CLEANING OF METHYLDIETHANOLAMINE AND DIETHANOLAMINE SOLUTIONS USED IN NATURAL GAS CLEANING

ABSTRACT

The work determined the composition and amount of heat-resistant salts contained in methyldiethanolamine and diethanolamine solutions, the secondary raw material of gas purification enterprises, as well as the purification and reuse of these solutions. Determining the reasons for the formation of thermally resistant salts in the vicinity of the absorption working solution for cleaning natural gas from sour components, researching the thermal methods of regeneration of amine absorbents, studying the processes of regeneration of ethanolamine absorbents in combined methods; improvement of natural gas purification technology with efficient regeneration and reuse of used alkanolamine solutions was studied

АННОТАЦИЯ

В работе определены состав и количество термостойких солей, содержащихся в растворах метилдиэтаноламина и диэтаноламина, вторичного сырья газоочистных предприятий, а также способы очистки и повторного использования этих растворов. Определение причин образования термостойких солей вблизи рабочего раствора абсорбции для очистки природного газа от кислых компонентов, исследование термических методов регенерации аминных абсорбентов, изучение процессов регенерации этаноламинных абсорбентов в комбинированных методах; исследовано совершенствование технологии очистки природного газа с эффективной регенерацией и повторным использованием отработанных растворов алканоламинов.

Key words. Gas purification, diethanolamine, methyldiethanolamine, hydrogen sulfide, absorption, thermal salts, technology

Ключевые слова. Газоочистка, диэтаноламин, метилдиэтаноламин, сероводород, абсорбция, термические соли, технология

It is an effective method to clean the natural gases produced worldwide from aggressive components with carbon dioxide and hydrogen sulfide in aqueous solutions of ethanolamines. At the same time, it is important to completely clean the thermally resistant salts contained in ethanolamines, to restore the initial activity of amines in the regeneration of working solutions of used absorbents by separating the organic part of detoxication products, and to improve the technological system of cleaning natural gases from sour components.

Today, in-depth research is being conducted in the world aimed at cleaning gases from acidic components, regeneration and reuse of the used working solution. In this regard, special attention is paid to comparing the chemical composition, physical and operational properties of ethanolamine solutions used in the purification of gases from sour components, determining the causes of thermal destruction of amine absorbents, accelerating and testing the technologies of regenerating the working solution by vacuum distillation, thermal decomposition, driving with water vapor, separation using adsorbates. . 

In our republic, special attention is paid to the improvement of the technological system with the regeneration of amines used in the purification of natural gases from sour components and the reuse of purified working solutions, and certain scientific results are being achieved. In this regard, the development of technologies for the regeneration of used working solutions contaminated with thermal destructuring products of amines, such as a number of oxocompounds, carboxylic acids, heterocyclic compounds with oxygen and nitrogen heteroatoms, and the creation of targeted areas of application of purified amines are highlighted.

In particular, at the Shortan Oil and Gas Production Department, a large gas processing enterprise in our country, gas is adsorbed using zeolite. The regeneration gas sent from these devices contains COS, CO2, H₂S gases, which are sent to absorption purification devices using amine-methyldiethanolamine (MDEA) and diethanolamine (DEA) and are cleaned of additional acidic gases. The presence of chlorine ions in the environment causes the formation of heat-resistant salts and an increase in the amount of MDEA and DEA solutions used in the device. As a result, metal ions present in the amine solution, such as iron ions, contain mechanical impurities, which form surfactants in MDEA and DEA solutions. This, in turn, causes the deterioration of pipes and equipment, and the need to replace the amine solution with a new one every two years. Also, when additional amine is poured into the device, one mole of pure amine neutralizes one mole of heat-resistant salts, that is, even in the case of burning pure amine, the concentration of the amine solution decreases due to the consumption of neutralization compared to the theoretical calculation.

Supersaturation of the amine solution, increase in pressure and temperature, increase in the amount of potassium and sodium chloride salts, the presence of metal compounds such as iron, chromium, manganese in the amine solution lead to the destruction of MDEA and DEA molecules, resulting in heat-resistant salts and organic leads to the formation of high molecular compounds, surfactants. Such compounds create a highly corrosive environment and friction. As a result, corrosion conditions are observed in the spare parts of pumps and internal parts of the equipment.

Cleaning and reuse of used amines in the process is one of the problems of enterprises in this direction [1-3].

In general, three different methods of purification of the amine solution used in the experiment were used:

1-simple driving of used MDEA,

2-vacuum-distillation of used MDEA

3-methods of cleaning the used MDEA using special sinks We used the following equipment in the simple method of driving the used MDEA: a heating device, a Vyurs flask, a thermometer, pipes for connecting the flask and the cooler during driving, an alonge with a grinder for connecting the cooler and the collecting container, a libix cooler, a collecting flask are needed.

If the used MDEA does not decompose at boiling temperature, a simple driving method is used at atmospheric pressure. An ordinary Würs flask is used as a driving flask. The mouth of the flask is closed with a stopper with a thermometer, the mercury part of the thermometer should be placed 0.5 cm below the mouth of the tube removed from the flask, and the amount of liquid put into the flask for driving should be up to 2/3 of the volume of the flask. In order for the liquid to boil evenly, a glass capillary with one end welded or a porous boiler (pieces of brick and porcelain) is inserted into the boiling flask. A 1-liter saturated aqueous mixture of MDEA was added to the process, and fractions from 50°C to 250°C were separated step by step using a simple driving method, and the solutions in each fraction were analyzed for elements.

In aqueous solutions of MDEA, the following processes occur between hydrogen sulfide and carbon dioxide and amines:

(HO – CH₂ – CH₂)₂ – NCH₃ + H₂O ↔ ((HO – CH₂ – CH₂)₂ – NCH₂⁺) + OH^-

H₂S + H₂O ↔ HS^- + H₃O⁺

((HO – CH₂ – CH₂)₂ – NCH₂⁺) + HS^- ↔ ((HO – CH₂ – CH₂)₂ – NCH₂⁺)HS^-

CO₂ + H₂O ↔ HCO₃^- + H₃O⁺

((HO – CH₂ – CH₂)₂ – NCH₂⁺) + HCO₃^- ↔ ((HO – CH₂ – CH₂)₂ – NCH₂⁺) HCO₃^-

Unlike tertiary amines, primary and secondary amines react directly with carbon dioxide to form carbamates. The process goes like this:

CO₂ + (HO – CH₂ – CH₂)₂ – NH + H₂O ↔ (HO – CH2 – CH2)₂ – NH + H₃O⁺

(HO – CH2 – CH2)₂ – NH + H₃O⁺ + (HO – CH₂ – CH₂)₂ – NH ↔ (HO – CH₂ – CH₂)₂ – N – COO^- + ((HO – CH₂ – CH₂)₂ – NH₂) ⁺ + H₂O

COS in the solution forms difficult-to-decompose compounds with amine:

2R₂NH + COS → R₂NCOSH NHR₂

R₂NCOSH NHR₂ → R₂NH + H₂S + R₂CNO

2R₂CNO + H₂O → (R₂N)₂CO + CO₂

In addition, aminoglycol, glycine, biocin, glycol, shovel and formic acids are formed, their forces increase corrosion and form less soluble, stable salts of metals such as calcium, magnesium, iron, chromium, manganese, nickel.

The following results were obtained when the samples of amine solutions of "Shurtan Oil and Gas Production Department" were analyzed for elements:

Figure 1. Is an unused MDEA IQ-spectrum in a clean state

Figure 2. Used MDEA IQ-spectrum [4]

Figure 3. IQ spectrum of regenerated MDEA

When we compared the IR spectra of pure, used and purified MDEA, it was found that the intensity of ON, C–O absorption bands in the spectrum of purified MDEA coincided with the intensity of the bands of the same group in the pure sample. In contrast to the clean sample, the presence of additional bands (1656, 1598 cm–1) was found in the IR spectrum of the purified sample.

Elemental analysis of MDEA solution.

The result of the analysis shows that the solution contains a high amount of sulfur, chlorine, potassium, calcium, chromium, manganese, iron elements and a small amount of phosphorus, silicon, and nickel elements. From these results, it can be concluded that elemental sulfur forms non-regenerative stable sulfur compounds.

Most of the unrefined products in the used alkanolamines are heterocyclic compounds, based on the formation of MDEA as a result of SO₂ reactions: it was determined that the reaction processes take place in different stages, and reaction schemes were suggested

Combined methods of cleaning the used working solution from organic additives based on thermal and ion exchange from durable components were recommended, and an absorbent with restored physical and operational properties (viscosity 2.8 sPz; surface tension 71.35 dynes/cm; absorption volume, 0.044 mol/mol) was obtained.

If it is shown in the literature that the formation of 2-oxazolidone in the working solution is the result of thermal changes of carbamates.

We can see that it is formed from N-hydroxyethylcarbamic acid:

The formation of reactive 2-oxazolidone with basic properties in the working solution leads to chemical changes of many other heterocycles. For example, 1-(2-oxyethyl)-2-imidazolidone is the reaction product of 2-oxazolidone with another molecule of DEA in the reaction medium:

A two-stage "vacuum-distillation" method is recommended as one of the most effective methods for cleaning amine solutions from heat-resistant salts and destruction products. At the first stage, the solution is concentrated at atmospheric pressure, and at the second stage, it is driven with water vapor in a vacuum (-0.9 atm).[5-6]

Figure 4. Technological scheme of a vacuum-distillation cleaning device for amine solutions with water vapor

1,2 - columns, 3,5,7 - coolers, 4,6,8 - separators, 9 – chiller, 10 – collector, 11,12,13 – heaters, 14-20 – pumps

Based on the given technological scheme, the amine solution is heated in a heat exchanger with medium pressure steam and fed to the column to concentrate the absorbent. In the contact elements in the column, water is separated from amines and hard-to-decompose salts. Water vapor is condensed in an air-cooled device and separated in a separator. Products such as non-condensed hydrocarbons, destruction products, hydrogen sulfide, CO₂ are sent to disposal from the separator. Condensed water is used in the preparation of absorbent with removable amine [7-8].

From the bottom of the column, heated inert nitrogen gas is supplied to extract the maximum amount of water from the solution. The liquid mass is sent to the vacuum driving rectification column. Evaporated amine is cooled in a refrigerator and collected in a barometric collector. Part of the condensate is used to saturate the column.

References:

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