Modeling of CO₂ conditioning processes

During the last decade, carbon capture, utilization, and storage (CCUS) technologies have received increasing attention from our society as solutions to reduce CO₂ emissions and mitigate global warming. While the carbon capture, transport and storage processes are being extensively researched, the CO₂ conditioning processes, which are very energy intensive, are comparatively overlooked. This study thoroughly studies CO₂ conditioning processes including CO₂ liquefaction and dehydration.
In the first part of this project, various designs based on the open and closed-cycle liquefaction concepts are modeled. For both concepts, the power consumption of the liquefaction designs can be significantly reduced by implementing the multistage expansion. In general, the closed-cycle liquefaction designs with ammonia are more energy efficient than the open-cycle liquefaction designs. In addition, Precool CO₂ before the liquefier can further reduce the energy consumption of the closed-cycle liquefaction design. Two transport cases, 8 and 16 bar, are considered. The most energy-efficient design for the 8 bar transport case is the hybrid design (where the open and closed-cycle liquefaction concepts are combined) with precoolers and multistage expansion. The energy consumption of this design is 62.0 kWh/tCO₂. The most optimal design for the 16 bar transport case is the closed-cycle liquefaction design with precoolers and multistage expansion. The energy consumption of this design is 55.7 kWh/tCO₂.
In the second part of this project, an economic analysis is carried out to evaluate the liquefaction cost of the designs. It is found that the design with the lowest liquefaction cost is not necessarily the design with the lowest power consumption. This is because the total plant cost also has a relatively high impact on the liquefaction cost. The LCC analysis shows that for the 8 bar transport case, the optimal design has a liquefaction cost estimated at 10.9 USD/tCO2. For the 16 bar transport case, this is 9.6 USD/tCO₂.
In the third part of this project, designs with various considerations are created to integrate the liquefaction process with the capture and transport processes in the 3D project. The considerations include recycling of the CO/CO₂ vapor stream, returning streams from the transport process, different dehydration methods etc. The final design, which is based on the closed-cycle liquefaction with ammonia as the refrigerant and precoolers, has an energy consumption of 79.5 kWh/tCO₂ and a liquefaction cost of 12.3 USD/tCO₂.
In the final part of this project, the CO₂ dehydration process with TEG is simulated. Three thermodynamic models including PR, TST and CPA from HYSYS are firstly compared. The results show that by comparing the model predictions to the thermodynamic data, CPA from HYSYS is a good model with systems containing TEG, but it could be improved for the CO₂-water system. Then, the conventional TEG design is compared to the enhanced TEG design. The results show that the conventional design is able to reduce the water content in the dried gas to around 200 ppm. On the other hand, the enhanced design can reduce the water content in the dried gas to below 30 ppm with the right model parameters. The sensitivity analysis shows that the TEG and the stripper gas flow rate can be adjusted to achieve the desired water content in the dried gas. To improve the separation in the absorber, the absorber pressure should be high while the temperature should be low. The reboiler temperature should be 204°C. The optimal stage numbers for the regenerator and stripper are around five.