Enhancement of thermal efficiency of a natural gas furnace in a petrochemical plant

This study consists two parts. In the first part, it was aimed to improve the thermal efficiency of an aromatic hydrocarbon furnace placed in a petrochemical plant located in Aliağa, İzmir. Exit temperature of the flue (stack) gas is being descreased to 259 °C by an air preheater (APH) existing in the plant. To increase the system effciency this temperature should be further decreased. For this purpose, two processes were investigated. 1) Increasing heat transfer area of existing APH, 2) placing a waste heat boiler after APH and it was seen that first method was not economically appropriate, since the remarkable fuel saving can only be achieved in very high area increase. In the second process, producing medium-low pressure steam, requires less heat transfer area in comparison with the first process and thermal efficiency of the combined system attains higher values with lower costs. In the second part of the study, the system was investigated from the second law of thermodynamics point of view. Conventional exergy efficiency of the furnace was calculated as 41.4%. By appling advanced exergy analysis, exergy efficiency of the unit can be increased up to 52.4% with suitable modifications. Conventional exergy efficiency of the air preheater was found as 37.0% and it was concluded that by improving working conditions and determining avoidable irreversibilities, the exergy efficiency of APH can be increased up to 83.5%. Likewise, for WHB, it was seen that conventional exergy efficieny was 45.7% and advanced exergy efficiency was 62.8%.