Page 98 - Separation process engineering

capacities are needed. More detailed design procedures and methods for horizontal drums are presented
by Evans (1980), Blackwell (1984), and Watkins (1967). Note that in industries other than
petrochemicals that sizing may vary.

Example 2-4. Calculation of drum size

A vertical flash drum is to flash a liquid feed of 1500 lbmol/h that is 40 mol% n-hexane and 60 mol%
n-octane at 101.3 kPa (1 atm). We wish to produce a vapor that is 60 mol% n-hexane. Solution of the
flash equations with equilibrium data gives x = 0.19, T drum = 378K, and V/F = 0.51. What size flash
H
drum is required?

Solution

A. Define. We wish to find diameter and length of flash drum.
B. Explore. We want to use the empirical method developed in Eqs. (2-64) to (2-68). For this we
need to estimate the following physical properties: ρ , ρ , MW . To do this we need to know
L
v
v
something about the behavior of the gas and of the liquid.
C. Plan. Assume ideal gas and ideal mixtures for liquid. Calculate average ρ by assuming additive
L
volumes. Calculate ρ from the ideal gas law. Then calculate u perm from Eq. (2-64) and diameter
v
from Eq. (2-68).
D. Do It.

1. Liquid Density

The average liquid molecular weight is

where subscript H is n-hexane and O is n-octane. Calculate or look up the molecular weights. MW =
H
86.17 and MW = 114.22. Then . The specific volume is
O
the sum of mole fractions multiplied by the pure component specific volumes (ideal mixture):

From the Handbook of Chemistry and Physics, ρ = 0.659 g/mL and ρ = 0.703 g/mL at 20°C. Thus,
H
O

Then

2. Vapor Density

Density in moles per liter for ideal gas is , which in g/L is .

The average molecular weight of the vapor is

where y = 0.60 and y = 0.40, and thus . This gives
O
H