The effect of chemical surfactants on the absorption performance during NH3/H2O bubble absorption process

The objectives of this paper are to visualize the bubble behavior by shadow graphic method, to examine the effect of surfactants on the bubble type absorption, and to find the optimal conditions to design highly effective compact absorber for NH₃/H₂O absorption system. The initial concentrations of NH₃/H₂O solution and the kinds and the concentrations of surfactants are considered as key parameters. By measuring the absorption rate for each condition, two effects of the addition of surfactants, the Marangoni and the barrier effect, are compared with each other. The results show that the addition of surfactant enhances the absorption performance up to 4.81 times. The experimental correlations of the effective absorption ratio for each surfactant, 2-ethyl-1-hexanol, n-octanol, and 2-octanol, are suggested within ±15, ±10, and ±10%, respectively.     

Absorption performance enhancement by nano-particles and chemical surfactants in binary nanofluids

The objectives of this paper are to visualize the bubble behavior during the NH₃/H₂O absorption process with chemical surfactant and nano-particles and to study the effect of nano-particles and surfactants on the absorption characteristics. Binary nanofluid which means binary mixture with nano-sized particles is tested to apply nanofluid to the absorption system. Cu, CuO and Al₂O₃ nano-particles are added into NH₃/H₂O solution to make the binary nanofluids, and 2-ethyl-1-hexanol, n-octanol and 2-octanol are used as the surfactants. The concentration of ammonia in the basefluid, that of nano-particles in the nanofluid, and that of surfactants in the nanofluid are considered as the key parameters. The results show that the addition of surfactants and nano-particles improves the absorption performance up to 5.32 times. It can be concluded that the addition of both surfactants and nano-particles enhances significantly the absorption performance during the ammonia bubble absorption process.     

Review of advanced absorption cycles: performance improvement and temperature lift enhancement

The objective of this study is to propose and evaluate advanced absorption cycles for the coefficient of performance (COP) improvement and temperature lift enhancement applications. The characteristics of each cycle are assessed from the viewpoints of the ideal cycle COP and its applications. The advanced cycles for the COP improvement are categorized according to their heat recovery method: condensation heat recovery, absorption heat recovery, and condensation/absorption heat recovery. In H₂O–LiBr systems, the number of effects and the number of stages can be improved by adding a third or a fourth component to the solution pairs. The performance of NH₃–H₂O systems can be improved by internal heat recovery due to their thermal characteristics such as temperature gliding. NH₃–H₂O cycles can be combined with adsorption cycles and power generation cycles for waste heat utilization, performance improvement, panel heating and low temperature applications. The H₂O–LiBr cycle is better from the high COP viewpoints for the evaporation temperature over 0°C while the NH₃–H₂O cycle is better from the viewpoint of low temperature applications. This study suggests that the cycle performance would be significantly improved by combining the advanced H₂O–LiBr and NH₃–H₂O cycles.     

Experimental researches on characteristics of vapor–liquid equilibrium of NH3–H2O–LiBr system

An improved system of NH₃–H₂O–LiBr was proposed for overcoming the drawback of NH₃–H₂O absorption refrigeration system. The LiBr was added to NH₃–H₂O system anticipating a decrease in the content of water in the NH₃–H₂O–LiBr system. An equilibrium cell was used to measure thermal property of the ternary NH₃–H₂O–LiBr mixtures. The pressure–temperature data for their vapor–liquid equilibrium (VLE) data were measured at ten temperature points between 15–85 °C, and pressures up to 2 MPa. The LiBr concentration of the solution was chosen in the range of 5–60% of mass ratio of LiBr in pure water. The VLE for the NH₃–H₂O–LiBr ternary solution was measured statically. The experimental results show that the equilibrium pressures reduced by 30–50%, and the amount of component of water in the gas phase reduced greatly to 2.5% at T=70 °C. The experimental results predicted much better characteristics of the new ternary system than the NH₃–H₂O system for the applications.     

The effect of oil-droplet on bubble absorption performance in binary nanoemulsions

The objectives of this paper are to study the absorption characteristics of NH₃ bubbles in binary nanoemulsions and to quantify the effect of oil-droplet on the bubble absorption performance. C₁₂E₄ and Tween20 are used as the surfactants and n-decane oil is added into NH₃/H₂O solution to make the binary nanoemulsion. The initial concentration of NH₃/H₂O solution and the concentration of oil are considered as the key parameters. The absorption rates are calculated by measuring the inlet and outlet mass flow rates per a given time period. In addition, the droplet size in the binary nanoemulsion is measured by the particle size analyzer (ELS-Z, OTSUKA ELECTRONICS). It is found that the effective absorption ratio for 2.0 vol% oil and 14.3 wt% NH₃/H₂O becomes 17% higher than that for the base fluid.     

Heat and mass transfer enhancement of binary nanofluids for H2O/LiBr falling film absorption process

The objectives of this study are to measure the vapor absorption rate and heat transfer rate for falling film flow of binary nanofluids, and to compare the enhancement of heat transfer and mass transfer under the same conditions of nanofluids. The key parameters are the base fluid concentration of LiBr, the concentration of nanoparticles in weight %, and nanoparticle constituents. The binary nanofluids are H₂O/LiBr solution with nanoparticles of Fe and Carbon nanotubes (CNT) with the concentrations of 0.0, 0.01 and 0.1 wt %. The vapor absorption rate increases with increasing the solution mass flow rate and the concentration of Fe and CNT nanoparticles. It is found that the mass transfer enhancement is much more significant than the heat transfer enhancement in the binary nanofluids with Fe and CNT. It is also found that the mass transfer enhancement from the CNT nanoparticles becomes higher than that from the Fe nanoparticles. Therefore, the CNT is a better candidate than Fe nanoparticles for absorption performance enhancement in H₂O/LiBr absorption system.     

Optimization study of the compression/absorption cycle

For each external situation optimum working conditions for the compression/absorption cycle can be found. The improvement in cycle performance which is gained by optimizing the temperature gradient in the absorber is considerable, particularly for situations with small external temperature gradients. Theoretically, the external and internal temperature gradients should be equal to maximize the cycle performance. The introduction of a solution loop, however, changes this and the optimum internal temperature gradient is always larger than the external gradients. The optimum point of operation is found by studying the changes in the compressor and pump and the heat loss obtained in the solution heat exchanger with the working conditions. A comparison of a compression/absorption cycle, using NH₃-H₂O, and a compression cycle working with pure R12, always results in a higher coefficient of performance for the former. The capacity of the NH₃-H₂O system is also considerably higher.     

Performance research of self regenerated absorption heat transformer cycle using TFE-NMP as working fluids

A heat transformer is proposed in order to upgrade low-temperature-level energy to a higher level and to recover more energy in low-temperature-level waste heat. It is difficult to achieve both purposes at the same time using a conventional heat transformer cycle and classical working pairs, such as H₂O–LiBr and HN₃–H₂O. The new organic working pair, 2,2,2-trifluoroethanol (TFE)-N-methylpyrolidone (NMP), has some advantages compared with H₂O–LiBr and NH₃–H₂O. One of the most important features is the wide working range as a result of the absence of crystallization, the low working pressure, the low freezing temperature of the refrigerant and the good thermal stability of the mixtures at high temperatures. Meanwhile, it has some negative features like NH₃–H₂O. For example, there is a lower boiling temperature difference between TFE and NMP, so a rectifier is needed in refrigeration and heat pump systems. Because TFE–NMP has a wide working range and does not cause crystallization, it can be used as the working pair in the self regenerated absorption heat transformer (SRAHT) cycle. In fact, the SRAHT cycle is the generator–absorber heat exchanger (GAX) cycle applied in a heat transformer cycle. In this paper, the SRAHT cycle and its flow diagram are shown and the computing models of the SRAHT cycle are presented. Thermal calculations of the SRAHT cycle under summer and winter season conditions have been worked out. From the results of the thermal calculations, it can be found that there is a larger temperature drop when the waste hot water flows through the generator and the evaporator in the SRAHT cycle but the heating temperature can be kept the same. That means more energy in the waste heat source can be recovered by the SRAHT cycle.     

Absorption chiller crystallization control strategies for integrated cooling heating and power systems

The concept of an air-cooled absorption chiller system is attractive because the cooling tower and the associated installation and maintenance issues can be avoided. However, crystallization of the LiBr–H₂O solution then becomes the main challenge in the operation of the chiller, since the air-cooled absorber tends to operate at a higher temperature and concentration level than the water-cooled absorber due to the relative heat transfer characteristics of the coolant. This leads to crystallization of the working fluid. The paper focuses on the crystallization issues and control strategies in LiBr–H₂O air-cooled absorption chillers. As a result a novel application opportunity is proposed for the integration of absorption chillers into cooling, heating and power (CHP) systems. This new methodology allows for air cooler operation while avoiding crystallization.     

Thermodynamic properties for the quaternary system H2O + CH3OH + LiBr + ZnCl2

The thermodynamic properties (solubility, vapour pressure, density, viscosity, heat capacity and heat of mixing) of the H₂O + CH₃OH + LiBr + ZnCl₂ (9:1 H₂O:CH₃OH and 1:1 LiBr:ZnCl₂ by mass) system using H₂O + CH₃OH as the working media and LiBr + ZnCl₂ as the absorbents were measured. The solubility data were obtained in the temperature range from 270.35 to 389.55 K. The measurements of vapour pressure, density, viscosity and heat capacity were carried out at various temperatures and absorbent concentrations. The differential heat of dilution and differential heat of solution at 298.15 K were measured for solutionw with absorbent concentrations from 0 to 75.2 wt%. The integral heat of mixing data at 298.15 K were obtained from both sets of experimental data. The integral heats of mixing for this quaternary system showed exothermic behaviour. The vapour pressure data were correlated with an Antoine-type equation. An empirical formula for the heat capacity was obtained from experimental data. The experimental data for the basic thermodynamic properties of this quaternary system were compared with those of the basic H₂O + LiBr system.     

Well‐Defined Cu2O/Cu3(BTC)2 Sponge Architecture as Efficient Phenolics Scavenger: Synchronous Etching and Reduction of MOFs in confined‐pH NH3⋅H2O

Fabrication of low‐dimensional nano‐MOFs as well as nanoparticles/metal‐organic frameworks (MOFs) hybrids has sparked new scientific interests but remains a challenging task. Taking Cu₃(BTC)₂ as a proof of concept, it is demonstrated thats NH₃?H₂O solution of a confined pH value can readily shape the bulk Cu₃(BTC)₂ into nanoscale Cu₃(BTC)₂, beyond the need to control the crystal growth kinetics of MOFs. Adjusting the pH of NH₃?H₂O within a much small range (10–11) allows fine tuning over the size and shape of nanoscale Cu₃(BTC)₂. Particularly at pH = 11, NH₃?H₂O exhibits weak reducibility that triggers a reduction of part of Cu₃(BTC)₂ into Cu₂O, while shaping the other into Cu₃(BTC)₂ nanowires. Benefiting from the coincidence of reduction and etching effects, the newly generated Cu₂O dots can in situ anchor onto adjacent Cu₃(BTC)₂ nanowires at highly dispersive state, forming a well‐defined sponge‐like architecture built of Cu₂O dots and nano‐Cu₃(BTC)₂. The CuO_x derived from annealing of the Cu₂O dots/nano‐Cu₃(BTC)₂ hybrid preserves the sophisticated sponge architecture and high porosity, and exhibits promising applications in phenol scavenging, with efficiency outperforming its counterparts and many other Cu‐based catalysts reported in literature. It is anticipated that the findings here pave the way for the rational design of intricate nano‐MOFs in a more efficient way.     

Improved thermodynamic property fields of LiBr–H2O solution

This article presents a thermodynamically consistent set of specific enthalpy, entropy, and heat capacity fields for LiBr–H₂O solution. The temperatures span from 0 to 190°C, while the concentrations span from 0 to 75 wt%. The work is based on the empirical inputs of Dühring's gradient and intercept, specific heat capacity data at a reference concentration of 50 wt% and density data. These properties have been evaluated using most of the experimental data available in the literature. The present approach circumvents the issue of negative dew point at low temperatures and high concentrations. The information provided in this article could be useful for designers of absorption chillers.     

Electromagnetic and microwave absorbing properties of magnetite nanoparticles decorated carbon nanotubes/polyaniline multiphase heterostructures

Magnetite nanoparticles decorated CNTs/PANI multiphase heterostructures were prepared by polymerization of aniline monomer and an additional process of the coprecipitation of Fe²⁺ and Fe³⁺. Scanning electron microscopy and transmission electron microscopy observation indicated that the monodispersed magnetite nanoparticles were uniformly decorated on the surface of CNTs/PANI. The formation of magnetite nanoparticles on CNTs/PANI was mainly through a preferentially position-selective precipitation process. More interestingly, a portion of Fe₃O₄ nanoparticles was found to form core–shell structures with PANI. The effects of different additional amounts of NH₂Fe(SO₄)₂·6H₂O reactant on the magnetic properties and microwave absorbing performances of CNTs/PANI/Fe₃O₄ heterostructures were investigated. The CNTs/PANI/Fe₃O₄ multiphase heterostructures were proved to be superparamagnetic. The microwave absorption measurement showed that the CNTs/PANI/Fe₃O₄ samples under 1.5 g of NH₂Fe(SO₄)₂·6H₂O condition exhibited much more effective absorption performance. These results suggested the novel CNTs/PANI/Fe₃O₄ multiphase heterostructures with PANI as the second phase may be potential candidate for microwave absorption systems.     

Evaluation of the column components size on the vapour enrichment and system performance in small power NH3–H2O absorption refrigeration machines

This paper presents an analysis of the influence of the distillation column components size on the vapour enrichment and system performance in small power NH₃–H₂O absorption machines with partial condensation. It is known that ammonia enrichment is required in this type of systems; otherwise water accumulates in the evaporator and strongly deteriorates the system performance and efficiency. The distillation column analysed consists of a stripping adiabatic section below the column feed point and an adiabatic rectifying packed section over it. The partial condensation of the vapour is produced at the top of the column by means of a heat integrated rectifier with the strong solution as coolant and a water cooled rectifier. Differential mathematical models based on mass and energy balances and heat and mass transfer equations have been developed for each one of the column sections and rectifiers, which allow defining their real dimensions. Results are shown for a given practical application. Specific geometric dimensions of the column components are considered. Different distillation column configurations are analysed by selecting and discarding the use of the possible components of the column and by changing their dimensions. The analysis and comparison of the different column arrangements has been based on the system COP and on the column dimensions.     

Absorption of nitrogen hemioxide in aqueous solutions of gas treatment systems upon dissolution of UN in nitric acid

The absorption of N₂O from an air flow in various aqueous solutions at 293–298 K was studied. The maximal N₂O absorption under the experimental conditions is reached for water (~22–24%) and saturated solution of K₂Cr₂O₇ in concentrated H₂SO₄ in the presence of Al₂O₃ and without it (~34 and ~30%, respectively). In concentrated HNO₃ and NH₄OH solutions and in 1.0 M NaOH and N₂H₄·nH₂O solutions, the degree of the N₂O absorption varied from ~7.5 to ~11.5%. Similar degree of absorption was obtained with 0.5 M (NH₂)₂CO (~11%). In the other solutions tested, the degree of the N₂O absorption did not exceed ~4.0%.     

Simulation of ternary ammonia–water–salt absorption refrigeration cyclesSimulation des cycles frigorifiques à absorption à ammoniac / eau / sel

This paper proposes a new working fluid for refrigeration cycles utilizing low temperature heat sources. The proposed working fluid consists of the ammonia–water working fluid mixture and a salt. The salt is used to aid the removal of ammonia from the liquid solution. This effect is a manifestation of the well known “salting-out” effect. While the addition of salt improves the generator performance, it also has a detrimental effect on the absorber. The overall effects on the performance of three absorption cycles using the NH₃–H₂O–NaOH working fluid have been investigated using computer simulations. The results indicated that salting out can lower the generator operating temperature while simultaneously improving the cycle performance. Furthermore, limiting the salt to the generator suggests potential for further improvement in cycle performance.     

Working fluids for an absorption system based on R124 (2-chloro-1,1,1,2,-tetrafluoroethane) and organic absorbents

The possibility of using R124 (2-chloro-1,1,1,2,-tetrafluoroethane, CHClFCF₃) and organic absorbents as working fluids in absorption heat pumps was investigated. Various classes of organic compounds, all commercially available, were tested as absorbents for possible combination with R124; the absorbents included DMAC (N′, N′-dimethylacetamide, C₄H₉NO), NMP (N-methyl-2-pyrrolidone, C₅H₉NO), MCL (N-methyl ε-caprolactam, C₇H₁₃NO), DMEU (dimethylethylene urea, C₅H₁₀N₂O), and DMETEG (dimethylether tetraethyleneglycol, C₁₀H₂₂O₅). To evaluate the performance of a candidate refrigerant-absorbent pair in a refrigeration or heat pump cycle, the thermophysical properties of the pure components and the mixture and the equilibrium and transport properties have to be determined, either from experimental data or by prediction methods. The thermal stability of the refrigerant-absorbent must also be tested. A method for the calculation of the concentration in the liquid and gas phases and the excess thermodynamic properties of the mixture as a function of the system temperature and pressure based on our experimental setup is described. On the basis of vapor-liquid equilibrium measurements, density and viscosity measurements and thermostability testing, enthalpy-concentration diagrams were constructed. The performance characteristics of the investigated working fluids in terms of the coefficient of performance (COP) and the circulation ratio (f) were calculated for a single-stage absorption cycle. In terms of overall performance (COP, f and stability) R124-DMAC was found to be the superior combination, followed by R124-NMP, R124-DMEU and R124-MCL (the three pairs for which stability problems were found at high temperatures), and finally by R124-DMETEG.     

Industrial absorption refrigeration plants with storage facilitiesInstallations frigorifiques à absorption industrielles avec dispositifs de stockage

Waste heat can be used for refrigeration purposes by means of either the absorption refrigeration system or the mechanical compression systems driven by a Rankine cycle. If the heating, or the duty, is intermittent the absorption refrigeration system can incorporate the storage of liquid refrigerant and weak solution with thermodynamic and operational advantages. The working principle of such an absorption refrigeration plant with the binary system NH₃H₂O are explained and compared with conventional methods.     

Preparation of hydroxyapatite ceramics by hydrothermal hot-pressing method at 300 °C

Hydroxyapatite (HA) ceramics were prepared by a hydrothermal hot-pressing (HHP) method at a low temperature (300 °C). DCPD (CaHPO₄·2H₂O) + Ca(OH)₂, OCP (Ca₈H₂(PO₄)₆·5H₂O) + Ca(OH)₂, DCPD + NH₃·H₂O, OCP + NH₃·H₂O or α-TCP (Ca₃(PO₄)₂) + NH₃·H₂O were used as the precursors. The mixture was treated by HHP under a condition of 300 °C/40 MPa. In sample DCPD + Ca(OH)₂ and OCP + Ca(OH)₂, the HA ceramics obtained showed a porous and homogenous microstructure, and the bending strength were 9.9 MPa and 10.9 MPa, respectively. In sample α-TCP+NH₃·H₂O, rod-like HA crystals produced. When the starting materials were DCPD + NH₃·H₂O, OCP + NH₃·H₂O, the HA particles produced exhibited plate-like features. It appeared that the plate-like HA particles stacked into a lamellar structure. The formation of the lamellar structure leads to a noticeable improvement in fracture property of the HA ceramic. The bending strength and the fracture toughness of the sample prepared from OCP and ammonia water reach 90 MPa and 2.3 MPam^1/2, respectively.     

Numerical design of ammonia bubble absorber applying binary nanofluids and surfactants

The objectives of this paper are to analyze the combined heat and mass transfer characteristics for the ammonia bubble absorption process and to study the effects of binary nanofluids and surfactants on the absorber size. The ammonia bubble absorbers applying binary nanofluids and surfactants are designed and parametric analyses are performed. In order to express the effects of binary nanofluids and/or surfactants on the absorption performance, the effective absorption ratios for each case are applied in the numerical model. The values of the effective absorption ratio are decided from the previous experimental correlations. The kinds and the concentrations of nano-particles and surfactants are considered as the key parameters. The considered surfactants are 2-ethyl-1-hexanol (2E1H), n-octanol, and 2-octanol and nano-particles are copper (Cu), copper oxide (CuO), and alumina (Al₂O₃). The results show that the application of binary nanofluids and surfactants can reduce the size of absorber significantly. In order to reach 16.5% ammonia solution under the considered conditions, for example, the addition of surfactants (2E1H, 700 ppm) can reduce the size of absorber up to 63.0%, while the application of binary nanofluids (Cu, 1000 ppm) can reduce it up to 54.4%. In addition, it is found that the effect of mass transfer resistance is more dominant than that of heat transfer resistance. That is, the enhancement of mass transfer performance is more effective than that of heat transfer performance.