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The adsorption of copper in a packed-bed of chitosan beads: Modeling, multiple adsorption and regeneration
No full textThe adsorption of copper in a packed-bed of chitosan beads: Modeling, multiple adsorption and regeneration
No full textIn this study, exoskeletons of Cape rock lobsters were used as raw material in the preparation of chitin that was successively deacetylated to chitosan flakes. The chitosan flakes were modified into chitosan beads and the beads were cross-linked with glutaraldehyde in order to study copper adsorption and regeneration in a packed-bed column. Five consecutive adsorption and desorption cycles were carried out and a chitosan mass loss of 25% was observed, after the last cycle. Despite the loss of chitosan material, an improved efficiency in the second and third cycles was observed with the adsorbent utilizing 97 and 74% of its adsorbent capacity in the second and third cycles, respectively. The fourth and fifth cycles, however, showed a decreased efficiency, and breakage of the beads was observed after the fifth cycle. In the desorption experiments, 91–99% of the adsorbed copper was regenerated in the first three cycles. It was also observed that the copper can be regenerated at a concentration of about a thousand fold the initial concentration. The first cycle of adsorption could be accurately described with a shrinking core particle model combined with a plug flow column model. The input parameters for this model were determined by batch characterization methods, with as only fitting parameter, the effective diffusion coefficient of copper in the bea
Dissolution kinetics of sorbents and effect of additives in wet flue gas desulfurization
No full textFlue gas desulfurization (FGD) technology has been adopted by a number of power stations for the removal of sulfur dioxide (SO2) from flue gas. The wet FGD system is the most commonly used process because of high SO2 removal efficiency and because of the availability of the sorbent used. This paper emphasizes the wet FGD process and the different types of sorbents used. Sorbent dissolution in the wet FGD process plays a significant role in the overall performance of the system. Factors such as temperature, solid-to-liquid ratio, pH, particle size, and additives can be optimized to improve the dissolution rate in the wet FGD system. Additives such as organic acids and inorganic salts can improve the dissolution rate and the desulfurization efficiency of the sorbent. Dissolution kinetics gives an understanding of the effects of reaction variables on the dissolution rate. The dissolution process is a heterogeneous reaction system consisting of fluid reactants and solid particles. This is best described using the shrinking core model that considers a reducing solid particle size as the reaction takes plac
Leaching kinetics of bottom ash waste as a source of calcium ions
No full textBottom ash is a waste material from coal-fired power plants, and it is known to contain elements that are potentially toxic at
high concentration levels when disposed in landfills. This study investigates the use of bottom ash as a partial substitute sorbent
for wet flue gas desulfurization (FGD) processes by focusing on its leaching kinetics in adipic acid. This was studied basing on
the shrinking core model that was applied to the experimental data obtained by the authors presented at the International
Conference on Industrial, Manufacturing, Automation and Mechanical Engineering, Johannesburg, South Africa, November
27–28, 2013) on dissolution of bottom ash. The leaching rate constant was obtained from different reaction variables, namely,
temperature, pH, acid concentration, and solid-to-liquid ratio, that could affect the leaching process. The solid sample of bottom
ash was characterized at different leaching periods using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It
was found that solid-to-liquid ratio had a significant effect on the leaching rate constant when compared with other variables.
The leaching kinetics showed that diffusion through the product layer was the rate-controlling step during leaching, and the
activation energy for the process was found to be 18.92 kJ/molEskom Power Plant
Engineering Institute (EPPEI
Effect of fly ash as an additive on the limestone dissolution rate constant
No full textLimestone dissolution is a very important factor in flue gas desulfurization systems because it determines its
reactivity toward SO2. Fly ash, a siliceous material, has been reported to improve sorbent reactivity. This study investigates the
effect of adding fly ash to limestone on its dissolution rate constant. The experiments were carried out using a pH stat apparatus
where the effects of the reaction variables, fly ash/limestone ratio, slurry pH, reaction temperature, and concentration of acid,
used were investigated. The central composite design (CCD) of the experiment was used to develop a model that correlates the
dissolution rate constant and the reaction variables. It was found that fly ash had a positive effect on the dissolution rate constant
of limestone, with the pH having the most significant effect. The dissolution rate constant was found to increase with an increase
in the temperature and acid concentration. X-ray diffraction (XRD) analysis showed products of hydration formed, which are
mainly calcium silicate hydrates, on the samples. This led to an increase in the specific surface area, as observed in the Brunauer−
Emmett−Teller (BET) analysisEskom Power Plant Engineering
Institute (EPPEI, South Africa
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The effect of acid demineralising bituminous coals and de-ashing the respective chars on nitrogen functional forms
Get PDFAn opportunity presented itself to compare changes in nitrogen functional forms brought by the acid treatment of South African bituminous coals and their respective chars. X-ray photoelectron spectroscopy (XPS) was used to determine functional forms of the raw coals, acid-treated coals, respective chars prepared at 740 and 980 °C in a bench-scale fluidised-bed (FB), and at 1000 and 1400 °C in a drop-tube furnace (DTF), as well as their corresponding de-ashed remnants. The XPS N 1s spectra for the raw coals were typically similar to previous widely reported bituminous coals, of which pyrrolic nitrogen was the predominant form of organically bound nitrogen, followed by pyridinic and quaternary nitrogen. In pyrolysed chars, quaternary nitrogen was the dominant form followed by pyridinic, pyrrolic and protonated-/oxidised heterocyclic nitrogen forms respectively. Nonetheless, XPS N 1s analysis for DTF severely pyrolysed chars (1000 and 1400 °C) prepared from high ash and vitrinite-rich coal, and also a char (1400 °C) from a relatively low ash and inertinite-rich coal, gave a spectra with only two sub-peaks corresponding to quaternary and pyridinic nitrogen. It seems that the HCl/HF/HCl sequential demineralising/de-ashing process had no effect on the nitrogen functional forms of raw coals and the entire chars prepared from the FB. De-ashing of DTF severely pyrolysed chars emanating from high ash and inertinite-rich coal exhibited no marked change to the nitrogen functional forms. However, acid treatment of DTF chars derived from a high ash and vitrinite-rich coal, a char from relatively low ash and inertinite-rich coal, which initially contained pyridinic and quaternary nitrogen resulted in additional nitrogen moieties of pyrrolic and protonated/oxidised nitrogen
The influence of particle size on the pore development of coal chars during gasification
Get PDFPhD (Chemical Engineering), North-West University, Potchefstroom Campus, 2019.In recent times, there is a strong drive towards the reduction and dependency of coal for the production of energy globally, and specifically in Europe and Japan. However, coal utilisation, includes gasification technology, will remain an important energy source in countries like China, India and South Africa for the coming decades. Environmental impact of coal utilisation is seen as the greatest driving force to reduce coal dependency and consumption, with optimisation of coal conversion technologies aimed at increasing technology efficiency and thereby minimising the environmental impact of the operation. The fundamental knowledge of the heterogeneous coal conversion processes is hereby pivotal in the retrofitting and design of clean coal technologies. One aspect in the basic description of coal reaction processes is the change in the solid structure of coal as the carbonaceous material reacts. The changing solid structure is known to influence the carbon reactivity and also impacts internal mass transfer limitations, mostly studied in detail for powdered coal chars. Studies have predominantly focused on single reaction gases, where pore structure development is studied by both gas adsorption and mercury porosimetry techniques. It is hereby useful to combine results from different
measuring techniques in order to evaluate a wider pore size range. However, the influence of time-temperature histories on the char behaviour and the dissimilarities between various techniques complicate the application of combining these methods. Small angle scattering techniques have been used to probe the pore structure of coal and char, with the main advantage of small angle scattering measurements probing a wide pore size range (from Ångström to millimetres) with a single experiment, mitigating the difficulties encountered when combining multiple measuring techniques required to evaluate a similar pore size range. A comprehensive study was therefore proposed to evaluate the pore development arising from CO2 and steam gasification using SAXS measurements, focusing on evaluating the effect of reagent gas, particle size and temperature. A typical inertinite rich South African coal with low ash values from the Witbank coalfield was selected, with the parent coal sample being a single source coal (Witbank seam 4) that was density separated at 30 mm. A low ash coal was selected in order to reduce the influence of mineral matter on experimental results. A detailed coal sampling procedure was developed to prepare representative samples for parent coal characterisation, smaller particle samples and 20 mm particles from the 300 kg bulk sample. Parent coal characterisation results showed typical values for Witbank seam 4 washed coal, specifically resulting in a low ash coal with high calorific value and petrographic classification as an inertinite-rich, medium rank C bituminous coal. A detailed char preparation procedure was developed to reduce the influence of charring parameters on experimental results. Char preparation was conducted at 1000°C and further processed using a detailed sampling
procedure which resulted in a three particle size classification (75 μm, 2 and 20 mm). The resulting chars were characterised and generally, it is concluded that the chemical composition of the three particle sizes does not differ significantly. However, pore structure parameters obtained for different sized chars using low pressure CO2 gas adsorption measurements show particle size dependent pore structure with an increase in micropore volume and surface area as char particle size increases. Partially gasified chars were prepared in a thermogravimetric analyser for the three sized samples using steam and CO2. The lowest gasification temperature (800°C for steam and 850°C for CO2) was experimentally proven to be in the chemical reaction controlled regime for the smallest particle size. The highest gasification temperatures were chosen as 950 and 1000°C for steam and CO2 gasification, to illustrate the effect of mass transfer limitations. The partially converted samples were scanned at the Australian Synchrotron in Melbourne using two detector positions, which resulted in a Q-range probed from 1.698•10-3 to 0.8333 Å- 1 and an assumed pore diameter between 0.25 and 147 nm. A novel ratio analysis technique was developed to study the pore development of individually sized pores from SAXS experimental data without requiring non-fractal pore modelling. Three separate ratios were specified from the SAXS data to compare results of different scans: scaled intensity (SI), intensity conversion ratio (ICR) and scaled conversion intensity ratio (SCIR). The scaled intensity allows direct comparison of changes in pore size distributions between samples. The ICR was calculated by dividing the scattering intensity of the gasified char at each Q by that of the unreacted char. The SCIR was calculated by dividing the scaled intensity of the gasified char with the scaled intensity of the char (at each Q). The proposed ratio analysis technique was used to qualitatively compare the relative extent to which the number of pores at different sizes grow, allowing multiple samples comparison, without requiring non-fractal modelling to
determine pore volume and/or surface area.. Chars were partially converted to prepare samples with specific conversions of 10, 25, 35 and 50%, with a precise time required for each specified conversion. The influence of reagent gas on the pore development was evaluated using partially gasified 75 μm chars. Comparing the trends observed using large lumped pore ranges (IUPAC) from
SAXS results compares well to literature, with broadening of meso- and macropores from onset of CO2 gasification being the only exception, likely to be maceral related. Further, steam shows greater development of pore structure for all conversions of 75 μm chars, with increased pore volume and surface area, compared to CO2. The ratio analysis results showed that, for both CO2 and steam, size dependent pore growth rate. In particular, the intermediate pore sizes (between 1 and 40 nm) showed increased pore growth when compared to other pore sizes. Compared to CO2, steam gasification resulted in an increased pore growth of pore sizes between 1 and 40 nm. The ratio analysis technique further resulted in classification of critical cross over pore size, where the critical cross over pore size indicates at what size the pore generation rate is observed to be below the largest pore size. Comparison of the critical cross over pore size results for steam and CO2 gasification showed that a smaller cross over pore size (0.6 nm for steam, compared to 1 nm for CO2) and a smaller critical cross over value is observed for steam, which may be a direct consequence of the smaller kinetic diameter of water molecules. It was also observed that particle size influenced the pore development of CO2 gasified chars over the entire pore scale studied, specifically the development of micro- and macropores, which prevailed over the intermediate sized pores. For steam gasification, particle size only influenced the growth rate in the macropore range. For both the steam and the CO2 gasified chars, larger particle sizes resulted in a decrease in growth rate for < 0.6 nm pores as well as an increase in critical cross over pore size. A novel application to evaluate the radial pore development of a 20 mm particle was developed, with the sampling spanning from the surface
to the interior of the particle. The surface sample showed the greatest pore development over the entire pore range evaluated, followed by interior and centre. For both CO2 and steam chars, the radial changes in growth rate for individual pore sizes confirmed intra-particle mass transfer limitations for 20 mm particles. The results obtained during this study further demonstrates the advantages of using small
angle scattering techniques over other techniques, due to increased pore size range and pore size resolution probed using a single measurement. Further, this study developed a novel ratio analysis technique to elucidate pore structure development of gasified char, with results showing different sized pores grow at different rates. The techniques developed here gave greater insight into dependence of reagent gas, with steam and CO2 studied here, on pore development of gasified chars. On a pore scale, the results for micropore surface area development suggest that reported trends (increased surface area development for steam compared to CO2) are mainly due to differences in pore growth of pores smaller than 7 nm and could be a direct implication of the smaller kinetic diameter of water, compared to CO2. The detailed evaluation also highlights the additional complexities that needs to be addressed in advanced dynamic single particle reaction modellingNational Research Foundation (NRF)Doctora
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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