THE INFLUENCE OF DIFFERENT PARAMETERS IN THE PROCESSES OF AMINE PURIFICATION OF NATURAL GASES

ABSTRACT

Today, natural gas is one of the main sources of energy in the world, and the processes of its extraction, primary preparation in deposits and deep chemical processing are improving every year. This article examines the influence of the main performance indicators in natural gas purification processes using amines on the efficiency and selectivity of the process.

АННОТАЦИЯ

Сегодня природный газ является одним из основных источников энергии в мире, процессы его добычи, первичной подготовки в месторождениях и глубокой химической переработки совершенствуются с каждым годом. В данной статье исследуется влияние основных рабочих показателей в процессах очистки природного газа с использованием аминов на эффективность и селективность процесса.

Keywords: gas, amine, purification, absorption, desorption, temperature, pressure, concentration, sulfur

Ключевые слова: газ, амин, очистка, абсорбция, десорбция, температура, давления, концентрация, сера

Choice of amine. In accordance with recommendations [1-2], when choosing absorption treatment parameters, one should keep in mind two main mechanisms of CO₂ absorption. Most of the carbon dioxide is absorbed by MEA and DEA solutions to form carbamate, achieving an absorption degree of 0.5 mol/mol. The transformation of the carbamate structure into a bicarbamate structure followed by an acid-base reaction allows an absorption degree of 1 mol/mol to be achieved. At the same time, the equilibrium concentration of CO₂ in the gas phase increases due to a slowdown in the rate of chemisorption. The interaction of CO ₂ with tertiary amines by the carbamate type is impossible due to the absence of a mobile hydrogen atom in nitrogen, which determines the selective extraction of CO ₂ in the presence of H _{2 S. If COS and CS 2} are present in the purified gas , then it is not advisable to use primary amines (MEA) due to the formation and accumulation of non-regenerable and difficult-to-regenerate compounds in the absorbent .

In general, primary amines are more reactive and the equilibrium pressure of CO₂ and H₂S over their solutions is lower than over solutions of secondary amines. Therefore, at a total excess pressure below 0.7 MPa, only primary amines are used. The maximum permissible absorption capacity of the absorbent is limited both by the standards of permissible corrosion of equipment and by the maximum permissible heat of chemisorption. Corrosion limits on the concentration of primary amines in solution are 0.5 mol/l, secondary amines - 0.85 mol/l. Finally, the heat of reaction of acid gases with primary amines is 25% higher than with secondary amines, which determines for each of the amines its own critical limitations when purifying gases with a high content of acidic components. Due to the differences noted above in the heat of reaction with acidic components, primary amines are more difficult to regenerate and require more steam consumption for regeneration than secondary and tertiary amines [3].

The efficiency of removing mercaptans from gas depends on their properties: methyl mercaptan (CH₃SH) in MEA and DEA is extracted from gas by 35–45%, ethyl mercaptan (C₂H₅SH) – 10–15%, propyl mercaptan (C₃H₇SH) – 0.1% [4].

Selecting the concentration of the amine solution. The use of amine solutions of high concentrations makes it possible to reduce the volume of the circulating solution and, as a result, reduce the cost of pumping the solution, but leads to a number of undesirable phenomena [5]:

• the amount of absorbed acidic components per unit mass of solution increases, which leads to an excessive increase in the temperature of the amine due to an increase in the total thermal effect;
• the boiling point of the solution increases, and, consequently, the steam consumption for regeneration increases;
• The viscosity of the solution increases, as a result of which the mass and heat transfer coefficients decrease and the energy consumption for circulation of the solution increases. Moreover, viscous amine solutions exhibit a greater tendency to foam;
• the vapor pressure of the amine solution increases, which leads to increased losses due to evaporation;
• concentrated solutions have a greater dissolving ability with respect to hydrocarbons, which simultaneously leads to the release of additional heat in the absorber and increased load on the expander .

Optimal mass fraction: MEA – 12–25%, DEA – 20–30%, MDEA – 30–50%. Requirements for the residual content of acidic components in regenerated amines are determined by the nature of the amine used and are set within the following limits: for MEA – 0.1 mol/mol, for DEA – 0.02 mol/mol, for MDEA – 0.03 mol/mol [6].

Effect of absorption temperature. A decrease in the absorption temperature leads to an increase in the extraction of target components, but reduces the selectivity of the process due to an increase in the solubility of hydrocarbons in amine solutions and increases the likelihood of hydrate formation . An increase in temperature increases the selectivity of the process with respect to acidic components, but can lead to an increase in the residual content of acidic components in the purified gas. In addition, an increase in temperature leads to an increase in the moisture content of the purified gas, which increases the consumption of glycol for its drying and increases the energy consumption for regenerating the desiccant. The degree of influence of temperature on the selectivity of the process is determined by the nature of the amine and is more noticeable when using tertiary amines.

The interaction of H ₂ S with any amines proceeds with the formation of hydrosulfide and sulfide instantly. An increase in temperature to a certain limit (up to 70 ^oC) will primarily affect the formation of low-stable carbonic acid, which leads to a significant decrease in the degree of CO _{2 extraction} . In this case, the degree of extraction decreases, although to a lesser extent than CO ₂ [7]:

Effect of pressure. Increasing the pressure at constant temperature and amine concentration increases the degree of gas purification from acidic components, since the driving force of the process increases. Therefore, if it is necessary to purify low-pressure gas, it is advisable to pre-compress it. Typically, gas purification with amine solutions is carried out at a pressure of 2 to 7 MPa. Analysis of the material described above shows that the natural gas desulfurization technologies developed and successfully implemented in the world are designed for high raw material throughput and cannot always be adapted for APG preparation. Relatively small volumes and compositional features (high content of heavy hydrocarbons C₅₊ , ratio of hydrogen sulfide to carbon dioxide) prevent the use of classical alkanolamine purification plants for removing hydrogen sulfide from associated gas. An integral stage of the amine gas desulfurization technology is the regeneration of the absorbent when it is heated to 110–130 ^oC with the formation of a regenerated amine solution and “acid gas” containing predominantly hydrogen sulfide and carbon dioxide [8]. Processing of acid gas requires additional technology for the utilization of hydrogen sulfide by converting it into elemental sulfur in the process of high-temperature catalytic oxidation at the Claus plant, which, due to high capital intensity and energy consumption, operates only as part of large gas processing complexes and has a capacity of at least 5000 tons/year of sulfur. Alkanolamine purification processes without a Claus installation solve the issue of hydrocarbon gas desulfurization. But at the same time, the problem of recycling hydrogen sulfide-containing “acid gas” remains. Flaring of amine regeneration gas with high concentrations of H₂S results in emissions of toxic sulfur oxides. An alternative to the Claus process for small installations may be processes based on the direct oxidation of hydrogen sulfide contained in acid gas.

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