Page 40 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 29

where ct = relative volatility between the two components in 1.c
the temperature range Tbl to Tb2
Tbl = normal boiling point of Component 1,”K ‘I
-
Tb2 = normal boiling point of Component 2, “K *
L1= latent heat of vaporization for Component 1 at c
Q
I
0
Tbl, kcal/kmole n
L2 = latent heat of vaporization for Component 2 at L
0
Tb2, kcal/kmole 3
.^
c
e
If a compound’s latent heat is not known, it can be esti- c
Q
c
0
mated from the normal boiling points and molecular 0
E
-
0
weight. 0 I I
c I
.- I I
D
Example 8-8: Relative Volatility Estimate by Wagle’s -1 I I
0 .-
E
Method [92] (used by permission) c I
V I
LL
The average relative volatility of benzene and toluene -
can be determined using the following data: Tbb = 353.3 r”
K, Tbt = 383.8 K, Lb = 7,352 kcal/kmole, and L, = ’1,930 I
kcal/kmole (where the subscripts b and t denote benzene I I
and toluene, respectively). Substituting these values into 0 1.0
Equation 8-52 above, we find that: Mol Froetion Light Cumponent in Liquid Phase, x
Figure 8-23. Fractionation of binary mixture at minimum reflux con-
dition.
abt = exp
x (7,332+ 7,930) = 2.375
This compares with a value of 2.421 for a determined
using vapor-pressure/ temperature charts. External reflux ratio = L/D
Slope of line from XD:
Minimum Reflux Ratio: Infiiite Plates
As the reflux ratio is decreased from infinity for the
total reflux condition, more theoretical steps or trays are
required to complete a given separation, until the limit- L/V = internal reflux ratio
ing condition of Figure 8-23 is reached where the operat-
ing line touches the equilibrium line and the number of For non-ideal mixtures the minimum L/V may be as
steps to go from the rectifjmg to stripping sections indicated in Figure 8-15, and hence not fEed as indicated
becomes infinite. above.
If the operating lines of Figure 8-23 intersect at x,, yc Figure 8-17 presents a convenient and acceptably accu-
outside or above the equilibrium line when insufficient rate nomogram of Smoker’s [66].
reflux is used, the separation is impossible.
This graphical representation is easier to use for non-
ideal systems than the calculation method. This is anoth-
er limiting condition for column operation, i.e., below
this ratio the specified separation cannot be made even where xc and yc are coordinates of intersection of mini-
with infinite plates. This minimum reflux ratio can be mum reflux “operating” line with equilibrium curve. At
determined graphically from Figure 8-23, as the line with Boiling Point xc = xp
smallest slope from XD intersecting the equilibrium line Underwood’s algebraic evaluation [ 731 for minimum
at the same point as the “q” line for mixture following reflux ratio is acceptable for handling ideal or near ideal
Raoult’s Law. sys tems: