Gas-Liquid separators

Roberto BubbicoPhD, Chem. Eng.Department of Chemical Engineering“Sapienza” University of Romebubbico@ingchim.ing.uniroma1.it

INTRODUCTION• In phase separation, two or more phases can be

separated because a given force will act differently on them, or because one of the phases impacts on a solid barrier.

• The forces are usually gravity, centrifugal, and electromotive.

• Examples are removal of a solid from a liquid by impaction (filtration), gravity (settling), centrifugal force (cyclones or centrifuges), and the attraction of charged particles in an electrostatic precipitator.

INTRODUCTION• One exception to these mechanisms is

drying by evaporating unbonded water from a solid. In this case, separation of a liquid from a solid occurs by mass transfer.

• Since many component separations require contacting two phases, like liquid-liquid extraction, component separation is frequently followed by phase separation.

• Phase separators can be classified according to the phases in contact: liquid-gas, liquid-liquid, liquid-solid, solid-gas, …

INTRODUCTION• In many cases separators will also have the

role of accumulators, with the aim of reducing fluctuations in flow rate, pressure and/or composition (improving process control)

• Where the carryover of some fine droplets can be tolerated it is often sufficient to rely on gravity settling in a vertical or horizontal separating vessel (K-O drum or knockout pot).

INTRODUCTIONReasons for using gas-liquid or vapor-liquid

separators are:• to recover valuable products,• improve product purity,• reduce emissions,• protect downstream equipment,• ...

Gas-liquid separators are used after flashing a hot liquid across a valve (flash drum)

INTRODUCTION

The forces acting on a liquid droplet suspended in a gas are:

VVpDD UDCF ρπ 22

8=

L

VLB

gMFρρ

=

gMF LG =• gravity (acting downward)

• buoyancy (acting upward)

• drag (acting upward).

INTRODUCTION

From a force balance:

net gravity force = drag force

FG = FD

The relative velocity is given by:

( )VD

VLpT C

gDU

ρρρ

34 −

=

INTRODUCTION• The drag coefficient C’ is a function

of the Reynolds number: µρ pTG DU

=Re

• Depending on the Reynolds number, the terminal velocity can be defined further:

• Re>500 (Newton’s law)

• Re<2 (Stokes’ law)

• 2<Re<500

G

GLpT

gDU

ρρρ )(

74.1−

=

µρρ )(2

GLpT

gDU

−=

43.029.0

71.014.171.0 )(54.3µρ

ρρ

G

GLpT

DgU

−=

INTRODUCTIONAs a matter of fact, the terminal velocity is calculated as:

where K is an empirical constant which depends on• properties of the fluids, • design of the separator, • size of the drops, • vapor velocity, • degree of separation required

( )V

VLT KU

ρρρ −

=

INTRODUCTION

• In general around 95 % separation of liquid from vapor is accomplished by an empty drum

• If greater separation efficiencies are required, or very small drops need to be separated an uneconomically large separator should be used

• Very small drops (down to 1 µm) can be separated by impaction using a wiremesh pad located at the top of the separator

INTRODUCTION

• Entrained liquid drops in the vapor impact on the wires and coalesce until the drops become heavy enough to break away from the wire and fall to the bottom of the separator

• The use a wire-mesh mist eliminator, installed near the vapor outlet allows to get separation efficiencies of about 99.9% or greater

INTRODUCTION

• The mesh usually consists of 0.011 in (0.279 mm) diameter wires interlocked by a knitting machine to form a pad from 4 to 6 in (0.102 to 0.152 m) thick.

• Because of the large free volume of the pad - 97 to 99 % - the pressure drop across the pad is usually less than 1.0 in of water

INTRODUCTION

• The sizing of a separator depends on the value of the empirical constant K (or KD).

• The value of KD is largely influenced by the presence of internals. Normally, the value provided by the internals manufacturer should be assumed

• In the absence of manufacturer data, literature data can be used

INTRODUCTION

INTRODUCTION

• The value of K also depends upon the operating pressure

INTRODUCTION

• For horizontal separators, the separation efficiency depends on the total vapor travel length within the vessel.

INTRODUCTION

• A longer vessel makes it easier to remove liquid droplets.

• The values of K usually reported for a horizontal vessel, refer to a vessel length of 3.05 m.

• A typical design K value for horizontal separators is defined as 56.0

05.3⎟⎠⎞

⎜⎝⎛=

LKKD

INTRODUCTION

• For a two-phase vapor–liquid separator, both vertical and horizontal configurations are used, and the selection should be made on a case-by-case basis

• Vertical separators have the advantage of lower space requirement and easy-to-install control systems, but horizontal drums are typically smaller for high liquid loading service

INTRODUCTION

• In a horizontal separator, with an increase in liquid level, the area of the vapor space is reduced and the possibility of liquid entrainment increases

• In a vertical separator the vapor-flow area remains constant and liquid entrainment is not an issue

INTRODUCTION

• For a relief KO drum, the horizontal separator is popular simply because of the use of split flow. In this design, one inlet nozzle is used at the vessel center with two outlets on either side. This split-flow advantage is available only in horizontal separators

Horizontal separatorsAdvantages:• Separation efficiency higher than for a vertical separator• The only choice for a single inlet and two vapor outlets• Easy to design for three-phase separation• More suitable for handling large liquid volumes

Disadvantages:• It requires a footprint area larger than a vertical one• At high liquid levels, the liquid entrainment rate

progressively increases with the increase in liquid level

Vertical separatorsAdvantages:• The liquid surface area does not change with liquid

height: liquid entrainment is reasonably constant• It requires a smaller footprint area• Easier to install level instruments, alarms, and shutdown

systems• Usually more efficient for high vapor/liquid ratios

Disadvantages:• Not suitable for three-phase separation• Less suitable for high liquid–vapor ratios

Vertical separators design

• The separator diameter must be determined first

• The gas velocitymust be low enough to allow the liquid droplets to settle out

Vertical separators design

• After defining the maximum droplet diameter, the critical gas velocity Uv can be calculated:

• the minimum vessel diameter is given by: v

vv U

QDπ4

=

• Dv=minimum vessel diameter, m• Qv=gas, or vapour volumetric flow-rate, m3/s• Kv= 0.07 m/s if a demister pad is used, and 0.15*0.07 without

a demister pad, m/s

( )V

VLvv KU

ρρρ −

=

Vertical separators design

The height of the vessel is composed of a number of terms:

• droplet settling length: it is the length from the center line of the inlet nozzle to the bottom of the mist eliminator. – 0.75 D or a minimum of 12 in (0.305 m), or

alternatively– a length equal to the diameter or a minimum of 3 ft

(0.914 m)

Vertical separators design

• height from the bottom of the inlet nozzle to liquid surface: it is required to prevent nozzle flooding.

– a minimum of 6 in (0.152 m) from the bottom of the nozzle to the liquid surface or a minimum of 12 in (0.305 m) from the center line of the nozzle to the liquid surface

– 12 in (0.305 m) plus 1/2 of the inlet nozzle outside diameter or 18 in (0.4570 m) minimum

– 0.5 D or 2 ft (0.610 m) minimum.

Vertical separators design

If a mist eliminator is present (demister pad), the following lengths must be added:

• thickness of the mist eliminator (usually 6 in (0.152 m))

• an additional 12 in (0.305 m) above the eliminator to obtain uniform flow distribution across the eliminator (if it is too close to the outlet nozzle, most of the flow will be directed to the center of the eliminator, with reduced efficiency)

Vertical separators design

• the liquid height: an appropriate residence time of the liquid (surge time) is required to dampen variations in the liquid flow rate.– 2 to 5 min– sometimes 10 min is selected.

2)4/( V

SS D

VHπ

=sLs tQV =

Vertical separators design

• there is a minimum liquid height required to prevent a vortex from forming. The design of the separator will have to include a vortex breaker. The minimum liquid level should cover the vortex breaker plus an additional liquid height

– 2 ft (0.61 m) should generally suffice

• The volume of the dished heads is not included in the design procedure

Vertical separators design

Calculation procedure for vertical separators

1. Select Kv based on the vessel configuration2. Calculate the maximum gas velocity, Uv

3. Calculate the cross-sectional area and diameter, A and Dv

4. Round off D in 6 in (0.152 m) increments, starting at 30 in (0.762 m). If D is less than 30 in (0.762 m), use standard pipe.

Vertical separators design

5. Select a liquid-phase surge time, ts6. Calculate the liquid-level height7. Calculate the total separator height. Round off L

in 3 in(0.0762 m) increments, for example, 5.0, 5.25,

5.5, 5.75 ft etc.8. If L/D < 3.0, then recalculate L so that L/D > 3.0

by letting L/D = 3.2. If L/D > 5 use a horizontal separator.

Vertical separators

Horizontal separators design

• Differently from a vertical separator, in the design of a horizontal separator the vessel diameter is not independent of its length.

Horizontal separators design

The vessel diameter and length, and the liquid level, must allow for:

• sufficient vapour residence time for the liquid droplets to settle out, and

• the required liquid hold-up time to be met• avoid liquid re-entrainment from the liquid

surface• allow enough space for the feed distributor

and the mist eliminator

Horizontal separators design

The vapor velocity might be larger than the drop terminal velocity, with the condition that:

with θ= residence time and τdrop= separation time

KV = 0.125 ft/s (0.038 m/s) no mist pad;KV = 0.437 ft/s (0.133 m/s) with demister

dropτθ >

Horizontal separators design

• In other words:

where– UAH= vapor velocity– UV= critical drop velocity– HV= height for vapor flow

V

V

AH UH

UL

≥V

VAH U

HLU ≤

Horizontal separators design

• Since the upward drag of the vapor is practically absent in a horizontal drum, the empirical coefficient K is the same as for vertical vessels but multiplied by a factor of 1.25

• The vapor velocity must be compared with the maximum velocity to avoid entrainment:

1.024)(

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎥⎦

⎤⎢⎣

⎡ −⎟⎟⎠

⎞⎜⎜⎝

⎛=

L

VL

VV

Le

gUµ

ρρρσ

ρρ

Horizontal separators design• The most economical length to diameter ratio will

depend on the operating pressure• As a general guide the following values can be used

5>35

420-35

30-20

Length/diameter, L/D

Operatingpressure, bar

Horizontal separators design

• For preliminary designs, set the liquid height at half the vessel diameter:

hv = Dv/2 and fv = 0.5

where fv is the fraction of the total cross-sectional area occupied by the vapour

• In general the minimum cross-sectional area for gas flow should be at least 20 %of the total cross-sectional area

Horizontal separators design

• In the presence of a mist eliminator, the main concern is avoiding its flooding because of a rising liquid level

• The distance from the bottom of the mist eliminator to the liquid level should be at least ½ - 2 ft (0.152 - 0.610 m) and should not be below the center of the separator

• A further distance of 12 in (0.3048m) must be allowed above the eliminator (6 in (0.152 m) thick)

Horizontal separators design

• Assuming the liquid level at the center of the separator, the above rules result in a minimum diameter of 5.5 ft (1.68 m)

Horizontal separators designPreliminary calculation procedure (no.1, see

example)1. Select Kv (e.g. 0.07*0.15) 2. Calculate the maximum vapor velocity, UV3. Calculate the cross-sectional area, A, assuming half total area4. Assume a L/D ratio (e.g. 4)5. Calculate D by equating residence time of the vapor and time to settle

out for the liquid.6. Calculate L.7. Select a liquid phase surge time and check for the actual liquid

residence time8. Adjust the vessel diameter and repeat calculations

Horizontal separators design

Calculation procedure for horizontal separators (no.2)

1. Select Kv2. Calculate the maximum vapor velocity, UV3. Calculate the cross-sectional area, A4. Calculate D. Round off D in 6 in (0.152 m)

intervals, starting at 30 in (0.762 m). If D is less then 30 in (0.762), use standard pipe.

Horizontal separators design

5. Select a liquid phase surge time, ts6. Calculate the separator length. Round off

L in 3 in (0.0762 m) intervals (for example, in feet, 5.0, 5.25, 5.5, 5.75 etc.)

7. If L/D < 3.0, then recalculate L so that L/D > 3.0 by setting L/D = 3.2. If L/D > 5.0, then recalculate D so that L/D < 5.0 by setting L/D = 4.8

Horizontal separators designA more accurate calculation procedure (no.3)

1. Calcolo

2. Calcolo

3. Calcolo

(in questo caso K non tiene conto dello snebbiatore)

VVV WQ ρ/=

LLL WQ ρ/=

V

VLT KU

ρρρ −

= TV UU 75,0=

Horizontal separators designA more accurate calculation procedure (no.3)

4. Fisso TH

5. Fisso L/D (con 1.5<L/D<6) e un grado di riempimento: calcolo un diametro di I° tentativo

(corrisponde ad assumere )

LHH QTV =

3/1

)/(6,0)(4

⎥⎦

⎤⎢⎣

⎡=

DLVD H

π

TH VV %60=

Horizontal separators designA more accurate calculation procedure (no.3)

6. La sezione totale:

7. Fisso il livello minimo di liquido HLL (da tabelle) oppure:

HLL = 0.5 D + 7 (D=ft, HLL=in) con un minimo di HLL=9 in per D < 4 ft

4

2DATπ

=

Horizontal separators design

8. Noto HLL/D, calcolo ALL/AT e ALL

in alternativa, con

x= ALL/AT ey= HLL/D :

( ) ( ) 2211 2121cos yyyyx −−−−= −ππ

Horizontal separators design

9. HV=max (0,2D; 1 ft) (senza demister)HV=max (0,2D; 2 ft) (con demister)Da HV/D calcolo AV/AT e quindi AV

10.Calcolo la L richiesta dall’hold-up liquido

LLVT

H

AAAVL

−−=

Horizontal separators design

11.Calcolo il tempo di caduta (‘drop-out time’):

12.Calcolo la velocità del vapore:

V

Vdrop U

H=τ

VVVA AQU /=

Horizontal separators design

13.Verifico che la velocità ottenuta sia minore di quella di trascinamento:

14.Calcolo la lunghezza minima richiesta per l’abbattimento del liquido:

1.024)(

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎥⎦

⎤⎢⎣

⎡ −⎟⎟⎠

⎞⎜⎜⎝

⎛=

L

VL

VV

Le

gUµ

ρρρσ

ρρ

dropVAUL τ=min

Horizontal separators design

15.– Se L<Lmin (differenza < 20%) =>

pongo L=Lmin (l’abbattimento del vapore è controllante)

– Se L<<Lmin si incrementa HV e si riparte da 9.

– Se L>Lmin => OK– Se L>>Lmin (hold-up controllante) si

può aumentare Lmin e ridurre L diminuendo HV (se maggiore del minimo in 9.)

Horizontal separators design

16.Si verifica L/D.– Se L/D>6 => si aumenta D e si riparte da 6.– Se L/D<1.5 => si riduce D e si riparte

da 6.

17.Si calcola il peso totale del recipiente (costo). Si varia il diametro D facendo variare 1.5<(L/D)<6 e si trova il rapporto ottimale

Horizontal separators design

18.Una volta identificate le dimensioni ottimali si calcolano le altezze effettive:

– ANL=ALL+VH/L

– Da ANL/AT HNL

– e quindi HHL=D-HV.