Page 58 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 47

7. For each value of x, and the values of (BTO/BT1)
found (8 - 84A)

above, calculate B~~ - B~l (loo), the percent of material
BTO
taken overhead.
8. A plot of the distillate composition, y versus percent (8 - 84B)
distilled (from Step 7) will show the value of the
instantaneous vapor composition. Solve for xsi by trial and error.

The usual Raleigh Equation form [130] is for the con- After this reflux runs down the column the desired
ditions of a binary simple differential distillation (no trays lighter components leave, and a desired residual composi-
or packing), no reflux, but with constant boilup. tion is left, following the Raleigh equation to express the
material balance.
Most batch distillations/separations are assumed to fol-
(8 - 80)
low the constant relative volatility vapor-liquid equilibrium
curve of
For a binary mixture the values of x and y can be ax
obtained from the equilibrium curve. Select values of XI y= l+x(a-1) (8 - 50)
and read the corresponding value of y from the equilib-
rium curve. Tabulate values of 1/ (y - x), and plot versus After filling the receiver, reflux runs down the column at
XI, resulting in a graphical integration of the function dx the same molar rate as the vapor back up (L = G) . The oper-
(y - x) [130] between xo and XI. This system would have ating line has a slope of 1 .O. Then there are “n” plates/tmys
no column internals and no reflux. between composition xp and XI (the mol fraction in distil-
late). As the distillation continues, the operating line moves
Simple Batch Distillation: Constant u, with Trays or closer to the 45” line of the diagram, and XI and xp (and &)
Packing, Constant Boilup, and with Reflux [ 1291 Using become richer and leaner, respectively, until at the end x1
x-y Diagram becomes XD and x, becomes xIv The required time is €4.
During a batch distillation at constant pressure, the tem-
The system material balance from Treybal [129] using a perature rises to accomplish the separation as the more
heated kettle and distillation column following a McCabe- volatile component’s concentration is reduced in the bot-
Thiele diagram, using reflux, but having only a batch (ket- toms (kettle) or residue.
tle) charge: For a batch differential distillation where no reflux is
used, there is only boilup of a mixture of the desired
F=D+W (8-81) lighter component, which leaves the kettle, and a desired
residual bottoms composition is left in the kettle. This type
FXF = DXD + WXW (8-82) of distillation follows the Raleigh equation to express the
material balance. However, while simple, not having tower
packing or trays or reflux does not offer many industrial
applications due to the low purities and low yields
G = mol/hr boilup overhead involved. Repeated charges of the distillate back to the
L = mols reflux in the column kettle and redistilling will improve overhead purity.
D = overhead receiver contents, mols
The minimum number of plates [ 1291, for infinite time
for separation:
Starting with an empty overhead receiver, the time 81 to
condense D mols of vapor to fill the receiver, when the
vapor boilup rate is G mols/hr.
(8 - 85)

For operating line with slope of unity, from Smoker’s
during which time the receiver is filling and there is no equation:
reflux and the kettle mixture follows a Raleigh distillation
[129]. Under this condition, when the distillate receiver
just becomes full, the composition of the kettle contents J ‘1 a-c’ ‘I
are xsi, and [ 1291, (8- 86)