Modeling and investigation of physical conditions of hydrate formation in gas transmission pipes

Dissertation for obtaining a master's degree (M.Sc.)

Field: Gas processing and transmission engineering

Abstract:

Today, one of the problems in gas transmission lines is the phenomenon of gas hydrate, which is a combination of light gases such as methane, ethane or carbon dioxide with molecules Under certain conditions of temperature and pressure, water forms a substance similar to ice, which contains a large volume of gas. Gas hydrates are generally deposited and eventually reduce the operational capacity of the line or even lead to total blockage of the pipeline. Examining the parameters, variables and influencing factors of the formation and removal of the phenomenon is very important, which was first analyzed in this research and the sepsis of three situations before, after and during the formation of hydrate was investigated. Previously, a look at the methods, processes, advantages and disadvantages of gas dehumidification units. The mass and heat transfer resistances during the formation were also fully investigated and showed that the rate of hydrate formation is controlled by the mass transfer mechanism and the faster the heat transfer is done, the more stable the formed hydrate is. Then, it has been simulated by a field modeling of velocity distribution, pressure, temperature, volume fraction for the fluid, as well as the concentration distribution of solid particles in a slow gas-solid two-phase flow inside a horizontal pipe, by the COMSOL Multiphysics software package. The results of the simulation show that the decrease in the average speed leads to a decrease in the dispersing forces and ultimately causes a higher concentration of solid particles at the bottom of the pipe. rtl;">Foreword

Natural gas is an almost clean, abundant and cheap source of energy, which is currently used on a large scale for industrial and domestic purposes, and its exploitation will expand in the coming decades. In the economic development of the world, different regions and countries, due to the huge resources and reserves available and the development of creative technologies, have reduced the costs and time of projects and thus improved the economy of gas development and transmission projects. Also, the global effort to reduce greenhouse gases and CO2 gas shows the advantage of using natural gas compared to other fuels.

Today, in gas transmission lines, the phenomenon of gas hydrate is a combination of light gases such as methane, ethane, or carbon dioxide, which combine with water molecules under certain temperature and pressure conditions and form a substance similar to ice, which contains a large volume of gas. has given Precipitated gas hydrates will eventually reduce the possible throughput or even lead to total blockage of the pipeline. It is very important to check the parameters, variables and factors influencing the formation and removal of the phenomenon. This research is divided into three parts before, during and after hydrate formation in order to examine all parameters. At the time of emergence, he has taken a comprehensive look at two parts: the resistances during the beginning of the phenomenon and the continuous emergence of the phenomenon. Examining heat and mass transfer resistances during initiation, modeling a section of a pipe forming hydrate and simulating a gas supply network could provide us with complete results of the phenomenon during formation. The selection of suitable inhibitors with salt and glycolic structures has also been investigated.

rtl;">1                    Chapter One

Gas hydrate and its effective factors

1-1           Hydrate

Gas hydrates are crystalline solid compounds that are part of the family of endorners or clathrate [1]. An endorner is a simple compound in which a molecule of a substance (guest molecule [2]) is trapped in a network made of a molecule of another substance (host molecule [3]). The internalizer related to water is called hydrate. In their structure, water molecules form a quasi-network structure by creating holes due to hydrogen bonding. This network, which is unstable, is known as the empty hydrate network, which can be transformed into a stable structure at a certain temperature and pressure (at low temperature and high pressure) with the presence of various gas components of suitable size and shape. In this type of crystals, no chemical bond is formed between water molecules and trapped gas molecules, and the only factor that stabilizes the crystals is the formation of hydrogen bonds between the host molecules (water molecules) and the van der Waals force that occurs between the host molecules and the guest molecules (gas molecules) [1-3].

The hydrate structure is similar to ice, except that the crystal Hydrate can remain stable and not melt at a temperature higher than the melting point of ice, in conditions where the pressure is higher than the ambient pressure. Another thing that causes the similarity between hydrate crystal and ice is the increase in volume and the release of heat during formation. 1-2 Formation of hydrates Formation of hydrates is the result of hydrogen bonding. Hydrogen bonding causes water molecules to be arranged in regular directions. The presence of certain compounds stabilizes regular molecules and precipitates a solid mixture. Water molecules are also called host molecules and other compounds that stabilize the crystal are called guest molecules. In this research, guest molecules are often called "constituents[4]". Hydrate crystals have complex three-dimensional structures in which water molecules act as cages and guest molecules are trapped in these cages.

The stability caused by guest molecules is attributed to van der Waals forces [5], which is due to the attraction between molecules, not electrostatic attraction. As explained earlier, hydrogen bonding is different from van der Waals forces because hydrogen bonding is based on strong electrostatic attraction, although some classify hydrogen bonding as a van der Waals force.

Another interesting point about gas hydrates is that there are no bonds between host and guest molecules. The guest molecules rotate freely within the cages made by the host molecules. This rotation has been measured through a spectroscopic instrument. Therefore, these compounds can be defined as solid solutions.

1-3 Conditions for hydrate formation

The formation of hydrate requires three conditions:

1- The right combination of temperature and pressure, low temperature and high pressure are favorable conditions for the formation of hydrate;

2- The presence of hydrate formation: The constituents of hydrate are: methane, ethane, propane, isobutane, hydrogen sulfide and carbon dioxide; 3- Sufficient water, neither too much nor too little. Low temperature and high pressure are favorable conditions for hydrate formation. The exact temperature and pressure depends on the composition of the gas. Hydrates are formed at a temperature higher than zero degrees Celsius, the freezing point of water.

To prevent the formation of hydrates, one of the three mentioned conditions must be eliminated. Normally, the hydrate constituents cannot be removed from the mixture. In the case of natural gas, hydrate formers are desirable products. Therefore, by eliminating the other two conditions, the formation of hydrate can be prevented [4-6]. rtl;"> mixing rate (turbidity and turbulence), kinetics, crystal formation level, nucleation location, accumulation rate and salinity of the system.