Page 269 - Applied Process Design for Chemical and Petrochemical Plants Volume I

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
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 [ 261 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). Sigales Method Happel Happel Thumb
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)1/[(3.4)(10)(5)1
hb 8 in. 12 in. 11.3 in. 24in. 16.7in.
a = 19.22 Interface 14in. 6in. 6.4in. 0 in. Oin.
b = (3.505)(2)(95)(2)(39)/[(3.4)2(10)(5)1 HC residence 1.1 min 4.4 min 4.6 min. 6.8 rnin. 10 min.
time
b = 89.87

Solving for D: Decanter [321

In most general applications, a decanter is a continu-
D [19.22/2 * [(19.22)2 - 4(89.87)]'/2/2]1/2 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 412). The concept of the
Area of segment at top of vessel = A,, substituting into decanter involves the balancing of liquid heights due to
Equation 422: 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(10)]/[(2)(95)1-38.4/(xD)]-' 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:
(4- 34)

Ab = 1.2(3.34) [(7.48)(3.34)(3.4)(5)1/[(2)(39)1 - (38)/
x( 3.34) ] -' 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(1 ft = 304, 800pm, or lpm =
0.00 1 mm)
Then, using Equation 421A 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. clC = viscosity of the continuous phase, lb/(ft) (sec)
=
Note: 1 cp = 6.72 X lb/(ft) (sec)
hb = 7.48(2.2448) [(3.34) (3.4)] (5)/(2) (39)l = 12.2 in.
pm = millimicron
Then, h,/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/(12)(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 hb are between 30% and 70% of the diam- Figures 412 and 413 illustrate the density relationships
eter, the solution is acceptable. and the key mechanical details of one style of decanter.