Hydrogen analysis in raw gas
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Recently, a 7890A gas analyzer in the factory developed a problem; this instrument is used for analyzing the composition of raw gas via dual channels, one at each end. The front channel is used for hydrogen analysis, with nitrogen as the carrier gas; it consists of one ten-way valve plus 2 P-Q columns, one 3-foot long and one 6-foot long, with the 3-foot column being a backflush column. The rear channel features a six-way valve and a ten-way valve, equipped with 2 P-Q columns and 1 13X molecular sieve; hydrogen is used as the carrier gas, and the main components analyzed are O2, N2, CH4, CO, and CO2. Initially, normalization was used for analysis, but it proved impossible to match the results with the standard gas data; therefore, the external standard method was adopted to calculate the CO2 content separately. Recently, the H2 content increased suddenly, and the analysis results for hydrogen using standard gas data could not be matched with those of the standard gases – the values were too high, by about 7 percentage points. I began to suspect that the issue was caused by an obstruction in the gas circuit, so I cleaned and purged the pipelines such as the metering ring, six-way valve, and ten-way valve, but no blockages were found. I’ve been struggling to figure out what’s going on, and I would be very grateful if the experts here could help analyze it.
I speculate that it might be due to the dirty sample, which caused blockage in the backflow column of the front channel and reduced the efficiency of backflow; as a result, other components could not be separated from hydrogen, leading to increased values in the data. Unfortunately, I don’t have columns of the appropriate length available to verify this hypothesis. I would be very grateful if the experts here could offer some guidance. Additionally, this instrument features two quantitative rings in series for simultaneous sampling
This post was last edited by qugd on 2018-2-23 at 10:43. Your analysis method involves a two-column system with dual carrier gases; if the same detector is used, normalization along with the use of correction factors should not pose any problems. If it is a dual-detector system, separate calibration of hydrogen purity or hydrogen content may be required. Check whether the baseline is stable; if it drifts upward during the analysis, it is likely that polar components in the sample (such as water, sulfides, etc.) are remaining in the column system. Allowing the column system to stabilize for a while before analyzing the next sample should yield better results. Also, is the purity of the carrier gas guaranteed? There are many cases where a decrease in the purity of the carrier gas leads to a reduction in detection sensitivity and reproducibility. It would be best if you could post a chromatogram for everyone to see.