14 replies to this topic

[abinai]

Dear all,
When oil is treated in an initial separator,the lighter hydrocarbon molecules(methane,ethane..) will flash to gas state at higher pressure or at lower pressure?
Please explain,

Thanks in advance,
with regards,
abinai

[kasri]

Dear abinai,
Higher Separation will takes place at lower pressure.
Regards,
Kasri

[Andree]

the more volatile (lighter) a hydrocarbon, the higher is the pressure at which it will flash

[abinai]

Dear kasri and Andree,
Thanks for your comments.
Please anyone can explain me the below quoted statement,

"If the pressure for initial separation is too high, too many light components will stay in the liquid phase at the separator and be lost to the gas phase at the tank. If the pressure is too low, not as many of these light components will be stabilized into the liquid at the separator and they will be lost to the gas phase.Here the tank refers to the stock tank that comes downstream of the separator".

Thanks in advance,

[Andree]

at low pressure more liquid is vaporised (starting from light fraction which is more volatile) - it means that if pressure is high then only small amount of liquid will be vaporised, if pressure is low then a lot of light HC will vaporise and not much of it will remain in liquid phase

[kasri]

Dear Abinai,
Under high pressure lighter components mixed with oil in a dissolved manner,that why when inlet separator operates at high pressure, it doesnot flash.If it is operates at lower pressure it pave way to flashing of gas and separates at inlet separator itself.Hope it will clarify your doubts.
Regards,
Kasri

[abinai]

Dear Andree,
Then when the number of stages of separation increases the amount of liquid produced also increases? In each stage we go on decrease the pressure then howcome the liquid produced increase?

Thanks in advance,

[Qalander (Chem)]

Dear kasri and Andree,
Thanks for your comments.
Please anyone can explain me the below quoted statement,

"If the pressure for initial separation is too high, too many light components will stay in the liquid phase at the separator and be lost to the gas phase at the tank. If the pressure is too low, not as many of these light components will be stabilized into the liquid at the separator and they will be lost to the gas phase.Here the tank refers to the stock tank that comes downstream of the separator".

Thanks in advance,

Dear abinai Hello/Good Afternoon,

Firstly the Andree's response is guiding to your most recent query.However I add my little bit as follows;

1)The phrase 'initial separation' is implying Pre-fractionation, Main Fractionation or Well head vicinity separation? This is unclear here.

2)As a general concept the pressure above the liquid surface impacts rate of evaporation as we increase or decrease the exerted pressure light ends evaporation rate is affected somewhat inversely keeping temperature constant.

3)This concept is utilized throughout the chemical processing industry and wherever the material to be separated can get damaged 'quality wise'we decrease surface exerted pressures to certain optimum value to ensure/affect evaporation as well as maintaining of quality.

Hope this helps
Regards
qalander

[abinai]

Dear Qalander,
The "initial separator" implies the well head vicinity separation".

regards,
abinai

[Qalander (Chem)]

OK, Then there are certain standard rule of thumbs employed by production related staff.

Most usually keeping somewhat higher pressures exerted on surface

to recover as much as possible 'NGL' or (Natural gas liquids) or 'Casing head Gasolines' or 'Condensate'

To reduce load on further downstream facilities whether handling gas(es) or liquid(s)

This also helps in taking care of nuisance volumes of any associated water/salt water streams
Hope this helps
qalander

[Andrei]

Gentlemen,

In a lot of situations the actual primary separation selected pressure is affected by local factors, like reservoir pressure, gas/oil/water ratio and properties, and existing local access roads and regulations.
I've worked in primary gas-oil separation plants projects in a lot of areas, starting with extreme North of North-America, Central and South America, Persian Gulf and Asia. What I found out is that there may be some local rules of thumb, but they are not general applicable to all the areas.
Of course the objective is to perform the primary separation at a pressure as low as possible to separate as much as possible light components in the gas phase. The limitation is the size of the separator/separators; you cannot go too low because the size of the separators will increase beyond reasonable values. Simply you do not have roads wide enough to transport them at the site; these separators are shop fabricated.
Saying that the light components not separated at the primary separation are lost to the gas phase I think it is too much. If the quantities are significant, a Vapour Recovery Unit in the storage tanks area will capture and return them to the gas phase.
In the same time there will be certain amounts of heavy components entrained in the vapour phase that will be separated at the compression phase, and returned to liquid.
There will be some optimization required in each individual case.

[Qalander (Chem)]

Gentlemen,

In a lot of situations the actual primary separation selected pressure is affected by local factors, like reservoir pressure, gas/oil/water ratio and properties, and existing local access roads and regulations.
I've worked in primary gas-oil separation plants projects in a lot of areas, starting with extreme North of North-America, Central and South America, Persian Gulf and Asia. What I found out is that there may be some local rules of thumb, but they are not general applicable to all the areas.
Of course the objective is to perform the primary separation at a pressure as low as possible to separate as much as possible light components in the gas phase. The limitation is the size of the separator/separators; you cannot go too low because the size of the separators will increase beyond reasonable values. Simply you do not have roads wide enough to transport them at the site; these separators are shop fabricated.
Saying that the light components not separated at the primary separation are lost to the gas phase I think it is too much. If the quantities are significant, a Vapour Recovery Unit in the storage tanks area will capture and return them to the gas phase.
In the same time there will be certain amounts of heavy components entrained in the vapour phase that will be separated at the compression phase, and returned to liquid.
There will be some optimization required in each individual case.

Dear Andrei Hello/Good afternoon,

Thanks, As expected you have done a marvellous job ideed.Since I had very little; if any knowledge on practical aspects and practices of production scenarios.
Hope this proves benificial to OP and others.

One cautious remark for OP to be more clear/informative to all while Opening the topic Please!
Best Regards
qalander

[abinai]

Dear all,
I have quoted the below question before,but not yet received any reply,

Then when the number of stages of separation increases the amount of liquid produced also increases? In each stage we go on decrease the pressure then howcome the liquid produced increase?

Please explain,

[Qalander (Chem)]

Dear, I understand this statement is a general overview to define yield and productivity increased;
however not unlimited.Hope now the issue gets clarified!
Regards
Qalander

[Andrei]

abinai,

I do not know who stated that in a multistage gas/liquid separation (any of them, not only oil separation) the liquid produced increases while the pressure decreases. Personally I consider the statement inaccurate.

What are the processes involved in a gas/liquid separation? Let's consider the innitial oil at high pressure, when everything is in liquid phase. If the system pressure decreases below the vapour pressure of a certain component/fraction that component will be released from liquid in the vapour phase. So, if system pressure is decreased further more components will be released in the vapour phase, so less liquid phase will be produced, that's the mass concervation law. At least this is the ideal process. In reality there will be some non-ideal processes happening, like components that remain in the liquid phase even if the system pressure is below their vapour pressure due to some molecular interactions, or components that based on their vapour pressure should remain in the liquid phase but are entrained in the gas phase due to system's hydrodynamics.
The system temperature is expected to decrease a little bit also with the pressure decreasing.