Gas-Liquid separators
Roberto Bubbico
PhD, Chem. Eng.
Department of Chemical Engineering
“Sapienza” University of Rome
bubbico@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: liquidgas, 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).
INTRODUCTION
Reasons 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:
• gravity (acting
FG = M L g
downward)
• buoyancy (acting
upward)
• drag (acting upward).
FB =
FD =
π
8
M L ρV g
ρL
C D D p2UV2 ρV
INTRODUCTION
From a force balance:
net gravity force = drag force
FG = FD
The relative velocity is given by:
UT =
4 gD p (ρ L − ρV )
3CD ρV
INTRODUCTION
• The drag coefficient C’ is a function
of the Reynolds number:
ρ GU T D p
Re =
µ
• Depending on the Reynolds number, the terminal
velocity can be defined further:
• Re>500
(Newton’s law)
gD p ( ρ L − ρ G )
U T = 1.74
ρG
gD p ( ρ L − ρ G )
2
• Re<2
• 2<Re<500
(Stokes’ law)
UT =
UT =
µ
3.54 g 0.71D p
1.14
( ρ L − ρ G )0.71
ρ G 0.29 µ 0.43
INTRODUCTION
As a matter of fact, the terminal velocity is calculated as:
UT = K
(ρ L − ρV )
ρV
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
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
0.56
⎛ L ⎞
KD = K ⎜
⎟
⎝ 3.05 ⎠
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-toinstall 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 separators
Advantages:
• 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 separators
Advantages:
• 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 velocity
must 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:
Uv = Kv
Dv =
(ρL − ρV )
ρV
4Q v
πU v
• 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
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.
Vs = QLts
VS
HS =
2
(π / 4)DV
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 configuration
2. 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, ts
6. Calculate the liquid-level height
7. 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:
θ > τ drop
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
Horizontal separators design
• In other words:
L
U AH
HV
≥
UV
where
– UAH= vapor velocity
– UV= critical drop velocity
– HV= height for vapor flow
U AH
L
≤
UV
HV
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:
⎧⎪ ρ
Ue = ⎨ L
⎪⎩ ρV
⎛σ
⎜⎜
⎝ ρV
4
⎞ ⎡ g ( ρ L − ρV ) ⎤
⎟⎟ ⎢
⎥
µ
L
⎦
⎠ ⎣
2
⎫⎪
⎬
⎪⎭
0 .1
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
Operating
pressure, bar
0-20
Length/diameter,
L/D
3
20-35
4
>35
5
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 design
Preliminary calculation procedure (no.1, see
example)
1. Select Kv (e.g. 0.07*0.15)
2. Calculate the maximum vapor velocity, UV
3. Calculate the cross-sectional area, A, assuming half total area
4. 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 time
8. Adjust the vessel diameter and repeat calculations
Horizontal separators design
Calculation procedure for horizontal
separators (no.2)
1. Select Kv
2. Calculate the maximum vapor velocity, UV
3. Calculate the cross-sectional area, A
4. 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, ts
6. 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 design
A more accurate calculation procedure (no.3)
1.
2.
3.
Calcolo
Calcolo
Calcolo
QV = WV / ρV
Q L = WL / ρ L
UT = K
ρ L − ρV
ρV
U V = 0,75U T
(in questo caso K non tiene conto dello snebbiatore)
Horizontal separators design
A more accurate calculation procedure (no.3)
V H = TH Q L
4.
Fisso
TH
5.
Fisso
L/D (con 1.5<L/D<6) e un grado di
riempimento: calcolo un diametro di I° tentativo
1/ 3
⎡ 4(VH ) ⎤
D=⎢
⎥
π
0
,
6
(
L
/
D
)
⎣
⎦
(corrisponde ad assumere
VH = 60%VT
)
Horizontal separators design
A more accurate calculation procedure (no.3)
6.
7.
La sezione totale:
AT =
πD 2
4
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
Horizontal separators design
8. Noto HLL/D,
calcolo
ALL/AT e ALL
in alternativa,
con
x= ALL/AT e
y= HLL/D :
x = π cos (1 − 2 y ) − π (1 − 2 y ) y − y
1
−1
2
2
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
VH
L=
AT − AV − ALL
Horizontal separators design
11. Calcolo il tempo di caduta (‘drop-out
time’):
τ drop
HV
=
UV
12. Calcolo la velocità del vapore:
U VA = QV / AV
Horizontal separators design
13. Verifico che la velocità ottenuta sia
minore di quella di trascinamento:
⎧⎪ ρ
Ue = ⎨ L
⎪⎩ ρV
⎛σ
⎜⎜
⎝ ρV
4
⎞ ⎡ g ( ρ L − ρV ) ⎤
⎟⎟ ⎢
⎥
µ
L
⎦
⎠ ⎣
2
⎫⎪
⎬
⎪⎭
0 .1
14. Calcolo la lunghezza minima richiesta
per l’abbattimento del liquido:
Lmin = UVAτ drop
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
– e
quindi
HNL
HHL=D-HV.