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Evaluation of Natural Gas Liquefaction Processes for Floating Applications Offshore
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Energy and Process Engineering.
2010 (English)MasteroppgaveStudent thesis
Abstract [en]
A literature survey of LNG processes suitable for offshore environment has been evaluated. The survey has been performed with emphasis on space requirements, complexity, efficiency and safety. Thermodynamics theory is described and used in the investigation of the NicheLNG process. The liquefaction part of the HLNG FPSO-1 has been evaluated with respect to its energy consumption, improvements and the possibility to expand the process to give an indication about improvement potentials. In addition, one alternative liquefaction process has been compared with the NicheLNG process. Dual expander processes based on nitrogen as refrigerant are the most proposed solution suitable for offshore applications. Therefore it was chosen as an alternative process to NicheLNG. In the investigation of the two processes the processes simulated were with equal conditions. The NicheLNG process, based on an open methane cycle and a nitrogen cycle, had a significantly lower mass flow rate resulting in 10% lower power consumption. Decision of chosen refrigerant gas (methane or nitrogen) has different specific heat capacity and hence an influence on the flow rate. Methane as refrigerant requires less mass flow rate than nitrogen for a given duty. In addition, higher pressure levels will contribute to increased efficiency and reduced unit sizes. In the open refrigeration cycle of the NicheLNG process, methane is cooled down to -1,5°C before it is expanded. If the internal heat exchange is extended to -10°C before expansion is it possible to achieve some efficiency increase for the methane cycle. Comparison criteria are important when the quality of liquefaction processes is to be determined. Feed and product specifications provide some restrictions on obtainable efficiency. With an increasing feed gas pressure, the whole liquefaction process (from feed to LNG) demands less work, but the overall process efficiency is reduced due to the high efficiency of the feed gas compressor and the low efficiency of the liquefaction part. Hence, the efficiency of the liquefaction process should not be calculated from its feed gas pressure but rather the liquefaction pressure. For the NicheLNG process, the exergy efficiency of the liquefaction part was calculated to 26,6%, with a liquefaction pressure at 75 bar. An increase of LNG production with emphasis on improvements to keep work consumption down was also discussed. The four evaluated solutions were utilization of End Flash Gas, liquid expander, additional compressor and increase of heat exchanger area. The liquid expander was the improvement that stands out as the highest contribution to the efficiency. With a 25% increase in LNG production and with new units and modifications of the design resulted in a reduction in the specific work consumption from 0,5502 kWh/kgLNG to 0,4791 kWh/kgLNG. These efficiency improvements can justify higher investment costs since the work consumption, with 25% higher LNG production, was 12,9% lower than for the original design of the NicheLNG process. Never the less, space and weight on a FPSO are limited and has to be considered when a more efficient process is desired.
Place, publisher, year, edition, pages
Institutt for energi- og prosessteknikk , 2010. , 82 p.
Keyword [no]
ntnudaim:5357, MTPROD produktutvikling og produksjon, Energi-, prosess- og strømningsteknikk
URN: urn:nbn:no:ntnu:diva-12894Local ID: ntnudaim:5357OAI: diva2:426936
Available from: 2011-06-27 Created: 2011-06-27

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