(244q) Exergy Analysis and Comparison of Traditional and Modified APCI Cycles Used for Natural Gas Liquefaction

Nowadays natural gas (NG) is one of the most important energy sources, mainly because of its reduced environmental damage, when compared to other fossil fuels. It is also an important raw material in many chemical processes. NG may be used to produce electrical energy and steam in power cycles and can be used as fuel in vehicles. Considering security, cost and other amenities that pipelines provide, they are commonly used for NG transportation between producers and final consumers. However, when very large distances need to be covered, a pipeline almost always becomes not feasible and a liquefaction process is necessary to reduce NG volume up to 600 times, so that it can be transported by special means, mostly by ships. The APCI cycle is the most used to perform the liquefaction of NG using coupled refrigeration sub-cycles that decrease the gas temperature until approximately -160 °C. These sub-cycles operate with different working fluids and are great energy consumers. Thus, the study of alternative cycles may be attractive, for example, to replace the propane pre-cooling sub-cycle by an absorption refrigeration system (ARS), in order to reduce the electric power demand of the original APCI cycle, using waste heat available in NG processing plants (exhausted hot gases from turbines and hot streams that could be cooled, for example). In this context, this study presents a comparative thermodynamic evaluation of the traditional APCI cycle and a modified one, which includes a NH₃/H₂O single-effect ARS to replace the propane sub-cycle. Simulations of both cycles were carried out using ASPEN Hysys Version 7.3 software. The thermodynamic performance evaluation is based on the First and Second Laws of Thermodynamics and through an exergy analysis, inefficiencies were computed and identified. On energy basis, the original APCI cycle is more advantageous since its global First Law coefficient of performance (COP) is significantly greater in comparison to that of the modified APCI cycle (1.690 against 0.527). Under the exergy point of view, the modified APCI presents a global efficiency slightly higher (0.872 against 0.836, a relative difference of only 4.3%). This demonstrates that, in view of fuel and product definitions adopted for both cycles (according to their purposes of purchasing and using), the exergy destruction rate is a little more pronounced in the original APCI. In this sense, for both cycles, the main cryogenic heat exchanger is the most expressive contributor to decrease the global exergy efficiency (accounting for 41.5% and 30.3% of total exergy destruction rates in original and modified cycles, respectively), immediately followed by compressors in original APCI (25.7% of total exergy destruction rate in this cycle) and other heat exchangers in modified APCI (26.0% of total). A detailed economic analysis is desirable since it can show that despite its reduced COP, the modified APCI may be a good alternative once it uses waste energy from the process, reducing primary energy sources consumption.