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Pressure stability of lipases and their use in different systems
Maja Leitgeb, Željko Knez, 2001, izvirni znanstveni članek

Opis: For the investigation of the solvent impact on the enzymes, lipases from different sources (Pseudomonas fluorescences, Rhizopus javanicus, Rhizopus niveus, Candida rugose and Porcine pancreas) were used. Stability and activity of these lipases in aqueous medium in supercritical $CO_2$ and liquid propane at 100 bar and 40°C were studied. On the basis of previous results lipases were used for their application in two different systems. The application of the polysulphone membrane in the continuous stirred tank membrane reactor was studied on the model system of the hydrolysis of oleyl oleate in propane at high pressure. As a catalyst the Candida rugosa lipase was used. The next utilization of lipases was the use of on silica arerogel self-immobilized lipase from Porcine pancreas as catalyst for esterification reaction in near-critical propane at 40°C and 100 bar.
Ključne besede: chemical processing, supercritical fluids, lipases, enzyme stability, high pressure membrane reactor
Objavljeno: 10.07.2015; Ogledov: 782; Prenosov: 92
.pdf Celotno besedilo (608,04 KB)
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Electrochemical impedance spectroscopy as a tool in the plate making process optimization
Tomislav Cigula, Regina Fuchs-Godec, Miroslav Gojo, Mojca Slemnik, 2012, izvirni znanstveni članek

Opis: The structure of the porous aluminium-oxide layer, which builds non-image areas, has the most significant influence on the quality of final graphical product. This paper presents the results of the application of EIS in the characterisation and detection of changes on the aluminium-oxide layer caused by chemical processing in highly alkaline solution. The Al2O3 layer was characterised using SEM, fractal dimension and surface free energy calculationand EIS analysis. The results of the investigation showed that chemical processing has a significant influence on the structure of aluminiumoxide which could lead to a decrease in the quality of the printing plate. EIS enables the detection of changes on the aluminium- oxide layer. Thetwo equivalent circuits are proposed. Based on modelling with the obtained EIS spectra, precise evaluation of developing time in which complete removal of the photoactive layer is achieved. This makes EIS a powerful tool in optimizing chemical processing of lithographic printing plates.
Ključne besede: printing plate, chemical processing, SEM, surface free energy, EIS
Objavljeno: 10.07.2015; Ogledov: 724; Prenosov: 65
.pdf Celotno besedilo (266,51 KB)
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Particles formation and particle design using supercritical fluids
Željko Knez, Eckhard Weidner, 2003, pregledni znanstveni članek

Opis: Major recent advances. Particle formation and design of solid particles and powdery composites with unique properties is at the moment a major developmentof supercritical fluids (synonymsČ dense gasses, dense fluids, highpressure) applications. This review will focus on recent advances and on fundamentals of these processes and their applications.
Ključne besede: chemical processing, high pressure technology, micronization, supercritical fluids, micro-particles, nano-particles, powder generation, crystallization, particles from gas saturated solutions
Objavljeno: 01.06.2012; Ogledov: 2683; Prenosov: 83
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Multifunctional composites by high-pressure spray processes
Eckhard Weidner, Marcus Petermann, Željko Knez, 2003, pregledni znanstveni članek

Opis: A series of spraying processes designed to generate powders and composites using supercritical fluids have been proposed in the past 15 years. In this review, thermo- and fluid-dynamic aspects and engineering principles are discussed and advantages of such technologies are demonstrated. These new techniques display convincing advantages, producing competitive high-quality products with tailor-made properties. Initial industrial applications have been achieved in the production of food products and fine-chemicals.
Ključne besede: chemical processing, high pressure technology, micronization, supercritical fluids, particles, powder generation, thermodynamics, fluid dynamics, PGSS
Objavljeno: 01.06.2012; Ogledov: 1744; Prenosov: 77
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Silica aerogels as support for lipase catalyzed esterifications at sub- and supercritical conditions
Zoran Novak, Maja Leitgeb, Vlasta Krmelj, Željko Knez, 2003, izvirni znanstveni članek

Opis: The enzymes (lipases from Candida rugosa and porcine pancreas) were immobilized on silica aerogels by sol-gel procedure followed by supercritical drying with CO2. Such immobilized enzymes were used as biocatalysts for esterification in supercritical CO2 and near critical propane at 40 °C and 100 bar. It was found out that the initial reaction rates in propane rose two to three times in comparison with the same reaction, catalyzed by free lipase. SC CO2 deactivated the non-immobilized lipase in reaction mixture while with the immobilized enzyme the conversion was 35%. The initial reaction rates in propane were 20 times higher than in water medium due to the properties of propane as a medium for esterification of fatty acids.
Ključne besede: chemical processing, biotechnology, esterification, immobilization of enzymes, supercritial CO2, propane, lipases, supercritical CO2 drying
Objavljeno: 01.06.2012; Ogledov: 1249; Prenosov: 17
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Phase equilibria in systems containing [alpha]-tocopherol and dense gas
Mojca Škerget, Petra Kotnik, Željko Knez, 2003, izvirni znanstveni članek

Opis: Solubility of ▫$alpha$▫-tocopherol in CO2 and propane was determined at temperatures 303, 313, 333 and 353 K and over a pressure range from 79 to 286 bar for CO2 and 16 to 112 bar for propane. A static-analytic method was applied. The solubility of ▫$alpha$▫-tocopherol in dense CO2 under the conditions investigated was in the range of 0.2-17.0 mg/g CO2. The maximal solubilities of ▫$alpha$▫-tocopherol in propane were approximately up to ten times higher than in CO2 and were in the range from 38.9 to 171.9 mg/g propane. Phase equilibrium data for Milk Thistle seed oil in supercritical CO2 were determined at temperatures 313, 333 and 353 K and pressures ranging from 100 to 300 bar. The oil was previously additionally vitaminized and contained 1.9 wt.% of ▫$alpha$▫-tocopherol and 1.7 wt.% of free fatty acids (FFA). The solubility of oil in CO2 was in the range from 1.3 to 17.9 mg/g CO2. The distribution coefficients of ▫$alpha$▫-tocopherol and FFA between light and heavy phase were determined on a solvent free basis. The obtained separation factors increased with temperature and pressure to approximately 160-200 bar and decreased with a further increase of pressure.
Ključne besede: chemical processing, supercritical fluids, phase equilibria, separation factors, vitamin E, CO2, dense gases
Objavljeno: 01.06.2012; Ogledov: 1223; Prenosov: 26
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Preparation of WO[sub]3 aerogel catalysts using supercritical CO[sub]2
Zoran Novak, Petra Kotnik, Željko Knez, 2004, izvirni znanstveni članek

Opis: Single tungsten oxide aerogels (WO3), binary oxide aerogels (WO3-Al2O3) and ternary oxide aerogels (WO3-SiO2-Al2O3) were prepared using standard sol-gel route. Tungsten oxide tetraethoxide (WO(OCH2CH3)4) was used as the sol-gel precursor. The excellent properties of the gels obtained by the sol-gel synthesis were preserved upon supercritical drying with CO2. After supercritical drying at 40 °C and 100 bar, all aerogels were calcined to 800 °C. The influence of the synthesis parameters on the catalytic activity of WO3as supported on silica andžor alumina aerogels was investigated through thetransformation of N-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine. Despite including WO3 into single and mixed silicaand alumina aerogels, high specific surface areas (284-653 m2 g-1) were preserved. Higher conversion was obtained for catalysts with higher ratios of WO3 in the mixed silica-alumina aerogels that were calcined at 800 °C.
Ključne besede: chemical processing, aerogels, catalysts, tungsten oxides, preparation of aerogels, characterisation of aerogels, supercritical CO2, supercritical drying, high pressure technology
Objavljeno: 01.06.2012; Ogledov: 1335; Prenosov: 26
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Isolation and concentration of natural antioxidants with high-pressure extraction
Majda Hadolin Kolar, Andreja Rižner Hraš, Davorin Bauman, Željko Knez, 2004, izvirni znanstveni članek

Opis: In present work, the purification of crude rosemary extract with supercritical fluids is presented. Carbon dioxide was used as a solvent. The crude extract was prepared from rosemary with conventional extraction process. The supercritical CO2 extraction of crude extracts was performed at pressures 10 and 20 MPa and temperatures 35 and 60 °C. The best results were obtained at pressure 10 MPa and temperature 35 °C. The content of carnosic acid in the samples before and after high pressure extraction was identified by high performance liquid chromatography. The content of carnosic acid in purified extract is higher compared to crude extract. The antioxidative efficiency of extracts was determined by measuring peroxide value. Activity ofpurified rosemary extracts is higher compared to crude extracts.
Ključne besede: chemical processing, high pressure technology, extraction, CO2, natural antioxidants, rosemary extract, food additives
Objavljeno: 01.06.2012; Ogledov: 1507; Prenosov: 84
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A strategy for MINLP synthesis of flexible and operable processes
Zorka Novak-Pintarič, Zdravko Kravanja, 2004, izvirni znanstveni članek

Opis: Abstract This paper presents a sequential two-stage strategy for the stochastic synthesis of chemical processes in which flexibility and static operability (the ability to adjust manipulated variables) are taken into account. In the first stage, the optimal flexible structure and optimal oversizing of the process units are determined in order to assure feasibility of design for a fixed degree of flexibility. In the second stage, the structural alternatives and additional manipulative variables are included in the mathematical model in order to introduce additional degrees of freedom for efficient control. The expected value of the objective function is approximated in both stages by a novel method, which relies on optimization at the central basic point (CBP). The latter is determined by a simple set-up procedure based on calculations of the objective functionćs conditional expectations for uncertain parameters. The feasibility is assured by simultaneous consideration of critical vertices. The important feature of the proposed stochastic model is that its size depends mainly on the number of design variables and not on the number of uncertain parameters. The strategy is illustrated by two examples for heat exchanger network synthesis.
Ključne besede: chemical processing, process synthesis, MINLP, mixed integer nonlinear programming, flexibility, operability, controllability, steady state model
Objavljeno: 01.06.2012; Ogledov: 1748; Prenosov: 69
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Pebax©/Polyethlylene glycol blend thin film composite membranes for CO2 separation : performance with mixed gases
Anja Car, Črtomir Stropnik, Wilfredo Yave, Klaus-Viktor Peinemann, 2008, izvirni znanstveni članek

Opis: The paper describes the performance of Pebax©/Polyethylene glycol (PEG) blend thin film composite membranes for CO2 separation from gas mixtures containing H2, N2 and CH4. Membranes were tested at different conditions; temperature andpressure dependence of gas flux and selectivity were explored. The temperature dependence was correlated with the Arrhenius equation to determinethe activation energy of single gas permeation. Single and mixed gas permeation was measured for different pressures at 293K up to 20 bar. Improvedpermeabilities and CO2/H2 selectivities were obtained in the newly developed composite membranes.
Ključne besede: chemical processing, membranes, membrane separation, CO2, Pebax, thin film composite membranes, gas separation
Objavljeno: 01.06.2012; Ogledov: 1541; Prenosov: 23
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