The comparison of regeneration energy of different solid adsorbents in temperature swing adsorption process: A review

The chapter is focused on state of the art of materials for adsorptive heat energy conversion basic principles for substantiation of working pair choice. Types of heat storage materials based on heat storage mechanism were compared. Sensible heat mechanism of thermal energy is based on increasing the temperature of the material. Phase-change mechanism of heat energy storage concerns with alternating reversible processes of phase-changing. As a rule, they are mainly melting-crystallization. Thermo-chemical heat energy storage mechanism is based on reversible chemical reactions. Limitations of conventional sensible heat storage are shown to lowest density of heat energy storage determined by sensible heat of materials, which led to large mass storage units and additional needs of large areas and building volumes, calculated according to heat storage density, constant changing the temperature when discharged, the need for a large overheating of heat storage media. The main defects of phase-change materials are instability of properties of heat-accumulating substances in multiple cycles of crystallization – melting, degradation in time, corrosion activity, the need for developed surfaces of heat exchange and environmental danger. Commercilisation of thermal chemical storage materials is strongly limited by high operating temperatures of thermal chemical storage materials, which are unacceptable for systems of district heating and decentralized heat supply due to sanitary regulations, impropriety for multifold cycling because of irreversibility of a wide range of chemical reactions. Perspective of adsorptive heat energy storage and conversion is shown. Interval of operating temperatures and heat storage density of conventional adsorptive materials are shown to be intermediate between phase-change and thermal chemical heat storage materials. Properties of probable adsorptive heat storage materials were analysed according with literary data. Low adsorptive capacity of conventional adsorbents results in low heat of adsorption and heat energy storage density. Salts forming crystalline hydrate occur to exhibit rather high energy storage density of 1.9–2.7 GJ/m3 of crystalline hydrate, but their application is strongly inhibited not only by physical and chemical instability along with the corrosive activity of these salts at high temperatures, but instability in multifold cycling, degradation in time, and an underdeveloped heat exchange surface. As engineering solution, modification of conventional adsorbents with salt can be considered. Composites ‘salt inside porous matrix' is shown to be promising alternative to conventional adsorbents. Main advantages of these materials are low regeneration temperature and high adsorptive capacity. Crucial impediments of industrial introduction of composite adsorbents ‘salt inside porous matrix' is shown to be complex technology of their production based on rather expensive dry and wet impregnation of porous media by crystalline hydrate solutions. As an alternative, sol gel method for obtaining composite adsorbents ‘silica gel – crystalline hydrate' developed by authors is suggested. The adsorption properties of the obtained composite adsorbents ‘silica gel – sodium sulphate' and ‘silica gel – sodium acetate' are shown to be non-linear combinations of characteristics of silica gel and massive salt. The key distinction of kinetics of adsorption of water vapor with massive salts and composites obtained with sol gel method is shown to be difference limitative stage of process. The adsorption of water with massive crystalline hydrates is shown to be complicated by kinetic limitations. For composite adsorbents limiting stage is water transport through the pore system. Composites ‘slilica gel – crystalline hydrate' are shown to be a promising material for adsorptive heat energy storage and conversion.

Performance analysis of a rotary desiccant wheel using polymer material with high water uptake and low regeneration temperature

Experimental studies on 3A and 4A zeolite molecular sieves regeneration in TSA process: Aliphatic alcohols dewatering–water desorption

Two representative metal–organic frameworks, Zn4O(BTB)2 (BTB3− = 1,3,5-benzenetribenzoate; MOF-177) and Mg2(dobdc) (dobdc4− = 1,4-dioxido-2,5-benzenedicarboxylate; Mg-MOF-74, CPO-27-Mg), are evaluated in detail for their potential use in post-combustion CO2 capture via temperature swing adsorption (TSA). Low-pressure single-component CO2 and N2 adsorption isotherms were measured every 10 °C from 20 to 200 °C, allowing the performance of each material to be analyzed precisely. In order to gain a more complete understanding of the separation phenomena and the thermodynamics of CO2 adsorption, the isotherms were analyzed using a variety of methods. With regard to the isosteric heat of CO2 adsorption, Mg2(dobdc) exhibits an abrupt drop at loadings approaching the saturation of the Mg2+ sites, which has significant implications for regeneration in different industrial applications. The CO2/N2 selectivities were calculated using ideal adsorbed solution theory (IAST) for MOF-177, Mg2(dobdc), and zeolite NaX, and working capacities were estimated using a simplified TSA model. Significantly, MOF-177 fails to exhibit a positive working capacity even at regeneration temperatures as high as 200 °C, while Mg2(dobdc) reaches a working capacity of 17.6 wt % at this temperature. Breakthrough simulations were also performed for the three materials, demonstrating the superior performance of Mg2(dobdc) over MOF-177 and zeolite NaX. These results show that the presence of strong CO2 adsorption sites is essential for a metal–organic framework to be of utility in post-combustion CO2 capture via a TSA process, and present a methodology for the evaluation of new metal–organic frameworks via analysis of single-component gas adsorption isotherms.

A water extraction device that takes water from air in dry area is proposed. This device is designed to meet domestic water demand in remote rural areas, where the climate is dry and fresh water is scarce. The device can be driven effectively by low-temperature waste heat and has the characteristics of large daily water production, low energy consumption per unit of water and high water quality. Because the moisture content of air in dry area is very low, the effect of direct condensation is limited. Solid adsorbent is able to adsorb water vapor from air at a low temperature and release water vapor at under high temperature, which can be used for water extracting from air. To improve its performance under dry circumstances, the key technical point of this device is to use solid adsorbent to collect water vapor from other air to raise its dew point temperature, and then use high temperature cold source to condense water vapor from it. In this paper, configurations of the solid adsorption are proposed, which can be driven with low regeneration temperature under the same humidity increasing amount. This device uses multi-stage desiccant wheels to realize humid increasing. Desiccant wheel can be driven with high temperature to take water vapor from dehumidification air and release water vapor to regeneration air. The multi-stage configuration is good for the reduction of regeneration temperature, making applications of low temperature waste heat form heat pumps possible. Then, influencing factors of water extracting rate are analyzed. The influencing of regeneration temperature, humid reduction amount of the humidified air and cooling and heating systems, etc., are analyzed. Last, air handling processes considering cold and heat sources are recommended to reduce energy consumption. The heat pump driven scenarios are discussed in particular. Through optimization, the water extracting rate can be increased and energy consumption per unit of water can be reduced. At present, this paper only studies air water extracting processes and thermal processes, and does not involve structure of the device, water purification and power consumption of fans, etc.

Techno-economic analysis of MOF-based adsorption cycles for postcombustion CO2 capture from wet flue gas

A Full-Solid-State Humidity Pump for Localized Humidity Control

The development of new materials with high adsorption capacity and selectivity is becoming attractive for the applications of clean energy and environment pollution control. Metal-organic frameworks (MOFs) are promising adsorbents for gas storage and separation, such as H2 and CH4 storage, and CO2 capture, due to their extraordinarily high porosity, adjustable pore sizes, controllable surface functionality and potential scalability for industrial applications. This thesis focuses on developing novel MOFs for selective gas adsorption with large adsorption capacities and high selectivity, as well as good thermal and chemical stability. The studies include optimizing the activation conditions for uniform and empty pores with MOFs Cu-BTC taken as a case study, designing novel MOFs structure with desired functional groups for high selectivity of CO2 over other gases, fabricating MOFs contained composites with desired electrostatic force with ZIF-8/CNTs as an example, and evaluating the selective gas adsorption performance of all the prepared materials. This thesis aims to establish the relationship between the structure features of MOFs (pore size, metal centres, surface functional groups and electrostatic force) and gas adsorption performance of MOFs (including adsorption capacity and selectivity). The first part of experimental chapters focuses on the preparation and activation of copper-based MOFs Cu-BTC. The materials were synthesized by solvothermal method and activated by six different solvents (chloroform, dichloromethane, acetone, ethanol, methanol and water) in the activation process. The effects of different activation solvents on the thermal stability, porous structure and CO2 adsorption of Cu-BTC were investigated. The more DMF molecules were evacuated from the pores of Cu-BTC, the better adsorption performance was reflected in the material. The high crystalline and nearly solvent-free frameworks with highest BET surface area (2042 m2/g), largest pore volume (0.823 cm3/g) as well as highest CO2 loading (11.60 mmol/g at 0 dC and 132 kPa) can be achieved while using methanol as activation solvent. Then the selective adsorption of CO2/N2 and CO2/CH4 on Cu-BTC were examined through the experimental measurement of equilibrium adsorption capacities from pure fluids (CO2, CH4 and N2) and mixtures of CO2/N2 and CO2/CH4. Grand Canonical Monte Carlo (GCMC) model was performed to predict the adsorption capacities from pure fluids and binary mixtures. The GCMC model gives reasonable predictions of the measured adsorption capacities for pure gases at low pressures (l5 bar), but significantly over predicts that at pressures greater than 5 bar. The GCMC model fails to provide a satisfactory fit of the binary adsorption measurements across the entire pressure range studied. The Ideal Adsorbed Solution Theory (IAST) model using best-fit parameters for Langmuir isotherms of each pure fluid provides more satisfactory predictions of CO2/N2 and CO2/CH4 than the GCMC model. This combined experimental and modelling approach can provide criteria to screen MOFs for the separation of gas mixtures at industrially relevant compositions, temperatures and pressures. The second part of experimental chapters mainly focuses on synthesising MOFs with enhanced affinity for CO2 and selectivity of CO2 over other gases. Three novel amino-functionalized MOFs with both open metal sites (OMSs) and Levis basic sites (LBSs) were synthesized by solvothermal reactions. Single crystal structure analysis showed that Mg-ABDC and Co-ABDC were isostructural comprising two-dimensional layer structures, while Sr-ABDC contained a three-dimensional motif. These amino-functionalized MOFs were further characterized by powder X-ray diffraction, thermal gravimetric analysis and N2 ads-desorption. Adsorption isotherms of CO2 and N2 were obtained at various temperatures (0, 25 and 35 dC) and then the adsorption capacity and CO2/N2 selectivity for these MOFs were compared. Based on results, both Mg-ABDC and Co-ABDC decorated by the -NH2 groups and the open metal sites exhibit high heat of CO2 adsorption (g 30 kJ/mol) and excellent adsorption selectivity of CO2 over N2 (g375). In contrast, Sr-ABDC displays poor adsorption properties due to small pore size, low surface area and small pore volume. Introducing desired electrostatic force into MOF structures by the incorporation of carbon nanotubes (CNTs) into MOFs can obtain better crystals and enhance the properties of composite. A series of ZIF-8/CNTs composites were successfully synthesized by solvothermal method. The contents of ZIF-8 and CNTs in the composites were calculated from Thermogravimetric Analysis data. CO2 and N2 adsorption at 273 K on the composites were also investigated and compared. Results show that there are interactions (synergetic effect) between ZIF-8 crystals and CNTs in the composites, reflected in the change of crystallinity, morphology, thermal stability, and adsorption properties. The surface area and adsorption capacities of ZIF-8/CNTs composites can be controlled by adjusting the CNTs content in the composites. In optimal CNTs loading ratio, the ZIF-8/CNTs composite showed improved adsorption capacities and selectivity of CO2/N2, illustrating that the incorporation of CNTs into MOFs synthesis is a promising approach to enhance the adsorption performance of MOFs.

Global warming threatens the entire planet, and solutions such as direct air capture (DAC) can be used to meet net-zero goals and go beyond. This study investigates using DAC in a 5-step temperature vacuum swing adsorption (TVSA) cycle with adsorbents' Li-X and Na-X, readily available industrial zeolites, to capture and concentrate CO2 from air in cold climates. From this study, we report that Na-X in cold conditions has the highest known CO2 adsorption capacity in air of 2.54mmol/g. This combined with Na-X's low CO2 heat of adsorption, and fast uptake-rate in comparison to other benchmark materials, allowed for Na-X operating in cold conditions to have the lowest reported DAC operating energy of 1.1 MWh/tonCO2. These findings from this study show the promise of this process in cold climates of Canada, Alaska, Greenland, and Antarctica to be part of the solution to global warming.

An All-Purpose Porous Cleaner for Acid Gas Removal and Dehydration of Natural Gas

Due to burgeoning carbon dioxide (CO2) emission, adsorption post-combustion capture technology gathers momentum. Cost-effective adsorbents with prominent performance draw more attention. Thus, a novel KOH activation biomass-derived activated carbon, i.e. BBC-KOH, is developed and investigated in terms of textural properties, adsorption/desorption temperatures, desorption heat and cyclic stability. A techno-economic assessment of a solar-assisted coal-fired power plant integrated with temperature swing adsorption process is conducted to evaluate potential merits in a practical application. Results demonstrate that BBC-KOH could perform an excellent adsorption capacity (1.50 mmol·g−1 at 25 °C) with a low desorption temperature (1.42 mmol·g−1 at 70 °C) and could be desorbed completely at 80 °C, i.e. 1.54 mmol·g−1. Compared with solar-assisted coal-fired power plant systems using monoethanolamine and polyethyleneimine/silica, carbon emission intensity, levelized cost of electricity and cost of CO2 removed of the system using BBC-KOH always show the best performance, corresponding to 93.22 g·kWh−1, 59.19 USD·MWh−1 and 14.12 USD·tonCO2-1, respectively. It reveals that BBC-KOH may be a potential solution to carbon capture with low capital cost, low regeneration temperature and excellent adsorption capacity.

The efficacy of carbon molecular sieve and solid amine for CO2 separation from a simulated wet flue gas by an internally heated/cooled temperature swing adsorption process

Systematic evaluation of materials for post-combustion CO2 capture in a Temperature Swing Adsorption process

Hydrogen storage in 1D nanotube-like channels metal–organic frameworks: Effects of free volume and heat of adsorption on hydrogen uptake

The use of desiccant-coated heat exchangers (DCHE) in air conditioning systems possesses great advantages in the independent control of both temperature and humidity, as well as low energy consumption and high coefficient of performance (COP). The paper presents a novel high temperature cooling system that uses metal-organic frameworks (MOFs) as sorbents for humidity control. MOFs are a new class of porous crystalline materials consisted of metal clusters and organic linkers, which have an excellent performance of water sorption due to the large specific surface areas and high porosity. In this research, we directly coat MOFs on the heat exchange surface of evaporator and condenser. The evaporator can simultaneously remove both the sensible and latent loads of the incoming air without reducing the temperature below its dew point. The regeneration of wet MOFs is completely driven by the residual heat from the condenser. We also make comparison between the MOF-coated cooling system and conventional desiccants coated ones (i.e. silica gel, zeolite) by way of tests and calculation. The results indicate that the dehumidification capacity of the MOF-coated heat exchanger outperforms other conventional desiccant coated ones under low regeneration temperature (∼50°C). The MOF-coated system has a high COP, up to 7.9, and can save 36.1% of the energy required, compared to the traditional vapour compression system with reheating. The amphiphilic MOFs used in the study have high water uptake and low regeneration temperature, and they thus have the potential for being scaled up for large-scale applications in energy efficient air conditioning systems.