5 replies to this topic

[Dmitriv]

Good day. I hope you can help me on this matter.

 

Consider a 3-phase overhead drum separator in a vacuum distillation unit. The feed to the drum is skimmed oil, sour water, and off-gas. There is a compartment for skimmed oil and a compartment for sour water in the drum. The off-gas is sent to a heater through a vent at the top of the drum.

 

Normally, the discharge in the skimmed oil compartment should be pure skimmed oil and the discharge in the sour water compartment should be water. However, this is not the case. There is water present in the skimmed oil compartment discharge and there is skimmed oil in the sour water compartment discharge. Note that the level controls of the drum and the discharge pumps are working properly.

 

What could cause this unusual condition? If it is due to drum internals, how can I prove it without first opening the drum? (We cannot just open a drum without proving that there is something wrong with its internals.) Are there any calculations to prove this? If so, what data do I need? 

 

Thank you very much.

[AlertO]

hi

Is your separator is weir or boot type?

It probably causes from the insufficient lenght of the separation zone and/or the fluid velocity is too high.
Any attached info of the seperator may help us being better to analyse your system. Anyway, you may check the separation by using stoke laws (search in google).

good luck

[Bobby Strain]

If you want help, you need to provide all the relevant information.

 

Bobby

[PingPong]

A 3-phase separator never gives a perfect separation between oil and water, no matter how big it is.

There will always be some very small water droplets dispersed in the oil, and vice versa.

 

Especially when surfactants are present there can be emulsion formation, which makes simple gravity separation very inefficient.

Amount and type of surfactants will depend on crude oil source and film temperature in feed (fired) heater coils.

[Fr3dd]

Hello,

 

As PingPong says, no perfect separation should be expected; however, I assume that you're asking because you're having abnormal high levels of hydrocarbon in the water phase and viceversa.

 

I dont think that this is a weir boot type separator but maybe a bucket and weir since you're expecting a biggest heavy fluid (water) proportion than light fluid (hydrocarbon).

 

If you are having excessive carry over of heavy liquid into light liquid section the probable causes are:

-Not enough time for gravity separation: the inlet flowrate is higher than expected or some design flaw lead to lack of lenght in the separation section.

- The light liquid content is lower than expected, so part of the heavy liquid is being "skimmed" into the light liquid weir.

- Lack of interface control, the interface level its not being monitored properly and the interface is too high.

 

If you're having excessive light liquid carry under on heavy liquid section, the probable causes are:

- Once again, not enough residence time for separation.

- As AlertO said, excessive fluid velocity (excess of turbulence PLUS lack of residence time).

 

I agree that there is lack on information on this. These are only assumptions that I can make based on your description. I hope it helps a bit.

 

Fr3dd

[122]

This problem happens most of the time when there is insufficient residence time for separation. Was there any throughput increase in the plant? Is this problem recent or very old? Look for the events like debottlenecking, turnaround etc. Good luck.