Tracy Q. Gardner

Analysis of transient permeation fluxes into and out of membranes for adsorption measurements

Abstract A relationship between the integrated fluxes into and out of a membrane following a positive or negative step change in feed concentration was derived. This analysis allows adsorption isotherms in the transport pathways through membranes to be determined from transient permeation responses to step concentration changes in the feed without measuring the retentate response. For Fickian diffusion through a membrane with Langmuir adsorption and zero coverage at the permeate boundary, the difference between the time-integrated flux into and flux out of the membrane is shown to be three times the time-integrated difference between the steady-state flux and the flux out. For Maxwell–Stefan diffusion, this ratio of integrated flux differences is 3 at low coverages and decreases towards 2 at saturation coverage. Mass transfer resistance at the permeate boundary increases the Fickian ratio above 3, and the ratio increases with decreasing Sherwood number. The ratio of integrated flux differences is shown to be identical to the steady-state replenishment time divided by the time lag. Thus, the ratio can be calculated directly from the steady-state concentration profile and the concentration-dependent diffusion coefficient. Surface diffusion through zeolite membranes was analyzed to demonstrate the calculation of the flux relationship for specific adsorption and diffusion models, but the method developed can be applied to membrane permeation in general.

Lessons learned when gathering real-time formative assessment in the university classroom using Tablet PCs

Worldwide, higher education instructors are exploring ways of enhancing classroom learning experiences with their Tablet PC-equipped students. To collect real-time formative assessment, instructors pose an open-format question to the class and the students utilize the digital ink of Tablet PCs to respond with answers in the form of handwriting, diagrams, graphs, equations, proofs, etc. Instructors receive the responses instantaneously. Built on principles well-grounded in educational research, this not only actively engages the students in their learning, but also increases student metacognition and provides valuable real-time formative assessment to guide the instructor. Several types of software are readily available both commercially and for free to facilitate this classroom exchange. This paper transcends the specifics of various types of software and discusses the experiences of instructors as they mesh their use of this technology-enabled feedback with the delivery of their undergraduate courses. The lessons presented here are drawn from our own experiences as well as input from instructors at other institutions on four continents, received by their voluntary completion of a written survey (n=19).

Adsorption and diffusion properties of zeolite membranes by transient permeation

Abstract Adsorption isotherms and diffusion coefficients for light gases and butane isomers were measured for the transport pathways involved in gas permeation through H-ZSM-5 membranes by a transient permeation technique. The permeate responses to step changes in the feed were measured, and the transport was modeled as Maxwell—Stefan diffusion with single-site Langmuir adsorption in the zeolite. Isotherms measured for N2, CO2, and CH4 at 295 K were nearly identical to those measured by calorimetry on H-ZSM-5 powders. Isotherms for butane isomers were also similar to isotherms for MFI powders and heats of adsorption and diffusion activation energies were in the ranges reported in the literature. Maxwell—Stefan diffusion coefficients for all gases studied increased slightly with feed partial pressure and were similar to those measured by other macroscopic methods for zeolite membranes and crystals. Effective membrane thicknesses were also determined non-destructively for tubular zeolite membranes by the transient permeation technique.

Using InkSurvey with Pen-Enabled Mobile Devices for Real-Time Formative Assessment II. Indications of Effectiveness in Diverse Educational Environments

InkSurvey is free, web-based software designed to facilitate the collection of real-time formative assessment. Using this tool, the instructor can embed formative assessment in the instruction process by posing an open-format question. Students equipped with pen-enabled mobile devices are then actively engaged in their learning as they use digital ink to draw, sketch, or graph their responses. When the instructor receives these responses instantaneously, it provides insights into student thinking and what the students do and do not know. Subsequent instruction can then repair and refine student understanding in a very timely manner.

Using InkSurvey with Pen-Enabled Mobile Devices for Real-Time Formative Assessment: I Applications in Diverse Educational Environments

InkSurvey is a free, web-based software designed to facilitate the collection of real-time formative assessment. Using this tool, the instructor can embed formative assessment in the instruction process by posing an open-format question. Students equipped with pen-enabled mobile devices (tablet PCs, iPads, Android devices including some smartphones) are then actively engaged in their learning as they use digital ink to draw, sketch, or graph their responses. When the instructor receives these responses instantaneously, it provides insights into student thinking and what the students do and do not know. Subsequent instruction can then repair and refine student understanding in a very timely manner.

High‐Efficiency Extraction and Utilization of Lunar Solar Wind Volatiles

A method of processing Lunar regolith for the extraction of solar wind volatiles has been developed to enable sustainable exploration of the Moon. The process incorporates efficient heat and mass transfer processes to provide thermal extraction and subsequent recovery of atoms implanted on the upper surface of Lunar regolith. End‐to‐end system modeling and component design establish a level feasibility and efficiency not previously achieved. Extracted molecular species are purified and stored on a mobile platform which delivers the resources to a central storage facility. The design concept results in several hundred times the mass of the facility produced in liquid product resources for life support and propulsion over the life of the system. Key developments include recuperative heating and cooling systems, rapid heating of regolith, and separation membranes capable of high selectivity due to thermal management. The ability to produce significant amounts of hydrogen, nitrogen, and helium from the Lunar ...

NATO-ASI FLUID TRANSPORT IN NANOPOROUS MATERIALS COURSE .a student’s perspective and explanations from a veteran

One of the first things mentioned in the introduction to many of the articles I’ve read about diffusion in zeolites, is that “the diffusivities reported in the literature vary by up to xxx orders of magnitude”. Although it is not always specifically stated, the implication is that these widely varying diffusivities have been measured or estimated for the same molecule/host system, and I’ve seen “xxx” as low as one and as high as four. (Interestingly enough, despite this, many authors, myself included, still report at least 2 or 3 significant figures in the diffusivities they presented in their own articles...but that is another issue.) Although most articles I’ve read have been about diffusion of relatively simple molecules in zeolites, we learned at the NATO course that large discrepancies exist between the reported diffusivities of species diffusing through nanoporous materials in general. While this can be useful to us relative newcomers to the field of diffusion, for instance when we are writing our first papers we can almost always say something like, “the diffusivities measured fell within the range reported in the literature” (if we can’t we are probably in serious trouble), but frankly it is disturbing and confusing to realize the magnitude of the discrepancies and to try to make sense of our measurements or estimates in this light. In the end, we as students are faced with the many obvious questions posed by the quote at the beginning of this paragraph: Why are there discrepancies? Why are they so large? What exactly do the different techniques for measuring diffusion actually measure and how are the diffusivities calculated from the measurements? All reported diffusivities are really estimates since no one has a digital diffusivity meter that reads out the error-free exact diffusivity of a species diffusing through a nanoporous material. Which values can we trust and to which should we compare our own estimates? What are the different diffusivities (self, transport, corrected, etc.) and how are they