Nucleation and growth kinetics of CaCO3 crystals in the presence of foreign monovalent ions

The aim of this work is to study the effect of the presence of different monovalent ions (Na+, NH4+ and K+) on the nucleation and growth rates of CaCO3 precipitation. There is currently great interest in the precipitation of CaCO3 particles reusing CO2 rich flue gases and calcium-rich wastes, which implies the presence of foreign ions that can affect the crystallization process. Unseeded and seeded tests were carried out in a batch system to estimate the nucleation and growth kinetics, respectively. Tests with Na2CO3 and CaCl2 as precursors led to the classical calcite crystallization mechanism via vaterite formation at high supersaturations. On the other hand, the use of (NH4)2CO3 entailed lower pH and the presence of NH4+, which stabilized the vaterite and avoided its transformation into stable calcite crystals. Thus, faster nucleation kinetics by using Na2CO3 were obtained. To estimate the growth rate, tests with two initial seed loadings and types (micro and nano seed) were performed. The growth rate increased with the crystal size and decreased with the magma density. The results indicate that the ion effect on the growth rate seems to be related to the ionic radius of the foreign ion

Insights on a methanation catalyst aging process: Aging characterization and kinetic study

Power to gas systems is one of the most interesting long-term energy storage solutions. As a result of the high exothermicity of the CO2 methanation reaction, the catalyst in the methanation subsystem is subjected to thermal stress. Therefore, the performance of a commercial Ni/Al2O3 catalyst was investigated over a series of 100 hour-long tests and in-process relevant conditions, i.e. 5 bar from 270 to 500 °C. Different characterization techniques were employed to determine the mechanism of the observed performance loss (N2 physisorption, XRD, TPO). The TPO analysis excluded carbon deposition as a possible cause of catalyst aging. The BET analysis evidenced a severe reduction in the total surface area for the catalyst samples tested at higher temperatures. Furthermore, a direct correlation was found between the catalyst activity decline and the drop of the catalyst specific surface. In order to correctly design a reliable methanation reactor, it is essential to have a kinetic model that includes also the aging kinetics. For this purpose, the second set of experiments was carried out, in order to determine the intrinsic kinetics of the catalyst. The kinetic parameters were identified by using nonlinear regression analysis. Finally, a power-law aging model was proposed to consider the performance loss in time

Optimization of CaCO3 synthesis through the carbonation route in a packed bed reactor

This article presents an investigation on the recovery of CO2 from the combustion gases of the cement industry through a carbonation route in order to obtain Calcium Carbonate Nanoparticles (CCnP), which could find application as either polymer or cement fillers. Two different experimental setups, a Continuously Stirred Bubbling Reactor (CSBR) and a Packed Bed Reactor (PBR), were studied in order to improve the final product and enhance the process yield. The influence of the experimental parameters on the particle size and morphology was tested for both reactors. The process was intensified by employing the PBR, where cubic calcite particles smaller than 300 nm were synthesized and higher CO2 conversions were obtained with respect to the CSBR performance

400 ans d'exploration botanique en zone méditerranéenne algérienne Une histoire méconnue et inachevée

Cet article fait une mise au point quasi exhaustive de l'exploration botanique en Algérie. Exploration couronnée, en 1962, par la publication de la flore de Quézel et Santa, dont la révision reste toujours d'actualité

Photo-catalytic activity of BiVO4 thin film electrodes for solar-driven water splitting

There is an ever-increasing attention directed to the development of solar fuels by photo-electrochemical water splitting, given the inexhaustible availability of solar energy. The water oxidation half-reaction is a critical step for the overall water splitting reaction, and the development of suitable photoanodes is therefore required. The present research work focuses on bismuth vanadate thin films’ deposition on FTO glass electrodes, through the dip coating technique, and discusses the influence of the film preparation technique on the electrode's photo-electrochemical performance. The bismuth vanadate thin films were synthesized with thicknesses ranging from 60 to 210 nm, depending on a number of dip coatings from 2 to 15. The structural and optical characterization of the films showed that monoclinic scheelite-type phase was obtained in all samples, with crystal sizes ranging from 24 to 65 nm, at increasing film thicknesses, and corresponding band gaps between 2.55 and 2.35 eV. A maximum photo-current density of about 0.57 mA cm−2 at 1.23 V vs. RHE under sunlight illumination was obtained for an electrode thickness of 160 nm. The electrochemical impedance spectroscopy elucidated the transport mechanisms occurring at the electrolyte–electrode interface, as well as inside the film. The estimation of the equivalent circuit parameters showed that an increasing film thickness decreased the resistance associated to the charge transfer between the electrolyte and the electrode (from 1100 to 450 Ω, from 60 to 160 nm layer thickness, respectively), given the higher number of active sites involved in the reaction. However, excessive film thicknesses increase the probability of charge recombination within the film and, in the specific case here investigated, can also be associated to film imperfections arising from several deposition-calcination cycles, which further act as traps. These concurring phenomena are of high relevance to isolate the rate-determining step of the water oxidation half-reaction, in the perspective of an optimization of bismuth vanadate film coating on FTO to obtain photo anodes

Contribution of ETM+ images and GIS for detecting dynamic changes in the steppe area of Naâma, Algeria

Ce travail met en évidence l’apport des images Landsat pour le suivi dynamique de l’espace steppique dans une aride  (Algérie). L’analyse qualitative et quantitative de l’évolution rapide du paysage et plus particulièrement l’évolution de l’ensablement dans les communes s’avère intéressante dans la mesure où elle permet de dresser un bilan provisoire sur l’état actuel de la zone d’étude. Pour l’analyse, les changements de l’état de surface intervenus entre 1957 et 2002 extraient à partir des images MSS, TM et ETM+ de Landsat ont été retenus. Dès lors, trois méthodes de détections des changements (identification rapide et quantification de l’évolution des changements, technique de la composition colorée diachronique et la méthode indice dynamique de couverture des sols) ont été utilisées afin de localiser les différentes zones qui ont subi des mutations profondes et plus particulièrement l’extension de l’ensablement et la dégradation des sols à l’intérieur des contrées de la wilaya de Naâma. Il est important de noter que nous nous sommes basés sur  le modèle d’Olsson (1994)  pour les corrections atmosphériques des images. La zone concernée par cette étude fait partie des hautes plaines oranaises. Elle se rattache administrativement à la wilaya de Naâma. Elle s’étend sur une superficie de 29 825 km2, entre 32°08’ et 34°16’ de latitude nord et 0°09’ et 1°43’ de longitude ouest. L’utilisation des données de télédétection au travers de l’indice dynamique de couverture des sols et de la composition colorée diachronique a permis de mettre en évidence les différents changements qui ont eu lieu de 1987 à 2002. Ces changements concernant les mutations des unités paysagères en zone steppique ont nécessité de tester plusieurs indices de changement (indice de végétation, indice de brillance, indice de curasse et l’analyse en composante en principale) afin de minimiser les erreurs sur l’appréciation des résultats sur les changements. En effet, ces différents traitements ont révélé que pour l’ensemble de la région d’étude, la surface des espaces ensablés est passée de 1,38 % en 1957 à 42,9 % en 2002, soit une superficie de 1 280 762 ha; ce qui correspond à une extension de 28 461,4 ha par an.  Mots clés: Dégradation, Détection de changement, Ensablement, Télédétection, Naâma, AlgérieIn this work we propose to highlight the Landsat images contribution for the dynamic monitoring of steppe area. We retained for our analysis, the changes  in the soil surface state occurred since 1957 up to 2002 from MSS, TM and ETM+ images of Landsat. From that time on, three methods of detection of changes were used so as to locate the various areas, which are affected, by a major changes and the sanding up extension inside the studied area of the department of Naâma. These images of Landsat showed an alarming of the sand extension.  Indeed, this analysis revealed that for the all the studied area, the sanding up of space surface passed from 1.38 % in 1957 to 42.9 % in 2002, that is a surface of 1 280 762 ha; what corresponds to an extension of 28 461.4 ha per year. Keywords:  change detection, sanding Up, remote sensing, Naâm

Les forêts d'Algérie, de Césarée la romaine à ce jour.

D'après de nombreux auteurs, les forêts d'Algérie furent jadis luxuriantes. L'état de déliquescence dans lequel elles se retrouvent aujourd'hui, pose la problématique de ce qu'elles ont subi durant les diverses époques de l'histoire de ce pays. Peu d'écrits existent sur le sujet a constitué et en fonction des convictions politiques ou religieuses des auteurs, chaque publication s'accorde à dire que c'est telle ou telle autre période de l'histoire fort tourmentée de ce pays, qui a été déterminante. La situation géographique de celui-ci, a fait qu'il a subi, de tout temps, des invasions, et que ce sera toujours la forêt qui payera le plus lourd tribut. Actuellement, la situation est encore plus complexe, du fait de la mondialisation

Aqueous phase reforming of pilot-scale Fischer-Tropsch water effluent for sustainable hydrogen production

Fischer-Tropsch (FT) synthesis produces an aqueous stream containing light oxygenates as major by-product. The low carbon concentration of the organics makes its thermal recovery unprofitable. Thus, novel processes are needed to utilize this waste carbon content. In this work, the aqueous phase reforming of the wastewater obtained from a 15 kWth Fischer-Tropsch plant was explored as a promising process to produce hydrogen at mild temperatures. The FT product water was firstly characterized and afterward subjected to the reforming at different reaction temperatures and time, using a platinum catalyst supported on activated carbon. It was observed that, besides activity, the selectivity towards hydrogen was favored at higher temperatures; equally, increasing the reaction time allowed to obtain the total conversion of most molecules found in the solution, without decreasing the selectivity and reaching a plateau at 4 hours in the hydrogen productivity. In order to get more insights into the reaction mechanism and product distribution derived from the APR of FT product water, several tests were performed with single compounds, finding characteristic features. The importance of the position of the hydroxyl group in the molecule structure was highlighted, with secondary alcohols more prone to dehydrogenation pathways compared to primary alcohols. Moreover, no interference among the substrates was reported despite the mixture is constituted by several molecules: in fact, the results obtained with the real FT product water were analogous to the linear combination of the single compound tests. Finally, the reuse of the catalyst showed no appreciable deactivation phenomena