(406f) Dynamic Modeling and Control of the Prico© Lng Process

For cases where the cost of laying pipelines is prohibitive, natural gas transportation over long distances is often done in the form of Liquefied Natural Gas (LNG). LNG is produced by fully condensing natural gas; by cooling it to around -163 C. Specific volume ratio of natural gas at atmospheric pressure to LNG is approximately 600 to 1, making transportation by special tanker ships feasible. The first commercial liquefied natural gas shipping was realized in 1964 with the export from Lacamel LNG Plant in Arzrew (Algeria) to Canvey Island, UK. With new gas discoveries being made in remote areas, LNG production is therefore becoming an increasingly important technology. Among ongoing developments is STATOIL's LNG plant for the Snohvit field in the far north of Norway (STATOIL is a Norwegian oil company). Clearly, cooling the gas to -163 C requires large amounts of energy. Therefore efficient and safe operation of the liquefaction plant is important. To achieve this, the plant requires a robust control system which ensures the quality of LNG and the safety of plant. The academic work on dynamic LNG simulation is limited. The only work found and studied are Hammer (2004), Zaim(2002) and Melaaen(1994). A significant part of these works focuses on modeling of LNG plant since modeling of LNG plant is a challenging task and consequently control of plant is not studied in detail. We aim at studying control challenges of LNG plant in detail. In this work the regulatory control system for the PRICO© (Poly Refrigerant Integrated Cycle Operations) process for LNG production is studied. The PRICO process is a common LNG production process, and is developed and marketed by Black and Veatch Company. The design of the control system is based on a detailed dynamical model of the process developed in gPROMS, using the Multi-flash physical property package for reliable physical property calculations at the low temperatures present in the process. In addition to enabling the use of model based tools for control structure design, this allows testing the effects of common model simplifications, such as assuming constant temperature of the refrigerant at the condenser outlet, or ignoring the flash drum and refrigerant holdup. The effects of these model simplifications for model based control structure development and controller tuning are described in the present work. Controllability analysis shows that the pairing of the two main control loops in the PRICO process should be reversed, when compared with established industrial practice which is done on the basis of proximity of controlled and manipulated variables. That is, compressor speed should be used to control LNG temperature and throttle valve opening should be used to control degree of superheat at the compressor suction. This conclusion is verified by controller tuning and dynamic simulation.

References: - Hammer, M. (2004) ?Dynamic simulation of a Natural Gas liquefaction Plant? Ph.D. diss., Department of Energy and Process Engineering, Norwegian University of Science and Technology. - Zaim, A. (2002) ?Dynamic optimization of an LNG plant, Case study: GL2Z LNG plant in Arzew, Algeria.? Ph.D. diss., RWTH Aachen University. - Melaaen, E. (1994) ?Dynamic Simulation of the liquefaction section in Base load LNG plants.? Ph.D. diss., Department of Refrigeration Engineering, Norwegian University of Science and Technology.