[mbeychok]

In English units:

A vapor-liquid separator drum is a vertical vessel into which a liquid and vapor mixture (or a flashing liquid) is fed and wherein the liquid is separated by gravity, falls to the bottom of the vessel, and is withdrawn.  The vapor travels upward at a design velocity which minimizes the entrainment of any liquid droplets in the vapor as it exits the top of the vessel. 
 
The size a vapor-liquid separator drum (or knock-out pot, or flash drum, or compressor suction drum) should be dictated by the anticipated flow rate of vapor and liquid from the drum. The following sizing methodology is based on the assumption that those flow rates are known.

Use a vertical pressure vessel with a length-to-diameter ratio of about 3 to 4, and size the vessel to provide about 5 minutes of liquid inventory between the normal liquid level and the bottom of the vessel (with the normal liquid level being at about the vessel's half-full level).

Calculate the vessel diameter by the Souders-Brown equation to determine the maximum allowable vapor velocity:

V = (k) [ (dL - dV) / dV ]^0.5

where:
V = maximum allowable vapor velocity, ft/sec
dL = liquid density, lb/ft³
dV = vapor density, lb/ft^0.3
k = 0.35  ft/s (when the drum includes a de-entraining mesh pad)
 
Then A, the cross-sectional area of the drum, in ft² = (vapor flow rate, in ft³/s) / (vapor velocity V, in ft/s)

and D, the drum diameter, in ft =  ( 4 A / 3.1416 )^0.5
 

The GPSA Engineering Data Book recommends the following k values for vertical drums with horizontal mesh pads (at the denoted operating pressures):

      0 psig:  0.35 ft/s
  100 psig:  0.35 ft/s
  300 psig:  0.33 ft/s
  600 psig:  0.30 ft/s
  900 psig:  0.27 ft/s
1500 psig:  0.21 ft/s

GPSA Notes:
1. K = 0.35 at 100 psig; subtract 0.01 for every 100 psi above 100 psig
2. For glycol or amine solutions, multiply above K values by 0.6 – 0.8.
3. Typically use one-half of the above K values for approximate sizing of vertical separators without mesh pads.
4. For compressor suction scrubbers and expander inlet separators, multiply K by 0.7 – 0.8

The drum should have a vapor outlet at the top, liquid outlet at the bottom, and feed inlet at somewhat above the half-full level. At the vapor outlet, provide a de-entraining mesh pad within the drum such that the vapor must pass through that mesh before it can leave the drum. Depending upon how much liquid flow you expect, the liquid outlet line should probably have a level control valve.

As for the mechanical design of the drum (i.e., materials of construction, wall thickness, corrosion allowance, etc.), use the same methodology as for any pressure vessel.

In Metric units:
 
A vapor-liquid separator drum is a vertical vessel into which a liquid and vapor mixture (or a flashing liquid) is fed and wherein the liquid is separated by gravity, falls to the bottom of the vessel, and is withdrawn.  The vapor travels upward at a design velocity which minimizes the entrainment of any liquid droplets in the vapor as it exits the top of the vessel.
 
The size a vapor-liquid separator drum (or knock-out pot, or flash drum, or compressor suction drum) should be dictated by the anticipated flow rate of vapor and liquid from the drum. The following sizing methodology is based on the assumption that those flow rates are known.

Use a vertical pressure vessel with a length-to-diameter ratio of about 3 to 4, and size the vessel to provide about 5 minutes of liquid inventory between the normal liquid level and the bottom of the vessel (with the normal liquid level being at about the vessel's half-full level).

Calculate the vessel diameter by the Souders-Brown equation to determine the maximum allowable vapor velocity:

V = (k) [ (dL - dV) / dV ]^0.5

where:
V = maximum allowable vapor velocity, m/sec
dL = liquid density, kg/m³
dV = vapor density, kg/m³
k = 0.107 m/s (when the drum includes a de-entraining mesh pad)

Then A, the cross-sectional area of the drum, in m³ = (vapor flow rate, in m³/s) / (vapor velocity V, in m/s)

and D, the drum diameter, in m =  ( 4 A / 3.1416 )^0.5
 

The GPSA Engineering Data Book recommends the following k values for vertical drums with horizontal mesh pads (at the denoted operating pressures):

    0 barg:  0.107 m/s
    7 barg:  0.107 m/s
  21 barg:  0.101 m/s
  42 barg:  0.092 m/s
  63 barg:  0.083 m/s
105 barg:  0.065 m/s

GPSA Notes:
1. K = 0.107 at 7 barg; subtract 0.003 for every 7 bar above 7 barg
2. For glycol or amine solutions, multiply above K values by 0.6 – 0.8.
3. Typically use one-half of the above K values for approximate sizing of vertical separators without mesh pads.
4. For compressor suction scrubbers and expander inlet separators, multiply K by 0.7 – 0.8

The drum should have a vapor outlet at the top, liquid outlet at the bottom, and feed inlet at somewhat above the half-full level. At the vapor outlet, provide a de-entraining mesh pad within the drum such that the vapor must pass through that mesh before it can leave the drum. Depending upon how much liquid flow you expect, the liquid outlet line should probably have a level control valve.

As for the mechanical design of the drum (i.e., materials of construction, wall thickness, corrosion allowance, etc.), use the same methodology as for any pressure vessel.

Milton Beychok
(Visit me at www.air-dispersion.com)

[Donaldson Tan]

Given the extent of liquid flashing depends on the operating pressure of the the seperator, how would you implement the control strategy for the seperator? In a typical industrial setting, what is the controlled variable? Is it the composition of the vapour phase or the molar flowrate of the vapour stream? I believe the manipulated variable is the operating pressure.

[technologist]

Geodome

Generally VL separators are used for V & L separation onyl not for controlling components

(in Most of the cases, I would love to learn if any specific cases r mentioned here where we control components flashing except in cryogenics)

The control strategy will vary depdning on process requirement e.g. if system is provided with upstream heating the temperature may also be a manipulated variable along with pressure.

Sometimes it is controlled by separator vapor phase pressure also due to criticality for D/S conditions.

Probably, specific case based discussions are more helpful than generalized considerations.

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[Donaldson Tan]

I understand now that composition control is definitely not employed for VL Seperator. After-all, its essential function is to seperate the vapour phase from the liquid phase. I wonder how the extent of flashing is controlled. Is it done by setting the entry feed condition or by setting the operating pressure? Also, in doing so, where should the valves should be installed. Can someone suggest a possible control loop?

[mbeychok]

Given the extent of liquid flashing depends on the operating pressure of the the seperator, how would you implement the control strategy for the seperator? In a typical industrial setting, what is the controlled variable? Is it the composition of the vapour phase or the molar flowrate of the vapour stream? I believe the manipulated variable is the operating pressure.

If the drum is being used primarily as an adiabatic flash drum (for example, in a vapor compression refrigeration system), the operating pressure or the desired temperature would usually be the control variable.

However, if the drum is being used primarily as a simple vapor-liquid separator, the control variable would usually be the liquid level in the drum.  For example, the primary purpose of a gas compressor suction drum (also known as a "knockout drum") is to provide "a wide space" in the compressor suction piping to allow any liquid droplets to drop out of the gas stream so as not to damage the rotor blades in an axial flow or centrifugal gas compressor. In such cases, the liquid level must not be allowed to be above the vapor inlet to the drum. The space in the drum below the vapor inlet must also be large enough to capture and hold any large "slugs" of liquid that may be anticipated, for the same reason (i.e., to protect the compressor).

[technologist]

I wonder how the extent of flashing is controlled. Is it done by setting the entry feed condition or by setting the operating pressure?

It seems that Geodome is not talking about KO drums where we normally control only liquid level & rest is taken care by design stage assumptions.

He is interested in operational control for Flash separators where pressure reduction is taking place & component separation is the motive for flashing, If I am correct.

In these cases Inlet Temp control is the major variable whereas you have little margins in the pressure (Generally) to play with due to effects on Up/St & Dn/St flow systems. However, pressure reduction valve is necessary at immediate inlet to this flash drum.

These are only general considerations, proper control philosophy depends on actual system.

[technologist]

I wonder how the extent of flashing is controlled. Is it done by setting the entry feed condition or by setting the operating pressure?

So basically your question is related to flash drum controls not V-L separator controls.

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[Donaldson Tan]

So basically your question is related to flash drum controls not V-L separator controls.

Isn't the flash drum a VL Seperator?

[mbeychok]

Geodome:

Yes, a flash drum is also a vapor-liquid separator, which is the way we spell it in American English. Is seperator a British English spelling??

[Borek]

Is seperator a British English spelling??

No, its Geodome English spelling

We are from so distant parts of the world it is kind of a miracle we are able to communicate

[eugenedakin]

Chuckle,

I always get a rough time from residence of other countries when I write words like:

centre
colour
labour
sulphur

Although the 'Chemical Forums' Spell-Check accepts them ...

Sincerely,

Eugene

[mailtoamol1]

i need to design the vertical vapor liquid seperator for the following compositionof CNG
 compositon is
Carbon Dioxide: 0.038
Methane: 0.91
Ethane: 0.0452
propane: 0.0055
n-Butane: 0.0002
n-Hexane: .0011

Critical temperaur and pressure are
critical temperature: 201.301 K
critical pressure: 4714.462 kPa
Molecular weight: 17.97 kg/kg-mole

the problem with me is how to find the liquid density for the operating condition

Please advice