Page 84 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 73

There are a total solutions of 8fi equal to one more ?i
than the number of split components between the Component,j wj a. - XD~ 0.x~. ( x ~ j )
~
J
J
J
keys. The 8f values will be spaced: 1 (light key) 0 0 aj . 0 J 0
.
h (heavy key) 0 0
a13 "4 ef4 a3 ef5 ah6 L1+1 1 lighter
w.
where a1 is the light key and component number 3, z wj xDj ZJ (xDj
and correspondingly for the heavy key, component a.
J
number 6. Determine e values as for constant volatili- 4. Calculate (L/V)min: (internal)
ty case of adjacent keys.
For some systems, the 8 values can be assumed with-
out further solution of the above relation, but using
these assumed dues as below. (8 - 139)
2. Calculate,

1
5. Calculate External (L/D)min:
(8- 137)
(8 - 140)

For variable CY conditions, the pinch temperature can
be used for a determinations as previously described.
which represents (for the hypothetical system set up
in (1)) the product of (8,) (8,) (0,) divided by the Example 8-23: MinimUm Reflux Ratio Using Underwood
product of (a5) (q), based upon the lightest compo- Equation; Calculate the Minimum Reflux Ratio
nent being numbered one, the next two, etc., the
heaviest components having the higher numbered Use 0.384 to begin, (assumed).
subscripts. P means product, and 1, i = h - 1, i = 1 + 1 Expanding to determine more exact value of b.
are limits for evaluation referring to components
between the keys, and the light and heavy keys. P $ = PQa + $ - $a 8' $a
3. Calculate,

(8- 138)

For the 8 example shown in (I) above:

1 0.10 0.025 -0.334 -0.0749 +0.224
2 0.225 0.1125 -0.084 -1.34 +1.5.9
3 0.430 0.450 +0.416 +1.08 +2.6
Also calculate o for all components lighter than 4 0.223 0.450 +1.416 +0.318 +0.224
light key. P = -0.016 +18.948 =
Q$a =-0.016 - (1 - 4)
$a =-0.016 - (1 - 1)
Qa =-0.016