Page 296 - Chemical process engineering design and economics

Separator Design 275

close to the side of the vessel as recommended by Patterson [16]. A vortex will
occur at a higher liquid level when a tank is draining with no inflow of liquid than
when a tank has both equal inflow and outflow. Finally, to minimize the forma-
tion of a vortex at low liquid levels, a vortex breaker is installed at a vessel outlet.
Vortex breakers may be flat plates, crosses, radial vanes or gratings. Although
Patterson [16] gives dimensions for a flat-plate design he recommends radial
vanes, as shown in Figure 6.4, or a grating. Another design, recommended by
Frank [75], is four vanes at right angles with a flat circular plate welded at the top.

ACCUMULATORS
Accumulators are not separators. In one application, an accumulator placed after a
total condenser provides reflux to a fractionator and prevents column fluctuations
in flow rate from affecting downstream equipment. In this application the accumu-
lator is called a reflux drum. A reflux drum is shown in Figure 6.3. Liquid from a
condenser accumulates in the drum before being split into reflux and product
streams. At the top of the drum is a vent to exhaust noncondensable gases that
may enter the distillation column. The liquid flows out of the drum into a pump.
To prevent gases from entering the pump, the drum is designed with a vortex
breaker at the exit line.
The total volume of an accumulator is calculated using a residence time,
also called surge time, which is obtained from experience, according to the type
and degree of the process control required. After examining 18 accumulators in
service, Younger [11] recommended a residence time of 5 to 10 min. Once a resi-
dence time is selected, size the accumulator for half-full operation to accommo-
date either an increase or decrease in liquid level. Thus, the accumulator volume
is calculated from Equation 6.5.1 in Table 6.5, where equations for sizing an ac-
cumulator are listed. The volumetric liquid flow rate, VL, is obtained from a mass
balance on the system. After calculating the total accumulator volume, calculate
the accumulator diameter and length by solving Equations 6.5.3 and 6.5.4. Equa-
tion 6.5.4 is a rule-of-thumb for L/D. According to Younger [11], for an L/D ratio
of 2.5 to 6 the cost varies by only 2%. After surveying several accumulators in
use, Younger [11] found that fifteen were horizontally placed and three were ver-
tically placed.
Table 6.6 outlines a calculation procedure for sizing an accumulator. Ac-
cording to Gerunda [4], the calculated diameter for a vessel is rounded off in six-
inch increments, starting with a 30 in (0.762 m) diameter vessel. Six-inch incre-
ments are required to match standard-diameter heads for the ends of a vessel (Aer-
stin, 6.5). The maximum vessel diameter is limited to about 13.5 ft (4.11 m), be-
cause of shipping limitations by rail or truck. If a larger diameter than 13.5 ft
(4.11 m) is required, then the process engineer must consider either specifying two
or more vessels in parallel or fabricating a larger diameter vessel at the construc-
tion site. If a vessel is less than 30 in (0.762 m) in diameter, use standard pipe.
After calculating the vessel length, round it off in three-inch increments.

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