Search Results

You are looking at 1 - 10 of 18 items for :

  • "activation energy" x
  • Refine by Access: Content accessible to me x
Clear All

energy(Δ E ) are used. Often these have positive values that, in a notation of exp(Δ E/RT ), can result in a misunderstanding (a negative value of activation energy). Using relation ( 17 ) and satisfying condition ( 8 ), one can derive the linear form

Open access

-step reaction [ 6 , 7 ], in which the Arrhenius parameters (activation energy and pre-exponential factor) vary with dehydration degree. Therefore, it's difficult to use a single pair of Arrhenius parameters to reveal the chemical mechanism and to predict the

Open access

methods [ 6 – 8 ]. These investigations enable the activation energies for decomposition, decomposition rate constants at different temperatures, and thermal ranges of decomposition to be determined. The kinetic data enables the conditions of

Open access

, formulas in forms ( 14 ) and ( 15 ) were used. Each analysis of compounds (CaC 2 O 4 ·H 2 O and CaCO 3 ) was preceded by determination of activation energy according to modified Kissinger law in version [ 17 ]: 16 to confirm whether own data are

Open access

heating rates or a set of isothermal measurements with different temperatures. We will consider here the set of different heating rates, because it can be analyzed by all model-free methods. Model-free analysis allows to find activation energy of

Open access
Journal of Flow Chemistry
Authors: Dominique Roberge, Christian Noti, Eberhard Irle, Markus Eyholzer, Bruno Rittiner, Gerhard Penn, Gottfried Sedelmeier, and Berthold Schenkel

After a short section of safety aspects related to 2-nitroethanol, the paper describes a powerful methodology for developing flow processes based on a proof of concept (1), an optimization and modeling analysis (2), and a long run study in a mini-plant (3). The proof of concept is the initial stage where the solubilities and concentrations are fixed, taking into account the rough kinetics with a mass transfer understanding. It is followed by a complete kinetic analysis including activation energy to model the reaction under various conditions to optimize different targets (yield not being the only driver!). The last section shows the operation of a mini-plant including a microreactor and work-up unit operations. The approach is extremely powerful as it enables the study at laboratory scale of all the features that are usually associated with a pilot plant namely: stability over time on stream, solvent recirculation, model prediction, and robustness.

Open access

Summary

The major objective of the present work was to compare the kinetic study of alcoholic fermentations conducted in the presence of wheat supported biocatalysts in laboratory scale and in a scale-up system of 80 L and to compare these results with those reported in literature. The kinetic study of fermentation processes was accomplished with the technique of reversed flow gas chromatography (RFGC), which is a version of inverse gas chromatography. The wine yeast species used was Saccharomyces cerevisiae AXAZ-1, and fermentations were conducted between 20 and 2°C. At low temperatures, maximal ethanol productivity and fermentation rate were reduced. The rate constants, determined through a mathematical model obtained from RFGC, were higher in the laboratory scale comparing to the scale-up system at the temperatures of 20 and 15°C. However, with the reduction of temperature, both systems presented almost similar values proving the great fermentative ability of immobilized cells even at extremely low temperatures. Activation energies of the alcoholic fermentations in the two systems presented their higher values at the second phase (stationary) compared to those observed at the other two phases (growth and decline).

Full access

Summary

A simple, sensitive, specific, precise, and stability-indicating high-performance liquid chromatographic (HPLC) method for determination of cefpodoxime proxetil as bulk drug and as pharmaceutical formulation was developed and validated as per the International Conference on Harmonization (ICH) guidelines. An isocratic separation was achieved using a Phenomenex Luna C18 (250 mm × 4.6 mm i.d., 5 μm particle size) column with a flow rate of 1 mL min−1 and a UV detector to monitor the eluate at 254 nm. The mobile phase consisted of acetonitrile and 50 mM ammonium acetate pH 6 (pH was adjusted with o-phosphoric acid) in the ratio of 45:55 (υ/υ). The linear regression analysis data for the calibration plots showed good linear relationship with r 2 = 0.9998 in the working concentration range of 1–80 μg mL−1. The LOD and LOQ were 0.17 and 0.5 μg mL−1, respectively. The drug was subjected to acid and alkali hydrolysis, oxidation, dry heat, wet heat treatment, and photodegradation. The standard drug peaks were well resolved from the degradation products' peaks with significantly different retention time (t R), and the resolution factor for cefpodoxime proxetil was found to be greater than 1.7. As the method could effectively measure the drug in the presence of all degradation products and excipients expected to be present in the formulation, it can be employed as a specific stability-indicating method. Moreover, the proposed HPLC method was utilized to investigate the kinetics of the acidic and oxidative degradation processes at different temperatures. An Arrhenius plot was constructed and the apparent pseudo-first-order rate constant, half-life and activation energy were calculated.

Full access

energy has been derived from the Arrhenius plot on the base of mean values of reaction rate constants from experiments at various temperatures. The values of activation energy E a and the values of pre-exponential factor k 0 are given in

Open access
Journal of Thermal Analysis and Calorimetry
Authors: Lech Nowicki, Anna Antecka, Tomasz Bedyk, Paweł Stolarek, and Stanisław Ledakowicz

-exponential factor and activation energy, respectively, and n is the reaction order in respect to the concentration of gasifying agent. Many different functions can be used to describe changes in the physical properties of the solid comprised in the r s

Open access