(203u) Optimal Design and Operation of CO2 Liquefaction Process Considering Variation in Cooling Water Temperature

In order to compactly transport large amounts of CO₂ via ship, liquefaction process is indispensable to reduce large volumes of CO₂. Even though several CO₂ liquefaction process models have been suggested in the field of carbon capture and sequestration (CCS), there are still some technical issues to be further investigated with regard to compressor power as an operation cost. The compressor power is determined by mechanical efficiency, molar flow and inlet temperature. The mechanical efficiency cannot be manipulated since it is generally set by manufacturers, whereas molar flow and inlet temperature can be. In the case of CO₂ liquefaction process, inlet temperature has a considerable impact on molar flow due to the knock-out separators in each compression stage. For this reason, inlet temperature entering the compressor is the key variable for optimal design and operation. Nonetheless previous studies only consider a constant cooling water temperature from cold seawater regardless of seasonal and daily variations. It is difficult to adjust the process configuration and operating condition whenever the cooling water temperature varies. Furthermore there exist several recycle loops which can also affect each unit operation. Thus, this study proposes more practical design guidelines and operation strategies considering available cooling water temperature with minimizing total compressor power. It was implemented by simulation-based optimization using flowsheet simulator. First, solubility of water in CO₂ and Triethylene glycol (TEG) dehydration column were considered at each compression stage to prevent hydrate formation. Second, on the basis of compression-cooling-expansion cycle, closed and open loop liquefaction cycles were compared. In addition, compression ratio and Joule-Thompson (J-T) expansion pressure were optimized considering several mechanical constraints such as compressor discharge temperature. In particular, the final stage pressure, which has a significant influence on overall operation energy, was determined to maximize the ratio of compression energy to cold energy generated by the J-T expansion ranging from 5 to 35℃ in temperature of seawater. The computed minimum CO₂ liquefaction energy was about 100kWh/tonCO₂ based on the 5℃ of cooling water.