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

244 Applied Process Design for Chemical and Petrochemical Plants

The same results can be achieved with internal flat plate h = distance from center to given chord of a vessel, ft
baffles and outlet nozzles. I = width of interface, ft
D = decanter diameter, ft
L = decanter length, ft
r = vessel radius, ft
zh = heavy phase outlet dimension from bottom of horizontal
decanter Horizontal vessels as cylinders are generally more suit-
zi = interface measured from bottom able for diameters up to about 8 feet than other shapes,
z1 = light phase outlet measured from bottom of decanter or vertical, due in part to the increased interfacial area
for interface formation. For a horizontal drum (See Fig-
Droplet diameter, when other data is not available: ure 412):

= 150pm (d = 0.0005 ft)

Reference [32] recognizes that this is generally on the
safe side, because droplets generated by agitation range
500 to 5000 pm, turbulent droplet range 200 to 10,000 AL = 1/2 nr' - h(r' - h')1/2 - r2 arc sin(h/r) (439)
pm. Due to limitations of design methods, decanters sized
for droplets larger than 300 pm often result in being too or use the methods from the Appendix to calculate
small to work properly [32]. area of a sector of a circle. The arc is in radians:
The continuous phase moves through the vessel on a Radians = (degrees) (x/lSO)
uniform flow equal to the overflow rate. To identify which
is the continuous phase (from [65] by 1321): AH = nr2 - AL

D, = 4 AL/(I + P)
0.3
e = -(--'I PLFH
QL
(4- 36)
QH DH = 4AH/(I + 2 xr - P)
8 Result where P = 2r arc cos (h/r)
< 0.3 light phase always dispersed
0.3-0.5 light phase probably dispersed Degree of turbulence [321:
0.5-2.0 phase inversion probable, design for worst case
2.0-3.3 heavy phase probably dispersed
> 3.3 heavy phase always dispersed (4-43)

where Q = dispersed volumetric flow rate, cu ft/sec c = continuous phase
= volumetric flow rate, cu ft/sec, light phase DH = hydraulic diameter, ft = 4 (flow area for the phase in
@ = volumetric flow rate, cu ft/sec, heavy phase question/wetted perimeter of the flow channel)
pL = density of light phase fluid, Ib/cu ft v, = velocity down the flow channel
PH = density of heavy phase fluid, lb/cu ft
p~ = viscosity of heavy phase, lb/(ft) (sec)
pL = viscosity of light phase, lb/(ft) (sec) Guidelines for successful decanters [32]:

To begin, there is a dispersion band through which the K Results
phases must separate. Good practice [32] normally keeps < 5000 little problem
the vertical height of the dispersed phase, HD < 10% of 5000-20,000 some hindrance
decanter height (normally a horizontal vessel), and: 20,000-50,000 major problem may exist
Above 50,000 expect poor separation
1/2H&/Q, > 2 to 5 min

where AI = area of interface assuming flat interface, sq ft Velocities of both phases should be about the same
AL = cross-sectional area allotted to light phase, sq ft through the unit. By adjusting mechanical internals, a
AH = cross-sectional area allotted to heavy phase, sq ft ratio of < 2:l is suggested (internals do not need to be
HD = height of the dispersion band, ft equal) [32]. Velocities for entrance and exit at the vessel
QD = volumetric flow, dispersed phase, cu ft/sec nozzle should be low, in the range of 0.5 to 1.5 ft/sec. The