The issue of nitrogen, hydrogen, and hydrocarbon contents in the continuous reforming cycle
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I am an instrumentation technician. A problem has recently arisen in our plant’s continuous reforming unit, and it’s causing quite a lot of headaches: the hydrogen-hydrocarbon analyzer for circulating nitrogen has several times shown excessively high hydrocarbon levels, triggering automatic shutdown procedures. When instrumentation technicians went to the site and used pure nitrogen to test the analyzer, its readings were accurate – there was no issue； The process shows no detectable combustible gases at the sampling port using a portable detector, and hydrocarbons are also absent in the sample analysis. Every time this happens, the process increases the nitrogen venting and supplies fresh nitrogen, and the measurement values gradually return to normal. The hydrocarbon analyzer has a measurement range of 0–15%; sometimes the measured values reach the full scale. Currently, the process team attributes this to a problem with the instrument, but when the analyzer is calibrated using standard gases, it functions properly. We suspect that the process gas contains certain gases that interfere with the thermal conductivity analyzer, resulting in abnormally high measurement values. However, we do not have any means to determine what these gases are, and the process team claims that there cannot be any other gases present. From an instrument perspective, if everything works fine when using a standard gas to calibrate the analyzer, then it’s likely not an issue with the instrument itself; it must be a problem with the sample gas. However, no hydrocarbons can be detected, and it’s not clear what gases are present. The situation returns to normal once the process vent is used and fresh nitrogen is introduced. Please help analyze this issue from various angles to determine what might be causing it.
It may be due to the maintenance of equipment with high pressure such as compressors; during replacement, contamination occurs as a result of the reverse flow of the medium.
This post was last edited by lijianhuai on 2011-8-16 22:15. Do you mean a hydrogen compressor by \"compressor\"? If hydrogen enters the circulating nitrogen, the hydrocarbon analyzer can detect hydrogen separately. In case of an abnormality, there is no hydrogen present, but only the hydrocarbon reading increases. What gases could interfere with the measurement?
The hydrogen and hydrocarbon analyzer in circulating nitrogen is a magnetic type of online analyzer, primarily used to detect trace amounts of hydrogen and hydrocarbon components. If samples are taken in the event of an anomaly, existing laboratory testing equipment cannot detect trace amounts of hydrocarbons, with the exception of precision instruments (I have seen such testing devices in the laboratory). If the online analysis instrument does not experience zero drift and has been calibrated using a standard gas, and it is functioning properly, it indicates that some medium has entered the system; in such cases, nitrogen purging is necessary. This is what replenishes fresh nitrogen and restores the normal level of hydrogen and hydrocarbon content. Magnetic hydrogen and hydrocarbon analyzers require weekly checks for zero drift, and after such checks, they need to be purged for a long time before being put back into use. Hydrocarbons are present when introduced into circulating nitrogen; otherwise, what is the purpose of the pressure-controlled valve designed for purging at the inlet of the nitrogen compressor? Primarily, the hydrocarbon components are present in the catalyst carbon deposit of the fourth reactor; they volatilize into the circulating nitrogen in the separation hopper. Over time, an increasing amount of hydrocarbons accumulates in the circulating nitrogen, which requires frequent replacement of this nitrogen. As long as regular inspections and maintenance of the instruments are carried out, magnetic hydrogen and hydrocarbon analyzers remain quite accurate; there’s no need to constantly doubt issues with the instruments – it’s important to observe carefully during operation.
This post was last edited by lijianhuai on 2011-8-16 22:15. Thank you for your helpful answers. The hydrogen and hydrocarbon analyzer we use is of the thermal conductivity type. Are you referring to the magnetic-type hydrogen and hydrocarbon analyzer made by Emerson? If that’s the case, then the analyzer we have chosen is likely not suitable. I would appreciate your further guidance.
It is said that the technology for 15% HC-1% H2 already exists in ROSEMOUNT’s possession, and the conversion formulas have not been made public. It’s not clear what the performance of thermal conductivity-based detectors is like; I hope those who use thermal conductivity hydrogen and hydrocarbon analyzers will share their experiences so that we can find a solution to this problem!
The last edit to this post was made by lijianhuai on 2011-8-16 at 22:14. A magnetometric oxygen analyzer cannot provide accurate readings in mixed gases containing nitrogen; the results obtained using such an instrument are basically useless for measuring paramagnetic oxygen； Emerson used a flow-compensated, three-element parallel dual-measurement-arm dual-reference-arm differential-mode sensored, highly resistant, intrinsically safe heat conduction detector
The instruments are too specialized; regarding the manufacturing process, I suggest checking as follows: 1) Is the interval between failures roughly the same? If there is a pattern, then identify the cause based on that pattern； 2) Is there a possibility that the hydrocarbon analyzer is faulty? The gauge in my previous device has been in use for 7 years without any problems ； 3) Starting from the third generation, UOP has installed a separate nitrogen buffer tank before the regeneration system; nitrogen and the compressor section are separated using nitrogen as soon as it enters the facility, so the possibility of contamination is low. As for the issue on the second floor, you can resolve it by checking whether the compressor was serviced or showed any abnormalities prior to the failure occurring ； 4) Check, for the reasons mentioned on the 4th floor, whether the amount of nitrogen added during regeneration has decreased. I’m not sure if it’s the second-generation or third-generation system; if it’s the third generation, check the volume and temperature of the purge gas at the bottom of unit 4, while for the second generation more checks are required ； 5) Have the manufacturer come to make a diagnosis.
I’m not sure which company’s continuous reforming technology your facility uses. If it’s UOP, there are generally no process-related issues, as the hydrogen and hydrocarbon analyzers are located on the main pipeline, while the regeneration nitrogen is supplied via a dedicated line； If it is of the LPEC type, and since its measurement point is at the inlet of the nitrogen compressor, it is possible that the pressure difference between the lower hopper of the fourth counterflow unit and the compressor is not properly controlled, which causes some material to flow into the separation hopper and thus into the nitrogen compressor.
This post was last edited by lijianhuai on 2011-8-16 at 22:16. Floor 7 is very professional; they must be in the instrumentation business, right? The estimates provided by the thermal conductivity analyzer we use differ from this value. I believe that some gas must be interfering with the instrument’s measurements. After checking the data on the thermal conductivity of various gases, it seems that among the gases that could be related to the current processing conditions, CO2 and Cl2 have thermal conductivities similar to those of hydrocarbons. I wonder if this is related to chlorine; such a situation did not occur in previous years. With the recent process modification involving dechlorination, we are not entirely sure about the details, and we wonder if this is connected to it
Was your previous dechlorination done through alkaline washing, or has it been changed to using dechlorination tanks now?