Heat production rate, exothermic

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. 

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

This assessment criterion shows very clearly the fundamental difference between BR and SBR. Especially for the isothermally operated BR the example discussed has shown, that the ratio of Damkoehler to Stanton number should be significantly smaller than 1 to be sure of safe operating conditions. Exactly the opposite is required for the SBR. Semibatch reactors must be operated in an ignited state in order to be able to utilize the degree of heat production rate. Batch reactors have to be operated in the extinguished state with the consequence already outlined in Section 4.3.1.3, that they are suitable for slow and moderately exothermic processes only. 

Typical pressure versus time curves for runaway reactions are illustrated in Figure 8-2. Assume that an exothermic reaction is occurring within a reactor. If cooling is lost because of a loss of cooling water supply, failure of a valve, or other scenario, then the reactor temperature will rise. As the temperature rises, the reaction rate increases, leading to an increase in heat production. This self-accelerating mechanism results in a runaway reaction. 

In DSC instruments, heat production (q) can be determined directly as a function of temperature. The shape of the heat production curve is also important for hazard identification. A sharp rise in energy release rate (i.e., a steep slope of the exotherm), whether due to a rapid increase of the rate constant with temperature or to a large enthalpy of reaction, indicates that the substance or reaction mixture may be hazardous. Figure 2.14 illustrates an example of a DSC curve with a gradual exothermic reaction, while Figure 2.15 is an example of a steep exothermic rise. 

For exothermic reactions, the value of AH is by convention negative and for endothermic reactions positive. For a set of R individual reactions, the total rate of heat production by reaction is given by... 

There are many different aspects to the field of turbulent reacting flows. Consider, for example, the effect of turbulence on the rate of an exothermic reaction typical of those occurring in a turbulent flow reactor. Here, the fluctuating temperatures and concentrations could affect the chemical reaction and heat release rates. Then, there is the situation in which combustion products are rapidly mixed with reactants in a time much shorter than the chemical reaction time. (This latter example is the so-called stirred reactor, which will be discussed in more detail in the next section.) In both of these examples, no flame structure is considered to exist. 

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. 

In this situation, the reaction cannot immediately be stopped by shutting the feed and further, the feed cannot be used to directly control the heat release rate or the gas release rate of a reaction. If, after a deviation from the design conditions, one decides to shut down the feed, the amount of accumulated B will react away despite the feed being stopped. If the reaction is accompanied by a gas release, gas production will continue and if the reaction is exothermal, heat will be released even after the interruption of the feed. 

The mixers discussed in Sections 4-6 are particularly suitable for reactions where the required heat input (endothermic reaction) or heat production (exothermic reaction) is modest (i.e., temperature changes on reaction would be only a few degrees in the absence of any heat transfer). HEX reactors can be used for rapid, highly exothermic (or endothermic) reactions not only are the mixing rate and residence time of a reactor matched to the kinetic rate and reaction time, but heat transfer performance is also matched to heat production (Figure 9). 

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. 

DMA and TMA. Product ratios can be varied to maximize MMA, DMA, or TMA production. The correct selection of the N/C ratio and recycling of amines produces the desired product mix. Most of the exothermic reaction heat is recovered in feed preheating (3). The reactor products are sent to a separation system where firstly ammonia (4) is separated and recycled to the reaction system. Water from the dehydration column (6) is used in extractive distillation (5) to break the TMA azeotropes and produce pure anhydrous TMA. The product column (7) separates the water-free amines into pure anhydrous MMA and DMA. Methanol recovery (8) improves efficiency and extends catalyst life by allowing greater methanol slip exit from the converter. Addition of a methanol-recovery column to existing plants can help to increase production rates. 

Figure 4.4 Relative heat production and heat withdrawal rates versus dimensionless pellet temperature 0p A) exothermic reactions, 0p, 0t and An > 0 B) endothermic reactions, 0p, 0t and An < 0 ( ) stable operating point (o) unstable operating point.

The rate of exothermic heat production and hence dissolution rate of oxides should be sufficiently slow to allow the phosphate gel to crystallize slowly into a well-ordered crystal lattice without interruption, and grow into a monolithic ceramic. 

Temperature is a key control parameter for adjusting production rate. Increasing the temperature increases both the propagation rate constant (according to an Arrhenius correlation) and the concentration of radicals due to increased rates of initiation. Another means of increasing rate is by the addition of chemical initiators to increase the radical concentration. Heat removal capabilities of the system provide a practical limitation to production rate since the reaction is exothermic and the rate increases with temperature, the reaction will run away if the cooling system is inadequate. 

The entropy production of a sulfur dioxide oxidation (exothermic) reactor with heat exchangers was minimised in two different cases.Case 1 was a four-bed reactor with intermediate heat exchangers of a given total area, see Figure 8. The entropy production rate was calculated from the entropy balance over the system. All inlet and outlet flow conditions were kept constant, except the pressure at the outlet. Tlie... 

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