ABSORPTION AND ADSORPTION METHODS FOR NATURAL GAS PURIFICATION FROM HYDROGEN SULFIDE AND MERCAPTANS

ABSTRACT

This article presents the results of various proposed new compositions of absorbents in the purification of gases from acidic components. At the same time, the corrosive effect of these absorbents on gas cleaning devices was studied, methods for reducing this corrosive activity were developed and proposed.

АННОТАЦИЯ

В данной статье представлены результаты различных предлагаемых новых композиций абсорбентов в очистке газов от кислых компонентов. В то же время было изучено коррозионное воздействие этих абсорбентов на устройства газоочистки, разработаны и предложены методы снижения этой коррозионной активности.

Keywords: amines, absorbent, carbon dioxide, sulfur, gas purification

Ключевые слова: амины, абсорбент, диоксид углерода, сера, очистка газов

When RSH mercaptans interact with alkalis in the presence of O₂ and CO₂, which are always, albeit in small quantities, in gases, di- and polysulfide’s are formed, which are poorly soluble in the absorbent. Carbon supplied, organic sulfides RSR' (and a number of other compounds) are neutral in nature and dissolve in these absorbents, although their sorption capacity is much less than that of RSH. The presence of CO₂ in gases above 0.1–0.3% leads to its preferential dissolution, significantly reducing the absorption of RSH. In natural gases, the CO₂ content is usually above 0.7%, which makes it difficult to use these methods for fine purification. The methods are also inefficient for purification from thiophene C₄H₄S and its derivatives [1].

Solutions of mono- and diethanolamine with various activating additives, such as N-methylpyrrolidone-2, dipropanolamine, etc., are also widely used to remove acidic sulfurous substances [2].

Adsorption methods have become widely used. They are based on the selective absorption (adsorption) of sulfur compounds by solid sorbents. As a rule, adsorption is carried out at a temperature of 20–50 °C and elevated pressure, and regeneration (desorption) of an adsorbent saturated with sulfurous substances is carried out at low pressure and a temperature of 100–350 °C. For regeneration, any of the inert gases, low-sulfur natural or petroleum gas, water vapor, etc. are passed through the adsorbent layer [3].

In a number of cases, catalytic reactions proceed simultaneously with adsorption, as a result of which sulfurous substances are converted into elemental sulfur, which is extracted during regeneration and then used.

As an adsorbent, activated carbon of the AR-3, SKT-1 and other brands, as well as coal with alkali additions, is used. At the same time, along with purification from sulfurous substances, benzene and toluene are also extracted from gases, which are then released during regeneration.

For the purification of polysulfurous natural and petroleum gases, molecular sieves (zeolites) of the CaA and, especially, NaX brands have become widely used as adsorbents. Their adsorption capacity largely depends on the content of H₂O, CO₂ and higher hydrocarbons in gases, operating conditions and the degree of purification and ranges from 2 to 18% [4].

The presence of heavy hydrocarbon vapors in the gas has a significant effect on the capacity of zeolites for sulfur compounds. According to the degree of sorption on zeolites, the compounds that are part of natural gas can be arranged in a row:

H₂O>RSH>H₂S>COS>CO₂.

The main problem of adsorption purification of gas on zeolites from hydrogen sulfide in the presence of CO₂ is that during the adsorption of CO₂ and H₂S, carbon sulfide (COS) is formed according to the reaction:

CO₂ + H₂S → COS + H₂O.

Although the equilibrium constant of this reaction is small and amounts to 6.6∙10-6 at 298 K, however, the almost complete removal of H₂O vapors in the frontal layer of the zeolite shifts the equilibrium to the right, and this leads to the formation of significant concentrations of COS. The regeneration of zeolites is carried out with nitrogen, low-sulfur natural or petroleum gas, and in the regeneration gases (regenerators) the content of sulfurous substances increases by 5–10 times compared to the initial one. In addition to coals and zeolites, aluminum oxide, bauxites, aluminosilicates, etc. are also used in the purification process. The advantage of adsorption methods is the possibility of carrying out the process at low temperatures, as well as fine purification of gases not only from hydrogen sulfide, mercaptans, organic sulfides, but also from substances that are difficult to remove by other means, such as thiophene and its derivatives.

This method also has a number of significant drawbacks. Almost all gases contain a certain amount of H₂O, CO₂ vapors, higher hydrocarbons, which are well adsorbed by coals and zeolites, which reduces the sulfur capacity of adsorbents. The periodic purification process requires the installation of several columns operating in parallel: on some, sulfurous substances are absorbed (adsorption stage), and on others, adsorbents are regenerated.

Chemisorption and catalytic methods

The disadvantages inherent in absorption and adsorption methods force the use of more universal catalytic and chemisorption methods.

They can be divided into the following groups:

a) catalytic: organosulfur substances undergo hydrogenolysis to saturated hydrocarbons C_nH_{2n + 2} and H₂S, destruction (cracking) with the formation of unsaturated hydrocarbons C_nH_2n and H₂S, hydrolysis with elimination of H₂S and its oxidation to SO₂;

b) chemisorption: interaction of sulfurous substances with metals or their oxides occurs with the formation of metal sulfides;

c) chemisorption-catalytic: in its first stage, chemisorption processes occur, in the second, after partial sulfidation of the contact, simultaneously chemisorption and catalytic processes on the formed metal sulfides as catalysts, and in the third, after complete sulphurization, only catalytic processes.

Among the catalytic methods, the methods of hydrogenolysis of organosulfur substances are the most widely used [3]. For this purpose, catalysts based on Ni, Mo, Co, W, etc. are widely used. At the same time, the following reactions can occur in the temperature range of 300–450 °C:

RSH + H₂ → RH + H₂S,

RSR' + 2H₂ → RH + R'H + H₂S,

C₄H₄S + 4H₂ → C₄H₁₀ + H₂S,

COS + H₂ → CO + H₂S,

COS + 4H₂ → CH₄ + H₂O + H₂S,

CS₂ + 2H₂ → C + 2H₂S,

CS₂ + 4H₂ → CH₄ + 2H₂S.

Catalysts based on elements of groups VI and VIII are widely used for hydrodesulfurization. Basically, Co or cheaper Ni (3–5%) and Mo (10–15%) deposited on active γ-Al₂O₃ are used. For stable operation of catalysts, it is necessary that the gas contains at least 5% (preferably 9–11%) hydrogen. The presence of CO and CO₂ in the purified gas in the amount of 1–2% does not affect the purification process.

A simplified approach to the calculation of the hydrogenation stage is as follows [5]. Assuming that: a) the reaction order in terms of organic total sulfur is the first; b) the temperature along the gas is constant; c) ideal displacement occurs in the catalyst layer, we get

V = k/ln(cin/cout),

where  V is the volumetric velocity of the gas flow; k is the rate constant; cin and cout are the input and output contents of organic total sulfur, respectively.

With different input contents c_1in and c_2in for the corresponding volumetric velocities V₁ and V₂, at which the values of cout are the same, we have:

V₂ / V₁ = ln(c_1in /cout)/ ln(c_2in /cout).

With an allowable content of organic sulfur after the hydrogenation stage of 1 mg/m³, nominal value c_1in = 80 mg/nm³, actual maximum single content (JSC Metafrax, Gubakha, Perm region, see section 3) c_2in = 11.1 mg/m³, V₁ = 1650 h^-1 (nominal load of the hydrogenation apparatus), we obtain for the maximum allowable hydrogenation volumetric rate at the actual sulfur content V₂ » 3000 h^-1. A more active catalyst, with a higher k value, can be loaded even less.

In gas and petrochemistry, desulphurization masses based on oxides of zinc, copper, chromium are mainly used, which have received significant distribution. Chemisorbents can also be used for single-stage purification, if the source gas contains practically only H₂S or organic sulfur substances in an amount of not more than 5–7 mg/m.

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