Development of Small Scale LNG plants: Utvikling av småskala LNG anlegg
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AbstractSINTEF Energy Research have for several years worked with a plant concept for small scale LNG production in the range from 5 to 20 tonnes per day. This process, called Mini-LNG, is based on mixed refrigerant and standard refrigeration components such as oil lubricated screw compressor and plate heat exchangers. The design has emphasized low costs and high robustness. In this master thesis, written in cooperation with SINTEF, production of LNG at a rate from 20 to 100 tonnes per day has been reviewed. The feed could be for instance associated gas.Focus has been put on evaluating existing processes, both mixed refrigerant and nitrogen expansion. Four criteria have been highlighted as increasingly important namely; operation and reliability, specific power consumption, cost and complexity and production rate. In order to evaluate the thermodynamic efficiency, the most interesting processes were simulated in the program PRO/II. Initial values were gathered from a literature survey. An exergy analysis was made to quantify the losses is each component. The assumptions were feed gas at 40 bar at a Norwegian climate with sea water temperature of 15 °C. It is not accounted for pressure drop losses in the heat exchangers and other equipment. This is due to uncertainly of two-phase pressure drop modeling and the thought of the simulations only to be considered as a benchmark.A novel LNG process has been proposed, based on the concept of SINTEF Mini-LNG, but with improved efficiency. The novel process includes a phase separator, two turbo compressors and several brazed plate heat exchangers. Through simulations it was calculated that is has lower specific power consumption than existing small scale LNG processes evaluated in this report. The exergy efficiency is over 40 percent, which is considered to be relatively high. At the given conditions this corresponds to specific power consumption slightly above 325 kWh/tonne. As the process is based on simulations, only further work can state if it can be realized. Uncertainty is linked with the size of the plate heat exchangers which may not be available, but perhaps can be tailor-made. The easiest mixed refrigerant process, PRICO, was calculated to have an exergy efficiency of 37 percent vs. 27 percent for the most advanced nitrogen dual expansion process, cLNG.Instability and maldistribution of mixed refrigerant in multichannel heat exchangers is a problem that can obstruct normal operation and reduce the plant efficiency. So called Ledinegg-instability is a problem that leads to uneven distribution of refrigerant in the channels due to several possible flow solutions. This is visualized as an N-shaped curve when plotting pressure drop versus mass flow. Simulation in Aspen Plate+ and S-Plate gave no clear answer to whether this may appear in the plate heat exchangers for the novel process. More work must be done in validating the models used for simulation. The complete evaporation progress has been compared with that of PRICO. The novel process was briefly evaluated for use for liquefaction of associated gas offshore. More work must be done to clarify the area footprint and plant components. It is unlikely to build such as plant on an FPSO unless it is included in a retrofit design. A more probable alternative is to place a plant on small floaters adjacent to existing FPSOs for oil and LPG production. An economic feasibility study must also be made in order to evaluate costs and income for operation after plateau production.