Heat production rate, exothermic reaction

Essential modelling for scale-up relates to heat production (ref.4), and the universally applied calculation relates to the disaster calculation where the runaway instant temperature rise is always calculated for any one-shot exothermic reaction. In addition, the normal heat production rate is calculated to determine optimum feed rates, safety margins on cooling coil and condensers, etc. Increasingly, kinetic models are used as these become available. 

Ignition is dependent on various physicochemical parameters, such as the type of reactants, reaction rate, pressure, the heat transfer process from the external heat source to the reactants, and the size or mass of the reactants. The rate of heat production is dependent on the heats of formation of the reactants and products, the temperature, and the activation energy. As the process of ignition includes an external heating and an exothermic reaction of the reactants, there is a non-steady heat balance during these phases. 

For exothermal reactions, the addition controls the heat production rate and therefore adjusts the reaction rate to the cooling capacity of the reactor. 

Depending on the plastic to be cast, solidification takes place at either room temperature or elevated temperatures. With room temperature systems chemical reaction occurs with the liberation of heat. The rate of heat dissipation can influence the performance and aesthetic characteristics of the hardened product. In thin sections, where a large area in relation to the total volume of the plastic is exposed, the heat of the exothermic reaction is dissipated rapidly and the temperature of casting is not very high. Thin sections can be cast at room temperature with no danger of cracking. When the rate of heat is excessive, application of heat may be necessary to properly control cure rate. 

In general, acetic acid production via acetaldehyde oxidation takes place continuously in a bubble column at 50-80 °C with pressures of 1-10 bar. The construction material of choice for the reactor is austenitic Cr-Ni-steel. The acetic acid product serves as process solvent and the concentration of acetaldehyde is kept at 3%. It is necessary to keep the temperature over 50 °C to obtain a sufficient peroxide decomposition and oxidation rate. To remove the heat of the exothermic reaction, the reaction mixture is circulated through an external heat exchanger. Accurate temperature control is important to decrease oxidative degradation of acetic acid to formic acid, CO2, and water. The reaction mixture is separated by several distillation units. The process yields are typically in the range of 90-97% and the purity of acetic acid is higher than 99%. Typical by-products are CO2, formic acid, methyl acetate, methanol, methyl formate, and formaldehyde. 

With batch reactors, it may be possible to add all reactants in their proper quantities initially if the reaction rate can be controlled by injection of initiator or acqustment of temperature. In semibatch operation, one key ingredient is flow-controlled into the batch at a rate that sets the production. This ingredient should not be manipiilated for temperature control of an exothermic reactor, as the loop includes two dominant lags—concentration of the reactant and heat capacity of the reaction mass—and can easily go unstable. 

Decompositions may be exothermic or endothermic. Solids that decompose without melting upon heating are mostly such that can give rise to gaseous products. When a gas is made, the rate can be affected by the diffusional resistance of the product zone. Particle size is a factor. Aging of a solid can result in crystallization of the surface that has been found to affect the rate of reaction. Annealing reduces strains and slows any decomposition rates. The decompositions of some fine powders follow a first-order law. In other cases, the decomposed fraction x is in accordance with the Avrami-Erofeyev equation (cited by Galwey, Chemistry of Solids, Chapman Hall, 1967)... 

Chemical reactions are sometimes conducted in a dilute solution to moderate reaction rates, to provide a heat sink for an exothermic reaction, or to limit maximum reaction temperature by tempering the reaction. In this example there are conflicting inherent safety goals—the solvent moderates the chemical reaction, but the dilute system will be significantly larger for a given production volume. Careful evaluation of all of the process risks is required to select the best overall system. 

To evaluate the heat exchange/productivity performances of the device and its environment, an acid-base neutralization involving sulfuric acid and soda has been performed. It is an instantaneous and exothermic reaction with AH = —92.4 kj moP (NaOH). Each experiment is characterized by the initial concentration of the reactants (from 10 to 30% in mass of soda and from 5 to 12% in mass of sulfuric acid). These concentrations are varied in order to evaluate the behavior of the reactor with respect to different amounts of heat generated (from 0.4 to 1.3 kW). Each run is performed with a variable utility flow rate (from 1 to 3 m h ). 

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