Conventional land based LNG plants utilizes hydrocarbon-based refrigerant systems driven by large compressors. For floating LNG plants, gaseous N2 refrigeration system is preferred due to the compactness of the refrigeration equipment and the less hazardous nature of gaseous N2 when compared with liquid hydrocarbon refrigeration system. Due to the low heat capacity of N2, companders (expander-compressor) are required to maximize the cooling duty instead of JT valves. Multiple compander units are required to handle the pre-cooling, liquefaction and sub-cooling of the natural gas. To provide process flexibility and equipment maintenance window, parallel N2 refrigeration trains are required for floating LNG plant. PETRONAS Floating LNG 1 facility will be the first floating plant in the world, utilizing only gaseous N2 as the refrigerant to produce LNG.
The controllability of the N2 liquefaction is the main concern due to the large number of rotating equipment as well as load balancing the parallel N2 refrigeration trains. Specific analyses need to be conducted on the liquefaction system due to the complexity and highly interactive nature of the process, leading to possible process instability. An integrated dynamic model using PETRONAS's iCON dynamic process simulation software was developed and utilized to conduct several operability studies, focusing on normal start-up and shut-down, refrigeration load switching from single to parallel N2 refrigeration trains, increase and decrease in LNG production, feed disturbance from production wells and equipment malfunction.
Result shows that there is a highly interactive dynamic response between the companders and the heat exchangers due to the tight process integration. Disturbance to one compander may cause cascading effect through the other companders, which can cause compander overspeed or surge condition. This necessitates prudent analysis of the operating scenarios to ensure operability and carefully planned controller design for stable operation. Additionally, being the first such refrigeration configuration in the world, correction of perception that operation will be similar to the conventional hydrocarbon-based refrigeration LNG facilities is an issue to address.
