Liquid Liquid Separators

Liquid-Liquid separators

Roberto Bubbico PhD, Chem. Eng.

Department of Chemical Engineering “Sapienza” University of Rome

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

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

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

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

Heavy phase dispersed >3.3

Heavy phase probably dispersed

2-3.3

Design for the worst case

0.5-2

Light phase probably dispersed

0.3-0.5

Light phase dispersed <0.3 Result θ 3 . 0

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

=

Q

Q

µ

ρ

µ

ρ

θ

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):

where µ_c is the viscosity of the continuous phase.

• The calculation of U_T requires the knowledge of the droplets diameter (reference D_p= 150 µm)

• A maximum of 4 mm/s is usually assumed

c L H p T gD U

µ

ρ

ρ

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) T I c c

U

A

Q

v

=

<

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:

(

)

z

z

z

z

+

−

=

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 problem 20000-50000 Some hindrance 5000-20000 Negligible effect <5000 Effect Re • The separator diameter is calculated to minimize turbulence • The Reynolds numbers must be

calculated for both the light and heavy

HORIZONTAL DECANTERS

The 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 i i i eq i

v

D

µ

ρ

Re

=

π

π

+

=

2

D

D

HORIZONTAL DECANTERS

• Assuming a limiting value of Re for each

phase, two values of D are obtained.

• The largest of the diameters is adopted.

(D

=10 cm due to wall effects)

• The velocity of the phases is calculated

as:

A

Q

v

=

8

D

A

=

π

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 D_p=150 µm)

• The time taken for the dispersed phase to reach the interface and the length of the settling zone are:

and

where v_d is the velocity of the dispersed phase

T s

U

D

t

2

=

_L

₌

_v

_t

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,

HORIZONTAL DECANTERS

• It is usually recommended that the thickness of the coalescing zone be H_c≤ 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 A_I is the area of the interface.

I c

c

H

A

INTRODUCTION

• The residence time, t

, of the drops in the

coalescing zone is given by:

where Q_D is the

volumetric flow rate

of the dispersed phase

• t_c is specified by experience, and the interfacial area required for coalescence (A_I) is calculated.

D I c D c c

Q

A

H

Q

V

t

2

1

2

1

=

=

HORIZONTAL DECANTERS

• The length of the coalescing zone, L

, is

calculated from (neglecting the actual shape

of the shell):

and thus the total length of the separator is

calculated:

D

A

L

=

/

L

L

L

=

+

SUMMARY OF THE SIZING

PROCEDURE

1.

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.

SUMMARY OF THE SIZING

PROCEDURE

6.

settling of the dispersed phase

8. Determine H_c, the coalescing-zone height

9. Calculate A_I, the interfacial area required for coalescing the dispersed phase

10. Calculate L_c, the decanter length required for coalescing the dispersed phase

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

SPECIAL CONFIGURATIONS

When separation is a problem, different arrangements are used:

– proprietary equipment (baffles, parallel plates, etc.) – centrifugal separation (centrifuges, hydrocyclones)

1/3 D_drum ≥1500 500 1000-1500 ½ D_drum ≤1000 D_pot (mm) D_drum (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