Specific heat of reaction

The latter, the specific heat of reaction, is practical for safety purposes, because most of the calorimeters directly deliver the specific heat of reaction in kj kg"1. Further, since it is a specific entity, it can easily be scaled to the intended process conditions. Both heat of reaction and molar enthalpy are related by... 

The central term in Equation 2.5 enhances the fact that the adiabatic temperature rise is a function of reactant concentration and molar enthalpy. Therefore, it is dependant on the process conditions, especially on feed and charge concentrations. The right-hand term in Equation 2.5, showing the specific heat of reaction, is especially useful in the interpretation of calorimetric results, which are often expressed in terms of the specific heat of the reaction. Thus, the interpretation of calorimetric results must always be performed in connection with the process conditions, especially concentrations. This must be accounted for when results of calorimetric experiments are used for assessing different process conditions. 

The reaction is performed in a diluted aqueous solution. Thus, its density can be assumed to be 1000 kg m-3. Then, the specific heat of reaction is... 

The specific heat of reaction is 125 kj mol"1 (aromatic compound) and the specific heat capacity of the reaction mixture is 2.8kjkg 1 K"1. What maximum temperature (MTSR) could the reaction mass reach if the heating cooling system fails to stabilize at 125 °C ... 

Mrf represents the mass of the reaction mixture at the end of the feed, MrW the instantaneous mass of reactant present in the reactor, and Xal the fraction of accumulated reactant The ratio of both masses accounts for the correction of the specific energy, since the adiabatic temperature rise is usually calculated using the final reaction mass, that is, the complete batch. In Equation 2.5, the concentration corresponds to the final reaction mass this is also the case for the specific heat of reaction obtained from calorimetric experiments, which is also expressed for the total sample size. Since in the semi-batch reaction, the reaction mass varies as a function of the feed, the heat capacity of the reaction mass increases as a function of time and the adiabatic temperature rise must be corrected accordingly. 

Specific heat of reaction q 450kjkg (final reaction mass)... 

Consider a reaction in a 16 m3 reactor at 100 °C. At this temperature, using a feed time of at least one hour, the reaction is feed-controlled. The feed rate must be adapted to the cooling capacity of the vessel. There are 15 000 kg of reaction mass in the vessel, the specific heat of reaction is 200 kj kg final reaction mass. During the reaction, the heat exchange area remains constant at 20 m2. Ambient pressure is 1013 mbar. 

From the specific heat of reaction of 180 kj kg-1 and the specific heat capacity of the reaction mass of 1.8kjkg K 1, we find an adiabatic temperature rise of 100 K. Since the accumulated energy is 30%, the MTSR is... 

Q - specific heat of reaction of oxygen that took part in the reaction ... 

Cp = specific heat of reaction and reference mixture U = heat transfer coefficient for total cell area... 

To assess the thermal controllability of the process, the value obtained for the heat of reaction by using one of the methods cited above has first to be multiplied by the mass firaction of the limiting component A of the process and divided by its molecular weight in order to obtain the specific heat of reaction. Dividing this result by the specific heat capacity of the reaction system yields the so-called adiabatic temperature rise ATad ... 

All polymerization reactions with monomer combination, as well as polyadditions are characterized by very high specific heats of reaction. Some examples, supplemented by the resulting adiabatic temperature rises, are given in Table 4-6. 

This relation can be used to define various other isochoric heat quantities such as integral and differential, molar and specific heats of reaction and the corresponding heat capacities. The most well known of these quantities is the (global or integral) heat capacity at constant volume or isochoric heat capacity, which we got to know briefly in Sect. 9.1 ... 

As we have seen in the case above of internal energy [Eq. (24.12)], this relation can be useful for defining various isobaric heat quantities such as integral and differential, molar and specific heats of reaction, transition, solution, mixing, etc. These are all produced similarly at constant p and T and, depending upon the process in question, each one can have various symbols and names. We will be content with only two examples, one integral quantity and one differential quantity ... 

On the basis of the detailed treatment in [2] some peculiarities of polymerization reactions are briefly described here (vid. Sect 2.2). Frequently such reactions are characterized by high values of the specific heats of reaction (appr. 500-3,500 kJ/kg). 

Differential scanning calorimetry A Perkin-Elmer DSC-2 calorimeter with Thermal Analysis Data Station was used. The calorimeter was calibrated according to manufacturer s specifications. Heats of reaction were calculated from the peak areas using indium as a standard (AH=6.80 cal/g). Tg was taken as the onset of the endothermic deflection. The heating rate was set to 20 /min. For DSC analysis, samples were prepared by two techniques a) vacuum drying of varnish and b) by flaking off resin from prepreg. 

Polymerizations are generally exothermal reactions with specific energies up to 3600 kJ kg , corresponding to an adiabatic temperature rise of up to 1800 K. Some typical reaction enthalpies are presented in Table 11.2, together with the specific heat of reaction and adiabatic temperature rise obtained for mass polymerization. Most free-radical and ionic polymerizations have negative standard enthalpies and standard entropies thus at higher temperatures these reactions must be considered reversible [Eq. (15)]. 

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