Gas-Liquid separators Roberto Bubbico PhD, Chem. Eng. Department of Chemical Engineering “Sapienz “Sapienza” a” University University of Rome Rome
[email protected]
INTRODUCTION • In phas phase e sepa separa rati tion on,, two two or more more phas phases es can can be be separated because a given force will act differently on them, or because one of the phases impacts on a solid barrier. • The The for forces ces are are usua usualllly y gra gravi vity, ty, centr centrif ifug ugal, al, and and electromotive. • Exam Exampl ples es are are rem remov oval al of a sol solid id from from a liq liquid uid by impaction (filtration), gravity (settling), centrifugal force (cyclones or centrifuges), and the attraction of charged particles in an electrostatic precipitator.
INTRODUCTION • One One exc excep epti tion on to thes these e mec mecha hani nism sms s is is drying drying by by evapora evaporatin ting g unbond unbonded ed water water from from a solid. In this case, separation of a liquid from a solid occurs by mass transfer. • Sinc Since e man many y comp compon onen entt sepa separa rati tion ons s req requi uire re contacting two phases, like liquid-liquid extraction, component separation is frequently followed by phase separation . • Phas Phase e sep separ arat ator ors s can can be clas classi sifi fied ed according to the phases in contact: liquidgas, liquid-liquid, liquid-solid, solid-gas, …
INTRODUCTION • In many many case cases s sep separ arat ator ors s wil willl als also o hav have e the the role of accumulators, with the aim of reducing fluctuations in flow rate, pressure and/or composition (improving process control) • Wher Where e the the carr carryo yove verr of of som some e fin fine e dro dropl plet ets s 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 reco recove verr valu valuab able le prod produc ucts ts,, • impr impro ove pro product duct pur purity, ity, • reduce emissions, • prot protec ectt down downst stre ream am equi equipm pmen ent, t, • ... 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 F G = M L g downward) • buoyancy (acting upward) • drag (acting upward).
F B
F D
=
=
M L ρ V g
π 8
ρ L C D D p2U V 2 ρ V
INTRODUCTION From a force balance: net gravity force = drag force FG = FD The relative velocity is given by:
U T
=
4 gD p ( ρ L − ρ V ) 3C D ρ V
INTRODUCTION • The drag coefficient C ’ is a function of the Reynolds number:
Re =
ρ GU T D p µ
• Depending on the Reynolds number, the terminal velocity can be defined further: • Re>500
U T
(Newton’s law)
gD p ( ρ L − ρ G )
= 1.74
ρ G 2
• Re<2 • 2
(Stokes’ law)
U T
U T
=
=
gD p ( ρ L
3.54 g
− ρ G )
µ 0.71
D p
ρ G
1.14
0.29
( ρ L − ρ G )
µ 0.43
0.71
INTRODUCTION As a matter of fact, the terminal velocity is calculated as:
U T
= K
( ρ L − ρ V ) ρ V
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 ⎞ ⎛ K D = 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:
U v
= K v
D v
=
( ρ L − ρ V ) ρ V
4Q v
π U 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.
V s
= Q Lt s
H S
=
V S 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
