Measurements of Isotopic-Transient Diffusion through Zeolite Membranes: A New Approach to Chemical Separation
Jeffrey Wyss UROP Proposal 7/20/04 Table of Contents
Introduction Pg 1
Background
Zeolite Membranes Pg 1-2
Previous Work on Pervaporation Pg 2-3
Methods
Chemicals Pg 3
Apparatus Pg 3-4
Time Schedule Pg 4-5
Collaboration with Faculty Sponsor Pg 5
References Pg 6-7 Introduction
Membranes, in general, are used to separate chemicals and are appealing because they require relatively little energy. Compared to distillation (the most widely used industrial separation process), membranes use substantially less energy [1]. In addition, membranes can achieve better separations than distillation. Unlike separations using membranes, separations by distillation can be limited by azeotropes, mixtures characterized by identical compositions of the gas and liquid phase in equilibrium. Zeolite membranes have advantages over other types of membranes because they can withstand high temperatures, thermal cycling, and harsh chemical environments that would ruin most membranes. Currently, however, zeolite membranes are only used industrially for organic dehydration because they are presently too expensive to be economical [2]. This expense is mainly attributed to low fluxes (amount leaving the membrane per unit area), which requires large membranes to produce reasonable amounts of product.
The transport mechanisms through zeolite membranes are not fully understood, especially for multi-component mixtures. In addition, little experimental data for pervaporation through zeolite membranes exists. Pervaporation is the process of a liquid contacting the membrane, diffusing through it, and a vapor (called the permeate) leaving the other side. Basic measurements of diffusion in zeolite membranes will improve our understanding of how zeolite membranes work, so that predictions can be made of membrane performance.
For this reason, I propose to measure diffusion through zeolite membranes by isotopic- transient pervaporation. Isotopically labeled molecules are molecules with almost identical chemical form and properties but different mass. Since the isotopically labeled molecules have a different mass they can be distinguished from the non-labeled molecules by a mass spectrometer.
This distinction allows for direct measurements of diffusion at steady state conditions. Background
Zeolite Membranes 1
Figure 1. Chemical Structure of MFI Type Zeolite [3].
Zeolite membranes are crystalline structures that have pores similar in diameter to single molecules. There are many different types of zeolites, each with different characteristics, including pore size. Zeolites are used to separate different molecules based on their size, shape, and other physical properties. In its simplest form, a zeolite membrane is a molecular sieve.
Figure 1 shows a typical structure for a zeolite membrane. The lines represent bonds between atoms that make up the crystalline structure. The large pores are about 0.55 nm in diameter [4].
For comparison, water and ethanol molecules have effective diameters of about 0.28 nm and 0.44 nm respectively [5].
Previous Work on Pervaporation and Zeolites
Little research has been done on pervaporation through zeolite membranes using isotopic-transient techniques. Tanaka et al. [6] used isotopic-transient permeation to study pervaporation through polymeric membranes. Polymeric membranes are different from zeolites because they are organic, their separation is by a different mechanism, and they cannot separate many of the mixtures that zeolite membranes can. Gardner et al. [7-9] studied transient absorption and diffusion in zeolites for gases, but did not use isotopic-transient techniques.
I studied pervaporation through zeolite membranes using isotopic-transient permeation with Travis Bowen in John Falconer and Richard Noble’s research group at UCB (spring and summer 2003). We measured diffusion rates of water, methanol, ethanol, 2-propanol, and
2 acetone through a Ge-ZSM-5 zeolite membrane. We also studied binary mixtures of ethanol/methanol and methanol/acetone.
We observed directly that ethanol slowed the diffusion of methanol and that methanol sped up the diffusion of ethanol for the ethanol/methanol mixture. For the methanol/acetone mixture; however, the diffusivities of both components were smaller than the pure component diffusion of acetone (the slower diffusing component). This behavior was a surprising result and may be attributed to more efficient packing of the molecules in the zeolite pores for the mixture than for the pure components. More efficient packing allows for less motion, thereby slowing the diffusion of the molecules.
Our results have been published in Microporous and Mesoporous Materials and
Industrial and Engineering Chemistry Research [10, 11] . I am a co-author of both with Travis
Bowen, John Falconer, and Richard Noble.
Methods
Chemicals
The research I propose will help understand diffusion of multicomponent mixtures by pervaporation. Based upon the recommendations section of Travis Bowen’s thesis, I propose to study mixtures of acetone/ethanol and acetone/2-propanol. He hypothesized that the diffusion of acetone in these mixtures will be faster than the diffusion of pure acetone because ethanol and 2- propanol are too large to pack between the acetone molecules [12]. This behavior is different from the acetone/methanol mixture we studied in the summer of 2003, where the diffusion of acetone in the mixture was slower than the diffusion of pure acetone. I plan to test his hypothesis and gain understanding of the transport mechanisms in zeolites.
Apparatus
The experimental apparatus I will use is shown in Figure 2. Feed chemicals are added to the reservoir and are mixed by a stir bar. The circulating pump sends the feed through a tubular membrane. The outside of the membrane is under vacuum, which provides the driving force for
3 diffusion through the membrane. The feed that does not diffuse through the membrane flows back into the reservoir. A small sample of the permeate is collected by a mass spectrometer, and the rest is condensed in liquid nitrogen traps. The collected permeate in the liquid nitrogen traps is weighed to determine the flux and a sample is injected in a chromatograph to determine the selectivity (relative composition of the permeate and feed). The flux and selectivity are important for describing the quality of the membrane used in the experiment.
Reservoir
Membrane Stir bar
Circulating To mass pump spectrometer
To LN traps 2 and vacuum pump
Figure 2. Experimental Apparatus for Isotopic-Transient Pervaporation [12].
When the system is at steady state, isotopically labeled molecules are injected into the reservoir. They diffuse through the membrane and are detected by the mass spectrometer. The mass spectrometer signal shows the partial pressure of the isotope in the permeate as a function of time. From these measurements relative diffusion rates are determined and diffusion coefficients can be readily calculated.
Time Schedule
For completing this research I will receive 4 credits for a senior thesis and 1 credit for independent study during the 2004-2005 school year. The university rule for lab classes is at
4 least 2 hours in the lab per week for every credit hour. From the rule, I estimate I will be working about 10 hours per week for a total of 30 weeks.
The research will be split up into four phases: apparatus maintenance (4 weeks), calibrations (3 weeks), experiments (15 weeks), and publishing (8 weeks). The phases are not necessarily in chronological order. For instance, experimental data could indicate nonphysical results which would then be investigated by re-calibrating the instruments and possibly performing apparatus maintenance.
Apparatus maintenance includes ordering new parts to replace broken ones or to improve the functionality of the system. In addition, membranes need to be calcined (heated in oven for 8 hours) periodically to remove absorbed impurities and this time is included in the 4 week estimate. Calibrations mainly apply to the mass spectrometer and the chromatograph. The mass spectrometer needs to be calibrated daily and the chromatograph needs to be calibrated for each new chemical and mixture studied. The experiments will take the majority of my research time because reaching steady state can take hours. Publishing includes writing a report to the UROP and senior thesis committees, as well as writing a paper for publication.
Collaboration with Faculty Sponsor
I will collaborate with John Falconer and Richard Noble, both Professors in the Chemical and Biological Engineering department at UCB. Both research zeolite membrane preparation, characterization, and application. They will provide the lab space, experimental apparatus, and equipment needed for the research. They will guide me in all aspects of the research, from experimental techniques and safety to data analysis.
I will have weekly meetings with Professor Falconer to discuss problems, ideas and update my progress. There will also be weekly group meetings with everyone in their research group, including Professor Noble. Each week different group members will give oral presentations on their research progress and problems. I will present once every 15 weeks. In addition, I will give a poster presentation and oral defense for my senior thesis. 5 References
[1] E. J. Henley, and J.D. Seader, Separation Process Principles, John Wiley & Sons, Inc: New
York, (1998) 11.
[2] Y. Morigami, M. Kondo, J. Abe, H. Kita, and K. Okamoto, “The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane,” Sep. Purif. Tech.,
25 (2001) 251.
[3] Database of Zeolite Structures. Retrieved March 16, 2004, from http://www.iza- structure.org/databases/.
[4] J. L. Falconer, and R. D. Noble, “Zeolite Membrane Research,” Retrieved March 16, 2004, from http://www.colorado.edu/che/FalcGrp/research/zeolite.html.
[5] M.J. Carmo, and J.C. Gubulin, “Ethanol water adsorption on commercial 3A zeolites: kinetic and thermodynamic data,” Braz. J. Chem. Eng., Sept. (1997), vol. 14, no. 3 ISSN 0104-6632.
[6] K. Tanaka, H. Kita, K.-I. Okamoto, R. D. Noble, and J. L. Falconer, “Isotopic-transient permeation measurements in steady-state pervaporation through polymeric membranes,” J. Membr. Sci., 197 (2002) 173.
[7] T. Q. Gardner, A. I. Flores, R. D. Noble, and J. L. Falconer, “Transient Measurements of Adsorption and Diffusion in H-ZSM-5 Membranes,” AIChE J., 48 (2002) 1155.
[8] T. Q. Gardner, J. B. Lee, R. D. Noble, and J. L. Falconer, “Adsorption and Diffusion Properties of Butanes in ZSM-5 Zeolite Membranes,” Ind. Eng. Chem. Res., 41 (2002) 4094.
[9] T. Q. Gardner, J. L. Falconer, R. D. Noble, and M. M. P. Zieverink, “Analysis of transient permeation fluxes into and out of membranes for adsorption measurements,” Chem. Eng. Sci., 58 (2003) 2103.
[10] T. C. Bowen, J. C. Wyss, R. D. Noble, J. L. Falconer, “Inhibition during Multicomponent Diffusion through ZSM-5 Zeolite,” Ind. Eng. Chem. Res., 2004, 43, 2598-2601.
[11] T. C. Bowen, J. C. Wyss, R. D. Noble, J. L. Falconer, “Measurements of Diffusion through a Zeolite Membrane using Isotopic-Transient Pervaporation”, Microporous Mesoporous Materials, 71, 2004, 199-210. [12] T. C. Bowen, Fundamentals and Applications6 of Pervaporation Through Zeolite Membranes, thesis submitted to the University of Colorado, 257 (2003).
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