An Experimental Investigation of the Hydrate Formation Mechanism in the Throttling of Carbon Dioxide-Containing Trace Moisture

Abstract

Carbon capture, utilization and storage are facilitated through carbon dioxide (CO₂) transport. Pipe transportation is the main method for transporting CO₂. However, hydrate blockages reduce transport efficiency in the pipelines, and the throttling devices are the main location of hydrate blockages. In this paper, the mechanism of hydrate formation in the throttling of CO₂-containing trace moisture was investigated. The throttling device in a pipe was mimicked using a cylindrical orifice plate. The work also studied the effects of moisture content, upstream pressure and upstream temperature on hydrate formation. The results indicate that the Joule–Thomson cooling effect is a key contributor, and promotes the condensation of trace moisture, resulting in the free water necessary for hydrate nucleation. Under the effect of gas flow back-mixing, it is easy for the hydrate to adhere to the inner surface of the pipe behind the orifice plate. When the moisture content in the gas increases from 123 μmol/mol to 1024 μmol/mol, the hydrate induction time decreases from infinity to 792 s. However, the moisture content has no effect on the adhesion strength of the hydrate to the inner surface of the pipe. When the initial upstream pressure increases from 2.0 MPa to 3.5 MPa, the hydrate induction time decreases from infinity to 306 s. When the upstream temperature decreases from 291.15 K to 285.15 K, the hydrate induction time decreases from infinity to 330 s. With the decrease in the initial upstream temperature, the adhesion of hydrate particles to the inner surface of the pipe is promoted. This study provides experimental evidence for the characteristics of hydrate formation in the process of CO₂ throttling.

1. Introduction

Carbon dioxide (CO₂), as a prominent greenhouse gas, plays a pivotal role in the exacerbation of global warming [1,2]. It is predominantly derived from fuel combustion [3,4]. The atmospheric CO₂ concentration in the year of 2024 surpassed 426.57 cm³/m³, with predicted continued upward trends for the foreseeable future [5]. It is necessary to control the concentration of CO₂ in the atmosphere using appropriate Carbon Capture, Utilization and Storage (CCUS) technology to avoid the harmful consequences of global temperature rise [6,7,8]. CCUS technology is one of the most critical technologies for reducing global warming. The transport of captured CO₂ plays a crucial role in the context of CCUS. Pipelines, as an economical and convenient carrier, are the primary way to transport CO₂. Improving gas recovery and reducing global emissions are the main driving factors of CO₂ pipeline construction [9,10]. However, in the CO₂ pipeline, gas hydrates may form and deposit, which may block the pipeline in serious cases [11,12].

Gas hydrates are non-stoichiometric crystalline inclusion compounds with a hydrogen-bonded cage structure of water molecules [13,14,15]. Gas hydrates form in high-pressure, low-temperature conditions, presenting a significant challenge in the field of flow assurance [16,17,18]. Because of hydrate formation and deposition, CO₂ poses a risk of hydrate blockage in pipelines during transport [19,20]. The CO₂ molecules adsorbed in large hydrate cavities can maintain the stability of the hydrate structure [21]. The quadrupole moment of CO₂ in large hydrate cavities has a net average destabilization effect [22]. However, due to potent short-range interactions between CO₂ molecules and water molecules, CO₂ is still a strong hydrate former [23,24]. This means that the formation, accumulation, deposition and blockage of CO₂ hydrates have a more adverse impact on transportation than natural gas hydrates. In the process of gas pipeline transport, the trace moisture in the gas phase is always a cause of concern in the fuel industry within the context of flow assurance [25,26]. It is a non-trivial challenge to ensure that there is no water in the CO₂ gas being transported. Under the confluence of the right temperature, pressure, moisture content and a particular gas composition, there exists the potential for the aforementioned moisture to undergo a phase transition, culminating in condensation from a gaseous state to a liquid state (free water) [27,28].

Free water is necessary for hydrate formation. In the conditions of favorable mass and heat transport, the presence of free water may lead to the formation of hydrate crystals [29]. This phenomenon is more likely to occur within the throttling device in the pipeline. From a thermodynamic point of view, throttling is the effect of pressure drop as a result of fluid temperature change when a liquid or gas flows through a narrow section of a pipe. The Joule–Thomson effect, which characterizes the temperature change experienced by a gas during throttling, is influenced by the specific thermodynamic properties and behavior of the gas [30]. The throttling process is isenthalpic, as the heat does not evolve nor is it absorbed. The process is categorized as an adiabatic process [31].

The J-T valve is the most used pressure valve for the throttling process of CO₂ gas. When a CO₂ stream flows through a J-T valve, the gas expands rapidly. The inevitable result is a drop in flow pressure, and the gas is cooled. Under conditions where the inlet gas temperature is not high enough or the pressure is greatly reduced, the condensation of moisture and the formation of hydrates may occur. Lv et al. used high-pressure circulation to study the hydrate plugging process and the real-time change of particle size [32]. Aman et al. studied the influence of gas velocity, the subcooling degree and other factors on hydrate formation and deposition [33]. Zhang et al. established a prediction model for hydrate deposition blockage by studying the effect of hydrate formation in droplets and liquid films on a tube wall [34]. Research on the hydrate blockage mechanism within CO₂ pipelines is lacking, especially regarding the throttling process. Therefore, investigating the hydrate formation and blockage mechanism in the throttling process of CO₂-containing trace moisture is essential for ensuring safe and economical operation of CCUS technology.

In this work, an experimental investigation was undertaken to elucidate the mechanism of hydrate formation and deposition during the throttling of CO₂-containing trace moisture. A cylindrical orifice plate was used to simulate a throttling device. The formation mechanism of CO₂ hydrate-containing trace moisture was analyzed through picture signal, pressure response, temperature response and hydrate induction time during throttling. In addition, the effects of moisture content, upstream pressure and upstream temperature on the hydrate formation during the throttling were also investigated. That investigation contributes substantively to understanding of the intricate processes governing hydrate formation in CO₂ systems containing trace moisture, filling the gap of experimental research in this field.

2. Materials and Methods

2.1. Materials

The materials used in this study are mainly CO₂ and deionized water. CO₂ with a purity of 99.9 mol% was purchased from Beijing Yong Sheng Gas Technology Co., Ltd. (Beijing, China). The gas composition was determined by Agilent 7890B gas chromatography (Agilent Technologies, Inc., Beijing, China). The deionized water was produced with a Smart-Q15 ultra pure water machine (Shanghai Hitech Instrumens Co., Ltd., Shanghai, China). The resistivity of the deionized water is 18.0 MΩ·cm.

2.2. Experimental Apparatus

The experimental apparatus is shown in Figure 1. The gas cylinder and the reduction valve are used to control the gas flow in the experiment. A flash tank and a water bath are used to generate the moisture. The moisture content of the gas flow is controlled by changing the temperature of the flash tank. The throttling pipe section consists of a front transparent tube, an orifice plate, and a rear transparent tube. Both the front and rear transparent tubes are made of organic plastic. The orifice plate is sealed to the transparent tube by the end face. The heat exchanger coil is placed in the air bath to adjust the upstream temperature of the gas in the throttle. The back pressure valve is used to adjust the pressure together with the reduction valve. The heating water bath is used in the back pressure valve to prevent hydrate formation. The paperless recorder has a recording frequency of 60 Hz. The temperature platinum resistor is OMEGA Pt100 (Omega Measurement Technology (Shanghai) Co., Ltd., Shanghai, China) with a measurement accuracy of ±0.02 K. The pressure transmitter is a Senex DG2111-C-20 Class 0.1 pressure transmitter (Guangzhou Senex lnstrument Ltd., Guangzhou, China) with a measurement accuracy of ±0.05 MPa. A SONY FDR-AX45 high-definition digital video camera (Sony Group Corporation, Beijng, China) is placed in the rear transparent tube section to observe and film the experimental phenomenon. The dew point meter and the rotor flow meter are placed behind the back pressure valve.

2.3. Experimental Procedure

After cleaning the flash tank, 50.0 mL of deionized water was injected into the flash tank. The cylinder valve, reduction valve and back pressure valve were slowly opened in sequence. CO₂ was used to purge the pipeline to replace the air. After purging for 20.0 min, the back pressure valve was closed. By adjusting the pressure reducing valve, the gas pressure in the pipeline reached the experimental value. The temperature of the cooling water bath was then set to the experimental value to control the flash tank to generate a certain amount of saturated moisture. The temperature of the air bath was also set to the experimental value to control the gas flow temperature at the inlet of the throttle section. The temperature of the heating water bath was set at 353.15 K to raise the temperature of the back pressure valve. When there was no significant change in the monitored temperature and pressure for 1.0 h, it was indicated that the system had stabilized. DV was activated to record the image signals of the transparent straight pipe section behind the orifice plate. The back pressure valve was opened when the target experimental differential pressure was reached. The data acquisition system recorded the temperature and pressure during the throttling process.

3. Results and Discussion

3.1. The Process of Hydrate Formation in the Throttling of CO₂-Containing Trace Moisture

Figure 2 shows the response of downstream pressure ($P_{\mathrm{r}}$), downstream temperature ($T_{\mathrm{r}}$) and the phenomenon of hydrate formation in the throttling of CO₂-containing trace moisture. The initial experimental conditions were as follows. The initial upstream pressure and downstream pressure of the throttling orifice plate were 2.50 MPa and 1.50 MPa, respectively. The initial upstream temperature of the throttling orifice plate was 289.15 K, and the moisture content was 445 μmol/mol.

It can be observed from the $P_{\mathrm{r}}$ and $T_{\mathrm{r}}$ curves that in the period from 300 s to 1735 s, there is a stable $P_{\mathrm{r}}$ regime without any significant fluctuations, and the $T_{\mathrm{r}}$ gradually decreases. During this time range, the obvious phenomenon of liquefaction of trace moisture in the CO₂ gas occurs, as shown in the figure at 1735 s.

From the 1735th second onwards, the $P_{\mathrm{r}}$ and $T_{\mathrm{r}}$ demonstrate a precipitous decline and exhibit fluctuations. Furthermore, it was observed that the liquid film in the annular direction behind the orifice plate transformed into a hydrate layer, as illustrated in the figure of at 3000 s. This transformation can be attributed to the successive reduction in the outlet temperature of the orifice plate, facilitated by the Joule–Thomson cooling effect, which brought the temperature and pressure conditions into the thermodynamic stable region for hydrate formation. As a result, the temperature and pressure conditions achieved the necessary level of driving force for hydrate formation. The hydrate structures prompted the initiation of nucleation, crystallization and adhesion under the influence of vigorous perturbations. The adherence of hydrate particles to the orifice plate aperture resulted in a reduction in the cross-sectional area available for flow, which in turn led to a subsequent steep decline in downstream pressure of the orifice plate.

Figure 2 at 4000 s provides clear evidence of an increase in the thickness of the hydrate layer. The larger hydrate particles and droplets dislodge from the orifice throat and experience gravitational settling onto the lower wall. Furthermore, the minute hydrate particles and droplets are transported upwards through the hydrate layer on the inner surface of the pipe by the reflux and back-mixing action of the gas flow. The water droplets serve as a medium for interconnecting the solid particles. The solid particles are held together by the sintering action of the connecting water. In considering the formation of a pre-existing hydrate layer on the surrounding annular wall, the hydrate particles demonstrate a heightened propensity for adhesion to the wall [28,29,30]. This results in a continuous increase in the thickness of the hydrate layer. A deceleration in the gas flow rate has a significant impact on the throttling process, which is now subject to heat transfer and cannot be assumed to be adiabatic. This results in an increase in temperature. Conversely, as the rate of gas flow increased, the throttling process was less affected by heat transfer and could be regarded as adiabatic throttling, resulting in a reduction in temperature. Therefore, the oscillatory behavior of temperature fluctuations reflected the degree of blockage in the throat region.

Once the hydrate layer reaches a specific thickness, the cohesive forces within the layer are no longer sufficient to resist the shearing forces exerted by the gas flow. Consequently, the partial hydrate undergoes fragmentation, resulting in its detachment from the hydrate layer surface. Subsequently, the fragmented hydrate is subject to the force of gravity, resulting in its transfer and deposition onto the inner surface of the downstream pipe. The gradual accumulation of deposits within the pipeline ultimately impedes the flow of substances through the pipe. The progressive accumulation process is illustrated in the image, as shown in Figure 2 at 5000 s.

Based on the above analysis, the conceptual model depicted in Figure 3 provides a comprehensive description of the hydrate formation and deposition mechanism in throttling of CO₂-containing trace moisture. The formation and deposition of hydrates in the throttling of CO₂-containing trace moisture can be divided into four stages. The initial phase is the phase transition from a moisture state to a free water state. The second stage is the phase transition from free water to hydrate. The third stage is the adhesion and deposition of hydrate particles to the inner surface of the pipe. The fourth stage is the exfoliation and deposition of the hydrate.

3.2. Effect of Moisture Content on Hydrate Formation in Throttling

This section examines the impact of CO₂ moisture content on the hydrate formation during throttling. The differing flash temperatures result in the CO₂ flow carrying varying quantities of moisture (1024 μmol/mol, 886 μmol/mol, 844 μmol/mol, 445 μmol/mol, 123 μmol/mol). The flow state within the pipe was evaluated through the observation of pressure and temperature responses. The initial experimental conditions were as follows. The initial upstream pressure and downstream pressure were 2.5 MPa and 1.5 MPa, respectively. The initial upstream temperature was 289.15 K.

Figure 4a illustrates the throttling characteristics of CO₂ with varying moisture content. The results of the five groups of experiments indicate that the $P_{\mathrm{r}}$ is stable at a moisture content of 123 μmol/mol, which suggests that there is no hydrate formation within the pipe. However, at moisture contents of 445 μmol/mol, 844 μmol/mol, 886 μmol/mol, and 1024 μmol/mol, the $P_{\mathrm{r}}$ exhibits pronounced cyclical fluctuations following a period of stable flow. The pressure fluctuations are attributed to the periodic adhesion and detachment of hydrate particles within the flow channel. Additionally, it was observed that the pressure exhibited fluctuation between 0.1 MPa and 1.0 MPa, with no discernible variation in the amplitude of these pressure fluctuations, as illustrated in Figure 4b.

Figure 5 illustrates the induction time of the hydrate under varying moisture content conditions. As the moisture content in the gas increases from 123 μmol/mol to 1024 μmol/mol, the hydrate induction time decreases from infinity to 792 s. A linear relationship is observed between the hydrate induction time and moisture content. Once a critical moisture content is reached, the moisture is liquefied. The liquefied moisture provides the necessary free water for the nucleation and growth of hydrate crystals, which ultimately leads to the formation and deposition of hydrate, resulting in pipeline blockage. From a thermodynamics perspective, the formation of hydrate at the gas–hydrate interface is possible in the absence of free water, albeit at a significantly prolonged rate [35]. However, in a gas pipeline system where a gas–hydrate interface is absent and the concentration of water in the gas phase is insufficient to meet the thermodynamic conditions required for the liquefaction of moisture, as exemplified by the 123 μmol/mol system, the experimental observations demonstrated that there was no liquefaction of the gaseous water. It is exceedingly difficult for water in the gaseous phase to form hydrates, which can lead to significant flow safety issues.

Furthermore, the temperature response illustrated in Figure 6 demonstrates that the downstream temperature also exhibits a corresponding fluctuation subsequent to the formation of hydrate in these four experimental conditions. The temperature fluctuation occurred concurrently with the onset of pressure fluctuations. Upon adhesion of hydrate particles to the orifice plate’s throat, a discernible reduction in the CO₂ flow is observed. It is not possible to categorize the throttling process as adiabatic throttling, given that the temperature of the CO₂ is significantly affected by the heat transfer within the pipe. An important turning point can be identified within the declining temperature trajectory. This inflection point is directly correlated with the moisture content, with higher moisture content precipitating an earlier occurrence of this pivotal juncture. The temperature response curve subsequently undergoes periodic fluctuations, oscillating within the narrow range of 285.9 K to 286.7 K. It is noteworthy that the observed temperature oscillations in the hydrate formation process of CO₂ throttling exhibit no overt dissimilarity across various moisture content scenarios.

In light of the preceding analysis concerning variations in pressure and temperature response, the findings indicated that the sole effect of moisture content on hydrate formation is to prolong the induction time, while the adhesion strength of hydrate particles to the inner surface of the pipe remains unaffected. This is primarily due to the fact that, when the throttle differential pressure and upstream temperature remain constant, the temperature and pressure distribution in the pipe during the throttling process are inherently determined. Consequently, disparate moisture content directly affects the feasibility of moisture liquefaction and the respective time required for liquefaction, as evidenced by the distinct induction times of hydrate formation. In circumstances where equivalent subcooling degrees prevail, the thermodynamic driving force for hydrate formation remains constant. Therefore, the degree of adhesion of hydrate particles to the inner surface of the pipe is uniform, resulting in identical pressure fluctuation amplitudes. These findings highlight the necessity of specific moisture content conditions for the formation and deposition of hydrates in gas-dominated throttling processes devoid of liquid water.

3.3. Effect of Initial Upstream Pressure on Hydrate Formation in Throttling

In order to investigate the effect of initial upstream pressure variations on the hydrate formation of hydrates during the throttling process, a controlled experimental framework was employed, maintaining a set of constant parameters. These encompassed an upstream temperature of 289.15 K, a differential pressure of 1.0 MPa, and the moisture content of 1024 μmol/mol. The initial upstream pressures ($P_{\mathrm{r}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$) of CO₂ entering the throttle orifice plate are 3.5 MPa, 3.0 MPa, 2.5 MPa and 2.0 MPa, respectively. A comprehensive analysis was conducted to elucidate the effects of upstream pressures on the pressure and temperature responses.

Figure 7a illustrates the temporal evolution of downstream pressure ($P_{\mathrm{r}}$) during the throttling of CO₂ in varying $P_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ conditions. In the four experimental groups, the P_r remained stable at the $P_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ of 2.0 MPa, indicating that there is no hydrate formation occurring within the pipe. However, at the $P_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ of 2.5 MPa, 3.0 MPa, and 3.5 MPa, the $P_{\mathrm{r}}$ demonstrates pronounced cyclical fluctuations following a period of uninterrupted flow, indicative of hydrate blockage within the pipe. As illustrated in Figure 7b, when the $P_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ is increased to 2.5 MPa, the $P_{\mathrm{r}}$ curve displays fluctuations within the range of 0.1 MPa to 0.9 MPa. These fluctuations are distinguished by relatively low pressures accompanied by a wide amplitude. The primary factor influencing this behavior is the adhesion strength of hydrate particles to the pipe wall. For a $P_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ of 3.0 MPa, the $P_{\mathrm{r}}$ oscillation curve displays fluctuations within the range of 0.3 MPa to 1.1 MPa. In this scenario, the pressure fluctuations are more stable and have a narrower amplitude. The adhesion strength of the hydrate particles on the pipe wall is in equilibrium with the thrust force generated by the pressure difference, thereby influencing the observed pressure behavior. With a $P_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ of 3.5 MPa, the $P_{\mathrm{r}}$ curve exhibits fluctuations ranging from 0.2 MPa to 2.1 MPa. These pressure fluctuations are typified by elevated pressures and a broad amplitude. In this instance, the primary factor influencing the pressure fluctuations in this case is the thrust generated by the pressure differential across the orifice plate. In conclusion, the discrepancy in fluctuation amplitude within the pressure curve can be attributed to the interaction between the adhesion force of hydrate particles and the pressure difference.

Figure 8 illustrates the induction time of hydrate under varying $P_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ conditions. As the $P_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ increases from 2.0 MPa to 3.5 MPa, the induction time of hydrate decreases from infinity to 306 s. The timing of pressure response is observed to decrease when the upstream pressures decrease. When the initial upstream pressure was set at 2.0 MPa, no hydrate formation was observed throughout the throttling process. As the initial upstream pressure is increased to 2.5 MPa, 3.0 MPa and 3.5 MPa, respectively, the corresponding hydrate induction times are 792 s, 320 s and 306 s, respectively. This suggests that there is an inverse relationship between the hydrate induction time and the upstream pressure. This phenomenon can be attributed to the acceleration of moisture liquefaction under higher upstream pressures, which causes the critical pressure conditions for hydrate formation to be reached at an earlier stage.

Figure 9 illustrates the temporal evolution of the downstream temperature. When the initial upstream pressure is set at 3.5 MPa, the temperature undergoes a continuous and substantial decrease from 289.15 K to 278.80 K, indicating a significant temperature difference of 10.35 K. For an initial upstream pressure of 3.0 MPa, the temperature descends persistently from 289.15 K to 284.35 K, demonstrating a temperature difference of 4.8 K. For an initial upstream pressure of 2.5 MPa, the temperature then declines gradually from 289.05 K to 286.65 K, with a relatively modest differential temperature of 2.4 K. It is evident that the upstream pressure has a discernible influence on the magnitude of the downstream temperature reduction. This discrepancy in temperature reduction can be attributed to variations in the Joule–Thomson coefficients exhibited under distinct upstream pressure conditions. It can be observed that the higher the $P_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$, the more pronounced the observed downstream temperature decrease, which reflects the disparate thermodynamic responses influenced by the distinct Joule–Thomson coefficients.

The preceding analyses, which encompassed alterations in pressure and temperature response, demonstrate that variations in upstream pressure exert a discernible influence on both the initiation time of pressure perturbations and their amplitude. In essence, the upstream pressure exerts a dual impact, influencing both the onset of hydrate nucleation and the tenacity of hydrate particle adhesion. Consequently, divergent upstream pressures directly impact upon the viability of moisture liquefaction and the temporal requirements for the liquefaction process, resulting in disparate induction periods for hydrate formation. An increase in initial upstream pressure results in a reduction in hydrate induction time. Moreover, the intricate interplay between the adhesion strength of hydrate particles and the flow shear force generated by differential pressure gives rise to distinct pressure fluctuation characteristics during throttling.

3.4. Effect of Initial Upstream Temperature on Hydrate Formation in Throttling

Conducting a comprehensive investigation into the impact of initial upstream temperature ($T_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$) on pressure and temperature response during the throttling process of CO₂-containing trace moisture is of significant importance. This part of the study aims to elucidate the effectiveness of pre-heating techniques applied to the flow stream for mitigating hydrate blockage issues in throttling components. Four discrete sets of $T_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ (285.15 K, 287.15 K, 289.15 K, 291.15 K) were meticulously implemented via a heat exchange coil. The remaining initial experimental conditions were as follows. The initial upstream pressure and downstream pressure were fixed at 2.5 MPa and 1.5 MPa, respectively. The moisture content was maintained at 1024 μmol/mol.

Figure 10a illustrates the temporal evolution of pressure during the throttling process of CO₂ containing trace moisture. At a $T_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ of 291.15 K, the downstream pressure demonstrates stable behavior, indicating a steady flow of CO₂ gas within the pipe. This stability can be attributed to the fact that the temperature conditions along the pipe remain outside the region in which hydrate formation is possible, thereby eliminating the risk of hydrate blockage in the view of thermodynamics. When the $T_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ was set at 289.15 K, 287.15 K and 285.15 K, the $P_{\mathrm{r}}$ exhibited successive fluctuations, indicating the occurrence of hydrate blockage within the pipe. As illustrated in Figure 10b, the amplitude of pressure fluctuations also differs, with lower $T_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ resulting in greater reductions in pressure magnitude and smaller amplitudes of fluctuation. This behavior can be attributed to the intensified subcooling effect experienced by the low-temperature point within the cold core region during the throttling process at lower upstream temperatures. Consequently, this greater subcooling degree facilitates deeper adhesion of hydrate particles and enhances the pore-blocking effects, thereby leading to more pronounced reductions in pressure, as demonstrated by the data at the temperature of 285.15 K.

Figure 11 illustrates the induction time of the hydrate under varying $T_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ conditions. An increase in the $T_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ from 285.15 K to 289.15 K resulted in a notable prolongation of the hydrate induction time, from 330 s to 792 s. However, the formation of hydrate was not observed at the $T_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ of 291.15 K. This phenomenon can be attributed to the fact that, under a constant throttle differential pressure and moisture content, lower $T_{\mathrm{f}-\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}$ accelerates the liquefaction of moisture, reaching the critical temperature conditions for hydrate formation at an earlier stage.

Figure 12 illustrates the temporal evolution of the temperature difference (ΔT) between the upstream and downstream regions. The temperature difference curves exhibit a distinct separation at 330 s, 401 s and 792 s, which coincides with the onset of pressure fluctuations. Under the upstream temperature condition of 291.15 K without hydrate formation, the temperature difference reaches 4.00 K. At the upstream temperatures of 289.15 K, 287.15 K and 285.15 K, the temperature difference is approximately 2.80 K, 2.20 K and 1.70 K, respectively. The rationale behind these variations can be attributed to the substantial influence of upstream temperature on the adhesive forces governing the interaction of hydrate particles with the inner surface of the pipe. A reduction in the upstream temperature results in a decrease in the temperature within the frigid core region post-throttling, which in turn amplifies the adhesion strength of hydrate particles to the inner wall surface. Therefore, the impedance imposed by these adherent hydrate particles precipitates a pronounced decline in CO₂ flow. The throttling process is more significantly affected by the heat transfer of the pipe, as reflected in the temperature difference curve, which shows a decrease in value. This observation aligns harmoniously with the deductions derived from the analysis of pressure curves.

Based on the above analysis of variations in pressure and temperature response, the results indicate that significant variations in upstream temperature directly lead to different CO₂ temperatures, i.e., different degrees of subcooling. The hydrate formation time is significantly reduced by a high degree of subcooling. Given the different degrees of subcooling, the thermodynamic driving force underpinning the initiation of hydrate formation remains clearly disparate. It is important to underscore that the adhesive forces governing the attachment of hydrate particles to the inner wall of the pipe exhibit a positive correlation with the degree of subcooling [36,37]. Under conditions of increased subcooling, the adhesion of hydrate particles within the orifice plate throat is increased, resulting in a concomitant contraction of the pressure fluctuation range. These empirical findings highlight the paramount importance of fully considering the temperature profile within the non-stable region during the adiabatic throttling process, as a measure to mitigating the detrimental effects of hydrate formation in this operational context.

4. Conclusions

In this study, based on the characterized deposition, pressure response, temperature response and hydrate induction time, we offered a conceptual framework outlining the mechanistic intricacies governing hydrate formation and deposition in the throttling of CO₂-containing trace moisture. The effects of moisture content, upstream pressure and upstream temperature on hydrate formation were also determined. The Joule–Thomson cooling effect emerges as a key contributor, causing a continuous reduction in the outlet temperature within the orifice plate throat. This facilitates the condensation of trace moisture at lower temperatures, providing the free water necessary for the formation of hydrate nucleation. The gas flow vortices cause hydrate deposition on the inner surface of the pipe, particularly at the orifice plate exit point. Increasing the moisture content has been shown to reduce the hydrate induction time, but has no effect on the adhesion strength of hydrate particles to the inner surface of the pipe. Increasing the initial upstream pressure results in a significant reduction in hydrate induction time, and the hydrate adhesion state depends on the competitive relationship between the hydrate adhesion force and the CO₂ flow shear force. The lower the initial upstream temperature, the higher the subcooling at the back of the orifice plate, which shortened the hydrate induction time and increased the hydrate adhesion strength to the inner surface of the pipe. The effects of factors during the throttling process on the hydrate formation are significant in the context of hydrate management in CO₂ transport.