Non-isothermal Dehydration Research Articles

Assessment of the moisture storage characteristics of lignite focusing on lignite type, moisture forms and drying mechanism

ABSTRACT To further study the occurrence form of moisture in coal, better understand the dehydration mechanism and the interaction between water and coal structure, as well as improve the utilization rate of lignite, non-isothermal and isothermal dehydration experiments were carried out on Zhaotong, Yimin and Zhundong coal samples under different initial conditions. The results showed that the dehydration rate of lignite is related to the temperature and initial water content. The effective moisture diffusion coefficient rises with an increase in temperature and water content. This indicated that the higher the temperature, the easier it is for water removal. In the isothermal dehydration experiment, the activation energy decreases with the increase of water content, indicating that the coal sample with large water content is easier to dehydrate, whilst the opposite result was obtained in non-isothermal dehydration experiment. According to the different drying mechanisms and characteristics at different stages, the total water content of lignite can be divided into four parts: chemical bound water, transitional bound water, physically bound water and free water. The removal mechanism and influencing factors of each have been analyzed in depth. The removal difficulty and required energy consumption is found to be successively reduced.

Effect of the swelling degree on the non-isothermal dehydration kinetics of a poly(acrylic acid)-g-gelatine hydrogel.

The kinetics of non-isothermal dehydration of a poly(acrylic acid)-g-gelatine hydrogel swollen to different swelling degrees is investigated. The thermogravimetric curves are recorded at different heating rates in the interval from 5 K mol-1 to 20 K mol-1 within the temperature range from ambient temperature to 480 K. It is found that all kinetics curves can be described using the Weibull function. Based on the suggested kinetics model, the values of the specific rates are calculated and the values of apparent activation energies are established. Distribution functions of activation energies are obtained. The changes found in the values of Weibull function parameters, dependences of the specific rate on temperature, and apparent activation energies on temperature and the dehydration degree, as well as different activation energy distributions, are explained using the fluctuation model of a hydrogel, as a dynamically disordered system that goes through changes during the dehydration process. The presented results can have both fundamental and practical significance in the fields where the application is based on the ability of the hydrogel to conserve and slowly release water.

The kinetics of non-isothermal dehydration of equilibrium swollen Ca-alginate hydrogel

Non-isothermal dehydration of Ca-alginate hydrogel was investigated for the first time. Thermogravimetric dehydration curves were recorded at heating rates from 5 to 20 K min−1 in the temperature range from 300 to 450 K. The dependence of apparent activation energy on the degree of dehydration was determined by the Friedman’s differential isoconversion method. The experimental conversion curves of dehydration are fully fitted with the statistical model of hydrogel dehydration. The values of the parameters of the statistical model and their changes with the heating rate were determined. The rate constant as the function of temperature was calculated at all heating rates, and it was found that these dependencies obey the Arrhenius equation. From the values of rate constant, the changes in the kinetic parameters (activation energy and pre-exponential factor) of non-isothermal dehydration of Ca-alginate hydrogel with temperature and dehydration degree were calculated. The obtained results indicate the existence of two phase states of water absorbed on the hydrogel.

Thermal dehydration kinetics involved during the conversion of gadolinium hydroxide to gadolinium oxide

Non-isothermal dehydration kinetics of gadolinium hydroxide (Gd(OH)3) was studied by thermogravimetry analysis (TGA), and differential thermal analysis (DTA). It was observed that the dehydration of Gd(OH)3 occurred in two steps. In step I, Gd(OH)3 dehydrates to GdOOH at ∼ 596 K. Finally, Gd2O3 is formed after complete dehydration of GdOOH at ∼710 K. The kinetic triplets (activation energy, pre-exponential factor and reaction mechanism) were calculated for step I and step II. The activation energy was determined by using the Kissinger–Akahira–Sunsose (KAS) iso-conversional method. The results indicated that the average activation energy was higher for step II (91.29 kJ/mol) as compared to step I (77.94 kJ/mol). The reaction mechanism was estimated by following two different master plot methods such as Criado and integral master plot method. Both the master plot methods have shown similar results. It was found that the Avrami–Erofeev A3 nucleation reaction mechanism dominated in both the dehydration steps.

A new facile synthesis, kinetic mechanism and some thermodynamic studies of thermal transformation of α-LiZnPO4·H2O

α-LiZnPO4·H2O was successfully synthesized via a new simple route in an aqueous media at ambient temperature. The as-prepared and the calcined powders were confirmed by standard characterization methods. The SEM micrographs of the title compound and its dehydration product were studied. The experimental enthalpy of dehydration was evaluated from DSC data, which was found to be close to the standard enthalpy of water vaporization. α-LiZnPO4·H2O dehydrates in a single step. Therefore, this compound is an additional ideal case for studying the decomposition kinetics. Non-isothermal dehydration kinetics was studied from TG data using Ozawa–Flynn–Wall (OFW) and Kissinger–Akahira–Sunose (KAS) methods including their iterative procedures, as well as some reliable solid-state kinetic equations of Coats–Redfern, Madhysudanan–Krishnan–Ninan (MKN), Tang and Wanjun. The most probable mechanism function was determined using the master plots method. The dehydration mechanism was found to be the chemical reaction with the one-third order reaction or the type F1/3 reaction model with the differential form of f(α) = 3/2 (1 − α)1/3 and the integral form of g(α) = 1 − (1 − α)2/3. The calculated activation energy value is exact and reliable. The discussions about the results of XRD, SEM, DSC, FTIR and the most probable mechanism function of dehydration are consistent.

Kinetics of nonisothermal dehydration of unirradiated and γ-ray irradiated neodymium (III) acetate hydrate

Abstract Kinetics of dehydration of unirradiated and γ-ray irradiated neodymium (III) acetate hydrate with 103 kGy total γ-ray dose absorbed in air atmosphere were studied by isoconversional nonisothermal method. The dehydration proceeds in two steps with the elimination of 0.8 and 0.4 mol of H2O, respectively. This result indicates that the investigated neodymium (III) acetate hydrate contains 1.2 mol of crystalline water in its structure. The dehydration reactions are best described by nucleation (A 2 model) and gas diffusion (D 4 model) for unirradiated and γ-ray irradiated samples, respectively. Analysis of the kinetic data using linear and nonlinear isoconversional methods showed that the apparent activation energy, Ea (kJ/mol) is dependent on the conversion degree, α, of the dehydration process. The Ea−α plots for both unirradiated and γ-ray irradiated neodymium (III) acetate hydrate showed that the dehydration is a complex process and contains multistep reactions. The results showed that γ-ray irradiation has a significant effect on the kinetics and thermodynamic parameters of the dehydration reaction. Powder X-ray diffraction showed that neodymium (III) acetate hydrate has a monoclinic system (SG P2/m) and no phase transformation was detected by γ-ray irradiation up to 103 kGy absorbed dose. The system maintains the same crystal structure before and after dehydration.

Thermal decomposition of un-irradiated and γ-ray irradiated holmium acetate tetrahydrate. Part 1: kinetics of nonisothermal dehydration of un-irradiated and γ-ray irradiated Ho(CH3COO)3⋅4H2O

Abstract Nonisothermal dehydration of un-irradiated and γ-ray irradiated holmium acetate tetrahydrate with 103 kGy total γ-ray dose absorbed was studied in air atmosphere. The thermal decomposition experiments were conducted at heating rates of (5, 7.5 and 10°C/min). The results showed that for un-irradiated material, the dehydration process proceeds in two decomposition steps with the elimination of 3.0 and 1.0 moles of H2O, respectively. The apparent activation energy, E a , as given by both linear and nonlinear isoconversional methods showed dependence upon the conversion degree, α, in the range of 0.2–0.75 for the two dehydration steps. In the first dehydration step, the Ea decreases from 228.0 kJ/mol at the beginning of the decomposition to ≈64.0 kJ/mol at the end of the process. In the second dehydration step, the Ea increases from 42.0 to 72.0 kJ/mol by progressively increasing in α. Compared with solid state reaction models, the two reactions are best described by diffusion (D 4) and nucleation (A 3) models for the first and second dehydration steps, respectively. The results derived from nonisothermal data present a reliable prediction of isothermal kinetics. Straight lines and reduced time plots methods were applied for the determination of the kinetic triplet [Ea , ln A, and reaction model f(α)] from predicted isothermal data. For γ-ray irradiated samples of Ho(CH3COO)3⋅4H2O with 103 kGy total absorbed dose, the dehydration proceeds in two overlapped steps controlled by D 3 model. X-ray data showed phase transformation from monoclinic (SG P2/m) to tetragonal phase (SG P4/mmm) by the elimination of water content from the entire structure of Ho(CH3COO)3⋅4H2O. γ-Ray irradiation effects on the thermal decomposition of Ho(CH3COO)3⋅4H2O were evaluated and discussed based on the formation of trapped electrons, point defects, cation and anion vacancies and cluster imperfections in the host lattice of Ho(CH3COO)3⋅4H2O.

Kinetic analysis of non-isothermal dehydration of poly(acrylic acid)-g-gelatin hydrogel using distributed activation energy model

The applicability of distributed activation energy model (DAEM) for describing non-isothermal dehydration kinetics of poly(acrylic acid)-g-gelatin (PAG) hydrogel has been examined. Thermogravimetric curves were recorded at different heating rates from 5 to 20 K min−1. The activation energy dependence on dehydration degree was determined by Kissinger–Akahira–Sunose and Vyazovkin’s methods. The possibility of applying thermodynamically inhibited nucleation and growth reaction model on investigating process has been examined. Miura–Maki method was used to establish the shape of activation energy distribution function and the dependence of pre-exponential factor on dehydration degree. Non-isothermal dehydration kinetics of PAG hydrogel can be successfully described by the DAEM. Here we defined the procedure by which DAEM can be successfully connected with kinetic models commonly used for reactions in solid state. The dependences of specific rate (i.e., rate constant) and reaction model function on dehydration degree were established for all of the investigated heating rates. It has been concluded that the specific rate changes with dehydration degree and that this is the one of the reasons why non-isothermal dehydration kinetics of PAG hydrogel is complex.

聚乙烯醇/壳聚糖复合水凝胶的物理化学性质

The physical properties such as the equilibrium water content, the melting enthalpy, the isothermal swelling kinetics and the non-isothermal dehydration kinetics of the composite hydrogels of poly(vinyl alcohol) (PVA) and chitosan (CS) were determined in this work. The influence of the composition and preparation parameters of the hydrogels on these properties was discussed. The results indicate that, PVA/CS composite hydrogel have a proper network structure to support water, and to work as an artificial cartilage material. Combining CS and PVA leads the crystallinity of gel to be weakened, but the interaction between water and gel scaffold to be enhanced. Though the mechanical tensile strength was lowered, but the biocompatibility and the degradation capacity of the gel were optimized. As a class of biomedical functional materials, PVA/CS hydrogel has a great potential application in cartilage scaffold. And as a model system of theoretical research, it promotes the application of thermodynamics in biomedical functional materials.

Thermal dehydration kinetics of potassium bis(oxalato)cuprate(II) dihydrate

The potassium bis(oxalato)cuprate(II) dihydrate was synthesized, characterized using FT-IR and its non-isothermal dehydration was studied by differential scanning calorimetry and thermogravimetry at different heating rates. The kinetics of the thermal dehydration was investigated using different isoconversional and maximum rate (peak) methods using differential scanning calorimetric data. The activation energy values were also evaluated from thermogravimetric data using different model-free methods and the complexity of the dehydration is illustrated by the dependence of Eα on the extent of conversion. The Eα values obtained from all isoconversional methods shows a decrease with conversion, passes through a minimum at α=0.4 and becomes almost steady in the range α=0.5–0.95. The variation of Eα with conversion function, α, established using these different methods was found to be similar. We obtained an Eα value of ∼76kJmol−1 from different isoconversional methods which is reasonable than the huge values (411.5±41.1kJmol−1) reported earlier.

Non-isothermal Dehydration Kinetics of Titania Gel

Titania gel was synthesized by sol-gel method and dehydration kinetics of the gel was studied using a thermo gravimetric analyzer in the temperature range of 35-700oC at three different heating rates. Dehydration of the gel was observed to take place at two different stages. Kinetic analysis for dehydration at both stages were performed by Friedman model fitting and Kissinger –Akahira –Sunose (KAS) model free methods to determine kinetic triplet. It was observed that dehydration of the titania gel sample followed second order reaction kinetics in both the stages.

The dehydration behavior and non-isothermal dehydration kinetics of donepezil hydrochloride monohydrate (Form I)

Powders of donepezil hydrochloride monohydrate (Form I) underwent isomorphic dehydration, losing 3% w/w water between 90% and 10% relative humidity (RH) without changing its powder X-ray pattern. Below 10% RH, additional dehydration occurred in conjunction with a reversible phase transition between the monohydrate state and a dehydrated state, with a 4.0% w/w loss to 0% RH. A combination of methods was used to understand the structural changes occurring during the desolvation process, including dynamic vapor sorption measurements, thermal analysis and powder X-ray diffraction. Form I showed the characteristics of the channel hydrate, whose non-isothermal dehydration behavior proceeds in two steps: (1) the loss of non-crystalline water adsorbed on the surface, and (2) the loss of one crystalline water in the channel. Dehydrated Form I is structurally similar to the monohydrate Form I. According to the heat of fusion and the crystal density criteria, the two crystal forms belonged to the univariant system, and the anhydrate (Form III) is stable. The dehydration kinetics was achieved from the TG-DTG curves by both the Achar method and the Coats-Redfern method with 15 frequently cited basic kinetic models. The dynamic dehydration processes for steps 1 and 2 were best expressed by the Zhuralev-Lesokin-Tempelman equation, suggesting a three-dimensional diffusion-controlled mechanism.

Study on the dehydration kinetics of tunellite using non-isothermal methods

The thermal behavior of tunellite has been examined by using thermal analysis data at different heating rates. Coats–Redfern, Horowitz–Metzger, Piloyan–Novikova, Doyle, and isoconversional Ozawa kinetic models have been used for the non-isothermal dehydration kinetic investigation of the mineral. The kinetic parameters are computed using the above-mentioned models for all dehydration stages. In addition, structural characterizations of tunellite are analyzed by using an X-ray diffractometer, an inductive coupled plasma-optical emission spectroscopy, Fourier transform infrared spectroscopy, and a scanning electron microscope.

Kinetic of non-isothermal dehydration of equilibrium swollen poly(acrylic acid-co-methacrylic acid) hydrogel

The kinetic of non-isothermal dehydration of equilibrium swollen poly(acrylic acid-co-methacrylic acid) hydrogel (PAM) has been investigated. Thermogravimetric and conversion dehydration curves were recorded at various heating rates 5–30 K min −1. The conversion dehydration curves at all investigated temperatures can be mathematically described using the logistic regression function in entire. It was found that activation energy complexly changes with the increasing dehydration degree. Physical meaning of the parameters of logistic function ( b and w) is given. It was established that, during the dehydration, changes in the fluctuating hydrogel structure occur, and that limiting step on the kinetics of hydrogel dehydration have rate of structural rearrangement of hydrogel (actual relaxation mechanism). A procedure for determining the dependence of effective activation energy on temperature and dehydration degree, based on logistic function, is exposed. Possible explanation for the existence of negative values of activation energy in the certain range of temperature, is given.

The comparative kinetic study of non-isothermal and isothermal dehydration of swollen poly(acrylic acid) hydrogel using the Weibull probability function

The isothermal and non-isothermal dehydration of swollen poly(acrylic acid) hydrogel has been investigated. The possibility of calculating the isothermal conversion curves from the non-isothermal conversion curves was examined. It was found that the experimental conversion curves under non-isothermal and isothermal conditions can be described by the Weibull probability distribution function. Changes in the values of distibution parameters with heating rate (non-isothermal dehydration) and operating temperature (isothermal dehydration) were identified. The mathematical procudere for determining the non-isothermal kinetic conversion curves, based on the functional relationship between distribution parameters and the heating rate was developed. In addition, the new mathematical procedure for calculating the isothermal kinetic curves from the non-isothermal ones at a certain temperature was established. It was found that the large deviation exists between the experimental and calculated isothermal conversion curves. In order to explain the difference in the mechanism of hydrogel dehydration under non-isothermal and isothermal conditions, the dependence of the apparent activation energy on the degree of conversion ( E a = E a ( α)) was performed using the isoconversional (model-free) methods. It was found that in the case of non-isothermal dehydration, the apparent activation energy exhibits a concave decrease with the degree of conversion ( α), while for the isothermal dehydration, the apparent activation energy exhibits a convex increase with α. Certain changes in a variety of E a with α are explained on the basis of different relaxation processes, that take place during non-isothermal and isothermal hydrogel dehydration. The characteristic features of density distribution functions of the apparent activation energies are derived in the case of non-isothermal and isothermal dehydration. It was found that the differences in characteristic properties of derived distribution functions are a consequence of different relaxation processes, which taking place under various experimental conditions.

Dynamic and controlled rate thermal analysis of halotrichite

Three halotrichites namely halotrichite Fe 2? SO4� Al2(SO4)3� 22H2O, apjohnite Mn 2? SO4� Al2(SO4)3� 22H2O and dietrichite ZnSO4� Al2(SO4)3� 22H2O, were analysed by both dynamic, controlled rate thermogravi- metric and differential thermogravimetric analysis. Because of the time limitation in the controlled rate experiment of 900 min, two experiments were undertaken (a) from ambient to 430 C and (b) from 430 to 980 C. For halotrichite in the dynamic experiment mass losses due to dehydration were observed at 80, 102, 319 and 343 C. Three higher temperature mass losses occurred at 621, 750 and 805 C. In the controlled rate thermal analysis exper- iment two isothermal dehydration steps are observed at 82 and 97 C followed by a non-isothermal dehydration step at 328 C. For apjohnite in the dynamic experiment mass losses due to dehydration were observed at 99, 116, 256, 271 and 304 C. Two higher temperature mass losses occurred at 781 and 922 C. In the controlled rate thermal analysis experiment three isothermal dehydration steps are observed at 57, 77 and 183 C followed by a non-isother- mal dehydration step at 294 C. For dietrichite in the dynamic experiment mass losses due to dehydration were observed at 115, 173, 251, 276 and 342 C. One higher temperature mass loss occurred at 746 C. In the controlled rate thermal analysis experiment two isothermal dehydra- tion steps are observed at 78 and 102 C followed by three non-isothermal dehydration steps at 228, 243 and 323 C. In the CRTA experiment a long isothermal step at 636 C attributed to de-sulphation is observed.

Nonisothermal Dehydration Kinetics of Tincalconite by Thermal Analysis Data

In the present study, the thermal decomposition kinetics of tincalconite was investigated by using thermo gravimetric (TG) and differential thermal analysis (DTA) data. The modified form of asymptotic expansion of the temperature integral ∫x∞(e–x/x2)dx, which is employed in a wide range of x = E/RT ratios, was used to perform the kinetic analysis. An empirical model was suggested for calculation of activation energy for non-isothermal decomposition kinetics of solids. The calculation of activation energy of tincalconite was carried out by means of different methods. It was determined that the process takes place in two steps and fits a first order reaction kinetic model. The results obtained from five methods were generally in good agreement.

Synthesis, characterization, crystal structure and non-isothermal dehydration kinetics of a copper(II) complex

A three-dimensional complex, [CuL2(H2O)]2 (HL = 3-hydroxy-1-adamantanecarboxylic acid), C44H64Cu2O14, was synthesized under hydrothermal conditions from CuCO3 · Cu(OH)2 and HL. The crystal structure of the complex was determined by X-ray crystallography and its thermal behavior and IR spectra examined. The non-isothermal dehydration kinetics of the complex were investigated using the Achar differential method.

Kinetic analysis on the non-isothermal dehydration by integral master-plots method and TG–FTIR study of zinc acetate dihydrate

The thermal decomposition of Zn(AC) 2·2H 2O has been studied by TG–DTA/DSC at the heating rate of 2, 3, 4, 5, 10 and 20 °C min −1 from ambient temperature to 350 °C in static air and flowing N 2, respectively. The TG–DTA/DSC experiments show both the decompositions under the two different atmospheres go through two well-defined steps: release of crystal water and subsequent decomposition of anhydrous zinc acetate into zinc oxide. Kinetic analysis was performed by using an integral composite procedure, which includes an integral isoconversional method and an integral master-plots method. The results of integral isoconversional method at various heating rates suggest that in both atmospheres the dehydration (0.3 < α < 0.9) followed simplex kinetic mechanism which can be described by an Avrami-Erofeev ( A 1.5) model. The evolved gas of non-isothermal decomposition of Zn(AC) 2·2H 2O in flowing N 2 was measured by TG coupled with Fourier transform infrared spectroscopy (FTIR). During 60–120 °C water was firstly detected. And in the range of 175–390 °C acetic acid, acetone and carbon dioxide were detected.

Investigating Dehydration from Compacts Using Terahertz Pulsed, Raman, and Near-Infrared Spectroscopy

The purpose of this study was to investigate the dehydration of piroxicam monohydrate (PRXMH) in compacts using terahertz pulsed spectroscopy (TPS), Raman spectroscopy, and reflectance near-infrared (NIR) spectroscopy. Compacts were prepared by using PRXMH and poly(tetrafluoro)ethylene powders and combining them in three different manners before compression to produce compacts in which the PRXMH was dispersed throughout the compact, deposited on one face of the compact, or included as a layer within the compact. TPS was a suitable technique to assess the effect of sample preparation on dehydration, whereas Raman and NIR spectroscopy were limited by their sampling depth and the interference of the polymer matrix. TPS revealed that the dehydration behavior depended largely on the compact preparation method. Non-isothermal dehydration was investigated with all three spectroscopic techniques, combined with principal component analysis (PCA) on samples where the PRXMH was deposited on one face of the compact. In addition, variable temperature X-ray powder diffractometry (VT-XRPD) was used to verify the transformation from PRXMH to anhydrous PRX form I, while thermogravimetric analysis (TGA) was used to monitor the water loss. All three spectroscopic techniques allowed in situ monitoring of the dehydration from the surface layers of the compacts. TPS and Raman spectroscopy detected structural changes of the crystal, while NIR spectroscopy was more sensitive to water loss. PCA of the TPS, Raman spectroscopy, and XRPD data revealed similar dehydration profiles. In contrast, the NIR spectroscopy profile was more similar to the TGA results. The spectroscopic techniques were more suitable than slower techniques such as VT-XRPD for monitoring rapid structural changes that occurred during the dehydration.