Experimental Investigation of Solid Formation under CO2 Liquefaction Conditions for Ship Transport at 7 and 16 Bar with Water Content up to 300 ppm

Abstract

Ship-based transport of CO₂ is crucial in developing a carbon capture and storage (CCS) infrastructure. Lowering the ship transport pressure of liquid CO₂ from the conventional 14–18 to 7 bar increases the vessel-based transport capacity, leading to significant cost reductions. However, this reduction in pressure necessitates stringent water dew point control measures to prevent the formation of ice and CO₂ hydrates, which could block pipelines and equipment. The capital and energy demands of complete dehydration of CO₂ increase the CAPEX and the OPEX of the system. It is therefore important to know the limits for water content in the CO₂ stream and the consequences if this system cannot reach the dehydration specification due to operational upsets. This study experimentally investigated possible solid formation under CO₂ liquefaction for ship transport conditions at 16 and 7 bar, containing varying water concentrations. Using a CO₂ stream from a postcombustion amine-based capture plant to represent a realistic CO₂ composition, five tests were conducted at different liquefaction pressures and water contents. The experimental results were compared to the hydrate equilibrium predictions for pure CO₂. It was found that, for low-pressure CO₂ liquefaction with 200 ppm water, signs of solid formation started to occur at about 6.7 bar, which led to complete blocking of the filter over the course of approximately 1 h as the pressure was further reduced to 6.5 bar. This is clearly above the triple point pressure, so the solids that formed were most likely hydrates. For low-pressure CO₂ liquefaction with 100 ppm of water operating very close to the triple point and the hydrate formation area, there was no increase in the pressure drop across the filter. For medium-pressure CO₂ liquefaction at 16 bar, no indication of solid formation was observed with a water content of 200 and 300 ppm. These findings show the effects of exceeding the current water specification during liquefaction of low- and medium-pressure CO₂ for ship transport.