11 replies to this topic

[Parker]

Dear All,

 

We are experiencing condensate dropout in our natural gas fuel lines. Natural gas is supplied at 15 barg and its pressure is reduced in two stages to 5 bar and 1 bar respectively. There is a knockout drum after first pressure reduction. Condensate does not collect in the knockout, but in the final Fuel header (at its end point).

 

What are the possible causes of condensate dropout. The collected condensate is Black in color with API gravity 30-35 degrees. When heated in the laboratory the whole condensate evaporated at 500 degC.

If it is to say that condensate has been dropped out due to the achievement of Hydrocarbon Dew Point then how is it possible that a temperature decrease of 5-10 degrees leads to a drop out of a condensate which gets vaporized at 500 deg C?

 

C6+ content in the natural gas is 0.71-1.02. 

 

 

[PingPong]

That "natural gas" seems to be of poor quality. What is its origin? Where does it come from? Is it saturated vapor from a vapor/liquid separator? If so, at what T and P is that separator operating?

 

What condenses are mainly the heaviest molecules in the gas. If that condensate is measured to have a boiling point of 500 ^oC then that is what it is.

 

The C₆⁺ content (is that vol% ?) does not tell you what exactly that C₆⁺ composition is. You should obtain a more detailed composition of that natural gas.

 

If composition of that natural gas cannot be improved then a heater in the gas line is a possible solution.

[thorium90]

I would agree with Pingpong.

The short term solution would be to check if the supplier of gas has deviated from their contracted terms of supply and request that they comply with with the agreed gas specifications.

If the problem is here to stay and that you are unable to change the source of supply, you might require a gas scrubber and/or dew point heater. A knockout drum will be insufficient for your purpose.

 

You can google for fuel gas scrubbers, dew point heaters, gas conditioning equipment etc. eg:

http://www.exterran....on/gas-scrubber

[Parker]

yes, the C6+ content is in vol%. The gas is a blend of LNG and Well natural gas. Detailed analysis of natural gas is not possible due to limitation of lab equipment.

 

Please answer a conceptual question that: If it is to say that condensate has been dropped out due to the achievement of Hydrocarbon Dew Point then how is it possible that a temperature decrease of 5-10 degrees leads to a drop out of a condensate which gets vaporized at 500 deg C?

Edited by Parker, 02 August 2015 - 10:00 AM.

[thorium90]

Hydrocarbon dew point is a function of gas composition and pressure.

As you have already observed there is condensation from the pipeline when reduced to 1 Bar, the need for gas conditioning is already necessary. Further analysis of the condensate is not always fruitful since the condensate tested at the lab is 100% condensate and not the partial gas and condensate at higher pressure that is in the pipeline.

If the quality of gas supply is important to your plant, I would suggest you start to look for vendors of fuel gas scrubbers for a start. Projects like these take time to be implemented and during the interim, you will continue to be affected by poor gas quality until concrete remedial actions are taken.

Edited by thorium90, 02 August 2015 - 10:27 AM.

[PingPong]

how is it possible that a temperature decrease of 5-10 degrees leads to a drop out of a condensate which gets vaporized at 500 deg C?

Very simple: a small temperature drop below the gas dew point results in condensation of only the very heaviest components in the dew point gas.

If one would cool the gas much deeper then also lighter components will condense.

So the more the gas temperature drops, the more condensate will form, but the lighter that condensate will be.

[Parker]

Dear PingPong and Thorium, I want the clarification of one more concept.

 

What is the phenomenon that at high pressures we don't have condensation, although, we see that for pure components, high pressures lead to a change of state from gas to liquid?

 

Are these heavier hydrocarbons present in the dissolved state with natural gas at high pressures? If, on pressure reduction we observe liquid dropout, will on increasing the pressure to the same value vaporize/dissolve that liquid again?

 

I want such conceptual clarification as I didn't understand the phenomenon.

[PingPong]

My best guess at this moment is retrograde condensation, in combination with Joule Thomson cooling.

 

But without a detailed composition of that natural gas your questions can not be answered with certainty.

Such detailed composition is required to make a phase envelope using a process simulator or similar software.

Edited by PingPong, 03 August 2015 - 04:18 AM.

[Parker]

  I have attached a phase envelope, which I generated by using ASPEN PLUS and SRK thermodynamic model.  

Attached Files

•  phase envelope.jpg   81.38KB   3 downloads

[PingPong]

If you don't have a detailed breakdown of the C₆⁺ (as you stated before) then the phase envelope can not be accurate in the region (bottom right corner) that matters.

 

As you have Aspen Plus simulator, you can test whether the NG composition that you used produces some heavy condensate downstream control valves. If it does not, then your NG composition does not have enough heavy tail to reproduce reality.

[Art Montemayor]

Parker:

 

As is usual, PingPong has made excellent observation and recommendations regarding the basic query.  The topic of Phase Equilibria is one that evades a lot of recent graduates and it is a shame that it is not dealt with more diligent effort at the university level, in my opinion.

 

Retrograde condensation is a subject that should be thoroughly understood by all who deal with hydrocarbon processing - upstream and downstream.  Some years back I uploaded an edited version of a work of Dr. Michale Adewumi’s on the subject and I am taking this opportunity to once more upload a copy for everyone’s review.

 

Read carefully the part pertaining to Retrograde Condensation and note how the phenomena applies to PingPong’s suspicions.  I believe the subject matter will be more than worthwhile of your efforts.

 

 

Attached Files

•  PNG 520 - Course on Phase Relations.docx   1.53MB   106 downloads

[Parker]

I must say Thanks to Art Montemayor for uploading this valuable document and to PingPong for helping me.