Iranian Journal of Oil and Gas Science and Technology

Abstract In this paper, an advanced analysis of a novel hybrid compression-absorption refrigeration system (HCARS) for natural gas dew point control unit in a gas refinery is presented. This unit separates the heavy hydrocarbon molecules in the natural gas, which is traditionally carried out by natural gas cooling in a compression refrigeration cycle (CRS). The power input required for the refrigeration cycle compressors is usually provided by gas turbines. The low efficiency of gas turbines and the excessive power required for running the CRS compressors have made it crucial to investigate different means to decrease the energy consumption of this cooling system.
The waste heat of gas turbines flue gas can be recovered and utilized as the heating source for running an absorption refrigeration system (ARS) to provide part of the needed cooling load; hence, a hybrid compression absorption refrigeration system (HCARS) is launched. In this work, the application of HCARS is extended to the Fajr-e-Jam gas refinery currently operating with a CRS, and an advanced exergetic analysis of the proposed ARS is performed to further improve the proposed system. The effect of different variables on the performance of the proposed HCARS is also inspected. The proposed system and these analyses are novel for the gas refinery dew point control unit. Real CRS operational data are utilized in all the investigations, and proper means are presented for the validation of the simulation results.
The proposed system resulted in 63% additional cooling capacity of the HCARS (12550 KW) in comparison to the current CRS (7670 kW) for the equal natural gas consumption, which overall saves about 50000 SCMD of natural gas. Based on the exergy analysis of all the equipment, the exergy efficiency of the proposed ARS is 0.155. In addition, the parametric study of the effects of the gas turbine flue gas exit temperature and flow rate, ambient temperature, partial load operation of CRS, absorption solution flow rate, and concentration on the HCARS performance is carried out. These studies should provide the information needed for operating the proposed system in different situations.

Abstract In this paper, exergy and exergoeconomic analysis is performed on the recently proposed process for
the coproduction of liquefied natural gas (LNG) and natural gas liquids (NGL) based on the mixed
fluid cascade (MFC) refrigeration systems, as one of the most important and popular natural gas
liquefaction processes. To carry out this analysis, at first, the proposed process is simulated, and then
the exergy analysis of the process equipment is performed; finally, an economic model is used for the
exergoeconomic analysis. The results include cost of exergy destruction, exergoeconomic factor,
exergy destruction, and exergy efficiency. The results of the exergy analysis demonstrate that the
exergy efficiency of the proposed process is around 53.83%, and its total exergy destruction rate is
42617.5 kW at an LNG and NGL production rates of 68.99 kg/s and 27.41 kg/s respectively. The
results of exergoeconomic analysis indicate that the maximum exergoeconomic factor, which is
69.53%, is related to the second compressor in the liquefaction cycle and the minimum
exergoeconomic factor, which is 0.66%, is related to the fourth heat exchanger in the liquefaction
cycle. In this process, demethanizer tower holds the highest relative cost difference (100.78) and the
first air cooler in liquefaction cycle has the lowest relative cost difference (1.09). One of the most
important exergoeconomic parameters is the cost of exergy destruction rate. The second heat
exchanger has the highest exergy destruction cost (768.91 $/Gj) and the first air cooler in the
liquefaction cycle has the lowest exergy destruction cost (19.36 $/Gj). Due to the high value of fuel
cost rate (as defined in exergoeconomic analysis) in heat exchangers, their exergy destruction cost is
much higher than other devices.