Heat capacity, exothermic reaction

Non-toxic Opportunity for replacing VOCs Naturally occurring Inexpensive Non-flammable High specific heat capacity -exothermic reactions can be more safely controlled Distillation is energy intensive Contaminated waste streams may be difficult to treat High specific heat capacity - difficult to heat or cool rapidly... 

With batch reactors, it may be possible to add all reactants in their proper quantities initially if the reaction rate can be controlled by injection of initiator or acqustment of temperature. In semibatch operation, one key ingredient is flow-controlled into the batch at a rate that sets the production. This ingredient should not be manipiilated for temperature control of an exothermic reactor, as the loop includes two dominant lags—concentration of the reactant and heat capacity of the reaction mass—and can easily go unstable. 

FIGURE 6J5 If the heat capacity ot the reactants is larger than that of the products, the enthalpy of the reactants will increase more sharply with increasing temperature. If the reaction is exothermic, the reaction enthalpy will become more negative, as shown here. If the reaction is endothermic, the reaction enthalpy will become less positive and may even become negative. 

Use the estimates of molar constant-volume heat capacities given in the text (as multiples of R) to estimate the change in reaction enthalpy of N2(g) + 3 H,(g) —> 2 NH.(g) when the temperature is increased from 300. K to 500. K. Ignore the vibrational contributions to heat capacity. Is the reaction more or less exothermic at the higher temperature ... 

In case of exothermic reactions, the heat-exchange capacities of the reactor allow to rapidly evacuate the heat generated by the reaction and therefore to perform a transposition of a pure batch operating mode into a continuous one. The main point is the ability to avoid, as far as possible, an initial increase of the temperature as soon as the reactants are mixed. 

For reversible exothermic reactions, the situation is more complex. Figure 6.5a shows the behavior of an exothermic reaction as a plot of equilibrium conversion against temperature. Again, the plot can be obtained from values of AG° over a range of temperatures and the equilibrium conversion calculated as discussed previously. If it is assumed that the reactor is operated adiabatically, and the mean molar heat capacity of the reactants and products is constant, then for a given starting temperature for the reaction Tin, the temperature of the reaction mixture will be proportional to the reactor conversion X for adiabatic operation, Figure 6.5a. 

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. 

For cases where AH0 is essentially independent of temperature, plots of in Ka versus 1/T are linear with slope —(AH°/R). For cases where the heat capacity term in equation 2.2.7 is appreciable, this equation must be substituted in either equation 2.5.2 or equation 2.5.3 in order to determine the temperature dependence of the equilibrium constant. For exothermic reactions (AH0 negative) the equilibrium constant decreases with increasing temperature, while for endothermic reactions the equilibrium constant increases with increasing temperature. 

The high heat capacity associated with the large mass of liquid facilitates control of the reactor and provides a safety factor for exothermic reactions that might lead to thermal explosions or other runaway events. 

Equation 12.3-16 is valid whether heat is transferred to or from the system, and whether the reaction is exothermic or endothermic. Note that each term on the left side of equation 12.3-16 may be an input or an output. Furthermore, CP is the molar heat capacity of the system, and is given by equation 12.3-13 as such, it may depend on both T and composition (through /A). The right side of equation 12.3-16 may also be expressed on a specific-mass basis (12.3-11). This does not affect the consistency of the units of the terms in the energy balance, which are usually J s-1. 

Some of the heat transferred to the surroundings during an exothermic reaction are absorbed by the calorimeter and its parts. In order to account for this heat, a calorimeter constant or heat capacity of the calorimeter is required and usually expressed in J °C 1. 

High thermal conductivity, heat capacity and heat of evaporation. High specific heat capacity means that exothermic reactions may be controlled effectively. 

Water offers a number of important properties as a solvent for polymerization reactions. As well as its high polarity, which gives a markedly different miscibility with many monomers and polymers compared to organic solvents, it is nonflammable, nontoxic and cheap. Water also has a very high heat capacity that sustains heat exchanges in a number of very exothermic polymerizations. Largely because of these factors, polymerizations are now widely carried out in aqueous media, and, for example, more than 50% of industrial radical polymerizations are carried out in water [19]. 

In catalysis applications, the tunable solvent properties result in a variety of effects, such as controllable component and catalyst solubilities. Moreover, it is possible that kinetic rates are affected by both temperature and pressure effects, equilibrium constants are shifted in favor of the desired products, and selectivity and yields are increased by manipulating the solvent s dielectric constant or by controlling the temperature in highly exothermic reactions through an adjustment of the solvent s heat capacity [18-23]. 

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