Browsing by Author "Radeva G."
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Item A correlation between the kinetic and thermodynamic adsorption characteristics of an optical brightener on a pulp surface(2008-01-01) Radeva G.; Veleva S.; Valcheva E.An attempt to find a correlation between the kinetic and thermodynamic terms (e.g. activation energy, pre-exponential factor, isosteric heat, entropy, etc.) of the chemisorption process involved in a commercial optical brightener/pulp system is presented. A linear inter-relationship between the kinetic and thermodynamic characteristics of the system was found. Based on the coefficients obtained in the thermodynamic investigation of the adsorption process, it was possible to calculate the kinetic coefficient of the surface inhomogeneity. The use of both a model employing uniformly inhomogeneous surfaces and an approach involving all the kinetic and thermodynamic data was confirmed by the results obtained.Item A correlation method in studying chemisorption processes on uniformly inhomogeneous surfaces(2010-03-01) Radeva G.; Valcheva E.; Veleva S.Chemisorption processes taking place on uniformly inhomogeneous surfaces in the presence of entropic inhomogeneity are examined. A new form of linearity relationship connecting the current rate and the equilibrium constant at definite degrees of surface coverage is derived. The relationship can be applied to complex chemisorption processes when determination of the rate constant is impossible. Linear correlations between the activation energy and the adsorption enthalpy, as well as between the entropy characteristics and pre-exponential factors, are obtained. The complex effect of the energy and entropy factors on the kinetics and the equilibrium is discussed theoretically and verified experimentally. These factors operate through a thermodynamic and a kinetic compensation effect. The present paper illustrates the correlation methods employing experimental results derived from the study of the chemisorption of the optical brighteners Leucophor AP and Tinopal UP onto pulp, of Fe 3+ and Cu2+ ions onto fibrous polyampholyte and of hydrolysis-modified polyacrylonitrile (HMP) onto pulp.Item Compensation effect in the kinetics of thermal aging of semi chemical pulp(2014-01-01) Boeva R.; Radeva G.The effect of thermal ageing of neutral sulfite semi-chemical pulp derived from poplar wood is followed on the ground of the comparative investigation of bleached and unbleached samples. The brightness reversion observed in the course of the process is studied at 90°C, 105°C and 120°C. The degree of brightness is determined at the 6-th, 12-th, 24-th and the 36-th hour of the process duration. The kinetic analysis carried out shows that the exponential kinetic equation valid for processes taking place on uniformly inhomogeneous surfaces describes the kinetics of the process investigated. The linear relation between the apparent pre-exponential factor and the apparent activation energy, as obtained from an Arrhenius analysis, indicates the existence of a compensation effect.Item Effect of bleaching procedure type on the aging resistance of chemical-mechanical pulp(2016-01-01) Boeva R.; Radeva G.Chemical-mechanical pulping methods are for producing pulp from high-yield fiber materials derived from poplar wood. The samples are subjected to a single-stage bleaching with H2O2 and a two-stage procedure combining the effect of H2O2 and Rongalyt C. A kinetic investigation of the bleaching pretreatment effect on the thermal ageing of chemical-mechanical pulp is presented. Comparative investigations are carried out with pulp chips which were not subjected to preliminary bleaching. The brightness reversion is followed within the interval from 6 to 48 hours at constant temperature. The ageing process is described by an exponential kinetic equation. The initial and the current rates of the process are determined. It is found that they decrease with increase of the extent of the process proceeding. Microscopic images of the samples are taken after preparation, bleaching and ageing, correspondingly.Item Investigation on the kinetics of coated paper thermal aging(2019-01-01) Todorova D.; Lasheva V.; Radeva G.; Cenkova R.The paper aging is primarily attributed to the presence of carbonyl groups formed as a result of the oxidation of the hydroxyl groups at the second and third carbon atom in the elementary unit of the cellulose macromolecule and the presence of both aldehyde and carboxyl groups in the cellulose materials. Reduction of paper whiteness degree is mainly connected with the presence of lignin, hemicelluloses, the presence of metals and the sizing agent type. The paper as an elasto-plastic, capillary porous material quickly breaks down at an elevated temperature. The latter accelerates the chemical reactions that occur in paper in the course of aging. The depth of the paper changes depends not only on the temperature but also on the duration of the impact. For different paper types differing in composition the reaction rate constant is different. Its temperature dependence is described by the Arenius equation. The aim of this work is to carry out a kinetic analysis of the influence of the temperature increase on the ageing of coated paper used for book covers. The accelerated thermal aging is conducted at three different temperatures-90°C, 105°C and 120°C with duration of 6 h, 12 h, 18 h, 36 h and 48 h. In order to examine the kinetics of the process the level of whiteness is determined. The data obtained from the thermal aging of the paper are treated using the laws of polychrome kinetics.Item The role of the entropy factor on the adsorption of an optical brightener on pulp(2009-09-21) Radeva G.; Valcheva E.; Veleva S.The present paper reports results of the thermodynamic study of adsorption of 2 optical brighteners (Tinopal UP and Leucophor AP) on pulp. The entropy non-homogeneity of the pulp surface was confirmed and the coefficients of entropy non-homogeneity were determined. The entropy effect on the variation of Gibbs free energy of the process studied was followed. © Tübi̇ tak.Item Valorization of a plant β-amylase: Immobilization and dataset on the kinetic process(2018-02-01) Lahmar I.; Radeva G.; Marinkova D.; Velitchkova M.; Belghith H.; Ben Abdallah F.; Yotova L.; Belghith K.The data presented in this article are related to the research article titled “Immobilization nd topochemical mechanism of a new β-amylase extracted from Pergularia tomentosa” (Lahmar et al., 2017) [1]. This article documented information on the determination of the molecular weight of the β-amylase, the method of its immobilization and a comparison of the kinetic mechanism between the free and the immobilized forms by a mathematical method. Fresh Pergularia tomentosa was collected from Tunisia and a special method for β-amylase extraction was followed (Yotova et al., 2000) [2]. Public dissemination of this dataset will allow further analyses of the data.