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- Herausgeber: Wiley-VCH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Comprehensive and technically detailed approach to addressing environmental and energy challenges through the advance of adsorption theory and techniques
Supercritical Adsorption for Cleaner Energy and the Environment delves into the novel theory of supercritical adsorption and its practical applications in energy and environmental management issues. The book addresses critical topics such as supercritical adsorption and sustainable energy solutions, provides a deep understanding of advanced theories and techniques of supercritical adsorption, and addresses innovative methods for fuel desulfurization, natural gas storage, hydrogen energy, and emission-free coal power generation in the energy industry.
The book is divided into two parts. The first part provides a comprehensive theory of supercritical adsorption, illustrated with examples that showcase significant progress in both applied and theoretical research due to recent advancements. Building on this theoretical foundation, the second part demonstrates how supercritical adsorption theory can address research questions in the fields of energy and environmental science.
Supercritical Adsorption for Cleaner Energy and the Environment includes information on:
- Solutions for theoretical problems of supercritical adsorption, such as determination of absolute adsorption and the volume or density of the adsorbed phase
- Adsorptive technology to enhance natural gas storage, and methane enrichment from low-quality gas
- Ideas, chemical reactions, and materials and adsorbents used in supercritical adsorption research with the potential to transform approaches in environmental challenges
- Efficient and feasible strategies for achieving carbon circulation within the energy consumption and generation cycle
Supercritical Adsorption for Cleaner Energy and the Environment is an essential forward-thinking reference for practitioners and researchers in the fields of chemistry, chemical engineering, energy, and environmental science.
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Seitenzahl: 386
Veröffentlichungsjahr: 2025
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
Part I: Progress in Adsorption
Chapter 1: Classic Adsorption Theory
1.1 Adsorption Definition
1.2 Type of Adsorption Isotherms
1.3 Henry Law
1.4 Langmuir Equation of Monolayer Adsorption
1.5 BET Equation of Multilayer Adsorption
1.6 Potential Equations of Multilayer Adsorption
1.7 Kelvin Equation of Capillary Condensation
References
Chapter 2: Acquisition of Supercritical Adsorption Isotherms
2.1 Volumetric Method
2.2 Gravimetric Method
2.3 Other Measurement Techniques
2.4 Principal Factors Affecting Adsorption Measurements
2.5 Compressibility Factor and Fugacity Coefficient
2.6 Cryostat
2.7 Illustration of Calculations
2.8 Generating Adsorption Isotherms by Molecular Simulation
References
Chapter 3: Collection of Supercritical Adsorption Isotherms
3.1 Adsorption of H
2
on Activated Carbon
3.2 Adsorption of CH
4
on Activated Carbon
3.3 Adsorption of N
2
on Silica Gel over a Large Temperature Range
3.4 Adsorption of O
2
on Activated Carbon
3.5 Adsorption of CH
4
and N
2
on Activated Carbon and Silica Gel
3.6 Adsorption of CO
2
on Activated Carbon for Near-Critical Region
3.7 Adsorption of H
2
, N
2
, O
2
, CH
4
, and CO
2
on Carbon Molecular Sieves
3.8 Adsorption of CO
2
, CH
4
, and N
2
on Silica Molecular Sieves
3.9 Adsorption of H
2
on Carbon Nanotubes
3.10 Adsorption of Hydrogen Isotopes on Micro- and Mesoporous Adsorbents with Orderly Structure
References
Chapter 4: Theoretical Basis of Supercritical Adsorption
4.1 Isotherm Types of Supercritical Adsorption
4.2 Theory Crux of Supercritical Adsorption
4.3 Evaluation of the Henry Law Constant from Experimental Isotherms
4.4 Determination of Absolute Adsorption
4.5 Evaluation of Volume or Density for the Adsorbed Phase
4.6 Isotherm Modelling
4.7 Adsorption Mechanism at Supercritical Temperature
4.8 Boundary of Supercritical Adsorption
4.9 Effect of Supercritical Adsorption Theory [26]
References
Part II: Effect of Adsorption Progress on Energy and Environment
Chapter 5: Carbon Reduction Makes Coal Power Emission-Free
5.1 Present State of Coal-Fired Power Plants
5.2 Theoretical Basis of the Zero-Emission Approach
5.3 Experimental Basis of the Zero-Emission Approach
5.4 Process to Realize Zero-Emission Power Plant
5.5 Concluding Remarks
References
Chapter 6: Adsorption/Reaction Compound Function Removes Sulfur from Oil Fuels
6.1 Introduction
6.2 Theoretical Basis
6.3 Experimental Study
References
Chapter 7: Adsorptive Approaches for Methane-majored Fuels
7.1 Adsorbed Natural Gas Limited by Adsorption Mechanism
7.2 Enhanced Storage of Natural Gas in Wet Adsorbents
7.3 Charging/Discharging Experiments of Wet Storage
7.4 Effect of Pore Size Distribution on Storage Capacity of Wet Activated Carbon
7.5 Removal of Trace H2S from Natural Gas
7.6 Enrichment of Methane from Impoverished Gas
References
Chapter 8: Studies on Hydrogen Energy
8.1 Introduction
8.2 Recovery of Hydrogen from Process Flue Gas
8.3 Routine Technology of Hydrogen Production
8.4 Decomposition of Water Through Redox Reactions Looping
8.5 Hydrogen Storage
References
Epilogue
Acknowledgments
Index
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1 Diagrammatic sketch for gas/solid adsorption [1]. The density profile () shown...
Figure 1.2 Isotherm types of vapor adsorption [2] / with permission of Elsevier.
Figure 1.3 The adsorption mechanism assumed by BET theory.
Figure 1.4 Typical Type-II isotherm showing point B.
Figure 1.5 The -plot for Type-II isotherms.
Figure 1.6 Sketch of the adsorption potential field.
Figure 1.7 The radii of curvature.
Figure 1.8 Definition of the Kelvin radius.
Figure 1.9 The generation process of Type-IV isotherm.
Chapter 2
Figure 2.1 Schematic structure of volumetric setup. Adapted from
Figure 2.2 Comparison of -values for N2 at 126.15 K [20] / with permission of Spri...
Figure 2.3 Comparison of -values for CH4 at 190.15 K [20] / with permission of Spr...
Figure 2.4 Schematic structure of the author’s cryostat [21] / with permission of ...
Figure 2.5 Compressibility factor of hydrogen [22] / with permission of Elsevier.
Figure 2.6 Square root mean deviations of -values from equations of state to that from th...
Figure 2.7 Adsorption isotherms of hydrogen on 5A zeolite evaluated from different source...
Figure 2.8 The DRK plot of CO2 adsorption on activated carbon AX-21 [29] / with permissio...
Figure 2.9 Pore size distribution of AX-21 carbon [29] / with permission of Chinese Journ...
Figure 2.10 Comparison of the excess with the absolute isotherms for the subcritical regio...
Chapter 3
Figure 3.1 Adsorption isotherms of H2 on AX-21 carbon. Adapted from [1]. Solid points: Ad...
Figure 3.2 Adsorption isotherms of methane on activated carbon [2] / with permission of S...
Figure 3.3 Adsorption isotherm of N2 on silica gel at 77 K [4] / with permission of Sprin...
Figure 3.4 Subcritical adsorption isotherms of N2 on silica gel, where ps is 0.953...
Figure 3.5 Supercritical adsorption isotherms of N2 on silica gel [4] Dots: Experimental...
Figure 3.6 The absorbed amount against gas phase density near the critical temperature ...
Figure 3.7 Adsorption isotherms of O2 on super-activated carbon [7] Dots: Experimental; ...
Figure 3.8 Adsorption isotherm of CH4 on activated carbon around the critical temperature...
Figure 3.9 Adsorption isotherm of N2 on activated carbon around the critical temperature ...
Figure 3.10 Adsorption isotherm of CH4 on mesoporous silica gel around the critical temper...
Figure 3.11 Adsorption isotherm of N2 on mesoporous silica gel around the critical tempera...
Figure 3.12 Adsorption isotherms for the subcritical region [11] / with permission of Amer...
Figure 3.13 Adsorption isotherms for the supercritical region [11] / with permission of Am...
Figure 3.14 Pore size distribution of the synthesized CMK-3-1.25 [14] / with permission of...
Figure 3.15 Adsorption isotherm of H2 at 273 K [14] / with permission of Elsevier.
Figure 3.16 Adsorption isotherms of N2, O2, CH4, and CO2 at 298 K [14] / with permi...
Figure 3.17 Adsorption isotherms of three gases at 298 K [17] / with permission of ...
Figure 3.18 Process to approach adsorption equilibrium [17] / with permission of Elsevier....
Figure 3.19 Adsorption isotherms of H2 on MWNT sample [20] / with permission of Elsevier. ...
Figure 3.20 Adsorption isotherms of H2 and D2 on 3A, 4A, 5A, and Y zeolites at 77 K....
Figure 3.21 Adsorption isotherms of H2 and D2 at 77 K [22] / with permission of American C...
Chapter 4
Figure 4.1 The virial plot of hydrogen adsorption data [2] / with permission of Peking Un...
Figure 4.2 Dependence of Henry constant on temperature for hydrogen adsorption on activat...
Figure 4.3 The isosteres of the H2 adsorption on activated carbon AX-21. Adapted from [1]...
Figure 4.4 Variation of the isosteric heat of adsorption with the adsorbed amount [1] / w...
Figure 4.5 The van’t Hoff plot of the H2 adsorption data on activated carbon AX-21...
Figure 4.6 General isotherm of the N2 adsorption on activated carbon for 138–298...
Figure 4.7 Linear isotherms of N2 adsorption on activated carbon for 138–298 K [16...
Figure 4.8 Comparison of the excess with the absolute isotherms for subcritical re...
Figure 4.9 Comparison of the excess with the absolute isotherms for supercritical ...
Figure 4.10 Adsorbed phase volume of hydrogen [14].
Figure 4.11 Pore size distribution of the carbon sample [14].
Figure 4.12 The volume of adsorbed N2 for 138–298 K [18] / with permission o...
Figure 4.13 The density of CH4 in the adsorbed phase [14].
Figure 4.14 Variation of the adsorbed phase density with temperature [14].
Figure 4.15 The density of CO2 in the adsorbed phase [17] / with permission of Ame...
Figure 4.16 Volume of the adsorbed phase for CO2 adsorption [17] / with permission of Amer...
Figure 4.17 Comparison of intermolecular distances [23] / with permission of Ameri...
Figure 4.18 Isotherms in adsorbed amount versus gas phase density [6] / with permis...
Figure 4.19 Adsorption amount against gas phase density [5] / with permission of W...
Figure 4.20 Schematic apparatus of dynamic measurement for mixture adsorption [50] / with ...
Figure 4.21 Temperature and pressure detected at the bed center during experiments [50] / ...
Figure 4.22 Adsorption isotherm of CH4 on activated carbon JX101 obtained by different met...
Figure 4.23 The mixture adsorption pattern.
Figure 4.24 The fitness of model to mixture adsorption data of Run 1a. Symbols: Experimenta...
Figure 4.25 The fitness of model to mixture adsorption data in Run 1b. Symbols: Experimenta...
Figure 4.26 The fitness of model to mixture adsorption data in Run 1c. Symbols: Experimenta...
Figure 4.27 The fitness of model to mixture adsorption data in Run 1d. Symbols: Experimenta...
Figure 4.28 The fitness of model to mixture adsorption data in Run 2. Symbols: Experimental...
Figure 4.29 The fitness of model to mixture adsorption data in Run 3. Symbols: Experimental...
Figure 4.30 The fitness of model to mixture adsorption data in Run 4. Symbols: Experimental...
Figure 4.31 The fitness of model to mixture adsorption data in Run 5. Symbols: experimental...
Figure 4.32 The comparison of models in predicting component adsorption [50] / with permis...
Figure 4.33 The comparison of models in predicting the composition of adsorbed phase [50] ...
Chapter 5
Figure 5.1 World marketed energy use by fuel type for 1980–2030 [1] / U.S. Departm...
Figure 5.2 Power generation by fuel type (2005) [2] / Peterson Institute for Internationa...
Figure 5.3 A review of capital cost ranges ($/kW) for wet FGD system. Adapted from...
Figure 5.4 The theoretical conversion of major reductions at different temperatures [22]....
Figure 5.5 The experimental setup without water in SFG [22].PRs: Pressure releasing valve...
Figure 5.6 The experimental setup of SFG with presence of water [22].Vs: Valves; F1: Filt...
Figure 5.7 Comparison of the experimental conversion of CO2 on corncob carbon with theore...
Figure 5.8 The composition of the oxygen-laden SFG effluent at different temperatures [22...
Figure 5.9 Effect of contact time on CO2 conversion in the oxygen-laden flue gas at 950...
Figure 5.10 Process to realize zero emission of coal combustion.
Chapter 6
Figure 6.1 Mechanism assumed for the desulfurization reaction.
Figure 6.2 Thiophene conversion rate at different temperatures, where AC-BY1 is loading f...
Figure 6.3 Plots for the determination of reaction order (AC-BY1 loading formaldehyde and...
Figure 6.4 Breakthrough curves of thiophene over the nanoreactor bed packed with seven ca...
Figure 6.5 Breakthrough curves over different silica gel beds. 1: SGB-2 loading nothing; ...
Figure 6.6 Breakthrough curves of model fuels containing thiophene [36] / with per...
Figure 6.7 Correlation between breakthrough capacity (in MXF fuel) of nanoreactor and por...
Figure 6.8 Variation of conversion time with nanoreactor dimension [34] / with per...
Figure 6.9 Compatibility of the desulfur function with fuel type [34] / with permi...
Figure 6.10 Breakthrough curves of gasoline at 70 °C and diesel at 80...
Figure 6.11 Breakthrough curves of commercial fuels over two consecutive sorption beds; ga...
Figure 6.12 Effect of regeneration on breakthrough times; H2SO4/activated carbon [30]...
Figure 6.13 Breakthrough curves passing the original and regenerated reagents; HCl/silica ...
Figure 6.14 Breakthrough curves of MXF fuel containing thiophene for different times of re...
Chapter 7
Figure 7.1 Adsorbed amount of CH4 on different materials at 3.5 MPa and 298 K [11]...
Figure 7.2 Effect of water content on storage enhancement of methane [16] / with permissi...
Figure 7.3 Effect of temperature on enhancement [16] / with permission of John Wiley &...
Figure 7.4 Experimental apparatus for the charging/discharging experiments [18] / with pe...
Figure 7.5 The released amount from wet carbon at different charging pressures [18] / wit...
Figure 7.6 Influence of packing density of wet carbon on the released amount [18] / with ...
Figure 7.7 Released amount of methane as a function of charging pressure [18] / with perm...
Figure 7.8 Variation of the apparent ratio with charging pressure [18] / wit...
Figure 7.9 Variation of charged amount and bed temperature during the charging process [1...
Figure 7.10 Variation of released amount and bed temperature during the discharging proces...
Figure 7.11 Adaptation of pore size to methane hydrates.
Figure 7.12 Adsorption isotherm of N2 on the tested activated carbon at 77 K [27] /...
Figure 7.13 Pore size distribution of the tested carbon [27] / with permission of Elsevier...
Figure 7.14 Activation progress as reflected in pore volume and pore size distribution [27...
Figure 7.15 Isotherms of CH4 on the tested carbon with different water ratios at 275...
Figure 7.16 Effect of water content on the gravimetric storage capacity (275 K, 9 MPa) [27...
Figure 7.17 H2O-to-CH4 molar ratio in wet carbons at equilibrium (275 K, 9 MPa) [27] / wit...
Figure 7.18 Effect of water content on the TVC of wet carbons [27] / with permission of El...
Figure 7.19 Recorded volumetric storage capacity [27] / with permission of Elsevier. Packa...
Figure 7.20 Effect of TEA-loading ratio on regeneration cost [43] / with permission of Joh...
Figure 7.21 Adsorption isotherm of H2S on TEA-coated silica gel at 298 K [43] / wit...
Figure 7.22 The experimental two-column flow sheet [43] / with permission of John Wiley &...
Figure 7.23 Effect of pressure on breakthrough times [43] / with permission of John Wiley ...
Figure 7.24 Effect of operation pressure on regeneration cost [43] / with permission of Jo...
Figure 7.25 Effect of P/F on process performance [43] / with permission of John Wiley &...
Figure 7.26 Effect of adsorption time on process performance [43] / with permission of Joh...
Figure 7.27 Effect of blow downtime on process performance [43] / with permission of John ...
Figure 7.28 Effect of purging time on process performance [43] / with permission of John W...
Figure 7.29 Process performance at continuous operation [43] / with permission of John Wil...
Figure 7.30 Recorded H2S content in product (curve 1) and exhaust (curve 2) during a cycle...
Figure 7.31 Flow chart of experimental setup [55] / with permission of Chinese Journal of ...
Figure 7.32 Mass balance over the adsorption bed [55] / Chinese Journal of Chemical Enginee...
Figure 7.33 Recorded pressures and flow rate [55] / with permission of Chinese Journal of C...
Figure 7.34 Comparison between static and dynamic methods of adsorption measurement [55] / ...
Figure 7.35 Breakthrough curves of CH4 on activated carbon K04 at 298 K and four pre...
Figure 7.36 Breakthrough curve on activated carbon JX-406 and the recorded temperature of t...
Figure 7.37 The flow sheet of experimental setup [82] / with permission of John Wiley &...
Figure 7.38 Adsorption isotherms of CH4, N2, and CO2 on activated carbon at 298 K. ...
Figure 7.39 Breakthrough curves of the He/N2/CH4 mixture passing the carbon bed at 298...
Figure 7.40 Pressure path in a cycle [82] / with permission of John Wiley & Sons. 1: D...
Figure 7.41 The two concentration profiles along the adsorption bed in step 3 [82] / John ...
Figure 7.42 Performance without CO2 displacement [82] / with permission of John Wil...
Figure 7.43 Composition variation of the effluent stream with CO2 displacement at adsorpti...
Figure 7.44 Composition variation of the effluent stream with CO2 displacement at ambient ...
Figure 7.45 Effect of adsorption time on displacement consequence with displacement at : ...
Figure 7.46 Effect of processed feed gas per cycle on product quantity and methane recover...
Figure 7.47 Composition of effluent stream at the optimal operation regime. CO2 displaceme...
Figure 7.48 Product concentration and methane recovery for CO2 displacement at 0.4 ...
Figure 7.49 Product concentration and methane recovery for CO2 displacement at 0.1 ...
Figure 7.50 CO2 concentration in tail gas at different purging rates [82] / with pe...
Figure 7.51 Effect of purging rate on purging time and regeneration cost evaluated for 10%...
Figure 7.52 Product concentration and methane recovery in consecutive cycles. White points...
Chapter 8
Figure 8.1 The new 4-column PSA process [8] / with permission of American Chemical...
Figure 8.2 Adsorption isotherms on OAC at 298 K [8]. with permission of American C...
Figure 8.3 Adsorption isotherms on SAC at 298 K [8] / with permission of Am...
Figure 8.4 Adsorption isotherms on ZMS-5A at 298 K [8] / with permission of...
Figure 8.5 Adsorption amount of CH4 on three adsorbents per unit volume at 298 K [...
Figure 8.6 Adsorption isotherms of N2 at 298 K per unit volume [8] / with p...
Figure 8.7 Breakthrough curves on OAC+ZMS bed at pressure 1.0 MPa and feed rate 6.42 SLPM...
Figure 8.8 Effect of on pressure level of surge tanks [8] / with permission of Am...
Figure 8.9 Run I with higher volume factor shows higher recovery of H2 [8] / with ...
Figure 8.10 Effect of operation plan on process performance [8] / with permission o...
Figure 8.11 Separation performance of OAC+ZMS combination at low pressure [8] / wit...
Figure 8.12 Separation performance of SAC+ZMS combination at low pressure [8] / wit...
Figure 8.13 Effect of P/F on product purity and recovery at pressure 0.4 MPa and feed flow...
Figure 8.14 Effect of P/F on process performance at operation pressure 0.6 MPa [8] ...
Figure 8.15 Effect of P/F on process performance at operation pressure 0.8 MPa [8] ...
Figure 8.16 Effect of P/F on process performance at operation pressure 1.0 MPa [8] ...
Figure 8.17 Result of 15 cycles continuous operation [8] / with permission of Ameri...
Figure 8.18 Hydrogen production through redox reactions looping at cost of carbon material...
Figure 8.19 The experimental setup for the redox reactions looping [30] / with perm...
Figure 8.20 Phase diagram in reducing ferric oxides with CO [34].
Figure 8.21 Composition of the reactor effluent streams [30] / with permission of American C...
Figure 8.22 Coal carbonization apparatus [30] / with permission of American Chemica...
Figure 8.23 TG curves of three coal samples [30] / with permission of American Chem...
Figure 8.24 Result of catalyzed carbonization of lignite at different temperatures [30]...
Figure 8.25 The schematic flow sheet of continuous operation [22] / with permission...
Figure 8.26 The off-gas composition during initial oxidation [30] / with permission...
Figure 8.27 Result of continuous running of hydrogen production via FeO/Fe3O4 (containing ...
Figure 8.28 The storage capacity of H2 by compression [43] / with permission of Els...
Figure 8.29 Adsorption isotherms of H2 on AC AX-21 at 77 K [43] / with permission o...
Figure 8.30 The storage capacity of H2 with compression and adsorption on AC AX-21 [43]...
Figure 8.31 Weight percentage of H2 stored on AC AX-21 powder and pellets.
List of Tables
Chapter 2
Table 2.1 Specification of AX-21 carbon by CO2 adsorption at 25 °C treated...
Chapter 3
Table 3.1 Adsorbents used in experiments.
Chapter 4
Table 4.1 Adsorbate layers in the adsorbed phase.
Table 4.2 Critical parameters and eccentric factors of tested gases.
Table 4.3 Experimental conditions for multicomponent adsorption.
Table 4.4 Parameter values used in the prediction model for multicomponent adsorption.
Table 4.5 The prediction error of the proposed model for four-component mixture adsorpti...
Table 4.6 Comparison of the proposed model with LRC, IAST, and GDSL for the data of Drei...
Chapter 5
Table 5.1 Thermodynamic data required for conversion calculation of reactions 5.1 and 5....
Table 5.2 Conversion of SFG components in the presence of water.
Chapter 6
Table 6.1 Structural property of carbon materials tested [34] / American Chemical...
Table 6.2 Result of corrosion tests [36] / John Wiley & Sons.
Chapter 7
Table 7.1 Materials presented in Figure 7.1.
Table 7.2 Adsorbents tested.
Table 7.3 Separation coefficient of adsorbents tested.
Table 7.4 Parameters of the carbon and adsorption bed.
Table 7.5 Composition of gas mixture.
Table 7.6 Effect of methane content in feed gas on adsorption time and process productiv...
Table 7.7 Effect of evacuation time on regeneration degree.
Table 7.8 Comparison with literature.
Chapter 8
Table 8.1 Description of adsorption columns and adsorbents.
Table 8.2 Cycling sequence for 2-bed PSA process.
Table 8.3 Cycling sequence of operation steps for conventional 4-bed PSA process.
Table 8.4 Cycling sequence for the new 4-bed PSA process.
Table 8.5 Standard molar formation enthalpy of related materials at 298.15 K.
Table 8.6 Information of coal samples.
Table 8.7 Catalyzed carbonization result of coking and lean coals.
Table 8.8 Most important families of hydrides forming intermetallic compounds [47]
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Epilogue
Acknowledgments
Index
End User License Agreement
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Supercritical Adsorption for Cleaner Energy and the Environment
Li Zhou
Author
Professor Li Zhou
Tianjin University
21-2602, Xin-yuan-cun
Tianjin, 300072
China
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Preface
Energy and environment are of world concern especially in recent years when global warming appears to be a hazardous trend. The origin of the problem was attributed to the combustion of fossil fuels, and hydrogen energy was put forward as a solution to the problem. The pollution species of burning fossil fuels are SOx, NOx, CO2, and other gases with small molecule weights. On the other hand, hydrogen and other cleaner fuels, such as methane-majored gases, are also of small molecule weights. In fact, both energy and environmental issues are intimately related to gases of small molecule weights, for which adsorption-based technologies are the most efficient and cost-effective to deal with them. However, the critical temperature of this group of gases is rather low, and the adsorption bearing market value occurs at much higher than the critical temperature; therefore, the adsorption of interest belongs to supercritical for such gases. Available adsorption theory applies only to the adsorption at subcritical temperatures when gases are condensable. Nonetheless, gases with small molecule weights are incondensable at temperatures of engineering interest. As such, research on the adsorption of this kind of gases is lack of theory to guide. A lot of works were dedicated to the study of supercritical adsorption; however, there are two barriers seemed difficult to overcome. The experimental barrier is lack of a cryostat that covers large range of temperature from sub- to supercritical temperature, without which the change in the adsorption mechanism of adsorption on crossing the critical temperature cannot be observed. The theoretical barrier is the determination of the absolute adsorption quantity due to the fact that supercritical gases are not condensable. Both barriers are overcome, and solutions are presented in present book. The first part of the book takes four chapters to elucidate related adsorption theory. I do believe that the present book is the unique source of complete theory for supercritical adsorption. It will show with examples that a series of progress is achieved in both applied and theoretical research due to the progress in the theory of adsorption.
In the second part of the book, it shows how the supercritical adsorption theory clears up or handles queries in the research of energy, and how the ideas, chemical reactions, materials/adsorbents that have been applied in the research of supercritical adsorption may change/improve the train of thoughts on dealing with problems of the environment. For example, there isn’t a magic material for hydrogen storage at all according to the mechanism of supercritical adsorption, but thermochemical reactions looping provides another route to decompose water with the energy gain index higher than unity; adsorbents with proper pore structure and treatment may possess a compound function of adsorption/reaction to effectively remove sulfur from oil fuels; it will show how the adsorptive approach cuts the storage pressure of natural gas by half while maintaining the same capacity as compressed natural gas (CNG). It also shows how the adsorptive process improves natural gas sweetening and enrichment of methane from impoverished gas. More importantly, it shows how coal power changes to be emission-free and yields methanol at the same time, so that a big loop in energy consumption and generation is formed, and the accumulated carbon cycles in the loop. As a consequence, the world’s pollution will get less and less since the chemical nature of pollution is the accumulation of carbon element on the Earth’s surface.
The present book proposes an alternative approach to deal with the environmental challenge. Instead of quitting fossil fuels, a general cycling of carbon element in the big loop of energy consumption and generation is suggested. Hydrogen energy may be of advantage for some special cases, but it cannot be the substitute of fossil fuels due to the fact that it is produced at cost of another available energy and perplexed with storage problem. Hydrogen energy has to pass two examinations: the energy balance and the cost balance.
Li Zhou,
August, 2024
Part IProgress in Adsorption
Chapter 1Classic Adsorption Theory
1.1 Adsorption Definition
Adsorption is a phenomenon that occurs at the interface between two phases. It might occur at the interface of gas/solid, gas/liquid, liquid/liquid, and liquid/solid, but only the adsorption that occurred at the gas/solid interface is of concern in this book. What is adsorption? It is the phenomenon that the concentration of gas at the interface is higher than that in the bulk phase. How it happens? There are two reasons. The first is due to the electrostatic attraction at the interface and yields so-called chemisorption; the second is due to the attraction of van der Waals force and yields physical adsorption. The former can only adsorb one layer of molecules because the chemical bond is no longer attractive once it is saturated, but each gas molecule can exert van der Waals force; therefore, physical adsorption is not limited to adsorbing just one layer of molecules, and the first layer of molecules adsorbed by chemical adsorption can still yield physical adsorption. The solid is named “adsorbent,” the substance to be adsorbed is named “adsorptive,” and it changes its name to “adsorbate” after being adsorbed. Either electrostatic force or van der Waals force will generate an adsorption potential field near the interface. As internal forces, their strength is limited, so the amount of adsorption is capped. The upper limit of the adsorption capacity is determined by the upper limit of gas phase pressure, which is the saturated vapor pressure, , when the adsorption occurs below the critical temperature because the vapor is liquefied when saturation pressure is reached. How about the situation of the adsorption occurring above the critical temperature when the gas cannot be liquefied? Is there not an upper limit for supercritical adsorption? The answer is Yes because there is an upper limit for the internal forces that yield adsorption. However, determining the upper limit is a problem for the adsorption at supercritical temperatures.
According to the Gibbs formalism, adsorption itself is an excess quantity. The situation at the gas/solid interface is schematically represented in Figure 1.1 [1]. The molecule’s density near the surface is higher than that in the bulk gas phase due to the adsorption, and a density profile is thus established between the solid surface and the ambient gas phase, but this density profile vanishes over quite a short range. As such, a layer on the solid surface, where adsorbate molecules are concentrated, is referred to the “adsorbed phase,” and a definite thickness is assumed for it.
Figure 1.1 Diagrammatic sketch for gas/solid adsorption [1]. The density profile shown by a solid line curve indicates it is a function of distance normal to the surface in the real system; the broken line shows the case without adsorption; and the chain-dotted line shows the boundary between phases. The shadowed area marks the adsorbed phase and the excess amount of adsorbed substance.
Based on the schematic representation, the adsorbed amount, , can be determined by either way of the following: First, provided the density profile can be determined or assumed, then
where is the density of the adsorbed phase at distance to the solid surface, and is the density of the bulk gas phase. The integral indicates that is a density excess amount of the adsorbed substance, which is usually referred to as “surface excess adsorption” or simply “excess adsorption.”
The density profile perhaps may be determined by molecular simulation, it can hardly be measured experimentally. Therefore, the quantity is often counted another way as is often cited in literature. Suppose the volume of the adsorbed phase in Figure 1.1 is , then the excess adsorption must be:
where is the total mass confined in the adsorbed phase, which is usually referred to as the “absolute adsorption.” The density of the adsorbed phase is implicitly assumed to be uniform in Eq. (1.2), and it is usually esteemed as the density of liquid for vapor adsorption. Since gas cannot be liquefied above the critical temperature, the quantification of the values of and is another big problem for the adsorption at the above-critical temperature.
The adsorption of gas on solid can be classified into three regions relative to the critical temperature according to the adsorption behavior:
Subcritical region
Near-critical region
Supercritical region
In the first region, isotherms show the feature of subcritical adsorption, and in the third region, isotherms will show the feature of supercritical adsorption. However, in the second region, the adsorption isotherms must show the feature of adsorption mechanism transition. The transition will occur more or less continuously if isotherms on both sides of the critical temperature belong to the same type; however, a discontinuous transition could happen on isotherms if there is a transformation of isotherm types. For all cases of it belongs to “supercritical region.” The decisive factor of such classification is only temperature, but irrespective of pressure. This is because a fluid cannot undergo a transition from gaseous to liquid phase at above-critical temperatures regardless of the pressure exerted. This fundamental law of physics determines the different adsorption mechanism for the adsorption at sub- or supercritical temperature.
Progress in basic science is driven by practical requirements. The appearance of gas bombs in the First World War greatly promoted the research and progress of the subcritical adsorption, but less attention was paid to the adsorption above the critical temperature because the adsorption amount is generally very low. However, this kind of adsorption attracted the attention of researchers when the energy crisis occurred in the 1970s. However, the key problem, i.e., the adsorption mechanism, is still unknown, and its application research in practice is restricted. To explain the relation and difference between the adsorption phenomena above and below the critical temperature, the core knowledge of adsorption theory below the critical temperature must be quoted first, which is the basis of the study on supercritical adsorption, and it also helps to understand the characteristics and problems of the latter.
1.2 Type of Adsorption Isotherms
The adsorbed amount of gas on a given adsorbent is a function of temperature and gas phase pressure. Adsorption isotherm is the functional relation between the adsorbed amount and the gas phase pressure at a constant temperature. The gas phase density is certainly fixed when temperature and pressure are unchanged; therefore, the adsorption isotherm can be also expressed as a function of the adsorbed amount against the gas phase density. Adsorption isotherms are usually acquired experimentally. Isotherms were first divided into five types, as shown in Figure 1.2. If the surface of nonporous adsorbent is completely uniform or close to completely uniform, the isotherm shows the stepwise type, and the stepwise isotherm can also be counted as the sixth type, which is not here concerned because of its limited application. The Type-IV and Type-V isotherms have a hysteresis loop, the lower branch of which is the measured value when the system gradually increases pressure (adsorption line), while the upper branch is the measured value when the system gradually decreases pressure (desorption line).
Figure 1.2 Isotherm types of vapor adsorption [2] / with permission of Elsevier.
Type-I is the monolayer adsorption isotherm, and also known as Langmuir-type adsorption isotherm. Adsorption on microporous adsorbents often exhibits a Type-I isotherm because saturation is reached when microporous space is filled. On an open surface or a porous adsorbent with a larger pore size, the adsorption isotherm usually shows a Type-II or Type-III feature, and the adsorption amount increases with the increase of adsorption pressure. When the adsorption pressure approaches the saturation pressure of the adsorptive , infinite multilayer adsorption can occur. When the molecular interaction force between the adsorptive and the adsorbent is greater than that among the adsorptive molecules, the adsorption isotherm appears as Type-II; if the interaction among the adsorptive molecules is greater than the interaction between adsorptive and adsorbent, it appears as Type-III. The Type-IV and Type-V isotherms correspond to the Type-II and Type-III isotherms, respectively, but the adsorption occurs on mesoporous adsorbents and the mesoporous space is gradually filled to achieve saturated adsorption at relative pressure . According to the International Association for Theoretical and Applied Chemistry [3], when the pore size is smaller than 2 nm it belongs to micropore; if the pore size is between 2 nm and 50 nm it belongs to mesopore; if the pore size is larger than 50 nm it belongs to macropore.
Quite a few adsorption theories were put forward to explain the mechanism of adsorption, based on which mathematical expressions describing different types of adsorption isotherms were established. Essential adsorption theories are introduced as that follows.
1.3 Henry Law
This is the adsorption theory at low surface coverage. For physical adsorption, the state of the adsorbed molecule does not change; the adsorbed molecule does not form a bond with the adsorbent, nor does it dissociate, therefore, it can be considered that the molecules adsorbed on the surface of a uniform solid are independent of each other, and have no interaction with each other when the surface concentration is very low. Under such conditions, there is a linear relationship between the adsorption amount and the gas phase pressure:
where is the adsorbed amount; is the gas phase pressure; and is the adsorption equilibrium constant. This relationship is called Henry law, and the adsorption equilibrium constant is called the Henry constant. The dependence of the Henry constant on temperature follows van’t Hoff equation:
where is the enthalpy difference between the adsorbed state of the adsorptive and the gaseous state. Ignoring the difference in heat capacity between different phases, Eq. (1.4) is integrated to give:
According to Eq. (1.5), if ln is plotted against , a linear relationship can be obtained over a wide temperature range, and thus can be evaluated from the slope of the straight line.
1.4 Langmuir Equation of Monolayer Adsorption
This theory describes the adsorption of a single molecular layer, proposed by Langmuir in 1916 and was originally used to describe chemisorption. Langmuir believed that the interaction force between the adsorbed molecules and the solid surface decreased rapidly with the increase of the distance between them so that the adsorption was limited to a single molecular layer. This applies to chemisorption as well as physical adsorption at low-pressure and high-temperature conditions.
To derive the isotherm equation of monolayer adsorption, Langmuir proposed the following hypotheses:
The adsorption is monolayer. Adsorption occurs on the adsorption sites of adsorbent surface. An adsorption site can only absorb one molecule, and the number of adsorption sites is definite. Adsorption reaches saturation when all adsorption sites are full.
The energy of the adsorbent surface is uniform. The adsorbed molecules are adsorbed equally at all sites on the surface, and there is no difference in energy.
There is no interaction among the adsorbed molecules, and the adsorption and desorption of each adsorbed molecule are independent and not affected by other molecules around it.
Adsorption is a dynamic equilibrium. When the adsorption equilibrium is reached at a certain temperature and pressure, the adsorption rate is equal to the desorption rate.
The Langmuir adsorption isotherm equation can be derived by kinetic method. The adsorption rate of adsorptive is related to the degree of blankness of the adsorbent surface. Suppose is the number of adsorptive molecules colliding on a unit area of solid surface in unit time, is the fraction of collision molecules being adsorbed, represents the fraction coverage of adsorbent surface, and is the adsorption rate, then we have
According to the kinetic theory of molecules, , where is the gas pressure, is the mass of a gas molecule, is the Boltzmann constant, and is the absolute temperature. The desorption rate of the adsorbed molecules on the surface depends on the extent of surface coverage and energy of the molecule. If the exothermic heat of adsorption is , only those molecules with energies above can fly away from the surface and return to the gas phase. According to Boltzmann’s law, the number of molecules that can fly off the surface is proportional to , so the desorption rate is:
where is the proportional coefficient. The adsorption and desorption rates are equal at adsorption equilibrium, , which leads to:
where
If represents the equilibrium adsorption capacity at a given temperature and pressure, and represents the saturated adsorption capacity of the monolayer, then , and is substituted into Eq. (1.8), then it leads to:
This is the well-known Langmuir monolayer adsorption isotherm equation, which can well describe the Type-I isotherms, and is a constant for a given adsorption system at a constant temperature and is called the adsorption equilibrium constant. At high pressures, when (only possible at supercritical temperature), , and then , . It means the adsorption capacity tends to saturate at high pressure and no longer changes with pressure. When the pressure is very low and the adsorption amount is quite small, Eq. (1.10) reduces to Henry law:
In order to obtain the saturation adsorption capacity from the adsorption isotherm, Eq. (1.10) is usually changed to a linear form:
When the experimental data of is plotted against , and a straight line is obtained, then the measured experimental data are in line with the Langmuir model. The saturation adsorption capacity and the adsorption equilibrium constant can be obtained from the slope and intercept of the straight line. The specific surface area of the solid can also be obtained from the saturated adsorption capacity :
where is the saturated adsorption amount per gram of adsorbent, is the cross-sectional area of each adsorbed molecule under monolayer adsorption, which can be estimated from the liquid density of adsorbate in practical application, and is the Avogadro constant.
1.5 BET Equation of Multilayer Adsorption
The Langmuir isotherm equation successfully explained Type-I adsorption isotherm, but it could not explain the rest types of adsorption isotherms. Brunauer, Emmett, and Teller (BET) extended Langmuir’s theory of monolayer adsorption to multilayer adsorption in 1938, which became known as the BET theory. The BET theory believes that the van der Waals force causes physical adsorption; therefore, in addition to the adsorption between adsorbed molecules and solid surface, adsorption can also occur when adsorptive molecules collide on the previously adsorbed molecules, so the adsorption can be multimolecular layers.
The BET theory accepts the Langmuir theory’s assumption that adsorption is localized and that the energy on the solid surface is uniform. Besides, further assumptions are made:
The adsorption heat of the first layer Q
1
is similar to in the Langmuir theory, which is a constant related to the solid surface. The adsorption heat of the second and subsequent layers is equal to the liquefaction heat of the vapor.
The hypothesis in the Langmuir theory that adsorption is dynamically balanced is applicable to the adsorption of each layer, and the adsorption of the second layer does not need to wait until the first layer is full, and the adsorption can be carried at the same time on each layer. So, the adsorption pattern is shown in
Figure 1.3
.
Figure 1.3 The adsorption mechanism assumed by BET theory.
Based on the above assumptions, the BET equation can be derived. Let be the percentage coverage of solid surface, then
Let be the saturated adsorption capacity of the first layer, then the adsorption amount, , is:
Because the adsorption rate of gas molecules on the zero layer to form the first layer is equal to the desorption rate of the first layer molecules to form the zero layer,
Based on the assumption introduced by BET, the adsorption heat from the second layer is equal to the condensation heat of the gas:
Let
It leads to
where
Because
Therefore
The linear BET equation is thus obtained:
If we plot against , then can be evaluated, and if the size of the adsorbed molecule is known, the specific surface area of the adsorbent can be evaluated.
1.5.1 Interpretation of BET Equation for Type-II and Type-III Isotherms
The adsorption of gas molecules on open solid surfaces at subcritical temperature usually shows isotherms of Type-II or Type-III, and Type-II isotherms are more common. Type-II and Type-III isotherms differ in shape with a difference in the value of C. The isotherm gradually transforms from Type-II to Type-III following the decrease of C value. When , i.e.,
