Page 69 - The engineering of chemical reactions

Adiabatic Reactor Temperature 53

Flow-reactor problems are just as simple as batch-reactor problems. In fact, they are
the same mathematical problem even though the reactor configuration and operation are
totally different.

THERMODYNAMICS AND REACTORS

A chemical reaction cannot be made to produce a conversion beyond that of chemical
equilibrium, AG = 0, where r = 0, as we discussed previously. This is an application of
the Second-Law of Thermodynamics.
Chemical reactors can liberate or absorb very large amounts of energy, and the
handling of this energy is a major concern in reaction engineering. This topic is an application
of the First Law of Thermodynamics, which says that mechanical and thermal energy is
conserved in any process. When we describe a chemical reaction, we designate its rate, but
we should also be very concerned about the heat of the reaction; so we need to specify A Hi
and AG; for every reaction.

5 VjAj = 0, r = kf fi t$.‘j -kb fj C,ybi, AH;, AG;
j=l j=l j=l
Table 2-2 lists some important chemical reactions along with their standard state enthalpies
and free energies of reaction, all in kJ/mole.
These are all industrially important reactions that we will discuss throughout this
book in the text and in homework problems. The student might want to try to identify the
type of reaction represented by each equation and why it is important.
These values of A HR are standard state enthalpies of reaction (all gases in ideal-gas
states) evaluated at 1 atm and 298 K. All values of A HR are in kilojoules per mole of the
first species in the equation. When A HR is negative, the reaction liberates heat, and we say
it is exothermic, while, when AHR is positive, the reaction absorbs heat, and we say it is
endothermic. As Table 2-2 indicates, some reactions such as isomerizations do not absorb
or liberate much heat, while dehydrogenation reactions are fairly endothermic and oxidation
reactions are fairly exothermic. Note, for example, that combustion or total oxidation of
ethane is highly exothermic, while partial oxidation of methane to synthesis gas (CO + Hz)
or ethylene (C&t) are only slightly exothermic.
Simple examination of A HR of a reaction immediately tells us how much heat will
be absorbed or liberated in the reaction. This is the amount of heat Q that must be added
or extracted to maintain the reactor isothermal. (This heat is exactly the enthalpy change in
any flow reactor or in a batch reactor at constant pressure, and it is close to this for other
conditions.)

ADIABATIC REACTOR TEMPERATURE

It is also important to estimate the temperature increase or decrease in a reactor in which
no heat is added or removed, which is called an adiabatic reactor. From the First Law of
Thermodynamics, we can construct a thermodynamic cycle to estimate the AT in going
from reactants at temperature Tt to products at temperature T2, as shown in Figure 2-10.
Assume that reaction occurs at T, with heat of reaction A HR per mole of a key reactant that