Influence of the ratio of water and gas on the process of hydrate formation during the explosive boiling of liquefied freon 134a in water with SDS

. This work is part of a comprehensive study of a method for obtaining gas hydrates, based on the explosive boiling of a liquefied hydrate-forming gas in a volume of water when the pressure is released. This method is characterized by high productivity. The paper presents the results of an experimental study of hydrate formation in the water + SDS system at different ratios of water and hydrate-forming gas. Freon 134a was used as the hydrate-forming gas. The change in temperature and pressure in the process of hydrate formation is shown.


Introduction
Gas hydrates, also known as clathrate hydrates, are icelike crystalline solid structures that form at low temperatures and high pressures.They consist of water molecules as "hosts" and guest molecules of light gases such as CO 2 , N 2 , CH 4 , H 2 , etc.The interaction of guest molecules and host molecules is carried out due to the van der Waals force, which ensures the formation and stable existence of the hydrate structure [1].
There are five different types of scaffolds that are used to form the three most common hydrate structures: sⅠ, sⅡ, and sH.Structure sⅠ consists of 2 pentagonal dodecahedral cells (contains 12 pentagonal faces) and 6 tetrakaidecahedral cells (contains 12 pentagonal and 2 hexagonal faces) 5 12 6 2 , sⅡ contains 12 pentagonal and 4 hexagonal faces 5 12 6 4 , and sH consists of 3 small cells 5 12 , 1 cell medium size 435663 (contains 3 square, 6 pentagonal and 3 hexagonal faces) and 2 large icosahedral cells 5 12 6 8 (contains 12 pentagonal and 8 hexagonal faces).It is important to note that the common cell type 5 12 in each of the three structures has different sizes.The diameter of the guest molecule is the determining factor for which structure can be formed.For example, CH 4 is suitable both for 5 12 6 4 sII and 5 12 6 2 sI, while C 3 H 8 is too large for sI, but can fit into 5 12 6 4 sII and forms it.
Research results show that in addition to light gases, some hydrophobic and water-soluble polar compounds, as well as ternary/quaternary alkyl ammonium salts, can form clathrate/semi-clathrate hydrates under certain conditions.This means that in addition to the typical structures of sⅠ, sⅡ or sH hydrates, other structures can be formed, as in the case of tetrabutylammonium bromide (TBAB), which forms a semi-clathrate structure.It is possible that other hydrate structures exist, but methods for their identification have not yet been implemented or are difficult [2][3].
The existence of structures such as gas hydrates has been known for over 200 years, but their existence was not given much importance until they caused large losses in the extraction and transportation of oil and gas [4].Organic carbon on Earth is largely stored as hydrate deposits.To determine the potential of hydrate deposits and their impact on the environment, the research community has attracted the attention of specialists from the fields of physics, chemistry and mechanical engineering to gas hydrate technologies [5][6].In addition, more and more attention is paid to other possible scenarios for the use of gas hydrates.In recent years, research has been carried out into potential technological applications of gas hydrates, including their use in natural gas or hydrogen storage and transportation, gas separation/capture, carbon dioxide capture and storage, seawater desalination, and cold storage [7][8][9][10][11][12][13][14].For example, one meter of cubic hydrate can contain more than 170 cubic meters of gas under standard conditions, which makes gas hydrate technology promising for gas storage and transportation.
The industry has shown great interest in gas separation and recovery technologies using hydrates due to their low flow requirements and energy intensity of the process.However, the low rate of natural hydrate formation limits their application.Therefore, increasing the rate of hydrate formation is a key factor for the successful application of hydrate technology.One of the ways to increase the rate of the process is the use of kinetic promoters, one of these substances can be sodium dodecyl sulfate (SDS) [15][16][17].In this paper, we study the dependence of the rate of hydrate formation on the mass of a solution based on SDS and water interacting with freon R-134a as a hydrate-forming gas.

Experimental setup
The study was carried out on an experimental complex, which includes an autoclave-type high-pressure vessel with a removable lid (Fig. 1, 2).The autoclave is designed for pressures up to 25 MPa, and has a working chamber with a diameter of 100 mm and a height of 300 mm.The tightness of the installation is ensured by a fluoroplastic seal and bolted clamps that ensure a snug fit of the lid to the tank.Maintaining the set temperature is ensured by the cooling system, which includes an external cooling jacket of the working chamber, through which antifreeze is pumped with a set temperature by the LOIP FT-316-40 thermostat.Monitoring of pressure and temperatures of gas and liquid media is carried out by sensors built into the cover and wall of the tank.The autoclave is equipped with a built-in stirrer with a speed of up to 1500 rpm, connected to the electric motor by means of a magnetic coupling.Controlled pressure relief is provided by the AALBORG flow regulator.

Experimental Method
For the experiment, the following research methodology was used.A solution was prepared by adding sodium dodecyl sulfate (SDS) powder of various weights to water in order to obtain a solution weighing from 50 g to 200 g in 50 g increments and with a constant SDS concentration of 500 ppm.Then the solution was placed in the working area of the cooled experimental setup, which was hermetically sealed.After cooling the solution to a temperature of 8 , the hydrate-forming gas freon 134a in gaseous form, weighing 100 g, entered the installation through an open valve.It is important to note that the stirrer was not used throughout the entire process.When the pressure reached saturation, freon began to condense on the cooled walls and flow down to the water surface, where it formed into drops and fell to the bottom of the experimental section.After reaching the water temperature of 6 , the pressure was released, accompanied by boiling and intense boiling of liquefied gas in the volume of water.Gas bubbles released from the water and rising to the surface contributed to the intensification of the hydrate formation process due to several factors: the involvement in the hydrate formation process of a large volume of gas that was in a liquefied state in the aquatic environment; cooling of the medium and removal of heat released in the process of hydrate formation; intensive mixing of the medium due to the release of gas bubbles and the development of an interfacial surface on which gas hydrate grew.The pressure drop to 0.1 MPa was stopped after not all the gas had passed into the gas hydrate state, which required additional releases to stabilize the pressure and completely boil off the liquefied gas, while maintaining the freon hydrate in a stable state.
The use of liquid freon significantly accelerates the process of hydrate formation, but does not allow using the pressure drop in the description of the process.Therefore, after reaching a pressure of 0.1 MPa in the system, the cooling of the system was stopped and the working section began to heat up.This leads to the fact that the hydrate begins to melt, releasing gas and creating excess pressure in the system, through which, using the ideal gas equation of state, the mass of gas that has passed into the hydrate is determined.
, (1) where -mass of gas converted to gas hydrate, molar mass of carbon dioxide, -pressure change during gas hydrate decomposition, -pressure in the system before decomposition, -universal gas constant, -temperature change during heating, -temperature of the system before heating (determined by a sensor that measures the temperature of the gas).
The mass of the synthesized hydrate is determined as follows: (2) , (3) where -molar mass of carbon dioxide, -molar mass of water.
Using the obtained data, the conversion coefficients of gas and water to the hydrate state were determined relative to the initial masses of gas and water.(4) , (5) where -the initial mass of carbon dioxide, -the initial mass of water.

Results
In this work, an experimental study of the effect of the ratio of water and gas on the process of hydrate formation was carried out by the method of explosive boiling up of a liquefied hydrate-forming gas in a volume of water during decompression.On Fig. 3 is shown a graph of the pressure in the system during the hydrate production according to the method described above with a water-to-gas ratio of 0.5 and 2. For both cases, the pressure first drops, and then begins to increase with continued pressure relief.This indicates the beginning of the process of explosive boiling, in which the liquefied gas is overheated, which leads to its volumetric boiling.After reaching the pressure in the system of 0.1 MPa, the discharge stops, however, liquefied gas is still present in the system, which leads to a repeated increase in pressure, therefore, an additional discharge was carried out until there was no liquefied gas left in the system.It should also be noted that the time of the discharge and re-discharge processes for the case with a lower ratio of water and gas is longer, this is due to the fact that there is less water in the system, with which the boiling liquefied gas interacts, as a result of which more gas remains.Fig. 4 shows the change in temperature in the system during the process of explosive boiling up of liquefied gas for the ratio of water and gas 0 and 2. It can be seen that the release of pressure is accompanied by cooling of the medium.And, accordingly, for the case with a lower ratio of water and gas, cooling is more active.I would also like to note that the temperature stabilization at 2.5 is caused by the release of heat in the process of hydrate formation.The results obtained for water conversion and gas conversion are presented in Fig. 5 and 6 and Table 1.It can be seen that the ratio of water and hydrate-forming gas significantly affects the process of hydrate formation in the method under study.At the same time, with a lower ratio of water (50 ml), about 47.2% of water passed into the hydrate, and 5.9% of gas.I would like to note that the great value of water conversion is directly related to its mass.The main indicator of the efficiency of the process will be gas conversion, which in the current case is minimal.The best result was obtained for ratio 2 (200 ml of water), where the proportion of water that passed into the hydrate was 18.5%, while about 9.1% of the gas was captured in the gas hydrate state.

Conclusions
In this work, the influence of the ratio of water and gas on the process of hydrate formation by the method under study is determined.It was shown that the ratio of water and hydrate-forming gas significantly affects the process of hydrate formation in the method under study.At the same time, with a lower ratio of water (50 ml), about 47.2% of water passed into the hydrate, and 5.9% of gas.I would like to note that the great value of water conversion is directly related to its mass.The main indicator of the efficiency of the process will be gas conversion, which in the current case is minimal.The best result was obtained for ratio 2 (200 ml of water), where the proportion of water that passed into the hydrate was 18.5%, while about 9.1% of the gas was captured in the gas hydrate state.
This work was carried out under the state contract with IT SB RAS (no.121031800216-1).

Fig. 3 .
Fig. 3. Pressure change in the system during the process of explosive boiling of liquefied gas at different ratios of water and gas: 0.5 (blue line) and 2 (orange line).

Fig. 4 .
Fig. 4. Temperature change in the system during the process of explosive boiling of liquefied gas at different ratios of water and gas: 0.5 (blue line) and 2 (orange line).

Fig. 6 .
Fig.6.Conversion of water to the gas hydrate state for various ratios of water and gas: 0.5; 1; 1.5, 2.

Table 1 .
Water and gas conversion in gas hydrate.