Page 30 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 19

he the conditions following the pattern of Figure 8-12 The requirement of bottoms temperature to avoid
E1491. It is essential to realistically establish the condens- overheating heat sensitive materials may become con-
ing conditions of the distillation overhead vapors, and trolling.
any limitations on bottoms temperature at an estimated 2. When separating volatile components such as a single
pressure drop through the system. Preliminary calcula- stream from low-volatility bottoms, use a molecular or
tions for the number of trays or amount of packing must unobstructed path process, either thin film or batch.
be performed to develop a fairly reasonable system pres- 3. When separating a volatile product from volatile
sure drop. With this accomplished, the top and bottom impurities, batch distillation is usually best.
column conditions can be established, and more detailed 4. Do not add a packed column to a thin film evapora-
calculations performed. For trays this can be 0.1 psi/actu- tor system, because complications arise.
a1 tray to be installed [149] whether atmospheric or
above, and use 0.05 psi/tray equivalent for low vacuum Note that good vapor-liquid equilibrium data for low
(not low absolute pressure).
pressure conditions are very scarce and difficult to locate.
Because low-pressure operations require larger diame- However, for proper calculations they are essential. See
ter columns, use pressures for operations only as low as References 151 and 152 dealing with this.
required to accomplish the separation. Studies with high-pressure distillation by Brierley [239]
provide insight into some FRI studies and the effects of
For high vacuum distillation, Eckles et al. [150] suggest pressure on performance as well as the impacts of errors
using a thin film or conventional batch process for indus- in physical properties, relative volatility, etc. This work pro-
trial type installations; however, there are many tray and vides important contributions to understanding and set-
packed columns operating as low as 4 mm Hg, abs Eckles ting operating pressures.
[l50] suggests "high vacuum" be taken as 5mm Hg, and
that molecular distillation be 0.3 - 0.003 mm Hg pressure,
and unobstructed path distillation occur at 0.5 - 0.02 mm Total Condenser
Hg. These latter two can be classed as evaporation process- In a total condenser all of the overhead vapor is con-
es. Eckles' [150] rules of thumb can be summarized:
densed to the liquid state. When the heat load or duty on
the condenser is exactly equal to the latent heat of the sat-
1. Do not use a lower pressure than necessary, because urated or dew point of the overhead vapor from the distil-
separation efficiency and throughput decrease as lation column, the condensed liquid will be a saturated
pressure decreases. bubble point liquid. The condenser and accumulator

-
Start
Distillate and bottoms
compositions known
or estimated

Calculate bubble-point pD < 215 psia (1.48 MPa)
(PD) of
at
Pressure distillate 1 Use total condenser
120 F (49 a c) (Reset P, to 30 psia
1
I
Calculate Dew-point P~ <365 Pia Estimate Calculate bubble-point decomposition or critical
-
pressure (PD) of (2.52 MPa) - bottoms temperature (TB) temperature -
distillate at Use partial - of bottoms
120 OF 149 OC) condenser at Ps
A A T,, > Bottoms
PD > 365 psia
decomposition or critical
11 temperature
Choose a refrigerant
so as to operate Lower pressure
partial condenser * PD appropriately
at 415 psia
(2.86 MPa)
Figure 8-12. Algorithm for establishing distillation column pressure and type condenser. Used by permission, Henley, E. J. and Seader, J. D.,
Equilibrium Stage Separation Operations in Chemical Engineering, John Wiley, 0 (1981), p. 43, all rights reserved.