Liquid Liquid Separators

Liquid-Liquid separators Roberto Bubbico PhD, Chem. Eng. Department of Chemical Engineering “Sapienza” University of Rome [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)

INTRODUCTION The aim of the separator is to provide a sufficiently large volume to allow: • sufficient residence time for the dispersedphase 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.

INTRODUCTION In 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 SIZING Even 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

QL θ= QH

⎛ ρ L µH ⎞ ⎜⎜ ⎟⎟ ⎝ ρ H µL ⎠

0.3

θ

Result

3.3

Heavy phase dispersed

Q= volumetric flow rate L= light phase H= heavy phase

VERTICAL DECANTERS • The decanter size is based on the settling velocity of the droplets of the dispersed phase (e.g. Stokes’ equation):

gD p ( ρ H − ρ L ) 2

UT =

18µc

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

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

Qc vc = < UT AI • Then, a hold-up time of 5 to 10 min is usually assumed (sufficient where emulsions are not likely to form)

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:

z2

( z1 − z3 )ρ L = +z ρH

3

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 • The separator diameter is calculated to minimize turbulence • The Reynolds numbers must be calculated for both the light and heavy phases

Re

Effect

50000

Probable poor separation

HORIZONTAL DECANTERS The Reynolds number must be calculated with the equivalent diameter:

Rei =

Deq ,i ρ i vi

where i = L, H

µi

Deq ,i

πDi = 2 +π

the light and heavy phases.

It is generally assumed that the liquid-liquid interface is at the center of the vessel.

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: 2 Qi π D vi = Ai = Ai 8 where

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:

D ts = 2U T

and

Ls = vd ts

where vd is the velocity of the dispersed phase

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:

Vc = H c AI where AI is the area of the interface.

INTRODUCTION • The residence time, tc, of the drops in the coalescing zone is given by: where QD is the 1 Vc 1 H c AI volumetric flow rate tc = 2 = 2 QD QD of the dispersed phase • tc is specified by experience, and the interfacial area required for coalescence (AI) is calculated.

HORIZONTAL DECANTERS • The length of the coalescing zone, Lc, is calculated from (neglecting the actual shape of the shell):

Lc = AI / D

and thus the total length of the separator is calculated:

L = Ls + Lc

SUMMARY OF THE SIZING PROCEDURE

1. Calculate θ to determine the dispersed phase 2. 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 time 7. Calculate Ls, the decanter length required for settling of the dispersed phase 8. Determine Hc, the coalescing-zone height 9. Calculate AI, the interfacial area required for coalescing the dispersed phase 10. Calculate Lc, the decanter length required for coalescing the dispersed phase 11. Calculate L, the total length of the decanter

PIPING ARRANGEMENT To 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 CONFIGURATIONS When separation is a problem, different arrangements are used: – proprietary equipment (baffles, parallel plates, etc.) – centrifugal separation (centrifuges, hydrocyclones) 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:

Ddrum (mm) ≤1000

Dpot (mm)

10001500 ≥1500

500

½ Ddrum

1/3 Ddrum