71. Supercritical fluids as solvents for enzymatic reactionsMaja Leitgeb, Mateja Primožič, Željko Knez, 2007, pregledni znanstveni članek Opis: Enzymes may act in different solvent systems. Water as the solvent in vivo may be replaced partially or mostly with other solvents, such as micro-emulsions, organic solvents, reversed micelles, ionic liquids and supercritical fluids (SCFs).Several types of enzymatic reactions were performed in SCFs. Influence of SCFs on enzyme stability and activity is presented on different examples; on different reaction systems (hydrolysis, transesterification...) and on the use of non-immobilized (Subtilisin carlsberg, Aspergillus niger...) as well as immobilized enzymes. Several types of high-pressure enzymatic reactors (batch-, stirred-tank-, extractive semibatch-, recirculating batch-, semicontinuous flow-, continuous packed-bed-, and continuous-membrane reactors) have been used for the performance of enzymatic reactions. In the studies on stability of biocatalysts in a high-pressure batch-stirred tank reactor changes in biocatalysts activity due to pressurization/depressurization steps were observed. Interesting alternative to overcome this inconvenience is the use of the high-pressure continuous membrane reactors, where just single compression and expansion step is necessary.
Ključne besede: enzymatic reactions, supercritical carbon dioxide, high-pressure, enzyme stability, high-pressure reactors
Objavljeno v DKUM: 31.05.2012; Ogledov: 3315; Prenosov: 150
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72. Supercritical fluid extraction of chammomile flower head flowers: comparison with conventional extraction, kinetics and scale-upPetra Kotnik, Mojca Škerget, Željko Knez, 2007, objavljeni znanstveni prispevek na konferenci Opis: Supercritical fluid extraction of chamomile flower heads was performed on semicontinuous extraction apparatus in the lab scale using carbon dioxide as solvent. The results of high pressure experiments were compared with those obtained with Soxhlet extraction, steam distillation and maceration. The obtained extracts were analysed by HPLC on ?-bisabolol, matricine and chamazulene content and by gravimetrical method on essential oil and waxes content. The highest content of active components in extracts and highest extraction yield were obtained using SFE at 250 bar and 40 °C. At this extraction conditions the two step separation was used to optimize the separation of essential oil from unwanted components. Dynamic behaviour of theSFE with single step separation runs were analysed using two mathematical models for describing the constant rate period and subsequent falling rate period. Based on the experimental data, external mass transfer coefficient, diffusion coefficient and diffusivity in solid phase were estimated. Results showed acceptable agreement of calculated and experimental data. Based on theparameters determined in the lab scale, the extraction process was successfully transferred to pilot scale.
Ključne besede: chemical processing, high pressure technology, supercritical extraction, chamomile, matricine, chamazulen, scale-up
Objavljeno v DKUM: 31.05.2012; Ogledov: 2249; Prenosov: 63
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74. Proteinase-catalyzed hydrolysis of casein at atmospheric pressure and in supercritical mediaMateja Primožič, Maja Leitgeb, Željko Knez, 2006, izvirni znanstveni članek Opis: In the presented work, reaction parameters for hydrolysis of casein, catalyzed by Carica papaya latex at atmospheric and high pressure, were optimized. Casein is a remarkably efficient nutrient, supplying not only essential amino acids, but also some carbohydrates, calcium, phosphorus and therefore is very important for the food industry. Different reaction parameters such as temperature, stirring rate, casein and enzyme concentration were studied to found the optimal conditions for the reaction. Reactions were performed at atmospheric pressure; an influence of temperature/pressure on the casein hydrolysis in supercritical carbon dioxide (SC CO2) was also investigated to improve the reaction rates. Higher conversions were achieved when the reactions were performed in SC CO2, even though casein was not soluble in this medium.
Ključne besede: chemical processing, high pressure technology, supercritical CO2, enzymatic reactions, proteinase, Carica papaya latex
Objavljeno v DKUM: 31.05.2012; Ogledov: 2249; Prenosov: 48
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75. Enzyme-catalyzed reactions in different types of high-pressure enzymatic reactorsMateja Primožič, Maja Leitgeb, Muzafera Paljevac, Željko Knez, 2006, izvirni znanstveni članek Opis: The enzyme-catalyzed hydrolysis of carboxy-methyl cellulose (CMC) was performed in three different types of reactors; in a batch stirred-tank reactor (BSTR) operating at atmospheric pressure, in a high-pressure batch stirred-tank reactor (HP BSTR) and in a high-pressure continuous tubular-membrane reactor (HP CTMR). In the high-pressure reactors aqueous SC C02 was used as the reaction medium. The aim of our research was optimization of the reaction parameters for reaction performance. All the reactions were catalyzed by cellulase from Humicola insolens. Glucose production in the high-pressure batch stirred-tank reactor was faster than in the BSTR at atmospheric pressure. The optimal temperature for the reaction performed in the BSTR at atmospheric pressure was 30°C, while the optimal temperature for the reaction performed in SC C02 was 32°C. The influence of the application of tubular ceramic membranes in the high-pressure reaction system was studied on the model reaction of CMC hydrolysis at atmospheric pressure and in SC C02. The reaction was catalyzed by cellulase from Humicola insolens covalently linked to the surface of the ceramic membrane. The hydrolysis of CMC in SC C02 and at atmospheric pressure was performed for a long time period. The reaction carried out in SC C02 was more productive than the reaction performed at atmospheric pressure.
Ključne besede: chemical processing, enzymatic reaction, cellulase, supercritical carbon dioxide, high-pressure batch reactor, high-pressure membrane reactor
Objavljeno v DKUM: 31.05.2012; Ogledov: 2164; Prenosov: 341
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76. The lipase-catalyzed synthesis of fatty acid fructose esters in organic media and in supercritical carbon dioxideSaša Šabeder, Maja Leitgeb, Željko Knez, 2006, izvirni znanstveni članek Opis: Sugar fatty acid esters are biodegradable surface active compounds in foodstuffs and cosmetics or pharmaceuticals. They have potential in replacing pollutant chemically synthesized surfactants. The enzymatic synthesis of fructose palmitate catalyzed by Candida antarctica B lipase was performed in different organic media in a batch react at atmospheric pressure. The influence of the organic solvent and temperature on the esterification was studied. Since supercritical carbon dioxide (SC CO2) has several advantages over organic solvents, such as high reaction rate, high mass transfer, non-toxicity, non flammability and low price, it was also chosen as a reaction medium for fructose palmitate production. The influence of temperature on immobilized lipase activity was studied at 10 MPa and the results were compared to the results obtained from reactions performed at atmospheric pressure under the same reaction conditions. The highest conversion (67%) was obtained after 24 hours of reaction in SC C02 at 80°C. A change of the particle size distribution and morphology of the untreated lipase and lipase treated with 2-methyl 2-butanol and SC C02 was observed.
This article was presented at 1st SEECChE, held in Belgrade, September 25-28, 2005
Ključne besede: chemical processing, high pressure technology, supercritical CO2, lipase catalyzed syntheses, esterification, fructose palmitate, biocatalysts, lipase, Candida antarctica B
Objavljeno v DKUM: 31.05.2012; Ogledov: 2712; Prenosov: 502
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77. Particle formation using supercritical fluids : a short reviewŽeljko Knez, 2006, pregledni znanstveni članek Opis: Particle formation and the design of solid particles and powdery composites with unique properties is at the moment one of major the developments of supercritical fluid (synonyms: dense gases, dense fluids, high pressure) applications. Conventional well-known processes for the particle-size redistribution of solid materials are crushing and grinding (which for some compounds are carried out at cryogenic temperatures), air micronization, sublimation, and recrystallization from solution. There are several problems associated with the above-mentioned processes. Some substances are unstable under conventional milling conditions, in recrysfallization processes the product is contaminated with solvent and waste solvent streams are produced. The application of supercritical fluids may overcome the drawbacks of conventional processes, and powders and composites with special characteristics can be produced. Several processes for the formation and design of solid particles using dense gases have been studied intensively. The unique thermodynamic and fluid-dynamic properties of supercritical fluids can also be used for the impregnation of solid particles, for the formation of solid powderous emulsions, particle coatings, e.g. for the formation of solids with unique properties for use in different applications. This review will focus on the fundamentals and on recent advances of particle formation and design processes using supercritical fluids on their applications and the technological advantages and disadvantages of various processes.
Ključne besede: chemical processing, high pressure technology, supercritical CO2, proteinase, thermal stability, pressure stability, enzyme activity, water content, dense gases
Objavljeno v DKUM: 31.05.2012; Ogledov: 2888; Prenosov: 162
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78. Kinetics of supercritical carbon dioxide extraction of borage and evening primrose seed oilPetra Kotnik, Mojca Škerget, Željko Knez, 2006, izvirni znanstveni članek Opis: In the present work, high-pressure extraction of borage (Borago officinalis L.) and evening primrose (Oenothera biennis L.) seed oil, containing the valuable -linolenic acid (GLA), has been investigated. Extraction was performed with supercritical carbon dioxide on a semi-continuous flow apparatus at pressures of 200 and 300 bar, and at temperatures of 40 and 60 °C. A constant flow rate of carbon dioxide in the range from 0.17 to 0.20 kgžhwas maintained during extraction. The extraction yields obtained using dense CO2 were similar to those obtained with conventional extraction using hexane as solvent. The composition of extracted crude oil was determined by GCanalysis. The best results were obtained at 300 bar and 40 °C for both seed types extracted, where the quality of oil was highest with regard to GLA content. The evening primrose seed oil extracted with supercritical fluid extraction was particularly rich in unsaturated fatty acidsČ up to 89.7 wt-% of total free fatty acids in the oil. The dynamic behavior of the extraction runs was analyzed using two mathematical models for describing the constant rate period and the subsequent falling rate period. Based on the experimental data, external mass transfer coefficients, diffusion coefficients and diffusivity in solid phase were estimated. Results showed good agreement between calculated and experimental data.
Ključne besede: chemical processing, high pressure technology, supecritical CO2, seed oil extraction, evening primrose, borage, free fatty acids, kinetics, modeling
Objavljeno v DKUM: 31.05.2012; Ogledov: 2348; Prenosov: 109
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79. Electrohydraulic pressure control by a self-adjusting proportional derivative controllerDarko Lovrec, Friedrich Faber, 2006, izvirni znanstveni članek Opis: The main characteristic of an electrohydraulic pump control system is the presence of an electrical signal path. It requires at least one sensor for the controlled value (in this case pressure), delivering feedback to be compared with the electrical or numerical reference in the controller. The additonal expense penalty for the sensor and the valve is offset by the advantage of flexibility in the structure and parameters of the controller in comparison to hydraulic - mechanical concepts. Thus, the transfer response can be adapted to demands, even in the operational state. Sophisticated control structures can be implemented easily in the form of computer or microcontroller programs. The layout of electrohydraulic pressure control discussed in this study includes the presented loading unit. The variable pump is designed to act together with a proportional valve having two metering edges. It is less expensive compared to the design with a synchronizing cylinder and four metering edges. However, only inferior static and dynamic performance can be expected. Hydraulic energy is supplied to the control valve directly from the output port of the pump - with the advantage of simple assembly, as there are no additional components required - but with the drawback that there is influence on the control loop`s properties at every change in the disturbance value or reference. The aim of the study presented in this paper is a more efficient and simple concept compared to other known methods for pressure control of the above mentioned pump including the dynamics of the distribution network, and also the characteristics of the hydraulic load with special focus on improvements in steady state and dynamic behaviour.
Ključne besede: automatic control engineering, electrohydraulic systems, pressure control, self-adjusting proportional controller
Objavljeno v DKUM: 31.05.2012; Ogledov: 2291; Prenosov: 141
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80. Laminar flow of a shear-thickening fluid in a 90° pipe bendJure Marn, Primož Ternik, 2006, izvirni znanstveni članek Opis: The non-Newtonian fluid flow in a sharp 90 curved pipe is studied numerically to obtain the pressure loss coefficient prompted by disagreement between the existing empirical correlations and results obtained by computer codes. This disagreement results from presumption of fully developed flow throughout the curvature (correlations) while the actual flow is partially developed for the Newtonian and sharp 90 curved bend non-Newtonian flows, and fully developed for slightly bent 90 curvature non-Newtonian flow. The Quadratic model is employed to accommodate the shear-thickening behavior of an electrostatic ash and water mixture. Numerical results are obtained for different values of Reynolds number. Finally, results for local pressure loss coefficient are compared with values obtained for the Power law rheological model.
Ključne besede: shear-thickening fluid, fluid dynamics, rheological model, pipe bend flow, numerical modelling, pressure losses
Objavljeno v DKUM: 30.05.2012; Ogledov: 1995; Prenosov: 89
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