Rate Of Heat Evolution Research Articles

Applications of conduction calorimetry to the setting reaction of zinc phosphate dental cement

The kinetics of the exothermic setting reaction of zinc phosphate dental cement can be successfully investigated by conduction calorimetry. The primary objective of this study is to examine the effects arising from the addition of various additives to the liquid and calcination of cement powder. The rate of heat evolution versus time shows the occurrence of merely one maximum. In the case of unmodified orthophosphoric acid a sharp peak occurs early after onset of mixing due to the formation of crystalline α-hopeite. Commercial cement liquids contain aluminum and zinc. From the calorimetric curves it has been found that these cations act as a setting retarders, and ensure sufficient working time in the surgery in this way. The peak of heat evolution broadens and shifts due to the formation of an amorphous reaction product. When the ingredients of the cement powder are calcined at sufficient temperature the reactivity is further reduced. The setting time proves to be sensitive to changes in specific surface area of cement powder. The observed effects are discussed in terms of chemical reactions taking place during setting.

Mechanisms of Reaction of Hydratable Aluminas

The reactivities of four hydratable aluminas were investigated. Rates of heat liberation during hydration from 15° to 55°C were determined by isothermal calorimetry. Rates of heat evolution show two peaks. An initial mixing peak is due to wetting and initial dissolution reactions. A second, main heat peak is associated with the hydration step. The hydration product was bayerite in all cases. Although the specific surface areas for the aluminas studied were as high as about 260 m2/g, they did not react completely. The origin of incomplete reaction is related to the microstructure of the hydratable alumina grains. The particles are approximately 10 μm in size. High surface areas are not the result of small particle size, but are the result of high internal surface areas. The Bayerite formation appears to block the porosity in the grains, thereby inhibiting further reaction.

Identification and control of oxidative metabolism in Ssaccharomyces cerevisiae during transient growth using calorimetric measurements.

The objective of this study was to characterize the dynamic adaptation of the oxidative capacity of Saccharomyces cerevisiae to an increase in the glucose supply rate and its implications for the control of a continuous culture designed to produce biomass without allowing glucose to be diverted into the reductive metabolism. Continuous cultures subjected to a sudden shift-up in the dilution rate showed that the glucose uptake rate increased immediately to the new feeding rate but that the oxygen consumption could not follow fast enough to ensure a completely oxidative metabolism. Thus, part of the glucose assimilated was degraded by the reductive metabolism, resulting in a temporary decrease of biomass concentration, even if the final dilution rate was below Dcrit. The dynamic increase of the specific oxygen consumption rate, qO2, was characterized by an initial immediate jump followed by a first-order increase to the maximum value. It could be modeled using three parameters denoted qjumpO2, qmaxO2, and a time constant tau. The values for the first two of the parameters varied considerably from one shift to another, even when they were performed under identical conditions. On the basis of this model, a time-dependent feed flow rate function was derived that should permit an increase in the dilution rate from one value to another without provoking the appearance of reductive metabolism. The idea was to increase the glucose supply in parallel with the dynamic increase of the oxidative capacity of the culture, so that all of the assimilated glucose could always be oxidized. Nevertheless, corresponding feed-profile experiments showed that deviations in the reductive metabolism could not be completely suppressed due to variability in the model parameters. Therefore, a proportional feedback controller using heat evolution rate measurements was implemented. Calorimetry provides an excellent and rapid estimate of the metabolic activity. Satisfactory control was achieved and led to constant biomass yields. Ethanol accumulated only up to 0.49 g L-1 as compared to an accumulation of 1.82 g L-1 without on-line control in the shift-up experiment to the same final dilution rate.

Determination of global parameters of gas-phase oxidation reactions from heat-evolution rates in a plug flow reactor

We have obtained curves of heat evolution in the process of gas-phase interaction of oxygen with 1,1-dimethylhydrazine, dimethyl formamide, kerosene (T-1 fuel), 1-methyl-1,2-dicyclopropylcyclopropane, n-butyl alcohol, and toluene in a plug flow reactor, with high dilution with nitrogen. The values of the kinetic constants for the processes have been determined using the data obtained. The adequacy of the kinetic models for chemical processes that cause self-ignition of combustible mixtures has been shown by comparing calculation results concerning self-ignition parameters with the available experimental data.

The effect of high temperature and salt on the metabolic heat rate of Chlorella cells

The metabolic heat rate of plants is an integral indicator of the state of the organism. The metabolic heat rate reflects changes in all the anabolic and catabolic processes in plant cells between optimal and extreme conditions. The present studies determined the effects of extreme temperatures (45°C) and high NaCl concentrations (450 mM) on the rate of heat evolution by Chlorella cells. At first, the rate of heat production increases as compared with the control; thereafter it decreases to a steady-state value which is lower than that of the the control. The decrease in metabolic rate may be connected with changes in metabolic processes associated with adaptation of the organism to stress condition.

Semilinear Elliptic Boundary Value Problems in Chemical Reactor Theory

This paper is devoted to the study of semilinear elliptic boundary value problems arising in chemical reactor theory which obey the simple Arrhenius rate law and Newtonian cooling. We prove that ignition and extinction phenomena occur in the stable steady temperature profile at some critical values of a dimensionless heat evolution rate.

Kinetic and Calorimetric Considerations in the Scale-Up of the Catalytic Reduction of a Substituted Nitrobenzene

A reaction calorimetric investigation was conducted to assess proposed scale-up conditions (30 psig, 10 °C, initial reactant concentration = 0.69 M, catalyst concentration of 6.66 × 10-3 g/g of reactant) for the batchwise reduction of a substituted nitrobenzene to give the corresponding aniline in the presence of H2, Pd/C catalyst, and ethyl acetate solvent. Heat evolution rates under these conditions were judged to be too large for safe plant operation. Alternative processing conditions were investigated, accounting for heat removal, pressure limitations, and gas−liquid mass transfer rates in the plant equipment as well as differences in reactivity between plant- and laboratory-prepared reactants. The recommended conditions (30 psig, 20 °C, and catalyst concentration of 7.53 × 10-3 g/g of reactant) were employed in the pilot plant, which resulted in conversion rates almost identical to those in the laboratory reactor, demonstrating the value of reaction calorimetry for scaling up a highly exothermic reac...

Study of early hydration of OPC-HAC blends by microwave and calorimetry technique

The early hydration of the Ordinary Portland Cement (OPC)-High Alumna Cement (HAC) system has been investigated by using the microwave and calorimetry methods. It is found that the hydration process of OPC-HAC systems vary with the HAC proportion. The pastes containing 20 and 25% HAC show a feature of fast setting. And the pastes containing 33% and 50% HAC show quite unusual patterns in the dielectric ϵ, conductivity σ and heat-evolution rate R curves. We find that the microwave method is more sensitive to the cement hydration than that of calorimetry method. The correlation of the microwave measurement results with the calorimetry measurement results is discussed.

Hydrothermal reactions of fly ash with Ca(OH) 2 and CaSO 4·2H 2O

Hydrothermal reactions of fly ash were investigated. Variations in reactivity depended on the presence of added Ca(OH) 2 or CaSO 4 · 2H 2O. Isothermal calorimetry determined the kinetics of the reactions between fly ash and these compounds. Fly ash was activated by these calcium salts and activation influenced hydration rates as determined by the rates of heat evolution. X-ray diffraction analysis determined the phases formed as a result of hydrothermal treatment. Calcium silicate hydrate, tricalcium aluminate hydrate and ettringite were observed. The mechanical properties developed by hydrothermal treatment demonstrates that hazardous ashes could be consolidated to be handled. Alternatively, non-hazardous ashes can be reacted to form useful products.

Correlation of microwave and conduction calorimeter measurements on the hydration of cement

Ordinary Portland cement (OPC) is one of the most widely used construction materials. The hydration of cement involves very complicated chemical and physical processes. A better understanding of the mechanisms of hydration and microstructural development in the cement paste is important for improving its mechanical strength. There are two methods to study cement hydration in situ. One is conduction calorimetry [1] which is a well-established method, and the other is electrical measurement [2±7] which is not yet well established because of the dif®culty in directly relating the results of the electric measurement to the chemical and physical changes which occur during cement hydration. We have successfully developed a microwave technique to study the early hydration of OPC [8, 9], slag cement [8, 9], mortar [10] and OPC±high-alumina cement (HAC) blend [11] by measuring the change in dielectric constant during cement hydration. The heat evolution rate of OPC in the ®rst 30 h of hydration measured using conduction calorimetry is shown in Fig. 1. There is a large broad peak in the heat evolution rate curve, which corresponds to the chemical reactions that have occurred during cement hydration. At the same time the rate of change in dielectric constant, -da=dt, was monitored using the microwave technique and the result is also plotted in Fig. 1. It is interesting to see that the two curves have similar shapes and features, showing that there is a correlation between them for OPC. Since the traditional de®nition of the four periods of cement hydration is based on the heat evolution rate curve, if we can correlate the rate of change in dielectric constant curve with the heat evolution rate curve for any cement system, then we can apply this welldeveloped de®nition of the four periods of cement hydration to the rate of change in dielectric constant curve and hence provide an accurate interpretation of the dielectric constant results. This paves the way for a novel method in studying the cement hydration process in situ. In this letter, we explore the correlation between the results measured by microwave technique and the results measured by conduction calorimetry. Several samples, namely, OPC, OPC±HAC, OPC± silica fume, OPC±pulverized fuel ash (PFA) and slag cement, have been measured in this study (Fig. 2). We used the water cement ratio, w=c, of 0.4 and measured samples at a microwave frequency of 9.5 GHz. The details of the measurement have been given in [8±11]. For the OPC±30% HAC blend, there are two different peaks in the heat evolution

Expanding the role of reaction calorimetry

Abstract Reaction calorimetry, a scientific tool designed to measure the rate of heat evolution occurring during a reaction or other process, has been employed in various capacities over the last 30 years. Recently, it has expanded to find a variety of uses in process development, process safety and basic research. This review acquaints the reader with the theoretical principles, measurements that can be made and surrounding issues of reaction calorimetry, and then methodically presents the different types of investigations which have been and are being performed in various laboratories around the world. This review also considers aspects of the equipment and how these features play a role in the quality and type of data obtained. The mathematical solution(s) to the energy balance in a variety of cases is presented along with examples of their use in the literature

Low temperature synthesis of a self-assembling composite: hydroxyapatite-poly[bis(sodium carboxylatophenoxy)phosphazene

The present study was undertaken to investigate the low temperature formation of a hydroxyapatite-polyphosphazene polymer composite likely to be biocompatible. The temperature range studied (25 to 60°C) was selected to bracket physiological temperatures. The composite precursors consisted of CaHPO4·2H2O, Ca4(PO4)2O, and poly[bis(sodium carboxylatophenoxy)phosphazene]. The results indicate that a synergistic relationship exists in the formation of a polyphosphazene network and hydroxyapatite (HAp) matrix phase during composite synthesis. Calcium from the HAp precursors participates in the formation of a Ca crosslinked polymeric network which influences the rate of HAp formation and its morphology. The mechanistic paths taken during composite formation were followed by determining variations in the concentration of species in solution (at physiological temperature), rates of heat evolution, and microstructural development. These analyses indicate that the polymer controls the kinetics of hydroxyapatite formation and the composite microstructure. Low reaction temperatures and a high proportion of polymer facilitate the formation of a highly interconnected composite. The presence of the polyphosphazene allows a metastable calcium phosphate solution to persist for extended periods prior to the formation of hydroxyapatite. The degree of supersaturation and the length of the induction period increase with an increase in polyphosphazene content. The temperature dependence of these induction periods obeyed an Arrhenius relationship.

Kinetics of anhydride and polyamide curing of bisphenol A-based diglycidyl ether using DSC

The cure reactions of a diglycidyl ether of bisphenol A-based epoxide resin (DGEBA) with dodecyl succinic anhydride (DDSA) as the curing agent and tris(dimethyl amino methyl)phenol (DMP-30) as the catalyst and also with a polyamide as curing agent were studied by differential scanning calorimetry (DSC) using an isothermal approach over a range of temperatures. The results indicate that the cure reactions are autocatalytic in nature. All isothermal DSC curves show the maximum rate of heat evolution around 30–40% conversion.

The kinetics of calcium deficient and stoichiometric hydroxyapatite formation from CaHPO4�2H2O and Ca4(PO4)2O

Isothermal calorimetry was performed on intimate mixtures of CaHPO4·2H2O and Ca4(PO4)2O constituted at Ca/P molar ratios of 1.50 and 1.67 to form the hydroxyapatite compositions Ca9HPO4(PO4)5OH and Ca10(PO4)6(OH)2, respectively, at complete reaction. The temperature range investigated was 15–70°C. The effects of the reaction temperature on the rates of heat evolution during hydroxyapatite formation were determined. Reactions were carried out utilizing a liquid-to-solids weight ratio of 1.0. A two-stage reaction mechanism was observed regardless of the Ca/P ratio as indicated by the presence of two reaction peaks in the plots of the rates of heat evolution against time. An Arrhenius relationship was found between the rate and temperature for each reaction stage for both compositions. Apparent activation energies of 120 and 90 kJ/mol (Ca/P=1.67) and 118 and 83 kJ/mol (Ca/P=1.50), respectively, were calculated for the first and second reaction peaks. An Arrhenius relationship was also found between the time of maximum rate and temperature. The following qualitative reaction mechanism is proposed for each of the two reaction stages for both compositions studied. The first stage involves the complete consumption of CaHPO4·2H2O and the partial consumption of Ca4(PO4)2O to form a noncrystalline calcium phosphate and nanocrystalline hydroxyapatite. During the second stage the remaining Ca4(PO4)2O reacts with the noncrystalline calcium phosphate to form the final product, stoichiometric or calcium deficient hydroxyapatite.

Some factors affecting early hydration of alkali-slag cements

This paper deals with the effect of activator dosage, temperature and water-to-slag ratio on early hydration of alkali-blast furnace slag cements activated with NaOH, Na 2CO 3 or Na 2SiO 3. A conduction calorimeter was used to monitor early-age hydration. The experimental results indicate that activator dosage, temperature and water-to-slag ratio have a significant effect on heat evolution rate, induction period and cumulative heat of hydration; the effect varies with the nature of activator. Hydration of Na 2SiO 3 activated slag appears to be the most sensitive to the variation of activator dosage, temperature and water-to-slag ratio among NaOH, Na 2CO 3 and Na 2SiO 3 activated alkali-slag cements.

Superplasticizers for Ciment Fondu. Part 2: Effects of temperature on the hydration reactions

In an investigation of the use of super plasticizers with Ciment Fondu, it was expected that they would have a profound influence on the interactions between temperature, setting time and hydration kinetics. The behaviour of Ciment Fondu in the absence of admixtures has been investigated using conduction calorimetry, supported by setting time measurements. The results confirm that hydration is retarded to the greatest extent at around 30°C, above which progressive acceleration occurs. Applying the Avrami—Erofe'ev equation showed that the maximum rate of heat evolution and the time at which it occurs are reliable kinetic parameters to quantify the hydration rate. In the range 30-45°C the activation energy for hydration is 120 kJ/mol.

Kinetic aspects of self-discharge of nickel-hydrogen batteries and methods for its prevention

Results of microcalorimetric experiments, in relation to self-discharge in Ni/H2 batteries are reviewed; the mechanism of self-discharge, as well as possible methods for its inhibition, are discussed. These studies indicate that: (i) the self-discharge is due to a direct reaction of hydrogen with the charged active material (nickel oxide); (ii) the presence of metallic nickel sinter particles does not affect the reaction rate; (iii) the reaction rate depends linearly on hydrogen pressure indicating that the reaction is first order with respect to hydrogen; (iv) the reaction rate is higher under starved-electrolyte rather than flooded electrolyte conditions, indicating that the rate is affected by a diffusion process of dissolved hydrogen; and (v) the microcalorimetric heat evolution rate correlates with that of a decrease in electrode capacity due to the self-discharge reaction. The effects of additives to the active material of the nickel electrode were tested as an approach to reduction of the intrinsic rate of self-discharge. An alternate method for minimizing this rate is by storing the hydrogen as a hydride and thereby lowering the cell operating pressure. Some alloys were thus examined for their hydriding/dehydriding characteristics.

Characterization of the hydration process of calcium sulphate hemihydrate by simultaneous evaluation of chemical and physical parameters

A new approach to follow the hydration reaction of calcium sulphate hemihydrate is described, based on two automated-computerized systems. The results of the study show that the hydration process can be divided into four stages: induction period, nucleation, crystal growth and diffusion-controlled reaction. It was found that the initial setting time obtained by the ‘knife’ test coincides with the end of the induction period, during which the degree of hydration is zero. Initial setting time obtained by the Vicat test coincided with the time when the degree of hydration was 10%. The length change curves could be interpreted in terms of four stages. Shrinkage occurred when water was consumed in the hydration reaction during the time of the Vicat initial setting; a high rate of expansion was initiated when the degree of hydration was about 20% and it was accompanied by a high rate of heat evolution. These two processes terminated at about the same time.

The relationship between the peak shape of a DTA curve and the shape of a phase diagram

DTA curves are simulated based on the heat transfer modeling of the DTA cells. The calculated results are compared with the experimental determinations. The rates of heat evolution or absorption of the specimens under the DTA scan are correlated with the shapes of phase diagrams. The relationships between the shapes of DTA curves and the shapes of phase diagrams are discussed.

On the Energy Evolution in Gaseous Detonation Waves

There is a definite inconsistency between the classical ZND theory of detonation and contemporary experimental observations and attempts to model real detonation waves. Nevertheless, the classical onedimensional model of detonation is still extensively used in interpreting measurements because of its simplicity and physical clarity. This naturally raises the questions, what does actually the classical model represent and how one can relate the real multidimensional wave structure to an effective onedimensional detonation? The answer to these questions is extremely important from the practical point of view, because it defines how simple the solution of the criticality problems in detonation could be (i.e. whether one has to solve the very cumbersome three-dimensional nonsteady gasdynamic problem with detailed kinetics of the chemical reaction comprising a few hundreds of elementary steps to predict such parameters as critical initiation energy, critical distances, and concentrations, or the solution of a simplified onedimensional gasdynamic problem with global chemical kinetic equations would suffice?). The present communication discusses the following topics with the aim of answering partly the above questions. (1) An analysis of the detailed kinetic calculations of the heat evolution rate in some fuel-air mixtures and comparison of these results with experiments reveals that (a) in the majority of detonable mixtures heat release can be described by global equations with an accuracy quite sufficient for assessing the critical detonation parameters, (b) chemical kinetics shows no so-called recombination zones with an appreciable heat evolution that follow the main reaction zone, (c) the ratio between the induction and the explosion (within which the major fraction of the stored energy is evolved) times exceeds unity under the conditions corresponding to detonation waves in fuel-air mixtures, which contradicts the shock tube measurements. All these results can be reasonably explained by the fact that the chemical reaction in detonation waves (in shock waves as well) proceeds in a gas with a highly inhomogeneous distribution of the parameters (temperatures, pressures, and particle velocity) which gives rise to the so-called hot-spot mechanism of chemical reactions in gaseous and two-phase media. It is shown that the hot spot ignition is the main reason of instability of detonation waves. Nonsynchronous mixture ignition at different points produces the effect of extended heat evolution zone and reduces the effective induction zone. Results of measurements of averaged and local parameters behind detonation waves in gases support the idea that the effective heat evolution rate in the onedimensional representation of the wave is much more complicated than that inherent in the chemical reaction alone, because it contains also a significant contribution of the kinetic and thermodynamic energy redistribution within the major reaction zone and downstream of it, which makes the onedimensional structure of detonation waves deviate significantly from the classical pattern leading to nonmonotonic heat release behind the shock front (the possibility of appearance of two or more effective sonic planes and heat rlease maxima). However, a onedimensional analysis of marginal detonation waves has demonstrated that critical parameters can be estimated quite accurately within simplified models, which is attributable to the fact that the criticality is associated with local termination of the reaction within narrow stream tubes where the flow pattern resembles closely the ZND non-CJ detonation wave structure. Unlike the chemical reaction zone in detonations far away from the limit, which is shorter than the detonation cell size, the reaction zone in marginal detonations is comparable with the cell size. The results of onedimensional calculations of critical diameters and minimal energies of direct initiation of detonation are compared with experim