Heat removal capacity

Combined thermal and multizone airflow models are needed for problems such as thermal comfort analysis in naturally ventilated buildings, determination of heat-removal capacity by natural ventilation, design and evaluation of passive cooling by nighttime ventilation. This is outlined in more detail in Section 11.5. 

Deterirunation of heat-removal capacity by natural ventilation... 

Clearly, the oxidation reaction could not have been implemented in a pure batch operating reactor. Indeed, heat removal capacity would not have been sufficient (100—1200 kW m removed versus 20 x 10 kW m generated). As a consequence, a semibatch mode is necessarily required. Besides, Table 12.10 shows that the feeding times are much higher than the residence time of the Shimtec reactor (around 15 s). 

The investigation team found that the reaction accelerated beyond the heat-removal capacity of the reactor. The resulting high temperature led to a secondary runaway decomposition reaction, causing an explosion that blew the hatch off the reactor and allowed the release of the contents from the vessel. 

A typical graph of k as a function of temperature is shown in Figure 3.6. The increasing slope shows the importance of determining a maximum allowable temperature in process equipment so that the heat removal capacity is not exceeded. Under adiabatic conditions, the temperature will reach the calculated maximum only if the reactants are depleted. The actual maximum temperature in a system with some heat dissipation will, of course, be somewhat lower than the calculated value. 

Another example of scale-up effects relates to the storage of chemically unstable substances. Well-established procedures can be followed on a small scale. In a commercial unit, the storage of such materials must be reviewed from the standpoint of critical mass. The heat removal capacity of the equipment must be substantially larger than the spontaneous exothermic rate of heat release in the bulk material. Temperature gradients must also be considered. 

The point where the heat production rate reaches its maximum value is of critical importance for a chemical process. This maximum value needs to be compared with the total given maximum heat removal capacity. A reaction going to completion can be considered safe, for normal operation, if the maximum heat removal capacity is greater than the maximum heat production rate. For more precise analysis see the literature 19, 10, 11/. 

For process reasons, and especially in this case because of the relatively high adiabatic temperature rise ATadiab (> 50 K) it is necessary that the heat removal capacity matches (or exceed) the reaction heat capacity. The cooling capacity has therefore been chosen in such a way that the heat of reaction that is released at the intended metering rate can be removed at the required process temperature. 

I b) Reactive power cannot be removed completely by the cooling system so that the temperature temperature increases beyond the required process temperature. The maximum temperature which can be reached is Tpr0cess + ATadiab- With Tpr0cess < 80°C it remains below Texo (even if the heat removal capacity = 0 )... 

The left side of Eq. (11.43) represents the heat generation. The right side represents heat removal. Thus our simple linear analysis tells us that the heat-removal capacity must be greater than the heat generation if the system is to be stable. 

Afterwards, the PP powder is fluidized by a mixture of ethylene, propylene and inert gas (propane) in the fluid bed reactor, FBR, which is cooled by convective gas flow. The monomer conversion per gas circulation is limited to a few percent, because of the adiabatic temperature rise of more than 10 K per % conversion. Neglecting the heat loss through the reactor wall and through the product withdrawal, and assuming a well mixed reactor, the heat removal capacity of a steady-state FBR limits the productivity and can be expressed by... 

Cooling Section For the cooling section, a quench cooler with adequate heat removal capacity is effective. Another technique is to inject the hot medium through an expansion valve into a vacuum chamber, which is known as flash cooling. Both of these take a very short time therefore, the sterilization during the cooling period can be assumed to be negligible. 

In the chapters devoted to reactors, it was considered that a situation is thermally stable due to the relatively high heat removal capacity of reactors compensating for the high heat release rate of the reaction. We considered that in the case of a cooling failure, adiabatic conditions were a good approximation for the prediction of the temperature course of a reacting mass. This is true, in the sense that it represents the worst case scenario. Between these two extremes, the actively cooled reactor and adiabatic conditions, there are situations where a small heat removal rate may control the situation, when a slow reaction produces a small heat release rate. These situations with reduced heat removal, compared to active cooling, are called heat accumulation conditions or thermal confinement. 

For batch reactors in which two reactants are involved, the problems encountered with a pure batch reactor can sometimes be reduced if only one of the reactants is initially charged to the vessel and the other reactant is fed gradually at a rate such that the heat removal capacity is not exceeded. We will study this mode of operation in the next section. 

Since hydrogenation reactions are very exothermic, the situation often arises where the heat removal capacity cannot maintain the desired temperature with the normal operating hydrogen pressure. This usually occurs early in the batch when the concentration of the other reactant is high because it has not yet been diluted by the formation of the product compound. This situation requires that the flowrate of hydrogen be restricted so that temperature control is maintained. Thus pressure control should be temporarily abandoned. 

Even with these improvements the heat-removal capacity of the reactor vessel is not sufficient when using fast initiators. By the addition of an external condenser the cooling capacity may increase by about 30% to 50% [1]. 

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