Dehydration column operation

[I2P]

Friends,
I work in a gas processing plant.We have a dehydration unit where gas is contacted with glycol(TEG).
The system is as under: Gas compressor to after cooler to dehydration coulmn. The temperature at after cooler outlet is 47 deg C, incoming lean glycol temperature is 49.7 deg.C.
The pressure at contactor inlet is 60 bar, and temp. is 47 deg. My problem is that the aftercooler temperature and hence the compressor trip on low temperature is 45 deg.c This as mentioned in the design is to avoid the corrosion due to wet CO2. So presently we have bypassed the trip.What I would like to know is how to determine whether at this temperature there is chance of wet corrosion. The gas comp. has 0.346% CO2.
How to calculate the dew point of this gas?
If I elevate the temp at aftercooler from 47 to 55 deg. C how would it affect my dehydration column?
Ideally a lower gas temperature would favour absorption, but then it would also mean knocking out more water and hence loading up the column too
Just for info, our glycol circulation rate is fixed,
how should we operate the column so that we are away from the wet Co2 corrosion and are on spec for water content of gas?

 

[TD2K]

I'm a little confused here. You say the purpose of the low temperature trip is to avoid wet CO2 corrosion which would imply that you don't condense any water in the aftercooler if you remain about this temperature. Then, you comment "would also mean knocking out more water and hence loading up the column too" if you raised the operating temperature to the TEG column which implies the compressor discharge condensor IS condensing some of the water.

Which is it?

You can do a rough estimate of the water dewpoint based on the steam tables. For example, if the suction gas to the compressor is 20 barg and 20 deg C, the water vapor pressure is 0.0234 barA and is 0.00111 mol fraction in the gas. After compression, if you cool down to saturation (which is supposed to be your 45C trip temperature or thereabouts), the mol fraction of the water is the same but its partial pressure has increased due to the increase in system pressure. If the system pressure is now 60 barg, the water vapor pressure will be about 0.068 barA or 32C saturation temperature using the steam tables. Note, I've neglected compressibility factors of the NG which for these pressures isn't the best assumption.

You can also check the dewpoint with the NG/water curve in the GPSA data book. For 20 barg and 20C, the water content is about 60 lbs/MMscf. That is about 0.00126 mol fraction.
If you then compress it to 60 barg, the dewpoint (when you have the same water content as the suction gas and the first drop is about to condense) is about 35C.

You can also do this with a computer program if you have one available and of course, you would need to do the work using your actual suction pressure and temperature. Whatever temperature you estimate as the water dewpoint of the gas, you want to leave some margin above that if you change the trip temperature to account for process variations, inaccuries, etc.

IF you aren't condensing any water at the compressor discharge (which seems to be the case implied with the existing low temperature trip), then you won't be increasing the water load on the TEG contactor when you raise the condenser outlet temperature. The higher gas temperatures to the TEG contactor will however increase the water content of the exiting gas from the column. The water content of the dried gas is proportional to the equilibrium water vapor pressure (and therefore temperature) of the lean glycol entering the column. Higher inlet gas temperatures would mean higher inlet glycol temperatures to avoid hydrocarbon condensation and therefore higher 'dried' gas water content.

One last comment, your setup is a little unusual. Most setups for TEG I've read about have the inlet gas being cooled as much as possible to remove the bulk of the water before the gas enters the TEG contactor since that reduces the TEG circulation rate and capacity of the reboiler system. In a system like this, increasing the inlet temperature increases the water load on the TEG system and can overload it. Your design appears to keep the gas superheated, in terms of water content. It may be the case, but I would to check over the original design pressures and temperatures and make sure that is consistent with this set-point.

 

[I2P]

TD2K,
Thanks for your reply. Yes I agree that the design and setup is unusual.Our problem right now is that the water content of the gas at the contactor outlet is gradually increasing, it is approaching the design value. Glycol purity is ok, it is 99.5%. As mentioned earlier the design inlet contactor temp is 55 deg, at 60 bar.Do u think we should raise the present temp of 47 deg to 55 deg? I have my reservations on this coz simply put the lower the temp better is the absorption. How would we go about trouble shooting this column with respect to increasing water content. It is a structured packing column.Pressure drop is ok.We cannot get the gas analysed at the contactor inlet as there is no provision. So based on temp and pressure how to arrive at the water content?

 

[TD2K]

Before we go too far down this path, do you keep the compressor discharge above the water saturation point?

Secondly, where are you now operating for a water dewpoint leaving the contactor and what is your specification?

 

[pronab]

Hirenjbhatt
We operate our TEG contractor at 39 deg.C @ 66 barg. Press. with inlet gas : 2.0mol% CO2, 0.237 mol% H2O. Normally there is a knock out drum before entering contractor,but if it is a tray column you can avoid it. Without KOD water content will increase ( with same TEG flow). We get our dew point for water / hydrocarbon are : -35 deg.C/-15 deg.C (design:-30/-10 deg.C). With high CO2 and water content there is a potential hydrate formation problems , which will plugging pipeline. For CO2 content , which you are mostly worried, we set our target LHV of pipeline gas ( our case it is 900 Btu/Scf). Over 3 years in operations we don't noticed any corrosion deu to wet CO2 . However you can check your corrosion coupon for corrosion result.

Best of luck.
Pronab.