Liquefaction Process Evaluation for Floating LNG
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The gas expander processes have attracted growing interest for FLNG facilities due to utilization of non-flammable fluids i.e. nitrogen as refrigerant and simplicity in configuration and operation. New developments of the processes show that the efficiency is getting closer to the established mixed refrigerant or cascade processes.The objective of this thesis is to do a fair and consistent comparison between three proposed expander process schemes for FLNG i.e. the MODEC/APCI process, the LINDE/SBM process and the CB&I/Randall with ?Niche? process. Each process was evaluated on the same basis for capacity, air temperature, cooling water temperature, heat exchanger sizing and component efficiencies. The DMR process was also used as the baseline. The analysis was primarily done using HYSYS and ASPEN-plus simulation programs in combination with the literature study.In general all the expander-based processes evaluated clearly have a disadvantage in term of process efficiency and capacity when they are compared to the DMR process. The primary difference concerns the power generation where one additional gas turbine is necessary for the expander-based process in order to get the intended capacity of 3 MTPA LNG production. However, the expander-based processes are excellent in safety and simpler in operation, which may offset this drawback particularly for FLNG operation.The Niche process, particularly the one with the precooling system, has the highest process efficiency among the expander processes. The evaluation reveals that in general we need higher complexity to achieve higher process efficiency. The MODEC/APCI process has the largest number of equipment units, and is considered as the most complex process and the heaviest one. The opposite of the MODEC/APCI process, the Linde/SBM is the simplest one, but it has the lowest process efficiency.The MODEC/APCI occupy more space due to the use of Coil Wound Heat Exchanger (CWHE), which is considered to be less compact than Plate Fin Heat Exchanger (PFHE), larger piping system due to low pressure and vacuum operation, and the higher need for cooling water giving a larger and more complex utility system. The Niche process may have smaller footprint due to smaller size of the pipes and utilization of compact PFHE. The use of CWHE in the MODEC/APCI is safer because it provides double containment. This is an advantage that offset the drawback in footprint aspect. In contrary, the Niche process is the lowest in term of safety among the expander-based processes due to the use of PFHE, which provides only single containment, and hydrocarbon as refrigerant even though only in gas phase. Each process has unique advantages over the others, and there are some trade-offs and compromises among efficiency, simplicity and safety. The process selection will therefore depend on the specifics of a particular FLNG project.