Page 135 - Elements of Chemical Reaction Engineering 3rd Edition

Sec. 3.5 Reactions with Phase Change 107
I and combining Equations (E3-9.2), (E3-9.4), and (E3-9.8) gives

dFB -
- rB = -2rA = 2kACTo
-
dV
Equations (E3-9.8) and (E3-9.9) can now be solved numerically, preferably by a
I software package such as POLYMATH or MATLAB. (See Chapter 4)

3.5 Reactions with Phase Change

When Equation (3-36) is used to evaluate E , it should be remembered from the
derivation of this equation that 6 represents the change in the number of moles
in the gas phase per mole of A reacted. As the last example in this chapt, er , we
consider a gas-phase reaction in which condensation occurs. An examiple of
this class of reactions is

Another example of phase change during reaction is chemical vapor dep-
osition (CVD), a process used to manufacture microelectronic materials. Here,
gas-phase reactants are deposited (analogous to condensation) as thin films on
solid surfaces (see Problem P3-25). One such reaction is the production of gal-
lium arsenide, which is used in computer chips.

GaC12(g)+~As2(g)+H2(g) --+ GaAs(s)+2HCl(g)

The development of continuous-flow CVD reactors where solid wafers and
gases continuously pass through the reactor is currently under way (see: Sec-
tions 10.8 and 12.11).
We now will develop our stoichiometric table for reactions with phase
change. When one of the products condenses during the course of a reaction,
calculation of the change in volume or volumetric flow rate must be under-
taken in a slightly different manner. Consider anoiher isothermal reaction:

A (g) + 2B (g) - 3- D (g, 1)
The vapor pressure of species D at temperature T is P,
The gas-phase concentration of the product D will increase until the cor-
responding mole fraction at which condensation begins is reached:
At PD = P",
YD =.YD,e and (3-50)
condensahon starts

Once saturation is reached in the gas phase, every mole of D produced con-
denses. To account for the effects of condensation on the concentrations of the
reacting species, we now write two columns for the number of moles (or irnolar