Controlled Production of Natural Gas Hydrates in an Experimental Device with an Internal Circulation Circuit

Abstract

For countries with limited access to conventional hydrocarbon gases, methane hydrates have emerged as a potential energy source. In view of the European Union’s requirements to reduce the energy intensity of technological processes and increase energy security, it appears promising to accumulate natural gas and biomethane in the form of hydrate structures and release them if necessary. Storing gas in this form in an energy-efficient manner creates interest in developing and innovating technologies in this area. Hydrates that form in gas pipelines are generated by a more or less random process and are an undesirable phenomenon in gas transportation. In our case, the process implemented in the proposed experimental device is a controlled process, which can generate hydrates in orders of magnitude shorter times compared to the classical methods of generating natural gas hydrates in autoclaves by saturating water only. The recirculation of gas-saturated water has been shown to be the most significant factor in reducing the energy consumption of natural gas hydrate generation. Not only is the energy intensity of generation reduced, but also its generation time. In this paper, a circuit diagram for an experimental device for natural gas hydrate generation is shown with complete description, principle of operation, and measurement methodology. The natural gas hydrate formation process is analyzed using a mathematical model that correlates well with the measured hydrate formation times. Hydrates may become a current challenge in the future and, once verified, may find applications in various fields of technology or industry.

1. Introduction

The world is witnessing a significant increase in energy demand to meet the ever-increasing industrial production caused by consumerism. Countries that do not have traditional gas or oil reserves are looking for possible alternatives. In view of this, in recent decades there has been a significant development of technologies in the process of energy storage and subsequent implementation when needed [1,2,3]. Foreign experimental studies have shown the real possibility of generating natural gas hydrates, thus creating the possibility of storing this type of primary energy in a different form in which energy storage in the form of natural gas has been used so far [4]. While storage in compressed natural gas (CNG) requires extremely high pressures (25 MPa) and storage in liquefied natural gas (LNG) requires extremely low temperatures (−162), storage in hydrates is considerably less demanding in terms of temperature and pressure. The main advantages of natural gas hydrates are their non-explosive nature and also their volumetric capacity, where they can accumulate 165 m³ of natural gas in 1 m³ [5,6,7]. However, the technology of forming methane hydrates in an energy-efficient manner is a current challenge for the future [8].

The problem of natural gas accumulation in hydrate structures is being addressed by a number of foreign institutions and authors around the world [9,10,11]. Technological devices used for the formation of hydrates are based on the principle of creating contact between the fluid and the gas [12,13]. Currently, we can achieve thermodynamic conditions in the stabilization zone of the hydrate on the basis of feeding gas bubbles into the aqueous phase, injecting water droplets into the gas phase or injecting water into a porous material [14,15].

A study of hydrate formation based on injecting precise amounts of methane into ionized water was undertaken by the Institute of Petroleum and Natural Resources [16]. The experiments were carried out in a reactor under isochoric conditions, where water of known volume was added to the reactor and then the air contained inside the reactor was removed. The reactor was pressurized with methane to a pressure of 70 bar and cooled to a temperature of 2 °C. Mechanical rotary stirring at 300 rpm was considered. The experiment lasted from 13 to 22 h, during which time a pressure drop was recorded with temperature changes. Hydrate formation was considered successful, but the use of the whole system was discouraged by the excessively long hydrate formation time, which was in the order of hours [16].

Foreign studies positively evaluate the injection of water into a gas-filled reactor. The National Energy Technology Laboratory (NETL) has investigated how the use of differently designed nozzles affects the rate of formation [17]. Nozzles are capable of separating liquids into droplets of small size, which causes an increase in the surface area of the water. The increased surface area of water interacting with the gas favors the rapid formation of hydrates. Measurements clearly showed that the nozzle with a full cone spray pattern synthesized methane hydrate in the shortest time of the commonly available industrial nozzles [18].

Among the conceptually new methods of hydrate production is a method based on the boiling liquid process [19,20]. The experimental apparatus consists of a cooled cylindrical reactor filled with water. The main principle is that the pressure in the gas vessel from which the gas is supplied to the experimental device is significantly higher than the pressure in the water-filled reactor. The delivered gas is cooled and condensed to a liquid state, which collects at the bottom or remains in the gaseous state on the surface of the water in the experimental apparatus. We then create decompression by venting the gas. As a result, the liquefied gas at the bottom of the chamber begins to boil explosively and mixes vigorously with the water. This leads to the formation of hydrate nuclei. I see the main disadvantage of this process as the fact that the area of hydrate formation is limited by the presence of the liquefiable gas and the methane in gaseous form is vented outside the experimental apparatus. Furthermore, the decompression process during which the hydrate nuclei are formed takes a few seconds, due to which it is not possible to form a large amount of hydrate [21].

A study by Filarsky et al. focused on hydrate production in an experimental facility constructed in three different conceptual designs. The hydrate formation rate and gas uptake were compared between the different designs. Hydrate formation based on rotary mixing, injection of a water-gas mixture using a nozzle, and a combination of injection and rotary mixing were considered [22,23,24]. In the combination of injection and mixing is evaluated as the most suitable of the three different technical methods of hydrate formation and this is due to the largest gas intake into the hydrate structures. Measurements clearly show that injection led to immediate nucleation of the hydrates and additional mixing disturbed the surface tension of the mixture, which also strongly promoted hydrate formation [25].

To accelerate the production of methane hydrates, mechanical rotary mixing is used in most cases to continuously disturb the water-gas mixture. However, during the hydration phase, the viscosity of the mixture increases and hence the energy required to mix it also increases. It should also be mentioned that mixing in the long term may cause disturbance of the structure of the resulting hydrate [26,27].

A similar study based on the disturbance of the mixture surface area by the reciprocating motion of the impactor was carried out by Xiao et al. [28]. In the reactor of an experimental device, a body is moved which continuously impinges on the hydrated mixture and displaces the excess water in the interspace. The main objective is to accelerate the hydrate formation and reduce the total volume of hydrate by displacing the excess water, which has a positive effect on the storage capacity [28].

The above-mentioned studies confirmed that the process of hydrate formation of hydrocarbon gases is feasible and therefore possible under laboratory conditions. Our proposed experimental setup, was based on the findings and measurements of the published articles. The main difference will lie in the use of a pump, which we will use to create an internal circulation circuit of the water-gas mixture. This unique experimental device concept will need to be further analyzed and finally evaluated to see how significantly the process of forming natural gas hydrates can be accelerated by the proposed modification.

2. Description of the Experimental Device

The hydrate formation process was carried out in two high pressure vessels. The developed prototype experimental device consists of a closed circuit, where the dynamic effect of water atomization in the nozzle is used to produce hydrate. Synthetic production of natural gas hydrates depends on the rapid formation of suitable gas-liquid bonds. For this reason, the method of injecting water droplets into a gaseous substance was chosen. Atomized water moves at a relatively high velocity in a volume of gas compressed to a high pressure (on the order of 1 to 10 MPa), coming into contact with the gas in a turbulent environment, which also causes a disturbance of the surface tension of the liquid in the form of a tiny droplet. The force at the interface of two liquids that do not mix increases until the surface tension cannot hold the mass of water together and droplets of even smaller diameters are formed by the action of the gas. By increasing the external surface area of the water and making it flow in a gas environment, we achieve more intense contact with the gas, which is a critical factor for rapid and favorable hydrate formation. This hydrate generation process is an exothermic reaction with the generation of reaction heat. The heat generated in hydrate formation can be efficiently dissipated in the liquid fraction of water due to its large heat capacity. The process of water-gas interaction causing hydrate formation occurs in two steps. The first step is nucleation to form a new thermodynamic phase. During nucleation, the natural gas spreads in the volume of water. The bound methane and other components of the natural gas crystallize in the aqueous component to form the initial hydrate structures. In a second step, the resulting nuclei increase in size and begin to clump together. Subsequently, the fine particles of the formed hydrate flow continuously into a second vessel, where the process of trapping the final product occurs using a micron sieve to filter the solid particles of hydrate from the water. The mixture of gas and water flows in a closed circuit, which makes it possible, among other things, to reduce the cost of driving the pump. The aim is to focus on a model for the formation of natural gas hydrates by using a circulation pump until the desired hydrate structure is formed. This will increase the interaction of hydrocarbon gas and water molecules, which can significantly accelerate the hydrate formation process. The last technological operation is the process of dehydration and cooling of the hydrate. Excess water is removed to reduce the total volume of the resulting hydrate. The resulting hydrate is dried by a cold stream of natural gas expanded behind the nozzle. The temperature of such gas is also significantly reduced due to the Joule-Thomson effect. Temperature sensing was carried out using NiCr-Ni thermocouples for which the uncertainty is defined: Class 1: ±1.5 °C or ±0.004 x /t/ (−40 to 1000 °C). The pressure was measured by a piezoresistive sensor with an accuracy of 0 …–400 bar ± 0.25% at a temperature range of 0…–+70 °C. Type B uncertainty. Due to the flow of the medium at high pressure, the flow measurement was carried out non-invasively using a Yokogawa ultrasonic flowmeter. The main advantage of this ultrasonic flowmeter is the surface installation directly on the piping system. Ultrasonic flow sensor: US300PM has a defined measurement uncertainty of 1%. Type B uncertainty. Figure 1 shows the experimental setup in schematic form.

The individual reference lines of the technological devices with numerical designations are specified in more detail in the following Table 1.

The plunger pump used in the experiment has an output of 5.5 kW, along with a working pressure adjustable between 40 and 250 bar. The motor has a speed of 1450 rpm. We used a frequency converter to control the motor speed, for the sake of setting the desired flow rate during the experiment. Distilled water and natural gas in transit with 94.61% methane were used in the experiment. The rest of the natural gas consisted of small percentages of various gases such as ethane, propane, isobutane and also carbon dioxide and nitrogen. In Figure 2 we can see the layout of the technological parts connected to each other and forming the circuit of the designed experimental device.

3. Determination of the Hydrate Formation Model

The difference between the chemical potential of the previous and the current phase being evaluated is called the supersaturation, which we refer to as ∆μ. We calculate the supersaturation as: [29]

(1)$\Delta \mu =kT ln\left(\frac{\left(P_r, T\right)P_r}{\left(P_e,T\right)P_e}\right)+\Delta v_e\left(P_r-P_e\right),$

where k is the Boltzmann constant [$\mathrm{J}·{\mathrm{K}}^{-1}$], T is the temperature [K], $P_r$ is the required reactor pressure [MPa], $P_e$ is the equilibrium pressure 2.6 [MPa]. and $\Delta v_e$ is the volume difference −4.4 × ${10}^{-2}$ [$\mathrm{n}{\mathrm{m}}^3$]. The value of supersaturation $\Delta \mu$ after calculation is 3.6 × ${10}^{-21}$ Joule [29].

The process of formation of methane hydrates in the system, water splashing in the gaseous environment is subject to a heterogeneous nucleation process. The kinetic parameter indicates the size of the heterogeneous nucleation particles and is calculated by:

(2)$A_I=Z\epsilon {(4\pi c)}^{\frac{1}{2}} v_h^{\frac{1}{3}} D{C}_{e}n{s}^{\frac{1}{3}}\frac{A_p{C}_p}{a_w},$

where Z is the Zeldovich factor 0.01 [−], $\epsilon$ is the coefficient of adhesion 1 [−], c is the numerical shape factor 4.8 [−], $v_h$ is the unit volume of hydrate 0.216 [$\mathrm{n}{\mathrm{m}}^3$], ns is the growth exponent for a stabilization period 50 [−], $A_p$ is the particle surface area ${10}^4$ [$\mathrm{n}{\mathrm{m}}^2$], $C_p$ is the concentration of active particles ${10}^{15}$ [${\mathrm{m}}^{-3}$] and $a_w$. is the area occupied by the water molecule on the surface of the particle 1.2 × 10⁻⁷ [$\mathrm{n}{\mathrm{m}}^2$]. $C_e$ is the concentration of dissolved gas in water at equilibrium conditions 0.03 [$\mathrm{n}{\mathrm{m}}^{-3}$]. The kinetic parameter $A_I$ describes the mechanism of binding of hydrate units to the hydrate nucleus and after quantification is 4.31 × 10²⁶ [${\mathrm{m}}^{-3}·{\mathrm{s}}^{-1}$] [30].

When water and gas particles mix, diffusion of gas through the surface layer of the liquid occurs. The growth constant that defines the growth of crystals during injection is given by the relationship:

(3)$G_I=\left(\epsilon v_{h}D{C}_e/\delta \right)\left(\exp \frac{\Delta \mu }{kT}-1\right),$

where D is the diffusion coefficient 1000 [$\mathsf{\mu }{\mathrm{m}}^2·{\mathrm{s}}^{-1}$] and δ is the thickness of the immobile layer on the crystal surface 10 [$\mathsf{\mu }\mathrm{m}$]. After quantification, the growth constant has the value $G_I$ = $1.28\times {10}^{-5}$ $\mathrm{m}·{\mathrm{s}}^{-1}$ [31].

The growth of hydrate crystals is a self-organizing process that leads to the formation of a new thermodynamic phase. The formula for calculating the rate of continuous nucleation of hydrate crystals is given by:

(4)$J_I=A_I\exp \left(\frac{\Delta \mu }{kT}\right)\exp \left(\frac{-4c^3{v}_h^2{\sigma }_{ef}^3}{27kT\Delta {\mu }^2}\right),$

where ${\sigma }_{ef}$ is specific surface energy [$\mathrm{J}·{\mathrm{m}}^{-2}$] and nucleation rate is $J_1$ = $3.30\times {10}^{12}$ $\left[{\mathrm{m}}^{-3}{\mathrm{s}}^{-1}\right]$ [32].

The rate constant of hydrate formation $k{e}_1\left(t\right)$ during the period of water injection into the gaseous environment of the pressure vessel is calculated using Equations (5) and (6). We first derive Equation (5) with respect to time, where all terms of the equation take known values [33].

(5)${\alpha }_I\left(t\right)=\frac{b{G}_I^{3m}{J}_I}{\left(1+3m\right)\left(2+3m\right)} t^{1+3m}$

where b is the crystal shape factor of 4.05 (-) and m is the growth rate exponent that characterizes the period of water injection into the pressure vessel and has a value of 1.6 [−] [34].

We calculate the rate constant of hydrate formation according to equation:

(6)$k{e}_I\left(t\right)=\frac{d{\alpha }_I\left(t\right)}{dt},$

The mole equilibrium of the injected water into the pressure vessel is a variable component, i.e., it varies in time and is given by the equation:

(7)$\frac{d{C}_w\left(t\right)}{dt}+\left[1+k{e}_I\left(t\right)\right]C_w\left(t\right)=C_w^0,$

The differential equation expresses the molar concentration of water $C_w\left(t\right)$, at each time during the period of water injection into the high pressure vessel. The initial molar concentration of water $C_w^0$ is at time 0, when the water is in its liquid state [35].

The number of moles of crystallized water $M_1\left(t\right)$ in the reactor during the experiment is calculated using the following equation:

(8)$M_1\left(t\right)=\left(C_w^0-C_w\left(t\right)\right) Qt,$

The initial amount of gas $M_2\left(t\right)$ does not depend on time since we do not supply the gas sequentially with some chosen flow rate but we put all the gas into the system right at the beginning and we do not supply any more during the experiment and therefore we have for all times $M_2\left(t\right)$ = $M_2\left(0\right)$ = constant value.

The number of moles of gas bound to water and subsequently to the hydrate structure is given by the following formula:

(9)$M_3\left(t\right)=M_1\left(t\right) \beta ,$

where β is the equilibrium molar ratio of gas and water binding to the hydrate structures. In the literature [36], it is reported that the hydrate ${\left(C{H}_4\right)}_4{(H_{2}O)}_{23}$ contains 1 mole of methane for every 5.75 moles of water, which corresponds to 13.4% methane. The value of β is 1/6 [−].

Water is supplied to the device at the required flow rate Q, where subsequently the number of moles of liquid water supplied to the reactor at any time is calculated by:

(10)$M_4\left(t\right)=Qt{C}_w^0,$

After the pump is taken out of operation, the stabilization phase of the hydrate/water mixture occurs. The temperature of the plant is kept below 5 °C so that the hydrate structure is not disturbed and the remaining water does not form ice. The crystallization of water expressed as a volume fraction is calculated as:

(11)${\alpha }_s\left(t\right)=\frac{b{G}_I^{3n}{J}_I}{\left(1+3n\right)} t^{1+3n},$

where n is replaced by 0.5 [36].

The rate constant of hydrate formation during the steady-state period of the system at any given time of the process during pump inactivity is calculated as:

(12)$k{e}_s\left(t\right)=\frac{d{\alpha }_s\left(t\right)}{dt}.$

The moles of crystallized water during the stabilization period, i.e., after the pump is taken out of operation, are calculated according to the relation:

(13)$M_1\left(t\right)=\left(M_w^c\left(ti\right)[1+(k{e}_2(t-t_i\right)\left(t-t_i\right)],$

where $M_1\left(t\right)$ is the number of moles of crystallized water during the injection period.

4. Measurement Process

The course of the measurement consisted in following a sequence of steps, the methodology of which will be subsequently described chronologically. The first step is to achieve the desired temperature conditions. By commissioning the cooling equipment, we start the cooling process. The time course of the cooling or lowering of the temperature of the vessels is shown in Figure 3.

Cooling consists of the following procedural steps. We rid the device of unwanted air by opening the exhaust vent in the uppermost part of the device so that the entire space can be filled with water. After filling the high-pressure vessels with water from the inlet tap, the temperature is between 13 °C and 14 °C. This is followed by the process of compressing the natural gas with a compressor until a needle gauge fitted to the tank indicated the required pressure of 80 bar. The natural gas filling process is complete. The initial compressed gas temperature recorded is 8 °C. The formation of natural gas hydrates is assumed from the equilibrium diagram at temperatures below 5 °C, therefore our objective is to cool the plant below the specified temperature limit. As can be noticed in Figure 3, the gas has cooled down to 1.5 °C after two hours. After eight hours, the temperature conditions have stabilized and the high-pressure vessels have reached a state of thermal equilibrium. Due to the high specific heat capacity of the water, the cooling process of the high-pressure vessels takes much longer than the cooling of the gas. The temperature was kept below 5 °C for the following time period. By opening a ball valve inserted before the nozzle inlet, the cooled compressed natural gas expands into the VN1 vessel. Before starting the measurement, a portion of the liquid is drained from the lowest point to the specified lower edge of the sapphire visor in the upper vessel VN1. The discharge of water is observed through the aforementioned sapphire visor. The vacated space is filled with compressed natural gas. When the desired pressure is reached, the ball valve for the natural gas is closed. The pump is then started, which ensures that the water particles in the gaseous environment of vessel VN1 are sprayed into the system. From this point on, the experimental measurement is started and the ongoing processes can be monitored through the sapphire visors. In the practical part, a series of experiments were carried out to reveal how the rate of formation of methane hydrates is affected by the setting of different temperature and pressure ratios.

5. Experimental Measurement

In the experimental measurement, by setting the pressure to 76.3 bar and cooling the equipment to temperatures between 4.7 and 4.9 °C, the conditions were set in the region of hydrate structure formation, but before the freezing point of water. After switching on the pump and adjusting the flow rate using a frequency converter, the ultrasonic flow meter indicated a flow rate of 0.06 L·s⁻¹. A gas volume of 1.3 m³ was forced into the device. The mathematical model (Figure 4) predicted the highest methane uptake from the beginning of the experiment to 610 s during the circulation of water through the high-pressure pump. During the experiment, the high-pressure pump was disconnected from operation as early as 480 s due to a visual change in the environment at the bottom of the VN2 vessel. The resulting mass of crystallized water and hydrate was trapped on the strainer at the bottom and clogged the circulation circuit. The ultrasonic flowmeter indicated a loss of signal (i.e., it did not signal any flow). The pump was subsequently disconnected from service to protect it. From a time, point of 610 s, a stabilization phase occurred. According to the simulation, only 121.9 g of unbound natural gas was present in the system. The stabilization phase lasted for 25 min after which the pressure started to rise slightly, indicating that a gas re-release process was occurring. According to the simulation, just by maintaining the temperature and pressure conditions in the non-pump phase, the presence of methane gas was reduced to a mass of 98.6 g. From an energy point of view, maintaining the stabilization phase is energetically disadvantageous since the amount of saturated natural gas is negligible compared to the energy expended on cooling. The time evolution of the different components involved in the hydrate formation can be seen in Figure 4.

As the water was supplied in excess, there was a pressure drop due to the saturation of the water with gas. The pressure conditions recorded during the measurement are shown in Figure 5.

In order to achieve a high rate of hydrate formation, it is necessary to stabilize the temperature conditions with as little deviation from the desired value as possible during the measurement. The hydrate formation is characterized by an increase in temperature, which was manifested by the heating of the vessel VN1 in Figure 6. The heat released by the hydrate formation is efficiently dissipated into the aqueous phase and dissipated on the vessel walls, which are continuously cooled. Due to the saturation of the water with gas, there is a sharp drop in pressure.

When the hydrate formation was considered complete based on visual assessment of the hydrate quality through the sapphire visor, the process of pressure reduction occurred. The depressurization is initiated by opening the drain valve at the bottom of vessel VN2. The pressure reduction is accompanied by the discharge of water from the system. After the water has been drained, the system is flushed with cooled natural gas until complete drying and compaction of the formed hydrate is achieved. The volume of water in the primary circuit system, after determining the volumetric capacities of the individual components, is 22.1 L. After draining and volume measurement, the water volume was 15.6 L. The simulation at the end of the experiment determined the number of moles of crystallized water to be 262 moles. This represents 4.7 L and 6.5 L of crystallized hydrate remained in the experimental apparatus. The deviation of the simulation from the measurement may be due to less accurate water capture in the measuring tank, as the entire process was complicated by high pressure and volume losses during collection. Also, the drain cock is not located at the lowest point of the device, so that some of the water content in the VN1 and VN2 vessels remains in the high-pressure hose. After unscrewing the flange of the sight glass located on the high-pressure vessel VN2, we can see in Figure 7 the formed hydrate dried by the compressed natural gas.

A representative sample was taken from the vessel for further assessment. Figure 7 shows a detail of the collected methane sample, which is exposed to atmospheric pressure and room temperature for stability testing. At the bottom of the tube, we can notice gas bubbles that are gradually released from the hydrate. The released gas is trapped in a balloon, which is visibly inflated after a small amount of the hydrate has completely melted Figure 8.

The course of the experiment was recorded, by scanning the visor of the vessel VN2. The photos in Figure 9 are taken chronologically from left to right and record the time in ten second intervals of 7:30, 7:40, 7:50 and 8:00 (minutes: seconds), until the time of complete light shading in the visor by the emerging hydrate.

6. Discussion and Conclusions

The obtained knowledge on the rapid generation of natural gas confirmed that it is possible to find a technological process that can generate natural gas hydrates in orders of magnitude shorter times compared to the classical methods of generating natural gas hydrates in autoclaves by water saturation only. While without mixing of water and gas it takes up to several hours to generate hydrates, our proposed and tested method of generating hydrates has times in the order of minutes.

Hydrates of natural gas or methane have interesting potential, and therefore the future need for energy originating from hydrocarbon sources may be a requirement for improving synthetic hydrate formation processes. A promising option is to accumulate standard natural gas in the form of artificially formed hydrates, with subsequent storage and, in cases of need to cover energy peaks, future release. Gas storage in this form is possible at relatively high temperatures and low pressures compared to other gaseous hydrocarbon storage technologies. The technique of rapid and continuous gas hydrate formation could rival current means of storing and transporting these energy reserves in a more than economically efficient aspect, but again, development is needed in these new technologies. For rapid and continuous hydrate formation technologies, knowledge of the typical mechanisms of hydrate formation is required. Methane hydrate beds are formed at locations where methane and water are present at temperatures and pressures that are favorable for hydrate formation. These conditions are most commonly found in marine sediments and Arctic permafrost. However, this natural hydrate has been forming for hundreds and perhaps thousands of years. In our case, we were able to produce it in times corresponding to minutes and hours.

I also consider the experimental verification of the energy-efficient formation of fast hydrates on a real experimental device to be a contribution. The proposed and implemented method of drying and cooling of the generated hydrate by means of gas flow cooled by the Joule-Thomson effect can also be evaluated as positive. The hydrate generated by us could be removed from the pressure vessel VN2 after removal of the sapphire visors and it showed stability for more than 1 h in air at an ambient temperature of about 30 °C.

Hydrates can find applications in various sectors of technology or industry. Specifically, the solution is suitable for application in the energy and industrial sectors to cover energy peaks that are financially demanding, in cogeneration and trigeneration, e.g., with the possibility of using waste heat for heating and subsequent release of stored energy when needed.

Very interesting applications of hydrates are emerging for carbon footprint reduction. Carbon dioxide together with water forms hydrates at even lower energy requirements than natural gas hydrates, and the hydrates thus formed are stable and keep carbon dioxide in a stable form for a long time. If the storage temperature of such CO₂ hydrate is not raised, the storage times of carbon dioxide are unlimited.

Author Contributions

Conceptualization, D.H. and P.M.; methodology, M.M.; software, D.H.; validation, N.Č.K., Z.K., and P.M.; formal analysis, P.B.; investigation, P.H.; resources, P.H.; data curation, M.M.; writing—original draft preparation, D.H.; writing—review and editing, D.H.; visualization, P.B.; supervision, M.M.; project administration, Z.K.; funding acquisition, N.Č.K. All authors have read and agreed to the published version of the manuscript.

Funding

This paper was supported under the project of Operational Programme Integrated Infrastructure: Independent research and development of technological kits based on wearable electronics products, as tools for raising hygienic standards in a society exposed to the virus causing the COVID-19 disease, ITMS2014+ code 313011ASK8. The project is co-funding by European Regional Development Fund. Also with support of the granting agency KEGA within the Project Solution No. 046ŽU-4/2021 with the title Innovative Methods for Increasing the Thermal Efficiency of Small Heat Sources by Heat Recovery through Phase Transformations.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are available in a publicly accessible repository.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Table

Figure 1. Schematic diagram of the experimental device. The refence lines are described in Table 1.

Figure 2. Visualization of the experimental device in 3D. The refence lines are described in Table 1.

Figure 3. Cooling system.

Figure 4. Prediction of hydrate formation by mathematical model.

Figure 5. Pressure curves.

Figure 6. Temperature curves.

Figure 7. (a) Natural gas hydrate formed in an experimental device, scale 2:1; (b) Sample of hydrate taken in a volumetric cylinder, scale 2:1.

Figure 8. Hydrate exposed to atmospheric pressure and room temperature, scale 1:3.

Figure 9. Images of the sapphire visor on VN2 during the experiment, scale 1:1.

Table 1

Technological specification of the experimental device.

Mark	Description	Specification
1	Nozzle	Angle of spray 51° and 155°
2	Sapphire visor	Diameter ϕ 50 mm
3	High pressure vessel VN 1	Diameter Φ 175, volume 5.7 L
4	High pressure vessel VN 2	Diameter Φ 175, volume 15 L
5	Plunger pump	Power 5.5 kW, pressure 40–250 bar
6	Water cooling container	Material plastic, volume 40 L
7	Julabo FL 2506	Cooling capacity 0 °C—1 kW
8	Compressor CNG	Working pressure 200 bar
9	Compressed natural gas tank	Maximum working pressure 400 bar
10	Julabo F34	Cooling capacity 0 °C—0.32 kW
11	Julabo FLW 11006	Cooling capacity 0 °C—7.3 kW
12	Julabo FP 40	Cooling capacity 0 °C—0.5 kW

References

1. Makogon, Y.F.; Holditch, S.A.; Makogon, T.Y. Natural gas-hydrates—A potential energy source for the 21st century. J. Pet. Sci. Eng.; 2007; 56, pp. 14-31. [DOI: https://dx.doi.org/10.1016/j.petrol.2005.10.009]

2. Linga, P.; Clarke, M.A. A review of reactors designs and materials employed for increasing the rate of gas hydrate formation. Energy Fuels; 2017; 31, pp. 1-13. [DOI: https://dx.doi.org/10.1021/acs.energyfuels.6b02304]

3. Lee, S.-Y.; Holder, G.D. Methane hydrates potential as a future energy source. Fuel Process. Technol.; 2001; 71, pp. 181-186. [DOI: https://dx.doi.org/10.1016/S0378-3820(01)00145-X]

4. Carroll, J. Natural Gas Hydrates: A Guide for Engineers; 2nd ed. Gulf Professional Publishing: Waltham, MA, USA, 2014; pp. 24-157.

5. Hao, W.; Wang, J.; Fan, S.; Hao, W. Evaluation and analysis method for natural gas hydrate storage and transportation processes. Energy Convers. Manag.; 2008; 49, pp. 2546-2553. [DOI: https://dx.doi.org/10.1016/j.enconman.2008.05.016]

6. Javanmardi, J.; Nasrifar, K.; Najibi, S.H.; Moshfeghian, M. Economic evaluation of natural gas hydrate as an alternative for natural gas transportation. Appl. Therm. Eng.; 2005; 25, pp. 1708-1723. [DOI: https://dx.doi.org/10.1016/j.applthermaleng.2004.10.009]

7. Thomas, S.; Dawe, R.A. Review of ways to transport natural gas energy from countries which do not need the gas for domestic use. Energy; 2003; 28, pp. 1461-1477. [DOI: https://dx.doi.org/10.1016/S0360-5442(03)00124-5]

8. Sloan, E.D. Fundamental principles and applications of natural gas hydrates. Nature; 2003; 426, pp. 353-363. [DOI: https://dx.doi.org/10.1038/nature02135] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/14628065]

9. Chen, T.S. A Molecular Dynamics Study of the Stability of Small Renucleation Water Clusters. Ph.D. Thesis; University Missouri-Rolla: Rolla, MO, USA, 1980.

10. Hesse, R.; Harrison, W.E. Gas hydrates (clathrates) causing pore-water freshening and oxygen isotope fractionation in deep-water sedimentary sections of terrigenous continental margins. Earth Planet. Sci. Lett.; 1981; 55, pp. 453-462. [DOI: https://dx.doi.org/10.1016/0012-821X(81)90172-2]

11. Lekvam, K.; Bishnoi, P.R. Dissolution of methane in water at low temperatures and intermediate pressures. Fluid Phase Equilib.; 1997; 131, pp. 297-309. [DOI: https://dx.doi.org/10.1016/S0378-3812(96)03229-3]

12. Zhong, D.L.; Liu, D.P.; Wu, Z.M.; Zhang, L. Natural Gas Hydrate and Growth on Suspended Water Droplet. Proceedings of the 6th International Coference on Gas Hydrates (ICGH 2008); Vancouver, BC, Canada, 6–10 July 2008.

13. Gumerov, N.A.; Chahine, G.L. Dynamics of Bubbles in Conditions of Gas Hydrate Formations; Dynaflow Inc.: Fulton, MD, USA, 1992; pp. 1-7.

14. Metal Journal Publishing. Available online: https://www.metaljournal.com.ua/basic-principles-of-gas-hydrate-technologies/ (accessed on 9 October 2021).

15. Lara, P. Onset of air entrainment for a water jet impinging vertically on a water surface. Chem. Eng. Sei.; 1979; 34, pp. 1164-1165. [DOI: https://dx.doi.org/10.1016/0009-2509(79)85025-3]

16. Rodrigues, L.F.; Ramos, A.; Araujo, G.; Silveira, E.; Ketzer, M.; Lourega, R. High-Pressure and Automatized System for Study of Natural Gas Hydrates. Energies; 2019; 12, 3064. [DOI: https://dx.doi.org/10.3390/en12163064]

17. Welander, P.; Vincent, T.L. Designing and Optimizing Gas/Liquid Reactions for: Environmental Processes/Chemical Reactions/Heat Transfer; Brochure from Exhibition at the Chem Show: New York, NY, USA, 1999.

18. Brown, D.T.; Taylor, E.C.H.; Bernardo, P.M. Rapid Gas Hydrate Formation Processes: Will They Work?. Energies; 2010; 3, pp. 1154-1175. [DOI: https://dx.doi.org/10.3390/en3061154]

19. Meleshkin, A.V.; Elistratov, D.S. Experimental investigation of the process of hydrate-formation by the method of explosive boiling of liquefied freon 134a in the water volume during decompression. MATEC Web of Conferences; EDP Sciences: Les Ulis, France, 2018; Volume 194, 01015.

20. Chernov, A.A.; Elistratov, D.S.; Mezentsev, I.V.; Meleshkin, A.V.; Piľnik, A.A. Hydrate formation in the cyclic process of refrigerant boiling-condensation in a water volume. Int. J. Heat Mass Transf.; 2017; 108, pp. 1320-1323. [DOI: https://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.12.035]

21. Chernov, A.A.; Pilnik, A.A.; Elistratov, D.S.; Mezentsev, I.V.; Meleshkin, A.V.; Bartashevich, M.V.; Vlasenko, M.G. New hydrate formation methods in a liquid-gas medium. Nature; 2017; 7, 40809. [DOI: https://dx.doi.org/10.1038/srep40809] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/28098194]

22. Qureshi, M.F.; Atilhan, M.; Altamash, T.; Aparicio, S.; Aminnaji, M.; Tohidi, B. High-pressure gas hydrate auto-clave hydraulic experiments and scale-up modeling on the effect of stirring RPM effect. J. Nat. Gas Sci. Eng.; 2017; 38, pp. 50-58. [DOI: https://dx.doi.org/10.1016/j.jngse.2016.12.023]

23. Filarsky, F.; Schmuck, C.; Schultz, H.J. Development of a Surface-Active Coating for Promoted Gas Hydrate Formation. Chem. Ing. Tech.; 2019; 91, pp. 85-91. [DOI: https://dx.doi.org/10.1002/cite.201800002]

24. Mori, Y. On the Scale-up of Gas-Hydrate-Forming Reactors: The Case of Gas-Dispersion-Type Reactors. Energies; 2015; 8, pp. 1317-1335. [DOI: https://dx.doi.org/10.3390/en8021317]

25. Filarsky, F.; Wieser, J.; Schultz, H.J. Rapid Gas Formation-Evaluation of Three Reactor Concepts and Feasibility Study. Molecules; 2021; 26, 3615. [DOI: https://dx.doi.org/10.3390/molecules26123615]

26. He, Y.Y.; Rudolph, E.S.J.; Zitha, P.L.J.; Golombok, M. Kinetics of CO₂ and methane hydrate formation: An experimental analysis in the bulk phase. Fuel; 2011; 90, pp. 272-279. [DOI: https://dx.doi.org/10.1016/j.fuel.2010.09.032]

27. Linga, P.; Daraboina, N.; Ripmeester, J.A.; Englezos, P. Enhanced rate of gas hydrate formation in a fixed bed column filled with sand compared to a stirred vessel. Chem. Eng. Sci.; 2012; 68, pp. 617-623. [DOI: https://dx.doi.org/10.1016/j.ces.2011.10.030]

28. Xiao, P.; Yang, X.-M.; Sun, C.-Y.; Cui, J.-L.; Li, N.; Chen, G.-J. Enhancing methane hydrate formation in bulk water using vertical reciprocating impact. Chem. Eng. J.; 2018; 336, pp. 649-658. [DOI: https://dx.doi.org/10.1016/j.cej.2017.12.020]

29. Kashchiev, D.; Firoozabadi, A. Driving force for crystallization of gas hydrates. J. Cryst. Growth; 2002; 241, pp. 220-230. [DOI: https://dx.doi.org/10.1016/S0022-0248(02)01134-X]

30. Kashchiev, D.; Firoozabadi, A. Nucleation of gas hydrates. J. Cryst. Growth; 2002; 243, pp. 476-489. [DOI: https://dx.doi.org/10.1016/S0022-0248(02)01576-2]

31. Mullin, J.W. Crystallization; 4th ed. Butterworth-Heinemann: Oxford, UK, 2001; pp. 216-284.

32. Kashchiev, D.; Firoozabadi, A. Induction time in crystallization of gas hydrates. J. Cryst. Growth; 2003; 250, pp. 499-515. [DOI: https://dx.doi.org/10.1016/S0022-0248(02)02461-2]

33. Kashchiev, D. Nucleation; 3rd ed. Butterworth-Heinemann: Oxford, UK, 2000; pp. 174-224.

34. Hečko, D.; Mičko, P.; Holučík, M.; Kapjor, A. Experimental Simulation of Hydrate Formation Process in a Circulating Device. Processes; 2021; 9, pp. 1529-1543. [DOI: https://dx.doi.org/10.3390/pr9091529]

35. Froment, G.F.; Bischoff, K.B. Chemical Reactor Analysis and Design; 3rd ed. John Wiley & Sons: New York, NY, USA, 1990; pp. 106-129.

36. Gnanendran, N.; Amin, R. Modelling hydrate formation kinetics of a hydrate promoter-water-natural gas system in a semi-batch spray reactor. Chem. Eng. Sci.; 2004; 59, pp. 3849-3863. [DOI: https://dx.doi.org/10.1016/j.ces.2004.06.009]