Article pubs.acs.org/IECR
Isotherms for Water Adsorption on Molecular Sieve 3A: Influence of Cation Composition Ronghong Lin,† Austin Ladshaw,‡ Yue Nan,† Jiuxu Liu,† Sotira Yiacoumi,‡ Costas Tsouris,‡,§ David W. DePaoli,§ and Lawrence L. Tavlarides*,† †
Department of Biomedical and Chemical Engineering, Syracuse University, 329 Link Hall, Syracuse, New York 13244, United States School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0355, United States § Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6181, United States ‡
ABSTRACT: This work is part of our continuing efforts to address engineering issues related to the removal of tritiated water from off-gases produced in used nuclear fuel reprocessing facilities. In the current study, adsorption equilibrium of water on molecular sieve 3A beads was investigated. Adsorption isotherms for water on the UOP molecular sieve 3A were measured by a continuous-flow adsorption system at 298, 313, 333, and 353 K. Experimental data collected were analyzed by the Generalized Statistical Thermodynamic Adsorption (GSTA) isotherm model. The K+/Na+ molar ratio of this particular type of molecular sieve 3A was ∼4:6. Our results showed that the GSTA isotherm model worked very well to describe the equilibrium behavior of water adsorption on molecular sieve 3A. The optimum number of parameters for the current experimental data was determined to be a set of four equilibrium parameters. This result suggests that the adsorbent crystals contain four energetically distinct adsorption sites. In addition, it was found that water adsorption on molecular sieve 3A follows a three-stage adsorption process. This three-stage adsorption process confirmed different water adsorption sites in molecular sieve crystals. The second adsorption stage is significantly affected by the K+/Na+ molar ratio. In this stage, the equilibrium adsorption capacity at a given water vapor pressure increases as the K+/Na+ molar ratio increases.
1. INTRODUCTION Professor D. Ramkrishna is highly distinguished through his major contributions and accomplishments to the engineering profession. My interactions with “Ramki” go back through his early publications in the 1970s, and subsequently, on his solutions with colleagues of population balance equations and the applications to liquid−liquid dispersions undergoing mass transfer with reactions. This subject was also important to our research activities in this area, at that time, and his work provided us with inspiration and friendly competition to achieve further accomplishments. We have continued to enjoy our meetings and discussions, both personal and professional, over the decades. It is a pleasure and privilege to contribute to this issue of Industrial & Engineering Chemistry Research in Professor Ramkrishna’s honor. The reprocessing of used nuclear fuels is a possible way to reduce the nuclear waste inventory and to recycle valuable fuel elements. It is understood that the reprocessing procedure will release a variety of radioactive species, including krypton-85, xenon-135, iodine-129, and tritium (3H). The removal and immobilization of tritium from used nuclear fuels is a desirable strategy for reducing tritium emissions into the atmosphere. Several technical solutions for capturing tritium from used nuclear fuels have been proposed, including volatilization and adsorption of tritiated water from off-gas streams, isotopic enrichment and collection from liquid streams, and aqueous recycle with removal and solidification of a small side stream.1 Voloxidation has been proposed for removal of tritium from spent fuels prior to dissolution, to avoid introducing tritium into the aqueous systems.2 The tritium released from this process further reacts with oxygen to form tritiated water.2 © XXXX American Chemical Society
Water adsorption by solid adsorbents represents the state-ofthe-art of tritiated water vapor capture from used fuel reprocessing off-gases.3−10 Over the past four decades, various adsorbents have been developed and tested for the capture of tritiated water vapor, including silica gel, molecular sieves (3A, 4A, 5A, 13X), anhydrous calcium sulfate (e.g., Drierite) and activated alumina. Among these adsorbents, molecular sieves have been the most favorable choice for capture and immobilization of tritiated water.11 The current work is a companion part of our recent work on kinetics of water adsorption on molecular sieve 3A12 and represents our continuing efforts to develop fundamental engineering data and advanced modeling tools to support the U.S. Department of Energy (DOE) Fuel Cycle Research and Development (FCR&D) Materials Recovery and Waste Forms Campaign.11 An example of the unit operations for removal of radioactive species from off-gas streams is a sequence of adsorption beds for the removal of tritium (in the form of tritiated water), krypton, xenon, and iodine.13 Development of highly efficient adsorption unit operation models requires experimental data on adsorption isotherms and kinetics. Although water adsorption on solid adsorbents is a relatively mature technology, adsorption isotherm data for molecular sieve 3A are still very limited in the open literature. Special Issue: Doraiswami Ramkrishna Festschrift Received: May 5, 2015 Revised: June 15, 2015 Accepted: June 16, 2015
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DOI: 10.1021/acs.iecr.5b01411 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research The objective of this work was to gather adsorption isotherm data for water on molecular sieve 3A and use the data to determine parameters for the Generalized Statistical Thermodynamic Adsorption (GSTA) model.14 Adsorption isotherms for water on the UOP molecular sieve 3A were measured in a continuous-flow adsorption system at four different temperatures from 298 K to 353 K. GSTA model parameters were determined by analyzing the experimental data with a comprehensive set of algorithms designed for GSTA equilibria analyses.14 Finally, the effect of cations on adsorption isotherms is discussed.
a brief description of the system is given below. The system is composed of three functional units: vapor generation, adsorption, and data acquisition (Figure 1). The core of the system is a microbalance head (Model MK2-M5, CI Precision, U.K.) that is used to measure mass gain or loss. The balance head was maintained at a constant temperature (301 K) to minimize inaccuracies caused by temperature variations in longperiod experiments. Water vapor was generated by bubbling water with dry air in a controlled manner. The dew point of the gas mixture out of the water vapor generation system was measured and monitored by a dew point meter (Easidew Online, Michell Instruments) before the mixture entered into a glass sample tube, where adsorption took place. Gas mixtures of desired water vapor pressures were generated by precisely controlling gas flow rates and water temperature. The gas mixture was preheated to a desired adsorption temperature by passing through a glass coil and then entered into the sample tube through an inlet located near the bottom of the sample tube. The sample tube was connected to the microbalance head, from which several connected suspension rods hung down to hold a sample pan. The adsorption system was run for a period ranging from hours to days to establish desired experimental conditions. After establishment of the desired condition, molecular sieve 3A was loaded onto the sample pan to start the adsorption process and data recording. The adsorbents were degassed using the ASAP 2020 Physisorption Analyzer to remove residual water prior to use. Degassing was performed under vacuum at 503 K for 8 h to a final pressure of 15 wt %), corresponding to different adsorption behaviors, as indicated by the slope change of the q vs log p plots. This pattern can possibly be interpreted by different adsorption behavior of water molecules adsorbed in the zeolite A framework. Previous studies showed that, in each zeolite 3A unit cell, there are 12 K+ cations distributed in the α-cage (supercage) in three different positions: site I (8 K+), site II (3 K+), and site III (1 K+) (see Figure 4).19 Adsorbed water molecules are distributed in the αcage and the β-cage (sodalite cage), forming clusters via hydrogen bonding, depending on the level of adsorption. Water molecules (or clusters) in the α-cage are bound to cations through the cation-oxygen linkage. Water molecules have adsorption site preference in the order III > II > I,17 which means that adsorbed water will first attach to cations on site III and then move onto cations on site II and site I. Theoretically, a maximum of 24 water molecules can be adsorbed by a zeolite 3A unit cell, among which 12, 3, and 1 water molecules are bound to cations on sites I, II and III, respectively, and 8 are located in the β-cage.20 In zeolite 4A, the distribution of 12 Na+ in each unit cell is similar, but a maximum of 28 water molecules can be adsorbed with a different distribution on site I and in the β cage,17 as shown in Table 3. This may be interpreted that the smaller size of Na+ makes more room for water to occupy, which would result in greater maximum water capacities (qmax). The UOP molecular sieve 3A beads contain 37−41 mol % K+ and 58−62 mol % Na+, according to the EDX and ICP-OES analyses (Table 4). Accordingly, in each unit cell of the UOP molecular sieve 3A, there are ∼5 K+ and ∼7 Na+ cations. Previous studies also revealed that Na+ has a site selectivity in the sequence I > II > III,17 which means that Na+ has priority
Table 2. Optimized Parameter Results for the GSTA Analysis on Water Adsorption Dataa n
ΔH°n (kJ/mol)
ΔS°n (J/K/mol)
1 2 3 4
−46.60 −125.0 −193.6 −272.2
−53.70 −221.1 −356.7 −567.5
a The maximum adsorption capacity (qmax) was assumed 21% by weight, which is the theoretical maximum.
and analysis agrees well with previous findings17 and demonstrates the strong capability of the GSTA model in describing water adsorption isotherms on molecular sieves. If we look closely at the data in Figure 2, there are three inflection points in the isotherm data that are visible within the regions of temperature and pressure shown. However, according to the GSTA model analysis, there should be four inflection points, which correspond to the four equilibrium parameters found in Table 2. The fourth inflection point is not visible in the data because the final inflection point only occurs at very high temperature and pressure, in the region of the isotherm curve where the adsorption capacity must be restricted to its theoretical maximum value of 21%. If we had used only a three-parameter solution to the GSTA model, then the model and data would both only show three inflection points. However, the modeling analysis concluded that the three-parameter solution was significantly less suitable than the four-parameter solution, even when factoring in the penalty for having an additional parameter. In addition to analyzing the current experimental data, the GSTA optimization routine was also used to provide results for the adsorption isotherms of water vapor on the Grace Davison zeolite.14 The Grace Davison isotherms16,18 are the only set of isotherms for water adsorption on zeolite 3A that we found in the open literature. Results of that analysis can be viewed below in Figure 3. The Grace Davison adsorbent is of a slightly different material makeup,16 which resulted in a different adsorption behavior. The optimization routines had determined that the least number of equilibrium parameters needed to
Table 3. Cation Distribution and Adsorbed Water Molecule Distribution and Accumulated Weight Percentage α cage adsorption site K-A (3A) cation distribution20 H2O/unit cell20 accumulated H2O (wt %)a Na-A (4A) cation distribution20 H2O/unit cell17 accumulated H2O (wt %)a KNa-A (UOP 3A) cation distributionb H2O/unit cellb accumulated H2O (wt %)b
Figure 3. GSTA analysis results for the Grace Davison water vapor adsorption curves from the literature.16,18 Solid lines show the results of the optimization routine against the data. Theoretical maximum capacity (qmax) for this adsorbent was 22.8% by weight rather than the 21% for the UOP-type molecular sieve 3A. (Reprinted with permission from Ladshaw et al.14 Copyright 2015, Elsevier BV, Amsterdam.)
Site I
β cage
Site III
Site II
1 K+ 1 0.9
3 K+ 3 3.8
8 K+ 12 15.2
0 8 22.8
1 Na+ 1 1.0
3 Na+ 3 4.2
8 Na+ 20 25.4
0 4 29.6
1 K+ 1 1
3 K+ 3 4
1 K+, 7 Na+ 12−20 15−25
0 4−8 23−30
a
Theoretical calculation. bData proposed in this work based on data for K-A and Na-A in this table.
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DOI: 10.1021/acs.iecr.5b01411 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research over K+ to occupy site I in the zeolite A structure. Table 3 summarizes the distribution of cations and adsorbed water molecules in zeolite A structure and the accumulated water weight percentage. According to these data, the first 4 wt % of adsorbed water is bound to cations on sites III and II. Upon further adsorption, water will attach to cations on site I, reaching an adsorption capacity of 15−25 wt %, depending on the type of cations (Na+ or K+) occupying site I. After completion of stages A and B, further increase in water vapor pressure will enable water molecules to enter into the β cage to reach a maximum adsorption capacity of 23−30 wt %. Therefore, it may be concluded that the three adsorption stages A, B, and C of the adsorption isotherms correspond to water bound to cations on sites III and II, water bound to cations on site I, and water in the β cage, respectively. In addition, it could also be inferred that the zeolite A framework would have four energetically distinct adsorption sites (α-I, α-II, α-III, and β), which is in agreement with the GSTA model analysis (section 4.1) for the UOP molecular sieve 3A. Thus, the modeling results help to confirm the crystal analysis based on the positioning of the cations shown in Figure 4.
Figure 5. Water adsorption isotherms for the UOP molecular sieves 3A and 4A crystal powder and a K+-exchanged (modified) UOP molecular sieve 3A crystal powder at 313 K.
When the water vapor pressure was
