Kinetics Of Thermal Dehydration Research Articles

The physico-geometrical reaction pathway and kinetics of multistep thermal dehydration of calcium chloride dihydrate in a dry nitrogen stream.

Several inorganic hydrates exhibit reversible reactions of thermal dehydration and rehydration, which is potentially applicable to thermochemical energy storage. Detailed kinetic information on both forward and reverse reactions is essential for refining energy storage systems. In this study, factors determining the reaction pathway and kinetics of the multistep thermal dehydration of inorganic hydrates to form anhydride via intermediate hydrates were investigated as exemplified by the thermal dehydration of CaCl2·2H2O (CC-DH) in a stream of dry N2. The formation of CaCl2·H2O (CC-MH) as the intermediate hydrate is known during the thermal dehydration of CC-DH to form its anhydride (CC-AH). However, the two-step kinetic modeling based on the chemical reaction pathway considering the formation of the CC-MH intermediate failed in terms of the reaction stoichiometry and kinetic behavior of the component reaction steps. The kinetic modeling was refined by considering the physico-geometrical reaction mechanism and the self-generated reaction conditions to be a three-step reaction. The multistep reaction was explained as comprising the surface reaction of the thermal dehydration of CC-DH to CC-AH and subsequent contracting geometry-type reactions from CC-DH to CC-MH and from CC-MH to CC-AH occurring consecutively in the core of the reacting particles surrounded by the surface product layer of CC-AH. The acceleration of the linear advancement rate of the reaction interface during both contracting geometry-type reactions was revealed through multistep kinetic analysis and was described by a decrease in the water vapor pressure at the reaction interface as the previous reaction step proceeded and terminated.

Thermal dehydration of calcium sulfate dihydrate: physico-geometrical kinetic modeling and the influence of self-generated water vapor.

Complex kinetic behaviors in the thermal dehydration of CaSO4·2H2O under varying water vapor pressure (p(H2O)) conditions impel researchers in the field of solid-state kinetics to gain a more comprehensive understanding. Both self-generated and atmospheric p(H2O) are responsible for determining the reaction pathways and the overall kinetic behaviors. This study focuses on the influence of the self-generated water vapor to obtain further insights into the complexity of the kinetic behaviors. The single-step mass-loss process under conditions generating a low p(H2O) was characterized kinetically by a physico-geometrical consecutive induction period, surface reaction, and phase boundary-controlled reaction, along with the evaluation of the kinetic parameters for the individual physico-geometrical reaction steps. Under the conditions in which more p(H2O) was generated, the overall reaction to form the anhydride was interpreted as a three-step process, comprising the initial reaction (direct dehydration to the anhydride) and a subsequent two-step reaction via the intermediate hemihydrate, which was caused by the variations in the self-generated p(H2O) conditions as the reaction advanced. The variations in the reaction pathways and kinetics behaviors under the self-generated p(H2O) conditions are discussed through a systematic kinetic analysis conducted using advanced kinetic approaches for the multistep process.

Non-Isothermal Dehydration Kinetics of Diphasic Mullite Precursor Gel

Aluminosilicate gel precursor having mullite composition was synthesized from inorganic salts of aluminum and silicon by employing the sol-gel method. Chemical analysis, surface area, and bulk density measurements were performed to characterize the dried gel. The course of the palletization was examined by FT-IR analysis which confirmed the diphasic nature of the gel. SEM and XRD analysis were performed to study microstructure and phase development. ThermoGravimetric (TG) analysis of the dried gel was performed at multiple heating rates and from the results obtained; kinetics of thermal dehydration was studied by applying Friedman differential and Kissinger-Akahira-Sunose integral isoconversional procedures. It was observed that the total dehydration process of the gel was accomplished by two different stages and both the stages followed second-order rate kinetics. The first stage was assigned to the dehydration of silicon hydroxide gel whereas the second stage was associated with aluminum hydroxide gel dehydration.

A thermogravimetric study of thermal dehydration of copper hexacyanoferrate by means of model-free kinetic analysis

The kinetics of thermal dehydration of K2x/3Cu[Fe(CN)6]2/3·nH2O was studied using thermogravimetry for x = 0.0 and 1.0. Data from both non-isothermal and isothermal measurements were used for model-free kinetic analysis by the Friedman and KAS methods. The water content was determined to be n = 2.9–3.9, plus an additional ~10% of water, likely surface adsorbed, that leaves very fast when samples are exposed to a dry atmosphere. The determined average activation energy for 0.2 ≤ α ≤ 0.8 is 56 kJ mol−1. The dehydration is adequately described as a diffusion-controlled single-step reaction following the D3 Jander model. The determined dehydration enthalpy is 11 kJ (mol H2O)−1 for x = 0.0 and 27 kJ (mol H2O)−1 for x = 1.0, relative to that of water. The increase with increasing x is evidence that the H2O molecules form bonds to the incorporated K+ ions.

Kinetic Studies on the Thermal Dehydration and Degradation of Chitosan and Citralidene Chitosan

The thermal dehydration and degradation of chitosan and citralidene chitosan was studied by differential scanning calorimetry at four different heating rates; 5, 10, 15 and 20 K min−1. The kinetics of thermal dehydration and degradation of chitosan and citralidene chitosan was investigated using different isoconversional and maximum rate (peak) methods viz. Kissinger–Akahira–Sunose (KAS), Tang, Starink1.95, Starink1.92, Flynn–Wall–Ozawa (FWO) and Bosewell. The activation energy values of thermal dehydration and degradation reactions obtained from isoconversional methods of FWO and Bosewell are slightly higher than that obtained from other methods. The variation of activation energy, E α with conversion function, α, established using these different methods were found to be similar. Compared to the FWO method, the KAS method offers a significant improvement in the accuracy of the E a values. All the maximum rate (peak) methods yielded consistent values of E α for the dehydration and degradation reactions of both chitosan and CIT-chitosan

Kinetics of thermal dehydration of sol-gel derived MgO–ZrO2 composite hydrogel

Abstract The kinetics of thermal dehydration of mixed hydroxide hydrogels in MgO–ZrO2 system was studied as a function of composition by following the isothermal heat treatment route. Kinetic parameters were calculated through the application of the Guggenheim equation. The expulsion of both loosely bound water and constitutional OH groups were not continuous processes but proceeded in steps. The applicability of 1st order reaction kinetics for the major portion of the reaction for all compositions suggests that during the dehydroxylation process the dehydration is essentially controlled by the orientation of H2O molecules and the mutual interaction of hydroxyl groups. Due to a decrease in the concentration of the reacting species the activation energy was always higher at the final stage of dehydration.

Kinetics of Thermal Transformation of Mg3(PO4)2 · 8H2O to Mg3(PO4)2

The kinetics of thermal dehydration of Mg3(PO4)2 · 8H2O was investigated using thermogravimetry at four different heating rates. The activation energies of the dehydration step of Mg3(PO4)2 · 8H2O were calculated through the isoconversional Ozawa and Kissinger-Akahira-Sunose (KAS) methods and iterative methods, which were found to be consistent and indicate a single mechanism. The possible conversion function of the dehydration reaction for Mg3(PO4)2 · 8H2O has been estimated through the Coats and Redfern integral equation, and a better kinetic model such as random nucleation of the “Avrami–Erofeev equation (A 3/2 model)” was found. The thermodynamic functions (ΔH*, ΔG*, and ΔS*) of the dehydration reaction are calculated by the activated complex theory and indicate that it is a non-spontaneous process when the introduction of heat is not connected.

A comparative study of non-isothermal kinetics of decomposition of calcium oxalate monohydrate

The kinetics of thermal dehydration and decomposition CaC 2O 4·H 2O was studied using thermogravimetry at four rates of linear increase of temperature and two different sample mass. Four calculation procedures based on single TG curves and iso-conversional method, as well as 35 mechanism functions were used. The comparison of the results obtained with these calculation procedures showed that they strongly depend on the selection of proper mechanism function for the process. Therefore, it is very important to determine the most probable mechanism function. In this respect the iso-conversional calculation procedure turned out to be more appropriate. In the present work, the values of apparent activation energy, pre-exponential factor in Arrhenius equation, as well as the changes of entropy, enthalpy and free Gibbs energy for the stages of dehydration, decarbonylation and decarbonation of calcium oxalate monohydrate are calculated. All calculations were performed using programs compiled by ourselves.

Role of some transition metal cations on the kinetics of thermal dehydration of synthetic zeolites

Zeolite was synthesized by the interaction of sodium silicate and sodium aluminate solution. The gel formed was purified and physico-chemically characterized. The gel was converted into different cationic forms, like, Ni 2+, Co 2+, Cd 2+, etc., by ion exchange process. Isothermal dehydration kinetics of the hydrogel was studied from thermo-gravimetry. The gel dehydration reaction was observed to be low energy diffusion controlled process and the major part of the process followed first order kinetics. Exchangeable cations affected the kinetic parameters of the dehydration process.

Effect of heat treatment on properties of steam cured fly ash-lime compacts

Four different varieties of class F fly ashes, collected from different sources from the state of West Bengal (India) were mixed with lime in 9 : 1 wt ratio, followed by compaction of the mixes. The compacts were subjected to steam curing to develop an optimum strength by the reaction between fly ash and lime. The steam cured compacts were heated at different elevated temperatures and free lime content, compressive strength, bulk density and water absorption tendency of these compacts were measured and FTIR spectral changes were studied as a function of the heating temperatures. Kinetics of thermal dehydration of the compacts was also studied from thermogravimetric measurements under non-isothermal condition to ascertain the order of dehydration process and the associated activation energy.

Kinetics of Thermal Dehydration and Decomposition of Hydrated Chlorides of Some 3d Transition Metals (Mn—Cu Series). Part 5. Dehydration and Decomposition of CuCl2×2H2O.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The kinetics of thermal dehydration of copper(II) acetate monohydrate in air

The thermal decomposition of copper(II) acetate monohydrate was studied in air using TG-DTG/DTA, DSC, XRD techniques. TG-DTA curves show that the decomposition occurs in four steps. TG and XRD data indicate the reduction of Cu(II) during the decomposition of CuAc 2·H 2O. DTG and DSC data imply at least two steps contained in dehydration reaction, which is verified by the activation energy values estimated with Friedman and Flynn–Wall–Ozawa (FWO) methods. The dependence of activation energy on conversions indicates the dehydration reaction contains an additional initial reversible step. The most-probable kinetic model has been estimated with multivariate non-linear regression method assuming a two-step consecutive reaction. Bna (expanded Prout–Tompkins equation) → Cn ( n order autocatalytic reaction) model fits the original data best with a high correlation coefficient of 0.9998, and the calculated apparent activation energies of the fitted models are consistent with those calculated by isoconversional methods using original data. The corresponding function f( α), activation energy E and preexponential factor A of Bna, are (1 − α) 0.7593 α 0.2867, 128.5 kJ/mol and 1.6 × 10 15, respectively. Those of Cn, are (1 − α) 1.0534(1 + 2.712 α), 80.5 kJ/mol and 6.9 × 10 7, respectively. The combination of model free isoconversional methods and multivariate non-linear regression can give more reasonable and applicable models than commonly used model fitting methods.

Kinetics of thermal dehydration and decomposition of hydrated chlorides of some 3d transition metals (Mn-Cu series). Part-V. Dehydration and decomposition of CuCl2.2H2O

Kinetics of thermal dehydration and decomposition of hydrated chlorides of some 3d transition metals (Mn-Cu series). Part-V. Dehydration and decomposition of CuCl2.2H2O

Kinetics of thermal dehydration and decomposition of hydrated chlorides of some 3d transition metals (Mn-Cu series). Part-IV. Dehydration and decomposition of NiCI2.2H2O

Kinetics of thermal dehydration and decomposition of hydrated chlorides of some 3d transition metals (Mn-Cu series). Part-IV. Dehydration and decomposition of NiCI2.2H2O

Kinetics of thermal dehydration and decomposition of hydrated chlorides of 3d transition metals (Mn-Cu series). Part-II. Dehydration of FeCl2.4H2O

Kinetics of thermal dehydration and decomposition of hydrated chlorides of 3d transition metals (Mn-Cu series). Part-II. Dehydration of FeCl2.4H2O

Dehydration kinetics of calcium aluminate cement hydrate under non-isothermal conditions

Calcium aluminate cement (containing 75% Al 2O 3), which finds extensive high temperature applications, was synthesized by sintering a mixture of calcined alumina and synthetic calcium carbonate. The cement was hydrated and its kinetics of thermal dehydration was studied under non-isothermal conditions by thermo-gravimetry using the integral approximation model. It was observed that on heating the cement hydrate followed multistage dehydration with different reaction orders at different stages. The activation energies of dehydration for these different stages were compared.

Kinetics of thermal dehydration and decomposition of hydrated chlorides of some 3d transition metals (Mn-Co series). Part-III. Dehydration and decomposition of CoCl2 hydrates

The kinetics of dehydration of NiCl 2 .2H 2 O have been studied by isothermal method in air and by non-isothermal method in air as well as in flowing nitrogen atmosphere. While in air (or self-generated atmosphere) the dehydration apparently takes place in single step, in nitrogen atmosphere two steps, corresponding to the losses of about 1.46 and 0.48 moles of H 2 O, can be identified in the TG curves. Whereas in static air three-dimensional diffusion (D3 or D4 model) appears to be the best fit mechanistic model, in nitrogen atmosphere the three-dimensional progress of the reaction phase boundary (R3) followed by the first order decay of partially dehydrated porous particles (Fl ) appear to be the best fit models for the two steps of dehydration. When the hydrated salt is calcined at temperature above 450, rapid dehydration renders the material porous and therefore, subsequent dechlorination also follows first order decay law (Fl). Generally, there is a broad agreement in mechanistic model as well as in Arrhenius parameters between isothermal and non-isothermal methods of dehydration.

Kinetics of thermal dehydration and decomposition of hydrated chlorides of some 3d transition metals (Mn-Cu series). Part-I. Dehydration of MnCl2.4H2O

The thermal dehydration of MnCl 2 hydrate has been studied by using simultaneous TG/DTA under the static air (or self-generated atmosphere) and also under the flowing nitrogen condition at different heating rates. In either of the above condition the salt dehydrates in three steps, with the losses of about 1.0, 1.2 and 1.1 moles of water in static air and 0.9, 2.1 and 1.1 moles of water in flowing nitrogen atmosphere. The heats of the three steps of dehydration in air have been estimate2d from area under the corresponding DTA peaks at the heating rate of 10°/min. The first step of dehydration is accompanied with the melting of the salt and therefore does not follow any kinetic model of solid state decomposition. For the other two steps of dehydration changes in the mechanism are observed and attempts have been made to identify these changes. All these have been possible by converting non-isothermal time into isothermal time which is more appropriate in the kinetic analysis. It is observed that either nucleation and growth or progress of the reactant/product interface is generally the most suitable kinetic model for the dehydration of manganese chloride hydrate.

The kinetics of thermal dehydration of calcium copper acetate hexahydrate

The kinetics of the thermal dehydration of crushed crystals of CaCu(CH 3CO 2) 4·6H 2O in a flowing nitrogen atmosphere have been studied by means of isothermal gravimetry. At higher temperatures than 85°C the dehydration seemed to be described by three-dimensional phase boundary reactions, R 3(α). At temperatures lower than 80°C, the initial part of the dehydration could be described by phase boundary reactions, R 3(α), that of the middle part follows the Avrami-Erofeev type reaction, A 2.5(α), and then the kinetics of the final part followed diffusion-controlled models. These transitions of the dehydration mechanisms have been discussed on the basis of the diffusion process of dissociated water molecules.

The effect of the water vapor pressure on the kinetics of thermal dehydration of manganese(II) formate dihydrate

The effect of the water vapor pressure on the thermal dehydration of manganese(II) formate dihydrate was studied by means of isothermal gravimetry under various water vapor pressure, ranging from 4.6 to 24.4 torr.