Adiabatic reaction temperatures

Polymerization processes are characterized by extremes. Industrial products are mixtures with molecular weights of lO" to 10. In a particular polymerization of styrene the viscosity increased by a fac tor of lO " as conversion went from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1,800 K (3,240 R). Heat transfer coefficients in stirred tanks with high viscosities can be as low as 25 W/(m °C) (16.2 Btu/[h fH °F]). Reaction times for butadiene-styrene rubbers are 8 to 12 h polyethylene molecules continue to grow lor 30 min whereas ethyl acrylate in 20% emulsion reacts in less than 1 min, so monomer must be added gradually to keep the temperature within hmits. Initiators of the chain reactions have concentration of 10" g mol/L so they are highly sensitive to poisons and impurities. 

Computed Adiabatic Reaction Temperature (CART) at constant pressure and/or volume... 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

Figure 2-82. Schematic representation to calculate the adiabatic reaction temperature (T ).

Determine the stability of all individual components of the reaction mixture at the maximum adiabatic reaction temperature. This might be done through literature searching, supplier contacts, or experimentation. 

The adiabatic reaction temperature reaches essentially the steady conditions, x = 0.426 and T = 784, after about 70 axial increments. 

Heat generated at the catalytic wall of the active channels is efficiently transmitted by conduction through the thin metal foil and is dissipated in the gas flow on both the catalytic and the non-catalytic side, allowing the wall temperature to be kept well below the adiabatic reaction temperature. The structure can be adjusted in order to tune the fraction of active channels in the monolith cross-section. 

If we assume the heat capacity Cp is equal to (a reasonable approximation for small molecules such as N2 at moderate temperatures), this predicts an adiabatic reaction temperature of... 

Energy considerations give the first estimate of the characteristics of combustion. As discussed in Chapters 2 and 5, the adiabatic reaction temperature (frequently called the adiabatic flame temperature) gives an approximation to the final temperature,... 

From the data shown in Fi re 1, two key points are evident (1) increasing the adiabatic reaction temperature increases the selectivity of synthesis gas formation, especially Sh2 and (2) Rh is a better catalyst than Pt for producing H2 by direct oxidation of CH4. 

Similarly, a Rh foam monolith with 0.56 wt.% Rh gave a lower optimal H2 selectivity than a Rh foam monolith with 9.83 wt.% Rh (75% vs. 87%). In both the Pt and the Rh experiments, the samples with the lower metal loa gs had significantly higher adiabatic reaction temperatures because of the heat generated by the formation of H2O. As demonstrated by these experiments, the formation of H2 occurs on the noble metal surface, not in the gas phase or on the catalyst support. 

In the second configuration (hybrid combustor), only a portion of the fuel is fed to the catalyst section. The inlet air/fuel ratio is carefully controlled to limit the adiabatic reaction temperature typically below 1000 °C, and accordingly, to reduce the catalyst thermal stresses. The remaining amount of fuel is fed to a... 

We will review the basic quantities of thermodynamics energy, temperature, heat, work, and the ideal gas law. These quantities will be used to explain the principles of thermophysics and thermochemistry, which will be applied to the specific reactions of combustion and detonation. Using the thermochemical data of heats of detonation or explosion, we will see how to calculate adiabatic reaction temperatures. These data in turn will be used to analyze or predict pressures of explosions in closed vessels. We shall also see how, using thermochemical data, to predict detonation velocities and detonation pressures. 

We are now equipped to determine what is called the adiabatic reaction temperature. This is the temperature obtained inside the process when (1) the reaction is carried out under adiabatic conditions, that is, there is no heat interchange between the container in which the reaction is taking place and the surroundings and (2) when there are no other effects present, such as electrical effects, work, ionization, free radical formation, and so on. In calculations of flame temperatures for combustion reactions, the adiabatic reaction temperature assumes complete combustion. Equilibrium considerations may dictate less than complete combustion for an actual case. For example, the adiabatic flame temperature for the combustion of CH4 with theoretical air has been calculated to be 2010 C allowing for incomplete combustion, it would be 1920 C. The actual temperature when measured is 1885 C. 

To calculate the adiabatic reaction temperature, you assume that all the energy liberated from the reaction at the reference temperature plus that brought in by the entering stream (relative to the same base temperature) is available to raise the temperature of the products. We assume that the products leave at the temperature of the reaction, and thus if you know the temperature of the products, you automatically know the temperature of the reaction. In effect, for this adiabatic process we can apply Eq. (4.40). 

Any phase changes that take place and are not accounted for in the heats of formation must be incorporated into the energy pool. Because of the character of the information available, the determination of the adiabatic reaction temperature or flame temperature may involve a trial-and-error solution hence the iterative solution of adiabatic flame temperature problems is often carried out on a computer. 

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