A grain size distribution model for non-catalytic gas-solid reactions

A new model to describe the non-catalytic conversion of a solid by a reactant gas is proposed. This so-called grain size distribution (GSD) model presumes the porous particle to be a collection of grains of various sizes. The size distribution of the grains is derived from mercury porosimetry measurements. The measured pore size distribution is converted into a grain size distribution through a so-called pore-tosphere factor whose value is also derived from the porosimetry measurements. The grains are divided into a number of size classes. For each class the conversion rate is calculated either according to the shrinking core model, involving core reaction and product layer diffusion as rate-determining steps or according to a new model in which some reaction at the grain surface is assumed to be limiting. The GSD model accounts for the phenomenon of pore blocking by calculating the maximum attainable conversion degree for each size class. In order to verify the model, two types of precalcined limestone particles with quite different microstructures were sulphided as well as sulphated. Furthermore, a single sample of sulphided dolomite was regenerated with a mixture of carbon dioxide and steam. For each reaction good agreement was attained between measured and simulated conversion vs. time behaviour

Study of Kinetics Involved in Oxidation of Nonferrous Metal Sulphides

In the present investigation, an attempt has been done to study the simultaneous effects of the major processing variables on the extent of oxidation of commercially pure sphalerite ore pellets. These process variables taken into consideration were time and temperature. The oxidation was carried inside a muffle furnace where there was mild oxidation in the presence of atmospheric air. The pellets were charged for roasting inside the furnace in a graphite crucible. This process of roasting was carried out at four temperatures 7500 C, 8000 C , 8500 C and 9000 C .The project goal is to compare the oxidation or roasting at different temperature and time. At all instance of comparison one of the parameter was kept constant. The percentage (degree) of oxidation of sphalerite pellets was calculated at 15, 30, 45 and60 minutes, after the furnace reached the predetermined oxidation temperature. The experiments were statistically designed such that the effect of each variable can be quantitatively assessed and compared. The results showed that, temperatures above 8500C, time remaining constant, with the increase in temperature there is increase in the rate of oxidation (roasting) of sphalerite ore pellets. Further more for a constant temperature with the increase in time of exposure, rate of oxidation of Sphalerite (ZnS) pellets increases. This is valid for temperature range above 8500C. Another observation was made that temperature remaining constant, the rate of oxidation of Sphalerite (ZnS) ore pellets increases with time of exposure to attain a maximum limit than suddenly decreases followed by increment in the rate again. This observation was made in the temperature range of less than 8000C

From post-combustion carbon capture to sorption-enhanced hydrogen production: A state-of-the-art review of carbonate looping process feasibility

Carbon capture and storage is expected to play a pivotal role in achieving the emission reduction targets established by the Paris Agreement. However, the most mature technologies have been shown to reduce the net efficiency of fossil fuel-fired power plants by at least 7% points, increasing the electricity cost. Carbonate looping is a technology that may reduce these efficiency and economic penalties. Its maturity has increased significantly over the past twenty years, mostly due to development of novel process configurations and sorbents for improved process performance. This review provides a comprehensive overview of the calcium looping concepts and statistically evaluates their techno-economic feasibility. It has been shown that the most commonly reported figures for the efficiency penalty associated with calcium looping retrofits were between 6 and 8% points. Furthermore, the calcium-looping-based coal-fired power plants and sorption-enhanced hydrogen production systems integrated with combined cycles and/or fuel cells have been shown to achieve net efficiencies as high as 40% and 50–60%, respectively. Importantly, the performance of both retrofit and greenfield scenarios can be further improved by increasing the degree of heat integration, as well as using advanced power cycles and enhanced sorbents. The assessment of the economic feasibility of calcium looping concepts has indicated that the cost of carbon dioxide avoided will be between 10 and 30 € per tonne of carbon dioxide and 10–50 € per tonne of carbon dioxide in the retrofit and greenfield scenarios, respectively. However, limited economic data have been presented in the current literature for the thermodynamic performance of calcium looping concepts

Structured Functionalized Active Carbon Sorbents for the Purification of Gas Streams

Energy demand is constantly increasing as the world population and rapid industrial development grow fast. The main source of energy is represented by fossil fuels, responsible for greenhouse gas emissions into the atmosphere, global warming and climate change. In order to foster sustainable development, renewable sources are gaining great interest. However, the use of both fossil fuels and renewable sources requires purification processes of the gas streams from the energy production plants. Hydrogen sulphide (H2S), from natural gas processing, oil refining, biogas production and coal gasification, is a highly toxic compound for humans, it represents a poison for many catalysts and downstream fuel treatment devices and it is responsible for acid rains. A valid solution in industrial practice for the abatement of this pollutant is represented by the adsorption technique which can be considered an economic process combined with versatility, simplicity and high efficiency. Among the various adsorbent materials, activated carbons are widely used to remove hydrogen sulphide, as they offer a high surface area, a high pore volume and a variety of organic groups on its surface. The addition of metal oxides dispersed on the activated carbons facilitates the removal of H2S due to its high chemical affinity with metal cations. The use of activated carbons in structured form such as monoliths or foams is required in practical application when high pressure drop must be avoided. Unfortunately, most of carbon materials are available as powders since activated carbon in structured form is difficult to obtain due to the poor adhesion properties that require the use of binders which, despite giving good mechanical properties, reduce the adsorption capacity of the activated carbon monolith compared to that of the starting carbon powder due to the partial blocking of porosity. This thesis focus on structured activated carbon as sorbents for gas purification. In the first part of the work, the promoting effect of dispersed metals on commercial activated carbon monoliths was studied in order to improve the absorption properties of this material which are lower than the granular activated carbon samples which do not contain a binder. The reactive adsorption of H2S on copper and/or magnesium oxides dispersed onto activated carbon monolith was investigated in the co-presence of O2 and H2O in the gas stream at room temperature and in a lab-scale fixed-bed reactor. H2S capture rate and capacity of sorbents and the nature of sulphur species formed upon adsorption were analyzed using different techniques. Adsorption performance changes significantly depending on the metal although the adsorption is reactive in both cases. Two types of mechanisms were identified on the Cu-modified monolith: a faster mechanism associated to the formation of sulphates promoted by copper oxide and a slower mechanism involving the oxidation of H2S to elemental sulphur. Otherwise, a single adsorption mechanism is activated by magnesium, occurring through the dissociation of H2S into HS- and H+ promoted by the basic character of MgO. Moreover, a slow transformation of elemental sulphur into additional sulphate species was identified in the presence of O2 and water for saturated Cu-containing sorbents. Thermal regeneration of the saturated AC monoliths was evaluated and it was found that for both copper and magnesium the porous structure of the AC monoliths was completely restored due to the decomposition of sulphate species at a lower temperature, especially for Cu-modified sorbents, and the evaporation of elemental sulphur at a higher temperature, prevailing for Mg-modified sorbents. No significant loss of capture capacity was detected for sorbents after the thermal treatment which can then be proposed as effective and regenerable materials for gas purification In order to overcome concerns related to the addition of a binder, in the second part of the work, a new methodology was developed for the production of activated carbon as a three-dimensional microporous foam without any binder that does not involve the common complex multi-step procedure for obtaining foamy carbon with an open porosity including the template synthesis using a replica technique. ZnCl2 or CuCl2 were used as Lewis acid activators for the polymerization of furfuryl alcohol, chosen as carbon precursor, directly providing a polymeric foam due to the rapid evaporation of water formed in the reaction. Various synthesis parameters as temperature of polymerization and the composition of pyrolysis gas were explored in order to produce activated carbons with different porosity and metal load. The temperature of polymerization was selected according to the Lewis acidity strength of the two metal chlorides: lower for CuCl2 and higher for ZnCl2. The following pyrolysis of the polymer was investigated in the absence and in the presence of O2 traces at 600 or 850 °C in order to produce activated carbons with specific textural features and different metal content. Carbons with larger surface areas, also related to the presence of some mesopores, were obtained using ZnCl2 to activate the polymerization whereas CuCl2 promoted the formation of narrower micropores. Furthermore, copper was mostly preserved even at high pyrolysis temperature in contrast to zinc which was almost totally lost at 850 °C due to the much lower evaporation temperature with respect to metallic copper. The study provided a methodology to produce materials with different features for the adsorption of different molecules by suitably tuning the process parameters

Direct reduction of copper-cobalt sulphide mineral concentrates for metal/alloy production

The investigation focuses on an alternative route to metallic phase extraction from complex Cu-Co-Fe mineral sulphide concentrates, from the Copperbelt region in Zambia. In the context of developing a novel process route for metal extraction without SO2 gas emission and slag waste generation, the reduction of Cu-Co-Fe mineral sulphide concentrates via carbothermic reduction in the presence of lime (CaO) by following the equilibrium: MS + CaO + C = M + CaS + CO(g), where M represents the metallic Cu, Co and Fe, was studied. The reduction experiments were carried out in a temperature range of 1073 K - 1573 K, under argon atmosphere. The extent of metallization was analyzed by plotting the percentage reduction (%R) for each reaction against time (t). The reacted and partially reacted samples were characterised by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques. SEM-EDX analysis of the reduced samples showed that the purity of the metallic/alloy phases were over 97 wt.%. The reduced samples were subjected to magnetic separation for separating out the metallic/alloy phases from CaS, excess CaO and gangue minerals. The regeneration of CaO from CaS and utilisation of CaS were investigated. The effects of reduction temperature, type of carbon and mole ratios of CaO and C were investigated. Complete reduction/metallisation occurred within 2 hours above 1173 K and 1273 K with carbon black and graphite, respectively. Carbon black was found to be a better reductant as there was excessive sintering between the metallic and CaS phases, in the reduction with graphite. However, the mole ratio of carbon black should be slightly higher than that of CaO, because of; (i) consumption of part of CaO by the gangue minerals (SiO2 and Al2O3) at lower mole ratio of C than CaO (e.g. MS:CaO:C = 1:2:1) and (ii) incomplete reduction at very high mole ratio of C than CaO CaO (e.g. MS:CaO:C = 1:2:4), at T ≤ 1323 K. The reaction mechanisms were studied by stopping the reduction experiments at different times and characterising the partially reacted samples. The metallisation of Fe occured via reduction of the intermediate phase of Fe-O (FeO and Fe3O4) at C(carbon black) ≥ CaO (mole ratio). There was broad agreement between the experimental results and the thermodynamic predictions. The low temperature (≤ 1123 K) metallisation of Cu, Co and Fe was achieved via lime roast – reduction process. The mineral sulphide concentrates were roasted in air (21 % O2), in the presence of CaO by following the equilibrium: MS + CaO + 2O2 = MO + CaSO4, in the temperature range of 773 K – 923 K. The lime roast calcine was reduced at 1073 K and 1123 K with; (i) carbon black for the selective metallisation of Cu and for retaining the CaSO4 phase and (ii) activated charcoal for the complete metallisation of Cu, Co and Fe by following the equilibrium: MO + CaSO4 + 4C = M + CaS + 4CO (g). Because copper smelting slag is the major source of Cu and Co, the low temperature recovery of Cu and Co from a 40wt.%SiO2-(30wt.%Fe,6wt.%Al)2O3-10wt%CaO-7wt%CuO-7wt%CoO slag over a temperature range of 1173 K to 1323 K, was investigated via; (i) carbothermic reduction of metal oxides according to MO + C = M + CO(g) reaction and (ii) sulphidisation in the presence of CaSO4 and C according to MO + CaSO4 +SiO2 + 4C = MS + CaSiO3 + 4CO reaction. In the direct reduction of oxides, the recovery of metallic phase was below 90 % at 1323 K. Nearly all Cu, Co and Fe converted to metal sulphides or matte phase during sulphidisation of the slag. The reaction kinetics for the sulphidisation of slag in the presence of CaSO4 and C were determined. The sulphidised slag was reduced in the presence of CaO and C to obtain metallic/alloy, CaS and CaO rich slag phases. The metallic particles produced via sulphidisation – carbothermic reduction route were larger than those produced by direct reduction of the slag. The sulphidisation of slag in the presence of CaS via MO + CaS + SiO2 = MS + CaSiO3 reaction, was also investigated

Electrodeposition and hot corrosion of cermets

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A study on Correggio wall paintings: characterization of technique and materials of Abbey Church of S. Giovanni Evangelista in Parma, Italy

This study deals the materials of the sub- arch of the Del Bono chapel of the Abbey Church of S. Giovanni Evangelista in Parma, Italy, datable around 1523. The author of the painting is Antonio Allegri, known as Correggio (1489 – 1534), considered one of the greatest painters of the sixteenth century. Optical microscopy, micro-Raman spectroscopy, micro-Fourier Transform Infrared spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and gas chromatography coupled with mass spectrometry were selected in order to provide the higher set of significant data, limiting as much as possible sampling. The analysis has allowed to identify the pigments, characteristic of the epoch. The Correggio’s palette is composed by mineral pigments, sometimes expensive such as lapis lazuli, azurite and cinnabar, together with a wide range of earths or by synthetic pigments like smalt blue. From the amino acid content determination through GC/MS it was shown that, for all samples examined, the protein fraction is to be attributed to the presence of a mixture of egg and animal glue, present as binders in the samples containing lazurite, smalt, hematite, green earth and goethite. This allows to assume the use of a secco technique, also with pigments that do not need the employment of organic binding media on wall. For the gilding sample the hypothesis is that Au foil had been applied on a brown background where the Au foil is applied on the oil-based missione