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Separator Design 337

erations affect the spacing. For example, when separating oxygen and nitrogen
from liquid air, heat transferred to the fractionator from the surroundings must be
minimized, and thus, the fractionator surface area must be a minimum. This con-
sideration results in a tray spacing of as low as 6.0 in (0.152 m) [48].
The height of a packed fractionator is equal to the number of equilibrium
stages times the height equivalent to a theoretical stage (HETS). Although this
method is not rigorous, Ulrich [50] remarked that it is disquieting to find that the
HETS does not vary much in commercial columns after having spend hours learn-
ing to calculate combined mass transfer coefficients. For fractionator diameters
less than 0.5 m (1.64 ft), Frank [33] recommends the rule of thumb that D =
HETS, and for column diameters greater than 0.5 m (1.64 ft), the HETS is given
by Equation 6.27.26 [50].
Besides the height occupied by trays or packing, additional height is needed
at the top and bottom of the fractionator. Henley and Seader [31] recommend
adding 4.0 ft ( 1.22 m) to the top of the fractionator to minimize entrainment and
10.0 ft (3.05 m) to the bottom for surge capacity. For fractionators or absorbers of
about three feet in diameter, Walas [51] recommends that 4.0 ft (1.22 m) be added
to the top and 6.0 ft (1.83 m) to the bottom of the column. Ulrich [50] recom-
mends that the volume below the bottom tray be sufficient for 5 to 10 min surge
time which results in 1.0 to 4.0 m (3.28 to 13.1 ft) of additional height. Thus, as
an approximation add 4.0 ft (1.22 m) to the top of the column and a surge height,
L s, to the bottom of the column. The surge height is calculated from Equation
6.27.16. The diameter of a fractionator or absorber is usually limited to 13.0 ft
(3.96 m) and the length to about 200 ft (60.9 m) because of shipping limitations.
If lengths larger than 200 ft (60.9 m) are necessary, then two vessels in series
could be used. Exceptions to rules-of-thumb sometimes occur. One of the largest
fractionators - made in Europe - is 356 ft (109 m) high and 21.0 ft (6.40 m) in
diameter [47]. Another large ethylene fractionator built in Deer Park, TX, is 328 ft
(100 m) high by 18 ft (5.49 m) in diameter [9]. This column was fabricated in
sections and assembled at the site.
For the relationships listed in Table 6.27, assume that the fractionator pres-
sure is constant. If needed, the pressure drop across the column can be estimated
by the rules-of-thumb given in Table 6.29.
Safety factors are needed in fractionator design because of uncertainty in
system property data, unsuspected trace components in the feed, difference be-
tween plant and design conditions - particularly in feed composition and flow rate
- and variable operating conditions caused by controllers and by plant upsets [54].
Besides, the reasons for safety factors stated above by Drew [54], the factors
should also depend on the uncertainties of the calculation procedure. Different
safety factors are required for large and small fractionators as shown in Table 6.30.
This occurs because engineering costs for small fractionators are comparable to
equipment costs, whereas for larger fractionators equipment costs dominate.
Therefore, for large fractionators a more thorough design is justified to save 5 to
10 % of equipment costs, which results in a smaller safety factor.

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