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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: 2810; Prenosov: 86
URL Povezava na celotno besedilo

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: 1842; Prenosov: 81
URL Povezava na celotno besedilo

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: 1443; Prenosov: 28
URL Povezava na celotno besedilo

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: 1646; Prenosov: 88
URL Povezava na celotno besedilo

Vapor-liquid equilibrium of binary CO2-organic solvent systems (ethanol, tetrahydrofuran, ortho-xylene, meta-xylene, para-xylene)
Željko Knez, Mojca Škerget, Ljiljana Ilić, Christoph Lütge, 2008, izvirni znanstveni članek

Opis: High pressure vapor-liquid phase equilibrium data (P-T-x-y) for the binary mixtures of organic solvent (ethanol, tetrahydrofuran, o-xylene, m-xylene, p-xylene) with CO2 have been measured at temperatures 313.2, 333.2, 353.2 K and pressures from 1 to 14 MPa using a static-analytic method. For the systemsethanol-CO2 and tetrahydrofuran-CO2, the experimental data at 313.2 and333.2 K are in a good agreement with literature data. The experimental results have been correlated by the Peng-Robinson equation of state in combination with van der Waals one fluid mixing rule with two adjustable parameters.
Ključne besede: chemical processing, high pressure technology, carbon dioxide, ethanol, tetrahydrofuran, xylene, vapor-liquid equilibrium, experimental data, PR equation of state
Objavljeno: 01.06.2012; Ogledov: 1738; Prenosov: 105
URL Povezava na celotno besedilo

Enzymatic synthesis of isoamyl acetate with immobilized Candida antarctica lipase in supercritical carbon dioxide
M.P. Romero, L. Calvo, C. Alba, Maja Leitgeb, Mateja Primožič, Željko Knez, 2005, izvirni znanstveni članek

Opis: Isoamyl acetate was successfully synthesized from isoamyl alcohol in supercritical carbon dioxide by enzymatic catalysis. First, the impact of the acyl donor was investigated. Among several reactants, including acetic acid and two different acetates, acetic anhydride gave best yields. Then, two different immobilized lipases (Novozym 435 from Candida antarctica and Lipozyme RM-IM from Rhizomucor miehei) as biocatalysts for the above-mentionedreaction were compared. An esterification extent of 100% was obtained in continuous operation using acetic anhydride as acyl donor and Novozym 435 as enzyme. The amount of enzyme preparation was optimised to 6.25 g/mol alcohol. The effect of substrates load in the solvent was investigated. Operating at a CO2žsubstrates molar ratio below 7.0, the conversion of alcoholdecreased, probably due to an inhibitory effect on enzyme by high concentration of acetic anydride or by produced acetic acid. Pressure in the range of 8-30 MPa showed no effect on this reaction, while an increase in temperature (over 313 K) led to lower production of isoamyl acetate. Novozym 435 was very stable not finding any loss of activity during one month of continuous operation. Finally, carbon dioxide was compared to a conventional organic solvent (n-hexane). Initial reaction rate was higher in SC-CO2, although final esterification extent was similar in both media.
Ključne besede: chemical processing, high pressure technology, superkritical CO2, ester synthesis, isoamyl acetate, enzymatic catalysis, immobilized lipase, isoamyl acetate production, natural flavors
Objavljeno: 01.06.2012; Ogledov: 1631; Prenosov: 100
URL Povezava na celotno besedilo

Stability of proteinase from Carica papaya latex in dense gases
Maja Leitgeb, Mateja Primožič, Željko Knez, 2005, izvirni znanstveni članek

Opis: Proteinase from Carica papaya latex was tested on its thermal stability at atmospheric pressure and in supercritical carbon dioxide, near-critical propane and dimethyl-ether. In supercritical carbon dioxide at 300 bar thermalactivation of the examined proteinase was improved in the comparison toatmospheric pressure. In propane and dimethyl-ether (300 bar) activity of the examined proteinase decreased. Influence of compressionžexpansion cycles on residual activity of the same proteinase in supercritical carbon dioxide (300 bar and 50 °C) was studied, as well. Different ways of transition from supercritical to low-pressure-state were used which affected residual activityof the proteinase.Addition of water in the system increased activity of proteinase from C. papaya, which was incubated in supercritical carbon dioxide for 24 h. Optimum amount of water was found to be between 0.5 and 0.7 g/L.
Ključne besede: chemical processing, high pressure technology, supercritical CO2, proteinase, thermal stability, pressure stability, enzyme activity, water content, dense gases
Objavljeno: 01.06.2012; Ogledov: 1306; Prenosov: 22
URL Povezava na celotno besedilo

Enzymatic reactions in high-pressure membrane reactors
Maja Leitgeb, Mateja Primožič, Željko Knez, 2005, izvirni znanstveni članek

Opis: In the studies on the stability of biocatalysts in a high-pressure batch stirred tank reactor, changes in biocatalyst activity due to pressurizationždepressurization steps were observed. An interesting alternative to overcome this inconvenience is using the high-pressure continuous membrane reactors, where just a single compression and expansion step is necessary. The aim of the research was to carry out the enzyme-catalyzed reactions in high-pressure membrane reactors. At first, high-pressure enzyme-catalyzed hydrolyses of oleyl oleate and sunflower oil were performed in a high-pressure continuous enzymatic flat-shape membrane reactor. Additionally, cellulase from Humicola insolens was covalently immobilized on tubular membrane, and enzyme-catalyzed hydrolysis of carboxy-methyl cellulose was carried out in a high-pressure continuous enzymatic tubular membrane reactor. The membrane-stability tests for flat-shape polysulfon membranes before and after treatment with supercritical CO2 or subcritical propane were done, as well.
Ključne besede: chemical processing, high pressure technology, membrane reactors, biotechnology
Objavljeno: 01.06.2012; Ogledov: 1135; Prenosov: 61
URL Povezava na celotno besedilo

Comparison of the esterification of fructose and palmitic acid in organic solvent and supercritical carbon dioxide
Saša Šabeder, Maja Leitgeb, Željko Knez, 2005, izvirni znanstveni članek

Opis: Sugar fatty acid esters are nonionic surfactants, which are used for personal care products, cosmetic applications, and as emulsifiers for food. In recent years, enzymatic synthesis of sugar fatty acid esters is attracting keen attention as a new manufacturing method for future application. Reaction parameters of lipase-catalyzed synthesis of fructose fatty acid esters in organic solvent were optimized in a batch reactor at atmospheric pressure. Optimum conditions for reaction performed in organic solvent at atmospheric pressure were 10% (wžw of substrates) of lipase from Candida antarctica B, 12.14% (wžw of reaction mixture) of molecular sieves at 60 C, and stirring rate of 600 rpm. Esterification of fructose and palmitic acid was performed insupercritical carbon dioxide with and without addition of cosolvent (organicsolvent) at 60 C. Effect of pressure on enzyme activity was studied.
Ključne besede: chemical processing, high pressure technology, supercritical CO2, esterification, organic solvent, lipase catalysed synthesis, fructose fatty esters, Candida antarctica B, enzyme activity
Objavljeno: 01.06.2012; Ogledov: 1637; Prenosov: 78
URL Povezava na celotno besedilo

Phase equilibira of permethrin and dicofol with carbon dioxide
Petra Kotnik, Amra Perva-Uzunalić, Mojca Škerget, Željko Knez, 2005, izvirni znanstveni članek

Opis: The solubilities of pesticides (permethrin and dicofol) in CO2 were measured by a static-analytic method at pressures ranging from 10.0 MPa to 25.0 MPa andtemperatures of 293.2 K, 303.2 K, and 313.2 K. Due to the nonconventional shape of some solubility isotherms, solid-liquid transitions of pesticides under pressure of CO2 were determined using a modified capillary method. Sincethe solubilities depend on solvent density, the experimental binary solid-fluid equilibrium data were correlated as a function of solvent density by two different models.
Ključne besede: chemical processing, high pressure technology, CO2, pesticides, phase equilibria, solubility
Objavljeno: 01.06.2012; Ogledov: 1339; Prenosov: 27
URL Povezava na celotno besedilo

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