrafiltration hollow fiber membranes. J. Membr. Sci. 240, 67. 

It is well known that polymer concentration plays an essential role in membrane formation through the phase inversion process. The effect of polymer concentration on the morphology of PVDF hollow-fiber membranes has been demonstrated in many studies [21,34,37-42]. Generally, a concept of critical polymer concentration is often employed as a guideline for a proper selection of polymer content in dope solutions [4,5]. The critical polymer concentration can be determined from the correlation of viscosity versus polymer concentration at a specific shear rate and temperature. An example of the critical polymer concentration of PVDF/NMP dope solutions is illustrated in Figure 7.5. Depending on the specific application, PVDF dopes with a polymer content below the critical concentration are usually adopted to fabricate microporous PVDF hollow fibers for water-related applications such as MF [43], UF [40], and MD [8,10]. On the other hand, dopes possessing a polymer concentration above the critical value are chosen to fabricate PVDF hollow fibers with a relatively dense selective skin for gas separation and pervaporation [12,13,39]. 

Ralf Kuriyel (Millipore Corporation) addressed some of the issues related to the use of Dean vortices, formed during the flow of fluids in curved conduits, to enhance the performance of cross-flow filters by increasing the back transport of solutes. Results were presented on coiled hollow fibers with a varying radius of curvature, fiber diameter, and solution viscosity, to characterize the relationship between the back transport of solutes and hydrodynamic parameters. A performance parameter relating back transport to the Dean number and shear rate was derived, and a simple scaling methodology was developed in terms of the performance parameter. The use of Dean vortices may result in membrane systems with less fouling and improved performance. 

The alignment of polymer chains in shear flow through the spinneret may explain why hollow fibers generally have lower permeabilities than flat-sheet membranes with the same retention rating. 

Hollow fiber High wall shear rates (4000-14,000 s- ) High surface area-to-volume ratio Low liquid holdup Low energy consumption Narrow channels more prone to blockage Crossflow velocity between 0.5 and 2.5 m/s Reynolds numbers 500-3000 Max. operating pressure limited to approx 2 bar High membrane replacement cost... 

Fig. 4 J3. Top view AFM image of outer surface of hollow fiber UF membrane with shear rate 1305 s . Reprinted from [58]. Copyright 2002, with kind permission from Elsevier...

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