Energy and exergy analysis of acid gas removal processes in the LNG production chain

In the energy transition towards a zero-carbon energy sector, natural gas grows much faster than either oil or coal, since it is an environmentally-friendly fuel supported by the continuing expansion of LNG, increasing the availability of gas globally. In recent years, the substantial growth in the world energy demand has increased the interest in the exploitation of natural gas reservoirs previously deemed undesirable due to their high acid gas content. Existing technologies for natural gas purification, such as chemical absorption with alkanolamine solvents, may be not suitable for treating highly contaminated natural gas due to the required higher solvent circulation rate and, consequently, to the energy demand for solvent regeneration. Over the last decades attention has been devoted to the study and development of low-temperature CO2 removal processes. With these new technologies, CO2 is separated as a high-pressure liquid making it easier to be pumped underground for sequestration or utilization in Enhanced Oil Recovery (EOR) projects. The aim of this work is to analyze natural gas purification technologies and liquefaction schemes for the production of LNG starting from the same acid natural gas stream. In particular, two CO2 removal technologies are considered to bring CO2 concentrations down to levels suitable for LNG production: the conventional chemical absorption technology with activated-MDEA (aMDEA) as solvent and the recently patented Dual Pressure Low-Temperature (DPLT) distillation technology. Different commercial technologies are taken into account for the liquefaction of the purified natural gas: Propane-Mixed Refrigerant (C3MR), Mixed Fluid Cascade (MFC), and Single Mixed Refrigerant (SMR). However, since these liquefaction processes are designed for a sweet gas obtained using a conventional acid gas removal technology, some adjustments have been made for their application to a low-temperature sweet gas. The choice to compare a conventional technology with a novel low-temperature one has been made to understand if the synergy between a CO2 removal technology operated at low-temperature and the downstream liquefaction process is advantageous, despite the need for refrigeration also in the CO2 removal step. The different process schemes resulting from the combination of the two CO2 removal technologies with the liquefaction ones have been simulated in Aspen HYSYS® V10 and their performances are assessed and compared by means of energy and exergy analyses, respectively based on the “net equivalent methane” approach and on the exergy efficiency concept. Results suggest that, although the aMDEA absorption process and the DPLT distillation one with downstream NGLs recovery have about the same specific energy consumption when applied to the natural gas stream taken into account in this work considering the CO2 removal step only, the overall process (including the liquefaction of the purified natural gas stream) involving the DPLT distillation technology is characterized by lower consumptions and a higher exergy efficiency.