3-Phase Separator: Bucket and Weir 1

[ChEMatt]

I'm designing a separation system. The system will separate vapor, oil, and water. The system will be installed in a skid where vertical height is somewhat limited, but as I have not calculated the dimensions of the vessel yet I cannot say whether or not it is a constraint. It's for a pilot plant, so the flow rate and liquids produced are relatively small (less than 500 gal/day produced liquids, vapor travel is probably 0.5 MMSCFD)

A co-worker is convinced that the bucket and weir system is the way to go, while I believe this would be expensive to manufacturer and unnecessary when compared with other designs. In my mind, you would select this type of system if you are limited on vertical space and cannot fit a boot under the vessel. I have not encountered this type of system before so I cannot say whether or not it would be a good idea for certain.

So between going with a 3-phase separator with a boot, or with the bucket-and-weir option, what should be the questions one would ask to determine between the two? Is it simply cost? Is one method more efficient (allows for a smaller vessel) than the other? For that matter, why even go with a boot? Why not just a weir? The fourth option is an empty vessel with vertically spaced flanges to take off the light and heavy liquid. Of what advantage is a weir?

Of all the articles I've seen (GPSA, CEP, API 12J) there doesn't seem to be much information on *which* system you choose, only how you perform calculations.

Thanks you!

Onwards,

Matt

 

[CJKruger]

ChEMatt, my view is that you should aways use a boot with raised vortex breaker if you can. It is however only practical if the water flow is much less than the hydrocarbon flow.

If the water flow is high compared to hydrocarbon, then you have to use a baffle or bucket-weir arrangement.

In both cases, these drums are fairly difficult to size, due to the numerous mechanical issues and standards clearances you have to consider.

Cilliers

 

[ChEMatt]

Cilliers,

Thanks for the response. As I am new to the specification of these things I have since learned that the high ratio of vapor to liquid dictates we must use a vertical separator. Another engineer was concerned about possible parrafin issues, as the hydrocarbon liquid is 99% straight chains.

In our particular case, the water flow is twice the HC flow.

Thanks again.

Onwards,

Matt

 

[LBLutino]

Matt,

Use vertical separator if there is large quantity of gas compared to the total liquid flow. Otherwise use horizontal.

Horizontal: If the heavy liquid phase concentration is < 5% then use boot for ease of control. Otherwise use weir. If with weir you must check whether there will be well-defined L-L interface. Single weir is not recommended if there is an ill-defined interface. reason is on interface control. In this case you must consider double weir (more complicated to design). To check if interface is well-defined or ill-defined: if the dispersed phase concentration is atmost 5% of total liquid volume then more likely the interface will be well-defined.

Vertical: Use weir or bucket type to eliminate problem with interface control on the heavy and light liquid.