Temperature and Conversion Profiles During Processing

The precursors of thermosetting polymers are usually one of the ingredients of complex formulations. They may be present in very small amoimts, as in the manufacture of abrasive disks where the thermoset acts as an aggluti-nant in medium amounts, as in the case of filler-reinforced thermosets or as the only components, in formulations used for encapsulation purposes. Apart from fillers, fibers, pigments, etc., some formulations contain rubber or thermoplastic modifiers that phase-separate upon the polymerization reaction (cure). 

The cure cycle is the temperature vs time schedule used to polymerize the thermoset precursors. The selection of an adequate cure cycle has several purposes. What is desired is to obtain the final part without strains exceeding design tolerances, with a uniform conversion (usually close to the maximum possible conversion), without degradation produced by the high temperatures attained during the cure, with convenient morphologies (in the case of heterogeneous materials), and all this, must be achieved in the minimum possible time for economic reasons. 

In this chapter, the evolution of conversion and temperature profiles during typical cure processes is discussed. This is useful for analyzing the possibility of attaining the maximum conversion, avoiding undesired high 

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Differential Eqs (9.10) and (9.11), with initial and boundary conditions (9.12) and (9.13), may be numerically solved for different sets of values of the four dimensionless parameters, W3-W4 (Williams et al., 1985). To illustrate the evolution of temperature and conversion profiles during the cure, values of W2-W4 will be kept constant and Wi will be varied to simulate the influence of the part thickness. The particular case of W2 = 40, W3 = 1.5, and W4 =0.125 will be analyzed. This represents a process characterized by high values of both the activation energy and the adiabatic temperature rise. 

Through the analysis of the particular selected examples it was shown that it is possible to get a good description of temperature and conversion profiles generated during the cure of a thermosetting polymer. Thermal and mass balances, with adequate initial and boundary conditions, may always be stated for a particular process. These balances, together with constitutive equations for the cure kinetics and reliable values of the necessary parameters, can be solved numerically to simulate the cure process. 

Fig. 1.8. Reverse-flow CATOFIN process at equal heat capacity fluxes during production and regeneration cycle periodic temperature profiles (top) and conversion profiles (bottom) at the end of the endothermic semicycle (t = tcyc/2) and the regeneration...

This program concentrated on the requirements and problems that are expected during typical refinery processing. The main concerted efforts, therefore, were to detect and differentiate levels of performance of the synthetic liquids compared with the performances of an appropriate petroleum counterpart stream. Temperature was used as the primary process variable. If catalyst on-stream life and conversion are to be optimized, then an accurate temperature profile performance must be the initial step. Hydro-treating performances were evaluated under simulated refinery conditions while varying the most practical primary process variable, temperature. All experimental work utilized American Cyanamid HDS-3A catalyst. 

Figure 5.49 The original (a) and heat pipe-modified (b) temperature profiles during the ammonia conversion process.

Many times, the conversion of a reactor depends on the operating temperature. Even if isothermal operation is recommended, sometimes it is not easy to remove the energy generated during the reaction, or because it can be an asset for the process, the reactor operates under adiabatic conditions. The example that we are trying to solve now presents the features of this last type. In the production of nitric acid from ammonia, the converter oxidizes NHj into NO. The conversion of that reactor varies with temperature and the conversion profile may be obtained from experimental data in the Ullmann s Encyclopedia [9]. 

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