Gas analysis sampling

Discussions about GC and other "gas phase" separation techniques.

5 posts Page 1 of 1
I am setting up some gas analysis for our fixed bed reactors. I understand the gas analysis naturally has bigger error bars, especially on the sampling loop pressure control. I'm currently have a BPR there, to control the sampling loop's pressure at 30 psig, but the pressure was not controlled consistently overtime, mostly because the input pressure was not consistent. I tried to add in pressure reducing regulators with other flow restrictions, it did not go well. I checked some of the swagelok setup (https://www.swagelok.com/en/blog/back-p ... -engineers) but the needle valve might need to be tuned relatively regularly.

I know people suggested atmospheric injection (stop flow, water seal, etc.), my biggest concern is if there is a pressure difference in the GSV, even 0.5 psi out of atm, that will be more than 3% difference (0.5 psi/14.7 psi). 3% could lead to bigger error if you calibrate multiple compounds.

Is my understanding on the atmospheric injection correct or it is overthinking?
0.5 psi above atmospheric pressure is 15.7 psi absolute. That's only about 3%. Most analytical techniques - even in the pharmaceutical industry where it's the most stringent I've ever experienced - only require 2% RSD. So, if you're measuring something that's ~100 ppm (gas phase), you're still within 3 ppm or so. It's probably tough to control any process on +/- 3 ppm of anything.

What is your sampling system? I've worked with GC's designed to analyze gases over the years, most recently determination of impurities in CO2. My system has a rotary, six-port injection valve mounted inside the GC oven with a fixed-volume loop on the valve. I fill the loop with a rapid flow of sample (use a rotameter on the outlet of the loop to ensure good flow while sampling). At the end of the loop-purge time, I stop the flow at the source. When the rotameter ball drops to zero (atmospheric pressure in the loop), I can inject. This has worked very reliably for years.

I've never had good success with getting good precision when making pressurized injections. It's too difficult to control. Atmospheric pressure is pretty much what it is.
Thank you, we are using gas stream on our mass and carbon balance, not trace analysis, so 3% will be huge error for the whole system. But I think you are right, the pressure control seems causing more deviation right now. Let us try atmospheric injection and I will keep you all posted.
I suspect that this might not be a good analysis technique for your application. Large concentrations are difficult to measure really precisely. Your chromatography will likely go nonlinear when the concentrations are large from overloading the column.

I'm a trace analysis guy. While that presents its problems, we also have a difficult time doing ethanol in wine and spirits by GC because it's so concentrated. If you cut the sample with water, the precision is still such that the error in the final result (after compensating for the dilution factor) is too large for the TTB to be happy. In that case, however, the water is the tough part. Water has a huge expansion volume and you have to inject really small amounts of sample so you don't overfill the inlet.

You just can't get something for free. There are trade-off's to be made in everything.
Once with UOP539 gas analyzer we received proper reproducibility only keeping atmosphere pressure at the end of sample loop.

I.e. gas line end was in water flask, sampling loop considered filled when at the end appeared 1 bubble in 3-5 sec.
Flushing sampling loop for 10-15 sec or more with higher flow required before injection. Then adjust pressure to near-stopped flow.

Certified gas mixture was used for testing.
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