liquid liquid separators
TRANSCRIPT
Liquid-Liquid separators
Roberto BubbicoPhD, Chem. Eng.Department of Chemical Engineering“Sapienza” University of [email protected]
INTRODUCTION
It is commonly encountered in different phases:
• in the unit operation of liquid-liquid extraction
• in the separation of small quantities of entrained water from process streams (oil)
• in the separation of water polluted by hydrocarbons, before discharge
INTRODUCTION
• The simplest form of equipment used to separate liquid phases is the gravity settling tank (decanter)
• they are designed for continuous operation, but batch operation is also possible
• the feed to the decanter is a mixture of a dispersed and a continuous liquid phase (they can be more or less stable)
INTRODUCTIONThe aim of the separator is to provide a
sufficiently large volume to allow:• sufficient residence time for the dispersed-
phase drops to separate and reach (rise or drop) the liquid-liquid interface, and
• sufficient residence time for the droplets to coalesce.
• Thus, the residence time has two components.
INTRODUCTIONIn an operating decanter three zones can be identified:• clear heavy liquid;• separating dispersed liquid (the dispersion zone);• clear light liquid.
The separating zone can be further subdivided in two zones:• a settling out dispersion zone, where dispersed drops rise
or settle out through the continuous phase• a zone of densely packed dispersed drops that coalesce
more or less quickly
DECANTERS SIZINGEven though settling and coalescing occur
simultaneously, it will be assumed that first the drops flow to the interface, and then the drops coalesce with the appropriate phase
DECANTERS SIZING
• There is no universally accepted procedure for decanters sizing.
• Accurate sizing must be supplemented by testing
DECANTERS SIZING• The first step in developing
a sizing procedure is to determine which phase is dispersed
• the parameter θ, can be used as a guide to determine the dispersed phase
Heavy phase dispersed
>3.3
Heavy phase probably dispersed
2-3.3
Design for the worst case
0.5-2
Light phase probably dispersed
0.3-0.5Light phase dispersed<0.3
Resultθ
3.0
⎟⎟⎠
⎞⎜⎜⎝
⎛=
LH
HL
H
L
µρµρθ Q= volumetric flow rate
L= light phaseH= heavy phase
VERTICAL DECANTERS• The decanter size is based on the settling velocity
of the droplets of the dispersed phase (e.g. Stokes’equation):
where µc is the viscosity of the continuous phase.• The calculation of UT requires the knowledge of the
droplets diameter (reference Dp= 150 µm)• A maximum of 4 mm/s is usually assumed
c
LHpT
gDU
µρρ
18)(2 −
=
VERTICAL DECANTERS• The velocity of the continuous phase must be less
than the settling velocity (plug flow is assumed)• The velocity of the continuous phase is calculated
using the area of the interface
• Then, a hold-up time of 5 to 10 min is usually assumed (sufficient where emulsions are not likely to form)
TI
cc U
AQv <=
INTERFACE• The position of the interface can be
controlled, with or without the use of instruments, by use of a siphon take-off for the heavy liquid
• The height of the take-off can be determined by making a pressure balance:
( )3
312 zzzz
H
L +−
=ρ
ρ
INTERFACE• An alternative approach for the automatic control
of the interface:
• The height of the liquid interface should be controlled accurately when the liquid densities are close, when one component is present only in small quantities, or when the throughput is very small
INTERFACE• Where one phase is present only in small amounts
it is often recycled to the decanter feed to give more stable operation
• Drain valves should be fitted at the interface so that any tendency for an emulsion to form can be checked; and the emulsion accumulating at the interface drained off periodically as necessary
HORIZONTAL DECANTERS
• Liquid-liquid separation is markedly influenced (hindered) by turbulence
Probable poor separation>50000
Major problem20000-50000
Some hindrance5000-20000
Negligible effect<5000
EffectRe• The separator diameter is calculated to minimize turbulence
• The Reynolds numbers must be calculated for both the light and heavy phases
HORIZONTAL DECANTERSThe Reynolds number must be calculated with
the equivalent diameter:
where i = L, H the light and heavy phases.
It is generally assumed that the liquid-liquid interface is at the center of the vessel.
i
iiieqi
vDµρ,Re =
ππ+
=2,
iieq
DD
HORIZONTAL DECANTERS
• Assuming a limiting value of Re for each phase, two values of D are obtained.
• The largest of the diameters is adopted. (Dmin=10 cm due to wall effects)
• The velocity of the phases is calculated as:
wherei
ii A
Qv =8
2DAiπ
=
HORIZONTAL DECANTERS• The total length of the decanter is the sum of the
lengths required for settling and coalescence• The settling velocity for the dispersed liquid drops is
calculated using the Stokes' Law (with Dp=150 µm)
• The time taken for the dispersed phase to reach the interface and the length of the settling zone are:
and
where vd is the velocity of the dispersed phaseT
s UDt
2= sds tvL =
HORIZONTAL DECANTERS
• The dispersed phase drops finally accumulate near the interface to form a coalescing zone
• The length of the coalescing zone of the decanter is determined by the time required for the dispersed phase to coalesce.
HORIZONTAL DECANTERS• No relationship can predict the time required
for coalescence (from seconds to many hours, by experiments)
• Coalescence is enhanced by:– low viscosity of the continuous phase,– large density difference between the phases,– large interfacial tension,– and high temperature.
HORIZONTAL DECANTERS• It is usually recommended that the thickness of the
coalescing zone be Hc≤ 10% of the decanter diameter
• It is also assumed that the drops occupy about half of the volume of the coalescing zone volume.
• If the liquid-liquid interface is at the center of the separator, the dispersion zone volume is approximately equal to:
where AI is the area of the interface.
Icc AHV =
INTRODUCTION• The residence time, tc, of the drops in the
coalescing zone is given by:
where QD is the volumetric flow rateof the dispersed phase
• tc is specified by experience, and the interfacial area required for coalescence (AI) is calculated.
D
Ic
D
cc Q
AH
Q
Vt 2
12
1==
HORIZONTAL DECANTERS
• The length of the coalescing zone, Lc, is calculated from (neglecting the actual shape of the shell):
and thus the total length of the separator is calculated:
DAL Ic /=
cs LLL +=
SUMMARY OF THE SIZING PROCEDURE
1. Calculate θ to determine the dispersed phase2. Calculate the inside diameter of the decanter,
assuming that the light phase determines the diameter.
3. Calculate the inside diameter of the decanter, assuming that the heavy phase determines the diameter.
4. The decanter diameter is the larger of the diameters calculated in Steps 2 and 3.
5. Calculate UT, the droplet velocity
SUMMARY OF THE SIZING PROCEDURE
6. Calculate ts, the dispersed-phase settling time7. Calculate Ls, the decanter length required for
settling of the dispersed phase8. Determine Hc, the coalescing-zone height9. Calculate AI, the interfacial area required for
coalescing the dispersed phase10. Calculate Lc, the decanter length required for
coalescing the dispersed phase11. Calculate L, the total length of the decanter
PIPING ARRANGEMENTTo prevent generating turbulence and entraining in
the vessel, both the inlet and outlet liquid streams velocities should be kept relatively low:
• the inlet velocity should be below 1 m/s• the liquid velocity in each outlet nozzle should not
be greater than 10 times the average velocity of each phase in the decanter
This rules allow sizing the inlet and outlet nozzles
SPECIAL CONFIGURATIONSWhen separation is a problem, different
arrangements are used:– proprietary equipment (baffles, parallel plates, etc.)– centrifugal separation (centrifuges, hydrocyclones)
1/3 Ddrum≥1500
5001000-1500
½ Ddrum≤1000
Dpot (mm)Ddrum(mm)
When a small amount of heavy phase is present, it is preferable to use a settling pot on the bottom of the drum to save space and cost: