Browsing by Author "Angelova D."
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Item 21st European Conference on Fracture(2018-01-01) Susmel L.; Carpinteri A.; Hattingh D.G.; Bozic Z.; Angelova D.Item Alternative Options for Ebullated Bed Vacuum Residue Hydrocracker Naphtha Utilization(2023-12-01) Stratiev D.; Shishkova I.; Ivanov M.; Dinkov R.; Toteva V.; Angelova D.; Kolev I.; Tavlieva M.; Yordanov D.The vacuum residue hydrocracker naphtha (VRHN) is a chemically unstable product that during storage changes its colour and forms sediments after two weeks. It cannot be directly exported from the refinery without improving its chemical stability. In this research, the hydrotreatment of H-Oil naphtha with straight run naphtha in a commercial hydrotreater, its co-processing with fluid catalytic cracking (FCC) gasoline in a commercial Prime-G+ post-treater, and its co-processing with vacuum gas oil (VGO) in a commercial FCC unit were discussed. The hydrotreatment improves the chemical stability of H-Oil naphtha and reduces its sulphur content to 3 ppm. The Prime-G+ co-hydrotreating increases the H-Oil naphtha blending research octane number (RON) by 6 points and motor octane number (MON) by 9 points. The FCC co-cracking with VGO enhances the blending RON by 11.5 points and blending MON by 17.6 points. H-Oil naphtha conversion to gaseous products (C1–C4 hydrocarbons) in the commercial FCC unit was found to be 50%. The use of ZSM 5 containing catalyst additive during processing H-Oil naphtha showed to lead to FCC gasoline blending octane enhancement by 2 points. This enabled an increment of low octane number naphtha in the commodity premium near zero sulphur automotive gasoline by 2.4 vol.% and substantial improvement of refinery margin. The processing of H-Oil naphtha in the FCC unit leads also to energy saving as a result of an equivalent lift steam substitution in the FCC riser.Item Analysis of fatigue behaviour of stainless steels under hydrogen influence(2016-06-01) Angelova D.; Yordanova R.; Yankova S.Three stainless steels – ASTM 304, 316 and 316L - used in hydrogen utilization equipment are under investigation at conditions of tension-compression, rotating-bending and fretting fatigue. Fatigue tests are carried out with hydrogen charged and uncharged specimens. Hydrogen charging includes cathodic type of charging and exposure to high pressure hydrogen gas. The experiments under rotating bending and tensioncompression fatigue are conducted under different frequencies in three different laboratories: at The University of Chemical Technology and Metallurgy, Sofia, Bulgaria; at Sandia National Laboratory, California and The University of Tufts, Medford, Massachusetts, USA; The HYDROGENIUS Institute at Kyushu University, Japan. The fretting fatigue tests are presented by The HYDROGENIUS Institute at Kyushu University, Japan. The obtained results are presented in Wöhler curves complemented by plots ``Short fatigue crack length–Number of cycles`` and “Tangential force coefficient–Stress amplitude”. The found fatigue characteristics are analyzed and compared at different loading conditions, showing the best performance of Steel 316L.Item Antimicrobial Properties of Chitosan-Modified Cotton Fabric Treated with Aldehydes and Zinc Oxide Particles(2023-07-01) Staneva D.; Atanasova D.; Angelova D.; Grozdanov P.; Nikolova I.; Grabchev I.Chitosan is a natural biopolymer with a proven ability to impart textile materials with antimicrobial properties when loaded onto them. The mechanism of its bacteriological activity depends on the contact between the positive and negative charges of the amino groups located on the surface of the microbes. Unfortunately, the type of microorganisms and pH influence this action–shortcomings that can be avoided by chitosan modification and by loading its film with substances possessing antimicrobial properties. In this study, chitosan was modified with benzaldehyde and crosslinked with glutaraldehyde to form a film on the surface of cotton fabric (CB). Also, another material was obtained by including zinc oxide particles (CBZ) synthesized in situ into the chitosan coating. The performed analyses (contact angle measurement, optical and scanning electron microscopy, FTIR, XRD, and thermal analysis) evidenced the modification of the cotton fabric and the alteration of the film properties after zinc oxide inclusion. A comparison of the antimicrobial properties of the new CB with materials prepared with chitosan without benzaldehyde from our previous study verified the influence of the hydrophobicity and surface roughness of the fabric surface on the enhancement of antimicrobial activity. The microbial growth inhibition increased in the following order: fungal strain Candida lipolytica >Gram-positive bacteria Bacillus cereus >Gram-negative bacteria Pseudomonas aeruginosa. The samples containing zinc oxide particles completely inhibited the growth of all three model strains. The virucidal activity of the CB was higher against human adenovirus serotype 5 (HAdV-5) than against human respiratory syncytial virus (HRSV-S2) after 60 min of exposure. The CBZ displayed higher virucidal activity with a Δlog of 0.9 against both viruses.Item Biodiesel production from high free fatty acid byproduct of bioethanol production process(2022-01-01) Kokkinos N.; Theochari G.; Emmanouilidou E.; Angelova D.; Toteva V.; Lazaridou A.; Mitkidou S.Biodiesel is a reliable and promising replacement of fossil diesel. It is stable, less toxic and can be produced from sustainable resources, including a variety of raw materials. Currently, the most widely used are vegetable oils (edible and nonedible), due to their availability. The present paper considers the potential of obtaining fatty acid ethyl esters (FAEE) from corn oil, which is a byproduct of bioethanol production process. The ultimate outcome would definitely increase the profitability of the initial bioethanol production process. The biodiesel production process was implemented in two steps, due to the high content of free fatty acids of the obtained corn oil. The first step includes an acid-catalyzed esterification process and the second step comprises an alkali-catalyzed transesterification process to receive FAEE. Two different catalysts (sulfuric acid and p-toluene sulfonic acid) were utilized and compared each other in the esterification process, in order to cope with high acid number of the raw material. A comprehensive qualitative and quantitative analysis of both feedstock and biodiesel was performed using gas chromatography-mass spectrometry method. The obtained biodiesel was characterized by a significantly lower cloud point compared to the feedstock and high acid number.Item Fatigue crack development in a low-carbon steel. Microstructure influence. Modelling(2016-01-01) Angelova D.; Yordanova R.; Yankova S.Fatigue in a low-carbon steel with ferrite and pearlite microstructure is investigated through testing of three groups of specimens. Two of the groups consist of cylindrical specimens subjected to tension-tension and rotating-bending fatigue; in this case surface microstructurally-short crack propagation is monitored by acetate-foil replica technique. The third group of specimens includes flat specimens preliminary notched by FIB-technique and then subjected to pure-bending fatigue. The study is focused on examining of crack paths in terms of interaction between the propagating short cracks and the microstructure. The obtained data for pure-bending fatigue show higher crack growth rates and shorter fatigue lifetimes than those found for rotating-bending fatigue. In comparison, the registered tension-tension fatigue data present the lowest crack growth rates, due to much lesser loading than that applied at rotating-bending and pure-bending fatigue. Based on data obtained, a Parabolic-linear model ``Crack growth rate - Crack length`` is used for describing and predicting adequately short crack propagation under the specified three types of fatigue. The model is supported by a comparison between the predicted and the actual fatigue lifetimes.Item Investigation on fatigue behavior and fatigue crack growth of spring steel. Part I. Wöhler curve and fracture surfaces(2014-02-25) Nikolova L.; Yordanova R.; Angelova D.Fatigue behavior of spring steel at different stress ranges was investigated. The specimens were machined in hour-glass shape with polished surface and were subjected to symmetric cyclic rotating-bending fatigue in air and room temperature to fracture. Fatigue life time data was obtained and Wöhler curve of the investigated steel at different conditions is plotted. A mathematical description of the obtained results is presented. The fatigue test results are filled out with images and analysis of the fracture surface made with scanning electron microscope. A replication technique is used for short fatigue-crack growth monitoring for two specimens. The experimental data are presented in plots ``Crack lengths, a - cycles, N``.Item Liquid-phase sorption of oil by carbonized rice husks: Impact of grain size distribution on the sorption kinetics(2013-11-05) Uzunova S.; Uzunov I.; Angelova D.The use of sorbents, obtained from renewable wastes to purify the hydrosphere from various hazardous pollutants is an innovative approach with multiplied environmental effect. In this regard, there is interest in the processing of rice husks and converting them into an effective biosorbent for purification of water from oil pollution. By slow pyrolysis of rice husk at 480°C pyrolysis charcoal was obtained. The product was divided into fractions with sizes in the range from 0,4 to 2,0 mm. Each fraction was tested with respect to adsorption of crude oil from water surface gravimetrically and as emulsified in aqueous media by a batch-method. The aim of this study was to determine the influence of the factor ``granulometry of the adsorbent`` on the sorption kinetics and efficiency of the material for both cases of contamination. An attempt had been made the kinetics of the process to be described by kinetic models, most often used for adsorption from aqueous media: Lagergren's equation; second order kinetics equation; Weber-Morris's diffusion equation and Elovich equation.Item METHODS FOR MODIFICATION OF COTTON FABRICS WITH GELATIN - GLUTARALDEHYDE AND ZNO NANOPARTICLES(2022-01-01) Zheleva D.; Koleva M.; Angelova D.; Grabcheva D.; Todorov P.Cotton fabrics have good water absorption properties, air permeability, non-toxicity, but microorganisms easily damage them. Collagen is a commonly used biomaterial that has properties such as biocompatibility, biodegradability, non-toxicity, but with poor mechanical properties. Glutaraldehyde is the main cross-linking agent for collagen and characterized by antimicrobial properties. ZnO nanoparticles exhibiting antibacterial, antifungal, anticorrosive and protective properties against UV, is widely applicable in medicine. The combination of all these components in one biocomposite with potential antimicrobial properties and healing effect can find application in medical practice. In this study, an attempt was made to improve the properties of cotton fabric by impregnating it with gelatin hydrogel cross-linked with glutaraldehyde and containing ZnO nanoparticles. Three methods of modification were applied, varying the mixing regimes of the components and the conditions. The composite materials were investigated by means of SEM, FTIR, and fluorescence analysis. The morphological analyzes of the samples modified by different methods show that the spher ical particles of ZnO have changed into a flower-like structures; the particles are covered by the collagen film; and dispersed and agglomerated in certain places. FTIR analyzes prove interactions between the organic and inorganic components. This is also confirmed by the observed fluorescent properties, which are of different intensity. All these observations suggest good antibacterial properties that will be the subject of future research.Item METHODS FOR THE SYNTHESIS OF TiO2 NANOPARTICLES. PROPERTIES OF TEXTILE MATERIALS TREATED WITH TiO2 NANOPARTICLES(2024-01-01) Koleva M.; Angelova D.; Zheleva D.In recent decades, interest in oxide nanomaterials with multifunctional properties has grown significantly. Titanium dioxide undoubtedly belongs to them, characterized by exceptional photocatalytic activity, non-toxicity, high availability, biocompatibility, antibacterial properties, and low cost. The treatment of textile materials with TiO2 nanoparticles is relatively simple, but the insufficient bonding efficiency between certain fibers and TiO2nanoparticles creates a problem regarding the stability and durability of the nanocomposites. In this study, an attempt was made to improve the properties of cotton modified with gelatin hydrogel, which was cross-linked by glutaraldehyde, and incorporated titanium nanoparticles. Three modification methods were applied, varying the mixing regimes of the components and the conditions. The composite materials were investigated via SEM, FTIR, UV-Vis and elemental analysis. For the first time, titanium nanoparticles obtained by the reduction of TiO2 with oxalic acid were used to modify cotton. Cotton samples were modified with gelatin hydrogel cross-linked with glutaraldehyde to increase the active groups of cellulose fibers that bind to Ti ions. Using the hydrogel, the nanoparticles are deposited on the surface of the textile substrate. The elemental analysis shows the presence of TiO2 nanoparticles. Lines for Ti atoms appear, which is evidence that the particles retain their composition after immobilization. Microscopic analyzes showed that TiO2 nanoparticles were distributed unevenly in the cotton matrix. In the UV analysis, the appearance of a new absorption at 890 cm-1 was observed due to the attachment of Ti-NPs. The appearance of the IR peak at 878 cm-1 confirms the formation of chelate complexes of the nanoparticles in the composite material.Item On factors influencing fatigue process in steel 316L used in hydrogen energy technologies(2014-02-25) Angelova D.; Yordanova R.; Lazarova T.Investigations of fatigue in steels exposed to hydrogen media is extremely important problem. In this work, an austenitic stainless steel ASTM 316L resistant to hydrogen destructive influence is examined. The experiments presented have used hydrogen charged and uncharged specimens and were carried out under rotating bending and tension-compression fatigue in three different laboratories: at The University of Chemical Technology and Metallurgy, Sofia, Bulgaria; at Sandia National Laboratory, California and The University of Tufts, Medford, Massachusetts, USA; The Institute Hydrogenous at Kyushu University, Japan. The results are presented in Wöhler curves complemented by ``Short fatigue crack length - Number of cycles`` curves and ``Frequency - Lifetimes`` plots, and compared respectively.Item On fatigue behavior of two spring steels. Part I: Wöhler curves and fractured surfaces(2016-06-01) Angelova D.; Yordanova R.; Lazarova T.; Yankova S.Symmetric fatigue in two spring steels is investigated in three groups of specimens. One of the groups (Steel EN10270-1SH/ DIN 17223C – C 0.82%, Mn 0.76%, Si 0.26%) has experienced rotating-bending fatigue in air, and the other two groups (Steel BS250A53/ DIN 55Si7 – C 0.56%, Mn 0.81%, Si 1.85%), torsion fatigue in-air and corrosion environment. Fatigue life-time data are obtained for both spring steels, and their corresponding Wöhler curves plotted and mathematically described. Surface short fatigue cracks are observed from origin to fracture by using acetate-foil replication technique, and their length, a, measured at the corresponding number of fatigue cycles, N. Those data are presented in plots “Crack lengths, a – Cycles, N” and a comparison made between both the steels. The fractured surfaces of all specimens have been studied and analyzed by the scanning-electron microscopy methods.Item On fatigue behaviour of two spring steels. Part II: Mathematical models(2016-06-01) Angelova D.; Yordanova R.; Lazarova T.; Yankova S.Symmetric fatigue in two spring steels is investigated in three groups of specimens. One of the groups (Steel EN10270-1SH/ DIN 17223C – C 0.82%, Mn 0.76%, Si 0.26%) has experienced rotating-bending fatigue in air, and the other two groups (Steel BS250A53/ DIN 55Si7 – C 0.56%, Mn 0.81%, Si 1.85%), torsion fatigue in-air and corrosion environment. All experiments include testing to fracture, applying acetate-foil replication technique, replica monitoring of short crack surface growth, length measuring of propagating cracks, a, at the corresponding number of fatigue cycles, N. Data obtained from replica monitoring are presented in plots “Crack lengths, a – Cycles, N”, and used for calculating fatigue crack growth rates, da/dN, and graphical presentations “Crack growth rates, da/dN – Crack lengths, a”. A mathematical description of da/dN – a is presented by introducing a parabolic-linear model in different versions for each of the steels. The model versions are verified through comparing the experimental fatigue lifetimes with those calculated by the proposed model version.Item On monitoring mechanical characteristics of rolled electrolytic copper(2016-01-01) Angelova D.; Yankova S.; Yordanova R.; Atanasova G.Rolling technology for producing high-electrical-conductivity-copper strips and their quality are under investigation. The technology is designed for and used in the Bulgarian Metallurgical Plant SOFIA MED SA, Sofia. The copper strips are produced in three different tempers - soft, half hard and hard - under different rolling and heat-treatment conditions. The copper strips from different tempers are subjected to specialized testing and their mechanical and high-electrical-conductivity characteristics analyzed. On the basis of mechanical rolled-strip parameters - yield strengths, R e , ultimate tensile strengths, R m , Vickers hardness, HV, and elongation after fracture, A, - some Stress-hardness and Stress-hardness-elongation spaces have been plotted. These Spaces can be used as an instrument for general evaluation of the applied rolling technology and for prediction of copper-strip mechanical behaviour under given exploitation conditions.Item On monitoring of mechanical characteristics of hot rolled S355J2 steel(2016-01-01) Angelova D.; Yordanova R.; Georgiev A.; Yankova S.Hot rolling normalization technology for producing sheets from low-carbon Steel S355J2, used in the Bulgarian Metallurgical Plant ``STOMANA Industry SA`` is under investigation. A newly introduced automatic application optimization procedure to this technology is an important step that leads to avoiding traditional heat treatment, improving steel mechanical characteristics, increasing production efficiency, all resulting in high quality final products. On the basis of the final mechanical rolled-sheet characteristics - yield strengths, Re, ultimate tensile strengths, Rm, absorbed energies in impact tests, K, and elongations after fracture - some energy-stress and energy-stress-elongation constructions-spaces have been plotted. These spaces can be used for general evaluation of the applied rolling technology and for prediction of steel-sheet mechanical behaviour.Item On monitoring of mechanical characteristics of hot rolled S355J2 steel(2016-06-01) Angelova D.; Yordanova R.; Georgiev A.; Yankova S.Hot rolling normalization technology for producing sheets from a low-carbon steel, Steel S355J2, used in the Bulgarian Metallurgical Plant “STOMANA Industry SA” is under investigation. A newly introduced automatic application optimization procedure in this technology is an important step that leads to avoiding of traditional heat treatment, improving of steel mechanical characteristics, increasing of production efficiency, all resulting in high quality final products. On the basis of the final mechanical rolled-sheet characteristics - yield strengths, Re, ultimate tensile strengths, Rm, absorbed energies in impact tests, K, and elongations after fracture - some energy-stress and energy-stress-elongation constructions-spaces have been plotted. These spaces can be used for general evaluation of the applied rolling technology and for prediction of steel-sheet mechanical behaviour.Item Selective adsorption of thiophene and its polyaromatic derivatives from fuels on pyrolyzed rice husks: A thermodynamic study(2019-01-01) Uzunova S.; Uzunov I.; Atanasova G.; Angelova D.; Ivanov I.This work presents the study of the adsorption affinity of biobased adsorbent prepared by pyrolysis of rice husks (PRH) toward thiophene (T), benzothiophene (BT) and dibenzothiophene (DBT) in single and mixed solutions in a batch mode using a model fuel and real diesel. A degree of desulfurization higher than 40 % is observed in case of an initial sulfur concentration in the fuel not higher than 1000 ppm. The adsorption selectivity of PRH from single solutions of the three aromatic sulfur heterocycles (ASHs) decreases following the line: T > BT > DBT. The theoretical equilibrium adsorption capacity of PRH in regard to the three sulfur compounds adsorption is determined using the adsorption isotherms of Langmuir, Freundlich, Dubinin-Radushkevich and Temkin. The calculated values of the thermodynamic parameters, such as the Gibbs energy, ΔG0, the entropy, ΔS0, and the enthalpy, ΔH0, indicate that the adsorption in PRH-ASHs-fuel system proceeds at normal and high temperature values through simultaneous physical and chemical interactions.