Integrated Process Design and Modeling for LNG Evaporation and CO2 Liquefaction Systems

Abstract

As the global economy continues to expand, the demand for natural gas has surged, leading to a significant increase in LNG consumption. LNG offers a pipeline-free transportation solution, but its integration with standard natural gas systems requires an evaporation process. Currently, LNG evap- oration is commonly achieved through self-consuming processes, seawater, or air evaporators. However, there is growing interest in leveraging the cold exergy of LNG for other processes. With carbon storage gaining prominence due to climate change concerns, integrating LNG evaporation and CO2 liquefaction systems could offer potential cost reductions in the overall process. Singapore could be an optimal location for such an integrated system due to the increasing LNG imports and the desire to become carbon-neutral requiring CO2 exports. The research of this thesis therefore will be: ”How can the LNG evaporation process be integrated with a carbon-dioxide liquefaction process at an LNG terminal in Singapore?” Existing research primarily focuses on self-consuming processes or CO2 purification, leaving a gap in understanding non self-consuming systems for integrating LNG evaporation and CO2 liquefaction at cryogenic temperatures. This study aims to fill this gap by comparing various heat transfer systems and evaluating their technical and economic feasibility. Three systems have been set up to be compared: a Direct heat exchanger system, an intermediate Propane loop and an Organic Rankine cycle. The Organic Rankine cycle has been optimized based on the net power output, first and second law efficiency of the system. Subsequently, the three systems have been compared showing that the Organic Rankine cycle is dominant in technical performance, both being self sustaining and holding a second law efficiency of 82.3%, which is 5% higher than the other designed systems. Additionally, the Organic Rankine cycle has a specific net power output of 7.3 kWh per ton CO2 liquefied, which can be utilized for other processes. After this, a financial assessment is performed on the three systems based on the internal rate of return and net present value of the systems. The Direct heat exchanger showed to be dominant in terms of internal rate of return and net present value ($313M and 534.1%, respectively). The Propane loop would be the second best system in terms of internal rate of return (79.5%). The Organic Rankine cycle could provide a higher net present value than the Propane loop ($285M and $267M, respectively), assuming that constant flows could be guaranteed and no additional evaporator is required. The Direct heat exchanger would be the financial best choice for Vopak but is limited by the high risk of frost formation in the exchangers, making it an unfit choice for this application. While the Organic Rankine cycle could provide an interesting alternative due to it’s higher net present value and power generation, the system complexity, generated electricity value uncertainty and initial investment make it a less attractive choice. Based on the techno-economic analysis in its entirety, the Propane loop was determined to be the best system to combine CO2 liquefaction and LNG evaporation.