Application of conventional and novel low-temperature CO2 removal processes to LNG production at different CO2 concentrations in natural gas

Future energy scenarios forecast an increase in the global energy demand. To meet that, although renewable energy resources will be the fastest growing ones, fossil fuels will remain the main source of energy. Natural gas, in particular, is considered an effective short to mid-term bridge fuel to a low-carbon future, as an environmentally-friendly fuel. These factors have led to consider the exploitation of those low-quality natural gas reserves that were left undeveloped in the past because of the high H2S and/or CO2 content and the absence of technologies suitable for their profitable exploitation. In recent years, many studies have been carried out to develop new technologies that make the production of natural gas from these reserves economically viable: the most promising technologies have turned out to be the low-temperature ones. However, no research has been carried out on the performances of the whole LNG (liquefied natural gas) production process, requiring the coupling between the acid gas removal step and the liquefaction one. This work aims at studying the energy demand of the purification and liquefaction of several acid natural gas streams composed of methane and CO2, which differ from each other because of the CO2 content. The conventional amine chemical absorption process, where MethylDiEthanolAmine (MDEA) is used as solvent, is compared with a recently developed dual pressure low-temperature distillation process (DCCDTM) for CO2 removal from natural gas. Downstream of the acid gas removal step, the Propane Precooled Mixed Refrigerant (C3MR) technology has been considered for the liquefaction of natural gas. The aim is to understand whether there is an advantage related to the coupling of a low-temperature purification technology with the liquefaction of the sweet natural gas in terms of energy consumptions and efficiency of the overall process. The results of the simulations performed in Aspen HYSYS® V9.0 are utilized to carry out the energy analysis (based on the net equivalent methane method), and the exergy analysis (based on the second-law efficiency). The handouts allow assessing which is the most convenient acid gas removal technology for LNG production, depending on the CO2 content in the raw natural gas.