(116e) Optimization of Floating Liquefied Natural Gas Process with Process Knowledge and Boxline Method

Optimization of Floating Liquefied Natural Gas (FLNG) process plant is presented. The developments in the offshore technologies and favorable market conditions led to the spot LNG market that drives the possibility of economic development of offshore stranded natural gas field and promoted the evolution of FLNG facilities in many countries. FLNG avoids the major infrastructure cost associated with onshore plants and offers a number of benefits such as elimination of the platform, pipelines and harbor jetty, and reduces the facility’s environmental impact that leads to significantly reduced LNG production costs. The design criterion of FLNG is different from an onshore plant in terms of plant safety, storage, simplicity of operation and modular design. Considering the FLNG project requirements and criterion, an integrated scheme of NG liquefaction and NG liquids recovery is devised. The single mixed refrigerant NG liquefaction cycle is integrated with modified NG liquids recovery scheme. The preliminary design has several optimizing variables and to obtain the maximum benefit from the existing design, optimization was carried out. Refrigerant composition and operating pressure levels were the main variables in liquefaction plant whereas feed tray location, reflux rate, feed split rate and column number of trays are the main variables in NG liquids recovery plant. Overall plant specific energy minimization was considered as the optimization objective which in turn comprises the minimization of compression energy and reboiler duty in columns. To facilitate optimization, process knowledge of mixed refrigerant system is embedded within the search algorithm that helps in fast and robust convergence to practical solution. The optimization results in the decrease of about 70% compression energy and reboiler savings of 18% from the initial design. Furthermore the capex and opex are also calculated for the integrated process and conclusion was drawn that the annualized operating cost in FLNG plant is higher than the capital cost.