Heat Of Chemisorption Research Articles

Numerical investigation on the operation strategy of a chemisorption heat pump system

ABSTRACT This paper presents an analysis of the operating characteristics of a chemisorption heat pump. 1-D transient model of a chemisorption heat pump was developed, for which an analysis of four alternating modes of operation was performed. The performance was analyzed with respect to the operating time of the adsorption and desorption operation modes during alternating operations, and the results showed that the cooling capacity has an optimum point depending on the operating times. It was also confirmed that the coefficient of performance increases as the operating time increases. The system performance was also analyzed according to the flow rates of the secondary fluids supplied to the main components. As the flow rates of the secondary fluids supplied to the main components increased, the cooling capacity improved, and the coefficient of performance was found to be compounded by the secondary flow conditions and the operating times of the adsorption and desorption operation modes. Based on the results, it was found that the cooling capacity, coefficient of performance, and specific cooling power of the chemisorption heat pump can be operated in a large range from 876.7 to 3070.0 W, 0.13 to 0.32, and 39.3 to 137.7 W/kg, respectively, depending on the operating conditions.

Experimental study of a chemisorption heat pump under different operation conditions

A chemisorption heat pump utilizes thermal energy for cooling. Chemisorption heat pump can be applied to a wider variety of applications when the system uses thermal energy at lower temperature. Thus, study on the performance of a chemisorption heat pump operated by thermal energy of 40 °C was conducted. 500 W system was built and its performance was analyzed through experiments. Chemisorption heat pumps are capable of continuous cooling operation through alternating operation, in which the system performance changes depending on the operating time of each operating mode. There are no existing studies have analyzed the performance of a chemisorption heat pump driven by a 40 °C heat source according to the operating time. Therefore, the performance change according to the operation time of each modes was identified. The cooling capacity, coefficient of performance and specific cooling power were measured in corresponding ranges of 272.2–543.2 W, 0.06–0.24 and 42.5–84.9 W/kg for each case. The cooling capacity is maximum when the operation times of the ‘adsorption/desorption’ and ‘preparation 1’ mode are 10 min and 10 s, respectively, and the coefficient of performance is maximum when the operation times are 30 min and 20 s, respectively.

A numerical study on the performance of chemisorption heat pump according to various design conditions

To achieve net zero, research on waste heat recovery systems is actively underway, and interest in chemisorption heat pumps is also increasing. In a chemisorption heat pump, the performance increases as the temperature of the heat source increases. However, in order to recover a larger amount of waste heat, it is necessary to develop a system that can utilize waste heat at a lower temperature. Most existing studies on chemisorption heat pumps have been conducted on systems operated using heat sources above 65 °C, and most studies have presented system performance results at specific design points. In this study, changes in system performance according to system design were quantitatively analyzed for a system driven by a heat source at 40 °C, which is lower than the existing literature studies. A 1-D transient analysis model was developed and an analytical study was conducted. Performance analysis was conducted according to pressure drop requirement, number of tubes inside the heat exchangers and reactors, and number of fins inside the reactor, and the optimal design point for each condition was suggested. The cooling capacity, coefficient of performance, specific cooling power at the optimal design point are 1100.6 W, 0.3, 45.7 W/kg respectively.

Extend ethylene aromatization single-event kinetic modeling with physical and chemical descriptor based on ZSM-5 catalyst

The ethylene aromatization is critical for the methanol to aromatics and light alkane dehydroaromatization process. The single-event microkinetic (SEMK) model combining the linear free energy theory and solid acid distribution concept were established and extend for the ethylene aromatization process, which can reduce the kinetic parameters and simplify the reaction network by comparison with the SEMK model including subtype elementary steps based on the type of carbenium ions. Further introducing deactivation parameters φ into the model and applying the linear free energy model to the deactivation experimental data, the obtained deactivation parameters φ indicate that the carbon deposition precursors have the greatest impact on reducing the reaction rate of single-molecular reactions and the smallest impact on the hydrogen transfer reaction. Meanwhile, according to the change of reaction enthalpy, effect of carbenium ion structure on methylation, ethylation, cyclization and endo-β scission was investigated by introducing linear free energy concept into the SEMK model. The effect of different acid strengths on elementary steps was investigated based on the acid strength distribution model, it was found that the methylation and oligomerization reactions, the ali-β scission reaction, endo-β scission reaction and the cyclization reaction were more sensitive to strong acidity sites. The physisorption and chemisorption heat are separated from the protonation heat in the linear free energy kinetic model and the acid strength distribution kinetic model, and the absolute values of the obtained physisorption and chemisorption heat increase with the carbon number of carbenium ions. Furthermore, the parameters of the acid strength distribution kinetic model were applied to propane dehydroaromatization on H-ZSM-5 and the ethane dehydroaromatization on Zn/ZSM-5 to confirm the independence of parameters in the SEMK model with the similar reaction network.

Transient modeling of a chemisorption heat pump using ammonia with expanded graphite – NaBr

Numerical and experimental studies of a chemisorption heat pump driven by a low-temperature heat source were conducted. A transient model of a heat exchanger and reactor was developed. Equations of mass, momentum, energy and chemical reaction were considered and modeled. One of the important parameters related to the chemical reaction is the non-equilibrium curve of the working fluid and adsorbent pair. An experiment was conducted to obtain the non-equilibrium curve for NH3 and expanded graphite – NaBr under a low-temperature condition. The correlation for the non-equilibrium curve was suggested and applied to the transient model. A transient analysis of the reactor was then carried out. By comparing the simulation results to the experimental results, it was confirmed that the model of the reactor is valid. An experiment and simulation of the entire system were conducted. The transient features of the chemisorption heat pump when operated three times in an alternating manner were analyzed based on the simulation and experimental results. The simulation and experimental results showed similar values and tendencies of the temperature and pressure. Hence, the simulation model for the entire system was validated. Based on the proposed model, performance analysis according to the reactor fin tube design conditions was performed. Depending on the fin tube design, the average cooling capacity differed 2.13 times from 70.2 to 149.2 W. Depending on the fin tube design, the COP differed 2.2 times from 0.15 to 0.33. Based on the results of this study, the design condition of fin tube to achieve the optimal cooling capacity or COP was confirmed.

Ammonia-based hybrid chemisorption-compression heat pump for high-temperature heating

Conventional compression heat pumps employing natural refrigerant of ammonia can barely meet the requirement of high-temperature heating above 90 °C, while ammonia-based chemisorption heat transformers characterised by high-temperature heating require driving heat source temperature above 80 °C. To address these issues, a novel ammonia-based hybrid chemisorption-compression high-temperature heat pump employing SrCl2-NH3 as the working pair is designed and established in this paper. Particularly, a conventional normal temperature compressor instead of an expensive high-pressure ammonia compressor can be adopted to upgrade the abundant 50–80 °C waste heat to 90–120 °C high-temperature heat. Simultaneously, due to that chemisorption reaction heat is larger than condensing heat of refrigerant, this heat pump has significant potential for improving coefficient of performance (COP). For the first time, regulation strategies of key parameters such as sorption pressure, desorption pressure, and sorption reaction exothermic time are proposed, and these provide the theoretical support for the stable and efficient operation of the heat pump. Moreover, its performance is evaluated by varying operating conditions. The results indicate that at heat output temperature of 100 °C, the optimal sorption pressure and reaction exothermic time are 1.70 MPa and 22 min, respectively. At waste heat temperature of 70 °C and heat output temperature of 110 °C, the COP of the heat pump is 4.58, i.e., its efficiency is 14.5% more than that of R245fa compression one.

Chemisorption heat pump governed by asynchronous start-stop method for stable heat output

To guarantee clean heating of buildings in cold regions, an off-peak electricity driven chemisorption heat pump employing SrCl2/sulfurized expanded graphite composite sorbent is developed. However, the heat output temperature of chemisorption heat pump fluctuates greatly, up to 30 °C, which seriously restricts its popularization and application. In this paper, the above-mentioned problems are solved from two aspects of sorption bed structure design and system operation strategy. The sorption bed utilized for thermal energy storage is especially composed of several unit reactors, and the asynchronous start-stop control method of unit reactors is innovatively proposed. By controlling their start and stop time, the temperature fluctuation range of hot water that the heat pump outputs is significantly reduced. Simultaneously, to improve the energy efficiency of heat pump, small temperature difference fan coil units are adopted indoors. The results indicate that when ten reactors operate asynchronously, the output temperature of hot water fluctuates by as low as 1.4 °C. With four reactors in asynchronous operation, the temperature fluctuation range can be reduced to 3 °C, and 35 °C hot water enters the small temperature difference fan coil units to release heat to the indoor space. Additionally, the heating performance of the chemisorption heat pump is relatively stable in cold regions. When the evaporating temperature decreases from 5 °C to −15 °C, its energy efficiency is almost constant, about 1.47. Eventually, this chemisorption heat pump can shift electricity from peak periods to off-peak periods, and also saves 0.7 to 6.3 yuan per day in space heating cost in comparison to the conventional vapor-compression one.

Plasma-Catalysis of Nonoxidative Methane Coupling: A Dynamic Investigation of Plasma and Surface Microkinetics over Ni(111).

A heterogeneous catalytic microkinetic model is developed and implemented in a zero-dimensional (0D) plasma model for the dynamic study of methane nonoxidative coupling over Ni(111) at residence times and power densities consistent with experimental reactors. The microkinetic model is thermodynamically consistent and is parameterized based on the heats of chemisorption of surface species on Ni(111). The surface network explicitly accounts for the interactions of plasma species, namely, molecules, radicals, and vibrationally excited states, with the catalyst active sites via adsorption and Eley-Rideal reactions. The Fridman-Macheret model is used to describe the enhancement of the rate of the dissociative adsorption of vibrationally excited CH4, H2, and C2H6. In combination with a previously developed detailed kinetic scheme for nonthermal methane plasma, 0D simulation results bring insights into the complex dynamic interactions between the plasma phase and the catalyst during methane nonoxidative coupling. Differential turnover frequencies achieved by plasma-catalysis are higher than those of equivalent plasma-only and catalysis-only simulations combined; however, this performance can only be sustained momentarily. Hydrogen produced from dehydrogenation of ethane via electron collisions within the plasma is found to quickly saturate the surface and even promote the conversion of surface CH3* back to methane.

Kinetic Features of the Hydrogen Sulfide Sorption on the Ferro-Manganese Material

The kinetics of hydrogen sulfide sorption by the surface of a ferromanganese material containing in its composition a mixture of iron (II) and (III) oxides FeO × Fe2O3, takanelite (Mn, Ca) Mn4O9 × 3H2O and quartz SiO2, and which is samples of unrefined ferromanganese ore, was studied in this work. Sorption rate constant and activation energy constant values were calculated. The catalytic effect of iron (III) oxide was established, the presence of which in natural material contributes to a decrease in the H2S sorption activation energy. Based on the results of X-ray phase and chromatographic analysis methods, a chemical (redox) reaction of the conversion of hydrogen sulfide into elemental sulfur and H2O was revealed. The overall process rate is expressed in terms of the physical sorption stage and chemical transformation of the components; the influence of the rate of the third stage—reaction products desorption—on the overall rate of the process is taken into account. The limiting stage of the process is determined—a chemical reaction. The relation between the heat and the activation energy of the chemical transformation is used according to the Bronsted—Polanyi rule for catalytic processes. It was found that with an increase in the chemisorption heat, the activation energy of the chemisorption stage decreases and, as a consequence, its rate increases. The sorption process parameters were calculated—the Fe2O3 coverage degree with the initial substances and reaction products providing the maximum sorption rate, which can be a criterion for evaluating the catalytically active sites of the catalyst surface for carrying out catalytic reactions.

Investigation on the air-source chemisorption heat pump for the severely cold regions

In severely cold regions, the air-source vapor compression heat pump can hardly meet the demands of space heating of buildings with a high efficiency. Simultaneously, the mismatch between electricity supply and demand causes enormous energy losses. If the off-peak electricity could be stored to satisfy the buildings’ heating needs throughout the day, it is quite beneficial to the efficient operation of the electricity grid. To achieve this goal, an air-source chemisorption heat pump suitable for the severely cold regions is specially designed in this paper. The proposed heat pump is driven by the off-peak electricity late at night whose working process includes desorption process and sorption process. Both the sorption heat released during the sorption process and the condensation heat released during the desorption process are utilized for the space heating of buildings. Experimental results indicate that this heat pump can work stably even at an evaporation temperature of −33 °C, i.e. it can operate in the severely cold regions. The coefficient of performance is about 1.33. Moreover, compared with the conventional vapor compression counterparts, this air-source chemisorption heat pump is not only conducive to the efficient operation of the electricity grid, but also slightly reduces the cost on the space heating of buildings when the time-of-use electricity price is implemented.

Design screening and analysis of gas-fired ammonia-based chemisorption heat pumps for space heating in cold climate

Abstract Thermally-driven ammonia-based chemisorption heat pumps (CSHP) have the potential to provide high-efficiency space heating in cold climates. Using the reversible chemical bond between sorbent salt and ammonia, CSHP thermochemically pumps heat from the cold ambient to the end-uses of space heating at 50 °C. The heating coefficient of performance (COP) of a CSHP is largely dependent on the selection of the sorbent salts, cycle configuration, and the system operation. This study uses a thermodynamic model to investigate the performance of six CSHP system configurations, including four single-effect and two double-effect cycles. The feasibility and performance of 121 available NH3/salt reactions are studied for each configuration. The thermal COP of the cycles and the primary energy COP of the gas-fired CSHP systems are evaluated assuming 50 °C supply temperature for building space heating and the optimal system designs are identified. The highest thermal COP for single-effect and double-effect cycles under −25 °C ambient temperatures are predicted to be 1.22 and 1.57, respectively. The corresponding primary energy COPs are above 1.0 and 1.15, which are 30% higher than condensing furnaces and is sustained into the same cold temperatures.

Electricity-assisted thermochemical sorption system for seasonal solar energy storage

The present paper investigated the seasonal solar thermal energy storage (SSTES) using solid-gas thermochemical sorption technology that has inherently combined function of heat pump and energy storage. The thermochemical reactions that can discharge heat at a higher temperature usually requires a relatively higher desorption temperature during charging process, which could be problematic to efficiently recover solar energy in high-latitude regions like the UK when using the most mature and economic solar thermal collector (flat-plate or evacuated tube type). The present work studied two hybrid concepts where an electric-driven compressor or an electric heater was introduced to supplement the thermochemical desorption process in terms of pressure rise and temperature lift, respectively, when the available solar heat was not sufficiently high. The SrCl2-8/1NH3 chemisorption was selected from 230 ammonia-chemisorption reactions due to its suitable adsorption/desorption temperature and large energy storage density. The performance of two hybrid systems using SrCl2-8/1NH3 chemisorption were evaluated and compared to determine the optimal solution. The results revealed that the hybrid thermochemical sorption with a compressor substantially improved the storage capacity compared to that with electric heater. With a compression ratio of 4, the SSTES system with 20 m2 solar collector under the weather condition of Newcastle upon Tyne can store 3226.8 kWh chemisorption heat in summer by charging 4465.4 kWh solar heat and 848.2 kWh electricity, indicating 60.7% storage efficiency; the corresponding energy density based on the overall system volume is 147.3 kWh/m3. Because of using the renewable solar heat and low carbon intensity electricity in summer, the proposed hybrid SSTES system has noteworthy reduction on carbon emission compared to gas boiler and conventional heat pump.

Interaction between Fluorine and Graphene Vacancy Defects

The results are presented from quantum-chemical simulations of the interaction between F– and FHF– ions and monovacancy and divacancy defects in graphene. The energy characteristics of fluorine chemisorption from ion associates with water molecules are determined. It is shown that the vacancies affect the parameters of chemisorption: the activation energy falls and the heat of adsorption rises, compared to those of an ordered graphene sheet. The relationship between the heat of chemisorption and the degree of fluorine coverage is studied. The characteristics of the reaction between vacancy defects and F–, FHF–, and hydroxonium ions are compared.

Adsorption Properties of the C(100)-(2×1) Diamond Surface with Vacancy Defects and “Nitrogen + Vacancy” Complexes

The results are presented from the quantum-chemical modeling of hydrogen chemisorption on a C(100)-(2×1) diamond surface that contains the most stable neutral monovacancies and negatively charged “nitrogen + vacancy” defects in the surface layers. The configurations of molecular orbitals are analyzed for the studied defects. It is found that a vacancy in the third layer and a “vacancy in the third layer, nitrogen in the fourth layer” complex alter the values of the activation energy and the heat of hydrogen chemisorption on the surface. The most active adsorption centers are represented by the atoms of the first and second layers located directly above the NV complex in the singlet state. The chemisorption of hydrogen on the same defect in the triplet state is the most complicated of the considered cases.

In-operando elucidation of bimetallic CoNi nanoparticles during high-temperature CH4/CO2 reaction

Dry reforming of methane (DRM) proceeds via CH4 decomposition to leave surface carbon species, followed by their removal with CO2-derived species. Reactivity tuning for stoichiometric CH4/CO2 reactants was attempted by alloying the non-noble metals Co and Ni, which have high affinity with CO2 and high activity for CH4 decomposition, respectively. This study was focused on providing evidence of the capturing surface coverage of the reactive intermediates and the associated structural changes of the metals during DRM at high temperature using in-operando X-ray absorption spectroscopy (XAS). On the Co catalysts, the first-order effects with respect to CH4 pressure and negative-order effects with respect to CO2 pressure on the DRM rate are consistent with the competitive adsorption of the surface oxygen species on the same sites as the CH4 decomposition reaction. The Ni surface provides comparatively higher rates of CH4 decomposition and the resultant DRM than the Co catalyst but leaves some deposited carbon on the catalyst surface. In contrast, the bimetallic CoNi catalyst exhibits reactivity towards the DRM but with kinetic orders resembling Co catalyst, producing negligible carbon deposition by balancing CH4 and CO2 activation. The in-operando X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements confirmed that the Co catalyst was progressively oxidized from the surface to the bulk with reaction time, whereas CoNi and Ni remained relatively reduced during DRM. Density functional theory (DFT) calculation considering the high reaction temperature for DRM confirmed the unselective site arrangement between Co and Ni atoms in both the surface and bulk of the alloy nanoparticle (NP). The calculated heat of oxygen chemisorption became more exothermic in the order of Ni, CoNi, Co, consistent with the catalytic behavior. The comprehensive experimental and theoretical evidence provided herein clearly suggests improvement to the catalyst design protocol by selecting the appropriate composition of Co-Ni alloy.

Effect of NO2 and water on the catalytic oxidation of soot

The influence of adding NO2 to 10vol% O2/N2 on non-catalytic soot oxidation and soot oxidation in intimate or loose contact with a catalyst has been investigated. In non-catalytic soot oxidation the oxidation rate is increased significantly at lower temperatures by NO2. For soot oxidation in tight contact with a Co3O4 catalyst a more reactive NO2-containg atmosphere did not change the oxidation profile significantly during temperature programmed oxidation. This is consistent with the expected Mars van Krevelen mechanism, where the rate limiting step is reaction between carbon and lattice oxygen from the oxide, and where the reaction thus has reached the zero order kinetics regime in the gaseous reactant. In loose contact with a catalyst the presence of NO2 causes a pronounced enhancement of the oxidation rate. The rate constants for loose contact soot oxidation in the presence of NO2 exhibited a volcano-curve dependence on the heat of oxygen chemisorption, and among the tested pure metals and oxides Cr2O3 was the most active catalyst. Further improvements were achieved with a FeaCrbOx binary oxide catalyst.

Features of reduction and chemisorption properties of nanosized iron(III) oxide

Nanosized iron(III) oxide with an average particle size of 8–10 nm has been synthesized by the thermolysis of iron(III) acetylacetonate using diphenyl ether as a dispersion medium. Reduction processes in a hydrogen atmosphere in the presence of nanosized iron oxide and natural mineral hematite (Fe2O3) have been studied by the temperature-programmed reduction method. The activation energies of the reduction processes have been determined by the Kissinger method; it has been shown that, in the case of nano-Fe2O3, the activation energies are several times higher than the respective values observed for hematite. The adsorption properties of nano-Fe2O3 and hematite have been studied; the isosteric heats of chemisorption of hydrogen and carbon monoxide on the surface of these samples have been determined.

Pd Supported on Carbon Nitride Boosts the Direct Hydrogen Peroxide Synthesis

Herein, the development of an improved Pd on carbon nitride catalyst for direct H2O2 synthesis from the elements is reported. Microcalorimetric CO chemisorption is used to characterize the chemical speciation of the Pd-selective and -unselective sites. Selectivity trends among the samples suggest that a bare metal surface with a differential heat of CO chemisorption ranging between 140 and 120 kJ mol–1 is responsible for the total O2 hydrogenation, while a maximum threshold value of differential heat of CO chemisorption of approximately 70 kJ mol–1 is necessary for the partial hydrogenation of O2 to H2O2. Such a low differential heat of CO chemisorption indicates a low exposed metallic Pd surface subjected to electron withdrawal from the surrounding ligands: i.e., the N functional groups on the carbon support. With respect to N-containing carbon nanotubes, carbon nitrides provide the following: a higher concentration of N sites, a flexible network of π-conjugated polymeric subunits with sp3 linking subuni...

Importance of the oxygen bond strength for catalytic activity in soot oxidation

The oxygen bond strength on a catalyst, as measured by the heat of oxygen chemisorption, is observed to be a very important parameter for the activity of the catalyst in soot oxidation. With both intimate contact between soot and catalyst (tight contact) and with the solids stirred loosely together (loose contact) the rate constants for a number of catalytic materials outline a volcano curve when plotted against their heats of oxygen chemisorption. However, the optima of the volcanoes correspond to different heats of chemisorption for the two contact situations. In both cases the activation energies for soot oxidation follow linear Brønsted-Evans-Polanyi relationships with the heat of oxygen chemisorption. Among the tested metal or metal oxide catalysts Co3O4 and CeO2 were nearest to the optimal bond strength in tight contact oxidation, while Cr2O3 was nearest to the optimum in loose contact oxidation. The optimum of the volcano curve in loose contact is estimated to occur between the bond strengths of α-Fe2O3 and α-Cr2O3. Guided by an interpolation principle FeaCrbOx binary oxides were tested, and the activity of these oxides was observed to pass through an optimum for an FeCr2Ox binary oxide catalyst, which exhibited a rate constant at 550°C that was 2.3 times higher than the one for pure α-Cr2O3 and 29 times higher than the one for pure α-Fe2O3.

Chemisorption heat storage in buildings: State-of-the-art and outlook

Chemisorption is a promising solution for long-term heat storage with a high energy density. Considering application to buildings, heat storage can be used for space heating and domestic hot water production. In the present work, the specification requirements of a heat storage system for buildings are given. A review on chemical heat storage materials is carried out and reactor development is discussed. Some future research topics are presented in the conclusions.