Page 269 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Mechanical Separations 239

API-Oil Field Separators

The American Petroleum Institute Manual on Disposal
of Refinery Wastes provides specific design and construc-
tion standards for APl-separators that are often used in
oilfield waste disposal [24].

Liquid/Liquid, Liquid/Solid Gravity Separations,
Decanters, and Sedimentation Equipment

Lamella Plate Clarifier; [see Figure 4-10]
This angle plate gravity separator removes suspensions
of solids from a dilute liquid. The unit is more compact
than a box-type settler due to the increased capacity
achieved by the multiple parallel plates. The concept is
fairly standard (U.S. Patent 1,458.805-year 1923) but
there are variations in some details. For effective opera-
tion, the unit must receive the mixture with definite par- Bo1h Sides
TyD!CaJ
QI
ticles having a settling velocity The units are not totally ..
Larnclta Prates
effective for flocculants or coagulated masses that may
have a tendency to be buoyant.
Flow through the plates must be laminar, and the criti- Wa.te•Conta111ng w.Hoc Particles Loauld Sludge
Cianhed Water
Bemcvat
F'oc Parnc'es
cal internal areas are [25,54,61]: Pcrroved
Figure 4-10. Lamella Clarifier. By permission, Graver Water Div. of
• Distribution of the inlet flow the Graver Co.
o Flow between each parallel plate surface
• Collection of clarified water clarifying, wastewater clarification and thickening. A good
summary discussion is given in Reference [27].
Because these units are essentially open, the "standard"
design does not fit into a closed process system without
adding some enclosures, and certainly is not suitable for a Horizontal Gravity Settlers or Decanters, Liquid/Liquid
pressurized condition. Many processes require the separation of immiscible
Although the main suspended flow is through a top liquid/liquid streams; that is, water/hydrocarbon. The
mixing chamber, the mixture flows around the angled settling unit must be of sufficient height (diameter) and
(avg. 55°) parallel plate enclosure and begins its settling length to prevent entrainment of the aqueous phase into
path from the bottom of the plates, flowing upward while the hydrocarbon and vice versa. Horizontal units are usu-
depositing solids that slide countercurrent into the bot- ally best for settling and possibly vented units for decanta-
tom outlet. The purified liquid flows overhead and out tion (but not always).
the top collecting trough.
Residence time of the mixture in the vessel is a func-
Manufacturers should be contacted for size/rating
information, because the efficiency of a design requires tion of the separation or settling rate of the heavier phase
proper physical property information as well as system droplets through the lighter phase. Most systems work sat-
capacity and corrosion characteristics. isfactorily with a 30 minute to 1 hour residence time, but
this can be calculated [26]. After calculation, give a rea-
sonable margin of extra capacity to allow for variations in
Thickeners and Settlers
process feedrate and in the mixture phase composition.
Generally, large volume units for dewatering, settling From Stokes' Law, the terminal settiing velocity:
of suspensions, and thickening of solids, and concentra-
tion of solids and clarification must be designed by the V, = gD/ (Pp - p)/18µ, ft sec (4-14)
specific manufacturer for the process conditions and
physical properties. Some typical processes involving this µ = viscosity of surrounding fluid, lb/ft sec
class of equipment are lime slurrying, ore slurrying, ore DP = diameter of particle, fl