RU2245897C1 - Method of treating of process gases to remove hydrogen sulfide - Google Patents

Abstract

FIELD: gas treatment.

SUBSTANCE: invention is directed to remove hydrogen sulfide from CO₂-containing process gases from enterprises involving heat treatment of sulfur-containing combustible fossils in reductive medium. Treatment comprises countercurrent contact of gases with liquid basic absorbent, absorption of hydrogen sulfide with absorbent, separating hydrogen sulfide from associated carbon dioxide, and converting hydrogen sulfide into usable products. Absorbent is selected from aqueous solutions or suspensions of alkali and alkali-earth metal hydroxides and oxides. Contact of gas with absorbent is performed in vortex chambers provided with rotating gas-liquid layer at residence from 0.001 to 0.1 sec.

EFFECT: increased degree of purification of gases and simplified treatment scheme.

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Description

The invention relates to the field of purification from hydrogen sulfide of CO ₂ -containing process gases, in particular off-gas from plants producing heat treatment in a reducing medium of sulfur-containing combustible minerals (coking and gasification of coal, etc.).

Known methods for purification of CO ₂ -containing process gases from hydrogen sulfide by contacting gases with aqueous solutions of the main compounds - iron hydroxide (+3), soda, amines (JHCurrey. Iron Steel Eng. 72, 10, 1995, P.10; Z. P Ismagilov, M. A. Kerzhentsev, S. R. Khairulin, V. V. Kuznetsov. One-stage catalytic methods for the purification of acid gases from hydrogen sulfide. Chemistry for Sustainable Development, 7, 1999, p. 376).

A known method of purification of hydrogen sulfide gases of coke production, which is simultaneously the prototype of the invention (M. S. Litvinenko. Purification of coke oven gas from hydrogen sulfide. Vacuum-carbonate method. Kharkov. Metallurgizdat, 1959). The method includes three stages. On the first of the gas, hydrogen sulfide is captured by countercurrent contact with a 5% aqueous solution of Na ₂ CO ₃ in an irrigation absorber tower at an absorbent flow rate of 2-4 dm ³ per 1 m ^{3 of} gas, the gas stays in the absorption zone of the tower for 3 seconds and the temperature 30-45 ° C. When the content of hydrogen sulfide in the feed gas is 19-20 g / m ³ (about 1.3 vol.%), The degree of extraction of H ₂ S in the absorbent is 83.5-93.2%, and the content of H ₂ S in the absorbent reaches 5.4 -9.9 g / dm ³ . Together with hydrogen sulfide, the accompanying carbon dioxide is partially extracted into the absorbent; the degree of its extraction from gas under the described conditions is 3-4% with an initial content of about 60 g / m ³ (3 vol.%). In the second stage, the absorbent saturated with hydrogen sulfide is subjected to vacuum treatment at a vacuum of 595-645 mm RT. Art. and a temperature of 60-84 ° C. During this treatment, the absorbed hydrogen sulfide 98% goes into the gas phase. Concomitant H ₂ S components also pass into it. As a result, the gas phase has a composition, vol.%: H ₂ S 83.5-86; CO ₂ 10.3-10.8; HCN 1.55-2.56; air 1.65-3.60. In the third stage, the resulting rich hydrogen sulfide gas is processed into sulfuric acid or elemental sulfur by known methods.

The disadvantages of the prototype are:

- low degree of purification of gases from hydrogen sulfide;

- multi-stage process.

The basis of the invention is the technical task of increasing the degree of purification of gases from hydrogen sulfide and simplifying the technological and equipment purification schemes.

To solve the problems in the known method of purification of process gases from hydrogen sulfide, including countercurrent contact of gases with a liquid primary absorber, absorption of hydrogen sulfide by an absorber, separation of hydrogen sulfide from associated carbon dioxide and conversion of hydrogen sulfide to utilized products, according to the invention, aqueous solutions or hydroxide suspensions are used as an absorber or oxides of alkali or alkaline earth metals, and hydrogen sulfide is separated from carbon dioxide at the stage of gas contact with absorber, removing only hydrogen sulfide by them, due to contact in the vortex chambers with a rotating gas-liquid layer with a residence time of gases in the layer from 0.001 to 0.1 sec.

The stated technical problem of increasing the degree of gas purification from hydrogen sulfide is solved by switching to more active absorbers and reducing the residence time of gases in the contact zone to a value insufficient for the transition of carbon dioxide from the gas phase to liquid, but exceeding the absorption time of hydrogen sulfide, which is achieved by intensifying the contact of gases with absorbers, namely, conducting contact in the vortex chambers with a rotating gas-liquid layer.

According to the prototype for the purification of gases from hydrogen sulfide as absorbers use aqueous solutions of Na ₂ CO ₃ . During the cleaning process, H ₂ S goes into absorbers according to the reaction:

The reaction is reversible and, at a constant concentration of Na ₂ CO _{3, it} shifts to the left as NaHCO ₃ and NaHS accumulate in the absorber. This prevents the deep cleaning of gases from hydrogen sulfide. The degree of purification can be increased by increasing the concentration of Na ₂ CO ₃ in the absorber, however, the degree of H ₂ S distillation from the absorber at the stage of its regeneration decreases. Given these circumstances, in the prototype a compromise concentration of Na ₂ CO ₃ (0.5 mol / dm ³ ) was chosen, at which a deep gas purification from hydrogen sulfide cannot be achieved.

According to the invention, the purification proceeds by reactions of the type:

These reactions are shifted to the right to a much greater extent than reaction (1), since the protons necessary for the formation of H ₂ S in water are held more strongly by hydroxylions than the protons in NaHCO _{3 by} carbonate ions (pK ₂ H ₂ CO ₃ = 10.24; pK Н ₂ О≈14 (I.M. Koltgof, V.A. Stenger. Volumetric analysis, v.1. Goskhimizdat, 1950, p.23, 307). As a result, when using absorbers based on alkali hydroxides (oxides) or alkaline earth metals under comparable conditions achieve a more complete purification of gases from hydrogen sulfide and more complete saturation of the absorbers with hydrogen sulfide than in the case of soda absorbers.

However, the use of the aforementioned absorbers for gas purification from hydrogen sulfide under the conditions of the prototype — the residence time of gases in the contact zone (τ _cont ) for at least 3 seconds — is practically impossible due to the interfering effect of carbon dioxide. The content of CO ₂ in the gases to be cleaned is usually at least 3 vol.%, Which is higher than the content of H ₂ S (0.1-1 vol.%). Under these conditions, CO _{2 is} intensively captured by hydroxide (oxide) absorbers, which sharply worsens the performance of gas purification from hydrogen sulfide, reducing the degree of purification, the level of saturation of the absorbers with hydrogen sulfide and increasing the consumption of hydroxides (oxides).

The disturbing effect of CO ₂ according to the invention is eliminated by contacting the gases to be cleaned with absorbers at a value of τ _cont , less _{than the} time of establishing equilibrium of hydration of CO ₂ (τ _hydr ):

In pure water, the value of τ _hydr is 20-60 s (M.S. Litvinenko. Purification of coke oven gas from hydrogen sulfide. Vacuum-carbonate method. Kharkov. Metallurgizdat, 1959, p. 108). In the presence of basic compounds, in particular alkali and alkaline earth metal hydroxides (oxides), the reaction protons are bonded (neutralized) of reactions (6), (7) with hydroxides (oxides) into water, which leads to a shift of equilibria to the right. The degree of shear increases with increasing basicity of hydroxides (oxides). According to the invention, the value of τ _cont does not exceed 0.1 s. In accordance with the results described in the examples, this virtually eliminates the transition of CO ₂ from the gas phase to the liquid.

A decrease in the value of τ _cont under comparable conditions should lead to a decrease in the degree of extraction of hydrogen sulfide. This decrease according to the invention is compensated by the intensification of the contact of the gases with the absorbers, namely, the contact in the vortex chambers with a rotating gas-liquid layer (RF Patent No. 2084269, 01 D 47/06, 1993). Such a layer is characterized by a large specific interfacial surface, due to the high dispersion and dense arrangement of liquid droplets, a high rate of updating of the contact surface associated with a cascade process of crushing / merging of the droplets, and the uniformity of the microstructure associated with the cyclical motion of the layer. All this together determines the high efficiency of gas and liquid contact, in particular, the high mass transfer rate. For low molecular weight gases such as hydrogen sulfide, the values of the volumetric mass transfer coefficient in the gas phase are 10 ³ -10 ⁴ 1 / s. This means that the characteristic time for the capture of hydrogen sulfide molecules from gas by the interphase surface does not exceed 10 ^-4 -10 ^-3 s. Unlike carbon dioxide, the absorption of which is limited by hydration reactions (4) - (7), for hydrogen sulfide, all reactions at the interface and in the liquid phase for the absorbers selected according to the invention are fast. The examples described below show that a contact time of the order of 10 ⁻³ s is sufficient for the process of absorption of hydrogen sulfide as a whole. Thus, when gases containing H ₂ S and CO _{2 are} in contact with solutions of alkali and alkaline earth metal hydroxides (oxides) in a rotating gas-liquid layer, there is a contact time interval of 10 ^-3 -10 ^{-1 -1} s, in which selective extraction of only hydrogen sulfide is ensured.

The solution to the technical problem of simplifying the technological and apparatus circuits for cleaning gas from hydrogen sulfide according to the invention is achieved by reducing the mass and size characteristics of absorbers (by reducing contact time) and eliminating the stages of vacuum distillation of hydrogen sulfide from a saturated absorber and processing the gas enriched during vacuum distillation into sulfuric acid or elemental sulfur. As a result, instead of the three stages of the process of purifying gases from H ₂ S and converting it into a utilizable form according to the invention, there remains only one stage - the extraction of hydrogen sulfide from gases by absorbers with their saturation.

The final products formed by the proposed method — solutions of alkali (alkaline earth) metal bisulfides — can be used as sulfidizing agents in hydrometallurgical processes, in particular, in flotation processing of ores, as well as in other traditional applications (artificial fiber production, leather industry, etc. )

The invention is illustrated by the following examples.

Example 1

Three vortex chambers (VK) with a rotating gas-liquid layer with a capacity of 5 m ³ gas per minute with a gas stay in the contact zone of each vortex chamber (rotating BC layer) for 0.1 seconds are connected in series in accordance with the diagram. The scheme provides sequential countercurrent transmission of the gas to be cleaned through VK 1-2-3 and an absorber through VK 3-2-1. In addition to the counterflow, the absorber is recycled in the circuit through each VK using intermediate tanks B1-3. Constant in time and identical for all contact stages, the absorber duct ensures that the liquid level in B1-3 is constant. In accordance with the scheme, at the outlet of VK3, a gas purified from H ₂ S is obtained, which is discharged into the atmosphere. Fresh absorber is supplied to tank B3, and an absorber saturated with hydrogen sulfide is obtained at the exit from B1.

For cleaning use an artificial gas mixture containing, vol.%: H ₂ S - 0.2; CO ₂ - 3; air is the rest. The mixture is prepared by dosing H ₂ S and CO ₂ from individual cylinders into the air stream entering BK1. As an absorber, an aqueous suspension of calcium hydroxide Ca (OH) ₂ with an initial content of 3 mol / dm is used. $\genfrac{}{}{0ex}{}{\text{3}}{\text{.}}$ 8 dm ^{3 of} absorber is poured into each of B1-3 tanks, 0.075 dm ³ / min (0.015 dm ³ per 1 m ^{3 of} gas) is fed into B3 of fresh suspension and recirculation is organized through each of the VC with a flow rate of 25 dm ³ / min (5 dm ³ per 1 m ^{3 of} gas).

The purification is carried out with continuous iodometric control of the content of H ₂ S in the gas mixture at the outlet of VK3. The process continues until the stabilization of the composition of the absorber in B1-3. The system is stopped, the content of H ₂ S in the liquid part of the contents of tank B1 is determined iodometrically and the calcium content in it is complexometric.

According to the results obtained, the gas after cleaning at the exit from the refinery contained no more than 0.0015 mol / m ³ H ₂ S (no more than 50 mg / m ³ ), the degree of purification was more than 98%. The content of H ₂ S in B1 (at the outlet of the absorber) was 5.85 mol / dm ³ , and calcium 2.90 mol / dm ³ . This means that H ₂ S is absorbed in the form of an acid salt Ca (HS) _2, a Ca (OH) ₂ saturated with this salt only, Competitors transition in the CO ₂ absorber in the form of CaCO ₃ has virtually no space.

Example 2

The same as in example 1, but the duration of the gas mixture in the contact zone of each of the vortex chambers is 0.002 seconds. The results obtained on the composition of the absorber at the outlet are similar to those shown in example 1. The degree of purification from hydrogen sulfide is about 97%.

Example 3

The same as in example 2, but the number of contact steps is five, and the consumption of the absorber in each of the recirculation circuits is 0.5 dm ³ per 1 m ^{3 of} gas. The results obtained are similar to those in example 2.

Example 4

The same as in example 3, but the duration of the stay of the gas mixture in the contact zone of each of the vortex chambers is 0.001 seconds. The composition of the absorber at the output is similar to that shown in examples 1-3. The degree of purification from hydrogen sulfide is approximately 95%.

Example 5

Same as in example 4, but using a 6 mol aqueous NaOH solution as an absorber. The results obtained are similar to those in example 4.

As can be seen from the above examples, the proposed method allows to increase the degree of purification of gases from hydrogen sulfide and to simplify the purification scheme and can be widely used for purification of hydrogen sulfide from CO ₂ -containing process gases, in particular, waste gases from plants that perform heat treatment in a reducing medium of sulfur-containing fuels minerals (coking and gasification of coal, etc.)

Claims (1)

A method for purifying process gases from hydrogen sulfide, including countercurrent contact of gases with a liquid primary absorber, absorption of hydrogen sulfide by an absorber, separation of hydrogen sulfide from associated carbon dioxide and conversion of hydrogen sulfide to utilized products, characterized in that aqueous solutions or suspensions of alkali or alkali oxides or oxides are used as the absorber alkaline earth metals, and hydrogen sulfide is separated from carbon dioxide at the stage of gas contact with the absorber, removing only hydrogen sulfide, and by carrying out the contact in the vortex chamber with a gas-liquid layer rotating at a residence time of gases in the layer of from 0.001 to 0.1.