COMPARISON OF THE MOISTURE ABSORPTION EFFICIENCY OF EG, DEG, AND TEG USED IN THE NATURAL GAS DRYING PROCESS

ABSTRACT

In this article, the absorbents used in the industrial process of drying natural gas, mainly glycols, and their sorption properties were studied through the research method. In the research work, some advantages and disadvantages of the sorbent properties of DEG and TEG, which are widely used in the process of drying natural gas today, were compared. The operating principle of the Easidew PRO IS hygrometer and the requirements for the dew point of moisture in natural gas were studied using the device. We can see that by adding anti-corrosion inhibitors to TEG, the corrosion of the equipment by the acids formed during the degradation process is significantly reduced.

АННОТАЦИЯ

В данной статье исследовательским методом изучены абсорбенты, используемые в промышленном процессе осушки природного газа, в основном гликоли, и их сорбционные свойства. В исследовательской работе проведено сравнение некоторых преимуществ и недостатков свойств сорбентов ДЭГ и ТЭГ, которые сегодня широко используются в процессе осушки природного газа. С помощью прибора изучен принцип работы гигрометра Easidew PRO IS и требования к точке росы влаги в природном газе. Мы видим, что при добавлении в ТЭГ ингибиторов коррозии коррозия оборудования кислотами, образующимися в процессе деградации, значительно снижается.

Keywords: Absorption, gas hydrate, EG, DEG, TEG, dew point, Easidew PRO IS device, freezing temperature, concentration, advantage, disadvantage.

Ключевые слова: Абсорбция, газовый гидрат, ЭГ, ДЭГ, ТЭГ, точка росы, прибор Easidew PRO IS, температура замерзания, концентрация, преимущество, недостаток

Introduction. Currently, many gas and gas condensate fields have entered a phase of declining production, which complicates gas preparation in fields, especially in terms of ensuring the quality indicators of natural gas. In order to comply with regulatory requirements for the quality of natural gas supplied to main gas pipelines, it is necessary to improve the calculation, technological and measurement methods for studying the efficiency of gas drying technological equipment. To clarify the thermodynamic methods for calculating the moisture content and dew point temperature for aqueous phases (liquid water, ice and gas hydrate), additional experimental studies of the phase equilibrium in the “natural gas - water - hydrate” system are required. Therefore, the development and improvement of methods for determining the technological parameters of natural gas drying devices is a topical research topic[1].

Literature analysis and results. The absorption method of gas drying is widely used in gas processing, based on the difference in partial pressures of water vapor in the gas and the dehydrator. Gas drying devices are used in gas processing plants to dry purified commodity gas before feeding petroleum gas to low-temperature absorbers or condensing units operating at temperatures not lower than -10 +30°C, as well as before feeding gas to main gas pipelines, i.e. mainly in the absorber unit[2].

The following parameters of glycol solutions are important for drying processes:

1. Ability to dry solutions at different temperatures and different concentrations during the absorption process
2. Density of glycol solutions of various concentrations at different temperatures.
3. The boiling point of glycols in aqueous solutions.
4. Freezing point of glycol solutions of various concentrations.

Comparing the physical and mechanical properties of ethylene glycol with diethylene glycol and triethylene glycol allows us to identify the following comparative characteristics[3].

1. The freezing point of ethylene glycol solutions is lower than that of DEG and TEG solutions.
2.  Ethylene glycol has a high level of prevention of hydrate formation, and it also has a low viscosity property at the working temperature during the drying process and has a low solubility in liquid hydrocarbons.
3. The vapor pressure of a 99% concentrated EG solution at 20°C is 2.5 times higher than that of TEG and 7 times higher than that of DEG. Due to this property, significant losses occur during the drying process of ethylene glycol, which is why it is not widely used in industry.[4]

The high-temperature heating property of TEG is an advantage over DEG. Due to this advantage, it is possible to regenerate it without using a vacuum even at a mass fraction of 98.2%. For DEG, this figure is 96.7% by mass[5].

With such glycol solutions, at a contact temperature of 25°C, under equilibrium conditions, the gas can be dried to a dew point of minus 18 and minus 1°C, respectively.[6] It follows that when drying gas with a DEG solution, if the vacuum system is ineffective when installing glycol regeneration, it will not be possible to obtain conditional gas. At the same time, when using TEG, the gas is close to the standard.

In addition, one of the important advantages of TEG is its low saturated vapor pressure. This ensures low TEG losses in the vapor phase during gas drying. We can see that during gas drying, TEG losses can be reduced to 0.3-1.6 g/1000 m³ at a temperature range of 10-20°C, which is typical for gas drying units operating in SCS (Compression Compressor Stations) before natural gas enters the unit during drying. TEG parameters are important for its contact even at temperatures of 30-40°C and higher. It should be noted that the low saturated vapor pressure of TEG ensures low losses with the reflux liquid. DEG losses of up to 12-16% are associated with the reflux liquid, i.e. switching to TEG reduces glycol losses by 6g/1000 m³ of dried gas. The most important difference between the glycols considered is their solubility in hydrocarbons in the liquid phase. This indicator is significantly higher for TEG, which is the main disadvantage of using TEG for drying.

Studies were conducted under stable operating conditions to determine the efficiency of the absorbers of the complex gas purification plant when using TEG as a dryer. The droplet formation of liquid with gas in the absorbers was determined using a method developed by the Central Design Bureau of Petroleum Equipment. Taking into account the small values ​​of TEG absorption with gas and to reduce the measurement time, the values ​​were determined by the gravimetric method. The dew point was determined using the EASIDEW PRO IS device.(Fig. 1)

Figure 1. Overview of the EASIDEW PRO IS structure

The operating principle of the Easidew PRO IS hygrometer is based on impedance technology (sorption measurement method): the ceramic base of the converter consists of three layers: an active adsorbent layer and two separated porous superconducting layers. Water molecules from the analyzed gas freely penetrate the adsorbent layer through the permeable layer, changing the dielectric constant of the medium between the conductive layers. Some of the main characteristics of the Easidew PRO IS hydrometer are presented in the table below. (Table 1)

Table 1.

Some features of the EASIDEW PRO IS device

The main principles of the advantage of using TEG instead of DEG in gas purification plants were confirmed; The technological losses of TEG (passage through absorbers and regenerators) were found to be 20 mg/m³; The contact temperature of the absorber was 7.2-7.4°C, the dew point of the moisture was not higher than minus 20 ° C, and the regeneration of TEG concentration was determined to be 98.5% by mass; It was proved that the TEG regeneration unit reaches a TEG concentration of 98.6% by mass. Taking into account the above, the main physical and chemical indicators of DEG and TEG are compared in the table below. Table 2.

Table 2.

Some physical and chemical characteristics of TEG and DEG

TEG is a more expensive sorbent, four times more expensive than DEG. However, calculations based on the fact that DEG has four times higher dry gas losses than TEG confirm the advantages of using TEG. When drying gas with DEG, additional energy is required to create a vacuum in the desorber, which further increases the cost of using DEG.

During the recovery of DEG, the difference between the decomposition temperature and the operating temperature of the system does not exceed 4^○C, which leads to its partial destruction and, accordingly, a decrease in the quality of gas drying. To neutralize the decomposed DEG elements, it is necessary to add special additives, which requires additional costs not only for the purchase, but also for storing the reagent.

There are some main disadvantages of absorption drying of gases using absorbents, such as insufficient water desorption depth, reduced absorption efficiency, glycol foaming in the absorber, corrosion of equipment with acids formed during glycol degradation, and loss of glycol with water vapor during desorption. When using TEG, it was found that when 0.01% of the INKORGAZ 114 anti-corrosion inhibitor was added to TEG, corrosion of equipment with acids formed during TEG degradation was significantly reduced. As a result of the research, the physicochemical properties of TEG and DEG were studied and the following advantages and disadvantages were presented.

The advantages of DEG. It has high hygroscopicity, good stability towards sulfur compounds, and stability towards oxygen and CO₂ at normal temperatures. Its advantage is that its concentrated solutions do not crystallize under absorption conditions.

The disadvantages of DEG. Compared to TEG, the loss due to gas leakage is much higher. The difficulty in obtaining a DEG solution concentration of more than 95% during regeneration is due to the smaller dew point depression compared to TEG. The price is also higher compared to them.

Advantageous aspects of TEG. It has high hygroscopicity. It provides a high degree of depression of the dew point of the drying gas (27.8-47.3°C). Good stability to sulfur compounds, stability to oxygen and CO₂ at normal temperatures. Its main advantage is that it is easy to obtain an active concentration of 99% during regeneration without any difficulties, and its concentrated solutions do not freeze.

Disadvantages of the TEG. High capital costs in operation. High tendency of TEG solution to foam with light liquid hydrocarbons. Higher solubility of hydrocarbons in TEG compared to DEG.

In conclusion: Thus, the analysis showed that from a technological point of view, TEG has a number of significant advantages over DEG, but has a higher cost. However, the combination of factors such as low consumption, low permissible concentration of the regenerated solution, and lower losses with dried gas provide favorable conditions for the use of TEG in absorption technology for drying natural gas.

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