Thermodynamics, regulations and real-world experience shape the handling of vapour return at carbon capture and storage terminals
The transport and storage of liquid CO2 is a core component of the carbon capture and storage (CCS) value chain. While attention is often directed towards pipelines, capture plants or injection sites, the transfer of liquefied CO2 between ships and shore terminals presents a technical and operational challenge that is quietly attracting scrutiny. Vapour return during loading and unloading operations is one such issue, requiring careful integration of ship design, terminal layout, pressure management and regulatory planning.
During a Riviera Maritime Media webinar, titled Addressing Vapour Return Challenges in CO2 Collection Terminals, panellists from Knutsen NYK Carbon Carriers, DNV and Vopak shared their views on what is required to develop reliable, flexible and economically viable systems for managing vapour return in this emerging sector.
CO2 vapour return is driven by thermodynamic conditions. When liquid is transferred from an onshore storage tank to a ship’s cargo tanks, vapour is returned from the ship to the terminal to maintain pressure equilibrium. The reverse occurs during offloading. However, the properties of CO2 complicate this otherwise familiar process.
CO2 is sensitive to small changes in temperature and pressure. Its phase diagram includes a triple point where solid, liquid and gas can coexist, introducing the risk of dry ice formation if system pressures fall below safe thresholds.
“There are quite a lot of considerations that have to be taken into account,” said for Knutsen NYK Carbon Carriers (KNCC) CEO, Oliver Hagen-Smith. “There are differences between the different transportation modes in terms of flexibility and which mode will allow you to do different operations.”
KNCC has developed a proprietary test facility to evaluate the behaviour of liquid CO2 under varying conditions. The facility includes a twin-tank setup with the ability to adjust temperatures, pressures and flow rates. Tests have included leak simulations, forced corrosion, and transfer scenarios using low, medium and elevated pressures. According to KNCC, results indicate that CO2 rapidly seeks equilibrium between vapour and liquid phases, which may allow for reduced reliance on vapour return lines, if systems are designed with this behaviour in mind.
“The reaction rates in terms of temperature differences or pressures — condensation and evaporation — are relatively fast,” said Mr Hagen-Smith. He suggested that, particularly for discharging operations, the natural recondensation or evaporation of CO2 might allow for a reduction in vapour return flows, provided that cargo tank conditions are allowed to vary and system envelopes are carefully modelled.
“Vapour return can carry cost consequences”
DNV principal engineer, Gabriele Notaro, provided a broader view of the role impurities play in vapour return handling. Unlike commercial liquefied gases such as LPG or LNG, captured CO2 is often not subject to purification unless required by the receiver. This leads to variable levels of impurities such as oxygen, nitrogen, argon, sulphur oxides and water. These impurities affect both vapour composition and the physical behaviour of the CO2 stream. Their presence also alters the safety case.
“The presence of impurities affects the physical properties, the phase envelope, water solubility and the solubility of other compounds,” Mr Notaro explained. “These introduce risks to the value chain — health, safety and environment concerns, as well as material selection challenges.”
Terminals must therefore make a choice. Should vapour lines be fully integrated into the system with reliquefaction and vapourisers? Or can pressure variation be tolerated in order to avoid vapour return entirely? In cases where vapour return is not used, a ship must either be capable of absorbing pressure swings, or it must be equipped with a reliquefaction plant or vapouriser to restore balance. The same applies at the terminal. Yet sizing such systems for real-world flow rates, and ensuring that they are energy-efficient and economically viable, remains a challenge.
Vopak project manager, Stijn Berbers, and part of the CO₂next terminal team in Rotterdam, addressed these considerations in the context of a large-scale facility currently under development. “Making the right design choices to ensure smooth operation is really essential,” he said.
The CO₂next terminal will begin operations with 5.4M tonnes of annual capacity, scaling up to 15M tonnes in the future. Six 8,000m³ spheres will store medium-pressure CO2 (15–18 bar), with loading and unloading via two jetties. Mr Berbers explained that Vopak opted for a single, commingled vapour return line equipped with gas analysers and mass flow meters, rather than segregating vapour streams from different tanks. This reduces infrastructure complexity but requires more stringent vapour specification monitoring.
“There’s a trade-off,” he said. “If vapours are comingled, it’s important to implement precise quality and quantity measurements not only on the ingoing liquid stream, but also on the vapour return lines.”
Mr Berbers emphasised the commercial dimension of vapour return. In the European Union, CO2 emissions fall under the Emissions Trading System (ETS), and vapour return can carry cost consequences. If impurities or non-condensable gases in the vapour stream need to be vented, the ETS cost liability must be accounted for. Whether this cost is assigned to the emitter, the terminal operator, or the transport company is still not clearly defined.
“The reaction rates in terms of temperature differences or pressures are relatively fast”
In a live poll during the webinar, 46% of respondents said the emitter should own the emissions associated with vapour return, 29% said it should be the terminal, and 25% preferred the vessel operator. This division reflects the current lack of clarity on regulatory ownership and may create disputes when vapour return carries a financial penalty.
The composition of the vapour stream itself introduces further complication. Vapour typically contains higher concentrations of light impurities — nitrogen, oxygen, methane — than the liquid phase. This creates problems when vapour is returned from one emitter’s cargo to a shared terminal, which may then redistribute it to another receiver. Both Mr Notaro and Mr Hagen-Smith pointed to the risk of cross-contamination and chemical incompatibility in such scenarios.
“The solution adopted by CCS chains under development is to create vapour return lines, but impurities are exchanged between ship cargo tanks and the onshore facility,” said Mr Notaro. “This can cause cross-contamination between emitters and receivers with different tolerances.”
The heel — that is, residual CO2 left in cargo tanks between voyages — is also a source of complexity. The heel is required to maintain tank pressure and prevent collapse, but it can mix with new cargo on the next loading. In a chain involving multiple emitters, this can lead to off-spec cargo if impurities from one source are not tolerated by the next receiver.
