الفهرس 
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Abstract
As the whole world is now moving towards reducing carbon emissions in most industries, natural gas has become significant in this transition. LNG, the main means of natural gas around the world, is a very energy-intensive industry and improving its efficiency is an essential area of study. This improvement is achieved in this thesis by studying two points: optimising the ambient cooling system and optimum utilization of waste heat from gas turbines driving refrigerant compressors. As the LNG process is sensitive to cooling media, conventional air cooling is replaced by proposed water cooling with a cooling tower. Alexandria, Egypt, is used as the base location for comparison between the two systems. The average temperature of August, the most severe ambient condition, is used for design calculations and for comparison between air- and water- cooled cycles. Hourly temperature records are then used to obtain the actual performance of the proposed system throughout the year. The proposed system achieved a total year improvement of 3.98% reduction in SPC, fuel gas and CO, emissions and 7.21% reduction in annual propane consumption. To assess the suitability of the system in different climates, different weather conditions in four different locations are assessed. Nigeria, Qatar, Egypt, and Russia are chosen to present these different weather conditions. Performance is calculated throughout the year on hourly calculations. The results revealed that power reduction of up to 4.26% can be achieved based on total year performance. The power reduction achieved is 1.24%, 1.6%, 3.98%, and 4.26% for Russia, Nigeria, Egypt, and Qatar, respectively. The main refrigerant compressors in LNG cascade cycle are driven by gas turbines where 30% of the heat is dissipated into atmosphere as exhaust gases at temperature in the range of 450-600 °C which is considered as waste energy. This waste heat is recovered to drive ammonia absorption refrigeration cycle. This refrigeration cycle is used in 18 different cases to determine the best way to utilize the cooling load generated by the ammonia cycle in LNG cascade system. In each case, all parameters are calculated: power consumption, refrigerant consumption, cooling area. Results revealed that the most efficient way to utilize ammonia absorption refrigeration cycle, driven by heat recovered from gas turbines, is to cool methane and ethylene streams, after air coolers, by ammonia. This modification resulted in a 13.53 % reduction in power consumption, fuel consumption, and COz emission and 34.02% reduction In propane consumption.