[nastybyte]

Greetings fellow chemists!
I'm considering trying to build my own apparatus for measuring the carbon and hydrogen (nitrogen would be a bonus, but not necessary) content of oil samples. The goal is to comply with ASTM standard D5291.

The sample is first pyrolyzed in a stream of argon/oxygen, but then i have a multitude of options for detection. If a TCD is to be used, excess oxygen must be purged from the gas stream before the detector, can this be done without condensing the water too? Another option would be to use a methanizer followed by a FID, but the methanizer catalyst is poisoned by sulfurous gases (and the machine will be used with fuel oil/vacuum gas oil which can contain over 3 mass % sulphur). Still, i think it would be easier to get rid of SO_x than O₂, right? Any other ideas on how to do the detection would be greatly appreciated! Of course, i will post every detail about the upcoming construction here on the forum. Information wants to be free, right? 

[Stepan]

Just an idea. If you are concerned about C and H2 only, you can burn the sample in dry air. Complete oxidation with a catalyst, and send gas stream to CO2 and H2O sensors. It will be way cheaper than to build a TCD GC. You can even add SO2 sensor for sulfur and likely HCl probe for chloride. I have seen them on internet. Alternatively you can detect them on gas FTIR.

If you need N2, than you need a TCD GC.   

[nastybyte]

Thank you very much for the input - i wasn't aware that a catalyst is necessary for complete combustion, have to read up on that! Also, any ideas on what kind of sensors one could use for CO₂, H₂O? Measurement is made in a stream of gas so response time must be fast. I was under the impression that FTIR wouldn't be specific enough.

[Stepan]

Gas flow need to be optimized. It cannot be too high or too low. It should be low enough to complete oxidation, and high enough for speedy analysis. Catalyst is required to convert CO into CO2. I believe, sometimes additional oxidizer/catalyst is added to the sample (Ag2O, MnO2).

Entire instrument require fine optimization of oxidation temperatures, catalyst temperature, gas lines temperature, sensors temperature, gas flow rates, catalyst type and amount, sample size, sample heating rate and so on.

FTIR is quite sensitive. Not sure if it is sufficiently fast (remember this is gust an idea :-))

Sensors are available:
http://www.futurlec.com/Gas_Sensors.shtml
http://www.figarosensor.com/
http://www.parallax.com/Store/Sensors/GasSensors/tabid/843/List/0/CategoryID/91/Level/a/SortField/0/Default.aspx

Good luck

[nastybyte]

I've looked at those sensors and precluded them because their response time is way too slow - they typically take more than a minute to reach 90% of the maximum output value. Won't work well for measuring in a gas stream, but maybe if one collected all gas, and measured its volume and CO₂ concentration...anyway, less ideal for water.

I'm quite sure that FTIR would be fast enough, the instrument we have takes a full spectra (400-4000 cm^-1) in just a second or two. But i also think that an FTIR spectrometer would be at least 100 times more expensive than any of the other options discussed so far. Also, one would need to have the sample compartment and everything before it heated so the water vapour won't condense, and i think there might be a problem finding a suitable IR-transparent material that doesen't dissolve (correct me if i'm wrong). In short, i think that FTIR detection would be real overkill for this application, expensive, complicated and probably sensitive to other gases (more chemical bonds = more IR absorption, for example SO_x absorptions probably overlap more or less).

This nut was a lot harder to crack than i had expected when i started to think about it...anyone know of a GC detector that is sensitive to CO₂ and/or H₂O, and isn't destroyed by oxygen?

[crosemeyer]

Dunno about Hydrogen, but combustion/catalyst TOC/TN instruments use NDIR.

http://www.teledynetekmar.com/products/TOC/Apollo/

[nastybyte]

Sorry to bring up an old topic, but this project got lost for a while and now i'm considering giving it a try again.

As mentioned in the previous post, NDIR detectors seem to be ideal for measuring CO₂. They have also become very affordable over the last years, and the precision is, from what i've gathered, more than useful. Limiting factor seems to be the sampling frequency - ½ Hz seems to be common, but there are models that can operate at 20 Hz, which should be more than enough (many chromatographs run at 10 Hz i think?).

Now the remaining problem is measuring the water vapour concentration. There are a plethora of simple & cheap relative humidity sensors but i don't think they're accurate enough, and who knows about interferences? Anyways, i think NDIR is the way to go for the water measurement also. Anyone against? Any other ideas?