RIGOROUS SIMULATION AND OPTIMIZATION OF THE COLD SECTION IN A REAL LNG PLANT: UPFRONT NITROGEN REMOVAL AS PROCESS ENHANCING CONCEPT

Abstract

The global demand for LNG is expected to continue increasing over the next decays. Qatar, as one of the largest LNG exporters with a 22% global share, announced in 2020 a plan to increase the production capacity by more than 60% over the next five years. The LNG industry relies on the liquefaction process to reduce the volume of natural gas (NG) by approximately 600 times. NG liquefaction offers a cost-effective way of transportation in tanks instead of relying on a network of pipelines for gas transportation. NG processing and liquefaction include complicated unit operations such as impurities removal (e.g. CO2, H2S), separation of heavy hydrocarbons (C3+), and then liquefaction. The cold section is the pillar of the liquefaction process and consumes roughly 60% of the total energy demand. This section is comprised of the refrigeration cycles (C3MR or SMR, etc.), fractionation unit, nitrogen removal unit (NRU), and helium extraction unit. Shaft work used to operate compressors accounts for the majority of the total energy demand in this section. Therefore, heat integration and process optimization are commonly used to reduce energy demand in the LNG process This thesis aims to establish an effective upfront nitrogen removal (UNrem) process in the hope to save energy, improve efficiency, increase production, and boosting the economic feasibility. This novel idea was validated via rigorous simulation of the cold section process using Aspen HYSYS integrated with Aspen EDR operated. Energy optimization, detailed exergy analysis, and economic analysis for several UNrem cases were conducted and compared. Results showed that the total power requirement and the production rate can be decreased upon the implementation of the UNrem concept. Compared with baseline operation, removing up to 87.5% of nitrogen from the feed was found optimal as it decreases the total power requirement by 0.24 MW and increases the LNG product flow rate by 4.4 %, while exergy loss decreased by 7.08 MW. UNrem of 87.5% scenarios showed a profit of 24.2 billion USD in 20 years. Results confirmed the importance of UNrem process for energy savings, process improvement, and profit without the need for structural changes.