Fluid exothermic/endothermic reactions

Tubular reactor Behavior close to plug flow High heat and mass transfer rates - suitable for highly exothermic /endothermic reactions Not suitable for slow reactions Difficult to use hig viscous fluids... 

Temperature gradient normal to flow. In exothermic reactions, the heat generation rate is q=(-AHr)r. This must be removed to maintain steady-state. For endothermic reactions this much heat must be added. Here the equations deal with exothermic reactions as examples. A criterion can be derived for the temperature difference needed for heat transfer from the catalyst particles to the reacting, flowing fluid. For this, inside heat balance can be measured (Berty 1974) directly, with Pt resistance thermometers. Since this is expensive and complicated, here again the heat generation rate is calculated from the rate of reaction that is derived from the outside material balance, and multiplied by the heat of reaction. 

With exothermic reactions, it may be necessary for reasons of safety to control the temperature rise in the reactor and in this case coils of tubing are normally provided through which cold fluid is circulated (see Fig. 1). On the other hand, with endothermic reactions, it may be necessary to heat the reactants to ensure a reasonable rate of reaction. Heating may also be required to enable a viscous mixture of reactants to be stirred without excessive power consumption. 

First of all, if the reaction is exothermic and if heat transfer is unable to remove all of the liberated heat, then the temperature of the reacting fluid will rise as conversion rises. By similar arguments, for endothermic reactions the fluid cools as conversion rises. Let us relate this temperature change with extent of conversion. 

For exothermic reaction, heat is released and particles are hotter than the surrounding fluid, hence the nonisothermal rate is always higher than the isothermal rate as measured by the bulk stream conditions. However, for endothermic reactions the nonisothermal rate is lower than the isothermal rate because the particle is cooler than the surrounding fluid. 

When strongly exothermic or endothermic reactions occur in the catalyst pellet, the temperature cannot be regarded as uniform throughout the catalyst particle. To describe heat transfer through a catalyst particle it is usually considered to be homogeneous. So, the heat flow is described by the conventional heat conduction equation used for isotropic solids or stagnant fluids. When chemical reactions take place, the energy balance equation is... 

The influence of the temperature distribution on selectivity varies according to the reaction scheme. Among such schemes, the ccmsecutive reaction (A —B — C) qualitatively represents many organic reactions with by-products. As shown in the previous section, the use of dilute phase is recommended for endothermic reactions, but prohibited for exothermic reactions. This conclusion agrees with the development of fluid bed reactors for partial oxidations (exothermic) and cracking (endothermic). This knowledge may help one to design or develop new fluid bed contactors. 

The heat transfer fluid will be a coolant for exothermic reactions and a heating medium for endothermic reactions. If the flow rate of the heat transfer fluid is sufficiently high with respect to the heal released (or adsorbed) by the reacting mixture, then the heat transfer fluid temperature will be constant along the reactor. 

Typical heat transfer fluid temperature profiles are shown here for both exothermic and endothermic reactions... 

The reactive fluid experiences exothermic chemical reaction. An endothermic chemical reaction occurs in the cooUng fluid to enhance its potential to remove heat generated by the reactive fluid. 

In the case of fast highly exothermic or endothermic reactions, temperature gradients inside the porous catalyst and temperature differences between the fluid phase and catalyst surface cannot be neglected. Depending on the physical properties of the fluid and the solid catalyst, important temperature gradients may occur. The relative importance of internal to external temperature profiles can be estimated based on the relationships presented in Sections 2.6.1.2 and 2.6.2.2. According to Equation 2.158 the temperature difference between bulk and outer pellet surface is ... 

Since G < 0, it is clear from the numerator of Equation 6.72 that reactant diffusion has a stabilizing effect, as expected. If the reaction is exothermic, heat transport in the fluid is away from the interface, G > 0, and heat transport is destabilizing. Finally, the effect of interfadal tension is stabilizing and of greatest importance for short wavelengths (small a), the same as found in Section 8 for solidification. Thus instabiUty is possible if the heat transport effect outweighs the combined effect of reactant diffusion and interfacial tension. Of course, G < 0 for an endothermic reaction, all three effects are stabilizing, and no instability is possible. 

Non-isothermal reactors are the reactors in which the temperature of the fluid in the reaction vessel changes significantly due to the heat of reaction AH. The temperature of a reacting fluid kept in a batch reactor would gradually increase with time for an exothermic reaction (AHj is negative) or decrease with time for an endothermic reaction (AHjj is positive) as shown in Figure 3.21. 

The term pbR(—AHr)AV describes the energy that is released in an exothermic reaction in catalyst particles. Alternatively, it describes the energy effect that is consumed in an endothermic reaction. AQ denotes the heat transfer to or from the surroundings, and the term mcp AT describes the change in the temperature of the flowing media (fluid). 

Not all reactions are exothermic. Thermal cracking is an endothermic reaction. Heat is absorbed. Good thing, too. If thermal cracking of crude oil was exothermic, all the earth s crude would by now have turned to coal and natural gas. Delayed cokers, visbreakers, and fluid catalytic cracking units are processes that are primarily endothermic in nature. A delayed coker operates with a zero order reaction. This means the rate of reaction depends on time in the coke drum and the temperature in the coke drum. The composition of the products of reaction have no effect. 

An important variant of the Fluid Bed system is under development. This variant eliminates use of air or oxygen in the actual gasifier. Steam and coal are the reactants. Since we know from Table 3 that the reaction of steam with coal is endothermic, a heat source must be provided. Hot solids in the form of char are heated in a combustor and are transferred to the gasification reactor as one these processes. In another, hot alkaline oxides react with the carbon dioxide in the gas to form carbonates. The exothermic reaction of carbonate formation supplies the heat requirements of the steam-carbon reaction. Both of these processes depend on a reactive coal or char to implement the steam-carbon reaction. 

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