Optimal heating rate

The injector is heated a few seconds after the injection when the solvent has already evaporated and has reached the column (Figure 2.68). Typically, this time interval is between 5 and 30 s. For high-boiling substances in particular, longer residence times have been found to be favourable. The heating rate should be moderate in order to achieve a smooth evaporation ofthe sample components required for transfer to the column. Heating rates of about 10-14°C/s have proved to be suitable. The optimal heating rate depends on the dimensions of the insert liner and the flow rate of the carrier gas in the insert. 

The morphology of globular type is the most favourable when superplastic deformation is to occur in AI78wt%Zn alloy. This type of structure is formed by decomposition of the a solid solution a -> a + P However, plates usually dominate in the structure of this alloy. To obtain the non-plate or globular type, a special heat treatment is neccesary i.e. the optimal cooling rate as well as the temperature and time of ageing. 

In order to answer these questions, the kinetic and network structure models were used in conjunction with a nonlinear least squares optimization program (SIMPLEX) to determine cure response in "optimized ovens ". Ovens were optimized in two different ways. In the first the bake time was fixed and oven air temperatures were adjusted so that the crosslink densities were as close as possible to the optimum value. In the second, oven air temperatures were varied to minimize the bake time subject to the constraint that all parts of the car be acceptably cured. Air temperatures were optimized for each of the different paints as a function of different sets of minimum and maximum heating rate constants. 

A network structure model has been developed from which a parameter that correlates well with physical measures of paint cure can be calculated. This model together with a kinetic model of crosslinking as a function of time and temperature has been used to evaluate the cure response of enamels in automotive assembly bake ovens. It is found that cure quality (as measured by the number and severity of under and overbakes) is good for a conventional low solids enamel. These results are in agreement with physical test results. Use of paints with narrower cure windows is predicted to result in numerous, severe under and over bakes. Optimization studies using SIMPLEX revealed that narrow cure window paints can be acceptably cured only if the bake time is increased or if the minimum heating rate on the car body is increased. 

This mode is seldom used except for extremely slow processes such as fermentation or for very small reactors where the surface area for heat transfer is large enough to maintain the reactor thermostatted at the temperature of the surroundings. In fact, we seldom want to operate a reactor isothermally, because we want to optimize the temperature and temperature profile in the reactor to optimize the rate and selectivity, and this is most efficiently achieved... 

Due to the irreversible heat exchange between the four heat reservoirs (for T0 = 300 K and T, = 600 K), the real maximum power is found to be close to a value of 0.3 rather than the ideal value of 0.5. The maximum power is found at an optimal flow rate (32 corresponding to an optimal set of temperatures T2 and T3/ satisfying the Carnot relation... 

The two parameters considered by Walters and Deming [612] were the initial temperature and the heating rate. They used a composite optimization criterion (see section 4.4.2) and imposed a time constraint of 5 minutes on the system by assigning a very unfavourable... 

Extraction rates in dependance of temperature and pressure are very different and are shown in figure 1. At 50°C the extraction efficiency is very low. Between 60°C and 70°C extraction is more effective and depends little on the carbon dioxide flow rate. For a given temperature the variation of pressure has only a small influence on the extraction rate, whereas the variation of temperature at a fixed pressure leads to great differences in extraction efficiency. The parts treated at 50°C often show cracks or bubbles. At 75°C they soften and deform. The optimal temperature range for this binder system is 60-70°C. Here it is possible to extract up to 70 vol.% of the binder without damaging the workpieces. As a result of the reached porosity it is possible to reduce the time of the furthermore necessary pyrolysis of the remaining binder components drastically (heating rate 10°C/min from r.t. up to 1000°C). The dimensions of the examined parts were 4 x 5 x 60 mm. 

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