Dynamic simulation and control of unloading and holding small-scale onshore LNG regasification processes

By Luthfi Kamal Bangkit Setyawan, Abdul Wahid, Widodo Wahyu Purwanto | Publication, Scientific Paper

The problem encountered on the regasification terminal is unsteady operating because of unloading from LNG carrier to storage and holding mode that involves the on-off operation of Ambient Air Vaporizer (AAV) caused by frosting. The purposes of this study are to simulate a dynamic operation of small-scale regasification terminal using AAV with and without controls by process simulator UniSim. The result of the simulation without controls, shows that the gas flow rate has deviated 14% from the target, after 4 hours of operation output gas temperature is below 2.5°C, it cannot meet the requirements of the gas power plant. For simulation with controls, a digital on-off type controller is used for pump during unloading and switch or on-off between AAV, while PI type controller is used to control LNG flow rates. For the digital on-off controller, the controller is set to stop the unloading process when the remaining LNG in the carrier is 10% (350m3) and the other digital on-off controller will be set to act on switching AAV when the AAV gas sent out temperature reaches 3°C. For PI controllers, the tuning parameters to regulate the flow rate is Kc = 0.00638 and Ti = 0.00043. The maximum operating time for 1 AAV is 5 hours 15 minutes before it will be switched for defrosting and replaced with other AAV. After the controller installed, the gas sent out specifications from the terminal able to meet the requirements of the power plant, a molar flow rate of 6.53 MMSCFD and a minimum gas temperature of 2.5°C throughout the operating time.