We are interested in finding dew point of a vapor containing
80 percent benzene and 20 percent toluene.
Vapor pressure data is provided:
log of P is equal to A minus B over t plus C. Note that P is the vapor pressure in millimeter of mercury and t
is in degrees Centigrade. Total system pressure is 1000 millimeter of mercury.
Solution:
Dew point is the temperature at which the first drop of dew
is formed upon condensation. During
calculation of dew point, it is assumed that the liquid phase is ideal, so for
a binary mixture, x1 plus x2 is equal to one. For the dew point, the sum of y1/K1
and y2/K2 should be equal to one. In other words, the F(t), defined by y1
over K1 plus y2 over K2 minus one should be
equal to zero. Remember that the K is
the volatility, a ratio of pure component vapor pressure, P, to the system
pressure, Pt.
For a guessed dew point value of 100 degrees C, the function
could be calculated as 0.23. Another
guess of 110 degrees C gives value of the function equal to minus 0.078. It can be seen that at 107 degrees, function
is almost zero. So, the dew point of
the vapor is 107 degrees Centigrade.