MODELING OF H2O ADSORPTION ON ZEOLITES
David Harris
Department of Biological Engineering
University of Hawai‘i at Mānoa
Honolulu, HI 96822
Jim Knox1
NASA Marshall Space Flight Center, Huntsville, AL, 35812
Hernando Gauto2
NASA Marshall Space Flight Center, Huntsville, AL, 35812
Carlos Gomez3
NASA Marshall Space Flight Center, Huntsville, AL, 35812
NOMENCLATURE
ci
ui
ɛ
q
q*
ρg
ρs
cpg
cps
kg
ks
hg
Tg
Ts
=
=
=
=
=
=
=
=
=
=
=
=
=
=
Concentration of sorbent, mol/m3
interstitial velocity, m/s
Void Number
pellet loading, mol/m3
equilibrium pellet loading, mol/ m3
density of gas mixture, kg/m3
density of sorbent bed, kg/m3
heat capacity of gas mixture, J/kgK
heat capacity of sorbent bed, J/kgK
Thermal Conductivity of gas mixture, W/mK
Thermal Conductivity of sorbent bed, W/mK
gas heat transfer coefficient of sorbent bed, W/m2K
Temperature of gas mixture, K
Temperature of sorbent
I.
ABSTRACT
Zeolites are common adsorbents used in industry. Their unique molecular structure
allows them to behave like sieves trapping molecules within their structure. Their ability to
adsorb molecules is dependent on pressure, temperature, surface geometry and packing
arrangements. This study determines the breakthrough curve, the time it takes for sorbents to
reach a saturation point, via COMSOL Multiphysics TM. This will inform researchers how
long to run a hydrothermal stability test. The model accounts for heat transfer, mass transport,
the geometry of the pellets and the initial inlet partial vapor pressures of 183.98 Pa, 93.33 Pa,
and 1.60 Pa. This study finds the breakthrough curve for RK 38 pellets (spheres), and ASRT
2005 pellets (cylinders).
1
Principle Investigator, Environmental Control and Life Support Systems, ES 62, Marshall Space Flight Center.
Co-Principle Investigator, Environmental Control and Life Support Systems, ES 62, Marshall Space Flight Center.
3
AST Heat Transfer, EV34, Marshall Space Flight Center.
2
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II.
INTRODUCTION
Estimates state that an astronaut produces 1.00 kg of CO2 and 2.28 kg of H2O from
respiration and perspiration(1). A buildup of these products can cause electrical problems, brain
damage and even death. CO2 and H2O are scrubbed from the air in the ISS by zeolites,
aluminosilicate, micro porous minerals that have a repeating molecular structure(2). This pattern
gives them the ability to behave like molecular sieves, trapping smaller molecules within the
pores (see figure 1) (3). These small molecules cannot be released back into the atmosphere
unless exposed to high temperatures or by a chemical reaction.
Figure 1: Artist rendition of zeolites depicting the repeated pattern of uniform sized hole for
trapping molecules
Zeolites are imbedded in clay pellets to form sorbents. The porosity of the clay allows
the zeolites to be exposed to the gaseous mixture even while imbedded. Clay sorbents come in a
variety of shapes and sizes, but cylinders or spheres are most common. The sorbent shape
effects packing density and hence the surface area available for adsorption.
An important aspect of the design is the packing arrangement of the pellets. The way the
pellets are packed in a canister determines the void fraction, or the amount of empty space left
after the canister is filled. The void fraction determines how much gas or fluid will move
through the canister and interact with the sorbents. Figure 2 below depicts a side view of spheres
in different arrangements.
Figure 2: 2 Dimensional representations of spheres packed in two different arrangements.
A cylinder with the same diameter will have a different volume and will pack
differently than spheres. The challenge in designing the sorbent canisters is picking an optimum
shape and size of pellet.
It is possible to measure the effectiveness of each size and shape of a pellet
computationally via the finite element method. The finite element method finds approximate
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solutions to complex differential equations by solving for the equation in a small subdomain, and
connecting them to a find an approximate value for a complex equation over a larger domain. (4)
This is similar to the idea of connecting a series of small straight lines to form a smooth curve
(see figure 3). As each finite element gets smaller, the approximate value becomes closer to the
actual solution.
Figure 3: An example of a few small straight lines forming an arc.
Many software packages include the finite element method, but the software most useful
for this project is COMSOL MultiphysicsTM. COMSOLTM requires minimal coding and can
solve for many partial differential equations simultaneously. It is used industry to model
problems in heat and mass transfer, stress and strain, and fluid mechanics. Figure 4 depicts a
typical COMSOLTM interface.
Figure 4: COMSOL MultiphysicsTM interface depicting different temperature gradients for
different time steps
In the case of sorbents, the finite element method is useful to determine the breakthrough
curve, or the time it takes for the sorbents to reach a saturation point for water. This will inform
future engineers on the best shape, size and packing arrangement for sorbents to use on the ISS.
III.
MATERIALS AND METHODS
A list of materials used for this project is shown below:
1. Test article canister- 165.10 mm in length and 6.35 mm radius
2. Snow storm funnel
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3. Packing rod- 6.35 mm radius
4. COMSOL MultiphysicsTM
5. RK 38 and ASRT 2005 Sorbent pellets
6. Dry glove box
7. Ruler
8. Scale
9. 50 mL beaker
10. Dry Glove box
The two sorbents used were RK 38 and ASRT 2005. RK 38 pellets are spherical in
shape and a have a diameter of 2.1 mm. ASRT 2005 pellets are cylindrical in shape with a
diameter of 2.05 mm and with varying heights (see figures 4-5).
Figure 4-5: RK 38 pellets (left) and ASRT 35 pellets (right).
The test article was held up vertically by a clamp, rod and stand. The sorbents were
poured into the test article through a snowstorm funnel by tapping a beaker against the edge of
the funnel. This method ensured a uniform packing arrangement in the cylinder. The process
continued until the packing rod would fit inside the test article and only the area below the
groove would be covered.
Once the right amount of pellets was in the test article, they were poured into a 50 mL
beaker. Seven small vials were filled with 75 pellets each, extracted by the sorbents from the test
article. The remaining pellets were poured back in the test article using the same procedure as
before. The packing rod was placed in the test article and was pushed down until it made contact
with the sorbents. A blue marker was used to mark the depth of the packing rod. A set of 75
pellets from one of the vials was poured in the test article and the depth of the packing rod was
measured again. This process was repeated until all of the sorbents were poured back in the test
article (see figure 5-6).
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Figure 5-6: Packing rod in test article (left) and packing rod with markings (right)
The markings were measured using a ruler to find the difference in height that 75 pellets
filled. This was important in finding the average volume of 75 pellets, and for designing spacers
to replace 75 when absent. The average difference in height was approximately 0.25 inch.
The next step in the procedure was to determine the mass of 75 pellets after a test. Eight
empty canisters were pre-weighed and filled with sorbents. Four were filled with ASRT 2005
and the other four were filed with RK 38. After activation, the sorbents were poured in a sieve,
sifted for dust and weighed again. 75 pellets were extracted, and the mass was measured again.
After subtracting the weight of the dust from the weight of the remaining sorbents, the final mass
of 75 pellets was found.
The sorbents were poured back into their respective canisters. A spacer with a quarter
inch diameter and a quarter inch length were placed in the canister to replace the volume lost
from the 75 pellets. The canisters were put back in the testing system.
The Model
The equations used were mass balance (eq. 1), mass transfer rate (eq. 2), fluid phase heat
balance (eq. 3), and sorbent bed heat balance (eq. 4):
𝝏𝒄
PDE Mass Balance: 𝝏𝒕 +
(1)
𝝏(𝒖𝒊 𝒄)
PDE Mass Transfer Rate:
(2)
Fluid Phase Heat Balance: 𝜀𝜌𝑔 𝑐𝑝𝑔
𝜕𝑇𝑔
𝜕𝑡
𝜕𝑞
𝜕𝑞
𝜕𝑡
+ 𝜀𝜌𝑔 𝑢𝑖 𝑐𝑝𝑔
(3)
Sorbent Bed Heat Balance: (1 − 𝜀)𝜌𝑠 𝑐𝑝𝑠
𝑑ℎ 𝜕𝑡 (1 − 𝜀)
𝝏𝒙
𝜕𝑇𝑠
𝜕𝑡
+
(𝟏−𝜺) 𝝏𝒒
𝜺
𝝏𝒕
=0
= 𝑘𝑚 (𝑞 ∗ − 𝑞)
𝜕𝑇𝑔
𝜕𝑥
𝜕
𝜕𝑇𝑔
= 𝜀𝑘𝑔 𝜕𝑥 ( 𝜕𝑥 ) +
𝜕
𝜕𝑇
= (1 − 𝜀)𝑘𝑠 𝜕𝑥 ( 𝜕𝑥𝑠 ) +
(1−𝜀)6
𝜀𝐷
(1−𝜀)6
𝜀𝐷
ℎ𝑔 (𝑇𝑠 − 𝑇𝑔 )
ℎ𝑔 (𝑇𝑔 − 𝑇𝑠 ) −
(4)
In COMSOL MultiphysicsTM, the input parameters came from a proposed test, The
Hydrothermal Stability Test, by Jim Knox. In this test, canisters filled with sorbents will be
connected in series and exposed to a flow of nitrogen gas with four set partial vapor pressures (4).
(see figure 7).
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Figure 7: The test system with eight test articles bundled in insulation and connected in series
The paper recommended the same void fraction for both ASRT 2005 and RK 38 at 0.40.
Both sorbents had a density of 1201 kg/m3. The flow rate of gas was assumed to be 1.384 L/min,
the initial temperature of gas at 24 C, the initial pressure at the inlet, to be 100.8 [kPa], change in
pressure was 0.36 [Pa], and the LDF to be 0.000625 [1/s].
Even though the study assumed a void fraction of 0.40, another method to calculate the
void fraction was used*. Dividing the total mass of pellets by the particle density yielded the
sorbent volume. The sorbent volume was divided by the canister volume to get the void number,
which was subtracted by one. The void fractions were 0.44 for RK 38 and 0.435 for ASRT
2005.
The model for this test ran for 3600 minutes with time steps of 30 seconds. It took two to
four minutes for the computer solve each problem
*The calculated void fractions were included in the project because of the suggestion that
not all sorbents will have the same void fraction. If there was a way to measure void fractions
that is the
IV.
RESULTS
For the packing experiment, the average height difference for the both ASRT 2005 and
RK 38 are in Table 1:
Table 1 : Average height of 75 pellets in the test article
Sorbent
Inches
meters
Average RK 38 for 75 pellets
0.22544643 0.005726
Average ASRT 2005 for 75
pellets
0.24776786 0.006293
The average height was 5.7 mm for RK 38 and 6.3 mm for ASRT 2004. This is why
quarter inch spacers were used.
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The average mass of 75 pellets of both ASRT 2005 and RK 38 are:
Table 2: Average weights of 75 pellets
Bulk weight of RK 38
A7
A5
A3
A1
Average
in g
in kg
13.42
13.58
13.52
13.41
13.4825
Bulk Weight of
ASRT 2005
A4
A6
A8
A2
0.0134825 Average
in g
in kg
13.02
13.76
13.86
13.69
13.5825 0.0135825
The curve was found for both ASRT 2005 and RK 38 at three different inlet partial vapor
pressures, 183.98 Pa, for 93.33 Pa, and 1.60 Pa. In the Hydrothermal Stability Test Protocol,
there was a test for 0.0097 Pa, but that inlet pressure never broke through in COMSOLTM. The
breakthrough times, assuming a void fraction of 0.40, are in Table 3:
Table 3: Times to reach breakthrough point assuming void fraction of 0.40
Sorbent
183.98 Pa
93.33 Pa
1.60 Pa
ASRT 2005
1542 min
2600 min
43100 min
RK 38
1542 min
2600 min
42000 min
The breakthrough curves, assuming a void fraction of 0.4 for ASRT 2005 are in Figures
8-10:
Figure 9: ASRT 2005 with
inlet pressure of 93.33 Pa with
void fraction of 0.40
Figure 8: ASRT 2005 with
inlet pressure at 183.98 Pa with
void fraction of 0.40
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Figure 10: Breakthrough curve
for ASRT 2005 with an inlet
temperature of 1.60 Pa with void
fraction of 0.40
The breakthrough curves for RK 38, assuming void fraction of 0.40 are in Figures 11-13:
Figure 11: Breakthrough curve for
RK 38 with an inlet partial pressure of
183.98 Pa with void fraction of 0.40
Figure 12: Breakthrough curve for
RK 38 with an inlet partial pressure of
93.33 Pa with void fraction of 0.40
Figure 13: Breakthrough curve for RK 38 with an inlet
partial pressure of 1.60 Pa with void fraction of 0.40
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Table 4: Times to reach breakthrough point assuming void fraction of 0.435 for ASRT 2005
and 0.44 for RK 38
The breakthrough curves for 1.60 Pa were not found
Sorbent
ASRT 2005
RK 38
183.98 Pa
1100 min
1100 min
93.33 Pa
1800 min
1685 min
1.60 Pa
---
Figures 14-15 are the breakthrough curves for ASRT 2005 assuming a void fraction of 0.44:
Figure 14: Break through curve for
ASRT 2005 with an inlet pressure of
183.98 Pa with void fraction of 0.435
Figure 15: Break through curve for
ASRT 2005 with an initial intlet pressure of
93.33 Pa with void fraction of 0.435
Figures 16-17 are breakthrough curves for RK 38: with a void fraction of 0.56:
Figure 16: Breakthrough curve for
RK 38 with an initial inlet partial
vapor pressure of 183.98 Pa with
void fraction of 0.44
Figure 17: Breakthrough curve
for RK 38 with an initial inlet
partial vapor pressure of 93.33Pa
with void fraction of 0.44
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V.
CONCLUSION
The breakthrough curves were predicted for three different inlet partial vapor pressures.
In all scenarios, sorbents break through faster at higher partial vapor pressures than at lower
ones. The solutions from the model vary when using different void fractions. The breakthrough
curves with a void fraction of 0.40 for both sorbents are almost identical. However, if the void
fractions were 0.44 for RK 38 and 0.435 for ASRT 2005, the results are drastically different.
Both models indicate that running a hydrothermal stability test for more than 2600 minutes is
unnecessary, as well as running a test with vapor pressures of 1.60 Pa or lower since they will
never breakthrough.
ACKNOWLEDGMENTS
D. H. Harris thanks Robert F. Coker for his consultation with COMSOL Multiphysics TM,
and David Watson for supervising the projects when others were unavailable.
REFERENCES
ECLLS staff, Human Needs and Effluents Mass Balance (per person) -Marshall Space Flight
Center Poster
Lobo, Jairo Antonio Cubillos –“Heterogeneous asymmetric epoxidation of cis-ethyl cinnamte
over Jacobsen's catalyst immobilized in inorganic porous materials” [Thesis P. 28}, 2005
Murphy, Donald W. and INterrante, Leonard V.- “Zeolite Molecular Sieves”, Inorganic
Synthesi, 2007s
Knox, Jim –“Requirements for Hydrothermal Stability Test,” NASA Planning Document, 2014
Image sources:
1. “The microporous molecular structure of a zeolite, ZSM-5”- December 28, 2013
http://commons.wikimedia.org/wiki/File:Zeolite-ZSM-5-3D-vdW.pn
2. “Sphere Packing”-2007-http://www.keplersdiscovery.com/Images/SpherePacking.jpg
3. “Finite Element method 1D illustration1”- March 8, 2006
http://en.wikipedia.org/wiki/Finite_element_method#mediaviewer/File:Finite_element_m
ethod_1D_illustration1.png
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