Quality
of the Feed: The thermal condition of the feed plays an important role
in the feed location and the number of plates required to achieve a particular
level of separation. For a given feed
composition, find the bubble point and dew point.
Example
6.8: A solution contains two components benzene
and toluene. The more-volatile
component, benzene, is present at composition of 66.7 % on mass basis, whereas
less-volatile component, toluene, is present at a composition of 33.3 %. The feed enters the column at 40 °C. Calculate the quality of the feed. The
following information applies:
Bubble point
of the solution is 94 °C
Specific heat
of benzene, cpB, 32.8 cal/(mol·°C)
Specific heat
of toluene, cpT, = 40.5 cal/(mol·°C)
Latent heat of
vaporization of benzene, λB, = 7566 cal/mol
Latent heat of
vaporization of toluene, λT, = 7912 cal/mol
Solution:
Heat
required to vaporize one mole of benzene at feed temperature, HGB
= cpB(TB - TF)
+ λB = 32.8´(94 - 40) + 7566 = 9337.2 cal
Heat
required to vaporize one mole of toluene at feed temperature, HGT
= cpT(TB - TF)
+ λT = 40.5´ (94 - 40) + 7910 = 10097 cal
Heat
required to vaporize one mole of feed at feed temperature, HG
= xFHGB + (1 -
xF)HGT = 0.667´9337.2 + (1 - 0.667) ´10097 = 9590.2 cal
Latent
heat of vaporization of feed at bubble point, Hλ
= xFλB +
(1 - xF)λT = 0.667´7566 + (1 - 0.667) ´7910 = 7680.6
cal
Quality
of feed, q = HG/Hλ = 1.25