เทคนิคการตรวจจับค่า Tetra Hydro Thiophene (THT) ใน CNG

แก๊สธรรมชาติ เป็นพลังงานสะอาดที่กำลังถูกกล่าวถึงมากในขณะนี้ และมีแนวโน้มความต้องการใช้งานเพิ่มขึ้นเรื่อยๆอย่างต่อเนื่อง เรามีการประยุกต์ใช้งานเครื่อง MicroGC ร่วมกับ Plasma Source เพื่อตรวจจับค่า TetraHydroThiophene (THT) ในระดับต่ำมากๆด้วยความแม่นยำ เป็นที่ทราบกันดีว่า ในการขนส่งแก๊สธรรมชาติในระบบท่อ (Pipeline Distribution) นั้น หากมีการรั่วไหลของแก๊ส TetraHydroThiophene (THT) จะสามารถนำมาใช้ตรวจหากลิ่นของ CNG ได้ และนำมาสู่การแก้ไขที่ทันท่วงที

A quick analysis to trace tetrahydrothiophene (THT) in natural gas is required for controlling the amount of this odorant added in the natural gas. Due to its odor, the THT is used to detect any presence of gas leakage in natural gas distribution networks. The THT is composed of a five-membered ring containing four carbon atoms and a sulfur atom. It is also known as thiophane or thiolane. The THT is generally used in mixtures containing tert-Butylthiol (TBM) which is an organosulfur compound with the formula (CH3)3CSH. In the presence of TBM in natural gas, it is required to measure its concentration at low ppb/ppm because of its strong odor that causes nausea. The permissible exposure limit (PEL) is in the range of 500ppb and it is the reason why a highly sensitive detection system is required for measuring both THT and TBM in natural gas.

LDETEK SOLUTION:

The use of a highly sensitive detection system (PlasmaDetek2) combined with a compact GC system (MultiDetek2) are required for such type of analysis where sensitivity, robustness and speed are necessary. The figure 1 shows the easy to operate 2-valve/2-column configuration used to achieve a quick analysis of THT at low ppb/ppm concentration with no interference from other gases present in natural gas. This type of configuration is straight and ensures a good stability of the measurement over time. The valves, tubing, fittings are made of coated stainless steel to ensure repeatability and durability of the system. The robustness of the injection and detection system makes this solution maintenance free on long-term operation.

According to the minimum detection limit that is required, the analysis time can vary. The figure 2 shows a chromatogram of such system having a limit of detection fixed at 31ppb. Analysis time is then fixed below 4 minutes, what is the average time generally required for this type of measurement. If quicker analysis time is requested, the system parameters can be easily modified to achieve lower cycle time analysis. That generally consists of sampling loop volume and system gain reduction to achieve a higher ppb detection limit.

The PED detection system can be configured either with Helium or Argon as carrier gas depending of the sensitivity requested. It is a unique carrier gas source with low gas consumption for cost saving.

The analysis of tert-Butylthiol (TBM) can be added in the same MultiDetek2 compact GC still using PlasmaDetek2 (PED) as the detection system.

With its built in industrial PC, the MultiDetek2 offers all the conventional communication protocols (analog output, Modbus, Profibus, RS232), data storage disk and alarms contacts required for a process GC.

Figure 1 : Application hardware configuration

Figure 2 : Chromatogram showing a standard gas containing 9.25ppm THT

Based on noise to ratio, LDL is calculated as follow:

Figure 3 : Historic values chart showing repeatability results better than 1% of reading

CONCLUSION:

Using a solution combining the PlasmaDetek2 and MultiDetek2 is the best way to achieve sensitivity, robustness and speed in an industrial compact system. This is exactly what is required from the natural gas industry.

#CNG
#TetraHydroThiophene
#THT