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Measurements of water vapour sorption isotherms for RD silica gel, AQSOA-Z01, AQSOA-Z02, AQSOA-Z05 and CECA zeolite 3A
Abstract
The water vapour adsorption isotherms of RD silica gel, AQSOA-Z01, AQSOA-Z02, AQSOA-Z05 and CECA zeolite 3A are measured for temperatures between 20 degrees C and 160 degrees C and vapour partial pressures between 1 and 500 mbar. The measurements were fit to a theoretical isotherm function and for some materials the RMS deviation was less than 5%. Since others had a poorer theoretical fit, all data was also fit to spline functions for use in numerical simulations. The variation of isosteric heats of adsorption as a function of uptake, calculated using a least squares fitting approach, are compared over a range of uptake values for each adsorbent. The heats of adsorption were between 1 and 1.6 times the heat of vaporisation for uptake values above 10% of the maximum uptake for each adsorbent. The AQSOA adsorbents display Type IV isotherm shapes and the silica gel and 3A zeolite display Type I shapes. The sorption hysteresis is investigated for the AQSOA adsorbents at a selected temperature. These adsorbents displayed a small degree of hysteresis with an H1 type behaviour.
... This technology has the unique advantages of little risk of damage, environment friendly refrigerant, no use of CFC, low electricity consumption, no moving parts and low maintenance effort [16,[23][24][25]27]. It can run on heat sources of relatively low temperatures ranging from 50-150 o C [28]. ...
... In the sorption enhanced CO 2 methanation, the water sorbent must be able to operate efficiently at a high temperature (>200°C) since the minimum working temperature of a conventional Ni-based catalysts, for example, is well over 300°C. Silica-gel cannot be used as its extremely low water capacity at the methanation reaction temperature (Goldsworthy, 2014;Wang & LeVan, 2009). The most promising sorbent class for CO 2 methanation is zeolites (van Kampen et al., 2019), which provides high water absorption capacity and are stable under the reactor and regeneration conditions. ...
Hydrogen produced by the electrolysis of water using sustainable electricity will play an increasingly important role as an energy and a feedstock vector. Shifting from fossil to renewable resources means that new industrial platforms have to be set up to provide carbon-based fuels and bulk base chemicals to replace the current fossil resources based routes. The global demand cannot be met by indirect use of carbon dioxide via biomass necessitating the use from point sources or direct air capture, which changes the value of CO2 from waste to commodity chemicals. The production of chemicals by hydrogenation of CO2 is typically hampered by the thermodynamic conversion being far from 100% under currently viable reaction conditions. The equilibrium can, however, be shifted to increase conversion by removing one of the reaction products, namely water, from the reaction mixture with sorbents like zeolites. Prerequisite to conversion enhancement and process intensification is the close proximity of sorption and catalytic sites. This review presents the state of the art in synthesis and application of these, in fact, bifunctional materials.
Air-conditioning (A/C) systems in tropical regions are characterized by significant energy consumption for latent load handling. Decoupling of the latent load from the A/C units can be achieved using a dedicated dehumidification system while the A/C systems handle only the sensible heat at high efficiencies. Desiccants are widely used in industry, and adsorbent materials that exhibit a unique isotherm shape, i.e. "S shape", have been developed extensively. Recently, activated carbons (ACs) have been discussed as effective adsorbents for dehumidification applications. Although pristine ACs are considered to be hydrophobic materials, certain surface treatments initiate surface phenomena that promote water vapour uptake at relative pressures above 0.4 due to microdroplet aggregation. This work reviews and reports the latest developments of sustainable activated carbons for dehumidification using a multiscale approach spanning from the sustainable precursor selection, “green” activation processes and surface functionalization, adsorption thermodynamics, and system-level developments. With the focus on sustainability, we demonstrate that water adsorption and viable adsorption range are gradually improving with the progressing research, and they are reaching operational values required for practical use. The unique adsorption process of water onto ACs is further explained in detail using solvation theory on the microdomains created by the hydrophilic functional groups while providing clarification of thermodynamic properties adopting the specificities of water/activated carbon adsorption pair. The predicted performance of a desiccant dehumidification system utilizing activated carbon is evaluated using the local weather conditions of numerous major cities worldwide. The highest dehumidification performances of activated carbon, as indicated by the unified SDP (specific dehumidification power) value, are reached particularly in cities that suffer from high humidity and temperature the most proving the viability of this cheap and sustainable material.
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In this study, the effect of using a hybrid solar thermal-activated adsorption desalination system for brackish water is evaluated under the climatic conditions of the Brazilian semiarid region. The proposed theoretical model utilizes climatic data from the meteorological station in Campina Grande, PB, and adsorptive kinetics data of Fuji Davison RD 260 silica gel to predict the performance indices of the specific daily water production (SDWP), specific cooling power (SCP), and coefficient of performance (COP) performance coefficients over a characteristic day. The SDWP value of 6.26 m3/ton, SCP ranging from 50 to 300 W/kg, and an average COP of 0.5 were obtained, considering variations in global horizontal irradiance in the ACDS system and transient ambient temperature. It was observed that both the production of desalinated water and the refrigeration effect increase with the rise in daily solar irradiance. The variation in the number of solar collectors used in the system and their optimality, as well as the variation in the salinity index of the feed source, impacted the evaluated performance coefficients.
N-Methyl-pyrrolidone (NMP) is an important coating solvent for the production of lithium batteries, and its water content will greatly affect the coating quality and energy density of lithium batteries, which needs to be reduced to 200 ppm. The current vacuum distillation technology suffers from high operating costs and high energy consumption, whereas the pervaporation technology only achieves solvent dehydration up to 99.5%. Therefore, it is of great significance to carry out the study of trace water removal from NMP solvents. In this paper, the A-type molecular sieve adsorption method was used to remove trace water from the NMP solvent, and the effects of molecular sieve type, particle size, adsorption temperature, feeding amount, and contact time on the dehydration performance of NMP system were first investigated. Adsorbed at 25 °C for 240 min at a feeding amount of 120 g/L, 3A molecular sieves were able to reduce the water content of the NMP solvent from 5000 to 140 ppm. Second, Langmuir and Freundlich equations were used to fit the static isothermal adsorption data, and the results showed a better correlation of the Langmuir equation. Then, the adsorption kinetics and diffusion mechanism were analyzed by the kinetic model and the Crank single-pore diffusion model. The R² of the pseudo-first-order kinetic model was 0.9993, which was more suitable for describing the process of adsorption of water from the NMP solvent by 3A molecular sieves, and the effective diffusion coefficient De = 2.986 × 10–8 cm²/s was calculated for the Crank single-pore adsorption model, which proved water molecules on the 3A molecular sieve. The diffusion of water molecules on the inner surface of the pores is the controlling step of the adsorption process. Finally, the fixed-bed dynamic penetration curves were investigated to obtain the experimental data of fixed-bed adsorption, and the experimental data were fitted using the Thomas and Yoon–Nelson models, which showed that both models could describe the adsorption behavior of trace water in NMP solvents on 3A molecular sieves. This study provides a new idea for the removal of trace water in NMP systems, and a series of model fitting parameters provide basic data for industrial scale-up.
By combining the advantages of desiccant dehumidification and vapor compression refrigeration, the desiccant-coated heat exchanger-based heat pump (DCHE HP) is regarded as a promising alternative to the traditional vapor-compression air conditioning system (VCAC). Selecting proper desiccants from a large number of candidates is of great importance to improve the performance of DCHE HP. However, this task is challenging using current experimental or modelling strategies. In this work, we developed an equilibrium model to evaluate the power consumption of 39 DCHE HPs coated with different desiccants under various operating conditions. Eventually, five desiccants with low power consumption were selected. It was also demonstrated that under given operating conditions, the DCHE HP based on the five selected desiccants can save 21.3%–32.9% power compared with the VCAC. The power consumption of the DCHE HP is largely dependent on the heat of adsorption, the cyclical water uptakes and the remained moisture contents of the coating desiccants. It was further revealed that the moderate heat of adsorption, the larger cyclical water uptake and the lower remained moisture content are preferable for reducing the system energy demand. This work reported a quick evaluation of 39 desiccants for DCHE HP by an equilibrium model, which may also offer insights into the choosing and designing of desiccants for DCHE HP.
The adsorption desalination and cooling system (ADCS), a promising technology for alleviating water scarcity, requires adsorbents with high performance, low cost, and low energy consumption. Enhancing the operating efficiency and minimizing the energy consumption of the system entails improving the adsorption performance and thermal conductivity of adsorbents. In this study, nanoparticles (NP) embedded silica gel (SG) composite adsorbents with CaCl2 were synthesized for use in ADCS. The effects of nano-Fe, nano-Cu and carbon nanotubes (CNT) as SG thermal conductivity enhancers were thoroughly compared through microscopic properties, thermal properties and adsorption properties. The experimental results show that SG/CNT/CaCl2 composite performs the largest specific surface area and pore volume of 180.72 m² g⁻¹ and 0.13 cm³ g⁻¹. Scanning electron microscopy (SEM), accelerated surface area and porosimetry (ASAP) and constant thermal analyzer (TCA) were used to characterize the various performance of the enhanced adsorbents comprehensively. SG/CNT/CaCl2 sample exhibits high thermal conductivity and thermal diffusion coefficient, measuring as high as 0.664 W m⁻¹ K⁻¹ and 0.701 × 10⁻⁶ m² s⁻¹, respectively. Based on dynamic water adsorption performance tests, the SG/CNT/CaCl2 composite had an adsorption capacity of 0.665 g g⁻¹, five times higher than SG. The composite’s adsorption capacity and thermal conductivity did not decline significantly during cycling, indicating a high degree of sustainability. Therefore, these results imply that the nanoparticles-embedded SG/CaCl2 adsorbent with excellent integrated properties has promising prospects in adsorption desalination systems.
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It was investigated how different embedded nanoparticles affected the thermal and adsorption performance of SG. The results showed that the composite SG/CNT/CaCl2 exhibited the best performance, with the thermal conductivity and adsorption capacity being enhanced to 0.664 W m⁻¹ K⁻¹ and 0.665 g g⁻¹, respectively.
In this paper, the use of Metal-Organic Framework (MOF) materials as an option for the energy efficiency enhancement of HVAC systems is investigated. In particular, the possibility of using MOFs as dehumidifying materials to reduce the latent load associated with the moisture content of the airflows is studied. A literature review is proposed, highlighting the benefits of using MOFs instead of other adsorbents (e.g., silica-gel) and discussing the unique features (high water uptake capacity and low regeneration temperatures) that make MOFs a preferential desiccant. The possibility to finely tune these properties is also underlined, reporting some explicative examples. A theoretical proposal of a psychrometric transformation, to be performed in a HVAC system equipped with a MOF-Assisted Dehumidifier (MAD), is presented. This transformation is compared with a traditional one (cooling and dehumidification operated by a cooling coil with low temperatures of the coolant). The preliminary numerical simulations, conducted on a reference case study in Florence, Italy, show an estimated energy saving of 30–50%, leading us to consider the use of this technology as a very competitive one in the air-conditioning sector.
Sorption thermal energy storage (STES) systems offer great potential for meeting the growing energy needs and
increasing the use of renewable energy resources. However, the complexity and large-scale of current STES
technologies lead to relatively low-efficiency systems, severely limiting their widespread implementation. To
address this challenge, this study presents a novel and compact smart-polymer sorbent (SPS) thermal battery
with passive material-enabled mechanisms for controlled low-temperature thermal energy storage applications.
A numerical model is established to investigate the coupled heat and mass transport processes underlying the
working behavior of the SPS battery. It is also used to reveal the interdependence of the two processes and the
limiting factors for the battery’s thermal performance. The heating performance of several SPS battery designs
was studied as an example, focusing on their application for cold environment thermal protection. Moreover, the
influence of various design parameters was examined, including sorbent selection, void fraction, relative hu-
midity, gate thickness, and gate-to-sorbent area, along with their implications to the coupled mass-heat transport
processes. The results revealed that diffusion-based mass transport is the limiting process affecting the SPS
battery thermal performance. The numerical simulation results also indicated that the sorbent selection should
consider their low recharge temperature (40 ◦C–100 ◦C) and less pronounced temperature-dependent adsorption
behavior to maximize the heating performance of the SPS battery. In addition, the SPS battery heating efficiency
could be enhanced by increasing the sorbent void fraction to 0.35, relative humidity (RH) to 40 %, and gate-to-
sorbent area to 60 % while decreasing the gate thickness to 0.1 μm effectively, which facilitate the diffusion-
controlled mass transport process. Lastly, a single layer SPS battery is beneficial for thermal regulation at low
temperatures; however, using distributed battery designs had a significant thermal performance enhancement
benefit of 90 % compared to the single layer design. Hence, our study reveals the complex coupling effects of the
heat and mass transport processes within the proposed SPS battery and provides insights and guidance (e.g.,
materials selections and SPS design) for engineering emergent small-scale passive STES applications.
Adsorption desalination is prescribed as a promising and eco-friendly solution for mitigating water scarcity, owing to its utilization of low-grade thermal waste and zero liquid brine discharge. The keystones that regulate the performance of the adsorption desalination system (ADS) include nature of adsorbents, system design, and operating conditions. The present study aims to provide a state of the art review on the keystones of ADS. Metal-organic frameworks (MOFs) hold remarkable adsorption capacity and tunable structure. However, hydrothermal instability, high cost, and complex synthesizing procedures are the potential challenges that need to be addressed. The technological advancements in ADS have been classified into: (i) Conventional Approach, (ii) Heat and Mass Recovery Approaches, (iii) Hybridization Approaches, (iv) and Adsorbent Substituting Approach. The study provides critical insight and compares the performance of each approach based on specific daily water production (SDWP), specific cooling power (SCP), and coefficient of performance (COP). The conventional ADS produce SDWP of 4.7 m3/ton/d, however producing zero and/or minimal SCP while using payable energy of 1.50 kWh/m3. In heat/mass recovery approaches, pressure equalization-valve delay schemes and master–slave configuration provide ∼ 5 % additional water adsorption/desorption on/from silica-gel and reduce ∼ 50 % thermal heating load, respectively. Evaporator-condenser amalgamation emphasizes the evaporator temperature of 30–42 °C leading towards ∼ 69 % higher SDWP with zero SCP. Dual stage, multi evaporators/condensers scheme is found supportive in cogenerating feature of ADS thereby improvising COP to ∼ 0.87. In hybridization approach, ejector integrated ADS produces SDWP of 80 m3/ton and COP of 2.22 using payable energy of 0.92 kWh/m3, however, needs experimental validation. In the adsorbent substituting approach, CPO-27(Ni), Emim-Ac/Syloid 72FP, and composite adsorbent manifest the SDWP to higher levels. The operating conditions are sensitive and need to optimize depending on the configuration of ADS. Possible future research directions may include efficient designing/ sizing of evaporators/ condensers, minimizing the heat and mass transfer resistances in adsorber/desorber reactor, optimize the thickness of the adsorbent layer in heat exchangers, and investigating wide range of adsorbent classes that can be driven with very low regeneration temperature.
Freshwater resources are being heavily depleted and not replenished at the same high rate, thus, atmospheric water vapor harvesting has earned growing interest. Development of high-performing desiccant or adsorbent materials offering high sorption capacity and selectivity as well as regeneration capability in atmospheric conditions is crucial to tackle water scarcity. The required properties to generate potent water sorbents include pore accessibility, high specific surface area and porosity to enable high capture capacity and kinetics, and hydrothermal resilience to resist cyclic sorption and desorption. Further, polarity, hydrogen bonding, adhesion ability of water molecules to adsorbent surface, and hydrophilic functional groups can boost water adsorption. A highly promising class of water vapor adsorbents is the aluminophosphate molecular sieves (AlPOs), which are microporous zeotype materials. In this review, AlPO-based adsorbents are discussed for water sorption applications and a link is established between performance of materials and their chemical and morphological properties. Synthesis-properties-performance relationships are elucidated in light of synthesis techniques and adsorption behavior, and prospects to enhance AlPOs' water vapor sorption performance toward large-scale water harvesting applications are highlighted.
Composite sorbents with hygroscopic salts (like LiCl and CaCl2) impregnated porous matrixes obtained special attention in the past years, especially for solid desiccant dehumidification. However, the drawback of strong corrosivity limited their practical implementation. Alternatively, ionic liquid (1-ethyl-3-methylimidazole acetate) with less corrosivity is confined into ordered mesoporous MCM-41 to fabricate composite sorbent in this study, overcoming the poor mass transfer or limited adsorption capacity of single sorbent. Comprehensive characterizations including SEM, TEM, XRD, FTIR and N2 adsorption prove that the ionic liquid is encapsulated in ordered channel of MCM-41. Sorption isotherms and kinetics indicate that the composite sorbent has superior water sorption performance (0.5 g/g at 20 °C/70% RH), fast sorption rate under wide humidity range and potential of low temperature-driven regeneration. Moreover, the leakage test and corrosion experiment reveal that the composite sorbent has long-term stability and is corrosion-free to metal components. Further, the dehumidification performance using desiccant coated heat exchanger with reported composite sorbent is evaluated, showing the 5 times higher dehumidification capacity than that of MCM-41 and 2 times higher than traditional silica gel (typical ARI summer, 35°C, 40% RH). This study identifies a feasible way using ionic liquids to develop energy-efficient, safety-ensured and low-cost composite sorbents for water sorption-related applications.
This study investigates the performance of double effect adsorption refrigeration cycle that utilizes the adsorption heat of a high-temperature adsorbent heat exchanger for desorption of a low-temperature one. Because there is a freedom to select an adsorbent for each adsorbent heat exchanger, the combination of adsorbents such as silica gel, FAM-Z01, FAM-Z02 and FAM-Z05 were examined. The temperature of an external heat source was assumed to be 90°C. Dynamic cycle simulation was employed to estimate the performance in terms of coefficient of performance (COP) and specific cooling power (SCP). The results show that the combination of the adsorbents FAM-Z01 and FAM-Z02 for the low-temperature heat exchanger and high-temperature one, respectively, is the most effective to achieve high COP as well as high SCP. The results also indicate that the COP=1.0 is attainable when the mass allocation of the adsorbents and the cycle time are selected appropriately. From the viewpoint of maximizing the cooling power, COP=0.9 is suggested for the best design, where the mass of high-temperature side is equal to the mass of low-temperature side.
This paper presents a study of the application of the properties of water vapor as a gas with high potential energy, strongly dependent on temperature and pressure. Analyses of water vapor sorption on two types of silica gels (SG) (90 wt.%) enriched with carbon nanotubes (CNTs) (10 wt.%), in the context of their application in the design of adsorption beds in adsorption cooling and desalination systems were conducted. The sorption experiments were performed by gravimetric method at a relative humidity of 0% < RH < 100% and temperatures of 298 K, 313 K, and 333 K. The addition of CNTs to SG caused a decrease in the sorption capacity and depended on the temperature. As the process temperature increased, a lower SG/CNT mixtures sorption capacity to vapor was obtained. The highest influence of CNTs was observed at the highest temperature, and the average decrease of sorption capacity was several percent. The ratio of SG/CNT sorption capacity to pure SG values was below 1 in most measurements.
This study explores enhancing adsorption system’s performance utilizing sodium polyacrylate (SP) as an adsorbent for the first time. Four innovated SP samples are explored: raw SP, SP/HCl, SP/(NH4)2CO3), and SP/CaCl2 composite for adsorption desalination and cooling applications. Different characterization methods, including X-ray diffraction, nitrogen adsorption isotherm, and water adsorption (isotherms and kinetics) of SP samples, are investigated. Water adsorption experimental results onto SP samples and their numerical fitting with the Dubinin-Astakhov equilibrium model for isotherms and linear driving force model for kinetics have been expressed. The composite SP/CaCl2 had the highest experimental adsorption uptake of 1.26 kgH2O/kg among the studied samples. At 85 °C regeneration temperature, water desalination production per day (SDWP) achieves 15 m³/ton, with a cooling power of 425 W/kg. SDWP could reach 41 m³/ton of SP/CaCl2 per day with heat recovery. The system can obtain an SDWP of 45 m³/ton per day at a regeneration temperature of 95 °C. The findings show that the system can run efficiently using renewable energy, waste heat, or geothermal energy as heat sources.
Adsorption cooling system (ACS) driven by engine waste heat is an attractive alternative to vehicle air conditioning (A/C) system. To find a compact and efficient solution for onboard application, a heat pipe-assisted finned adsorber unit tube (AUT) packed with FAM-Z02 adsorbent was studied numerically. Based on the integrated physical prototype, a three-dimensional numerical model was developed to investigate adsorption/desorption characteristics combined with experimentally validated heat pipe model and adsorption isotherm model. Moreover, specific cooling power (SCP), coefficient of performance (COP) and adsorber bed to adsorbent mass ratio (AAMR) were quantified to evaluate cooling capacity and system performance. It is found that, compared with adsorber without heat pipe, a 15.3% increment of SCP and 19.1% increment of COP can be achieved in the FAM-Z02/water based adsorber due to the contribution of heat pipe, and the AUT has an optimal SCP of 64.19 W/kg at the cycle time of 8510 s. The smaller adsorbent particle size leads to more refrigerants adsorbing/desorbing under the same cycle time, and FAM-Z02 with 0.1 mm diameter provides optimal SCP of 66.7 W/kg for ACS due to smaller particles possess less intra-particle mass transfer resistance. When the cool source temperature and heat source temperature are 300 K and 365 K respectively, higher SCP can be achieved due to the larger cyclic water uptake. Heat pipe-assisted finned adsorber offers a viable solution for coolant’s application in onboard ACS, and a 6-fin adsorber packed with 0.1mm-diameter FAM-Z02 is considered optimal under lower adsorption temperature and higher desorption temperature.
Due to the ever-increasing cooling demand, alternative refrigeration is being explored for the last few decades that are less energy-intensive and can utilize ozoenvironmental friendly working fluid. Vapor adsorption refrigeration is one of the most promising alternatives among all heat-driven refrigeration systems. One of the most important requirements in an adsorption-based cooling system is to enhance the adsorption uptake capacity of the adsorbent materials for their impactful, effective, and economical operation. However, there are several working pairs explored during the recent decades but none of them could be suitable as an ideal one, for all the operating conditions. In this article, the various characteristics of different adsorbents including the highly porous activated carbons derived from waste biomass, the composites, and compounds developed, are thoroughly reviewed. Further, the synthesization, the optimal assortment, regeneration, coatings on adsorber, and commercial application of novel adsorbents including the advanced adsorption reactors and the current scenario of industrial adsorption chillers are discussed in detail. Finally, the impact of operating parameters, such as adsorbent-adsorbate mass ratio, desorption pressure, operating temperatures, and adsorption/desorption cycle time on the specific cooling power, coefficient of performance, and the chiller efficiency are summarized.
Abstract: Energy, freshwater, and the environment are interrelated factors that permeate all our activities on the earth. They have be-come the most important and popular topics in research fields now-adays. This paper studies improving the performance of the adsorp-tion desalination system by improving a commercial silica gel as an adsorbent. Acid treatment has been used to activation for the silica gel. Then, a composite adsorbent is prepared chemically using silica gel as a host matrix and impregnated in CaCl2 salt hydrate. Water vapor adsorption isotherm and kinetics are investigated for raw, treated, and composite silica gel. The Dubinin-Astakhov (D-A) and linear driving force models (LDF) have fitted experimental isotherm and kinetic results. The system is driven by renewable energy such as solar energy. The results illustrated that the silica gel/CaCl2 achieved water uptake (0.95 kgH2O/kg). It is also observed that the composite silica gel based-adsorption desalination cycle provides specific daily water production of 70 % higher than the raw silica gel-based ad-sorption desalination cycle.
In this study, a crossflow-type heat exchanger coated with aluminophosphate zeolite was tested for desiccant dehumidification with direct-supplied hot water and evaporative cooling using remaining hot water. This evaporative cooling process could accelerate dehumidification by removing the heat of adsorption. Therefore, the influences of the cooling air velocity, regeneration hot water temperature, and adsorption/regeneration cycle time were experimentally investigated. The result showed that the minimum humidity of the adsorption outlet air decreased with an increase in the cooling air velocity and adsorption/desorption time, indicating that the complete regeneration of coated adsorbents is required to significantly decrease humidity. However, the maximum time-averaged amount of dehumidification appeared at an appropriate cycle time, which avoided adsorption saturation during the adsorption period. Further, in any case, increasing the cooling air velocity had a positive impact on both the momentary observed minimum humidity and time-averaged amount of dehumidification.
At all cooling air velocity, the temperature of the hot water contributed to the increase in the time-averaged amount of dehumidification. However, when the hot water temperature was 55 °C, the cooling effect of evaporative was not observed, even when the cooling air velocity reached 1.5 m/s. Considering that the actual temperature of the coated adsorbent was expected to be much lower than that of the supplied hot water, the hot water temperature should have been ≥65 °C.
Zeolite-based molecular sieves are applied in industrial dehydration units for their high water uptake capacities and extremely low equilibrium pressure of water vapor. During their operational life, they tend to lose their water vapor adsorption capacity slowly. To optimize the usage of molecular sieves in dryer units, it is vital to understand the mechanism(s) leading to deactivation. In this work, the capacity loss was studied by exposing LTA- and FAU-type zeolites to methanol and heptane vapors under relatively harsh conditions using repetitive adsorption/regeneration cycles. A simple microflow unit was designed and used for the deactivation experiments. The water vapor adsorption capacity of the resulting samples was measured using a gravimetric analyzer. In addition, they were characterized by classic XRD, 13C NMR, and TGA techniques. The crystallinity of fresh and spent zeolite XRD patterns was not drastically affected even after exposure to the contaminants. It was found that methanol easily gave rise to a severe loss of water vapor adsorption capacity, much more so than heptane. Water vapor uptake in the methanol exposed samples is ∼50% lower than that for the fresh zeolites. This is attributed to nonvolatile, residual hydrocarbons.
Atmospheric water harvesting has been inexorably proliferated as a potential source of freshwater, notably for remote areas that lack access to water and electricity. This technology could be significantly operated with renewable energy sources. The current study comprehensively reviews the state-of-the-art atmospheric water harvesters and their desiccant materials. Firstly, a detailed survey on desiccant materials, silica gel, metal-organic frameworks (MOFs), hydrogels, zeolite, hygroscopic salts and composite desiccant materials is illustrated. The review particularly focuses on the materials adsorption capability, kinetics, proper matching with climate conditions. Moreover, the most suitable adsorbents are thoroughly surveyed for a wide range of climate conditions, especially for water scarcity regions (i.e., arid zones) that are characterized by low relative pressures. Moreover, various designs of solar-powered atmospheric water harvesters are comparatively summarized, including fixed and portable installations. It can be concluded that MOF-801, MOF-808, MOF-841, HKUST-1, and CPO-27(Ni) have a superior potential for water harvesting in arid areas. Additionally, MIL-101(Cr) has superior water uptake and kinetic at high relative pressure (i.e., humid areas), and it is irrelevant for water harvesting at dry zones. It is found that the cost of the collected water from atmospheric water harvesting technology is about 0.062-0.86 $/kg of adsorbent. This work provides beneficial perspectives for selecting the most relevant desiccant materials beside the appropriate solar system for water harvesting applications.
The concept of coupled storage for dry cooler enhancement, based on the coupling of heat and humidity storages in a single module, has been experimentally studied and characterized in a previous study by the authors. The experimental results have highlighted that such storage can be effective to reduce the drop of performances for air-cooled condenser under high ambient temperatures and that fine understanding and interpretation of what happens inside the storage tank requires a dedicated numerical model. This paper presents in details the modelling of the coupled heat and moisture storages. The model is first validated on data from the literature in purely sensible and sorption operations. When compared to experimental data of coupled heat and moisture storage, the model exhibits very good qualitative agreement and good to correct quantitative agreement, depending on the considered parameters and the tests. The model allows to give physical insight of coupled sensible and moisture storage by giving local information and behaviours that are not available from measurement devices. The model needs further enhancements but is already of upmost importance for characterization and design of such coupled storage.
We report a simulation-based feasibility study of a single-stage temperature swing adsorption (TSA) process operated on a metal-organic framework (MOF) adsorbent, UTSA-16, for CO2 capture and concentration from a wet flue gas without pre-drying or a desiccant pre-layer. Two cycles comprising four and five-steps are compared. A non-isothermal and non-isobaric in-house simulator including suitably modelled equilibrium and kinetics of H2O-CO2-N2 ternary system is used to investigate the cyclic processes. The variables investigated are moisture-saturated flue gas at two feed and three regeneration temperatures. Both TSA cycles can meet 95% purity-90% recovery targets for postcombustion streams representative of wet flue gas from a coal-fired power plant (after SOx/NOx removal). The maximum productivity attained is 91.8 kg CO2 m⁻³ adsorber h⁻¹ for 15% CO2, 82% N2, and 3% H2O feed at 25 °C (i.e., 95% relative humidity) operated on a five-step cycle and regenerated at 150 °C.
Desiccant coated heat exchanger (DCHE) systems have significant potential in improving dehumidification performance. The moisture transfer occurring in one single air channel of DCHE can dominate the overall system performance. In this study, the adsorber with an approximate air channel of DCHE was designed to experimentally investigate the kinetics of silica gel and two aluminophosphate zeolites (FAM Z01 and FAM Z05). Their dehumidification performances also were simulated at regeneration temperature range of 50–80 °C to reveal the applicability in air-cooled cross-flow DCHE. Results showed that linear driving force (LDF) model could well describe the dynamic water uptake behaviors. Adsorption rate coefficient kads increased with increasing inlet air humidity ratio and velocity, but decreased with increasing coating thickness. It is worth noting that increasing regeneration temperature had a little effect on kads, while desorption rate coefficient kdes increased significantly. Simulation results showed that moisture removal capacity (MRC) and dehumidification performance of coefficient (DCOP) of FAM Z05 coated DCHE could reach 0.495 g/kg and 0.57 at regeneration temperature of 50 °C, which were 2.3 and 15.4 times higher than these of silica gel and FAM Z01 coated DCHEs. It demonstrated that FAM Z05 was more promising to utilize lower-grade heat energy.
The main objective of this study is to design a low-grade heat driven ethanol-silica gel adsorption chiller. The low-grade waste heat is utilized in an adsorption chiller. For this purpose, an innovative bed heat exchanger including a condenser embedded inside the adsorbent bed is uniquely designed. The silica gel/ethanol pair is examined both analytically and numerically in this article. The cycles of the adsorption chiller are performed experimentally. The obtained isotherm for the silica gel/ethanol fitted to Type III isotherm behavior. The obtained equations of isotherm are also validated numerically. The new adsorption chiller design used in this study has reduced the desorption temperature for silica gel/ethanol pair to 37°C which will widen the application area of adsorption chillers. The COMSOL Multiphysics program is used for 2-D numerical analysis of adsorbent bed. The mass transfer inside of the particle, the heat transfer in porous media, and Darcy law are used for analyzing the heat and mass transfer of the bed. Temperature and concentration distributions of adsorbent bed during the duration of adsorption and desorption processes are examined numerically. The specific cooling power and volumetric cooling power values of the system are found as 20.2 Wkg-1 and 4.5 kWm-3, respectively.
In the present study, benefits of utilizing acid activated montmorillonite as a proposed new adsorbent material in adsorption desalination-cooling systems-have been experimentally expressed. Montmorillonite is a natural clay mineral which is composed mainly of alumina-silicate. Effect of acid activation on montmorillonite has been illustrated using infra-red spectra analysis. Adsorption characteristics (isotherm and kinetic) of acid activated montmorillonite (with 2 mole of hydrochloric acid/water vapor pair have been expressed. Isotherms results have been fitted with Dubinin-Astakhov and Sun-Chakraborty models. An experimental adsorption desalination-cooling test rig has been erected to explore the adsorption desalination-cooling systems performance with montmorillonite /water pair and axial finned tube adsorption bed design. The experimental results indicates that the daily water production is about 4.4 m 3 /ton of montmorillonite, its specific cooling power is 110 W/kg and the coefficient of performance is 0.41 at a driving temperature less than 100 • C. Solar energy can drive adequately the investigated system. The experiment also illustrates that the ADCS is very significant in removing all forms of salts, as proven by the weighty drop of the total dissolved salt, TDS (measured by TDS analyzer), level from approximately 40,000 ppm in seawater intake to less than 30 ppm.
For many applications, the possibility of controlling heat flow by “thermal switching” could be very beneficial. Several concepts for heat switches were already proposed and tested, however, many drawbacks of these concepts are evident. In this work, we present a novel approach for thermal switching using a water-loaded adsorbent as part of the evaporator of a heat pipe. The basic idea is that the adsorbent releases water upon exceeding a certain evaporator temperature, and thus “activates” the heat pipe by providing the working fluid for thermal transport. The first part of this work concentrates on the adsorbent characterization by analyzing the adsorption isobars and isotherms and thus understanding the behavior of the system. Furthermore, a model to predict the release of water from the adsorbent in dependence of temperature was developed. Subsequently, the adsorbent was integrated into an actual heat pipe demonstrator to verify these predictions and demonstrate the thermal switching ability. Overall results revealed a very good agreement between the predictions concerning water release and the heat pipe’s thermal behavior. The obtained thermal switching ratio depends on the heating power and temperature range that is considered. Depending on whether evaporator/condenser or the adiabatic zone are considered, average switching ratios of circa 3 and 18 were found, respectively.
Keywords: heat pipe; zeolite; adsorbent; thermal switch; heat switch; thermal management
Due to the discoveries of large reserves of oil and gas in unconventional reservoirs, the production of natural gas (NG) has increased significantly, becoming a component of the world's nonrenewable energy matrix, which has grown the most in the last 10 years. In Brazil, these reserves, called the pre-salt layer, corresponded to 47% of the total NG produced until 2020. However, several technological challenges need to be overcome, including the development of more efficient strategies for gas conditioning and transport, including the removal of large amounts of water in the NG stream. Water with gaseous hydrocarbons under specific conditions of temperature and pressure result hydrate formation, causing corrosion, depletion, and loss of localized pressures, compromising their transport through pipelines. Among the processes used to remove water from the NG stream, dehydration using solid desiccants stands out for the high selectivity with the adsorbate due to the high affinity in the separation of water molecules from the gas stream, high recovery of products, and being a reversible process. Mesoporous silica and metal-organic frameworks (MOFs) are highly porous adsorbents that emerge as promising alternatives for water adsorption, as they present high chemical and mechanical stability, good renewability, support various adsorption/desorption cycles, and present high selectivity and capacity with water adsorption. It is worth noting that these characteristics result in more compact adsorption units, which is valuable, especially in offshore exploration. Therefore, since practical studies using mesoporous silica (SBA-15, MCM-41, xerogel and aerogel) and MOFs in the dehydration of NG are scarce, this work will present the characteristics of these adsorbents in relation to their textural, structural, and physical-chemical properties and the challenges for their use in separating water from gases.
The enhanced energy performance of desiccant dehumidifiers through new material synthesis and optimized design have extended their potential to several state-of-the-art energy-related applications including heat transformation, adsorption chilling, energy storage, and water harvesting. A review of the latest research trends in these system-level applications is presented in this chapter. Additionally, controlling methods to yield precise moisture levels for human thermal comfort and various industrial manufacturing processes are described. For human well-being, both extreme low and high humidity levels can cause health issues such as dryness and nausea including critical illnesses due to the airborne spread of bacteria, fungal, and viral infections. In industrial operations, the presence of high moisture levels may damage machines, deteriorate product quality, and ultimately lead to significant revenue losses. For such critical applications, desiccant dehumidifiers offer close and precise moisture control in an energy-efficient and cost-effective manner.
The dynamic uptake of adsorbate onto the porous adsorbent plays a crucial role in determining the performance of the adsorption-based cooling system. Therefore, it is imperative to know the kinetics parameters of an adsorbate – adsorbent pair to design a system to be operated at variable working conditions. The kinetics models of adsorption, used to simulate the adsorption rate of different pairs, are derived and presented in this paper. Besides, the limitations and advantages of the models are also mentioned. Moreover, the dynamic performance of different adsorption pairs is analyzed, and the values of kinetics parameters, determined through experimental procedures and fitting of kinetics models, are also summarized. It is opined that during the initial unsaturated condition of adsorption, the semi-infinite model can be preferred to determine the diffusion time constant. The modification of different models, e.g., Langmuir and linear driving force models, can significantly overcome the drawbacks of the models, as shown by several researchers. However, research may be carried out to investigate different models' fitting errors from a statistical perspective. Furthermore, to evaluate the dynamic performance of different adsorbates, a lot of research needs to be done, specifically, on the adsorption of the newly developed environment-friendly refrigerants, onto the promising composite adsorbents possessing high thermal conductivity and significantly improved adsorption uptakes.
Owing to its high porosity and high surface area, zeolite has a good adsorption effect on liquid adsorbates such as water, ammonia, organic liquids and gas adsorbates such as volatile organic compounds, hydrocarbons, etc. A large amount of heat is released during the adsorption period when the zeolite adsorbs these gases and liquids. The energy storage, the heat and mass transfer performance of zeolite adsorption is influenced by the selection of adsorbent and adsorbate as well as the design of zeolite bed. In this paper, the mechanism of zeolite adsorption is discussed, and equations that describe the adsorption isotherm and the heat and mass transfer of adsorbate on zeolite are reviewed. From the view of system aspect, the factors (such as the structural design of zeolite bed, the mass flow rate of adsorbate, the operating temperature, and the cycle time) that will affect the heat storage and transfer in adsorption are concluded. In terms of adsorbent/adsorbate, the properties of modified zeolites and zeolite composites are presented, and the reaction behavior of different zeolites and adsorbates is discussed. As for the application of zeolite adsorption system in the energy storage and heat transfer field, zeolite-based heat exchanger (HX), energy storage system (ESS), dehumidifier, energy absorption system (EAS), volatile organic compounds (VOCs) removal system and hydrocarbon (HC) trap are reported. Research results demonstrate that the zeolite adsorption system is promising no matter used alone or in combination with other equipment in energy-saving and environmentally friendly fields.
In this study, consolidated composite adsorbents comprising highly porous zeolite of type AQSOA-Z02 and two types of graphene nanoplatelets (GNPs), namely H25-GNPs and M25-GNPs, are developed to enhance the performance of a thermally powered adsorption heat pump. The thermophysical properties such as specific heat capacity, packing density, thermal conductivity, and porosity of the synthesized composite adsorbents are rigorously investigated. The addition of GNPs into zeolite improves the packing density as well as the thermal conductivity significantly. The highest thermal conductivity is observed for 50 wt% H25-GNPs contained composite, which is almost 127 times higher than the parent zeolite. Porous property investigation reveals that the fabricated composites are microporous, like the parent zeolite. Besides, the equilibrium adsorption uptake of water onto the composite adsorbents is measured gravimetrically at three different temperatures, 30, 50, and 70 °C. The experimental uptake data are correlated with the modified Langmuir, Sun and Chakraborty, and Universal models to describe the adsorption process mathematically. It was found that 20 wt% H25-GNPs contained composite provides almost 43% improvement of effective volumetric uptake over the parent zeolite. All the presented findings would carry significant importance for the development of an efficient and compact adsorption heat pump system. Besides, the obtained results would be very supportive for designing zeolite composite based gas separation and storage, desalination, and thermal energy storage systems.
Desiccant-coated heat exchanger (DC-HX) is a promising technology for greenhouse dehumidification since both latent and sensible loads can be addressed simultaneously. DC-HX can be operated using low-grade heat, with temperatures less than 90 °C, from renewable energy sources, which makes them attractive from environmental and operating cost perspectives. In this study, a new DC-HX coated with AQSOA™-FAM-Z02 is built and studied experimentally and theoretically under typical greenhouse conditions for the first time. For this purpose, a custom-built experimental setup is developed to examine the dehumidification performance of the proposed DC-HX. The effects of greenhouse airflow rate, temperature, and relative humidity (RH), as well as regeneration and cooling heat transfer fluid (HTF) temperatures, on the average moisture removal capacity (MRC) and thermal coefficient of performance (COP) are analyzed. Moreover, the contributions of the removed latent and sensible heats by the present DC-HX are discussed. It is found that the custom-built DC-HX can achieve cyclic MRC and COP in the range of 2.5–4.0 and 0.18–0.3, thereby providing satisfactory dehumidification performance for greenhouse applications. Moreover, optimal operating parameters to maximize COP and MRC are determined by performing a multi-objective optimization using genetic algorithm approach.
In winter conditions air humidifiers are particularly important because low indoor relative humidity can lead to occupants’ discomfort, such as dry throat, skin and eyes and, in addition, it can facilitate virus and bacteria transmission among persons. In this work, a study of a novel humidification system based on silica gel packed beds is carried out. In the proposed device, vapour is adsorbed from the exhaust airflow leaving the building and it is released to the fresh air stream supplied to the indoor environment. The analysis is carried out through both experimental and numerical approach: a specific test rig has been implemented to evaluate performance of the packed bed apparatus and obtained data have been used to validate a phenomenological model. The adsorption isotherm and the heat of adsorption of the adopted silica gel have been evaluated though a gravimetric approach. Numerical results highlight the proposed system can provide satisfactory air humidification even when it is driven by low temperature heat sources. Depending on bed thickness, airflows arrangement and air velocity, the humidity ratio of airflow supplied to the building can be increased from 1.5 g kg⁻¹ to 4.8–5.8 g kg⁻¹ when the indoor humidity ratio is 5.8 g kg⁻¹ and the regeneration temperature is around 50 °C.
Low‐grade heat is abundantly available below 100 °C, while industry mainly needs heat above 100 °C. Thus, the industry cannot directly utilize low‐grade heat to save primary energy and emissions. Low‐grade heat could be utilized by Adsorption Heat Transformers (AdHT); however, closed adsorption heat transformers to upgrade heat above 100 °C have only been investigated by idealized steady‐state analyses, which indicate the maximal theoretical performance. For evaluating the performance achievable in practice, this work studies a closed adsorption heat transformer in a one‐bed configuration using dynamic simulation. For the working pair AQSOA‐Z02/H2O, the performance is optimized via the design of the adsorber heat exchanger and the control of the adsorption heat transformer cycle. When heat is upgraded from 90 °C to 110 °C, releasing waste heat at 35 °C, the maximum exergetic Coefficient Of Performance (COPexergetic) is 0.64, and the maximum Specific Heating Power (SHP) is 590 W kg−1. The maximum specific heating power could increase by 35% when releasing waste heat at 25 °C. Both performance indicators strongly depend on design, control, and the available temperature of the waste heat. Overall, adsorption heat transformers with optimized design and control are promising to utilize low‐grade waste heat. This article is protected by copyright. All rights reserved.
The kinetics of water adsorption on loose grains of Fuji Davison RD silica gel was studied by a TG differential step method in the temperature range 29–64°C and in the pressure range 6.5–34mbar. Three grain sizes were selected, 0.3–0.325, 0.355–0.425mm and 0.8–1.0mm. Furthermore, adsorption isobars at PH2O=9, 18 and 48mbar were measured over the temperature range of 30–150°C by a TG technique to determine pertinent equilibrium parameters which are used to calculate the coefficients of diffusion. The equilibrium uptake was described as a linear function of the Dubinin–Polanyi adsorption potential.It was found that the influence of particle size, temperature and pressure on the adsorption kinetics can be well described in terms of the Fickian diffusion model. The apparent water diffusivity Dap was found to be an Arrhenius function of temperature with the apparent activation energy Ea=41.5kJ/mol and the pre-exponential factor Dap0=2.9×10−4m2/s. The apparent diffusivity of water in silica pores was measured to be De=(3.7–4.7)×10−7m2/s and possessed a slight increase with temperature. This value is close to the Knudsen diffusivity, calculated for a cylindrical pore of radius rp=1.0nm.For smaller grains at T>39°C the contribution of thermal effects was revealed, which decreases the rate of water sorption. In this case, application of non-isothermal kinetic model of Lee and Ruthven allowed good description of experimental uptake curves as well as the estimation of parameters which determine simultaneous heat and mass transfer.
This article presents results of the experimental investigation on the adsorption of the water vapor on silica gel. Two independent
experimental methods has been used, viz. the constant-volume-variable-pressure (CVVP) system and variable pressure thermogravimetry
(TG). Results from these two methods are compared with each other. Also the isosteric heat of adsorption of this system has
been determined from the equilibrium data. The silica gels investigated here are Fuji Davison type 'A' and type 'RD'. Adsorption
isotherm of water vapor have been measured under a variety of conditions all referring to chiller operation cycles, i.e. temperatures
from 303 to 358 K and pressures from 500 to 7000 Pa. The data collected from the two independent experiments compare very
favorably with each other and their trends are consistent with those of the adsorption chiller manufacturer. This lends significant
weight to our experimental data on silica gel+water systems as being valuable to the adsorption chiller manufacturers and
the scientific community.
A pilot scale adsorber apparatus was designed and constructed to investigate water and ethanol adsorption/desorption kinetics on 3A zeolite for the design purposes of a fuel ethanol dehydration pressure swing adsorption (PSA) process. Equilibrium studies have shown that 3A zeolite adsorbed a significant amount of water while very weak ethanol adsorption was observed. The breakthrough curves were utilized to study the effects of column pressure, temperature, flow rate, pellet size, and adsorbate concentration on the overall mass transfer resistance. Based on experimentally observed trends, both macropore and micropore diffusion were identified as relevant mass transfer mechanisms. A mathematical model for a bench scale adsorption bed included the linear driving force (LDF) adsorption rate model and the variation of axial velocity. A detailed heat transfer model was a necessity since the bed dynamics was affected by heat transfer in the bed wall. The model was used to analyze the experimental data and extract values of pertaining diffusion coefficients.
Mise au point comportant des definitions generales et la terminologie, la methodologie utilisee, les procedes experimentaux, les interpretations des donnees d'adsorption, les determinations de l'aire superficielle, et les donnees sur la mesoporosite et la microporosite
New theoretical expressions to model the five adsorption isotherm types have been established. Using the grand canonical ensemble in statistical physics, we give an analytical expression to each of five physical adsorption isotherm types classified by Brunauer, Emett, and Teller, often called BET isotherms. The establishment of these expressions is based on statistical physics and theoretical considerations. This method allowed estimation of all the mathematical parameters in the models. The physicochemical parameters intervening in the adsorption process that the models present could be deduced directly from the experimental adsorption isotherms by numerical simulation. We determine the adequate model for each type of isotherm, which fixes by direct numerical simulation the monolayer, multilayer, or condensation character. New equations are discussed and results obtained are verified for experimental data from the literature. The new theoretical expressions that we have proposed, based on statistical physics treatment, are rather powerful to better understand and interpret the various five physical adsorption type isotherms at a microscopic level.
Desiccant cooling is an environmentally attractive alternative to conventional mechanical air-conditioning. The heart of the process is the rotary desiccant wheel which is used to dehumidify air. Recent experimental measurements of wheels with proposed alternative materials at low regeneration temperatures (<80C) have shown fewer benefits than anticipated based on the material adsorption characteristics.Here a numerical model of a desiccant wheel was used to investigate the specific influence of the desiccant equilibrium adsorption isotherm on the overall wheel performance. The heat of adsorption, moisture diffusion rate, desiccant specific heat capacity and density were varied to provide further insight into the limiting heat and mass transfer mechanisms for low temperature regeneration. In addition, an optimization analysis of the desiccant adsorption isotherm shape was performed for a range of process conditions.The results show that the extent of dehumidification is limited primarily by a combination of thermal affects caused by both the exothermic adsorption process and the carryover of heat from the regeneration stream. Braunuer Type 1 isotherms increase supply air dehumidification over a linear shape, though this is mostly due to the air inlet conditions which are more typically in the lower relative humidity range. The tendency toward Type 1 behaviour is greater when the heat of adsorption is a stronger function of the adsorbed moisture content. At moderate to high face velocities desiccant layer moisture diffusion kinetics also become important. Critically, the absolute moisture capacity has very limited influence on the performance. These findings have important implications for the design of desiccant wheels.
For a sorbent bed through which a single-phase fluid mixture is flowing and transferring energy and an adsorbate between bed and fluid, and where thermal and sorption equilibrium exists between bed and fluid at every location, the analysis in Part I has shown that the process can be described in terms of characteristic potentials and specific capacity ratios. Following transformation of the energy and adsorbate conservation equations into a pair of equations, each having the form of the equation for heat transfer alone, these new properties are seen to be analogous to temperature and bed/fluid specific heat ratio.
The paper focuses on development of new adsorbent material of aluminophosphate (FAM-Z05) which could be subsequently used in utilization of waste beat below 373 K. To evaluate this new material, water vapor adsorption isotherms of FAM-Z05 were measured at several temperatures. Furthermore, an attempt was made to improve the heat exchanger by coating FAM materials onto rin tube type heat exchanger. Then, the dehumidification performance of direct cooling and heating desiccant system was evaluated. It was found that the water vapor adsorption isotherms of FAM-Z05 were characterized by S-shape and the adsorption character of FAM-Z05 was dependent on temperature. Further, the new adsorbent of FAM-Z05 was able to adsorb water vapor in lower regeneration temperature than FAMZ02 and FAMZ01, which was attributed to FAM-Z05 ability to adsorb water vapor in high humidity range. However, when FAM-Z05 is used in low humidity range, the dehumidification performance of FAM-Z05 might be lower than those of FAM-Z01 and FAM-Z02.
AFI-type structure ferroaluminophosphate zeolite (FAPO-5) was examined as an AHP adsorbent. FAPO-5 with iron content of 2-8mol% (Functional Adsorbent Material-Zeolite 01; FAM-Z01) was selected for further testing. The water vapor adsorption isotherm of FAM-Z01 was S-shaped and highly dependent on temperature, and almost no hysteresis was observed with adsorption/desorption at 333 K and 348 K. No changes were observed in the properties of FAM-Z01 after 200,000 cycles of water vapor adsorption-desorption, indicating that FAM-Z01 is suitably durable for practical use. When the AHP was operated under conditions of TL/TM/TH= 283 K/303 K/333 K, the adsorption capacity of FAM-Z01 was 4 times that of silica gel.
CHA-type structure silicoaluminophosphate zeolite (SAPO-34) was examined as an AHP adsorbent and the influence of silicon content on its adsorption performance was evaluated. SAPO-34 with silicon content of 7.5 mol% (Functional Adsorbent Material-Zeolitc 02; FAM-ZO(2)) was selected for further testing. The water vapor adsorption isotherm of FAM-ZO2 was S-shaped and highly dependent on temperature, and a small hysteresis was observed with adsorption/desorption at 363K. No changes were observed in the properties of FAMZO(2) after 100000 cycles of water vapor adsorption-desorption, indicating that FAM-ZO(2) is Suitable durable for practical use. When the AHP was operated under conditions of T,(L)/T-M/T-H=283 K/313 K/363 K, the adsorption capacity of FAM-ZO(2) was 4.8 times and 3.8 times those of Y zeolite and silica gel.
Vapor adsorption on porous materials for non-wetting adsorbent–adsorbate pairs, characterized by a Type 5 isotherm, results in low loading at low relative pressure and increased loading due to capillary condensation behavior at higher relative pressure. The relationship between capillary condensation and the Kelvin equation suggests that a simple expression can be developed in which the adsorption equilibrium is obtained from an integrated pore size distribution function. This new expression is shown to describe the Type 5 isotherms of water adsorption on activated carbon. In addition, this expression has useful properties including: (1) finite saturation limit, (2) readily calculated Henry’s constant, and (3) readily calculated heat of adsorption, (4) can be written explicitly in terms of partial pressure or adsorbed phase loading and (5) parameter values attributable to the distribution function.
Increasing awareness of global warming forces policy makers and industries to face two challenges: reducing greenhouse gas emissions and securing stable energy supply against ever-increasing world energy consumption, which is projected to increase by 71% from 2003 to 2030. In addressing these two issues simultaneously, renewable energies prove themselves attractive, as they are independent from the fossil fuel supply and do not contribute to greenhouse gas emissions. Along with the global warming impacts and climate changes, the demands for air conditioning and refrigeration have increased. Therefore, providing cooling by utilizing renewable energy such as solar energy is a key solution to the energy and environmental issues. This paper provides a review of the available cooling technologies assisted by solar energy and their recent advances. Thermally powered cooling technologies were classified into three categories: closed cycles, open cycles, and thermo-mechanical cycles. Adsorption and absorption cycles represent the closed cycle. The adsorption cycle typically needs lower heat source temperatures than the absorption cycle. Solid and liquid desiccant cycles represent the open cycle. The liquid desiccant cycle has a higher thermal COP than the solid desiccant cycle. The ejector cycle represents the thermo-mechanical cycle, and has a higher COP but requires a higher heat source temperature than other cycles. Based on the thermal COP of each cycle, the absorption cycle is preferred to the adsorption cycle, and the liquid desiccant system is preferred to the solid desiccant system. Although these cycles have similar thermal COP values, advanced solar collector technologies such as the mini dish or trough collectors open a new opportunity of achieving higher overall system efficiency with these thermally powered cooling technologies and enable the solar thermal system to outperform the solar electric system for the time being. Therefore, next to improving efficiency and compactness of solar-driven cooling technologies, research on advanced solar collectors is the most important research topic.
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.
The adsorption characteristics of pure water vapor onto two different types of silica gel at temperatures from (298 to 338) K and at different equilibrium pressures between (500 and 7000) Pa were experimentally studied by a volumetric technique. The thermophysical properties such as the skeletal density, Brunauer−Emmett−Teller surface area, pore size, pore volume, and total porosity of silica gel were determined. The Tóth isotherm model is found to fit all of the experimental data within the experimental errors. The experimental isotherms and the computed enthalpies of adsorption are compared with those of various researchers and found to be consistent with a chiller manufacturer's data.
A packed bed of 3 Å zeolite is used to dry ethanol solutions initially at 24°C and containing up to 6.6 wt.% water. In view of the highly exothermic nature of the process, the progress of the thermal wave through the bed is followed and its effect on product dryness identified. The thermal wave is found to leave the bed around the time at which breakthrough of water begins to occur. For a given flow rate, the water concentration in the initial effluent and the peak temperature rise are found to be directly proportional to the water concentration in the feed. For a given feed concentration, both the initial effluent water concentration and the peak temperature rise can be reduced by decreasing the feed velocity. However, very low water concentrations should be achievable by removing the exotherm. In addition, equilibrium isotherms are reported for the adsorption of water on 3 Å molecular sieve zeolite, and the individual isotherms conform closely to Langmuir and Freundlich models.
In designing adsorption chillers that employs silica gel–water as adsorbent-adsorbate pair, the overriding objective is to exploit low temperature waste-heat sources from industry. This paper describes an experimental approach for the determination of thermodynamic characteristics of silica gel–water working pair that is essential for the sizing of adsorption chillers. The experiments incorporated the moisture balance technique, a control-volume-variable-pressure (CVVP) apparatus and three types of silica gel have been investigated, namely the Fuji Davison Type A, Type 3A and Type RD. As evidenced by the experimental results, the Henry-type equation is found to be suitable for describing the isotherm characteristics of silica gel–water working pair at the conditions of adsorption chiller. The regeneration of adsorbent depends on the correct allocation of temperature as well as the amount of regeneration time. From the experiments, the isotherm characteristics of silica gel–water in the low- to high-pressure regimes and hence, its isosteric heat of adsorption will be determined. Key parameters for optimizing the amount of heat recovery such as the cycle and switching time of chiller can also be implied from the measured results.
A simple model to fit experimental data of adsorption of gases and vapours on
microporous adsorbents (type I isotherms) is proposed. The main assumption is
that the adsorbate phase can be divided into identical and non-interacting
effective subsystems. This gives rise to a simple multiparametric isotherm
based on the grand canonical ensemble statistics, whose functional form is a
ratio of two polynomial functions. The parameters are interpreted as effective
equilibrium constants. A simplified isotherm that reduces the number of
adjustable parameters with respect to the general isotherm is also proposed. We
show how to use these isotherms to fit the adsorption data in such way that the
parameters have statistical significance. Due to their high accuracy, both
isotherms can be used to estimate thermodynamic properties like isosteric and
differential heats of adsorption. A simple method is presented for systems that
show an apparent variation in the coverage limit with temperature. This method
avoids overparametrization and improves fitting deviations. Finally, several
applications to fitting data, taken from literature, of adsorption of some
gases on activated carbon, molecular sieving carbon, silica gel, and pillared
clays are presented.
Siliporite Molecular Sieves -NK10 AP powder
- Ceca
CECA. Siliporite Molecular Sieves -NK10 AP powder. [Online] April 2005. www.siliporite.com.