CO/CO2 analysis with methaniser /FID

[tfmann]

In an ethylene plant that requires very low levels of CO/CO2 we have both on-line and lab GC with methaniser /FID detection.
We consistanty see 200 ppb CO2 on lab test but only 20 ppb on on line analyser, when both systems are calibrated to the same 2ppm standard. obviously something strange is going on at the lower levels but I dont know if the on line analyser is giving false low results or the lab giving high results. Ideally a certified standard in the ppb range would help me resolve it but no one seems to make such low standards. To take a sample to the lab, we use a stainless steel cyclinder with quick connect valves. I dont believe we are getting contamination here. Someone suggested carrier gas purity can effect the result but I dont understand how. if there was CO2 in the lab carrier gas then wouldnt this just shift the baseline rather than contribute to the peak from the sample? Any ideas appreciated.
Terry

[chromatographer1]

Interesting problem.

I would recommend a test of sampling another gas in the same manner and the same hardware you sample the ethylene now. This gas must contain a known and a low amount of CO2.

If your sampling does not give a result higher than expected then it would appear your on line analyzer is incorrect. The problem is duplicating the situation using another gas that has no or very low levels of CO2.

CO2 can absorb onto metal and be desorbed slowly. I don't know it that is the problem here or not.

Good luck.

Rod

[willnatalie]

It's possible the activity of your, I believe its nickel, catalyst could be low and only converting part of the CO2 coming online.

[chromatographer1]

Good catch.

I agree this is a possibility. There should be vendors which provide standards for calibration.

Even at this low level. Not that they will be cheap.

best wishes,

Rod

[mr3573]

It could be related to carrier gas purity. We have similar problem and usually used helium ‎grade 5 as carrier a when test that gas, it show about 100-150 ppb CO2. so we changed ‎our carrier and replaced by helium grade 6.‎

[chromatographer1]

Methanizers on ethylene analyzers after a time can 'coke' up and lose sensitivity. Removal and heating the unit using air carrier at 600-800C will generally fix them, but can be dangerous if not in the proper lab with precautions.

Much cheaper to buy new and reinstall.

CO2 standards must surely be found in industry. Process control analyzers have to be certified over a 24 hour run before shipping and installation in an ethylene plant. Even if you have to use a permeation tube you should be able to make a std for yourself. At these low ppm levels spectroscopy is generally used instead of a GC to certify stds.

They don't come cheap though, of that I am certain.

best wishes,

Rod

[AICMM]

tfmann,

Are you gasifying or is this liquid LPG? How much are you shooting? What does a pressurized helium injection look like? At this low level I would first be suspicious of a leak as the source of the difference, especially since you have to grap a lab sample.

Best regards,

AICMM