gas liquid separators

Gas-Liquid separators

Roberto BubbicoPhD, Chem. Eng.Department of Chemical EngineeringSapienza University of [email protected]

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 228=

L

VLB

gMF =

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

3

4 =

INTRODUCTION The drag coefficient C is a function

of the Reynolds number: pTG DU=Re

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

Re>500 (Newtons law)

Re

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

=

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 m) 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 vaporliquid 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 liquidvapor 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


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

the minimum vessel diameter is given by: v

vv U

QD 4=

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

=


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)


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.


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)


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/( VS

S DVH =sLs tQV =


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


Calculation procedure for vertical separators

1. Select Kv based on the vessel configuration2. Calculate the maximum gas velocity, Uv3. Calculate the cross-sectional area and

diameter, A and Dv4. 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.


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.


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


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

with = residence time and drop= 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

drop >


In other words:

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

V

V

AH UH

UL V

VAH UH

LU


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

gU

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


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


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 - 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)


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


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.


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 /=LLL WQ /=

V

VLT KU

= TV UU 75,0=


4. Fisso TH

5. Fisso L/D (con 1.5


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

2DAT=


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

in alternativa, con

x= ALL/AT ey= HLL/D :

( ) ( ) 2211 2121cos yyyyx =


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 dallhold-up liquido

LLVT

H

AAAVL =


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

12.Calcolo la velocit del vapore:

V

Vdrop U

H=

VVVA AQU /=


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

14.Calcolo la lunghezza minima richiesta per labbattimento del liquido:

1.024)(

=

L

VL

VV

Le

gU

dropVAUL =min


15. Se L

pongo L=Lmin (labbattimento del vapore controllante)

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

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


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


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.

gas liquid separators

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