Cooling rate effects steps

The real power of the model developed in this work lies in the transient or dynamic simulations such as those necessary for control system design. The model we have developed can be used to simulate the effects on the reactor of various process disturbances and input changes. Under normal reactor operating conditions, step or pulse changes in inlet gas temperatures, concentrations, or velocity or changes in cooling rates can significantly affect... 

To keep the temperature low the heat of reaction must be removed in an appropriate way, and to achieve a sufficient reaction rate effective catalysts have to be applied. The process is therefore performed in steps, with intermediate heat removal between the individual catalyst beds, in which the reaction runs adiabatically. Recently, quasi-isothermal reactors have been developed in which cooling tubes run though the catalyst layers. 

The recommended stirring rates are 200 rpm during dissolution and stabilization and 100 rpm during crystallization. The temperature of solution is decreased at a constant cooling rate (typically 0.1-0.2°C/min) during the crystallization step. The slow cooling rates are essential to avoid co-crystallization effects. Sample aliquots are taken out of the vessels at a given sequence and directed to the IR detector and the concentration of solution is recorded as a function of temperature. The plot of polymer concentration as a function of temperature (see Fig. 8) is called cumulative CRYSTAF profile. 

In this chapter, the influence of the number of recycling steps on the opacity of films of a commercial-grade i-PP is explored. The material was extruded several times to mimic the effect of recycling procedures. After extrusion, films were obtained by cooling samples of material at different cooling rates [18]. The opacity of the obtained films was then measured and related to their crystallinity and morphology. 

Figure 6.1 Effect of cooling rates and number of recycling steps on film opacity. Reproduced with permission from F. De Santis and R. Pantani, The Scientific World Journal, 2013, Article ID 354093. 2013, Hindawi Publishing Corporation...

In view of the broadening of the step and the rate effects, it does not seem appropriate to introduce a sharply defined Tg. For the practical use as material parameter and for comparisons it is sufficient to conduct the measurements with a standard heating or cooling rate ( T = 10 — 1 Ks ) and to pick out some temperature near the center of the step, for example that associated with the maximum slope. The so-obtained values of Tg have a tolerance of some degrees but this must be accepted regarding the physical nature of the phenomenon. 

---