LPG analysis

This post was last edited by dalian*aochao on 2011-1-17 at 20:24. First of all, the error associated with micro-injection of gas samples is relatively large. If the gas valve or the injection port is used in such a way as to ensure that the quantitation tube is fully filled, then the volume of that quantitation tube is certainly many times larger than that of the micro-injection needle. If splitting is also employed in the analysis, the actual amount of sample that reaches the chromatographic analyzer is truly minimal; in this case, the errors will be even greater. Furthermore, what is the reason for the high content of carbon tetra-compounds? It is mainly a matter of whether it is due to chromatographic analysis or the composition of liquefied gas. Liquefied petroleum gas is a by-product of the catalytic or thermal cracking of crude oil; the composition of LPG varies greatly depending on the process used and even between different manufacturers. Additionally, carbon atoms with numbers 4 and higher exist in liquid form. Is there any truth to your suspicions? Could your suspicion be that you think the error lies in the sampling?

This post was last edited by qugd on 2011-1-18 13:18. The analysis of LPG shows that sampling and injection methods have a significant impact on the results of sample analysis. 1. If it’s a gas sample taken from the gas phase of a fluidized gas, no analytical method can yield accurate results, as the composition of the gas phase differs significantly from that of the liquid phase. However, in this case, the content of C3 components is likely to be relatively high. 2. If a liquid-phase sample is used, and the entire volume of the liquid sample is directly vaporized for analysis, the results will be more accurate and reliable; however, such facilities may not be available in ordinary laboratories. 3. A liquid-phase sample of liquefied gas is taken, and a small amount of sample is obtained for analysis through vaporization; this process relies heavily on the operator’s experience and skills. If the operation is not done properly, the vaporization of C2 and C3 compounds results in significant heat absorption, which causes the vaporization rate of C4 to be lower than that of hydrocarbon components such as C3. As a result, the analysis results will show an elevated concentration of C4 components. It is estimated that the original poster used a syringe to take samples from the gas bag; this method is not reliable. It is recommended to install a sampling valve between the gas bag and the sampling port. It is also recommended that the poster use automatic valve sampling, and that the containers for sampling and injection as well as the connection methods be properly arranged; an appropriate approach must be chosen. For this kind of liquefied gas analysis, you can go to the Quality Inspection Center of Qilu Petrochemical to learn about it*.

Reply to 1# songzhen: This issue has also been bothering me. It can’t be said that using a syringe for sampling with liquefied gas results in inaccurate measurements; it’s just that the reproducibility of such sampling isn’t very good. Weren’t syringes used for sampling before the introduction of six-way valves? The way I solve this problem is to directly obtain the desired volume from the air bag, then pull it out and introduce it into the vaporization chamber for sampling, without having to collect the desired volume in the air first. The results obtained in this way are relatively accurate. When using a six-way valve, some manufacturers’ valves have a relatively large dead volume, which results in poor separation performance.

Well, the sampling error is quite large; sometimes the repeatability from one person to another is very poor. By reducing the amount of sample used, things have improved a lot compared to before. Thank you!

Well, yes, the accuracy is unreliable and the repeatability is poor. There is a six-way valve, but due to the large capacity of the dosing tube, it’s difficult to achieve proper peak separation, so it hasn’t been used. By the way, I’d like to ask why the carbon content is high when taking gas samples. Thank you.

Carbon 3 has a lower boiling point than carbon 4 and is more volatile, so its proportion in the gas phase is higher.

Reply to 5# songzhen: I now use a micro-syringe for sampling. For aluminum oxide capillary columns of 50 meters in length, about 5 microliters of sample is sufficient. With domestically produced columns and constant-temperature chromatography at a column temperature of 140°C, the separation performance and peak shape are quite good. Also, if you use a bladder or an aluminum foil composite membrane sampling bag for sampling, releasing the sample midway due to expansion can result in higher measurement values. So the sampling must succeed on the first try; it’s best not to release air! It seems we’re in the same field; you work at that organization, right? I am in Dongying, Shandong

Reply to 6# songzhen: Additionally, the results obtained by different people are not very comparable, as various chromatographs, detectors, and manufacturers can all introduce uncertainties in the quantification. Therefore, it’s best for everyone to follow GB10410 or relevant industry standards so that the results can be compared. For example, both industry standards and relevant regulations stipulate that when sampling liquefied gas, a steel cylinder must be used for sampling; the sample is then introduced into an 80-degree Celsius water bath to ensure it is completely vaporized before being fed as a gas phase into the chromatograph for analysis. However, in practice, we often directly take samples from tank trucks or storage tanks using a balloon bag, and then analyze them right away in the laboratory. This approach carries the risk that the components may not be fully vaporized, leading to inaccurate results.

Reply to 1# songzhen: Another point is that, during the process of collecting liquefied gas samples, it is necessary to use the sample itself to replace the sampling bulb or bag. Consider this: if the level of carbon-5 components in the liquefied gas sample is high, these components may not vaporize completely during sampling in winter, or their vaporization rate will be slow. During the replacement process, it is the carbon-3 and carbon-4 components that vaporize first; the more times this replacement process is carried out, the more carbon-5 components will accumulate. As a result, the analysis results for carbon-5 upon returning to the laboratory might be higher than actual values. Therefore, sampling must be done taking into account the specific conditions at hand, and an appropriate sampling method should be used to obtain representative samples. If the samples are not representative, then the analysis results have no meaning.