Average heat of reaction

In terms of an average heat of reaction over a modest temperature range, the often adequate equation is... 

The polymerization process is exothermic. The average heat of reaction when motor polymer is made from propylene is 670 B.t.u./lb. In contrast, the average heat of reaction of butylenes to motor polymer is only 400 B.t.u./lb. It is easier, therefore, to control the temperature of a butylene polymerization unit than to control propylene polymerization. 

The function of q( ) is the heat of reaction relative to the chemical reaction under consideration [Ref. 1.]. We further define the average heat of reaction for the change from to as expressed by Eq. 4.15. Integrating Eq. 4.14 for one equivalent extent of the reaction, we obtain the average heat of reaction g f°r the chemical change as shown in Eq. 4.15 ... 

This equation indicates that the average heat of reaction Q ° at constant Tand V equals the change in the internal energy (AU)t v for one equivalent extent of the reaction. 

This is the direct relation connecting the average affinity and the average heat of reaction. 

The average heat of reaction at constant T and V is thus equal to the decrease in internal energy in that reaction. 

Equations (4.8) and (4.9) can also be written in terms of the average affinity and average heat of reaction. We start from the Gibbs-Helmholtz relation (4.33) and apply this to one state in which = 1, and another in which T and V have the same values but = 0 ... 

Average heat of reaction 1, AFTi = 53,600 Btu/lb mole Average heat of reaction 2, ATfj = 43,900 Btu/lb mole Average heat of reaction 3, = —27,700 Btu/lb mole... 

Pressure DSC was used by Hassel (285) to evaluate catalyst activities. Hydrogen gas was added to a DSC cell containing the PdO/carbon catalyst, previously pressurized at 150 psig with helium. As the hydrogen replaced the helium gas, an exothermic reaction was observed, the peak area of which was proportional to the heat of reaction. The average heat of reaction, as obtained from six runs, was 19.5 0.25 mcal/mg of catalyst... 

Figure 5.21 shows a typical DSC curve obtained an initial endothermic blip occurs when the gas is introduced, followed by a large exothermic peak due to the chemisorption reaction. The PdO/carbon catalyst shown in Figure 5.21 was measured six times and the average heat of reaction was 81.6 +1.05 J g (1.25% rel.). A shoulder peak is observed on the exothermic peak at around 5 min, which is attributed to physical adsorption on the carbon. However, it was difficult to separate the heat of chemisorption and physical adsorption on the carbon from Figure 5.21. 

If an average heat of reaction equal to 225 kj/mol (valid at 700°C) is used, it is found that in this case 75% of the total heat added is used in the reaction and the rest is used for heating of feed and product. The split between the two parts changes, of course, with H2O/CH4 ratio and other operating conditions. 

If one looks at the average bond dissociation energies for X2, C-X, H-X, and C-H bonds (Table 2.2), an average heat of reaction for the halogenation of alkanes can be calculated. The results in kcal/mol are as follows F = -101, Cl = -22, Br = -4, and I = 16. The variation in these numbers comes from a continual decrease in H-X and C-X bond strengths in the series F, Cl, Br, and I. These heats of reaction reflect a dramatic change in reactivity. Free radical fluorination is so exothermic that it occurs spontaneously and very explosively. Chlorination and bromination can be controlled and are useful reactions. Free radical iodination rarely occurs. 

The heat of reaction requires cooling water at the rate of 10 to 40 L/(1,000 L holdup)(h). Vessels under about 500 L (17.6 fF) are provided with jackets, larger ones with coils. For a 55,000-L vessel, 50 to 70 m" maybe taken as average. 

It should be emphasized that the enthalpy change includes not only sensible heat effects but also a heat-of-reaction term and in some cases pressure effects. If there are multiple inlet and outlet streams, appropriate averaging techniques must be used to employ this equation. 

Specific heat of steam is 0.52, and the average of the other constituents is 0.41. Heat of reaction is assumed constant at AHr = 60,000 Btu/lbmol. 

An alternative means of estimating the heat of reaction is to take the sum of the average bond energies of the reactant molecules and subtract the sum of the average bond energies of the product molecules. 

B Average burning time seconds /gram Heat of reaction cal /gram... 

A general requirement is that the average molecular weight of exhaust gas should be low. This restricts the choice of solid propellant components to these containing elements of low atomic weight. Low atomic weight elements often release a greater heat of reaction. 

Autoclave reactors are operated adiabatically, which means that the heat of reaction must be removed by the fresh ethylene entering the reactor. The conversion is related, therefore, to the difference in temperature between the feed and the reactor temperature. This limits the conversion to 15 - 20%. Taking into account the fact that the percentage conversion, Ax, can be approximated by using the average specific heat, cp, the enthalpy of the polymerization,... 

The heat of reaction released is composed of the 15 820 kJ/kg nitrogen monoxide reacted and 6060 kJ/kg nitrogen dioxide reacted. Average heat capacity of the reaction gases in this temperature range is 1. 1 7 kJ/(kg K). The mass flowrate is unchanged. 

---