Blowdown of CO2 Vessels at Low and Medium pressure conditions: Experiments and Simulations
12 Pages Posted: 2 Dec 2022 Last revised: 16 Dec 2022
Date Written: December 2, 2022
Abstract
For Carbon Capture and Storage (CCS) applications, transporting CO2 by ship at low pressures (ca.7 barg) may lead to more cost-effective ship design with larger transportation capacity. However, this concept has not been technically demonstrated so far. One of the challenges to overcome is process safety, and in particular, depressurization. Safe and rapid depressurization to
reduce pressure and evacuate inventory is a common practice during emergencies. In the case of CO2, this operation may lead to very low temperatures and to the formation of dry ice.
This paper presents the comparison of simulations carried out using Unisim Design®, with experimental measurements obtained during controlled depressurization of a CO2 vessel. The goal is to quantify the accuracy of UniSim depressurization models, underline required model improvements, and establish best engineering practice to model depressurization scenarios.
Both gas phase (from the top of the vessel) and liquid phase (from the bottom) depressurizations were performed. The measured parameters during depressurization tests were: i) pressure, ii) fluid temperature, iii) wall temperature, and iv) mass of the inventory.
During gas phase depressurization, dry ice formation occurs. Unisim accurately simulates the process parameters until the beginning of dry ice plateau, then the gap between experiments and simulations becomes large due to Unisim lack of models for solid phase.
During liquid phase depressurization, there is no dry ice in the system. Unisim models could not accurately reproduce CO2 mass flowrate through the restriction orifice. We explain this discrepancy by a delayed equilibrium mechanism, or by the formation of a metastable liquid not taken into account by the current model for flow through restriction orifices.
Keywords: Carbon Capture and Storage, CO2 vessels, Low-Pressure Ship Transportation, Depressurization, Process Safety, Process Simulation
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