1.  Introduction

In recent decades, there has been a substantial increase in the need for natural gas, primarily driven by its recognized environmental benefits, energy efficiency, and versatility as a fuel source. To accommodate this escalating demand, there has been a parallel expansion in the development of a resilient transportation infrastructure, facilitating the movement of natural gas across vast geographical expanses. There are some methods of transporting natural gas like pipelines, compressed natural gas (CNG), liquified natural gas (LNG), and the conversion of gas to liquids (GTL). Nonetheless, the constraints and issues linked to these conventional approaches have encouraged the exploration of substitute technologies [1-11]. Natural gas hydrates (NGH), which are firm crystalline substances comprising water and gas molecules, have attracted interest due to their capacity to potentially transform the natural gas transportation landscape [12-15]. Natural gas hydrates provide an effective method for the long-distance transportation of gas. Their notable energy density establishes them as a favorable choice for the storage and conveyance of gas. Gas hydrates, sometimes referred to as clathrate hydrates, are structured like ice crystals, arising from the entrapment of gas molecules within water molecule cages. This phenomenon materializes at low temperatures and high pressures, facilitated by hydrogen bonding among water molecules. These molecular cages can accommodate suitably sized molecules [16-18]. Lately, researchers have been exploring practical uses of gas hydrates in diverse fields, including the storage and conveyance of natural gas [19,20], desalination of saline water [21,22], greenhouse gas sequestration [23,24], serving as a cold storage medium [25,26], and concentrating juices and coffee [27,28]. Nevertheless, the extended initiation time, sluggish growth of hydrate formation, and the substantial pressure requirements pose significant obstacles, hindering the widespread production of gas hydrates. One effective approach to surmounting these challenges is the application of kinetic and thermodynamic enhancers [29-37]. Javanmardi et al. (2005) conducted a study to determine the overall capital investment, operational expenses, maintenance costs, and the complete expenses associated with producing natural gas hydrates. Their research delved into how various operational factors influenced the economic aspects of transporting natural gas hydrate from Asaluyeh port in southern Iran to diverse gas markets [12]. Zhang et al. studied the impact of sodium dodecyl sulfate on methane hydrate formation within a nonstirred batch reactor. Their findings revealed that the inclusion of SDS effectively decreases the induction time. However, no consistent pattern emerged between induction times and varying SDS concentrations [38]. Liu et al. conducted a comparative analysis to assess the kinetic promotion effects of sodium dodecyl sulfate and L-methionine (L-Met) on CO₂ hydrate formation. The results from their experiments demonstrated that L-Met significantly enhances the formation of CO₂ hydrates, with a gas uptake in CO₂ hydrate formation being five times higher compared to SDS at equivalent concentrations [39]. In a study conducted by Sun et al., a comparison was made between the effectiveness of anionic sodium dodecyl sulfate (SDS) and dodecyl polysaccharide glycoside (DPG) in promoting CH₄ hydrate formation. The results indicated that SDS outperformed DPG in terms of both the hydrate formation rate and storage capacity [40]. In another investigation by Ugur Karaaslan et al., the promotion efficiency of different surfactants on natural gas hydrate formation was examined. The findings revealed that the anionic surfactant linear alkyl benzene sulfonic acid exhibited superior promotion efficiency compared to the quaternary ammonium salt and nonionic nonylphenol ethoxalate [41]. In 202, Mohammadi et al. conducted a study to explore the impact of SDS and tetra n-butylammonium fluoride on the kinetic parameters of methane hydrate formation. Their findings indicated that when SDS and tetra n-butylammonium fluoride were used simultaneously, it had an adverse effect on the kinetics of hydrate formation, as opposed to the results observed with TBAF and SDS solutions [25]. Researchers also employed nano-fluids characterized by high thermal conductivity to enhance the process of gas hydrate formation [29,42-46]. In a study conducted by Arjang et al., silver nanoparticles were utilized to enhance methane hydrate formation at pressures of 4.7 and 5.7 MPa. The results demonstrated that compared to pure deionized water, the presence of nano-silver led to a reduction in the induction time by 85% and 73.9%, respectively, at the aforementioned pressures. In addition, the gas consumption was improved by 33.7% and 7.4% under the same conditions [44].

The hydrate volume fraction (HVF) is a kinetic parameter that represents the volume fraction of formed hydrate in an aqueous solution. This parameter has garnered limited attention from researchers. The aim of this study is to examine the impact of different concentrations (350 and 700 ppm) of SDS on the hydrate volume fraction during the process of methane hydrate formation.

2. Experimental

2.1. Materials

In the hydrate formation experiments, we employed double-distilled water as the primary solvent. The methane gas utilized in these tests was sourced with a 99.95% purity level and was procured from Kavian Gas Company. We acquired SDS, with a purity of 98 wt%, from Sigma-Aldrich Merck Company. The chemical structure of SDS is depicted in Fig 1.

Fig 1.SDS chemical structure

The methane gas was stored in a 50-liter container, initially pressurized at 16.5 MPa.

2.2. Apparatus and Procedure

To facilitate our experiments, we employed a jacketed stainless-steel reactor constructed from SS-316, boasting a 169 cm³ internal capacity and capable of withstanding an operational pressure of 200 bar. Within this reactor's inner chamber, four valves were installed; each rated at a pressure tolerance of 6000 psi. Two of these valves were ball valves, serving the purpose of injecting the solution and discharging the water-gas mixture post-experiment.

The remaining two valves were needle valves, one facilitating gas injection, and the other serving as a connection point to a gas chromatograph device for gas sampling. We integrated two ports into the reactor's outer wall. These ports allowed for the entry and exit of a cooling fluid, thus affording precise temperature control within the reactor. Our choice for the coolant was a 30% weight solution of ethylene glycol. To minimize energy loss, we took measures to insulate the reactor, connections, and fluid transfer pipes effectively. To monitor the internal temperature of the reactor, we utilized a Pt-100 platinum temperature sensor with an accuracy level of ±0.1 K. For reservoir pressure measurements, we employed a BD type pressure sensor with an accuracy of approximately 0.1 MPa. To ensure proper mixing within the primary hydrate formation chamber, an autoclaving stirrer was implemented, while a vacuum pump was utilized to create a vacuum inside the cell. A photo of the hydrate formation apparatus employed in this research is depicted in Fig 2.

Fig 2.Visual representation of the experimental apparatus

3. Results and Discussion

At the commencement of each kinetic hydrate formation experiment, the cell is subjected to a cleansing procedure involving tap water followed by distilled water. Subsequently, the interior of the reactor is evacuated of air by means of a vacuum pump, and 25 cm³ of the solution is introduced into the cell. The reactor's temperature is precisely set at 275.15 Kelvin via a temperature bath. The hydrate-forming gas, in this case, methane, is introduced into the cell until the desired pressure is attained. Following this, the electromotor is activated and adjusted to a speed of 10 rpm.

Over a certain duration, hydrate formation initiates, leading to a decrease in system pressure until it reaches a state of stability where fluctuations within the reactor become minimal (0.05 bar/hr). The quantification of gas uptake is accomplished through the utilization of the Peng-Robinson equation of state [47]. Throughout the hydrate formation process, temperature and system pressure readings are gathered on a computer. The physical interaction between water and gas, in the context of hydrate formation can be described as follows:

Where, M represents the hydration number [48].

Within this formula, the variables θ_S and θ_L denote the fractional occupancy of small (S) and large (L) cages, respectively. The computation of these parameters is carried out in accordance with the principles of Langmuir adsorption theory, as detailed below [48]:

The Langmuir constants for both large and small methane hydrate cavities are presented in Table 1.

The PR EoS is written as follows [47]:

Within Equation (4), the variables "a" and "b" represent the energy and volume characteristics, respectively. The particulars of the PR EoS parameters are outlined below:

Within Equations (5) and (6), "v" represents the molar volume. The expression for the PR EOS parameter is articulated as follows:

Due to variations in molar volumes between the solution and gas hydrate, the gas volume within the cell diminishes as gas hydrate forms and expands. Consequently, we can calculate the instantaneous gas volume within the cell, labeled as V_t, using the subsequent equation [48]:

Where, V_cell and V_s0 denote the cell volume (169 cm³) and the initial volume of the feed solution (25 cm³), respectively. In addition, V_RWt and V_Ht represent the volume of reacted water and the volume of produced hydrate, respectively. The subscript "t" in the equation signifies the time-dependent nature of these parameters.

The calculation for the volume of the converted water is determined using the subsequent equation [48]:

The molar volume of aqueous solution, denoted as , is computed using Equation (11) [48]:

Where, T (temperature) and  are represented in K (Kelvin) and m³/kmol (cubic meters per kilomole), respectively. For structure sI empty hydrate lattice, , , and conversion can be obtained from the following equations [48]:

The hydrate volume fraction, HVF, is evaluated by the following equation [49]:

Table 2 presents the calculated values for water to hydrate conversion in SDS (350 ppm and 700 ppm) solutions. After 1 hour of hydrate growth, the water to hydrate conversion amount were calculated as 12.02%, 82.75%, and 83.73% in pure water, SDS (350 ppm) solution, and SDS (700 ppm) solution, respectively. It is evident that the SDS utilization at concentrations of both 350 ppm and 700 ppm significantly enhances the conversion of water to hydrate. According to research findings, SDS stands out as one of the most effective gas hydrate formation promoters [38-39,50-51]. SDS, as an anionic surfactant, reduces the surface tension of water molecules, leading to an increased gas uptake both in terms of quantity and rate. Fig.s 3 and 4 illustrate the SDS influence on the volume of the remaining solution and the formed of hydrate during methane hydrate growth. As can be seen in these figures as the reaction of hydrate formation proceeds, the volume of unreacted water decreases, and the volume of hydrate increases. As can be found from Figs. 3 and 4, the disparity in molar volume between water and the formed hydrate results in the combined volume of unreacted water and formed hydrate exceeding the initial volume of the injected solution (25 cc). Following 1 hour of hydrate growth in a pure water solution, the volume of unreacted water and formed hydrate were recorded as 22.03 cc and 3.734 cc, respectively. In pure water, the combined volume of unreacted water and formed hydrate reached 25.77 cc.

Fig 3.The volume of remained solution versus time in methane hydrate formation process at P₀ = 7.5 MPa and T = 275.15 K

Fig 4.The volume of formed hydrate vs. time in methane hydrate formation process at P₀ = 7.5 MPa and T = 275.15 K

Conversely, in the presence of SDS at concentrations of 350 ppm and 700 ppm, the respective combined volumes were measured as 30.33 cc and 30.38 cc. It is evident that the SDS utilization significantly increases the combined volume of unreacted water and formed hydrate when compared to the initially injected solution. Table 3 presents the impact of SDS with concentrations of 350 ppm and 700 ppm on the hydrate volume fraction (HVF) during hydrate growth. These findings are also visualized in Fig 5. As depicted in Fig 5 and Table 3, the utilization of both SDS concentrations leads to a significant increase in HVF compared to pure water. Fig 6 displays the calculated HVF values at different times during methane hydrate growth (t= 5, 10, 20, 60, and 200 min) under an initial pressure of 7.5 MPa and a temperature of 275.15 K. As evident from the figure, the use of 350 and 700 ppm SDS significantly enhances HVF within the first hour of hydrate growth.

Fig 5.The amount of hydrate volume fraction vs. time in methane hydrate formation process at P₀ = 7.5 MPa and T = 275.15 K

Fig 6.The effect of SDS on the quantities of hydrate volume fraction at various times of the methane hydrate growth at P₀ = 7.5 MPa and T = 275.15 K

After 20 minutes of hydrate growth, HVF values were calculated as 0.0238 in pure water, 0.8347 in the SDS (350 ppm) solution, and 0.8487 in the SDS (700 ppm) solution. This indicates a noticeable enhancement in HVF when SDS is present. Furthermore, the SDS utilization with concentrations of 350 ppm and 700 ppm resulted in an increase in HVF by 491.2% and 495.7%, respectively, after 1 hour of hydrate growth, compared to pure water.

The SDS utilization reduces the surface tension of water molecules, resulting in an accelerated hydrate formation process. This increase in hydrate formation process leads to a greater quantity of formed hydrate, ultimately enhancing both water to hydrate conversion and HVF.

4. Conclusion

The amount of water to hydrate conversion and hydrate volume fraction were calculated in aqueous solutions of SDS (350 ppm and 700 ppm). The experimental findings indicated that when compared to using only pure water, the inclusion of 350 and 700 ppm SDS led to a significant boost in HVF, with increments of 491.2% and 495.7%, respectively. Utilization of SDS, noticeably, increases the amount of water to hydrate conversion, and the amount of combined volume of unreacted water and formed hydrate during hydrate growth.

Orcid

Abolfazl Mohammadi

0000-0002-0623-4815

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Declarations

Conflict of interest: The authors have no relevant financial or non-financial interests to disclose.

Ethical approval: Not applicable.

Consent to participate: Not applicable.

Consent for publication: Not applicable