Overshooting, temperature

Modem process facHities are computer controHed. Temperature and AT are programmed upward during the start of a growth cycle, pressure and temperature are monitored and controHed, and pressure and temperature overshoot alarms and overrides are provided. Such systems also store data from previous mns for correlations with properties or for identical repHcation of past conditions. 

The turbine temperature, flow, and speed increases in a very short time of about three to five minutes to the full rated parameters. There is usually a short period of time where the temperature may overshoot. If supplementary firing or steam injection for power augmentation is part of the plant system, these should be turned on only after the gas turbine has reached full flow. The injection of steam for power augmentation, if done before full load, could cause the gas turbine compressor to surge. 

Once the mould is removed from the oven the mould starts to cool at a rate determined by the type of cooling - blown air (slow) or water spray (fast). There may be a overshoot in the internal air temperature due to the thermal momentum of the melt. This overshoot will depend on the wall thickness of the plastic product. In Fig. 4.61 it may be seen that the inner air temperature continues to rise for several minutes after the mould has been taken out of the oven (at about 13.5 minutes). 

Individual room temperature control is difficult with warm air systems, as any temperature control damper is likely to pass air at all times. Air thermostats are not accurate detectors of temperatures and temperature overshoot occurs, with consequent increases in energy consumption... 

The curves in Figure 5.2 are typical of exothermic reactions in batch or tubular reactors. The temperature overshoots the wall temperature. This phenomenon is called an exotherm. The exotherm is moderate in Example 5.2 but becomes larger and perhaps uncontrollable upon scaleup. Ways of managing an exotherm during scaleup are discussed in Section 5.3. 

Clearly, the extent of exotherm-generated temperature overshoot predicted by the Chiao and finite element models differs substantially. The finite element results were not markedly changed by refining the mesh size or the time increments, so the difference appears to be inherent in the numerical algorithms used. Such comparison is useful in further development of the codes, as it provides a means of pinpointing those model parameters or algorithms which underlie the numerical predictions. These points will be explored more fully in future work. 

Current Developments. A number of low-cost proprietary temperature loggers are being trialled in conjunction with the above IS Controller. In one form (14) these produce only a strip chart data table. Although convenient for statistical analysis these require keying into a further microcomputer plotter to draw a complete process temperature profile, as shown in Figure lb. As an illustration of the IS Controller s performance, statistics for the 150 minutes after exothermic overshoot indicate a mean temperature within 0.1"C of the set point and a standard deviation of 0.4°C. 

By an industrial investigation of a gas-phase reaction, the chlorination of alkanes, thermal management (faster temperature ramping, avoidance of overshoots) was improved and, hence, control over radical formation was exerted. As a result, a significant increase in space-time yield to about 430 g h 1 was achieved using a hybrid micro-reactor plant compared with the conventional performance of 240 g h [127, 161]. 

In this way, the operating temperature is reached without thermal overshoots... 

As mentioned before, the vast majority of accidents in batch processing arise when the control of the temperature of the reaction mixture is lost. This situation often leads to a temperature thermal) runaway, i.e. a temperature overshoot that can result in undesired reactions (decompositions), evaporation, or gas formation. As a consequence, pressure is built up inside a reactor and this can cau.se an explosion. The explosion is usually accompanied by damage to the equipment and release of hazardous (toxic, explosive, or flammable) species to the. surroundings. 

Figure 8 illustrates how delicate the impact of residence time and temperatures is. Optimal performance is only achieved at short times not exceeding a few minutes and overshooting the temperature by roughly 10°C can already notably decrease the yield. 

A chemical reaction can be designated as oscillatory, if repeated maxima and minima in the concentration of the intermediates can occur with respect to time (temporal oscillation) or space (spatial oscillation). A chemical system at constant temperature and pressure will approach equilibrium monotonically without overshooting and coming back. In such a chemical system the concentrations of intermediate must either pass through a single maximum or minimum rapidly to reach some steady state value during the course of reaction and oscillations about a final equilibrium state will not be observed. However, if mechanism is sufficiently complex and system is far from equilibrium, repeated maxima and minima in concentrations of intermediate can occur and chemical oscillations may become possible. 

However, there is a temperature overshoot when the baskets are released and the autoclave with the laiger overshoot produces the higher conversions to lower bpt material. 

Procedure (ash-free MAD fibre). Place the crucible and its contents of MAD fibre in a cool muffle furnace. Increase the temperature of the furnace to 475-500°C keep at this temperature for at least 30 min and until ashing is complete. It is important not to overshoot the temperature, because the sintered crucibles will be damaged at 515°C. Remove the crucible from the muffle furnace, cool in a desiccator and weigh. 

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