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"Heat of adsorption"
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Adsorption heat transformer cycle using multiple adsorbent + water pairs for waste heat upgrade
Adsorption heat transformers (AHTs) are considered as promising systems for upgrading waste heat to a higher temperature. The cycle operates among three temperature reservoirs: (i) heat sink at the low temperature (
T
L
), (ii) heat source at the medium temperature (
T
M
), and (iii) heat supply at the high temperature (
T
H
). In the present study, the performance the AHT cycle was analyzed for possible applications in the waste heat upgrade and thermal desalination. An equilibrium model was developed using adsorption characteristics and isotherm data. Five types of commercially available silica gels and three types of zeolites were investigated as adsorbents. Nonlinear optimization technique was utilized for the determination of the intermediate pressure and uptake for preheating and precooling phase of the AHT cycle. The performance parameters in terms of useful heat ratio and condensation heat ratio were determined and compared for the reservoir temperatures at 30 °C (
T
L
)—60 °C (
T
M
)—80 °C (
T
H
). Parametric evaluation of the performance parameters was carried out based on the variation in gross temperature lift, as well as the heat exchanger mass ratio. It was found out that reduction in the gross temperature lift had a positive impact on the useful heat ratio and a negative influence on the condensation heat ratio of the AHT cycle. Significant variations in the maximum adsorption capacity and slope of the isosteric heat of adsorption across various adsorption pairs containing zeolites were observed. As a result, AQSOA-Z01 zeolite exhibited the highest heat exchange values of the AHT cycle in the range of ~ 320–370 kJ per kg of adsorbent. On the contrary, type AQSOA-Z02 zeolite displayed the lowest corresponding values in the range of ~ 60–90 kJ kg
-1
of adsorbent. On the other hand, variation across the different silica gel adsorbents was comparatively smaller because of similar isotherm and isosteric heat of adsorption characteristics. This study will assist the research on the theoretical development of the AHT cycle via material selection and system design optimization.
Journal Article
Estimation of isosteric heat of adsorption from generalized Langmuir isotherm
Simulation and design of adsorptive separation units demand accurate estimation of thermodynamic properties. Isosteric heat of adsorption as calculated from generalized Langmuir (gL) isotherm coupled with Clausius–Clapeyron expression for pure component and mixed-gas adsorption equilibria is presented in this work. The estimated isosteric heat of adsorption as functions of surface loading and composition is validated against the experimental data for various adsorption systems. Furthermore, the gL results are compared against classical Langmuir (cL) and Toth isotherm for pure components and with Ideal Adsorbed Solution Theory (IAST) for mixed-gas adsorption equilibria. The comparison highlights that gL outperforms cL and Toth for pure component adsorption and IAST for mixed-gas adsorption, and gL reliably captures the loading dependence and the composition dependence for isosteric heat of adsorption.
Journal Article
Adsorption of organic solvent vapours on pristine and doped few-layer graphene nanoflakes
Heterosubstitution is widely used to control the surface properties of graphene materials. The knowledge of the mechanism of organic solvent vapour sorption on doped graphene materials is necessary for development of air purification technologies, volatile organic compounds sensors, metal-free catalysis and for many other applications. The effect of N, S and Si doping and oxidative functionalization of few-layer graphene nanoflakes on the adsorption of organic solvent vapours was measured. The nanoflakes were also analyzed by TEM, XPS, Raman spectroscopy and low-temperature nitrogen physisorption. Special attention was paid to the dependence of the isosteric heat of adsorption on the surface coverage for various adsorbate-adsorbent pairs, which carry information about the energy inhomogeneity of the surface, the hierarchy of adsorbate-adsorbate, adsorbate-basal plane and adsorbate-functional groups interactions, and the mechanism of adsorption. This dependence for the hexane sorption can be used to detect hydrophilic groups on the surface, and to compare the degree of curvature of graphene layers in different heterosubstituted graphene materials.
Graphical Abstract
Journal Article
Thermodynamic analysis of promising MOF/water pairs for adsorption cooling systems
Metal-organic frameworks (MOFs) are a class of porous coordination polymers constructed from co-ordinately binding metals and organic linkers. Aluminium fumarate, a microporous MOF composed of aluminium and fumaric acid, has a high affinity toward water vapor adsorption. The adsorption process in such porous materials could be featured for cooling by harnessing thermal energy, reducing the need for an energy-intensive vapor compression refrigeration system. The thermodynamic property field analysis is crucial for calculating the adsorption cooling system’s energetic performance. In this work, the thermodynamic property fields of pristine aluminium fumarate and nickel and cobalt-doped aluminium fumarates were analyzed, and the results were compared from the viewpoint of adsorption cooling application. The water adsorption isotherms on these samples were correlated with the Sun and Chakraborty and the Universal models. Thermodynamic properties, isosteric heat of adsorption, and specific heat capacity of the MOF/water pairs were investigated, and results were analyzed with respect to the adsorbate uptake. The performance parameters, specific cooling effect, and coefficient of performance were studied and compared for the samples. Nickle and cobalt-doped aluminium fumarates have presented a higher specific cooling effect than the pristine MOF. This analysis provides crucial findings contributing to design practical MOF/water-based adsorption cooling systems.
Journal Article
Modelling Methane Adsorption Isotherms on Shale at Different Temperatures
Prediction of adsorption isotherms under different temperatures is significant to reserve estimation of shale gas reservoirs. Based on the Polanyi adsorption potential theory and Langmuir adsorption theory, a method was presented to predict adsorption isotherms at different temperatures. In this method, the relationship between Langmuir pressure and temperature was quantified. By utilizing this method, we predicted adsorption isotherms of Longmaxi shale from 45 °C to 120 °C according to experimental data at 30 °C. Meanwhile, by considering the pressure and temperature gradients, we also predicted the adsorption capacity of Longmaxi shale with geologic depth. Results show that the adsorption data predicted by our method are in accord with experimental data and the error coefficient is less than 5%. In addition, the isosteric heat of adsorption for Longmaxi shale can be calculated by predicted adsorption data based on the Clausius–Clapeyron equation of capillary systems. Therefore, our study illustrates theoretical foundations for the efficient evaluation of adsorbed gas content in shale gas reservoirs at geologic conditions.
Journal Article
Investigation of the isosteric heat of adsorption for supercritical methane on shale under high pressure
The isosteric heat of adsorption (IHA) is one of the key thermodynamic variables for evaluating the interaction between shale and methane, which is rarely studied especially under high pressure. In this work, we conducted methane adsorption experiments at pressures up to 30 MPa and different temperatures on shale samples collected from Longmaxi formation in Sichuan Basin, China. Based on the definition of IHA and Langmuir adsorption model, we proposed a new method to analyze the IHA of methane on shale under four conditions. The calculated results show that the commonly used Clausius–Clapeyron equation overestimates the true isosteric heat of shale, especially under high pressure. IHA under four conditions yield a fixed order as qst,i-va > qst,r-va > qst,i+va > qst,r+va, indicating both the real gas behavior and the adsorbed-phase volume have a negative influence on it, and the effect of adsorbed-phase volume is dominant. Moreover, IHA at zero coverage (
q
st
0
) in Henry region determined by linear fitting can be regarded as a maximum value in the above four cases, which is independent of pressure and temperature. Therefore,
q
st
0
can be used as a unique descriptor to evaluate the adsorption affinity of the shale. This work modified the method to obtain the true IHA of supercritical methane on shale more accurately, which lays the foundation for future investigations of the thermodynamics and heat transfer characteristics of the interaction between high pressure methane and shale.
Journal Article
Efficient Adsorption of Ammonia by Surface-Modified Activated Carbon Fiber Mesh
In view of the characteristics and risks of ammonia, its removal is important for industrial production and environmental safety. In this study, viscose-based activated carbon fiber (ACF) was used as a substrate and chemically modified by nitric acid impregnation to enhance the adsorption capacity of the adsorbent for ammonia. A series of modified ACF-based adsorbents were prepared and characterized using BET, FTIR, XPS, and Boehm titration. Isotherm tests (293.15 K, 303.15 K, 313.15 K) and dynamic adsorption experiments were performed. The characterization results showed that impregnation with low concentrations of nitric acid not only increased the surface acidic functional group content but also increased the specific surface area, while impregnation with high concentrations of nitric acid could be able to decrease the specific surface area. ACF-N-6 significantly increased the surface functional group content without destroying the physical structure of the activated carbon fibers. The experimental results showed that the highest adsorption of ammonia by ACFs was 14.08 mmol-L−1 (ACF-N-6) at 293 K, and the adsorption capacity was increased by 165% compared with that of ACF-raw; by fitting the adsorption isotherm and calculating the equivalent heat of adsorption and thermodynamic parameters using the Langmuir–Freundlich model, the adsorption process could be found to exist simultaneously. Regarding physical adsorption and chemical adsorption, the results of the correlation analysis showed that the ammonia adsorption performance was strongly correlated with the carboxyl group content and positively correlated with the relative humidity (RH) of the inlet gas. This study contributes to the development of an efficient ammonia adsorption system with important applications in industrial production and environmental safety.
Journal Article
Adsorption of selected GHG on metal-organic frameworks in the context of accompanying thermal effects
Thermal effects accompanying gas sorption on micro- and mesoporous materials provide unique insights into the type, course, and efficiency of sorption. In this study, metal-organic frameworks (MOFs) with different topologies and chemical structures were synthesized and investigated: HKUST-1, Ni-MOF-74, UiO-66, and MIL-140A. These MOFs were characterized structurally and sorptively with respect to selected greenhouse gases (GHGs). Sorption capacities for CO2 and CH4 were determined at several temperatures and measurement pressures, and the maximum sorption capacity was determined using the Langmuir-Freundlich model. Thermal effects accompanying adsorption were quantified through the isosteric heat of adsorption parameter. For each MOF, the values of isosteric heat of adsorption were higher for CO2 than for CH4. The values of this parameter was determined in the following order: HKUST-1 > Ni-MOF-74 > UiO-66 > MIL-140A. Energy homogeneity of the adsorbent surface was observed in nearly all cases, except for UiO-66 during CO2 adsorption. Changes in the determined isosteric heat of adsorption of CO2 with increasing sorption capacity were in the range of 5-15 kJ/mol, while for CH4 they ranged from 1.4 to 17 kJ/mol, respectively. The level of thermal selectivity of CO2 over CH4 was determined in the following order: UiO-66 (1.9) > Ni-MOF-64 (1.7) > MIL-140A (1.5) > HKUST-1 (1.1).
Journal Article
Application of the adsorbent CR-100 for Ammonium Removal: Thermodynamic and Kinetic Studies
Groundwater with increased ammonia concentration is a constant concern regarding the preparation of drinking water. The affinity of ammonia to be adsorbed on the surface of different solid materials significantly influences the selection of its removal process and has been the motivation for this investigation. Crystal-Right(TM) (CR-100) was used for the removal of ammonia from aqueous solution in batch adsorption procedure. The kinetics of adsorption followed the pseudo-second-order model. The Elovich model suggested that chemisorption rate decreased with the temperature increase. The liquid film diffusion and intra-particle diffusion models revealed that heterogeneous adsorbent surface energy had a particularly pronounced impact on the overall mass transfer rate. The Arrhenius and Eyring's equations suggested spontaneous and endothermic nature of complex adsorption/ion exchange removal process. The isosteric heat of adsorption revealed that with the increase in surface loading lateral interactions between the adsorbed molecules occurred. Keywords: groundwater treatment, synthetic mesoporous adsorbent, adsorption, isosteric heat of adsorption.
Journal Article