Browsing by Author "Peev G."

Now showing 1 - 11 of 11
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    Antioxidant activity of membrane-fractionated coffee extracts in dependence of the storage conditions
    (2016-11-15) Mitev D.; Peshev D.; Peev G.; Peeva L.
    Present paper aims at one of the important aspects of the application of products with antioxidant activity: namely the preservation and change of their properties during the storage in different conditions, as well as their reliable characterisation. The tests of antioxidant properties were conducted with membrane-separated coffee extracts, isolated using a ``Microdyn Nadir NP030P`` type of commercial nanofiltration membrane (30% retention of NaCl; MWCO∼400). Prepared coffee permeates and retentates were stored 0/10 days in cool/warm conditions, with/without air access and at different illumination conditions. The kinetics of content changes was evaluated according to Folin-Ciocalteu method of total phenolic/reducing content determination.
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    Antioxidant activity of membrane-fractionated coffee extracts in dependence of the storage conditions
    (2016-11-15) Mitev D.; Peshev D.; Peev G.; Peeva L.
    Present paper aims at one of the important aspects of the application of products with antioxidant activity: namely the preservation and change of their properties during the storage in different conditions, as well as their reliable characterisation. The tests of antioxidant properties were conducted with membrane-separated coffee extracts, isolated using a ``Microdyn Nadir NP030P`` type of commercial nanofiltration membrane (30% retention of NaCl; MWCO∼400). Prepared coffee permeates and retentates were stored 0/10 days in cool/warm conditions, with/without air access and at different illumination conditions. The kinetics of content changes was evaluated according to Folin-Ciocalteu method of total phenolic/reducing content determination.
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    Depot effect of bioactive components in experimental membrane filtrations
    (2017-02-06) Mitev D.; Peshev D.; Peev G.; Peeva L.
    Depot effects were found to be accompanying phenomena of membrane separation processes. Accumulation of target species in the membrane matrix during feasibility tests can hamper proper conclusions or compromise the filtration results. Therefore, we investigated the effects of delayed membrane release of chlorogenic acid and caffeine, considered as key compounds of interest in spent coffee products' recovery treatment. Permeate fluxes and key components release were studied in course of 24 hours via nanofiltration of pure solvent, both immediately after the mock solution filtration and after idle stay. Conclusions are drawn and recommendations advised for proper analysis of experimental data on membrane screening.
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    Depot effect of bioactive components in experimental membrane filtrations
    (2017-02-06) Mitev D.; Peshev D.; Peev G.; Peeva L.
    Depot effects were found to be accompanying phenomena of membrane separation processes. Accumulation of target species in the membrane matrix during feasibility tests can hamper proper conclusions or compromise the filtration results. Therefore, we investigated the effects of delayed membrane release of chlorogenic acid and caffeine, considered as key compounds of interest in spent coffee products' recovery treatment. Permeate fluxes and key components release were studied in course of 24 hours via nanofiltration of pure solvent, both immediately after the mock solution filtration and after idle stay. Conclusions are drawn and recommendations advised for proper analysis of experimental data on membrane screening.
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    Extraction of phenolic and flavonoid compounds from solid wastes of grape seed oil production by cold pressing
    (2018-01-01) Saykova I.; Tylkowski B.; Popovici C.; Peev G.
    The objective of this investigation was to assess the advantages of phenolics removal from the grape seeds coldpressing waste in comparison with their extraction from whole seeds. Series of batch solid-liquid experiments were carried out in order to determine: (a) at two temperature levels (25 and 60°C) the most convenient solvent from seven ethanol-water mixtures in the range 0 - 95 % tested (choosing flavonoid extraction capacity as criterion) and the extraction kinetics using the best solvent; (b) the effect of liquid to solid ratio in the range 2.5 - 50 ml g-1 on the total phenolic and flavonoid concentrations in the extracts from pressing waste; (c) the number of extractions necessary to attain a practically complete extraction from whole seeds and pressing waste. The data obtained from these experiments allowed to be evaluated the losses of phenolics and their flavonoid fraction in the course of the cold pressing process and their removal. The radical scavenging properties of the extracts from both sources were also investigated and correlated with the current total phenolic and flavonoid concentrations. The study has shown that the waste of grape seeds cold pressing can be classified as valuable phenol-rich resource. The fast one stage removal (within 20 min) at ambient temperature providing sufficiently high phenolic and flavonoid concentrations at reduced energy and solvent costs compensates for the losses and encourages the combination of seeds cold pressing with the liquid extraction of its solid wastes in industrial practice.
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    How to avoid concentration unsteadiness with a CVD precursor delivery system employing bubbler?
    (2016-11-15) Peev G.; Peshev D.
    An approach of great certainty is suggested as to avoid concentration unsteadiness with a CVD delivery system employing bubbler. Assuming that the carrier gas has to be saturated of precursor vapour during the entire CVD experiment a set of equations has been selected from the literature for calculating the height of liquid layer in the bubbler which is necessary to this aim. The selection is made on the prerequisites that gas bubbles are spherical, diffusion of precursor vapour in them is molecular and it takes place during their rising only, which has a velocity equal to the terminal bubble velocity of rising. The errors at levels lower than the one for saturation are discussed. Heights of Al(CH3)3 calculated using the approach are compared to those at which the bubbler controlling LED indicator of Dow Electronic Materials lights green, amber or red.
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    How to avoid concentration unsteadiness with a CVD precursor delivery system employing bubbler?
    (2016-11-15) Peev G.; Peshev D.
    An approach of great certainty is suggested as to avoid concentration unsteadiness with a CVD delivery system employing bubbler. Assuming that the carrier gas has to be saturated of precursor vapour during the entire CVD experiment a set of equations has been selected from the literature for calculating the height of liquid layer in the bubbler which is necessary to this aim. The selection is made on the prerequisites that gas bubbles are spherical, diffusion of precursor vapour in them is molecular and it takes place during their rising only, which has a velocity equal to the terminal bubble velocity of rising. The errors at levels lower than the one for saturation are discussed. Heights of Al(CH3)3 calculated using the approach are compared to those at which the bubbler controlling LED indicator of Dow Electronic Materials lights green, amber or red.
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    On the concentration unsteadiness of chemical vapour deposition with a precursor sublimated from packed bed of solid source. Problems with multicomponent coatings
    (2013-01-01) Peev G.; Tsibranska I.
    In some CVD processes the precursor is introduced into the reactor as vapor obtained by sublimation from particles of solid source forming packed bed in a column, where through a carrier gas passes. A typical example is the deposition of metal or metal oxide using its carbonyl compound. The study evaluates by mathematical modeling the effect of particle size reduction due to sublimation on the composition of the gas phase entering the reactor. A set of differential equations describing the convective solid to gas mass transfer process in the column is formulated. Equations for precursor equilibrium partial pressure, mass transfer coefficient, diffusivity and axial dispersion coefficient found from literature are added and the model is solved numerically by the method of finite differences. On the example of W(CO)6 the influence of the carrier gas flow rate, temperature and CVD duration on the change of the precursor concentration is revealed. The calculated alterations are significant and may spoil the running experiments. As to prevent this problem three approaches are suggested allowing the maintenance of the reactor inlet precursor concentration in reasonable limits: (i) to use a sufficiently high initial bed for saturation of the carrier gas during the entire particular experiment; (ii) to increase step-bystep in time the bed temperature; (iii) to increase step-by-step in time the carrier gas velocity in the column on the account of its stream by-passing the column. In all three approaches the model allows to calculate the necessary factors, i.e. the sufficient bed height, the temperature and velocity increment in time. Some numerical experiments employing the suggested approaches with approximately constant precursor concentrations are illustrated. Approaches how to proceed when multicomponent coating is to be deposited are discussed. The main conclusion is that the CVD processes should be carefully prepared accounting for a possible unsteadiness and the concentration of the precursor at the reactor inlet should be monitored.. © 2013 The Authors.
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    On the concentration unsteadiness of chemical vapour deposition with a precursor sublimated from packed bed of solid source. Problems with multicomponent coatings
    (2013-01-01) Peev G.; Tsibranska I.
    In some CVD processes the precursor is introduced into the reactor as vapor obtained by sublimation from particles of solid source forming packed bed in a column, where through a carrier gas passes. A typical example is the deposition of metal or metal oxide using its carbonyl compound. The study evaluates by mathematical modeling the effect of particle size reduction due to sublimation on the composition of the gas phase entering the reactor. A set of differential equations describing the convective solid to gas mass transfer process in the column is formulated. Equations for precursor equilibrium partial pressure, mass transfer coefficient, diffusivity and axial dispersion coefficient found from literature are added and the model is solved numerically by the method of finite differences. On the example of W(CO)6 the influence of the carrier gas flow rate, temperature and CVD duration on the change of the precursor concentration is revealed. The calculated alterations are significant and may spoil the running experiments. As to prevent this problem three approaches are suggested allowing the maintenance of the reactor inlet precursor concentration in reasonable limits: (i) to use a sufficiently high initial bed for saturation of the carrier gas during the entire particular experiment; (ii) to increase step-bystep in time the bed temperature; (iii) to increase step-by-step in time the carrier gas velocity in the column on the account of its stream by-passing the column. In all three approaches the model allows to calculate the necessary factors, i.e. the sufficient bed height, the temperature and velocity increment in time. Some numerical experiments employing the suggested approaches with approximately constant precursor concentrations are illustrated. Approaches how to proceed when multicomponent coating is to be deposited are discussed. The main conclusion is that the CVD processes should be carefully prepared accounting for a possible unsteadiness and the concentration of the precursor at the reactor inlet should be monitored.. © 2013 The Authors.
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    Potential possibilities of a new method for yield stress determination
    (2016-01-01) Peev G.; Peshev D.; Nikolova A.
    One-D solution for the hydrodynamic characteristics of waveless film flow of an Ellis liquid on a horizontal rotating disk is obtained and considered for determination of the liquid yield stress. Several versions of the method based on the film thickness of the remaining liquid on the disk at a zero feed flow rate are proposed. Verification that such layer remains is provided. The limitations of the method advanced which refer to wave formation, film breakup, liquid adhesion and disk wetting are discussed in view of its application in rheometry.