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Abstract

Reaction calorimetry strongly penetrated process development laboratories in the fine chemicals industry. Applications of calorimetry to different fields of process optimization, chemical reactions and physical unit operations were developed. Applications were first developed in the field of process safety. The thermal data of reaction obtained in the calorimeters allow us to check if a reaction will be controllable at full scale under normal operating conditions and in case of equipment failure. Further, the accurate temperature control and heat flow measurement opened the door to more engineering related data, in the fields of phase equilibria like vapour liquid, solubilities, crystallization and also in the mixing techniques. Some examples of developments in these different fields will be reviewed.

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Abstract  

When chemical reactions are performed in semi-batch mode and the reaction rate is relatively low, the reactant added may be accumulated. The resulting thermal accumulation is of major concern regarding process safety, as a fault in the cooling system may lead to a run-away reaction. The feed rate in semi-batch processes is usually constant, but this paper discusses methods of optimizing the feed rate interactively, based on the measured heat flow and the calculated amount of compound that has actually reacted. The prerequisite of such procedures is to run the experiments in a reaction calorimeter in which the heat flows can be measured accurately and continuously. For this purpose a ChemiSens reaction calorimeter CPA202, which is calibration free and gives stable, flat ‘zero-line-type’ baselines, was employed.

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Abstract  

Chemical polymerization of pyrrole (Py) was carried out in a reaction calorimeter by using FeCl3 or CuCl2 as an oxidant in an acetonitrile medium. The formation heat of polypyrrole (PPy), determined under a wide range of reactant concentrations and reaction temperatures, is directly related to the PPy yields and to the degree of polymerization. Due to the negative values of both the entropy and enthalpy of the reaction the gravimetric yield is inversely related to the temperature and directly to the Py concentration. The yields to the PPy and the related reaction heats, are close to zero when the ceiling temperatures are reached (T ceil=348 K for Fe-doped and T ceil=313 K for Cu-doped PPys). It was observed that a ceiling concentration corresponds to each ceiling temperature and only light oligomers are formed if Py concentration is too low. The electric conductivity values of the products were also determined and a direct relationship to the yields was found as well. The highest electric conductivity value (C=0.6 S cm–1) was related to the PPy fresh synthesized from a 0.017 M Py solution.

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The RC1 calorimeter revealed itself a suitable instrument to obtain information about safety and mechanisms involved in the reaction between cyclohexanecarboxylic acid (AEB) and oleum. A previous hypothesis about the existence of an unstable intermediate was confirmed and its heat of formation was calculated. The heat of sulphonation related to undesirable by-products production and the heat of protonation of AEB with H2SO4 were also evaluated. Therefore, it was possible to distinguish the reactions involved in the process and, through their thermal behaviour, to determine the limit conditions to avoid the by-products formation.

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Abstract  

Isothermal titration calorimetry (ITC) and reaction calorimetry (RC) have been used to construct the solid-liquid equilibrium line in ternary systems containing the solute to precipitate and an aqueous mixed solvent, and to study polymerization reactions under real process conditions, respectively. Phase diagrams have been established over the whole concentration range for some benzene substituted derivatives, including o-anisaldehyde, 1,3,5-trimethoxybenzene and vanillin, in {water + alcohol}mixtures at different temperatures. Acrylamide polymerization in aqueous solution using potassium permanganate/acid oxalic redox system as initiator was investigated on a homemade calorimeter, which works according to the isoperibolic mode. A Calvet type differential RC was used to illustrate the applicability of temperature oscillation calorimetry (TOC) for the evaluation of the heat transfer coefficient during the course of reaction.

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Abstract

For a great number of European safety groups, reaction calorimetry is the key technique for analysis of the main reaction in the risk assessment of chemical processes. A comparison of calorimetric studies of model reactions, the N-oxidation of two substituted pyridines with hydrogen peroxide, made by several European groups, can open the door to standardization of the methodologies used. However, the intrinsic experimental complexity of the model reactions, which included dosing at high temperature, a multiphase system and evaporation, and the different evaluation criteria, produced a considerable dispersion between the results obtained by the various groups.

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Abstract  

Materials with high surface areas and small particle size (nanophases), metastable polymorphs, and hydrated oxides are increasingly important in both materials and environmental science. Using modifications of oxide melt solution calorimetry, we have developed techniques to study the energetics of such oxides and oxyhydroxides, and to separate the effects of polymorphism, chemical variation, high surface area, and hydration. Several generalizations begin to emerge from these studies. The energy differences among different polymorphs (e.g., various zeolite frameworks, the - and -alumina polymorphs, manganese and iron oxides and oxyhydroxides) tend to be small, often barely more than thermal energy under conditions of synthesis. Much larger contributions to the energetics come from oxidation-reduction reactions and charge-coupled substitutions involving the ions of basic oxides (e.g., K and Ba). The thermodynamics of hydration involve closely balanced negative enthalpies and negative entropies and are very dependent on the particular framework and cage or tunnel geometry.

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Journal of Thermal Analysis and Calorimetry
Authors: R. Nomen, M. Bartra, J. Sempere, E. Serra, J. Sales, and X. Romero

Summary  

Estimation methods developed over years by S. W. Benson and co-workers for calculation the thermodynamic properties of organic compounds in the gas phase are applied to a pharmaceutical real process with all type of non-idealities. The different strategies used to calculate the reaction enthalpy of a chemical process, in the absence of data for complex molecules, using the Benson group additivity method are presented and also compared with the experimental value of reaction enthalpy obtained using reaction calorimetry (Mettler-Toledo, RC1). We demonstrate that there are some strategies that can be followed to obtain a good estimation of the reaction enthalpy in order to begin the safety assessment of a chemical reaction. This work is part of an industrial project [1] in which the main objective was the risk assessment of chemical real and complex processes using the commonly available tools for the SMEs (with limited resources).

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Abstract  

A small scale (100 mL) calorimeter is developed. It includes a glass vessel submerged in a thermostatic bath, a compensation electrical heater, and a control system. The typical operation mode consists on introducing the solvents and part of the reactants into the vessel, to stabilise a temperature of the bath (T j) some degrees below the desired process temperature (T p) and to adjust the reaction mass temperature (T r) to T p using the electrical heater. An oscillating set point is established for Tr, which produces an oscillating response of the applied compensation power (Q c). Finally, the rest of reactants are dosed to the vessel. A small deviation of T r and T p is observed. Even though it can be avoided improving the tuning of the controller, it can be useful for enhancing the calculation of the heat capacity of the reaction mixture (C P). The signals of T r, Q c and T j are processed on-line using the FFT (Fast Fourier Transform) method as the mathematical tool used to analyse the data obtained, producing accurate values of the heat evolved (Q c) by the process, the heat transfer coefficient (UA), and the heat capacity of the reaction mixture (C P).

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Abstract  

Initial plant scale trials of the nitrosation of an amino acid revealed a number of issues: _ Much lower yield compared to laboratory scale _ Considerable loss of mass balance _ Large excess of nitrosating agent required for complete reaction _ Highly reactive off-gases produced causing fires in the carbon absorber _ Reaction sensitive to agitation speed _ The by-product produces an impurity in the next process stage which has high human toxicity A kinetic and mechanistic study of the nitrosation reaction, using isothermal power compensation calorimetry and GC/mass spectrometry, has been undertaken in order to understand the above observations and to produce an improved manufacturing process - more robust, higher yielding, reduced effluent volumes and toxicity.

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