MECHANISM OF ABSORPTION OF H2S, CO2 AND OTHER SULFUR COMPONENTS BY AN AQUEOUS SOLUTION OF MDEA

ABSTRACT

Oil production and refining is a complex process consisting of a huge complex of processes and apparatus. Despite the fact that oil in the modern world is one of the most popular substances that continues the technological process and makes people’s lives easier, the oil industry poses a huge danger to the environment at its various levels: water, air, soil, etc. d.

This article describes the mechanism of absorption of H₂S, CO₂ and other sulfur components by an aqueous solution of MDEA.

АННОТАЦИЯ

Добыча и переработка нефти является сложным процессом, состоящим из огромного комплекса процессов и аппаратов. Несмотря на то, что нефть в современном мире является одним из самых востребованных веществ, которое продолжает технологический процесс и облегчает жизнь людей, нефтяная промышленность несет в себе огромную опасность для окружающей среды на самых разных ее уровнях: вода, воздух, почва и т.д.

В этом статьи приведено механизм поглощения H₂S, CO₂ и других сернистых компонентов водным раствором МДЭА.

Keywords: gas, amine, carbon dioxide, sulfur, hydrogen sulfide, absorption, chemical mechanism

Ключевые слова: газ, амин, углерод диоксид, сера, водород сульфид, абсорбция, химическая механизм

Reserves of fuel and energy resources will meet humanity's needs for the next century. However, certain environmental and economic restrictions are imposed on meeting these needs. These are, first of all, fairly strict restrictions on CO₂ emissions in the world at the 1990 level, as well as difficulties that have arisen in recent years with planning the use of nuclear energy. Tightening environmental requirements leads to the fact that the most competitive of all types of primary fuel and energy resources is natural gas. This is due to the fact that CO₂ emissions from gas combustion are lower than from the combustion of fuel oil and coal: gas combustion per 1 kWh of primary energy is accompanied by CO₂ emissionsin the amount of 0.2-0.22 kg of fuel oil, respectively – 0.26-0.28 kg, hard coal – 0.33 kg, brown coal – 0.4 kg. It should be taken into account that when using natural gas in large power plants with gas turbines, the efficiency exceeds 50%, and new generation heating boilers have an efficiency of over 90%, which accordingly reduces the consumption of primary fuel and specific CO₂ emissions per unit of final energy. If over the past 20 years, global energy consumption has increased by 38%, then natural gas – by 65%, while oil – by 12% and coal – by 28%. During this time, the share of natural gas in the balance of primary energy resources increased from 17 to 21%, the share of oil decreased from 49 to 40%, and coal from 30 to 27% [1].

Thus, the role of gas, as the most environmentally friendly type of fuel, is noticeably increasing and, according to expert forecasts, its share in the world’s energy balance could reach 28-30% by the middle of the 21st century.

The mechanism of absorption of H₂S and CO₂ by aqueous solutions of amines is considered in works [1-2]. Alkanolamines, being alkalis, easily react with acidic gases H₂S (CO₂), forming water-soluble salts. In this case, the following reactions occur [3]:

All amines react with H₂S in the same way to form the amine hydrosulfide or sulfide, the reaction being classified as instantaneous. Primary and secondary amines can react with CO₂ to form carbamate (salt of substituted carbamic acid – amine COO^- * H⁺), which is interpreted as a fast second-order reaction. In addition, carbonates and bicarbonates of amines are formed with CO₂ , but their formation is preceded by a slow dissolution reaction of CO₂ in water with the formation of carbonic acid H₂CO₃. It is believed that at moderate degrees of amine carbonization (up to 0.5 mol CO₂/mol amine), the reaction of carbamate formation occurs predominantly. Carbamates amines are unstable compounds and in a slightly alkaline environment they slowly decompose to form bicarbonate (using the example of a secondary amine) [4]:

where R – HOCH₂ CH^-.

Tertiary alkanolamine does not have a mobile H⁺ atomin the amine group, so it becomes impossible for a direct and rapid reaction with CO₂ to occuraccording to the carbamate type, and the interaction occurs through a preliminary and slow stage of formation and dissociation of carbonic acid [5]:

The end products of the reaction are bicarbonate and carbonate. Thus, the difference in the rates of reactions of tertiary amines with H₂S (instantaneous reaction) and CO₂ (slow reaction) is much greater than for primary and secondary amines. This makes it possible to use tertiary amines in practice for the selective extraction of H₂S from its mixtures with CO₂.

In accordance with the given chemical reactions of H₂S and CO₂ with amines, the concentration of active (free) amine in solution can be calculated using the equation:

where С_ж – isthe concentration of free amine, mol/l; С_ж0– initial amine concentration, mol/l; α_А, α_B– amine saturation, respectively, H₂S (A) and CO₂ (B), mol/mol; n – stoichiometric coefficient (for primary and secondary amines n – 2, for tertiary amines n – 1).

The reactivity of alkanolamines varies in the order: primary > secondary > tertiary and correlates with their alkalinity.

Carbon dioxide forms various byproducts with alkanolamines. The mechanisms of their formation have not been fully studied. Some of them are destroyed at the stage of absorbent regeneration and again release alkanolamine, the other part is not regenerated, causing loss of amine. The largest number of non-regenerable compounds is characteristic of primary alkanolamines.

Mercaptans, being acids, react reversibly with alkanolaminesto form water-soluble mercaptides:

[Amin] + RSH⇄[Amin* H ]^- *RS^-,

whereR is a hydrocarbon radical ( CH₃; C₂H-; C₃H- etc.).

This reaction is preceded by the dissolution of mercaptans in the absorbent and dissociation into ions:

The acidity of mercaptans is significantly lower than H₂S and CO₂ , as a result of which the latter displace mercaptans from their compounds with amines. Mercaptides are unstable compounds that are easily destroyed when heated.

Carbon sulfide in aqueous solutions of amines is hydrolyzed :

COS + H₂O ⇄CO₂ + H₂S.

The resulting H₂S and CO₂ react with amines. However, due to the fact that the concentration of COS in the source gas is small compared to H₂S and CO₂, the direct reaction never reaches completion. The limiting stage of the process is the dissolution of COS in the absorbent. Carbon sulfide can react directly with primary and secondary amines to form thiocarbamates (similar to CO₂). COS is also capable of entering into other reactions with amines, the mechanism of which is not fully understood. In this case, non-regenerated products are formed with primary amines; Secondary and tertiary amines react with COS reversibly.

Also reacts with alkanolamines in a similar way. It forms by-products with both primary and secondary amines. Sulfides and disulfides do not interact with alkanolamines.

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