Stabilization of Light crude in Wellhead locations 2

[reena1957]

Guys, this might not be the right forum but still being a refinery person forced to work on a wellhead crude stabilization project simulation, I want to know how to reduce the RVP of light crude to about 5psig using only gas-oil separators? In refinery stabilizers, we always went for a distillation column at high temperatures. How do we do this in field using temperatures below 50 Deg C? Even flashing to flare pressure gives RVP higher than 15 in my simulator!!

 

[monaco8774]

reena1957

being more of an upstream guy I might be able to help. Typically to achieve RVP specification you need to do a combination of:

Heat to a temperature above ambient

Flash off at lowest pressure achievable to allow for pressure drops thereafter

Recool if necessary to get below any design temperatures

If the crude is above ambient conditions when it arrives then a low pressure stage flash should normally be enough to achieve RVP specification, since thereafter it will cool, if it is below ambient then you will probably have to heat it.

In any case if you need to seperate water from this crude heating may be required.

 

[reena1957]

Dear Monaco, my flowsheet is same as what you have suggested. I am having the last separator( the first two are at 20 and 7 kg/cm2 respectively) open to flare at 50 Deg C. But I am not getting the RVP required in the simulation. Any further ideas, especially if you have someone who has some experience in simulating this? The crude API is 55 and I want an RVP of 5. Could we have some stripping action in the last separtor? Thanks.

 

[MortenA]

Remember that RVP is usually reported in PSIA - so 15 psia is just above 0 psig. Check for this in your simulator.

The RVP wont change when cooling because its a reference value.

Simulators usually predics a higher RVP than what is measured. I dont know why but i suspect it has something to do with the way samples are collected/treated.

Best regard

 

[reena1957]

Dear Morten, RVP is expressed in psig as it is the gauge pressure measured with 80% air and 20% liquid in a container at 37.8 Deg C, when it reaches equilibrium. It is the air pressure plus the True vapor pressure(TVP) of the liquid mesured together. So the TVP is the RVP minus atmospheric pressure expressed as psia. This particular subject of RVP being gauge pressure and TVP being abs pressure has been the cause of many design basis confusions and pressure tanks being installed instead of simple atmospheric tanks. However, TVP is slightly less than (RVP-1 atm), say about 90-95% of this value. Thank you brought this up.
Let us have some good hot spicy discussion on this with some feedback from the great designers like MonteMeyer, 25362, SSHEP and TD2K!!

 

[reena1957]

Dear MortenA, Thanks for the right reply. I understand RVP is reported in psia in the West and not psig as in India. I made a big(??) typo. RVP is smaller than TVP!! I checked my simulator, Viola, as psia the value of RVP is ok. Thanks once again.

 

[monaco8774]

reena

Typically we need something around 80°C + to achieve those kinds of RVP specifications, it depends on how many C3s and C4s you have in the crude. Im afraid you just need more heat. Stripping is not normally used as always everything offshore is kept as simple as possible. Remember that if you are taking to these temperatures you'll probably end up needing epoxy lining or glass flake in your last stage seperator

 

[MortenA]

As far as i can see you found that the RVP was indeed in psia and NOT psig?

What caught my attention was that 15 psig is a pretty high RVP especially at 50 deg C and atm.

I actually usually sees RVP in bara since my HYSYS is set up for "metric" units,

Best regards

Morten

 

[UmeshMathur]

MortenA:

A minor point: It's not hard to define (and save as the default) options in Hysys to change the standard units to English. You can even do your own definitions for selected sub-sets of properties. For example, specify viscosity in cP, or standard volume flow in BBL/Hr, while using the "standard" units for everything else.

 

[MortenA]

i know - but im a creature of habit

(an excuse for being lazy ;-) )

Best regard

Morten

 

[KazakhJeff]

Guys, interesting exchange about RVP abs vs gauge. Thanks for the clarifications. Let me follow this through and change the thread a little (if you allow me please) and seek advice for what you would do with this associated gas, now that the crude is stabilised. Our oilfields are in a remote location, away from pipelines and civilisation, flaring is now banned, so the only solution appears to be gas reinjection, which to a refiner like me is a waste. Does anyone out there have any other suggestions for imaginative ideas for what we might be able to do with this gas?
Thank you