Page 272 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

242 Applied Process Design for Chemical and Petrochemical Plants

The terminal (highest calculated) settling velocity of the In summary:
aqueous droplet in/ through the hydrocarbon phase is:

Design Calculation Practical Design Use
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Vhc = (1.2) (5 in./min) (95/39 GPM) = 14.6 in./min
Diameter 3.34 ft ( 40.08 in.) 3.5 ft. ( 42 in.) or 3.83 ft ( 46 in.)
Length HC inlet/outlet: 11 ft 12 or 14 ft
Because this is more than the 10 in./min recommend-
ed earlier, then use:
Abernathy [26] has compared several design methods
as follows:
Vhc = 10 in./min

Assume for design: fhc = fag = 2 (from earlier discus- This Modified Rule-of-
sion). Si gales Method Happel Happel Thwnb
Diameter 2.67 ft 3.34 ft 3.36 ft 4.01 ft 4.1 ft
ht 10 in. 22 in. 22.6 in. 24 in. 32.5 in.
Then, a= (1.889((10)(2)(39) + (5)(2)(95)]/[(3.4)(10)(5)]
hb 8 in. 12 in. 11.3 in. 24 in. 16.7 in.
a = 19.22 Interface 14 in. 6 in. 6.4in. O in. O in.
b = (3.505)(2)(95)(2)(39)/[(3.4) 2 (]0)(5)] HC residence 1.1 min 4.4 min 4.6min. 6.8min. lOmin.
time
b = 89.87

Solving for D: Decanter [32]

In most general applications, a decanter is a continu-
D = [19.22/2 ± [(19.22)2 - 4(89.87))11 2 /2] l/2
ous gravity separation vessel that does not run full, as con-
trasted to a settler that usually runs full, with one stream
D = 3.34 ft or -2.83 ft (latter is an unreal negative number, exiting at or near the top of a horizontal vessel. For most
so use 3.34 ft) decanters, one phase of a two-plane mixture overflows out
of the vessel (see Figure 4-12). The concept of the
Area of segment at top of vessel = At, substituting into decanter involves the balancing of liquid heights due to
Equation 4-22: differences in density of the two phases, as well as settling
velocity of the heavier phase falling through the lighter,
A,= 1.2 D [(7.48) (3.4)D(l0)]/[(2)(95)]-38.4/(nD)]- 1 or the lighter rising through the heavier.
Settling Velocity: Terminal [32]
Using: L/D = 3.4:

For the bottom segment of the vessel, aqueous layer: • (pd - Pc)
v = gd· ft I sec (4-34)
d 18 µ c ,
Ab = l.2(3.34) [ (7.48) (3.34) (3.4) (5) ]/ [ (2) (39)] - (38) I
n(3.34)r 1 where vd = terminal settling velocity of a droplet, ft/sec
Ab = 2.2448 sq ft g = acceleration due to gravity, 32.17 ft/sec-sec
d = droplet diameter, ft(l ft= 304, 800µm, or Iurn =
O.OOlmm)
Then, using Equation 4-21A: Pd = density of fluid in the droplet, lb/cu ft
Pc= density of fluid continuous phase, lb/cu ft
h, = 7.48(4..942) (3.4) (10)/(2.0) (95) = 22.1 in. µc = viscosity of the continuous phase, lb/ (ft) (sec)
Note: 1 cp = 6.72 X 10- lb/(ft)(sec)
4
hb = 7.48(2.2448)[(3.34)(3.4)](5)/(2)(39)] = 12.2 in.
µm = millimicron
Then, h 1 /D = (22.1)/(12)(3.34) X 100 = 55%
For a decanter that operates under gravity flow with no
instrumentation flow control, the height of the heavy
hb/D = 12.2/ (] 2) (3.34) X 100 = 30% phase liquid leg above the interface is balanced against
the height of one light phase above the interface [23].
Since h. and h , are between 30% and 70% of the diam- Figures 4-12 and 4-13 illustrate the density relationships
eter, the solution is acceptable. and the key mechanical details of one style of decanter.