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Kinetics of Water Vapor Adsorption on Single-Layer Molecular Sieve 3A: Experiments and Modeling
Abstract
The objective of the current work was to shorten the gap for fundamental adsorption kinetic data required for the development of advanced adsorption unit-operation models to be incorporated into an overall plant-level model for spent nuclear fuel reprocessing. The kinetics of water-vapor adsorption on molecular sieve 3A was investigated at 25–80 °C and water dew points from −69 to 17 °C. Water uptake curves were fitted with three kinetic models including the linear-driving-force model, the shrinking-core model, and the Langmuir kinetic model. The results suggest that the water-vapor adsorption on molecular sieve 3A under the investigated experimental conditions was controlled by both external film resistance and internal macropore resistance. The contribution of the external film resistance varied from 25% to 50% of the total mass-transfer resistance depending on the adsorption temperature. It was also found that the Langmuir kinetic model fitted individual sets of kinetic data very well, but the Langmuir adsorption constant obtained from curve fitting decreased with increasing adsorption temperature and with increasing water vapor pressure. This result indicates a significant surface heterogeneity of molecular sieve 3A and also implicitly verifies that the Langmuir isotherm model is unable to represent isotherms of water adsorption on molecular sieve 3A.
... The flow is described by the axial-dispersed plug flow model. The linear driving force model (LDF) is considered for description of the water adsorption kinetics [54]. The changes in the gas flow due to the occurring chemical reactions and adsorption phenomena are considered by the overall mass balance equation. ...
... The kinetics of water adsorption onto the 3 Å molecular sieve is described with the LDF model [54]: ...
... The axial diffusion coefficient value was determined with the correlations reported by Edwards and Richardson [83]. Evaluation of the film mass transfer coefficient value was done through calculation of the dimensionless Sherwood, Schmidt, and Reynolds numbers [54]. The constants used for the model equations are provided in Table A1. ...
Synthesis of methanol from carbon dioxide is affected by thermodynamic limitations and excessive formation of water that might have a detrimental impact on methanol production rate and catalytic activity. To overcome these effects, sorption enhanced (SE) carbon dioxide hydrogenation to methanol with selective in-situ adsorption of water is investigated both experimentally and via process simulation. A significant improvement in the process performance due to the thermodynamic equilibrium shift, achieved as a result of selective water removal, is reported. Depending on the process conditions, during the SE phase, outlet methanol concentration is ca. 150-290 % of the steady state values recorded after the adsorbent saturation. For carbon monoxide this factor is ca. 220-510 %. The effect of process parameters such as reactor pressure, temperature, overall gas flowrate and catalyst-to-adsorbent ratio is thoroughly addressed. Reactor pressure is demonstrated to affect methanol production during the SE phase to the greatest extent. Particularly, increasing reactor pressure above 40 bar enhances methanol formation during the SE phase at the cost of decreasing carbon monoxide production, thus improving process selectivity. Although increasing reactor temperature above 250 oC favors carbon monoxide formation, it results in suppressed methanol production and considerable shortening of the SE phase duration. Because of the RWGS reaction fast kinetics, the variations in gas hourly space velocity and amount of the adsorbent loaded in the reactor affect production of methanol more than carbon monoxide. The parameters describing kinetics of the involved reactions and water adsorption are adjusted based on the acquired experimental data.
... 1,3 For the microscale, adsorption is governed by film mass transfer from bulk solution to the outside of the adsorbent, pore, and surface diffusion inside the adsorbent domain, and surface reactions or adsorption equilibria. 4,5 To model these mechanisms requires a mass balance on each adsorbate as it travels between, around, and inside the particles. In other words, the model is required to include multiple mechanisms (diffusion, advection, etc.) on multiple scales (macroscale being transport around particles and microscale being transport in pore spaces) for multiple species (gas mixtures require tracking of many adsorbates). ...
... [23][24][25][26] Physical properties and chemical characteristics of the MS3A and Ag 0 Z were described previously. 5,23 In addition to those reported properties, the average macropore radii of MS3A and Ag 0 Z were measured in this work using mercury porosimetry, which were 35 nm and 26.5 nm, respectively. Tables 2 and 3 below provide some important structural and physical parameters for modeling adsorption kinetics in both MS3A and Ag 0 Z adsorbents. ...
... Both the H 2 O and I 2 uptake experiments were performed with continuous-flow gas adsorption systems, which have been reported previously. 5,23 Each of the systems had a H 2 O/I 2 vapor generating unit, a microbalance unit, a heating unit, and a data acquisition system. Changing the temperature of the generating units and varying the flow rates of the carrier and dilution gas streams controlled the H 2 O and I 2 vapor concentrations. ...
Adsorption is a complex physicochemical process involving interparticle transport, interphase mass-transfer, intraparticle diffusion, and surface reactions. Although the exact description of the adsorption process will inevitably vary from system to system, it will always be governed by those primary mechanisms. Therefore, by devising a model framework that can inherently include those mechanisms, it would be possible to create a modeling platform on which many different adsorption problems could be solved numerically. To accomplish this task, a generalized 1-D conservation law model was created to include the necessary mechanisms of adsorption on several different geometrical domains. Specific model applications for adsorption were developed under that framework and validated using experimental data available in literature or obtained in this work. This modeling platform makes it easier to model various adsorption problems and develop new adsorption models because of the common treatment of the mathematics governing the physical processes. © 2017 American Institute of Chemical Engineers AIChE J, 2017
... After being filtered, decanted, and redistilled to remove sieve dust and achieve anhydrous bioethanol with less than 1% water. 27 ...
... The most favorable pH of S. cerevisiae for ethanol production ranges from 4.0 to 5.0. 27 Nitrogen limitation has been shown to affect cell growth and biomass formation as well as directly affect the fermentation rate. 38 Although most of the nutrients needed for yeast development are present in molasses, sufficient nitrogen is frequently added to promote yeast growth and the production of ethanol. ...
Background Yeast strains that are tolerant to several environmental stresses are extensively employed to produce bioethanol from sugar-rich feedstock. Methods In our previous research findings, five wild indigenous strains of multi-stress tolerance yeast were isolated from traditional fermented alcoholic beverages (Tella, Tej, and Areke) in Ethiopia to the purpose of evaluate their potential use for bioethanol using sugarcane molasses as a substrate. Result The optimal parameters for maximum ethanol production were pH of 4.5, 25°Brix, 30°C, 4.0 g/l ammonium sulfate, supplement with yeast extract, and 72 h of incubation period. Under these optimum conditions, 13.13 ± 0.08% (v/v) ethanol was obtained. The yield of hydrous and anhydrous bioethanol from fermented molasses samples were produced at 94.20 – 95.60% (v/v) and 99.05 – 99.56% (v/v), respectively. Conclusion According to this finding, Saccharomyces cerevisiae isolate MUT15F, Saccharomyces cerevisiae isolate MUT18F , and Saccharomyces cerevisiae isolate R9MU strains have been good candidates for production of industrial bioethanol.
... Although dehydration with a molsieve packed bed is a relatively mature technology, there is a still a data gap in the open literature regarding the kinetics of water in 3A. 15 Water kinetics in zeolite LTA were studied in binder-containing pellets or beads form by means of batch-type uptake or dynamic breakthrough experiments for a packed column with water concentration profile monitored. Depending on the mathematical model considered for the relative contribution from various mass transfer steps in interparticle, interphase, and intraparticle, along with possible heattransfer resistances, the unknown kinetic parameters can be estimated by fitting experimental data to postulated kinetic models. ...
... Teo and Ruthven 10 found that diffusion transport of water into 3A was controlled by the intraparticle pore diffusion, with some contribution from external film resistance at low fluid velocities by varying particle size and velocities in experiments. The effective diffusion time constant was evaluated to have a low value of 4.3 Â 10 À5 s À1 , giving an effective pore diffusivity of 2 Â 10 À10 m 2 /s, which is slower than the value reported by Gorbach et al. 18 Lin et al. 15 investigated the kinetics of water vapor on single-layer 3A pellets in a breakthrough unit at adsorption temperatures of 25-80 C and water dew points from À69 to 17 C. The water uptake curves were fitted with three kinetic models (the linear-driving-force model, the shrinking-core model, and the Langmuir kinetics model) with time constants ranging from 5 Â 10 À3 to 3 h À1 , suggesting kinetics controlled by both the external film resistance and internal macropore resistance. ...
A new concentration‐swing frequency response system was developed to evaluate water kinetics in non‐binder containing zeolite Linde Type A (LTA) crystals with frequencies up to 1 Hz. For commercial micron‐sized 3A crystals, the mass transfer rate is too fast to be determined accurately due to reduced sensitivity near the system limits. Larger LTA crystals (~15 μm) were synthesized in house and ion exchanged to three potassium concentrations (0, 48, and 77 wt%) to aid in the accurate determination of mass transfer mechanism and rates. Micropore diffusion with parallel sites, not a surface barrier, described all the data well, consistent with the bimodal distribution of crystal sizes from scanning electron microscope images. The transport diffusivity for water in zeolite 3A crystals near saturated loadings at 25°C is about 1 × 10⁻¹² m²/s, 10 times slower than 4A crystals around 10⁻¹¹ m²/s, both supporting fast micropore diffusion time constants >1 s⁻¹ for micron‐sized crystals.
... The continuous flow adsorption system was used for iodine and water vapor adsorption on multiple materials previously by Nan et al. 19,38,39 and Lin et al. 40,41 and has been modified for performing the adsorption of CH 3 I on Ag 0 -Aerogel experiments. Figure 1 shows the schematic diagram of the modified adsorption system. ...
... Therefore, the adsorbate is limited at the reacting surface and no significant concentration gradient exists in the pellet. This model has been widely used in the nuclear waste treatment area including water adsorption on molecular sieves 3A, 40 water adsorption on Ag 0 Z, 16,38 I 2 adsorption on Ag 0 Z, 16,19 CH 3 I adsorption on Ag 0 Z, 18 and so forth. ...
The low concentration methyl iodide (CH3I) adsorption process on reduced silver‐functionalized silica aerogel (Ag⁰‐Aerogel) was studied. The kinetic data were acquired using a continuous flow adsorption system. Because the corresponding physical process was observed, the shrinking core model (SCM) was modified and applied. An average CH3I pore diffusivity was calculated, the CH3I‐Ag⁰‐Aerogel reaction was identified as a 1.40 order reaction instead of first order reaction, and the nth order reaction rate constant was determined. This modified SCM significantly increases the accuracy of adsorption behavior prediction at low adsorbate concentration. Modeling results indicate that the overall adsorption process is controlled by the pore diffusion. However, at low adsorbate concentration (ppbv level), the CH3I adsorption is limited to the surface reaction due to the low uptake rate in a predictable time period.
... , respectively (Ruthven 1984;Sircar 2018). Other authors also confirmed the predictive capability of the LDF model for the water adsorption on zeolite 3A (Simo et al. 2009;Lin et al. 2014). The overall mass transfer resistance has been attributed to a combination of the external film resistance and the macropore resistance (Lin et al. 2014), or to a combination of the macropore and micropore resistances (Simo et al. 2009). ...
... Other authors also confirmed the predictive capability of the LDF model for the water adsorption on zeolite 3A (Simo et al. 2009;Lin et al. 2014). The overall mass transfer resistance has been attributed to a combination of the external film resistance and the macropore resistance (Lin et al. 2014), or to a combination of the macropore and micropore resistances (Simo et al. 2009). If we calculate the external mass transfer resistance and the macropore resistance according to Eqs. 10-15, we observe that the data presented here are solely limited by the micropore resistance (Figs. 10, 11). ...
Steam adsorption enhanced reaction processes are a promising process intensification for many types of reactions, where water is formed as a byproduct. To assess the potential of these processes, adequate models are required that accurately describe water adsorption, particularly under the desired elevated temperatures and pressures. In this work, an adsorption isotherm is presented for H2O adsorption at 200–350 °C and 0.05–4.5 bar partial pressure on molecular sieve (LTA) 3A. The isotherm has been developed on the basis of experimental data obtained from a thermogravimetric analysis and integrated breakthrough curves. The experimental data at lower steam partial pressures can be described with a Generalized Statistical Thermodynamic Adsorption (GSTA) isotherm, whereas at higher steam partial pressures the experimental data can be adequately captured by capillary condensation. Based on the characteristics of the adsorbent particles, a linear driving force relation has been derived for the adsorption mass transfer rate and the apparent micropore diffusivity is determined. The isotherm and mass transport model presented here prove to be adequate for modelling and improved evaluation of steam adsorption enhanced reaction processes.
... The experiments of water adsorption on Ag 0 Z pellets were performed with a continuous-flow adsorption system, which was described in detail previously. 29,30 A diagram of the adsorption system is shown in Figure 1. The system included a water vapor generation unit, which was capable of generating humid gas streams with a dew point ranging from −70°C to 20°C , a heating unit with a working capacity of 25°C to 300°C, an analytical microbalance head with a sensitivity of 0.1 μg, and a programmed data acquisition system. ...
... Our previous studies have demonstrated its ability to model the adsorption kinetics of I 2 on Ag 0 Z and water on molecular sieve 3A. 13,30 This model simplifies the adsorption process to three sequential steps: (a) diffusion through the external gas film, (b) diffusion through the shell saturated by adsorbate, and (c) adsorption at the surface of the sorbate-free core. 37 Assuming that adsorption step (c) occurs sufficiently rapidly, compared to the two diffusion steps (a) and (b), the uptake curve can be expressed by eq 13: ...
Reduced silver-exchanged mordenite (Ag0Z) has been recognized as the state-of-art adsorbent for iodine retention in the nuclear fuel reprocessing off-gas treatment. It has also been shown to have a considerable adsorption capacity for water vapor, which is also a major component in the off-gases of spent nuclear fuel reprocessing facilities. Therefore, understanding the kinetics of water vapor adsorption on Ag0Z is necessary for a better design of off-gas treatment systems. The kinetics was studied through adsorption experiments of water vapor on Ag0Z pellets and analyses of the kinetic data with adsorption models that describe processes of mass transfer and inter/intra-crystalline diffusion. Uptake curves of water vapor on Ag0Z pellets were obtained with a continuous-flow adsorption system at temperatures of 25, 40, 60, 100, 150, and 200 oC and dew points from -53.6 to 12.1 oC. The diffusion controlling factors were determined experimentally and analytically. It was found that the diffusion process of water vapor in Ag0Z pellets was controlled by macropore diffusion. Gas film mass transfer resistance also contributed to the adsorption process of the 0.9 mm Ag0Z pellets, but it could be minimized with a high gas velocity and small pellet radius. Kinetic models including macropore diffusion (MD), linear driving force (LDF) and shrinking core (SC) were used to fit the uptake curves. The macropore diffusivity for water vapor adsorption on Ag0Z pellets was determined using the three models. It was found that the LDF and SC models could well describe the kinetic process, while the fitting with the MD model was not quite as good due to the existence of external mass transfer resistance for the 0.9 mm Ag0Z pellets.
... More details of the microbalance and adsorption column were described previously. 55,56 Description of the procedure Wide ranges of reduction temperatures (443-773 K) and reduction times (24-336 h) were studied to determine the optimal reduction conditions. At each set of conditions, about 10 g of AgZ were first pretreated with N 2 (500 mL/min) for 4 h at the reduction temperature to remove the residual moisture in the pellets, and then reduced with 4% H 2 /96% Argon at the same flow rate for the desired period of time. ...
... This model has been widely used to describe gas-solid adsorption systems. [56][57][58][59][60] In a previous study, 56 the SC model Iodine molecules mainly diffuse through the macro pores between the crystals to react with silver nanoparticles. Figure 6. ...
The adsorption process of iodine, a major volatile radionuclide in the off-gas streams of spent nuclear fuel reprocessing, on hydrogen-reduced silver-exchanged mordenite (Ag0Z) was studied at the micro-scale. The gas-solid mass transfer and reaction involved in the adsorption process were investigated and evaluated with appropriate models. Optimal conditions for reducing the silver-exchanged mordenite (AgZ) in a hydrogen stream were determined. Kinetic and equilibrium data of iodine adsorption on Ag0Z were obtained by performing single-layer adsorption experiments with experimental systems of high precision at 373 – 473 K over various iodine concentrations. Results indicate approximately 91% to 97% of the iodine adsorption was through the silver-iodine reaction. The effect of temperature on the iodine loading capacity of Ag0Z was discussed. The Shrinking Core model describe the data well, and the primary rate controlling mechanisms were macro-pore diffusion and silver-iodine reaction. This article is protected by copyright. All rights reserved.
... The mass transfer coefficient in Eq. (11) accounts for micro-and macropore diffusion as well as film diffusion phenomena. The coefficient is evaluated as follows (Lin et al., 2014): ...
Sorption enhanced synthesis has been previously shown to improve carbon dioxide hydrogenation to methanol by mitigating the thermodynamic limitations. This work investigates the efficiency of methanol synthesis via sorption enhanced carbon dioxide hydrogenation focusing on determining the optimal process parameters. The study is based upon a fully dynamic experimentally validated model of the process which is extended to account for adsorbent regeneration, downstream product separation and recirculation of the unreacted gases. An additional reactor configuration with a guard adsorbent layer is proposed for production of high purity methanol product. A multi-objective optimization study is performed to investigate the tradeoff between methanol production rate and product purity. The obtained results indicate that for synthesis of high purity methanol product, the optimal values of reactor temperature and catalyst mass fraction in the bed are 215 oC/0.65 and 235 oC/0.50 for the adiabatic and quasi-isothermal reactors, respectively.
... The continuous flow adsorption system was used for iodine and water vapor adsorption on multiple materials previously by Nan et al. 19,38,39 and Lin et al. 40,41 and has been modified for performing the adsorption of CH 3 I on Ag 0 -Aerogel experiments. Figure 1 shows the schematic diagram of the modified adsorption system. ...
The low concentration methyl iodide (CH3I) adsorption process on reduced silver-functionalized silica aerogel (Ag0-Aerogel) was studied. The kinetic data were acquired using a continuous flow adsorption system. Because the corresponding physical process was observed, the shrinking core model (SCM) was modified and applied. An average CH3I pore diffusivity was calculated, the CH3I-Ag0-Aerogel reaction was identified as a 1.40 order reaction instead of first order reaction, and the nth order reaction rate constant was determined. This modified SCM significantly increases the accuracy of adsorption behavior prediction at low adsorbate concentration. Modeling results indicate that the overall adsorption process is controlled by the pore diffusion. However, at low adsorbate concentration (20-30 ppbv), the CH3I adsorption is limited to the surface reaction due to the low uptake rate in a predictable time period.
... Jain and Gupta (1994) investigated the dehydration of isopropyl alcohol using 4A zeolite, wherein they provided a bi-disperse model to describe the uptake kinetics on a zeolite pellet. In more recent developments, Lin et al. (2014) compared three different types of kinetic model, including shrinking core model to describe the water uptake in 3A zeolite. In the bi-disperse model, the transport of species is separately modeled in the zeolite and the binder phase. ...
This paper dwells into the diffusional transport of water inside a pelleted zeolites wherein the transport behavior in binder phase as well as in the zeolite phase is explored. It is noted that both the phases contributes towards the separation process. A new variant of bi-disperse diffusion model is proposed for the water transport inside the adsorbent pellets. The equilibrium uptake of water is separately encountered in the binder and zeolite phases using a dual site Henry-Langmuir isotherm and a thermodynamically consistent correlation is developed for the boundary layer concentration of the zeolite phase with respect to the concentration within the binder phase. Adsorption kinetics measurements are reported for various adsorbent sizes and process temperatures, and their influence on transport diffusivity is explored. The proposed model prediction is in good agreement with the experimental observation in which the pellet diffusion controls the overall transport behaviour.
... Al Wahedi, Rabie, Al Shaiba, Geuzebroek, Daoutidis (2016) suggested optimizing the natural gas dryer using mixed integer nonlinear programming (MINP) and reported savings of up to 37 %. Lin et al. (2014) presented a kinetic model for water adsorption using molecular sieve 3A and suggested applying the surface heterogeneity on 3A. Taheri Qazvini and Fatemi (2015) studied simultaneous removal of water and mercaptan over molecular sieve 13X in a pressure-temperature swing adsorption process. ...
The 3A zeolite is widely used as an excellent adsorbent for industrial scale gas dehydration, due to its low energy demand for regeneration and ease of operation. The dehydration process of an industrial feed stream of ethane and water was numerically studied using an in-house code that included an appropriate equilibrium adsorption isotherm. The validated computational model was used and the impact of particle size on the process dynamics and the corresponding pressure drop were studied. In addition, the water concentration along the adsorption column was investigated. To increase the process capacity, the packed adsorption bed was divided into two distinct layers operating with different particle sizes and the length of each layer was determined according to the performed parametric study. The best breakthrough time (of 107,800 s) at the pressure drop allowance was obtained for the case that the length of first and the second layers were 4.5 m and 1 m, respectively. Finally, it was found that the new two-adsorption bed can save around 33.8 % in total energy requirement in comparison with the single size bed.
... and % & is solid porosity. is concentration at solid at equilibrium with initial gas concentration ( ). Lin et al. (2014) proved that for gas services, the micropore resistance is negligible especially for molecular sieves because of cation charge. ...
In this work, a novel multi-layer pressure-temperature swing adsorption (PTSA) process was designed for efficient simultaneous water and mercaptans removal from natural gas (NG) to less than 0.1 ppmv and 3 ppmv in a mini liquefied NG unit. The proposed multi-layer PTSA consists of a three-layer fixed bed including activated alumina, molecular sieves 4A and 13X. To gain in-depth insights about the process, a descriptive model considering mass, energy and momentum balances, along with the kinetic and equilibrium equations was developed. After validating the model with the experimental and operational data from the literature, the total energy requirement and long-term operational requirements (e.g. maximum water and mercaptan removal during regeneration process) were optimized. Results of the multi-objective optimization revealed that substitution of present series of dehydration and mercaptan removal columns with an integrated multi-layer PTSA for NG purification can decrease 5.1% of energy consumption, which is approximately equivalent to 137 GJ each year. The outcomes of this study can be used as an innovative design strategy for NG purification (i.e. the combination of dehydration and mercaptan removal columns in a single multi-layer PTSA bed) and can also provide process engineers with a cost-effective tool for the optimization of regeneration parameters in the present PTSA systems.
... The water adsorption isotherms were obtained with a continuous-flow adsorption system similar to the system reported in detail previously. 19,20 A schematic diagram of this system is shown in Figure 1. The system included a water vapor generation unit, a heating unit, an adsorption unit, and a data acquisition unit. ...
This work is related to the removal of tritiated water and radioactive iodine from off-gases released during spent nuclear fuel reprocessing. Specifically, it is focused on the adsorption equilibrium of water on reduced silver mordenite (Ag0Z), which is the state-of-art solid adsorbent for iodine retention in the off-gas treatment. As the off-gases contain different gas species, including iodine and water, Ag0Z would take up iodine and water simultaneously during the adsorption process. Therefore, understanding the adsorption of water on Ag0Z is important and necessary for studying the performance of Ag0Z in off-gas treatment processes. The isotherms of water (nonradioactive water) on Ag0Z were obtained at temperatures of 25, 40, 60, 100, 150, and 200 oC with a continuous-flow adsorption system. The data were analyzed using the Heterogeneous Langmuir and Generalized Statistical Thermodynamic Adsorption (GSTA) models, and thermodynamic parameters of the isotherms were obtained from both models. Both models were found capable of describing the isotherms. Isotherms of water on the unreduced silver mordenite (AgZ) were also obtained at 25, 40, and 60 oC and parameterized by the GSTA model. Through the comparison of the isotherms of Ag0Z and AgZ, it was found that Ag0Z had a higher water adsorption capacity than AgZ. The comparison of their thermodynamic parameters suggested that the interaction of water molecules with the H+ in Ag0Z was stronger than that with the Ag+ in AgZ.
... 6 The value of the surface diffusion coefficient lies in the range of 10 −12 −10 −11 m 2 /s, which corresponds well to values reported in the literature for water molecules. 6,13,37 The mathematical model describes the breakthrough time (when C out /C in = 0.10) for all mass flows, within 7% accuracy. All calculated data are within 15% of the measured concentration values (see Figure 6). ...
The drying of dichloromethane with a molecular sieve 3A packed bed process is modeled and experimentally verified. In the process, the dichloromethane is dried in the liquid phase and the adsorbent is regenerated by water desorption with dried dichloromethane product in the vapor phase. Adsorption equilibrium experiments show that dichloromethane does not compete with water adsorption due to size exclusion; the pure water vapor isotherm from literature provides an accurate representation of the experiments. The breakthrough curves are adequately described by a mathematical model that includes external mass transfer, pore diffusion, and surface diffusion. During the desorption step, the main heat transfer mechanism is the condensation of the superheated dichloromethane vapor. The regeneration time is shortened significantly by external bed heating. Cyclic steady state experiments demonstrate the feasibility of this novel drying process.
... Single-layer adsorption data were obtained for both iodine and water on Ag o Z, containing 11.9 wt% silver, over a wide range of possible operating conditions using a continuous-flow adsorption system. [2][3][4] Iodine adsorption experiments were conducted at adsorption temperatures of 100, 150 and 200°C, over iodine concentrations of 9-52 ppmv. Water adsorption experiments were conducted at the same temperatures and over water vapor concentrations in terms of dew points ranging from -60 to 20 o C. ...
Radioactive iodine (I129) and tritium (tritiated H2O) are
the major volatile radionuclides released in the off-gas
streams of spent nuclear fuel reprocessing that have to be
captured. The co-adsorption of iodine and water by reduced
silver-exchanged mordenite (AgoZ) is studied to support the
development of multi-component adsorption models for the
separation and recovery of radio isotopic gases in off-gas
streams in nuclear fuel reprocessing. Single-component
adsorption equilibrium data of iodine and water were
obtained to determine the saturation capacity and evaluate
the parameters of the generalized statistical thermodynamic
adsorption (GSTA)1 model being developed for modeling of
co-adsorption. Co-adsorption data acquisition is in progress
for model validation. Kinetic data were obtained to
determine the intraparticle mass transfer and reaction
parameters for the adsorption models. In addition, work is in
progress to obtain data for the Ag-I reaction at various
conditions to study the reaction kinetics, which will provide
insight into the chemisorption process of iodine on AgoZ.
... A continuous flow adsorption system was used to obtain adsorption isotherm data. While details on the system were reported previously, 12,15 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). ...
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.
... In our system, the formed water was excluded by adding molecular sieve 3A with excellent adsorption of water into the reaction solution. 26 shown in Figure 3d, the introduction of molecular sieve 3A removed the produced water in the reaction and hardly affected the reaction behavior. The system of benzyl alcohol, 18 O 2 , and molecular sieve 3A in CH 3 CN gave unlabeled benzaldehyde (Figure 2d), obviously different from the system without molecular sieve. ...
The oxidation of alcohols to the corresponding carbonyl compounds is a pivotal reaction in organic synthesis. Under visible light irradiation, the homogeneous CuCl2 and cheap solvent oxidized benzyl alcohol into benzaldehyde with a selectivity higher than 95% using molecular oxygen as oxidant. The formation of a visible light responsive complex between Cu(II) and solvent is responsible for the occurrence of the oxidation of benzyl alcohol. During the photocatalytic process, molecular oxygen wasn't incorporated into the final benzaldehyde, and only involved in the oxidation of Cu(I) into Cu(II) in which it served as a terminal hydrogen acceptor to form H2O. A similar role of molecular oxygen has also been observed in heterogeneous TiO2 photocatalytic system. The understanding of the role of molecular oxygen helps us to further design new classes of synthetic organic reactions by photocatalytic process.
The CO2 and N2 adsorption on small-pore zeolite |Na12-xKx|-A was hypothesized to be affected by the ion exchange sequence used for the zeolite preparation. Zeolites were prepared by ion exchange of a commercially available zeolite |Na12|-A (4A) and a zeolite |K12|-A (3A) composition prepared from zeolite 4A. The CO2 and N2 adsorption properties were studied experimentally, and the binary CO2-over-N2 selectivity was estimated from single-component adsorption data using the apparent Henry's law coefficients. It was observed that the level of CO2 adsorption was reduced by increasing the K content for both series of zeolite NaKA. Zeolite |Na12-xKx|-A-from-4A had the highest CO2 adsorption capacity (at 1 atm and 273 K) for a given K content. At low K content, zeolite |Na12-xKx|-A-from-3A had the highest CO2-over-N2 selectivity. At an intermediate K content, the zeolites prepared from 4A had the highest selectivity. These differences show that non-equilibrium processes during the ion exchange are important for the CO2 and N2 adsorption properties of the derived zeolites. As of now, we refrain from speculating whether they relate to the detailed positioning of K+ and Na+ cations in the local structure of the zeolite or to mass-transport-related concentration gradients of the cations in the structure. Irrespectively, it was observed that the ion-exchange sequence affects the CO2 and N2 adsorption properties of the zeolites, which could be of general importance when it comes to the tuning of the properties of cation-rich zeolites.
The necessity of ultrapure water and water-transport infrastructure pose grand challenges in renewable-energy-assisted water electrolysis to produce green hydrogen. Directly accessing atmospheric water should offer a decisive solution because it provides ∼13 trillion kiloliters of pure water at any given instant. We show that the central challenge for atmospheric water electrolysis is related to the water-sorption kinetics of the proton-conducting membrane where state-of-the-art membranes critically fail. A proof-of-concept atmospheric water electrolyzer is demonstrated with a graphene oxide proton-conducting membrane, which has nearly three times higher water-sorption kinetics and ten times higher hydration number than a Nafion membrane due to capillary water condensation and the abundant presence of hydrophilic functionalities. At a wind velocity of ∼50 km/h, this electrolyzer delivers nearly 18 mL/h/cm² of green hydrogen directly from the feedstock of atmospheric water. Because this electrolyzer does not require water-transport infrastructure, it can be placed almost anywhere, which offers opportunities for decentralized green hydrogen production.
Atomic layer deposition (ALD) is a technique capable of depositing conformal coatings in highly tortuous 3D nanostructures. One configuration that has attracted attention is nanocrystal (NC) based nanocomposite films, whereby a 3D network of randomly packed nanocrystals is infilled via ALD to yield a dense nanocomposite. In this work, we demonstrate criteria for predicting three important thermal ALD process parameters necessary to completely infill 3D NC networks: cycle number, precursor pulse time, and purge time. A description of representative pore geometry is developed using parameters of the film comprised of nanocrystals before infill, specifically NC diameter, NC volume fraction, and film thickness. This geometric description allowed for prediction of required precursor pulse times to saturate the NC film surface. A finite-difference model of water vapor transport during purging revealed that desorption kinetics can be used to predict purge times required to achieve complete infill. The model predictions show good agreement with experiments carried out by infilling films comprised of GaN NCs with ZnO by the diethylzinc/water process and films comprised of Al2O3 NCs with Al2O3 by the trimethylaluminum/water process.
Off-gases produced during the reprocessing of used nuclear fuel (UNF) include ¹²⁹I2, ³HHO, ¹⁴CO2, ⁸⁵Kr, and ¹³⁵Xe, which are volatilized out into the off-gas. In order to meet regulatory requirements for reprocessing plant emissions, these gases must be captured and removed from the off-gas stream prior to off-gas emission. Of particular interest are the noble gases, Kr and Xe, which can be fairly difficult to remove from the off-gas due to their low chemical reactivity. Thus, this work is focused on utilizing engineered adsorbents, AgZ-PAN and HZ-PAN, to capture Kr and Xe from a mixed-gas stream at relatively low temperatures (191–295 K) and various flow rates (50–2000 mL/min). Isothermal data for Kr and Xe on each adsorbent are analyzed to produce the Langmuir parameters needed to model the mixture adsorption capacities at relevant temperatures using the Extended Langmuir model. Those parameters are then incorporated into a fixed-bed adsorption model developed in this work using the Mulitphysics Object-Oriented Simulation Environment (MOOSE). That model is used to simulate breakthrough times for Kr and Xe in packed columns of AgZ-PAN and HZ-PAN, ranging in length from 6 to 20 in., at relevant temperatures and flow rates. Breakthrough times varied from nearly instantaneous for Kr in AgZ-PAN to 30 h for Xe in HZ-PAN. After the developed model was validated by comparisons with experimental breakthrough data, the model framework was used to simulate the performance of multiple fixed-bed columns connected in series.
The effectiveness of zero-valent iron (ZVI) nanoparticles in applications from water remediation to catalysis is intimately tied to adsorption and oxidation processes at the nanoparticle surface. Understanding water sorption and ZVI oxidation as a function of surface-sorbed organic ligand properties can provide new fundamental insights into tuning the reactivity of the nanoparticles. In this work, ZVI nanoparticles were synthesized in the presence of four different organic ligand molecules: two carboxymethyl cellulose polymers of different molecular weights and two phosphonate chelators with different known iron chelation strengths. The resulting ZVI nanoparticles were similar in size (~100 nm), and adsorption and oxidation behavior are compared on the basis of the properties of the ligand sorbed to the surface of the ZVI nanoparticles. Adsorption and oxidation processes are studied via quartz crystal microbalance (QCM) measurements, where the change in nanoparticle mass is followed over time as the nanoparticles were exposed to varying levels of relative humidity in air or oxygenated water. A clear dependence was shown between measured change in mass and either chelation strength or polymer molecular weight. An increase in either the ligand size or the chelation strength reduced oxidation in oxygenated water. Ligand size resulted in an increase in water vapor adsorption. Reversible mass changes were observed for RH values ≤ 50% and as a function of ligand, suggesting water sorption, while irreversible mass changes were observed for RH values ≥ 50% and suggest ZVI oxidation. QCM results were further corroborated with dynamic light scattering, zeta-potential measurements, and scanning electron microscopy. Our results suggest that water adsorption on and oxidation of ZVI nanoparticles may be engineered to a suitable degree through a more thorough understanding of ligand-ZVI interactions.
Methanol vapor as a containment presents in industrial streams, leads to catalysts deactivation. Therefore, it needs deep removal with adsorption technique. The isotherms and effective diffusion coefficients of methanol vapor are essential to design the purification process. The equilibrium sorption of methanol steam on commercial zeolites NaX, KA, CaA and activated alumina were tested using intelligent gravimetric analyzer, respectively. The equilibrium experiments were carried on at 303.2 K and pressures of up to 19.6 kPa and the results obtained were fitted with the Dubinin-Astakhov (D-A) model. Moreover, the diffusion of methanol vapor in the adsorbents was investigated and the effective diffusion coefficients were obtained with fitting the Fick diffusion model．The adsorbents are compared on the basis of adsorption capacities for methanol steam and the descending order is zeolites NaX>CaA>KA>Al2O3. The sorption equilibrium experimental results of methanol steam on NaX, KA and CaA agree well with D-A isotherm model，while it is not very well consistent on Al2O3. The equilibrium adsorption capacities of methanol vapor are consistent with the effective diffusion coefficients on the adsorbents.
This research highlights the dehydration of dimethyl amino ethyl azide (DMAZ), parametric optimization, and describes equilibrium and kinetics of water adsorption on 3A sieves.
Central composite design (CCD) was successfully employed for experimental design and analysis of the results. It was indicated
that the optimum adsorption capacity (191.3 mg/g) was obtained
at contact time = 150min, liquid/solid ratio = 10:1, initial concentration of water = 3wt% and agitating rate = 150 rpm. Langmuir-Freundlich and intra-particle diffusion models were appropriate to describe isotherms and kinetics of dehydration, respectively. Thermal stability and regeneration behavior of the adsorbent were investigated using XRD and TGA/DSC methods. It was revealed that the best regeneration occurred in the range of 250-300°C and the zeolite structure was stable up to 400 °C. Also, the thermal analysis verified that DMAZ molecules cannot diffuse into the aforementioned zeolite pores.
The laminated structure of graphene oxide (GO) confers unique interactions with water molecules which may be utilised in a range of applications that require materials with tuneable hygroscopic properties. Precise roles of the expandable interlayer spacing and functional groups in GO laminates are not fully understood till date. Herein, we report experimental and theoretical study on the adsorption and desorption behaviour of water in GO laminates as a function of relative pressure. We have observed that GO imparts excellent water uptake capacity of up to 0.58 gram of water per gram of GO (g g-1), which is much higher than silica gel a conventional desiccant material. More interestingly, the adsorption and desorption kinetics of GO is one order of magnitude higher than silica gel. The observed extraordinary adsorption/desorption rate can be attributed to the high capillary pressure in GO laminates as well as micro meter sized tunnel like wrinkles located at the surface.
In the removal of volatile organic compounds (VOCs) from gas, the unavoidable presence of water can degrade the performance of adsorbent because water competes well with VOCs. In this study, the adsorption kinetic behavior of water vapor on hypercrosslinked polymeric adsorbent (HPA) at three temperatures (308 K, 318 K and 328 K) was studied through thermogravimetric analysis (TGA), and compared with granular activated carbon (GAC) and activated carbon fiber (ACF). The results showed that the kinetic rate constants decreased with the increase of relative pressure, but increased with the increase of temperature for three adsorbents. Under the same condition, the kinetic rate constants were as following: HPA > ACF > GAC, which was just opposite with the concentration of surface functional groups. Activation energies were found to increase with the increase of relative pressure, indicating the development of barriers to diffusion associated with the formation of water clusters. A compensation effect was observed, where activation energy and ln(preexponential factor) parameter exhibited a linear correlation.
We report about the synthesis and surface functionalization of MCM-41 like mesoporous silica nanoparticles (MSNs) with spheroidal shape and particle size of 141 ± 41 nm. The success of surface functionalization with aminopropyl and sodium ethylcarboxylate groups (giving amino-MSN and carboxy-MSN, respectively) was ascertained by infrared spectroscopy and ζ potential measurements. The former showed the decrease of surface silanol groups and the corresponding appearance of signals related to NH2 bending mode (δNH2) at 1595 cm-1 and COO- stretching (νas and νsym) at 1562 and 1418 cm-1. The latter showed a change in surface charge, in that the isoelectric point, IEP, changed from pH 3-4.5 to 8.5 when MSN was functionalized with the amino groups, while carboxy-MSN showed a more negative charge in the whole pH range with respect to MSN. The hydrophilic character of the prepared materials was ascertained by quantitative microgravimetric measurements, allowing the calculation of the average isosteric adsorption heat
In 1971 Wexler and Greenspan published a formulation for the vapor pressure of water encompassing the temperature range 0 to 100 °C. In this paper a revision is made of that earlier formulation to make it consistent with the definitive experimental value of the vapor pressure of water at its triple point recently obtained by Guildner, Johnson, and Jones. The two formulations are essentially identical at temperatures from 25 to 100 °C.
For temperatures below 25 °C the new formulation predicts values that are higher than the 1971 formulation. At the triple point, the vapor pressure given by the new formulation is 611.657 Pa whereas the value given by the 1971 formulation is 611.196 Pa. A table is given of the vapor pressure as a function of temperature at 0.1-deg intervals over the range 0 to 100 °C on the International Practical Temperature Scale of 1968, together with values of the temperature derivative at 1-deg intervals.
Effect of co-existing H2O (isotopic swamping effect) in the adsorption of tritiated water vapor on molecular sieves was experimentally studied by two means. One was by breakthrough experiment. It was found that H2O-HTO two components Langmuir type adsorption isotherm is satisfactory to explain the HTO breakthrough curve. The other was by using a divisible column, where behavior of HTO at much lower concentration than at the inlet and the axial distribution of tritiated water on the adsorbent were studied. Concentration of tritiated water vapor at the column outlet was found to decrease with the increase of H2O concentration of the input. However the improvement of decontamination factor was little.
Experiments were conducted on the dynamic adsorption of tritiated water vapor, using a column packed with either silica-gel, activated alumina, or molecular sieve 5A, The column was 4 mm l.D. and 5~30 cm long. The runs were performed at 30°~70-C. Tritiated water vapor (HTO) was loaded into the column with a concentration of 170 µ/µCi/ml, either in pulses of predetermined duration or continuously (breakthrough). The concentrations of HjO and HTO at column outlet were measured. Particular interest was attached to observing the effect of differences in the pretreatment applied to the adsorbents (whether dried or saturated with water), and in the H2O partial pressure of the carrier gas. It was found that the adsorption characteristics shown for HTO were not influenced to any appreciable extent by differences in the pretreatment applied to the adsorbents. On the other hand, adsorbent performance depended sensitively on the HjO pressure in the carrier gas. Adsorption models for a two-component mixture composed of H2O and HTO were sought for describing the adsorption mechanism. For the particular case of the molecular sieve 5A, the Langmuir-type mixed adsorption model was found to give values agreeing fairly well with experiment. A model postulating chemisorption of H2O into hydroxyl group on the adsorbent surface, to be subsequently replaced by tritium, was found suitable for explaining the adsorbent behavior in the case of silica-gel.
This book presents a critical review of various estimation procedures for a limited number of properties of gases and liquids: critical and other pure component properties; PVT and thermodynamic properties of pure components and mixtures; vapor pressures and phase-change enthalpies; standard enthalpies of formation; standard Gibbs energies of formation; heat capacities; surface tensions; viscosities; thermal conductivities; diffusion coefficients; and phase equilibria. For most cases, estimated properties are compared to experiment to indicate reliability. Most methods are illustrated by examples.
Tritium and iodine-129 are two major radioactive elements that are present in off-gases from spent fuel reprocessing plants. Adsorption by solid sorbents is the state-of-the-art technique for removal of these species from off-gases. Modeling and simulating adsorption processes require accurate adsorption equilibrium and kinetic data to permit reasonable estimates of process parameters. We have developed a continuous flow single-pellet adsorption system to gather accurate adsorption equilibrium and kinetic data for adsorption of water by molecular sieve 3A and for adsorption of iodine by silver exchanged mordenite. In this paper, the design of the water and iodine adsorption experimental systems are briefly described and results of water adsorption experiments are presented and discussed. Water uptake curves are fitted with the linear-driving force model and the shrinking-core model to determine kinetic parameters. Preliminary results from iodine adsorption experiments will be presented in the conference.
The absence of industrial scale nuclear fuel reprocessing in the U.S. has precluded the necessary driver for developing the advanced simulation capability now prevalent in so many other countries. Thus, it is essential to model complex series of unit operations to simulate, understand, and predict inherent transient behavior. A capability of accurately simulating the dynamic behavior of advanced fuel cycle separation processes is expected to provide substantial cost savings and many technical benefits. To support this capability, a modeling effort focused on the off-gas treatment system of a used nuclear fuel recycling facility is in progress. The off-gas separation consists of a series of scrubbers and adsorption beds to capture constituents of interest. Dynamic models are being developed to simulate each unit operation involved so each unit operation can be used as a stand-alone model and in series with multiple others. Currently, an adsorption model has been developed within Multi-physics Object Oriented Simulation Environment (MOOSE) developed at the Idaho National Laboratory (INL). Off-gas Separation and REcoverY (OSPREY) models the adsorption of off-gas constituents for dispersed plug flow in a packed bed under non-isothermal and non-isobaric conditions. Inputs to the model include gas composition, sorbent and column properties, equilibrium and kinetic data, and inlet conditions. The simulation outputs component concentrations along the column length as a function of time from which breakthrough data can be obtained. The breakthrough data can be used to determine bed capacity, which in turn can be used to size columns. In addition to concentration data, the model predicts temperature along the column length as a function of time and pressure drop along the column length. A description of the OSPREY model, results from krypton adsorption modeling and plans for modeling the behavior of iodine, xenon, and tritium will be discussed.
Part 1 Equilibria: fundamentals of pure component adsorption equilibria practical approaches of pure component adsorption equilibria pure component adsorption in microporous solids multicomponent adsorption equilibria heterogeneous adsorption equilibria. Part 2 Kinetics: fundamentals of diffusion and adsorption in porous media diffusion and adsorption in porous media - Maxwell-Stefan approach analysis of adsorption kinetics in a single homogeneous particle analysis of adsorption kinetics in a zeolite particle analysis of adsorption kinetics in a heterogeneous particle. Part 3 Measurement techniques: time lag analysis in diffusion and adsorption in porous media analysis of steady state and transient diffusion cells adsorption and diffusion measurement by a chromatography method kinetics of a batch adsorber.
Tritiated water vapor in radioactive off-gas streams is determined by ; adsorption on silica gel and counted directly in a Tri-Carb liquid scintillation ; spectrometer. The tritiurm count rate varies with time as the tritium is ; extracted into the scintillator liquid and contains contributions from atoms ; adsorbed on the silica gel surface. (auth);
This document provides descriptions of the off-gases evolved during spent nuclear fuel processing and the systems used to capture the gases of concern. Two reprocessing techniques are discussed, namely aqueous separations and electrochemical (pyrochemical) processing. The unit operations associated with each process are described in enough detail so that computer models to mimic their behavior can be developed. The document also lists the general requirements for the desired computer models.
The results of an experimental program conducted to aid in the design of a tritium retention system to remove tritiated water from voloxidizer off-gases are presented. The retention system is expected to be a fixed-bed adsorption unit using a commercially available desiccant, such as molecular sieves, to dry the off-gases. The presence of iodine in the off-gas stream somewhat complicates the drying process since some iodine will be retained in the drying bed along with the tritiated water. The present work represents a follow-up to a study in which a small-scale (2-in.-diam by 30-in.-long) packed column of Linde Molecular Sieves (LMS) type 3A was repeatedly loaded and regenerated using a non-radioactive simulated voloxidizer off-gas containing water and iodine vapor. Both water and iodine loadings were measured and the regeneration characteristics of the bed were observed. The following studies were carried out: (1) testing of other desiccants showed LMS type 3A to be superior because of its high water loading and low iodine retention; (2) development of a column-mounted moisture detector; (3) adsorption isotherms; (4) iodine analysis using a commercial oxidant monitor; (5) tests on cartridge-type beds - a series of tests were conducted using three small drying beds connected in series. One further finding of this study was the importance of the clay binder (used in pelletized molecular sieves) in obtaining satisfactory or acceptably low iodine retention.
A new formulation is presented for the vapor pressure of ice from the triple point to minus 100 degree C based on thermodynamic calculations. A table is given of the vapor pressure as a function of temperature at 0. 1-degree intervals over the range 0 to minus 100 degree C, together with the values of the temperature derivative at 1-degree intervals. The formulation is compared with published experimental measurements and vapor pressure equations. It is estimated that this formulation predicts the vapor pressure of ice with an overall uncertainty that varies from 0. 016% at the triple point to 0. 50% at minus 100 degree C.
The solid-film-resistance linear-driving-force model (l.d.f.) for mass transfer provides a mathematically simple but adequate description of non-isothermal gas sorption kinetics in a differential test. Analytical equations for mass uptake and adsorbent-temperature–time profiles are derived. The model allows direct calculation of a mass-transfer coefficient from the experimental uptake data to be used in the design of adsorbers. Interactions between the equilibrium and transport properties of the adsorption system predicted by the l.d.f. model are equivalent to those given by other mathematically more complicated non-isothermal models.
Experimental uptake curves for adsorption of n-butane and n-pentane on 5 Å zeolite and i-octane on 13X zeolite were quantitatively described by the l.d.f. model. The analysis shows that a ‘surface barrier’ may be the principal resistance to mass transfer for these systems.
Differential heats of adsorption of water vapour on zeolites LiNaX, NaX, KNaX, RbNaX and CsNaX were measured calorimetrically over a wide range of adsorption. The heat of adsorption of water is determined mainly by the energy of specific interaction of the water molecules with the exchange cations, with the negatively charged oxygen ions of the zeolite framework, and with each other. The contributions of these interactions change with change of cation and with degree of filling; hence the dependence of the heat of adsorption of water on the amount adsorbed is wave-shaped in nature. This is due apparently to the presence of some groups of energetically different sites for water adsorption in the cavities of zeolites. Alternation of decrease and increase in the heat of adsorption of water with increasing coverage was particularly evident with KNaX zeolite. At low fillings the heats of adsorption of water decrease from LiNaX to CsNaX zeolites. Removal of Li+ by washing, sharply decreases the heat of adsorption of water at small coverages.
Removal mechanisms of HTO from variously pretreated and non-pretreated silica gel columns were investigated with pulse-loading with tritiated water vapor. With non-pretreated silica gel, the HTO physisorbed on the upper part of the column comes into contact with surface hydroxyl groups while passing downward the column, so that in each equilibration a part of the tritium is incorporated into hydroxyl groups by isotopic exchange reactions. With the silica gel pretreated at a temperature below 400°C, most of tritium in the applied HTO is easily incorporated into surface hydroxyl groups in the upper part of the column either by isotopic exchange reactions or by rehydration of the dehydrated surface (siloxyl linkage). In the pretreatment above 400°C, essentially all the tritium is trapped by siloxyl groups of various stabilities. The ease of rehydration of siloxyl groups by applied HTO depends on their stabilities, which, in turn, depend on the pretreatment temperature. As a general trend, treatment at higher temperature promotes annealing of the constrained siloxyl groups and thus the rate of rehydration become slower.
Experiments were conducted on the dynamic adsorption of tritiated water vapor, using a column packed with either silica-gel, activated alumina, or molecular sieve 5A. The column was 4 mm I.D. and 5–30 cm long. The runs were performed at 30°–70°C. Tritiated water vapor (HTO) was loaded into the column with a concentration of 170 μμCi/ml, either in pulses of predetermined duration or continuously (breakthrough). The concentrations of H2O and HTO at column outlet were measured. Particular interest was attached to observing the effect of differences in the pretreatment applied to the adsorbents (whether dried or saturated with water), and in the H2O partial pressure of the carrier gas. It was found that the adsorption characteristics shown for HTO were not influenced to any appreciable extent by differences in the pretreatment applied to the adsorbents. On the other hand, adsorbent performance depended sensitively on the H2O pressure in the carrier gas.Adsorption models for a two-component mixture composed of H2O and HTO were sought for describing the adsorption mechanism. For the particular case of the molecular sieve 5A, the Langmuir-type mixed adsorption model was found to give values agreeing fairly well with experiment. A model postulating chemisorption of H2O into hydroxyl group on the adsorbent surface, to be subsequently replaced by tritium, was found suitable for explaining the adsorbent behavior in the case of silica-gel.
Equations are presented which relate saturation vapor pressure to temperature for moist air. The equations are designed to be easily implemented on a calculator or computer and can be used to convert in either direction. They are more accurate than the commonly used Goff-Gratch equations for the meteorologically interesting region of 80 to +50°C. Equations also are given for the enhancement factor.
Effect of co-existing HâO (isotopic swamping effect) in the adsorption of tritiated water vapor on molecular sieves was experimentally studied by two means. One was by breakthrough experiment. It was found that HâO-HTO two components Langmuir type adsorption isotherm is satisfactory to explain the HTO breakthrough curve. The other was by using a divisible column, where behavior of HTO at much lower concentration than at the inlet and the axial distribution of tritiated water on the adsorbent were studied. Concentration of tritiated water vapor at the column outlet was found to decrease with the increase of HâO concentration of the input. However the improvement of decontamination factor was little.
The adsorption of tritiated water vapor on molecular sieves 5A and 13X, silica gel and activated alumina was investigated in the small column experiments. The results were analyzed by a simple first order kinetic equation. The adsorption characteristics of the adsorbents are rationalized by the two important parameters, maximum adsorption capacity and adsorption rate constant, which were determined at various temperatures for each adsorbents. The column exhaustion experiments were also carried out. An equation was derived to express the correlation between breakthrough time and column length. A reasonable consistency has been noticed between the calculated and the observed results.
The adsorption of water vapor on BaY and BaX zeolites is studied by thermogravimetry and calorimetry under isothermal or isobaric conditions. The faujasite zeolite exhibits a stronger adsorption affinity for water molecules with barium than with sodium or potassium as exchangeable cations. The adsorption–desorption isotherm at 150°C shows a hysteresis loop attributed to the migration of compensating cations during their interaction with water molecules. Calorimetric measurements and the Dubinin–Radushkevich’s model both show that the adsorption of water occurs via a three-step mechanism: adsorption on compensating cations, formation of a monolayer on the walls and multilayer adsorption in the cavities. Water probes the complete microporosity of the zeolite. After several isobaric adsorption–desorption cycles, the structure of BaX undergoes severe damages (complete loss of the microporosity and amorphization of the solid) whereas that of BaY remains intact.
Due to its low energy consumption and ease of operation, the zeolite 3A molecular-sieve pressure-swing adsorption process is currently becoming the method of choice for the production of fuel ethanol worldwide. Accurate correlation of the equilibrium adsorption isotherms of water vapor in zeolite 3A is required for the reliable modeling and simulation of that process. In this paper, we firstly show that popular adsorption isotherm models such as those of Langmuir, Sips, Toth, UNILAN, and Dubinin-Astakhov are uncapable of correlating the available equilibrium adsorption isotherms of water vapor in zeolite 3A. Hill's statistical thermodynamic adsorption model, which was originally expressed in terms of absolute activities and canonical partition functions for sites with a variable number of molecules adsorbed, is rewritten here in terms of the pressure and a set of temperature-dependent adjustable parameters that turn out to correspond to the equilibrium constants of adsorption in those sites. In contrast to other recast versions of Hill's model, our version does neither parameterize nor decouple the canonical partition functions or the configurational integrals, and can be applied to the adsorption of either non-polar or polar molecules. Our generalized model allows not only an easier interpretation of the isotherms fit but also the prediction of thermochemical quantities such as the differential and integral heats of adsorption. From the application of our generalized model, we obtain a very accurate correlation of the available equilibrium adsorption isotherms of water vapor in zeolite 3A in the temperature range from 0 to 100 degrees C. We also provide thermochemical and structural interpretations of the isotherms fit and made predictions for the isosteric heat of adsorption and the integral heat of immersion that are in excellent agreement with the experimental data.
Thesis (Ph. D.)--University of Tennessee, Knoxville, 1994. Vita. Includes bibliographical references (leaves [251]-257).
A review of a certain class of theoretical models describing the kinetics of pollutants sorption onto various sorbents is presented. These assuming the rate of surface reaction as the rate-limiting step are considered. A special attention is paid to possible theoretical grounds of the most commonly applied mathematical expressions, such as the pseudo-second and the pseudo-first order equations. Simple theoretical considerations based on some fundamental theories suggest that these two formulae do not correspond to any specific physical model. They simply approximate well the behaviours predicted by many different theoretical approaches.
Spent Fuel Reprocessing. Presented at theIntroduction to Nuclear Chemistry and Fuel Cycle Separations Course
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