High Pressure Knock Out Drum 3

Dear all
Do you know how it work?(In Oil&Gas industrial)

 

[quan1tum1]

Hi Roger

From my understantding a knockout drum is simply a liquid- vapour separator or flash vessel / and the only difference the pressure can makes is to shift the equilibrium so that more liquid condenses out at a given temperature, and to the construction. The liquid or liquid vapour stream enters, and the vapour (with a certain degree of droplet entrainment) leaves from the top of the vessel while the liquid leave from the bottom.

If you are wanting to size the vessel, using simple methods : Walas - Chemical process design (i'm not sure of the exact title but its something like that)is a good place to start. Otherwise start looking in the standards ASME or BS or the like

King regards

 

[vsvenkat7]

I agree with quantum1. There are also different types based on the principle of separation - like Gravity type, cyclonic type & demister type. You can also refer well known - Chemical Engineering design book written by E. Ludwig (title I am not so sure..) for further reference. Also Chemical Engineering Handbook - Perry has some information.

Regards
vsvenkat7

 

[Shaheryar]

Hi,

For separator sizing the in the two phase (K-O Drum), a quick estimation can be made by looking at API -521 and API 12J. For three phase separator design the best place to look is "Gas Conditioning and Processing" by John M. Campbell , that is if you can get your hands on one.

Shaheryar

 

[Buchi]

in addition, you might want to have a feel of a FWKO (free-water Knockouts). these are usually intended to remove only large pecentage of free water. by free water, i mean water that is carried in the produced crude oil streams, but not emulsified in the oil. Free water settles easily in less than 10-20min.

the FWKO is often used in conjuction with other methods as a preliminary step or first stage brine removal. Crude oil leaving the FWKO may still contain from 1 to as much as 30 or 40 percent emulsified water. These devices usually have little or no internals, unlike other separators that requires baffles, demister pads e.t.c. Often, they are only pressurized settling tankswith a water level control and discharge control valves. Water is removed by gravity separation. Removing free water before the crude - brine mixture (emulsion) into a fired heatersaves considerable fuel. It takes 350 Btu to raise 1bbl of crude 1 degree F but only 150 Btu to raise 1 bbl of crude oil 1 degree F!!! can you beat that?

FWKOs are not a panacea; they remove only free water. Emulsion breaking chemicals can, offcourse, be added upstream of the FWKO.

cheers Roger,

i hope that helps.
Buchi

 

[waadhassan]

dear all
i want to aske you abuot the (High Pressure Knock Out Drum only two phase oil and water ) espically about
design
parameters affecting efficincy
regards

 

[Montemayor]

I also recommend you go to "Gas Conditioning and Processing"; John M. Campbell; Volume I; Chapter 8, Process Vessel Design. My copy is the 3rd printing (April, 1979). There, you will find all the essential theoretical and empirical design information regarding separators in natural gas service.

Dr. Campbell's book is well known for many years here in the "oil patch" area of the USA where a lot of the state of the art was conceived for the petroleum and natural gas industry. You will basically find that the Souders-Brown relationship is the basis for the process design and that various schemes, internals, and flow configurations are employed in different manners to achieve successful liquid-vapor separation. I presume you are in an oil producing area and have access to this book. If not, it is published by Campbell Petroleum Series; 121 Collier Drive; Norman, Oklahoma 73069.

The size of a knockout drum should be dictated by the anticipated flow rate of vapor and liquid from the drum. The following sizing guidelines are based on the assumption that those flow rates are known:

For the maximum vapor velocity (which will set the drum's diameter), use the following version of the Souders-Brown equation:

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

where:
Vmax = maximum vapor velocity, ft/sec;
dL = liquid density, lb/ft3 at the operating pressure;
dV = vapor density, lb/ft3 at the operating pressure;
k = 0.35 (when the drum includes a de-entraining section)

In a Vertical separator, k can vary between 0.06 to 0.35.
In a Horizontal separator, it can vary between 0.4 to 0.5.
These values pertain to density units of lb/ft3 and the units of k are ft/sec.

I have used k = 0.25 with stainless steel mesh as internals on vertical vessels. In your application, I would recommend 0.15 for k, with no steel mesh or internals.

By calculating the allowable superficial velocity and using the vapor flowrate, you calculate the cross-sectional area of the vessel. Be sure to allow generous vapor disengagement space (minimum 1-2 feet) and a liquid inventory level in the vessel. I usually use a height to diameter ratio of 2-3:1 on vertical vessels. In your assumption of a k consider that the lower k will yield a larger and more "conservative" design.

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 quarter-full level). Depending upon how much liquid flow you expect, the liquid outlet line should probably have a level control valve and you should provide for a high level alarm.

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.


Art Montemayor
Spring, TX

 

[25362]

to waadhassan, do you mean two liquid phases, oil and water ?

 

[25362]

Often the fluid entrance to a vertical KO drum is made either tangential or pointing downwards to enable other forces (impaction, centrifugal) to help out in the V/L separation of a KOD.

One important item -not mentioned above- is where is actually the vapour offtake going. If it is steam from a boiler going to be used in reboilers, a little bit of entraiment wouldn't do harm. However, if the vapour is going to be fed to a reciprocating compressor, entrainment is of much consequence and a demister, or any other entrainment reducer, becomes an obligatory addition.

The formula given by Montemayor is of much practical use, but it is not the ultimate truth, it is rather an educated guess based more on experience than on precise engineering calculations: we don't know the liquid particles' size distribution to apply Stokes' Law or similar.

When the mixture has a high degree of dispersion and turbulence, and especially, when there is a tendency to foam or froth, even the safe recommended Souders-Brown "k" values should be taken down by, say, 30%.

When there is foam, carryover of liquid may then not be a function of vapour velocity but of foam height, since vapour may be blowing foam if its level is sufficiently high. This is one particular reason to ensure that there is sufficient distance between the inlet nozzle and the true liquid level. The external stagnant liquid measuring/controlling instrument could be showing a false level, since the foam level in the KOD may be 1/3 higher than that shown in the external level gauge glass.

This would have to be taken into account when designing a normal liquid level height at the bottom of the KOD, as well as the distance between the vapour top offtake nozzle and the inlet nozzle.

When clean naphtha mists are fed to a wet-gas centrifugal compressor a bit of entrainment may help to keep the rotors clean, however, large and sudden slugs of liquid, such as when there is foaming in the KOD, would surely damage the machine.

 

[waadhassan]

dear all

I am looking for information about (FWKO)(liquid-liquid separetion) in the oil industry -espically about

1-design and principal of work.

2-Parameters affecting efficiency.

regards

 

[Hookem]

If the vessel is horizontal, be sure to increase the calculation of critical velocity by the (gas flow length/20) ^.56; to check that the size of the vessel isn't dictated by liquid retention time; and, to check that the size of the vessel is not dictated by the maximum velocity through a mesh pad.