Comparative analyses on isothermal kinetics of water evaporation and hydrogel dehydration by a novel nucleation kinetics model

A comparative analysis of the isothermal kinetics of water evaporation and equilibrium swollen poly(aciylic-g-gelatin) hydrogel (PAAG) dehydration was performed. The isothermal thermogravimetric (TG) curves of water evaporation and PAAG hydrogel dehydration were obtained in temperature range from 313 K to 353K, at the same conditions. We found that the kinetics of water evaporation could be mathematically described by the model of zero order chemical reaction. The values of the kinetic parameters: E-e,E-a = 29 kJ/mol and ln(A(e)/min = 12.1) for water evaporation were calculated. By the application of the isoconversional method it was found that water evaporation was a kinetically complex process. It was shown that the kinetics of hydrogel dehydration could be described by a linear combination of logistic functions. A novel model, called the nucleation model, able to describe the kinetics of hydrogel dehydration and water evaporation was presented. The rate constants values for nucleation and the multiplication of the nuclei centers (bubbles) at different temperatures were calculated. The values of activation energy of the multiplication of the nuclei centers were calculated. The difference found between the kinetics of water evaporation and hydrogel dehydration was explained by different values for the constant rates of nucleation and multiplication of the nuclei centers during the water evaporation and hydrogel dehydration

Comparison of adsorbent materials for herbicide diuron removal from water

Diuron herbicide is a persistent and frequently detected compound in surface and groundwater. In this work, a comparative study of the performance of four types of adsorbent (two precipitated silica, zeolite Y and carbon molecular sieves (CMS)) for removing diuron from water was carried out. Precipitated silica samples were obtained by controlled precipitation of SiO2 with sulphuric acid from water glass; zeolite Y was synthesized by a microwave-assisted hydrothermal method; CMS were synthesized by the method of controlled pyrolysis of wheat straw and chemical vapour deposition of organic matter. It was found that adsorption isotherms for precipitated silica and zeolite type Y are of linear shape, whereas the one for CMS exhibits at low concentrations a concave shape followed by an inflection point which suggests cooperative adsorption and the formation of adsorbate multilayers. CMS adsorbent showed the highest adsorption capacity and this was likely due to its high hydrophobicity. Temperature has a negligible effect on precipitated silica and zeolite Y while it strongly affects adsorption properties of CMS. Adsorption is not thermodynamically favoured at high temperature. An unexpected decrease in the rate of adsorption was observed with increase in temperature. This is possibly related to the different adsorption behaviour of diuron conformers and aggregates

Comparative kinetics of the alkali-catalyzed sunflower oil methanolysis with co-solvent under conventional and microwave heating with controlled cooling

Abstract The kinetics of the alkali-catalyzed transesterification of sunflower oil with methanol in the presence of co-solvent (TSMPC) were investigated. The kinetics curves of the alkali-catalyzed TSMPC, in the temperature range of 26°C–55°C, were measured for conventional heating (CH) and microwave heating with controlled cooling. The results showed that for both heating modes, the kinetics of the alkali-catalyzed TSMPC reaction can be described with the kinetic model of the pseudo first-order reaction with respect to the concentration of the triglycerides. The values of apparent reaction rate constants, activation energies, and pre-exponential factors are also calculated. The existence of a linear correlation (compensation effect) between the values of apparent kinetic parameters determined for CH and microwave heating with controlled cooling conditions is established. The results confirmed that the increase in the transesterification rate in the microwave heating with controlled cooling conditions is not caused by overheating nor by the existence of hotspots. The model of mechanism of the impact of microwave heating on the kinetics of transesterification is hereby proposed.</jats:p