MODELLING THE CASCADE CYCLES AND THEIR IMPACTS ON DOWNSTREAM FRACTIONATION IN NATURAL GAS LIQUEFACTION PROCESS
The study was designed to further advance our earlier model on natural gas processing. The objective is to efficiently evaluate all field natural gas in a region in Nigeria and decide on processing and utilization requirements. The region has large reserve of natural gas: associated and non-associated from the production fields. These need to be recovered, processed and utilized, to the benefit of all stakeholders.A generalized algorithm was developed and modelled on ASPEN HYSYS, in the context of processing field natural gas of varied compositions: C1H4 of range 78-97% or wider, and the balance other components. The model employs the LNG unit operation with cascade refrigerant cycles. Each cycle which uses refrigerants: methane, ethane and propane separately and/or mixed in appropriate ratios, is in a process loop consisting of a compressor, condenser, a throttle valve and the LNG unit operation. Along the process flowsheet sequence, downstream the cascades cycles loop is the fractionation train which separates the desired components. Overall, identified critical parameters, such as refrigerants compositions in the cycles and compressors process conditions, are used to optimize the model. The model was tested and applied to various field data in the region and confirmed to be robust and reliable. This model now provides solution to specific and specialized natural gas processing; supplies data and process information for both engineering and investment decisions on the hydrocarbon utilization, optimization and applications.