Online Monitoring of Methane in Ethylene Impurity Control

Ethylene production has stringent product purity requirements and off specification ethylene must be sold at a considerable discount or disposed of via a flare system.  Methane is a byproduct of modern ethylene production processes and is considered one of the main impurities of the process. Even at small concentrations methane acts as a poison to the catalyst used for production of polyethylene from ethylene. The maximum acceptable concentration of methane in ethylene product is only a few hundred parts per million.

Monitoring these very low-level concentrations of methane requires precise and reliable analytical solutions. Gas chromatography (GC) has historically been used to provide methane monitoring in the ethylene production process, but its cycle time (often 3-5 minutes) results in gaps in measurement that can ultimately lead to the production of significant amounts of off-spec product. Tunable diode laser absorption spectroscopy (TDLAS) is a real time analysis technology that combines a non-contact optical technique with long term stability, high specificity, and considerable sensitivity. Real time monitoring during operation startup and other unscheduled events (upsets), can identify sudden, and negative changes in the concentration of methane and save the producer money. It has been experimentally demonstrated in ethylene plants that, during fast fluctuations, a gas chromatograph either misses the event entirely or reports it with delay after analysis completion. In contrast, TDLAS based systems provide real time monitoring of the process without delay.

The ideal location for a TDLAS analyzer measurement is at the demethanizer bottom stream. As previously mentioned, methane is an impurity to the final product and must be kept at extremely low concentrations. In the demethanizer, methane should be removed from the stream being processed and vented through the overheads. The demethanizer bottom’s stream should consist of ethylene and heavier components that are sent to the to the deethanizer for further processing. Any methane that gets to the deethanizer will ultimately end up in the final ethylene product stream as an impurity and should be identified prior to that happening. In some instances, end users may choose to monitor ethane at the outlet of the C2 splitter to confirm ethylene product purity. See Figure 1 for potential measurement locations.

Figure 1. Ethylene plant fractionation section

The AMETEK 5100HD (shown in Figure 2) is an extractive type methane analyzer designed for hot/wet sample analysis. There is no sample conditioning for the analyzer system, just a sample transport system. The 5100HD uses a sealed reference cell for continuous on-line analyzer verification and offers high specificity, and sensitivity.  

Figure 2. 5100HD TDLAS analyzer

The data shown in Figure 3 represent the response of the 5100HD to a series of methane challenges in the concentration range of 0 -2000ppmv. The duration of each of the challenges was from 10 to 20 minutes. The background gas used during testing was ethylene. As can be seen, the sudden changes in methane concentration are identified by the 5100HD almost immediately.  The use of a TDLAS-based  analyzer provides very quick – but still accurate - responses when compared to gas chromatography based solutions, as GCs require the samples be routed through separation columns.  As mentioned previously, the GC measurement time can take 3-5 minutes, but the TDLAS responds in seconds.  

Figure 3. Methane readings in ethylene.

The results of the methane measurement validation test in the ethylene stream are summarized in the Table 1.

Table 1. Methane readings in ethylene.

For more information about this topic refer to these documents:

1. Carbon Monoxide Measurements with Tunable Diode Laser Absorption Spectroscopy (TDLAS) in the Ethylene Manufacturing Process
2. Acetylene Measurements in Ethylene Production using Tunable Diode Laser Absorption Spectroscopy (TDLAS)
3. Hydrogen Sulfide and Carbon Dioxide Measurements with Tunable Diode Laser Absorption Spectroscopy (TDLAS) in the Ethylene Manufacturing Process
4. Use of TDLAS Based Analyzers for Real-Time Process Control in Ethylene Oxide Production

Learn more about AMETEK’s TDLAS solutions.