• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 149
  • 56
  • 17
  • 3
  • 3
  • 2
  • 1
  • 1
  • 1
  • Tagged with
  • 251
  • 155
  • 122
  • 84
  • 70
  • 45
  • 41
  • 38
  • 35
  • 31
  • 28
  • 26
  • 25
  • 24
  • 24
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Biotransformations using lipase enzymes in organic solvents

Sanvoisin, Jonathan January 1993 (has links)
Enzymatic transformations in organic solvents have received increasing attention over the past 10 years and lipases have become by far the most popular enzymes in this area.The initial aim of the research was to assess the effect of small modifications to the acyl donor in the transesterification reactions mediated by the Candida cylindracea lipase. 2.2.2-Trichloroethyl butyrate (TCEB) was used as a standard for the rate studies. In the event the acyl donor, trichloroethyl methoxyacetate (TCEMA), accelerated the transesterification reaction with hexan-1-ol by an order of magnitude over that with TCEB. This observation, and the absence of an acceleration with trichloroethyl methoxypropionate (and ethyl 2- fluorobutyrate over ethyl butyrate) suggested that the effect is due to the ~oxygen. A solvent activity profile indicated that the most hydrophobic solvents supported faster initial rates. This was attributed to the ability of the hydrophilic solvents to strip the hydrated water from the enzyme surface thus deactivating it. The switch to organic solvents allowed a wider temperature range to be studied. For the reactions between heptan-2-ol and TCEMA the reaction could be conducted in the temperature range -23 C to 80 C. It was of interest to assess how the alcohol moiety effected the transesterification reaction. A series of alcohols were presented to the enzyme and a pattern emerged with substrates containing an acetylene functionality being processed faster than those with a vinyl group, which were faster than those containing a methyl group (all other groups being the same). A series of heterocyclic alcohols were presented to the enzyme and it was observed that the order of reaction was thiophene > furan > pyridyl. The secondary alcohols in this series, 2-thiopheneethan-1-ol and o pyridylethan-1-ol, were resolved at various temperatures from -1soc to sooc with no variation in. enantioselectivity. These are the first resolutions to be accomplished at temperatures below zero degrees.
2

ENZYMATIC TRANSESTERIFICATION OF WASTE ANIMAL FATS FOR PRODUCTION OF BIODIESEL

Kumar, Santhosh 03 July 2013 (has links)
The process of transesterification is the exchange of the organic group R” of an ester with the organic group R’of an alcohol, often catalyzed by acid, base or enzyme. Biodiesel, a mixture of monoalkyl esters of long chain fatty acids, is produced from vegetable oils, animal fats and fish oils by transesterification in presence of alcohol. Biodiesel is a fuel which can be used in a mixture of other fuels or alone. The base catalyzed transesterification method of biodiesel production is not suitable for waste animal fat as it contains 10–15% free fatty acids which result in higher soap formation and cause extensive downstream processing. Enzyme catalyzed transesterification can overcome the problem of soap formation and multi-step purification of end products and results in a higher purity biodiesel. Lipase is the enzyme widely used in the process of enzymatic transesterification. Various lipases have been used to transesterify triglycerides with short chain alcohols to alkyl esters. The objectives of this study were to screen lipase enzymes for the transesterification process and to use the best lipase for biodiesel production from waste animal fat. Enzymatic transesterification by individual and combined enzyme catalysts (Novozyme 435 and NS88001) was first carried out to investigate the effects of reaction time (4, 8, 12 and 16 hour), oil : alcohol molar ratios (1:1, 1:2, 1:3, 1:4 and 1:5), the effects of alcohol type (methanol and 2-butanol) and reaction temperature (35, 40, 45 and 50°C) on biodiesel yield in solvent and solvent-free systems. The highest conversion yield of biodiesel (96.67%) was obtained from a combination of Novozyme and NS88001 lipase with the optimal reaction condition of oil : 2-butanol molar ratio of 1:4, enzyme concentration of 25% (12.5% w/w of each enzyme), hexane as solvent, a 45°C reaction temperature, a reaction time of 16 h and a mixing speed of 200 rpm. The reusability of lipase enzymes by individual and combination of enzyme catalysts (Novozyme 435 and NS88001) with solvent and solvent-free systems was also investigated in order to reduce the cost of the process. The lipase enzymes lost their activity after being reused for 30 cycles in solvent-free systems and after 10 cycles in solvent system.
3

Structure and properties of reactive polycarbonate-poly(ethylene terephthalate) blends

Nita, Elisabeta Maria Iuliana January 2000 (has links)
No description available.
4

Biodiesel production from fryer grease

Issariyakul, Titipong 19 June 2006
Biodiesel is a renewable, biodegradable, environmentally benign fuel for use in the diesel engines. It can be produced from renewable sources such as vegetable oils or animal fats. Although this fuel has gained worldwide recognition for many years, it is not being widely commercialized mainly because it is more expensive than petroleum diesel. A cheaper feedstock, such as fryer grease, may be used to improve the economics of biodiesel. <p>Methanol is the most common alcohol used in the transesterification process due to its low cost. However, recently, ethanol has been promoted as an alcohol for use in transesterification since it can be produced from renewable resources such as switchgrass, corn and wood, thereby reducing the dependency on petroleum sources (Pimentel and Patzek, 2005). A mixture of methanol and ethanol is hypothesized to take the advantages of both methanol and ethanol. The present work is focused on the production of biodiesel from fryer grease via transesterification with various mixtures of methanol and ethanol. Also, the kinetics of transesterification from fryer grease was studied. <p> Since fryer grease contains a high concentration of free fatty acids (FFA) (5.6 wt. %) and water (7.3 wt. %), a two-step acid/alkaline transesterification process was used to produce the esters. Sulfuric acid and potassium hydroxide were used as acid and alkaline catalysts, respectively. The methanol to ethanol molar ratio was varied from 3:3 to 5:1, whereas alcohol to oil molar ratio was maintained at 6:1. After the fryer grease was transesterified, all esters met ASTM standard D-6751. The viscosity of these esters ranged from 4.7 to 5.9 mm2/s. The heating value of the esters was approximately 10% less than that of petroleum diesel. The cloud point and pour point were in the range of 1 to -1 oC and -3 to -6 oC, respectively. When the mixed alcohol was used ethyl esters were also formed at a lower concentration along with methyl esters. The dominant fatty acid in fryer grease esters was found to be oleic acid. The lubricity of kerosene fuel was improved by as much as 33 % through the addition of these esters at rates as low as 1 %. <p>For the kinetic study of alkali-catalyzed transesterification of fryer grease, the alcohol to oil molar ratio, the reaction temperature, and the catalyst loading were varied as 6:1, 9:1, 12:1; 30, 40, 50 oC; and 0.5, 1.0, 1.5 %, respectively. The ester concentration was found to rise with an increase in the catalyst loading or the reaction temperature and with a decrease in the alcohol to oil molar ratio. The overall forward and backward reaction orders were assumed to follow first and second order kinetics, respectively. The kinetic parameters were calculated using MATLAB. The conversion of triglyceride to diglyceride was found to be the rate determining step (RDS) of the overall reaction, with an activation energy of 36.9 kJ/mol.
5

Study of factors influcencing the quality and yield of biodiesel produced by transesterification of vegetable oils

Ares Gondra, Zaloa January 2009 (has links)
Biofuels are a developing kind of fuel whose origin is biomass. Among them, many different kind of fuels can be found: bioethanol, biobutanol, biodiesel, vegetable oils, biomethanol, pyrolysis oils, biogas, and biohydrogen. This thesis work is focused on the production of biodiesel, which can be used in diesel engines as a substitute for mineral diesel. Biodiesel is obtained from different kinds of oils, both from vegetable and animal sources. However, vegetable oils are preferred because they tend to be liquid at room temperature. The process to obtain biodiesel implies first a reaction between the oil and an alcohol, using a catalyst and then a sedimentation, where the biodiesel and the glycerol, the two products that are obtained, can be separated because of their difference in density. After the separation, raw biodiesel is obtained and a treatment with either water bubbling or dry cleaning products is needed to obtain the product which will be ready to use. Many methods are available for the production of biodiesel, most of them require heat for the transesterification reaction, which converts the oil into biodiesel. Apart from that, in many cases biodiesel is produced by big companies or by individuals but using complicated and expensive installations. This work is an attempt to develop a way of producing biodiesel without any use of external heat, using a simple procedure which could be used by people with a low knowledge of chemistry or chemical processes. It also seeks to set an example on how biodiesel can be easily made by oneself without the use of any industrial systems, with a low budget and limited need of supervision over the process. In order to achieve that, many trials were undertaken, introducing changes in the different parameters that are responsible for the changes in the final product. Among them, changes in the amount and type of catalyst, the way the catalyst is added, the type of oil used, the time of reaction and the temperature were made. Apart from that, different types of cleaning were tried, starting by water cleaning and then using powder type products, Magnesol, D-Sol and Aerogel. A centrifuge was also tried to test its utility when separating impurities from liquids or different liquid phases. The results of the different trials were analysed using various tests, the most important being the 3:27 test, the solubility test, the soap titration and pH measurements. To sum up, it could be said that the investigation was a success, since it was proved that biodiesel can be made without the use of any external heat with both alkali and acid catalysts, as well as with different ways of adding the catalyst. As for the cleaning, good results were obtained with both dry products and water cleaning, since the soap content of the biodiesel was reduced in both cases. Apart from that, the centrifuge proved to be valid to eliminate impurities from raw oil.
6

Biodiesel production from fryer grease

Issariyakul, Titipong 19 June 2006 (has links)
Biodiesel is a renewable, biodegradable, environmentally benign fuel for use in the diesel engines. It can be produced from renewable sources such as vegetable oils or animal fats. Although this fuel has gained worldwide recognition for many years, it is not being widely commercialized mainly because it is more expensive than petroleum diesel. A cheaper feedstock, such as fryer grease, may be used to improve the economics of biodiesel. <p>Methanol is the most common alcohol used in the transesterification process due to its low cost. However, recently, ethanol has been promoted as an alcohol for use in transesterification since it can be produced from renewable resources such as switchgrass, corn and wood, thereby reducing the dependency on petroleum sources (Pimentel and Patzek, 2005). A mixture of methanol and ethanol is hypothesized to take the advantages of both methanol and ethanol. The present work is focused on the production of biodiesel from fryer grease via transesterification with various mixtures of methanol and ethanol. Also, the kinetics of transesterification from fryer grease was studied. <p> Since fryer grease contains a high concentration of free fatty acids (FFA) (5.6 wt. %) and water (7.3 wt. %), a two-step acid/alkaline transesterification process was used to produce the esters. Sulfuric acid and potassium hydroxide were used as acid and alkaline catalysts, respectively. The methanol to ethanol molar ratio was varied from 3:3 to 5:1, whereas alcohol to oil molar ratio was maintained at 6:1. After the fryer grease was transesterified, all esters met ASTM standard D-6751. The viscosity of these esters ranged from 4.7 to 5.9 mm2/s. The heating value of the esters was approximately 10% less than that of petroleum diesel. The cloud point and pour point were in the range of 1 to -1 oC and -3 to -6 oC, respectively. When the mixed alcohol was used ethyl esters were also formed at a lower concentration along with methyl esters. The dominant fatty acid in fryer grease esters was found to be oleic acid. The lubricity of kerosene fuel was improved by as much as 33 % through the addition of these esters at rates as low as 1 %. <p>For the kinetic study of alkali-catalyzed transesterification of fryer grease, the alcohol to oil molar ratio, the reaction temperature, and the catalyst loading were varied as 6:1, 9:1, 12:1; 30, 40, 50 oC; and 0.5, 1.0, 1.5 %, respectively. The ester concentration was found to rise with an increase in the catalyst loading or the reaction temperature and with a decrease in the alcohol to oil molar ratio. The overall forward and backward reaction orders were assumed to follow first and second order kinetics, respectively. The kinetic parameters were calculated using MATLAB. The conversion of triglyceride to diglyceride was found to be the rate determining step (RDS) of the overall reaction, with an activation energy of 36.9 kJ/mol.
7

Technical aspects of production and analysis of biodiesel from used cooking oil—A review

Enweremadu, CC, Mbarawa, MM 08 January 2009 (has links)
Abstract The increasing awareness of the depletion of fossil fuel resources and the environmental benefits of biodiesel fuel has made it more attractive in recent times. The cost of biodiesel, however, is the major hurdle to its commercialization in comparison to petroleum-based diesel fuel. The high cost is primarily due to the raw material, mostly neat vegetable oil. Used cooking oil is one of the economical sources for biodiesel production. However, the products formed during frying, can affect the transesterification reaction and the biodiesel properties. This paper attempts to review various technological methods of biodiesel production from used cooking oil. The analytical methods for high quality biodiesel fuel from used cooking oil like GC, TLC, HPLC, GPC and TGA have also been summarized in this paper. In addition, the specifications provided by different countries are presented. The fuel properties of biodiesel fuel from used cooking oil were also reviewed and compared with those of conventional diesel fuel.
8

Study of factors influcencing the quality and yield of biodiesel produced by transesterification of vegetable oils

Ares Gondra, Zaloa January 2009 (has links)
<p>Biofuels are a developing kind of fuel whose origin is biomass. Among them, many different kind of fuels can be found: bioethanol, biobutanol, biodiesel, vegetable oils, biomethanol, pyrolysis oils, biogas, and biohydrogen. This thesis work is focused on the production of biodiesel, which can be used in diesel engines as a substitute for mineral diesel. Biodiesel is obtained from different kinds of oils, both from vegetable and animal sources. However, vegetable oils are preferred because they tend to be liquid at room temperature.</p><p>The process to obtain biodiesel implies first a reaction between the oil and an alcohol, using a catalyst and then a sedimentation, where the biodiesel and the glycerol, the two products that are obtained, can be separated because of their difference in density. After the separation, raw biodiesel is obtained and a treatment with either water bubbling or dry cleaning products is needed to obtain the product which will be ready to use.</p><p>Many methods are available for the production of biodiesel, most of them require heat for the transesterification reaction, which converts the oil into biodiesel. Apart from that, in many cases biodiesel is produced by big companies or by individuals but using complicated and expensive installations.</p><p>This work is an attempt to develop a way of producing biodiesel without any use of external heat, using a simple procedure which could be used by people with a low knowledge of chemistry or chemical processes. It also seeks to set an example on how biodiesel can be easily made by oneself without the use of any industrial systems, with a low budget and limited need of supervision over the process.</p><p>In order to achieve that, many trials were undertaken, introducing changes in the different parameters that are responsible for the changes in the final product. Among them, changes in the amount and type of catalyst, the way the catalyst is added, the type of oil used, the time of reaction and the temperature were made. Apart from that, different types of cleaning were tried, starting by water cleaning and then using powder type products, Magnesol, D-Sol and Aerogel. A centrifuge was also tried to test its utility when separating impurities from liquids or different liquid phases. The results of the different trials were analysed using various tests, the most important being the 3:27 test, the solubility test, the soap titration and pH measurements.</p><p>To sum up, it could be said that the investigation was a success, since it was proved that biodiesel can be made without the use of any external heat with both alkali and acid catalysts, as well as with different ways of adding the catalyst. As for the cleaning, good results were obtained with both dry products and water cleaning, since the soap content of the biodiesel was reduced in both cases. Apart from that, the centrifuge proved to be valid to eliminate impurities from raw oil.</p>
9

Reação de transesterificação do óleo de soja utilizando o catalisador CTA+-MCM-41 para a produção do biodiesel. / Reaction of soybean oil transesterification using the CTA+ -MCM-41 catalyst for the production of biodiesel.

SANTIAGO, Alyson Silvestre. 23 April 2018 (has links)
Submitted by Lucienne Costa (lucienneferreira@ufcg.edu.br) on 2018-04-23T20:34:24Z No. of bitstreams: 1 ALYSON SILVESTRE SANTIAGO – DISSERTAÇÃO (PPGEQ) 2014.pdf: 1626315 bytes, checksum: a4f283a010ca8f77bb130e704407b33c (MD5) / Made available in DSpace on 2018-04-23T20:34:24Z (GMT). No. of bitstreams: 1 ALYSON SILVESTRE SANTIAGO – DISSERTAÇÃO (PPGEQ) 2014.pdf: 1626315 bytes, checksum: a4f283a010ca8f77bb130e704407b33c (MD5) Previous issue date: 2014-12-18 / A produção de biocombustíveis no Brasil tem atingido patamares elevados, levando o país a ganhar destaque em nível mundial, no que se refere à produção e desenvolvimento de energias renováveis, incluindo definitivamente os biocombustíveis na matriz energética do país. Este trabalho tem como objetivo avaliar a peneira molecular CTA+-MCM-41 na reação de transesterificação do óleo de soja através da rota metílica, utilizando catálise heterogênea para produzir o biodiesel, além de avaliar a dispersão do óxido de magnésio na superfície deste catalisador. A peneira molecular CTA+-MCM-41 é obtida por um processo térmico a 60 °C por 24h utilizando o brometo de cetiltrimetilamônio, o hidróxido de tetrametilamônio, etanol e água deionizada. Para a síntese do gel que deu origem à peneira molecular, foi utilizada a seguinte composição molar: 1TEOS:0,3 CTABr:11NH3:144H2O:40ETOH. O biodiesel foi obtido a partir da reação de transesterificação do óleo de soja por rota metílica, cujas condições de operação seguiram um planejamento experimental 2² com três repetições no ponto central, onde os fatores estudados foram os efeitos das variáveis temperatura e tempo de reação. Para a caracterização dos materiais obtidos, utilizou-se uma combinação de técnicas de Difração de Raios-X e Microscopia Eletrônica de Varredura. Através do DRX pode-se observar a presença dos picos característicos da peneira molecular antes e após os processos de impregnação do metal e sua respectiva calcinação, que comprovaram o não ordenamento do material a longas distâncias no eixo dos canais mesoposoros analisados. Na reação de transesterifição sem a presença do catalisador, obteve-se conversões máximas de 84,1%, tendo sua performance melhorada na presença do CTA+-MCM-41 chegando a conversões próximas a 100%. Diante dos resultados obtidos concluiu-se que os catalisadores sintetizados têm grande potencial na produção de biodiesel. / The production of biofuels in Brazil aims to increase the productivity and development of renewable energy, including biofuels in the country's energy matrix. This work aims to evaluate the molecular sieve CTA+-MCM-41 in the transesterification reaction of soybean oil through the methyl route by using heterogeneous catalysis to produce biodiesel and also to evaluate the dispersion of magnesium oxide on the surface of this catalyst. The molecular sieve CTA+-MCM-41 is obtained by a thermal process at 60 °C for 24 hours, using cetyltrimethylammonium bromide, tetramethylammonium hydroxide, ethanol and deionized water. For synthesize the gel which results in the molecular sieve, the following molar composition was used: 1TEOS:0.3CTABr:11NH3:144H2O:40ETOH. Biodiesel has been obtained by the transesterification reaction of soybean oil through the methyl route, whose operating conditions followed a 22 experimental design with three replicates in the center point, where the investigated factors were the effects of temperature and reaction time variables. For characterization of the obtained materials, a combination of X-Ray Diffraction and Scanning Electron Microscopy techniques was used. Through the XRD, it is possible to observe the presence of the characteristic peaks of the molecular sieve before and after the processes of impregnation of the metal and its respective calcination, which prove the non-ordering of the material at long distances in the axis of the analyzed mesophores channels. At the absence of the catalyst, the conversions were up to 84.1%, with a performance increasing at the presence of the CTA+-MCM-41, reaching values close 100%. Based on the results obtained it was concluded that the synthesized catalysts have a great potential for use in the production of biodiesel.
10

Molecular necklaces: polyester rotaxanes

Liu, Shu 27 February 2007 (has links)
Polyrotaxanes consisting of 30-60 membered aliphatic crown ether macrocycles whose cavities are pierced by polysebacate chains were synthesized by several polymerization approaches including transesterification polymerization, the acid chloride method, and interfacial polymerization. The polyrotaxanes were purified by multiple reprecipitations into good solvents for the crown ethers. In some cases the threaded macrocycles are constrained onto polymer chains by the incorporation of monofunctional blocking groups at polymer chain ends, or by the copolymerization of a difunctional blocking group with other monomers. The compositions and the physical properties of the polyrotaxanes were determined by a variety of characterization techniques including NMR, UV, VPO, GPC, DSC, TGA, and intrinsic viscosity measurements. Significant amounts, up to 51 mass %, of the macrocyclic components were incorporated. Because of incorporation of flexible and polar macrocycles, polyrotaxanes display special behavior in solution and in the solid state. In solution the linear components of polyrotaxanes are stiffened by the threaded macrocycles, resulting in increased hydrodynamic volumes. The solubilities of linear polymers in polar solvents are enhanced by the incorporated crown ethers. The glass transitions are also affected by the crown ether component. Due to the movement of the macrocycles along the backbone, the macrocycles are able to aggregate and crystallize without dethreading from the polysebacate backbones. The threading and dethreading processes were systematically studied in the diol/diacid chloride system. Due to the hydrogen bonding between the cyclic and linear species, the macrocycle contents of unblocked polyrotaxanes are significantly affected by the feed ratio of macrocycle to linear monomers up to a value of 2 but are independent of reaction time and the length of diol monomers. The macrocycle content of polyrotaxanes increases non-linearly with the size of the macrocycles, presumably due to changes in the fraction of threadable conformations of the macrocycles. Although some macrocycles near the unblocked polymer chain ends are apparently susceptible to the dethreading from polymer chains, most of the macrocycles are prevented on resonable time scales, e.g., months in solution, from the dethreading by the entanglement of cyclic and linear species. / Ph. D.

Page generated in 0.1603 seconds