ANALYSIS OF THE CONDITIONS FOR THE FORMATION OF GAS HYDRATES DURING THE TRANSPORTATION AND PROCESSING OF HYDROCARBON GAS, AND THE PREVENTION OF HYDRATE FORMATION

АННОТАЦИЯ

В статье рассматриваются условия газогидратообразования при транспортировке и хранении газа, их техногенные свойства, а также классификация и характеристика методов предотвращения и борьбы с гидратообразованием.

ABSTRACT

The article discusses the conditions of gas hydrate formation during the transportation and storage of gas, their technogenic properties, as well as the classification and characteristics of methods for preventing and combating hydrate formation.

Ключевые слова: природные газы, влага газа, газогидраты, газогидратообразование, контроль гидратообразования, ингибиторы гидратообразования.

Keywords: natural gases, gas moisture, gas hydrates, gas hydrate formation, hydrate control, hydrate inhibitors.

It is known that the gas from the field contains impurities such as water vapor, , ,  and mechanical particles. Purification of such additives will lead to a number of disadvantages in the transfer of gas to the gas processing plant. This is because when the gases are cooled or the pressure is increased, the vapor moisture can condense and form free water, ice or hydrate.

Gaseous fuels are make use of in compressed and liquefied form. Hydrocarbons with a critical temperature is higher air temperature change from a gaseous state to a liquid state at lower pressure. Such natural gases are called liquefied gases (namely,  is remanded to liquefy propane at ,  for normal butane).

Drops of gas (clathrates) are lucid admixtures that form lower certain thermobaric circumstances. The term "clathrates" was coined in the mid-20th century by Professor Powell, who learned them. The phrase translates from Latin as "closed fence".

Drops of gas are considered to be non-stoichiometric compounds as a result of they do not have a constant content.

A gradual decline in pressure during gas extraction and transmission leads to a decline in the temperature of the moisture-saturated gas. Under these circumstances, the gas particle interacts with water to form solid molecule particles.

Condensation and accretion of water vapor in baths and pipelines Under certain conditions, each molecule of natural components () has the ability to bind to   molecules (Figure 1). The physicochemical combo of  with hydrocarbon gases - a hard crystalline substance - is named crystal hydrates.

In appearance, hydrates views yellowish loose snow or ice. They are unstable combinations that decompose rapidly into gas and water when heated or, when pressure is decreased. The hydraulic operation of the technological mechanism can be carried out under the following conditions:

 and

Where: ,  –are the balance pressure, temperature of hydrate form determined experimentally.

Figure 1. The morphology of a gas hydrate

Table 1.

Hydrate form temperature

That is high the pressure, the high  beneath high pressure conditions, hydrates do not form at values aloft the critical temperature.

Not only hydrocarbon components though also non-hydrocarbon compounds in the gas are involved in hydrate form. An rise in the quantity of  and  in the gas leads to an increase in the balance temperature of hydrate form and a decrease in equilibrium pressure (namely, for pure , hydrate form temperature at  is ,  its value is  when the amount increases).

Nitrogen-containing hydrocarbon gases are hydrated at lower temperatures, which increases the stability of the hydrates.

High press and low temperature are required for hydration of liquid natural gases. Unlike natural gases, liquid hydrocarbon gases increase hydration as the pressure in the system increases.

In addition, as in natural gas, heat is released and the temperature in the system rises. As the volume remains constant, the pressure in the system increases with increasing temperature.

Hydrate of liquid hydrocarbons is observed with a decrease in pressure due to a decrease in volume. This is because the formation of hydrates in the presence of liquid hydrocarbons is much more difficult. For the process to begin, the system must be in equilibrium for a certain period of time. However, when small ice crystals are formed at negative temperatures, hydrates begin to form quickly. Interestingly, the hydrates of liquid hydrocarbons are lighter than water.

 of gas hydrate can contain up to  of gas. Methane gas hydrate burns in air. Natural gases of purely gas fields (with  and isobutane content less than ), as well as gases containing a significant amount of non-hydrocarbon components ( and ) form hydrates CS-I, while natural gases of gas condensate fields are characterized by formation of hydrates CS-II (figure 2).

Figure 2. The main types of crystal lattices of gas hydrates

Compressed gases are hydrocarbons with a critical temperature below air temperature. A temperature of  is required to liquefy methane, the main component of compressed gas. At atmospheric pressure, methane liquefies at . Methane does not liquefy at any high pressure above .

During the refining of crude oil, factory gases are formed from each destructive process. Factory gases differ in their hydrocarbon content. Thermal cracking gases are rich in methane and other unsaturated hydrocarbons. Catalytic cracking gases are characterized by high levels of butanes and butylene.

The hydrate-forming components are mainly light hydrocarbons in natural gas - ,, , isobutane, as well as nitrogen, carbon dioxide and hydrogen sulfide. Natural gas hydrates have the following formula:

Table 2.

Formula of Natural Gas Hydrates

Hydrates are white crystalline substances that, depending on the conditions of formation, resemble ice or compacted snow. In hydrocarbon gas hydrates, the aqueous crystal lattice is mostly filled with liquid  and iso-butane, while a small portion contains , , , and .

These additives in the gas cause hydrates to form in the pipelines, making it difficult to transport and process.

Hydrate formation related on the structure of the gas and the thermodynamic conditions (pressure and temperature) (Figure 3). Hydrate form also related on the amount of salts in the water. As the amount of salt increases, the temperature of hydrate form decreases.

Figure 3. Pressure-temperature phase diagram of methane hydrate

The hydrate is in the form of ice or compacted snow. Hydrates decrease the permeability of the pipe, overload the compressor, and cause corrosion of pipes and equipment (Figure 4).

Figure 4. Composition  gas hydration equilibrium graph

In addition, the negative consequences of this process include:

• Corrosion of pipelines, gas processing and operating equipment;
• accumulation of liquid in gas pipes;
• obstruction of the pipeline permeability and narrowing of process equipment due to hydrated plugs;
• The worst thing is the interruption of gas supply to consumers.

In order to eliminate such negative consequences in the industrial gas supply system, the gases are dried to the required level. Drying depth is determined according to industry standard requirements and future processing technology. All gases provided to the main gas pipelines must be dehumidified.

There are inhibitory and dehydrating techniques to combat hydrate form. Inhibitors of hydration temperature are added - methanol, glycol. When an inhibitor is added to the gas, it dissolves in water, the water vapor pressure decreases, and the temperature of hydrate formation decreases.

Different schemes and techniques of drying and inhibition of gases are used in the gas industry. Inhibition (introduction of an inhibitor into the gas stream) is widely used to combat the form of gas hydrates. The essence of this techniques is that the inhibitor introduced into the wet gas stream is freely soluble in water, which curtail the pressure of water vapor and the temperature of hydrate form. Hydrates break down into water and gas as pressure decreases and temperature increases.

Glycols are widely used in the drying of methanol and gases as inhibitors against the form of hydrates.  is a methyl alcohol that, when introduced into a gas stream, absorbs water vapor and converts it into an aqueous alcohol solution at low freezing temperatures.

Methanol has a high saturated vapor pressure, which is difficult to separate and regenerate from the gas, and the losses are high. Therefore, methanol is mainly used to remove hydrated clogs in wells, pipelines and main pipelines. Methanol used as an inhibitor is not regenerated. It is also used in low-temperature separation to eliminate hydration in throttling and compression (for the separation of heavy hydrocarbons).

Glycols EG and DEG are also widely used as inhibitors (although methanol is expensive) and are easy to regenerate (evaporate).

Glycols are also separated in water separators after saturation with water vapor and then regenerated.

Calcium chloride () solution and are also widely used as inhibitors. A more effective way to prevent the form of hydrates is to dry these gases, which dramatically reduce the amount of moisture.

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