Temperature rise, adiabatic, exothermic

Adiabatic operation. If adiabatic operation leads to an acceptable temperature rise for exothermic reactors or an acceptable fall for endothermic reactors, then this is the option normally chosen. If this is the case, then the feed stream to the reactor requires heating and the efiluent stream requires cooling. The heat integration characteristics are thus a cold stream (the reactor feed) and a hot stream (the reactor efiluent). The heat of reaction appears as elevated temperature of the efiluent stream in the case of exothermic reaction or reduced temperature in the case of endothermic reaction. 

Reactor heat carrier. As pointed out in Chapter 7, if adiabatic operation is not possible and it is not possible to control temperature by indirect heat transfer, then an inert material can be introduced to the reactor to increase its heat capacity flowrate (i.e. product of mass flowrate and specific heat capacity). This will reduce temperature rise for exothermic reactions or reduce temperature decrease for endothermic reactions. The introduction of an extraneous component as a heat carrier effects the recycle structure of the flowsheet. Figure 13.6a shows an example of the recycle structure for just such a process. 

A useful way to depict how highly energetic polymerization reactions are is the computation of their adiabatic temperature rise. Adiabatic temperature rise, is defined as the maximum temperature rise achieved during an exothermic chemical reaction when all heat generated by the reaction is adsorbed by the reacting mass in a closed system with... 

Figure 3.28 Adiabatic temperature rise (termed exotherm here) of reformate containing 53.1 vol.% hydrogen, 7.7 voL% carbon monoxide, 7.5 vol.% carbon dioxide, 31.4 vol.% steam and 0.3 vol.% methane versus carbon monoxide conversion by water-gas shift [57. ...

CO oxidation tests on Au supported on various metal oxides were undertaken at low CO concentrations, where the adiabatic temperature rise in the bed is negligible. Since CO oxidation is highly exothermic, when high CO concentrations are present in the feed 1%), and at high conversions, the adiabatic temperature rise in the catalyst bed due to the heat of reaction may be as high as 100 C. Therefore, it is important to monitor the catalyst bed temperature when high CO concentrations are present in the feed. 

In order to develop the safest process the worst runaway scenario must be worked out. This scenario is a sequence of events that can cause the temperature runaway with the worst possible consequences. Typically, the runaway starts with failure that results in an adiabatic course of exothermic reaction, inducing secondary reactions that proceed with a high thermal effect. Such a. sequence of typical events is shown in Fig. 5.4-55 (after Gygax, 1988-1990 Stoessel, 1993). It starts with, for instance, a cooling failure at time tx when the temperature is at the set level, Tv ,- Then the temperature rises up to the Maximum Temperature for Synthetic Reaction (MTSR) within the time Atn. Assuming adiabatic conditions MTSR = + ATad,R... 

Many kinetic data can be collected from ARC experiments the exothermic onset temperature, the rate of temperature rise, the rate of pressure rise, and the apparent activation energy. The basic data obtained are, however, thermodynamic properties the adiabatic temperature rise, the maximum pressure potential, the quantity of gaseous products generated, and the heat of reaction can be obtained in one run. The heat of reaction is estimated from ... 

The temperature rise due to this exothermic reaction then approaches the adiabatic temperature rise. The final steady state is always characterized by conditions T = T, and c = 0. A batch reactor, in which a zero order reaction is carried out, always has a unique and stable mode of operation. This is also true for any batch and semibatch reactor with any order or combination of reactions. 

These tests can also be used to evaluate the induction time for the start of an exothermic decomposition, and the compatibility with metals, additives, and contaminants. The initial part of the runaway behavior can also be investigated by Dewar tests and adiabatic storage tests. To record the complete runaway behavior and often the adibatic temperature rise, that is, the consequences of a runaway, the accelerating rate calorimeter (ARC) can be used, although it is a smaller scale test. 

To get an idea of the possible effects of a runaway, it is useful to calculate or to determine experimentally the adiabatic temperature rise, and to consider the effect of this temperature increase on the system. An adiabatic temperature rise of 150°C or above is considered a strongly exothermic situation that could result in loss of containment. 

For small vessels and slow reactions, corrections must be made because of the heat content of the reaction vessel itself. For large-scale reaction vessels and for rapid reactions, the system will be close to adiabatic operations. This aspect must be taken into account in scale-up. In effect, the extrapolation of data obtained in small-scale equipment has limitations as discussed in [193]. In case of a runaway, the maximum temperature in the reaction system is obtained from the adiabatic temperature rise, that is, Tmax = (Tr + ATad). In reality, the adiabatic temperature rise is significantly underestimated if other exothermic reaction mechanisms occur between Tr and (Tr + ATad). Therefore, a determination must be made to see if other exothermic events, which may introduce additional hazards during a runaway, occur in the higher temperature range. This can determine if a "safe operating envelope" exists. 

The reaction is strongly exothermic, with a heat of reaction AH equal to 59.97 kj mol-1, which is equivalent to an adiabatic temperature rise of 1.9-2.5°C per wt% of sodium hypochlorite in the feed solution. 

In each case the reaction is exothermic and the reaction enthalpy AHR known thereby the defining the adiabatic temperature rise which is always greater than ATadiab > 50 K. Below 60°C the reaction becomes dormant and an undesirable accumulation of reactants is to be expected. Major reaction energy releases are to be anticipated if the reaction re-initiates. 

The reaction is highly exothermic. Thus, if the reaction is conducted under adiabatic conditions the temperature rises and AG approaches zero, at which point the reaction reaches equilibrium and the temperature will not increase further, preventing further escalation ... 

As we see, for a specific reaction, the higher the inlet concentration, the higher the conversion and the exit temperature. This is a result of the positive effect of the temperature rise, due to the exothermic nature of the reaction, on the rate coefficient and thus on the reaction rate and conversion. Note that for higher inlet CO concentration, the conversion for the isothermal operation is the same, while for the adiabatic operation the conversion is higher for higher inlet concentrations. Furthermore, the conversion in the adiabatic fixed bed is always higher in comparison to the isothermal fixed bed. Of course, these results are such because the reaction is of first order in respect to CO. 

The addition of solvent makes it possible to control the temperature in the reactor despite the exothermic reactions and high reaction rates. The reactor operates nearly adiabatically, but the temperature rise in the reactor can be controlled, because the solvent acts as an internal cooling medium. The concentration of substrate determines the maximal temperature rise and therefore, by controlling the concentration, the maximal temperature rise is controlled. In this way the amount of unwanted side-products can be reduced. 

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