G. Glenn Lipscomb

Visualization of concentration fields in hemodialyzers by computed tomography

Abstract We present experimental measurements of concentration fields within a hemodialyzer using X-ray computed tomography (CT). The measured concentrations represent a volume average value for the shell and lumen fluids. Although average values for a cross-section decrease uniformly from lumen inlet to outlet, large variations in concentration occur within a cross-section. We attribute these concentration variations to variations in shell flow; regions of lower shell flow have higher concentrations. The regions of lower shell flow do not always correspond to regions of higher fiber packing. Distribution of fluid within the fiber bundle also is important.

Effect of shell-side flows on the performance of hollow-fiber gas separation modules

Abstract A theoretical analysis of shell-side flow effects on the performance of hollow-fiber gas separation modules is presented. The theory uses Darcy’s law to relate fiber packing, pressure fields, and velocity fields within the shell. The resulting shell conservation equations are coupled to the lumen conservation equations through the permeation relationship. This two-dimensional (2-D) analysis quantifies the performance penalty associated with gas distribution across the fiber bundle at the shell inlet and outlet. Theoretical predictions for the production of nitrogen from air in a commercial shell-fed module are closer to experimental data than predictions obtained assuming one-dimensional (1-D) plug flow. Fluid flows primarily across fibers near the inlet and outlet ports, and along fibers between ports. Nitrogen composition increases along fluid streamlines, which leads to axial and radial concentration variations within the fiber bundle. Diffusional contributions to shell mass transfer are small for the modules considered here.

Well-developed mass transfer in axial flows through randomly packed fiber bundles with constant wall flux

Abstract An analysis of the effects of fiber packing on mass transfer coefficients for axial fluid flow through fiber bundles with uniform wall flux in the well-developed limit is presented. The finite element method is used to solve the governing momentum and conservation of mass equations. The effective mass transfer coefficient depends strongly on fiber packing. Randomly packed fiber bundles have much lower mass transfer coefficients than regularly packed fiber bundles. Mass transfer rates are controlled by the lowest fiber packing regions through which most of the flow occurs.

Mass transfer in axial flows through randomly packed fiber bundles with constant wall concentration

Abstract An analysis of the effects of fiber packing on shell-side mass transfer coefficients for axial fluid flow through fiber bundles with uniform wall concentration is presented. A combined finite element (FE)–finite difference (FD) method is used to solve the governing conservation of momentum and mass equations. Results are given for values of the Graetz number that range from the entry mass transfer limit to the well-developed limit. Effective mass transfer coefficients for randomly packed fiber bundles can be much lower than for regularly packed fiber bundles for a given fiber packing fraction and Graetz number. Regions of higher fiber packing have lower flow and experience greater mass transfer than regions of lower fiber packing. The changes in the low packing regions dominate, because of the higher flow, and lead to a reduction in the overall mass transfer coefficient.

Effect of fiber variation on the performance of countercurrent hollow fiber gas separation modules

Abstract A theoretical and experimental study of the effects of variable fiber properties on countercurrent hollow fiber gas separation module performance is presented. Variations in ID, slow gas permeance, and selectivity are considered. Variability in any of these properties is detrimental to performance. The drop in performance increases as either property variation or product purity increase, but permeate mixing can improve performance. Fibers with higher permeation to feed rate ratios may stop producing a retentate product and may consume product from other fibers. The theory agrees well with experiments for the production of nitrogen from air.

Effect of random fiber packing on the performance of shell-fed hollow-fiber gas separation modules☆

A theoretical analysis of the effect of random fiber packing on shell flow distribution, associated shell-side concentration boundary layers, and ultimate performance of shell-fed hollow-fiber gas separation modules is presented. Specifically the relationship between recovery and product oxygen mole fraction is evaluated for nitrogen production from air. Results show that for commercial hollow-fiber membrane modules, random fiber packing is not detrimental to performance relative to the performance of an ideal module. However, poorer performance is predicted for modules with higher permeances. Permeances will have to increase by over an order of magnitude, though, before the effect becomes significant.

Effect of fiber variation on the performance of cross-flow hollow fiber gas separation modules

A quantitative analysis of the effects of variable fiber properties on the performance of a cross-flow hollow fiber gas separation module is presented. The effects of variations in size, permeance, and selectivity are considered. Fiber variability is detrimental to performance. The recovery and flow rate of an enriched retentate stream decrease as variability increases. Some fibers may actually stop producing product as purity increases. Additionally, performance is poorer if the permeate from all fibers is not well mixed. The results of this work can be used to determine quality control guidelines for fiber manufacture and evaluate process enhancements.

Direct measurement of the carbon dioxide‐induced glass transition depression in a family of substituted polycarbonates

We present a method for the direct measurement of the glass transition temperature of compressed gas–polymer systems. The technique utilizes a Setaram C80D microcalorimeter equipped with high-pressure cells. Pressurizing the cells and running in scanning mode allows direct determination of the glass transition temperature. To validate the method, Tg measurements of the CO2–poly(methyl methacrylate) system as a function of gas phase pressure were made. The results compare favorably with literature values. However, the effects of foaming appear to interfere with Tg measurement at the highest gas pressures. The CO2-induced Tg depression of a series of polycarbonates was also measured. The magnitude of the Tg depression increases with decreasing glass transition temperature, reflecting an increase in intrinsic chain mobility, as evidenced by the glass transition temperature. The data correlate well with the Chow model.

Cleaning membranes with focused ultrasound beams for drinking water treatment

Traditional methods for water treatment are not effective to remove micro pollutants such as harmful organics and cannot meet the demand for high-quality drinking water. Membrane technologies are known to produce drinking water of the highest quality. However, membrane fouling is a significant problem, which limits a widespread use of these technologies. Currently, chemical cleaning is used to control fouling, which interrupts the water production process during cleaning, produces secondary pollutants, shortens membrane life due to chemical erosion, adds costs of cleanup, handling, and transporting dangerous chemicals, and waste energy and the cleaned water. Ultrasound has been demonstrated effective for membrane cleaning and does not have the problems of chemical cleaning. However, current ultrasound methods have high energy consumption, require transducers that can handle high power, and are expensive to clean a large membrane area needed for a typical water treatment plant. In this paper, a focused ultrasound beam is used to create a high intensity at focus to produce cavitations for membrane cleaning. This method may save energy and potentially allow inexpensive low-power transducers such as polymeric transducers to be used. Combined with the beamforming technology that is widely used in medical ultrasound, the focused beams can be swept over a large surface area of membranes for cleaning. An experiment was performed and preliminary results show that the method is promising for membrane cleaning.

Sources of Non‐ideal Flow Distribution and Their Effect on the Performance of Hollow Fiber Gas Separation Modules

Abstract Hollow fiber membrane gas separation modules are used predominantly in membrane gas separation processes. The performance of these modules is often captured by simple flow and mass transfer models. However, as processes are pushed to the limits of their commercial viability, non‐ideal performance is often observed. Potential sources of this non‐ideality due to non‐uniform flows through the module are summarized and their effect on module performance evaluated.