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 isotopictransient 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
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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
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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 2propanol 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
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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
To mass
spectrometer
Circulating
pump
To LN2 traps
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
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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.
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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.izastructure.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 Applications
of Pervaporation Through
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Zeolite Membranes, thesis submitted to the University of Colorado, 257 (2003).
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