Discussion on the large discrepancy between the analysis data of propylene content in catalytic liquefied gas and that in storage tanks
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Colleagues, I’ve recently encountered a problem that I hadn’t noticed before; it’s quite troubling, and I’m raising it here for the expertise of all experts out there! Due to internal renovations, our company is removing the gas fractionation unit today, and the catalytically produced liquefied gas will be sold directly overseas. However, there is a significant difference between the propylene content measured in samples taken from the liquefied gas storage tanks and that measured in the catalytically produced liquefied gas before it leaves the plant; the average propylene content in the catalytically produced liquefied gas is 29% (M), with no values below 27% (M). In contrast, the propylene content in samples taken from the storage tanks varies greatly, ranging from as low as 9% (M) to as high as 30% (M), with most values falling around 22% (M). This level is much lower than that of the gas at the exit of the catalytic process, which creates significant difficulties in shipping the product and results in substantial losses in efficiency. We would like to know how other companies sample and monitor the propylene content in catalytically produced liquefied gas during storage. Our company currently does not have any gas separation units; the liquefied gas produced through catalysis is sent directly to the storage area, so there is no risk of cross-contamination between different storage areas.
This post was last edited by xwtsq on 2010-11-7 08:05. I have encountered this situation as well; first, make sure that the sampling and injection methods for your two samples are the same. By the way, does \"M\" refer to the mass fraction? Are you using a mass correction factor? Or is your detector a hydrogen flame type, and for lower hydrocarbons the mass correction factors are more or less the same, so can you simply consider the area-normalized results as the mass fraction? The catalytic mass fraction should be around 35 at the very least; yours is too low. 2 If the results obtained using the same person, the same equipment, and the same sampling methods still differ significantly, it’s quite unbelievable. Then check whether there are any manipulations involved with your liquefied gas tank trucks; it’s possible that other gases were loaded into those trucks, which in turn reduced the propylene content in your storage tanks. Try to analyze the nitrogen, carbon monoxide, and carbon dioxide present in the liquefied gas to see if such gases are present. If not, then this problem will be quite difficult to resolve. 3 Another point is that if sampling is done using a bladder, the sample must be analyzed immediately; it should not be left aside for some time before analysis. Even after sampling, one should not release air midway due to concerns that the gas expansion might cause the balloon to burst; such actions will lead to inaccurate analysis results. By the way, where are you from?
Reply to 2# chenhaidong5758: Thank you for your patient guidance! The (M)% and (V)% I mentioned refer to mass percentage and volume percentage respectively ； I’m from Hohhot, and I followed the sampling process throughout it. There were no issues with the people who conducted the analysis or the methods used. After thinking about it for a long time, I believe there are several possible reasons; please advise: a) Catalytic liquefied gas is always sampled while it’s in motion, whereas sampling in the storage tanks is done when the liquid is relatively stationary. b. The liquid levels in the various storage tanks of the catalytic liquefied gas desulfurization and refining system remain relatively stable, and the operating conditions such as pressure, temperature, and flow rate are also fairly constant; in other words, the partial pressures of oil and gas in the liquefied gas sampled from the catalytic refining system remain stable ； The liquid level in the storage tanks in the tank farm is not very constant; operational parameters such as liquid level, pressure, flow rate, and temperature are not stable at a single value during sampling. The partial pressure of oil and gas inside the storage tanks should change continuously with pressure and liquid level (i.e., the volume available for vaporization). C. The sampling points in the tank area are all located at the bottom of the large tanks; when storing a single type of medium such as C-4 or propylene, there are no issues with unevenness or localized phase separation, as the purity of such single mediums is very high, typically above 80% (by volume) ； Catalytic liquefied gas is an unseparated mixture containing various gas components such as C-2, C-3, and C-4. Due to the influence of the partial pressure of oil and gas within the storage tank, these components separate to varying degrees; as a result, the samples taken from the bottom of the tank are not representative.
What you say makes a lot of sense; I didn’t expect you to know so much about manufacturing processes. Yes, there are two methods of sampling: sampling from storage tanks and sampling from pipelines, and there are relevant standards that can be referred to. As for the possibility of phase separation you mentioned, it is even more likely if your storage tank has held only carbon tetra compounds or carbon tria compounds. Since the quality checks of the two samples yielded different results, and assuming there is nothing wrong with the analysis equipment, then there is a problem: namely, what caused the differences between the samples taken. Or it’s possible that both sample results are actually correct, but there are differences in the equipment and the storage areas, and the cause of these differences could be as you mentioned.