Master Degree / Yüksek Lisans Tezleri

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  • Master Thesis
    Synergistic Effect of Zinc Stearate and Natural Zeolite on Pvc Thermal Stability
    (Izmir Institute of Technology, 2004) Atakul, Sevdiye; Balköse, Devrim
    At high temperatures (about 100 °C), PVC decomposes by the removal of hydrogen chloride (HCl) gas and the decomposition is accompanied by polymer discoloration going from yellow to orange, brown and black. In order to prevent decomposition of PVC, thermal stabilizers are added to PVC. The synergistic effect of zinc stearate (ZnSt2) and/or natural zeolite (clinoptilolite) on PVC thermal stability was investigated in this project. For this purpose PVC plastisol was prepared by mixing poly(vinyl chloride) (PVC) and dioctyphthalate (DOP) and stabilized with different amounts of metal soaps and zeolite.The materials and the prepared PVC plastisols were characterized by spectroscopic and thermal analysis techniques. The gelation of the plastisols was observed by optical microscopy and the photographs were taken by a digital camera. The complete gelation was observed at 185 °C. The morphology of PVC plastigel films were studied by scanning electron microscope (SEM). SEM with energy dispersive X-ray (EDX) analysis was used to obtain elemental compositions in polymer and particle phases. The thermal stability of PVC plastigels in terms of color were obtained by heating them in a static vaccum oven. When zeolite was used as a primary stabilizer it prevented the early blackening of the film. Kinetic study of dehydrochlorination was determined by 763 PVC Thermomat equipment for unstabilized and stabilized PVC plastigels. When PVC plastigels were heated in PVC Thermomat in the presence of nitrogen gas, the conductivity of water which nitrogen gas was passed due to the evolved HCl, changed with respect to time. The period when conductivity starts to increase is called as induction time, and the period when the conductivity value reaches to 50 uS/cm is called as stability time. The induction and stability time values of unstabilized and stabilized PVC plastigel films were obtained at 140 and 160 °C. The stabilizing effect of zeolite on the increase in the induction period of the sample was considered the result from the absorption of HCl which was thought to reduce the autocatalytic effect of HCl evolved at the initial stages of dehydrochlorination. Since the induction time of the sample having 0.53% ZnSt2 and 0.53% zeolite was higher than the PVC plastigels having only ZnSt2 or zeolite the synergistic effect on thermal stability was observed at low levels of them.
  • Master Thesis
    Effects of Trace Elements on the Production of Baker's Yeast
    (Izmir Institute of Technology, 2002) Üzelyalçın, Berna; Harsa, Hayriye Şebnem
    Baker.s yeast has been used widely in traditional bakeries to produce different kinds of baked products and for many biotechnological purposes. The principal raw materials used in producing baker.s yeast are the pure yeast culture and molasses. Cane molasses and beet molasses contain 45 to 55 % sugar in the forms of sucrose, glucose, and fructose. Molasses, which is a rich carbon and mineral source, contains most of the nutrients and minerals required for baker.s yeast production. Nutrients and trace elements (magnesium, copper and zinc) that are not present in sufficient quantities in molasesses are added to the fermentation medium. In this study, the effects of Ca2+, Mg2+, Cu2+ and Zn2+ on baker.s yeast production were investigated. The method of Food Composition Laboratory (FCL), Human Nutrition Research Center, Beltsville, US, was used for the preparation of the fermentation samples prior to ICP-AES analysis. The concentrations of Ca2+, Mg2+, Cu2+, Zn2+ were determined in the raw materials entering the fermentors and the products. FCL digestion method was evaluated on precision and accuracy; linear standard curves were obtained for the studied concentration ranges of Ca2+, Mg2+, Cu2+ and Zn2+. Standard addition curves for each element in 45 brix molasses and dry yeast samples were constructed and spike recovery percentages were calculated. High spike recovery percentages were obtained for all elements in both dry yeast and molasses with the FCL method. In industrial fermentations, uptakes of Ca2+, Cu2+, Mg2+ and Zn2+ by the yeast cells were calculated as mg/kg dry yeast at 4h time intervals. The uptake rates were evaluated based on the available element concentration in the medium and the uptake of the elements. Results showed that, as the uptake of Ca2+ increased with time, uptake of Mg2+, Cu2+ and Zn2+ decreased. The yeast cells used the available elements with decreasing uptake rates except Ca2+. During the first hours of fermentation, uptake rates were high. The maximum uptake rates observed for each element in two different industrial fermentations were: 17.73-23.32 mg Cu2+/ kg dry yeast cells , 195.64-413.46 mg Zn2+ / kg dry yeast cells, 2106.46-2051.74 mg Mg2+ / kg dry yeast cells. Ion concentrations of biomass in industrial fermentations revealed that same amounts of Mg were taken by the cell from the fermentation medium under similar conditions. This was verified by the studies done in lab-scale using pure culture of S.cerevisiae, grown on different concentrations of Mg2+. It was found that uptake of Mg2+ was constant and very close to the uptake values of industrial fermentations. 806.7 mM Mg2+ was taken by 1 kg of dry yeast cells. This is in agreement with the stated Mg2+ uptake concentration range 500-1000 mM of Saccharomyces cerevisiae in literature. In the industry, magnesium is added in the form of Mg.SO4.7H2O to the fermentation medium. SO42- ions coming from the MgSO4.7H2O cause settling problems in the waste treatment system of the plant. Generally, molasses contain sufficient concentration of Mg2+ ions required for baker.s yeast production in the industrial scale. Since Mg2+ has toxic effects at high concentrations and its addition is increasing the cost of the process and causing difficulties in the waste treatment of the plant, it may be concluded that addition of Mg2+ as MgSO4.7H2O to the fermentation medium should be done after determining the Mg2+ concentration of molasses. Instead of using excessive amounts of MgSO4.7H2O, only the required quantity should be added to the fermentation medium.
  • Master Thesis
    Process Development for Metal Soaps
    (Izmir Institute of Technology, 2003) Gönen, Mehmet; Balköse, Devrim
    Zinc stearate (ZnSt2) is an important compound among the metallic soaps. It has many applications e.g., in resins, paints, cosmetics, textile, lubricants and Langmuir-Blodgett films. Double decomposition (precipitation) and fusion processes are widely used techniques in ZnSt2 production. The product purity has been a major concern in most of the ZnSt2 applications such as PVC stabilization, coating of textile goods, additive in cosmetic products. In this study, the production of ZnSt2 using precipitation, fusion and modified fusion processes was investigated based on product purity. Raw materials and ZnSt2 were characterized by using various techniques.In the precipitation process, in order to maximize the solubility of sodium stearate and to minimize the water evaporation, the reaction was carried out at 70°C. 2.5% (w) NaSt was used in the reaction at this temperature. The equivalent, excess Zn and deficient Zn cases were studied to determine the raw materials ratio on product purity. Equivalent raw materials produced highest product purity. From washing experiment, it was seen that Na2SO4 adsorption did not take place on to wet ZnSt2. The washing water to zinc stearate ratio was found to be 40 dm3/kg for effective removal of by products and raw materials unreacted at room temperature. Any further increase in the amount of water did not bring any significant removal results. In the drying of wet ZnSt2 at 100°C, it was determined that it has 85% (w) water. In IR spectra, characteristic ZnSt2 peak was observed at 1540 cm-1. ZnSt2 obtained by this process did not contain any free Na+ and SO4-2 ions as indicated by ICP, EDX and elemental analysis results. Only two of the characteristic peaks of ZnSt2 at 2. values of 6.40 and 19.58 were obtained in XRD pattern of the dried product due to low crystallinity. From SEM micrographs, it was seen that zinc stearate has lamellar structure and particle size changes between 2-4 .m. Melting point of the zinc stearate was found to be about 122°C using optic microscopy with temperature controlled hot stage.In fusion process, reaction was carried out at 140°C in equivalent amounts for different stirring rates 400, 600 and 750 rpm. The increase in mixing rate decreased the delay time occurring at the beginning of the reaction. The conversion was found to be 80% using the developed method from IR spectra. In the comparison of the experimental conversion data with shrinking core model no relation was established. In IR spectra, two peaks observed at 1540 cm-1 and 1700 cm-1 which belong to ZnSt2 COO- stretching and stearic acid C.O stretching vibrations, respectively. All of the characteristic 2. values of zinc stearate were observed for product, which means that the crystallinity of the product is high. From SEM micrographs, it was seen that zinc stearate structure is in the form of layered lamella and particle size change between 4-6 .m. The melting point of zinc stearate samples from fusion process was found to be slightly lower than 122°C by optic microscopy with temperature-controlled hotstage.In the modified fusion process, reaction was carried out at 80°C for 1 h. with equivalent amounts of stearic acid and zinc oxide in the presence of H2O. Sodium stearate 1.5% (w) was added into reaction mixture as a surfactant and its effect was examined. At the end of reaction it was seen from IR spectra that it does not significantly increase the reactants dispersion. The presence of unreacted raw materials was determined in IR spectras and XRD patterns. This result was confirmed by SEM micrographs too. In TGA analysis, thermal decomposition temperature of zinc stearate was found to be 250°C. The use of zinc stearate in n-paraffin wax shifted the thermal decomposition temperature of wax 10°C. Increasing the amount of zinc stearate in n-paraffin increased the decomposition temperature of wax.According to the results of this study, for pure zinc stearate production precipitation process should be preferred in spite of high wash water consumption. The fusion and modified fusion processes needs to be studied further to increase the conversion and decrease the delay time.