Heat of reaction terms

Here R=r is the parameter for radiative heat transfer in K units, p is a heat of reaction term, in K/atm units tj is the fluid temperature in the j-th axial position e is the particle emissivity 1 is the celt dimension in m 6 is the clock time in minutes... 

Example 6.6 Suppose the reactions in Equation (6.1) are exothermic rather than endothermic. Specihcally, reverse the sign on the heat of reaction terms... 

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. 

AC0 = 0. With this assumption, pthe last two terms in Equation (1) vanish.- Pit can be shown that the resulting equation is equivalent to the integrated form of the van t Hoff equation where the heat of reaction term is assumed to be constant,... 

The recombination reactions consume free radicals to create stable species, resulting in a net reduction of radicals. Since these recombination reactions are very exothermic, they cause the temperature to increase. The lower panel of Fig. 16.11 shows the contribution of various reactions to the temperature rise. Specifically, it shows the contribution of each reaction i to the heat-of-reaction term in the thermal-energy equation (Eq. 16.98) ... 

Note that the above equation does not contain any heat of reaction terms. These are automatically included as will become clear in next few lines. 

Note that heat of reaction terms are not required if the elements are chosen as references The heats of reaction are implicitly included when the heats of formation of the reactants (included in the Hj terms) are subtracted from those of the products (in the Horn terms) in the expression for AH. Substituting the calculated n and H values into Equation 9.5-2 yields AH = -1.26 X IIP kJ/s. 

The energy balance in the fixed bed includes convective, conductive and radiative heat transfer and heats of reactions terms. The equation is described below ... 

This result is then used to rewrite the surface enthalpy substantial derivative term in (3.102), and so the required form of the jump enthalpy balance is obtained. However, in practical applications of the jump condition for catalytic surfaces, one generally neglects the terms describing the interfacial transport phenomena and retains only the heat of reaction term. Therefore, the approximate jump heat balance is written as ... 

However, describing the chemical reactor performance under industrial operation conditions the heat balance is normally dominated by the heat of reaction term, the transport terms and the external heating/cooling boundary conditions, hence for chemical processes in which the phase change rates are relatively small the latent heat term is often neglected. 

The corresponding temperature equation for the interstitial gas is given by (11.6). To define the alternative model versions the effective heat of reaction term St = Pcat — in the basic model heat balance is... 

For the pseudo-homogeneous model St denotes an effective heat of reaction term. The source term for the pseudo-homogeneous model consists of two effective heat of reaction terms, one for the reforming reactions and one for the C02-acceptor kinetics ... 

Now, an exact temperature measurement of the air before and after the reaction is difficult. However, if we could insulate a portion of the surroundings, to isolate and trap the heat, we could calculate a useful quantity, the heat of the reaction. Experimental strategies for measuring temperature change and calculating heats of reactions, termed calorimetry, are discussed in Section 8.2. 

Suppose the reactions in Equation 6.1 are exothermic rather than endothermic. Specifically, reverse the sign on the heat of reaction terms so that the adiabatic temperature rise for complete conversion of A to B (but no C) is 4-30 K rather than —30 K. How does this change the results of Examples 6.2-6.57... 

SOLUTION The time-dependent and heat-of-reaction terms are omitted from Equation... 

It is of interest to note that the familiar heat of reaction term is included in the sum of the djihj-s. Evaluation of this term by use of Eq. (8-28) gives... 

If elemental species were taken as references, the heats of formation of each molecular species would have to be taken into accoimt in the enthalpy calculations and the heat of reaction term would not have been included in the calculation of AH. 

B). Under normal circumstances, the thermal loss term will be small compared to the heat of reaction term and we may obtain a reasonable first estimate of AT by neglecting the thermal loss term. On the basis of Illustration 12.8, we shall... 

All we have done here is notice that, because work becomes a fixed quantity in constant pressure processes, then heat does too, by the first law. And because constant pressure processes are so common (including all reactions carried out at atmospheric pressure, such as most biochemical reactions), it is convenient to have a state variable defined to equal this heat term. Defining enthalpy as in (3.15) accomplishes this, and we now have a heat of reaction term, which will be useful in all constant pressure processes. 

To define the two model versions, the effective heat of reaction term Sp = (1 - Sp,cat)pcat Y.r( R,r)rr in the basic pellet model heat balance is substituted with a modified source term Sp. ... 

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