Minimum Desorption Research Articles

Reversible de-/hydriding characteristics of a novel Mg18In1Ni3 alloy

The present work demonstrates the reversible hydrogen storage properties of the ternary alloy Mg18In1Ni3, which is prepared by ball-milling Mg(In) solid solution with Ni powder. The two-step dehydriding mechanism of hydrogenated Mg18In1Ni3 is revealed, namely the decomposition of MgH2 is involved with different intermetallic compounds or Ni, which leads to the formation of Mg2Ni(In) solid solution or unknown ternary Mg–In–Ni alloy phase. As a result, the alloy Mg18In1Ni3 shows improved thermodynamics in comparison with pure Mg. The Ni addition also results in the kinetic improvement, and the minimum desorption temperature is reduced down to 503 K, which is a great decrease comparing with that for Mg–In binary alloy. The composition and microstructure of Mg–In–Ni ternary alloy could be further optimized for better hydrogen storage properties.

Lead(II) adsorption by peach palm waste

Abstract The aim of the present study was to investigate the efficiency of chemical treatments for peach palm sheaths, targeting the treatment of Pb(II) contaminated aqueous effluents. The modification was done with sodium hydroxide and citric acid to introduce carboxyl groups. Pb(II) adsorption was evaluated in function of pH, contact time, metal concentration, desorption ability and thermodynamic factors. An infrared adsorption peak was observed at 1740 cm–1 due to the presence of carboxyl groups. The required time for the system to reach equilibrium was about 150 min, and the Pb(II) adsorption followed a pseudo-second-order kinetic model. The Langmuir model showed the best fit for the adsorption data. The maximum adsorption capacity and minimum desorption rate values of 65.32 mg g–1 and 74.72%, respectively were obtained for the NaOH-modified peach palm sheath. After using the chemical treatment, the adsorbent properties improved and the system became more spontaneous, as confirmed by low Gibbs energy values.

Realistic minimum desorption temperatures and compressor sizing for activated carbon + HFC 134a adsorption coolers

A low thermal diffusivity of adsorption beds induces a large thermal gradient across cylindrical adsorbers used in adsorption cooling cycles. This reduces the concentration difference across which a thermal compressor operates. Slow adsorption kinetics in conjunction with the void volume effect further diminishes throughputs from those adsorption thermal compressors. The problem can be partially alleviated by increasing the desorption temperatures. The theme of this paper is the determination the minimum desorption temperature required for a given set of evaporating/condensing temperatures for an activated carbon + HFC 134a adsorption cooler. The calculation scheme is validated from experimental data. Results from a parametric analysis covering a range of evaporating/condensing/desorption temperatures are presented. It is found that the overall uptake efficiency and Carnot COP characterize these bounds. A design methodology for adsorber sizing is evolved.

Effect of C (graphite) doping on the H 2 sorption performance of the Mg–Ni storage system

Binary Mg–Ni mixtures and ternary Mg–Ni–C (graphite) samples with fixed proportions of metals (Mg 85%–Ni 15% by weight) and amount of C increasing in increments of 5 wt % from 5 wt % to 15 wt % were prepared by high energy ball milling (BM) in Ar for t BM = 2 h. The purpose of the study was to evaluate the effect of C addition on the reactivity, the sorption activation and the storage performance of the Mg–Ni system. Increasing the amount of C had the effect of decreasing (from 10 to 3) the number of cycles needed for activation (performed at 623 K and 40 bar/0.9 bar charging/discharging H 2 pressure). After full activation, the 5 wt % C-containing sample exhibited the best absorption kinetics performance: the average rate to charge up to 5 wt % H 2 was about 3 times higher than that observed for the undoped sample. Unfortunately, increasing the amount of C had a negative impact on the desorption behaviour, causing an increase in the dehydrogenation activation energy and a decrease in the discharging rates. Within the present study, C reacted neither with H 2 nor with the H 2 active phases (the two discharged phases Mg and Mg 2Ni and the related hydrides) and consequently did not lead to variation in the sorption enthalpies of the Mg–Ni system. But, its presence did cause a small increase (4 K at 0.9 bar H 2) in the minimum desorption temperatures of the hydrides and a consequent minor decrease (0.2 bar) in the equilibrium pressures. The best sorption properties were obtained for the 5 wt % C-sample, that on the whole worked better than the binary mixture.

Evaluation of minimum desorption temperatures of thermal compressors in adsorption refrigeration cycles

The purpose of this paper is to identify the minimum desorption temperatures required to operate thermally driven adsorption beds of a solid sorption refrigeration system. The method is based on the evaluation of uptake efficiency of the adsorption bed and estimating there from conditions under which the compressor ceases to provide any throughput. The difference in the densities of the refrigerant between the inlet and outlet, the adsorption characteristics of the adsorbate–refrigerant pair and the void volume in the thermal compressor are the contributors to the manifestation of the desorption state. Among them, the void volume is a controllable parameter whose role is analogous to the clearance volume in a positive displacement compressor. The methodology has been tested out with three systems, namely, silica gel + water, activated carbon fiber + ethanol and activated carbon + HFC 134a systems. It is shown that waste heat at as low as 60 °C can operate these systems which make them good energy conservation devices through recovery of low grade process waste heat.

The inhibition of carbon deposition on stainless steel by prior selective oxidation

Techniques, developed to determine the surface concentration of monolayer adsorbates on metal substrates by means of chemical mapping Auger spectrometry, have been used to measure desorption of barium from Ba-W, Ba-C-W, and Ba-O-W surfaces. Desorption data were obtained by measuring the change in surface concentration with time at a constant temperature. By making these measurements over the temperature range 900 to 1400 K, activation energies for desorption were acquired. Higher barium desorption rates were obtained from Ba-W and Ba-C-W surfaces than from Ba-O-W surfaces. The activation energy for barium desorption from a Ba-O-W surface with about a monolayer of oxygen on the tungsten substrate was 4.8 eV and was about 1 eV lower for the other two types studied (Ba-W, Ba-C-W). To correlate electron emission with surface chemistry, time sequential measurements of both electron emission and barium (and oxygen) desorption were made on both barium-covered oxygenated and clean tungsten surfaces. The most stable thermionic emission and minimum desorption rate were obtained from Ba-O-W surfaces with a chemisorbed monolayer of oxygen. Exposing the tungsten surface to higher oxygen concentrations before monolayer barium deposition created an apparent subsurface oxygen concentration which led to poor electron emission characteristics. This effect increased with oxygen content on the Ba-O-W surfaces. Similar measurements of desorption and electron emission were made on an unimpregnated tungsten cathode, covered by a barium monolayer, and these results were collated with those obtained from Ba-O-W surfaces.

Correlation of electron emission with changes in the surface concentration of barium and oxygen on a tungsten surface

Techniques, developed to determine the surface concentration of monolayer adsorbates on metal substrates by means of chemical mapping Auger spectrometry, have been used to measure desorption of barium from Ba-W, Ba-C-W, and Ba-O-W surfaces. Desorption data were obtained by measuring the change in surface concentration with time at a constant temperature. By making these measurements over the temperature range 900 to 1400 K, activation energies for desorption were acquired. Higher barium desorption rates were obtained from Ba-W and Ba-C-W surfaces than from Ba-O-W surfaces. The activation energy for barium desorption from a Ba-O-W surface with about a monolayer of oxygen on the tungsten substrate was 4.8 eV and was about 1 eV lower for the other two types studied (Ba-W, Ba-C-W). To correlate electron emission with surface chemistry, time sequential measurements of both electron emission and barium (and oxygen) desorption were made on both barium-covered oxygenated and clean tungsten surfaces. The most stable thermionic emission and minimum desorption rate were obtained from Ba-O-W surfaces with a chemisorbed monolayer of oxygen. Exposing the tungsten surface to higher oxygen concentrations before monolayer barium deposition created an apparent subsurface oxygen concentration which led to poor electron emission characteristics. This effect increased with oxygen content on the Ba-O-W surfaces. Similar measurements of desorption and electron emission were made on an unimpregnated tungsten cathode, covered by a barium monolayer, and these results were collated with those obtained from Ba-O-W surfaces.