LNG: Natural Gas Liquefaction
Published on : Tuesday 12-09-2023
The Raman Rxn5 analyser with an MR method is an ideal solution for optimum efficiency of the liquefaction plant.
The majority of natural gas (NG) is transported in gaseous form via onshore or offshore gas pipelines. For stranded gas reserves where there is little or no local demand, or for long transport distances (>1000 km offshore, >3000 km onshore), it becomes more economical to liquefy the natural gas prior to transportation. In addition, due to the nearly 600-fold reduction in volume for LNG versus NG, LNG is often the preferred method for local storage.
Measurement of refrigerants used in liquefaction
While there are multiple liquefaction processes in use at LNG facilities today, the majority of plants use some variation of a mixed refrigerant (MR) process. Since NG liquefaction can constitute over 50% of the CAPEX and 40% of the total OPEX of a baseload LNG plant, it is essential that this process unit be run as efficiently as possible. The ability to adjust the composition of the MR is a primary control parameter to allow a plant to maintain optimum efficiency during large turndowns, changes in feed composition, and accommodation of design uncertainties and off-design equipment performance. The Raman Rxn5 analyser with a Rxn-41 cryogenic probe is a unique solution to the measurement of MR in the liquid phase. The ability of this system to measure in the pipe eliminates sample vaporisation, transportation, and reduces lag time, providing composition updates in seconds, not minutes. When coupled with the Rxn-30 probe, the Raman Rxn5 analyser can also measure MR in the gas phase.
Issues with traditional measurements
Refrigerant composition for liquefaction of natural gas is typically measured with a process gas chromatograph (GC). Over time the MR composition can change due to losses. Changes as little as 0.02% Mol in the MR component composition can produce over a 5% reduction in energy consumption. Measurement uncertainties in the vaporiser-GC combination, combined with update times typically between 5 and 10 minutes, compromise its ability to provide an optimal level of control. In addition, heated sample transfer lines are required, adding lag time to the analysis.
Solution: Raman Rxn5 analyser with a mixed refrigerant method
The Raman Rxn5 analyser with the Rxn-41 cryogenic probe is capable of measuring the full composition of the mixed refrigerants, including N2, at cryogenic temperatures, in the liquid phase and in the sample pipe, eliminating the need to vaporise the sample or transport it to the analyser. With precision over 10 times better than a process GC/vaporiser solution, the Raman Rxn5 analyser with an MR method is an ideal solution for optimum efficiency of the liquefaction plant under various operating conditions and with a wide range of NG feedstock.
The Raman Rxn5 analyser with an MR method consists of the following:
i. Raman Rxn5 analyser base unit
ii. Rxn-41 probe for liquid-phase measurements
iii. Optional Rxn-30 probe for gas-phase measurements
iv. Fiber optic cable (length from 15 to 250 metres, customised to your plant requirements), and
v. Dedicated mixed refrigerant method.
