A METHOD FOR REDUCING CORROSION DURING GAS PURIFICATION FROM SULFUR COMPONENTS

ABSTRACT

At the Gazli Gas Processing Plant, the process of purification of gases from acidic components with metildietanolamin solution leads to the occurrence of corrosion in technological systems. It was therefore tested by gravimetric methods with an anticorrosion inhibitor at a temperature of 120 ^○C and several different concentrations, and inhibition technology was proposed.

АННОТАЦИЯ

На Газлинском ГПЗ процесс очистки газов от кислых компонентов раствором метилдиэтаноламина приводит к возникновению коррозии в технологических системах. Поэтому он был испытан гравиметрическим методом с антикоррозионным ингибитором при температуре 120 ^○С и нескольких различных концентрациях и предложена технология ингибирования.

Keywords: hydrogen sulfide, carbon dioxide, corrosion, gas condensate, hydrocarbon, inhibitor, methylethanolamine, gravimetrik, sulfur, desorber, TFO (thiosemicarbaside-based inhibitor) desorber, division.

Ключевые слова: сероводород, диоксид углерода, коррозия, газовый конденсат, углеводород, ингибитор, метилдиэтаноламин, гравиметрик, сера, десорбер, десорбер ТФО (тиосемикарбазный ингибитор), разделение.

Introduction

Corrosion damage to metal products, equipment, structures and devices worldwide account for 25-30 % of annually produced metals[1-3].

Therefore, it is possible to study the causes of corrosion in each production plant and achieve high efficiency in protection with anti-corrosion coatings and effective inhibitors suitable for this environment[4-8].

In particular, the main goal of the study was to reduce the corrosion rate at the Gazli GPP. An aqueous solution of methyldiethanolamine 35-40 % is used to purify natural gases from acidic components at the Gazli GPP[8-13].

When cleaning a solution of saturated methyldiethanolamine, H₂S and CO₂ included in its composition have an aggressive effect on the metal, leading to chemical corrosion of the metal and forming the following mechanisms of corrosion reactions[14-17].

СО_2(g) ↔ СО_2(l)

СО_2(l)+H₂O_(l) ↔ H₂CO_3(l)

H₂CO_3(l) ↔ HCO₃^-_(l)+H⁺_(l)

HCO₃^-1_(l) ↔ CO₃^2-_(l) + H⁺_(l)

2H⁺_(l) +2e^- ↔ H_2(l)

H₂S_(g) ↔ H₂S_(l)

H₂S_(l) ↔ HS^-_(l) + H⁺ _(l)

HS^-_(l) ↔S^2-+ H⁺ _(l)

2H⁺_(l)+ 2e^- ↔ H_2(g)

The composition of the methyldiethanolamine solution in the desorber of the Gazli Gas Processing Plant is shown in Table 1 below.

When cleaning gases from acid components, corrosive products in instruments and equipment are exposed to hydrogen sulfide, along with moisture, carbon dioxide, and therefore it is necessary to take measures to protect process equipment from corrosion.

Table 1.

The composition of the methyldiethanolamine solution for gas treatment of the Gazli GPP

Desorbers at the Gazli GPP are made of steel 09G2S and 12Х18Н10Т, and the corrosion rate is high, mainly due to the absence of alloying elements in steel 09G2S. It is for this reason that the studies considered measures to reduce the corrosion rate of steel grade 09G2S.

It is noteworthy that the corrosion properties of 09G2S steels under the action of CO₂ and H₂S gases were not considered. Alloys containing 09G2S steel in the presence of dissolved hydrogen sulfide and carbon dioxide in various media have a significant effect on the formation kinetics and composition of corrosion products. The question of the joint influence of these two factors on the development of corrosion processes is still open.

Based on this, studies of the corrosion process were carried out in time, temperature, pH and various concentrations of the medium.

Research method

On the basis of the given data, studies of laboratory work on the gravimetric method of corrosion rate according to SST 9.905-82 were carried out. Then 0.5 l of a saturated solution of methyldiethanolamine (pH of the medium <6.8) is transferred into the flask. After thorough cleaning and drying, a plate made of 09G2S steel with an area of ​​22 cm² is weighed on an analytical balance and placed in a solution. At a temperature of 120 ^○C and 360 h without inhibitors and inhibitor solutions, tests were carried out. A corrosion inhibitor of the TFO brand (thiosemicarbazidebased inhibitor) was used as an inhibitor (Fig. 2).

Figure 2. Laboratory instrument for the gravimetric study of corrosion inhibitors
1-heater; 2-flask; 3-metal plate; 4-refrigerator; 5-thermometer; 6-where the product falls

The metal plate is then cleaned with grinding paper and weighed on an analytical balance. The results are recorded and calculated using the following formula.

According to these formulas, the rate of corrosion (K_grav), the coefficient of braking () and the degree of protection from corrosion (Z).

where: m₁- is the mass of the metal plate at the beginning of the study, g: m₂ - is the mass of the metal plate after exposure, g: W₀ - is the decrease in the mass of the metal in an inert solution, W_inh- is the decrease in the mass of the metal in the solution under consideration, S - is the surface of the metal plate, cm²: t₁-exposure time, hour.

Table 2.

Results obtained by the gravimetric method on plates made of steel 09G2S in solution with and without inhibitors

As a result of the research, the image of metal plates in inhibitory and non-inhibitory media with an AD106-C microscope approaching 80 times is shown in Figure 3.

Figure 3. Microscopic view of 09G2S metal samples in a) inhibitor and b) inhibitor solutions

According to the results of gravimetric analysis, it was found that with inhibitor a) there is no corrosion of the inhibitor complex in the steel sheet, and without inhibitor b) the solution has uneven corrosion, wounds and pits on the metal sheet.

Based on the results of the study, a technological scheme was developed for the use of a new type of corrosion inhibitor TFO in technological devices in order to sharply slow down corrosion in gaseous media. (Figure 4).

Figure 4. Technology of application of anticorrosive corrosion inhibitor TFO, which occurs during amine gas purification from acidic components
1-MDEA solution saturated with acidic components;  2-capacity for corrosion inhibitor; 3-diaphragm pump; 4-desorber; 5-reboys; 6-flare gases; 7-purified MDEA; 8-water vapor

In Scheme 4, since desorber 1 is the main corrosion site, corrosion inhibitor TFO 3 with a saturated solution of methyldiethanolamine, which must be regenerated in reboiler 2, is fed into the desorber by diaphragm pump 4.

Results and its discussion

Changing the concentration of the TFO inhibitor had a high impact on the anticorrosion efficiency and corrosion rate, which was reflected in the results obtained. For example, at a temperature of 120 ^○C, the corrosion rate in a metal plate without inhibitors was 0.1111 g/(cm²∙h), and the corrosion rate decreased to 0.0058 mg/(cm²∙h) when the inhibitor concentration reached 100 mg/l.

Correspondingly, the anti-corrosion efficiency of the inhibitor was 94.7 %, the braking coefficient was 19.13%.

This is due to the fact that at high temperatures the adsorption capacity of the inhibitor is lower than the desorption capacity, and an increase in temperature breaks the electrostatic bonds between the inhibitor and iron atoms on the surface of the steel sample. The results of this study showed that the TFO inhibitor is a good effective inhibitor in amine gas cleaning, and the use of 0.1% corrosion inhibitor against corrosion in gas cleaning from sulfur compounds showed high efficiency.

Conclusion

It can be concluded from the article that the concentration of the corrosion inhibitor TFO of 0.1% turned out to be quite effective when cleaning gases with methyldiethanolamine solutions to reduce the corrosion rate in devices and technological systems under the influence of acidic components.

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