A Strategy for Reliable Cargo Loading of Low-Pressure Liquid Carbon Dioxide Carriers

Abstract

This study addresses the control challenges associated with loading low-pressure liquid carbon dioxide carriers (LCO2Cs), which are crucial components of the carbon capture, utilization, and storage (CCUS) chain. It explores the need for stable pressure and temperature control to prevent dry ice formation and ensure efficient cargo handling. The research employed HYSYS dynamic simulations to assess three different control strategies. The simulations assessed each strategy’s effectiveness in maintaining stable operating conditions and preventing risks, such as dry ice formation and valve blockages. The study concluded by examining the necessity of pressurization for safe and efficient LCO₂ loading and by determining which control strategy is most effective and reliable based on the simulation outcomes. Among the three scenarios examined, Case A, which utilized two control valves, exhibited initial instability due to significant flow coefficient differences, resulting in temperature drops below the CO₂ triple point and increasing the risk of dry ice formation. Case C, operating without pressurization, experienced severe pressure fluctuations and prolonged exposure to temperatures below the triple point, posing risks of valve blockages. In contrast, Case B, which uses a remote pressure-reducing valve and a control valve, demonstrated the most stable performance, effectively avoiding dry ice formation and pressure fluctuations, making it the most reliable method for safe LCO₂ cargo loading.

1. Introduction

In the wake of escalating global efforts to combat climate change, the industry of low-pressure liquid carbon dioxide carriers (LCO2Cs) has emerged as an indispensable link in the carbon capture, utilization, and storage (CCUS) and onboard carbon capture system (OCCS) chains [1]. As nations have increased their commitments to reducing CO₂ emissions, the demand for efficient CO₂ transportation from capture terminals to injection sites has surged [2]. The expansion of CCUS and OCCS technologies, driven by international climate policies and decarbonization goals, has inevitably led to a heightened need for specialized transportation solutions that can safely and efficiently handle large volumes of captured CO₂ [3]. This has positioned LCO2Cs as a critical component in the global carbon management strategy [4].

The rationale behind the adoption of low-pressure systems for LCO2Cs lies in the necessity to transport vast quantities of carbon dioxide efficiently. While industrial CO₂ transport is still in its early stages, prior experience has primarily focused on medium-pressure CO₂ systems (15–30 bar a), which are less suitable for transporting large volumes. These systems, operating at higher pressures, did not require special consideration of the CO₂ triple point. However, for low-pressure systems (under 10 bar a), maintaining stable pressure becomes crucial due to the risk of dry ice formation near the triple point. Low-pressure configurations allow reduced tank thickness, providing a more efficient and safe solution for CO₂ transport compared with high-pressure systems [5]. Furthermore, operating at low pressures enhances the overall efficiency of the transport process, enabling carriers to handle larger capacities while maintaining cargo stability and safety through the sophisticated control systems of the cargo handling system [6]. This is particularly important as the CCUS and OCCS industries continue to grow and require scalable and cost-effective solutions for long-distance CO₂ transport [7].

Several key challenges arise in the practical implementation of low-pressure LCO₂ carriers. One challenge is that the loading pressure supplied by each terminal can vary, and terminals may increase the pressure to compensate for pipeline pressure losses. This can create discrepancies between the terminal-supplied pressure and the ship’s operational pressure, requiring specialized equipment to regulate and stabilize the cargo pressure in the ship’s tanks [8]. Additionally, to minimize pressure loss in CO₂ pipelines, valves with a high flow coefficient (Cv) are often used. However, the significant difference in flow coefficients between the loading line (lower flow coefficient) and the vent line (higher flow coefficient) can cause the loading valve to abruptly fluctuate between 0% and 100%, triggered by movements in the vent line control valve [9]. These factors highlight the need for effective control strategies to ensure safe and efficient cargo handling.

There have been various studies examining the broader field of low-pressure LCO₂ carriers. For instance, Pérez et al. [10] conducted research on the applicability of low-pressure CO₂ transport in marine environments, while Byong-Hee [11] proposed the use of multi-lobe tanks, which are typically found in small-sized LPG carriers, to maximize storage capacity for LCO₂. Damoon [12] compared low- and medium-pressure solutions for LCO₂ transport and storage. However, while these studies address aspects of storage and transportation, there is still a need for more research into the specific control challenges encountered during the loading process of LCO2Cs.

Research on carbon dioxide storage has predominantly focused on issues related to CCUS technologies. Zhang et al. [13] examined carbon capture, storage, and utilization technologies, while Middleton [14] developed an optimization model for CO₂ capture and storage networks. In terms of loading, Safarzadeh et al. [15] modeled the flow of crude oil and CO₂ mixtures to explore the effects of carbon dioxide storage and enhanced oil recovery. Despite these studies, further investigation is required into the specific issues related to the control of cargo handling systems during loading, particularly those concerning control valve behavior and pressure regulation to prevent drops below the triple point and to ensure safe operating conditions within the cargo tanks.

Given these gaps, the primary objective of this study is to propose a control strategy to address the challenges encountered during the loading process of low-pressure LCO₂, particularly issues such as dry ice formation and pressure fluctuations. To this end, we present a schematic diagram of a cargo handling system designed to maintain LCO₂ under optimal operating pressure during loading. Additionally, we explore whether pressurization is necessary to simplify cargo handling procedures on LCO₂ carriers.

To address the challenges of dry ice formation and pressure fluctuations during LCO₂ loading, we propose a novel control strategy involving two distinct modes. The research employs HYSYS dynamic simulations to assess three different control strategies. The first strategy involves two valves in a control mode and focuses on managing pressure through coordinated valve operation. The second strategy uses a remote pressure-reducing valve along with a control valve, providing pressure management in the cargo tank. The third strategy evaluates whether pressurization is necessary; it explores the feasibility of proceeding without pressurization while assessing potential risks such as dry ice formation and valve blockages.

We validate the effectiveness of this control method using HYSYS dynamic simulations (Aspen Technology, Bedford, MA, USA), which have shown a high degree of accuracy when compared with actual operational results [16]. Although actual experiments were conducted to further validate these simulations, the results cannot be disclosed due to confidentiality agreements. By addressing this issue upfront, we aim to clarify any potential concerns regarding the nature of the experimental data used in this study. Through these simulations, we assess whether the proposed solution adequately resolves the identified issues and maintains stable operating conditions. Additionally, the necessity of pressurization before LCO₂ loading is examined through the same dynamic simulation approach.

2. Process Configuration and Control Strategy

In the CCUS value chain, the interrelations among emitters, carbon capture and liquefaction processes, LCO₂ loading and unloading on transport vessels, and injection facilities are of paramount importance for effective operation. Emitters, such as industrial facilities and power plants, generate significant volumes of CO₂ during their processes, which are then captured using advanced carbon capture technologies [17]. This captured CO₂ is prevented from entering the atmosphere and can be transported to storage sites in either gas or liquid form, depending on the specific requirements. While CO₂ can be transported as a gas, it often undergoes liquefaction in order to be transformed into a liquid state, which is optimal for long-distance transportation due to its higher density and reduced volume. The liquefied CO₂ is transferred into specialized carriers, known as LCO2Cs, which are specifically designed to safely and efficiently transport LCO₂. These vessels play a pivotal role in conveying the LCO₂ from capture facilities to designated injection sites, ensuring that the CO₂ remains stable and secure throughout the transportation process [18].

At the injection sites, the LCO₂ is permanently stored underground in geological formations, such as depleted oil and gas fields or saline aquifers, ensuring secure and long-term storage. This process not only supports the sequestration of CO₂ but also helps mitigate the impacts of climate change [19]. The coordinated operation of the entire system—including the emitter sites, capture facilities, LCO2Cs, and injection terminals—is essential for the efficient and seamless functioning of the CCUS process and ensures the reliable capture, transport, and storage of CO₂ [20]. This integration ensures that CO₂ emissions are effectively reduced and thus contributes to global efforts to combat climate change. Figure 1 illustrates the organic and interconnected operation of the CCUS value chain.

The challenges associated with the practical implementation of low-pressure LCO₂ carriers are closely related to the processes illustrated in Figure 2. This schematic diagram depicts the flow from carbon capture and liquefaction to the loading of LCO₂ onto LCO2Cs. A key issue highlighted in this study is the variance in loading pressure supplied by the terminals. This discrepancy between terminal pressure and the ship’s operational pressure can be managed using specialized equipment, such as control valves, which regulate pressure during LCO₂ loading to maintain stability inside the cargo tanks.

As indicated in the schematic, efficient cargo handling is critical during the loading process, especially in managing the pressure of the LCO₂ being supplied to the cargo tanks. After adjusting the pressure of the supplied LCO₂, it is essential to appropriately control the vapor pressure accompanying the LCO₂ to ensure that the tank pressure remains within safe limits, which is vital for the safety of the vessel. Due to the significant difference in flow coefficients between the high-flow valve in the vapor return line and the low-flow valve in the LCO₂ loading line, any sudden movement of the trim in the vapor return valve can lead to abrupt fluctuations in the loading valve, potentially causing instability during the transfer process. These factors underscore the need for proper control strategies during the loading process. By effectively managing varying flow rates and pressures, these controls prevent sudden fluctuations, ensuring both stability and operational safety throughout the LCO₂ transfer.

2.1. Case A: Control Mode Application on Two Valves for Cargo Tank Pressure Adjustment

As illustrated in Figure 2, both the LLV (LCO₂ loading valve) and VRV (vapor return valve) are equipped with controllers. The LLV on the LCO₂ loading line is designed to respond more quickly than the VRV on the vapor return line, with a carefully designed trade-off in the controller to prevent excessive fluctuations. This is necessary because the two valves have distinct flow coefficients. Since the VRV has a larger flow coefficient, even a small adjustment of the VRV can result in significant flow changes from the perspective of the LLV, making it more sensitive to these variations.

For the gassing up and pressurization of the LCO₂ tank, nitrogen gas at 20 °C and 7.8 bar a was supplied into the cargo tank. Nitrogen is chosen over carbon dioxide for this process because filling the tank with pressurized CO₂ during actual experiments is challenging. The supply conditions for LCO₂ are limited to a pressure of 26 bar a and a temperature of −20 °C, making pressure reduction necessary. LCO₂ may be stored at either medium pressure or low pressure; in the case of low pressure, it is boosted by a pump to account for pressure losses in the loading arm and piping, making pressure reduction essential. After pressure reduction by the LLV, the LCO₂ is stored at 7.8 bar a and −46.8 °C.

2.2. Case B: Control and Manual Mode Applications on Two Valves for Cargo Tank Pressure Adjustment

Unlike in Case A, the LLV is a remote pressure-reducing valve with a fixed flow coefficient; it is responsible for regulating and supplying LCO₂ to the tank at a reduced pressure. The supply pressure and temperature of the LCO₂ are the same as in Case A, as are the pressure and temperature at which it is stored. To ensure that the vapor generated during the loading process is continuously returned to the terminal, the system is configured to route the vapor through the control valves. The VRV retains the same PI controller settings as in Case A, with no changes in its flow coefficient. As loading progresses, the VRV continuously manages the tank pressure. As in Case A, nitrogen gas at 20 °C and 7.8 bar a is used for pressurization.

2.3. Case C: Control Mode Application on Two Valves Without Pressurization for Cargo Tank Pressure Adjustment

Loading LCO₂ directly into a tank pre-filled with dry CO₂ at 20 °C and 1.01 bar a would greatly simplify the cargo loading process. Normally, the procedure involves gassing up to remove oxygen and moisture from the tank at atmospheric pressure, followed by a separate pressurization step to prepare the tank for cargo loading. However, if direct cargo loading can be achieved without complications, the CO₂ vapor generated during the loading process could be used for pressurization, reducing the overall time required for the cargo handling sequence. This study aims to investigate this possibility through simulation, using Case A, where both valves operate in control mode as the reference point.

Table 1. Summary of key parameters for each case.

Parameters	Case A	Case B	Case C
Tank Pressure [bar a]	7.8	7.8	1.01
Tank Temperature [°C]	20	20	20
Control Valve	LLV, VRV *	VRV	LLV, RVR
Remote Valve	-	LLV	-

* LLV: LCO₂ loading valve, VRV: vapor return valve.

summarizes the key parameters and results of Cases A, B, and C, allowing for a clearer comparison of their performance and control strategies.

Given that a typical tank on an LCO₂ carrier will have a capacity of approximately 10,000 m³, Case A was chosen as the baseline. In tanks of this size, fluctuations are likely to occur, especially if pressurization is not properly established, regardless of the control setup in Case A or Case B. By focusing on Case A, we can more effectively evaluate how pressure fluctuations might develop and impact the system. If no issues arise in Case C, it may be possible to eliminate the pressurization step from the cargo handling procedure altogether, thus reducing cargo loading time and streamlining the process.

3. Design Basis

This section provides a brief overview of the gas management system (GMS) and proportional–integral (PI) control parameters used in the dynamic simulation. Key factors such as pressure settings within the GMS, tank size, and chemical composition are discussed, as these are critical inputs for the simulation and affect the overall system performance.

3.1. Gas Management System Parameters

In this study, the storage of LCO₂ in the LCO2Cs at pressures below 10 bar a is defined as low pressure [22,23]. Storage at low pressure offers the advantage of thinner cargo tank walls, which in turn reduces the overall weight of the ship, thus benefiting operational expenses (OPEX) [24]. The thickness calculation for the tanks follows Equations (1) and (2) [25]:

$$\mathrm{Thickness} \mathrm{for} \mathrm{Cylindrical} \mathrm{Shell} (\mathrm{mm})={\displaystyle \frac{p·d}{2\sigma ·E+p}}$$

(1)

$$\mathrm{Thickness} \mathrm{for} \mathrm{Ellipsoidal} \mathrm{Heads} (\mathrm{mm})={\displaystyle \frac{p·d_h}{2\sigma ·E-0.2p}}$$

(2)

where $p$ is the design pressure (bar), $d$ is the internal diameter of the tank (mm), $\sigma$ is the allowable stress of the material (N/mm²), $d_h$ is the head diameter (mm), and $E$ is the efficiency of the weld (typically between 0.7 and 1).

Reducing the ship’s overall weight has a direct impact on its fuel consumption, which is a major component of OPEX. Lighter ships require less energy to maintain speed, and this relationship can be expressed as Equation (3):

$$P=C_f·V^3·D$$

(3)

where $P$ is the power required to propel the ship, $C_f$ is a coefficient based on the ship’s design and resistance, $V$ is the ship’s speed, and $D$ is the displacement representing the ship’s weight.

A reduction in displacement results in a lower power requirement for propulsion. As the power demand decreases, fuel consumption is also reduced, which directly impacts operational expenses since fuel costs represent a significant portion of OPEX.

However, it is not without drawbacks. The storage temperature drops significantly compared with medium-pressure storage, which is typically around −46 to 50 °C [26,27,28]. Furthermore, the operating pressure approaches 7.8 bar a, while the triple point of CO₂ occurs at 5.18 bar a. Comprehensive details on the triple point of CO₂ are provided in Figure 3.

If either the pressure or temperature falls below this point, there is a risk of dry ice formation, which can create hazardous conditions. This study addresses these challenges by designing a GMS that considers these factors; the operational settings are outlined in Figure 4 and

Table 2. Alarm set points for gas management system operational parameters.

Pressure (bar a)	Alarm Type	Action
9.8	-	Operation of Safety Valve
9.2	PAHH *	Vent Mode
8.5	PAH *	Alarm for cargo vaporizer operation stop
7.0	PAL *	Alarm for cargo vaporizer operation start
6.2	PALL *	Stop all equipment and close all ESDs

* PAHH: pressure alarm high–high, PAH: pressure alarm high, PAL: pressure alarm low, PALL: pressure alarm low–low.

.

The simulation incorporates a horizontal IMO Type C tank, featuring a length of 2.8 m and an inner diameter of 1.4 m. The tank, with a total volume of 5 cubic meters, is specifically designed for the safe storage of LCO₂.

In this study, the chemical composition presented in

Table 3. Chemical composition used in the HYSYS simulation.

Component	mol.%	Component	mol.%
H2O	0.000001	N2	0.0002
O2	0.0003	CO2	0.999499

was selected based on the range of gases typically available from industrial suppliers. This composition was used in the HYSYS simulation to ensure that the analysis reflects practical conditions and aligns with the industry’s real-world gas supply capabilities.

3.2. PI Controls

In this study, PI control is chosen over PID to prevent the kick phenomenon during the control process [30]. The proportional–integral (PI) controller is commonly used to reduce steady-state errors in control systems by integrating the error over time, which leads to more precise control of the process variables. Although advanced techniques like model predictive control (MPC) offer greater adaptability, PI controllers are still predominantly used on ships due to their simplicity and reliability in marine applications. In HYSYS, the PI controller adjusts the output to minimize the difference between the setpoint (SP) and the process variable (PV) [31,32]. The P and I values in this study were determined empirically to prevent overly rapid responses, as such adjustments could lead to instability by causing control valves with high flow coefficients to vent excessive flow, compromising system stability. These tuned values were applied in HYSYS, where dynamic simulations were conducted to confirm that the control behavior remained stable without adverse fluctuations.

The behavior of the PI controller is mathematically represented by combining the proportional and integral actions in the following form of Equations (4) and (5):

$$u\left(t\right)=k_P·e\left(t\right)+k_I·{\int }_0^{t}e\left(\tau \right)d\tau$$

(4)

$$e\left(t\right)=SP-PV$$

(5)

Here, $u\left(t\right)$ is the control signal, $k_P$ is the proportional gain, $k_I$ is the integral gain, and $e\left(t\right)$ is the error between the setpoint ($SP$) and the process variable ($PV$). The proportional term $k_P·e\left(t\right)$ responds to the current error, as shown in Equation (6):

$$u_P\left(t\right)=k_P·e\left(t\right)$$

(6)

while the integral term $k_I·{\int }_0^{t}e\left(\tau \right)d\tau$ accumulates past errors over time, eliminating steady-state errors, as represented in Equation (7):

$$u_I\left(t\right)=k_I·{\int }_0^{t}e\left(\tau \right)d\tau$$

(7)

By empirically adjusting $k_P$ and $k_I$ based on real-time process feedback in HYSYS, the controller’s performance can be optimized without the need for complex modeling, ensuring flexibility in tuning.

4. Simulation Results

This simulation focuses on evaluating two distinct control strategies for managing cargo tank pressure during LCO₂ loading operations, as well as examining the necessity of pressurization.

The first control strategy involves both valves operating in a control mode, ensuring precise management of the cargo tank pressure under pressurized conditions. This approach utilizes both valves to stabilize pressure during the LCO₂ loading process.

The second control strategy configures one valve as a remote pressure-reducing valve while the other operates in a control mode. This method is applied under pressurized conditions and is designed to manage pressure transitions effectively by leveraging the different functionalities of each valve.

Additionally, the simulation investigates whether pressurization is required for optimal pressure control during LCO₂ loading or if it can be omitted without compromising the stability of the system.

4.1. Case A: Performance Evaluation of Two-Valve Control Mode

During the LCO₂ loading process, the pressure downstream of the loading valve (LLV) fluctuates between 5.29 bar a and 8.43 bar a, as shown in Figure 5a. Although this remains within the operational limits of the GMS, the minimum pressure momentarily approaches the triple point. This phenomenon is attributed to the opening of the vapor return valve (VRV) as it attempts to prevent an initial pressure rise, leading to an increased outflow from the tank. The system responds to these fluctuations to maintain stability, but the momentary drop in the LCO₂ pressure raises potential risks related to the formation of dry ice in the piping.

As the LCO₂ loading process begins, the LLV opens, and the LCO₂ in the pipeline is exposed to the heat from the warm pipe, which has been influenced by the ambient temperature, as shown in Figure 5b. This causes a small amount of LCO₂ to evaporate, leading to a rapid pressure spike, which causes the valve to shut abruptly. After the valve closes, the pressure begins to drop quickly, and the valve reopens. As the valve adjusts the pressure to match the flow rate, the supply pressure decreases from 26 bar a to 7.8 bar a, and the Joule–Thomson effect causes a significant temperature drop [33]. Due to sudden fluctuations in the flow rate, the temperature can, in some cases, fall as low as −90.45 °C. Based on FAT results and actual experiments, this rapid pressure drop does not immediately result in dry ice formation, but small granules of dry ice may form and be carried into the tank. If this condition persists, there is a risk of blockages in the valves. As time goes on and the valve operation stabilizes, the system eventually settles at the target pressure and temperature of −46.8 °C. Figure 5 illustrates the pressure and temperature behavior downstream of the LLV.

As the flow rate through the VRV increases, the pressure rises slightly above atmospheric levels before stabilizing as the pressure inside the cargo tank reaches equilibrium, as shown in Figure 6a. During the initial phase, fluctuations in the outflow rate become pronounced, causing the vapor temperature to abruptly drop below −100 °C; this is similar to what occurs during the initial loading pipelines, as shown in Figure 6b. Under these conditions, there is a strong possibility that granular dry ice particles may form and be expelled from the tank. Considering the practical experimental setup, the pressure was lowered to atmospheric levels, as vapor was vented to the atmosphere rather than being maintained at the higher pressure required by the terminal. This adjustment was made to reflect the conditions of the actual experiment. However, given the loading conditions, there is a definite possibility that the pressure could drop in this manner. With regard to the evaluation of the behavior of the pressure safety valves in the cargo tanks, this setup holds significant value.

Regarding the liquid volume in the tank, the LLV allows a higher-than-expected inflow, resulting in the tank reaching 2.25 m³ within less than four hours—faster than the planned rate. The initial plan estimated a volume increase of 0.5 m³ per hour, but as the vapor return flow through the VRV increased, the overall supply flow rate also rose, accelerating the loading process. This faster loading rate can be attributed to the increased vapor return, which inadvertently boosts the inflow of LCO₂.

Figure 6 illustrates the vapor pressure and temperature profiles in response to VRV operation during the vapor’s return to the atmosphere, while Figure 7 shows the volume of LCO₂ accumulated in the cargo tank during the loading process.

4.2. Case B: Performance Evaluation of Remote Pressure-Reducing and Control Valve Modes

Unlike Case A, where the valve’s initial movements caused fluctuations in temperature and pressure, the LCO₂ loading process from a tank lorry is significantly more stable. The fixed Cv value of the valve allows the LCO₂, reduced to 7.8 bar a, to be supplied steadily and without significant fluctuations in the flow rate, ensuring a stable transfer into the cargo tank. Throughout the loading process, there are no major variations in pressure and temperature, indicating a more controlled operation compared to Case A.

While Case A experienced some instability due to the initial movements of the LLV, resulting in temperature and pressure fluctuations, the steady flow of LCO₂ into the tank in this scenario reduces such variances. The vapor entering the cargo tank is regulated through the vapor return valve (VRV), maintaining a controlled environment within the cargo tank. The overall process shows minimal fluctuation, ensuring efficient and stable loading conditions. Figure 8 shows the pressure and temperature of the LCO₂ supplied to the cargo tank during the loading process.

The loading process proceeded smoothly, and it was confirmed that the pressure released to the atmosphere remained stable, with a consistent flow rate. During the initial loading phase, there were no significant pressure differences, although the initial release of CO₂ vapor caused a slight increase in pressure. However, after more than two hours, the pressure stabilized with minimal fluctuations. Similarly, the temperature inside the cargo tank remained steady throughout the process, with almost no variation, confirming stable conditions within the tank. Figure 9 presents the temperature and pressure of the vapor released into the atmosphere.

As vapor was stably vented into the atmosphere, the volume of LCO₂ accumulated in the cargo tank increased steadily, following a linear pattern. While the loading process was slower than in Case A, the inflow rate closely matched the planned rate of 0.5 m³ per hour, accounting for the amount vented. After five hours of loading, the volume in the tank reached 2.25 m³, which was consistent with the initial loading plan. Figure 10 presents these observations and highlights the stable accumulation of LCO₂ during the loading process.

4.3. Case C: Performance Evaluation of Direct Loading Without Pressurization

Considering the significant fluctuations in LCO₂ flow rate, pressure, and temperature observed in Case A, the loading process for this scenario began with the LLV pre-set to the opening ratio required for the planned flow rate. This approach was aimed at reducing the need for pressurization, giving Case C a favorable condition compared with Case A. By simplifying the cargo loading procedure and eliminating the pressurization step, the overall loading time could be shortened. As a result, the temperature and pressure remained relatively stable compared with Case A, though the pressure still increased to 8.62 bar a. While this was within the manageable range of the GMS, the pressure rise point showed more instability compared with Case A. On the low-pressure side, the pressure dropped to 6.92 bar a, staying well away from the triple point. Similarly, the temperature remained within the controllable limits of the GMS, staying safely above the triple point. It is important to note that setting the LLV to the planned flow rate of 0.5 m³ from the start contributed to a more stable process compared with Case A. Figure 11 outlines the corresponding pressure and temperature profiles during this process.

During the initial loading phase, there were significant fluctuations in pressure, accompanied by an increase in the amount of vapor vented to the atmosphere, leading to unstable pressure variations. From a heat perspective, the temperature dropped to as low as −73.10 °C, falling below the triple point for an extended period. This suggests a high likelihood of freezing occurring at the vapor discharge point, highlighting the need for pressurization to ensure stable cargo loading. In Case A, despite starting under less favorable conditions—such as opening the valve from a 0% opening ratio—this issue of the temperature dropping below the triple point did not occur, making it an important consideration for the current cargo handling procedure in Case C. Figure 12 presents the corresponding pressure and temperature profiles during this phase and illustrates these fluctuations.

As a large amount of vapor was vented to the atmosphere to build pressure in the cargo tank, this case exhibited the slowest loading rate among the proposed scenarios. This demonstrates that, even when the pressurization process is skipped, a significant amount of LCO₂ is lost as vapor during the pressurization of the cargo tank, indicating the need for a controlled pressurization step. Figure 13 illustrates this point.

5. Discussion

The simulation results highlight the critical role of control strategies in managing the pressure and temperature during low-pressure LCO₂ cargo loading. Among the three scenarios examined, Case B, which used a remote pressure-reducing valve and a control valve, provided the most stable results, avoiding the risks associated with dry ice formation and pressure fluctuations that can arise when nearing the CO₂ triple point.

Although actual experiments were conducted, the results cannot be disclosed due to the confidentiality agreement. However, it is important to note that the experimental results closely aligned with the simulation findings. The validation of Case B through real-world applications further strengthens its reliability as a preferred control strategy. The equipment setup used in these experiments is illustrated in Figure 14 below.

The key performance metrics for each control strategy are summarized in

Table 4. Summary of key metrics for different control strategies.

Parameters		Case A	Case B	Case C
Loading	Minimum Temperature [°C]	−90.45	−48.68	−49.36
	Average Temperature [°C]	−46.89	−46.79	−46.75
	Minimum Pressure [bar a]	5.29	7.25	6.91
	Average Pressure [bar a]	7.77	7.79	7.80
	Likelihood of Dry Ice	Medium	Low	Low
Venting	Total Time Below Triple Point to 2.25 m3 [S]	12,082	18,360	18,780
	Minimum Temperature [°C]	−136.27	−59.09	−73.09
	Likelihood of Dry Ice	High	Low	High
Loading	Time to 2.25 m3 [S]	12,506	18,360	22,823

below. This comparison highlights the differences in temperature control, dry ice formation risk, and overall stability, providing a clear overview of the effectiveness of each approach.

• Case A: While operationally feasible, it showed instability at the initial stages due to the significant difference in flow coefficients between the LLV and the VRV. This led to temperature drops below the CO₂ triple point, increasing the likelihood of the formation of sand-sized dry ice particles, which pose risks to the integrity of the system. Although the system stabilized over time, these initial fluctuations make this method less reliable, and the potential formation of dry ice particles means that it is not recommended for long-term application.
• Case B: This configuration maintained a steady flow and stable pressure without approaching the triple point and thus prevented the formation of dry ice. This stability makes it a more reliable and safer choice for LCO₂ cargo loading, especially given the safety concerns associated with CO₂ handling under low-pressure conditions. The target was to load up to 2.25 m³ within approximately 5 h, and this configuration showed results that closely matched this objective. The similarity between the simulation results and actual experimental data further validates this control method.
• Case C: Operating without the pressurization step resulted in significant instability. The temperature remained below the triple point for an extended period, increasing the risk of dry ice formation, which could block valves or damage critical components. The pressure fluctuations were also considerable, suggesting that operating without pressurization introduces substantial risks. These results clearly show that pressurization is essential for maintaining safe and efficient operations during LCO₂ loading, as it ensures stable operation conditions within the cargo tanks and prevents hazardous situations such as valve blockages.

Based on these findings, it is evident that pressurization should remain a key part of the cargo handling procedure for low-pressure LCO₂ carriers, as it ensures both the safety of the vessel and the stability of the cargo. The approach used in Case B, with its ability to avoid reaching the triple point and maintain operational stability, is the most recommended method for practical implementation, especially in large-scale LCO₂ transport operations.

In addition, adopting this strategy could significantly reduce operational risks and enhance the reliability of LCO₂ loading systems, making it a robust solution for emerging carbon capture and storage applications. Future research should focus on ensuring the overall stability and efficiency of the cargo handling system used across LCO₂ carriers. This includes exploring empirical testing under varying operational conditions and investigating alternative control techniques that could further improve energy efficiency and reduce costs in these operations.

6. Patents

The patent for the mechanical configuration and control methodology is currently in the process of being registered with the Korean Intellectual Property Office. The patent applicant is Hanwha Ocean, and the patent developer is Soon-kyu Hwang.