Heat of reaction parameter

Viscosity has not been found to be a meaningful. The hot spot effect in poorly mixed viscous reactors can be included in the heat of reaction parameter. New phase generation and catalysts have not been chosen either because both parameters are considered by hazardous properties of those substances (chemical interaction, explosiveness etc.). This applies also to waste products parameter. 

Temperature control for laboratory reactors is typically easy because of high heat transfer area-reactor volume ratios, which do not require large driving forces (temperature differences) for heat transfer from the reactor to the jacket. Pilot- and full-scale reactors, however, often have a limited heat transfer capability. A process development engineer will usually have a choice of reactors when moving from the laboratory to the pilot plant. Kinetic and heat of reaction parameters obtained from the laboratory reactor, in conjunction with information on the heat transfer characteristics of each pilot plant vessel, can be used to select the proper pilot plant reactor. 

By introducing suitable approximations, simple analytical relationships have been derived to represent criticality in terms of the heat transfer parameter, as a fimction of the reaction order, n, and the heat of reaction parameter, a. Some of the expressions obtained are summarized in Table 1. 

Table 3, Critical values of the heat of reaction parameter, for various sensitivity criteria n= 1, i=0, 7=20, Le=l, A3=0.01, < =0.

The influence of inter and intraparticle transport resistances on the region of generalized parametric sensitivity is shown in Figure 4. For sufficiently large values of and the reactor becomes transport - limited, and runaway does not occur. On the other hand, in the region of small Ag values, as Ag increases (with Le fixed), transport limitations increase, so that it becomes more and more difficult to remove heat from the particle. This leads sooner to runaway, i.e. for lower values of the heat of reaction parameter a. 

In Figure 8, the normalized sensitivity of the outlet reactor temperature, co, is reported as a function of the dimensionless heat of reaction parameter, a, with respect to each of the four independent parameters of the model (14) to (15). The critical value is again the same for any choice of the input model parameter, [Pg.455]

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... 

The calculation of heat balance around the reactor is illustrated in Example 5-6. As shown, the unknown is the heat of reaction. It is calculated as the net heat from the heat balance divided by the feed flow in weight units. This approach to determining the heat of reaction is acceptable for unit monitoring. However, in designing a new cat cracker, a correlation is needed to calculate the heat of reaction. The heat of reaction is needed to specify other operating parameters, such... 

Based on the experimental data kinetic parameters (reaction orders, activation energies, and preexponential factors) as well as heats of reaction can be estimated. As the kinetic models might not be strictly related to the true reaction mechanism, an optimum found will probably not be the same as the real optimum. Therefore, an iterative procedure, i.e. optimization-model updating-optimization, is used, which lets us approach the real process optimum reasonably well. To provide the initial set of data, two-level factorial design can be used. 

There is considerable variation in the heat of reaction data employed in different articles in the literature that deals with this reaction. Cited values differ by more than an order of magnitude. If we utilize heat of combustion data for naphthalene and phthalic anhydride and correct for the fact that water will be a gas instead of a liquid at the conditions of interest, we find that for the first reaction (equation 13.2.3) the standard enthalpy change will be approximately — 429 kcal/g mole for the second reaction it will be approximately — 760 kcal/g mole. These values will be used as appropriate for the temperature range of interest. Any variation of these parameters with temperature may be neglected. 

Most of the illustrative examples and problems in the text are based on actual data from the kinetics literature. However, in many cases, rate constants, heats of reaction, activation energies, and other parameters have been converted to SI units from various other systems. To be able to utilize the vast literature of kinetics for reactor design purposes, one must develop a facility for making appropriate transformations of parameters from one system of urtits to another. Consequently, I have chosen not to employ SI units exclusively in this text. 

The standard deviation between experimental and calculated heats of reaction are between 0.5 and 1 kcal/mol for those classes of compounds where enough experimental heats of formation are available to allow a full parameterization. For those classes of compounds where insufficient heats of formation are known to allow the determination of all parameters for 1,2- and 1,3-interactions, an estimate can be given for the bond energy terms which are the dominating parameters. Even here, therefore, a reasonable value for the reaction enthalpy is available. 

Heat of reaction, selected by Heikkila et al. (1996), measures the energy available from the reaction. A high heat of reaction may generate higher temperatures and dangerous runaway reactions. Another parameter to consider controllability of a reaction is reaction rate. Reaction rate does not directly express the hazardousness of a reaction (e.g. when the heat of reaction is low). Thus it has been excluded from the list of chosen parameters. 

In deriving these equations we have made many assumptions to keep them simple. We have assumed constant density so that we can use concentration as the composition variable. We have also assumed that the parameters in these systems are independent of temperature and composition. Thus parameters such as AAr, pCp, and JJ are considered to be constants, even though we know they all depend at least weakly on temperature. To be exact, we would have to find the heat of reaction, heat capacity, and heat transfer coefficient as functions of temperature and composition, and for the PFTR insert them within the integrals we must solve for temperature and composition. However, in most situations these variations are small, and the equations written will give good approximations to actual performance. 

Table 2.7 shows some of the kinetic models for vinyl ester and some polyester systems, where one can notice the large differences among the kinetic parameters even for the same resin and range of temperatures. When a reinforced material is added to DERAKANE resin, for example, Palmese [206] found that the heat of reaction, frequency factor, and order of reaction, m, are 63 percent, 295 percent, and 16.5 percent higher, respectively, and that the activation energy as well as the order of reaction, n, are 26.5 percent and 22 percent lower, respectively, than are those from the paper of Lam et al. [98],... 

The effectiveness is a measure of the utilization of the internal surface of the catalyst. It depends on the dimensions of the catalyst particle and its pores, on the diffusivity, specific rate, and heat of reaction. With a given kind of catalyst, the only control is particle size to which the effectiveness is proportional a compromise must be made between effectiveness and pressure drop. In simple cases t] can be related mathematically to its parameters, but in such important practical cases as ammonia synthesis its dependence on parameters is complex and strictly empirical. Section 17.5 deals with this topic. 

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