Preparation of Papers for AIAA Technical Conferences

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

1. NOMENCLATURE

3 ci = Concentration of sorbent, mol/m ui = interstitial velocity, m/s ɛ = Void Number q = pellet loading, mol/m3 3 q* = equilibrium pellet loading, mol/ m 3 ρg = density of gas mixture, kg/m 3 ρs = density of sorbent bed, kg/m cpg = heat capacity of gas mixture, J/kgK cps = heat capacity of sorbent bed, J/kgK kg = Thermal Conductivity of gas mixture, W/mK ks = Thermal Conductivity of sorbent bed, W/mK 2 hg = gas heat transfer coefficient of sorbent bed, W/m K

Tg = Temperature of gas mixture, K

Ts = 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 1 Principle Investigator, Environmental Control and Life Support Systems, ES 62, Marshall Space Flight Center. 2 Co-Principle Investigator, Environmental Control and Life Support Systems, ES 62, Marshall Space Flight Center. 3 AST Heat Transfer, EV34, Marshall Space Flight Center.

1 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).

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.

2 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 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.

3 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 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).

4 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):

PDE Mass Balance: (1)

PDE Mass Transfer Rate: (2)

Fluid Phase Heat Balance: (3)

Sorbent Bed Heat Balance: (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).

5 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 0.2254464 0.0057 3 26

Average ASRT 2005 for 75 0.2477678 0.0062

6 pellets 6 93

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.

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 in g in kg Bulk Weight of in g in kg ASRT 2005 A7 13.42 A4 13.02 A5 13.58 A6 13.76 A3 13.52 A8 13.86 A1 13.41 A2 13.69 Average 13.4825 0.0134825 Average 13.5825 0.01358 25

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 COMSOL TM. 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:

7 The breakthrough curves for RK 38, assuming void fraction of 0.40 are in Figures 11-13:

8 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 183.98 Pa 93.33 Pa 1.60 Pa ASRT 2005 1100 min 1800 min -- RK 38 1100 min 1685 min --

Figures 14-15 are the breakthrough curves for ASRT 2005 assuming a void fraction of 0.44:

Figures 16-17 are breakthrough curves for RK 38: with a void fraction of 0.56:

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.

3. 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.

4. 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

10 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_ method_1D_illustration1.png