Coal Gasification Syngas Analysis

[isircar]

Hello,

I am a graduate student at Purdue. I have been trying to evaluate the gas composition in a coal-gasification product sample. I am using an old GC-8A with a TCD. Helium is the carrier gas and I have installed a new Supelco MolSieve 5A for separating H2, N2, O2, CO and CH4 and a Supelco Chromasorb 102 for separating CO2 and higher mass hydro-carbons.

The two columns are separate since there are different injection ports. I use both columns separately to detect the H2, N2, O2, CO, CO2 and CH4 concentrations.

My questions are regarding the operation of this system and interpretation of the results.

Is it adequate to perform the sample analysis at an Injection port temperature of 150 deg. C and a column temperature of 120 deg. C? There is condensed water in my sample bag, but I extract the gas using a syringe and some water vapor will be present in the analysis sample.

Secondly, in which order will the gases be separated? A manual written by a previous student states that the order of separation is H2, O2, N2, CO, and CH4 for the MolSieve 5A column but some forums have stated differently. Would trace amounts of other gases such as H2S complicate the data?

Furthermore, according to the same manual, we are told that the first peak (preceding O2) is the concentration of H2. However, in my analysis, I am receiving 2 peaks preceding O2. Is this a known issue for measuring hydrogen with a helium-carrier gas? Furthermore, the concentration value for the supposed H2 peaks is an order of magnitude lower than what is told to contain in a calibration sample that I used.

Thank you

[chromatographer1]

Wow,

Do I get any credit for all this work? Just joking, isircar.


quote:

"Is it adequate to perform the sample analysis at an Injection port temperature of 150 deg. C and a column temperature of 120 deg. C? "

Yes, but the separation on the mole sieve will be far from optimal.

"There is condensed water in my sample bag, but I extract the gas using a syringe and some water vapor will be present in the analysis sample."

True, and water is not good for the MS column but will not harm the Chromosorb 102 column.

"Secondly, in which order will the gases be separated? "

"A manual written by a previous student states that the order of separation is H2, O2, N2, CO, and CH4 for the MolSieve 5A column but some forums have stated differently. "

If the MS5A is very saturated with water that order is possible.
Usually, using a properly conditioned packing, it is: H2 O2 N2 CH4 CO with CO having a relative retention factor to H2 in the neighborhood of 8 - 12. Supelco bulletin 890A, figure 116 page 64

"Would trace amounts of other gases such as H2S complicate the data?"

H2S elutes after C2 on the Chrom 102, and is absorbed on the MS5A.

"Furthermore, according to the same manual, we are told that the first peak (preceding O2) is the concentration of H2. However, in my analysis, I am receiving 2 peaks preceding O2. Is this a known issue for measuring hydrogen with a helium-carrier gas? Furthermore, the concentration value for the supposed H2 peaks is an order of magnitude lower than what is told to contain in a calibration sample that I used. "

This is the well known " M " or " W " effect of hydrogen peak being detected on a TCD using Helium carrier at around 5-10% concentration. This is corrected by using nitrogen carrier to measure H2.

You will NOT have a linear response for Hydrogen using helium carrier on a TCD,

which answers your last question.

Have a good weekend.

Rod

[isircar]

Rod,

Thanks for your response. Your help will definitely be acknowledged.

I have a follow-up question regarding the temperature. I visited the document that you cited but could not get an explicit value for the column temperatures. Is there a recommended temperature at which to maintain the columns for the application that I had stated?

Thanks

[chromatographer1]

With fixed gases on MS 5A, the cooler the oven the better for separation.

Anything over 70°C makes the separations difficult.

I would prefer 35°C to 50°C as a working range.

Room temperature is better, but H2, O2, N2, C1, CO can be done on a 6 ft column @ 40°C quite well.

Be careful with the conditioning of the MS5A or your CO peak will tail badly. Once the molecular structure of the 5A is damaged it cannot be repaired only replaced. Remove the water SLOWLY at low temperatures. (100-150C) Then after several hours SLOWLY increase the temperature. Use the fastest carrier flow you can afford during conditioning.

There is no need ever to exceed 250°C, and 200°C will give you 80-90% of the maximum separation of the packing. Good enough if you realize that you can lose 80% of the separation with wet samples or carrier gas in a relatively short time. Again slow, cool conditioning of the packing is best. And more often it is done the better as well. Daily for example.

Rod

[AICMM]

isircar,

Two minor points to add to Rodney's discussion. First, I don't think you can get very good O2, N2 numbers when you sample using a syringe from a gas bag.

Second, have you considered running your sieve with nitrogen or argon while you run your Chromasorb with helium carrier? Would allow you much better hydrogen analysis.

Best regards,

AICMM