PURIFICATION OF COKE OVEN GAS FROM HYDROGEN SULFIDE BY THE VACUUM-SODA METHOD

K. K. STRAMKOVSKAYA, P. E. BOGDANOV, E. F. SERKOVA,
N. M. BELOGUROV, N. K. OVCHAREVICH

(Presented by the scientific-methodological seminar of the KhTF)

The vacuum-soda method has been introduced in the Soviet Union for the purification of coke oven gas from hydrogen sulfide at low pressure. However, the advan—

Fig. 1. Process flow diagram of a vacuum-soda purification unit for coke oven gas.
1 — absorber, 2 — expander, 3 — settling tank, 4 — pump, 5 — spent-solution tank, 6 — pump, 7 — filter, 8 — heat exchanger, 9 — heater, 10 — regenerator, 11 — circulation heater, 12 — spray trap, 13 — condenser-cooler, 14 — phase separator, 15 — mixing condenser, 16 — rotary wet compressor, 17 — moisture separator, 18 — intermediate tank, 19 — barometric tank, 20 — centrifugal pump, 21 — cooler, 22 — service tanks for regenerated solution, 23 — pump, 24 — fresh-solution tank, 25 — soda bunker.

advantages of this method—simplicity of the process scheme and reliability in operation, low reagent consumption and the virtual absence of harmful effluents; the possibility of comprehensive automation of the entire process with transfer of the equipment to remote control and, finally, the use of sulfur for producing sulfuric acid—indicate the expediency of applying this method for purifying coke-oven gas under pressure in process schemes at nitrogen-fertilizer plants.

This article presents the results of a study of the operation of an installation for purifying coke-oven gas from hydrogen sulfide by the vacuum-soda method at one of the nitrogen-fertilizer plants, where the existing bog-ore towers did not provide the required degree of purification. In the installation mounted in 1961, the process scheme of which is shown in Fig. 1, coke-oven gas after dry purification at a pressure of 12 atm is fed to a packed absorber 750 mm in diameter with a packing height of 2000 mm. The packing consists of 50 × 50 × 5 mm ceramic Raschig rings. Such absorbers are installed at each purification unit. The coke-oven gas from the absorbers goes to further purification, to water and alkaline absorbers, while the spent solution from all absorbers is sent to a collecting header and to the regeneration unit. The regenerator is of the tray type, with nine trough-type trays, one for all absorbers.

The purpose of our investigations was to determine the optimum regime for the processes of gas absorption and solution regeneration.

Study of the Absorption Process

The process of absorbing hydrogen sulfide by an aqueous soda solution is, according to modern concepts, a heterogeneous diffusion process accompanied by a very rapid reversible chemical reaction in the liquid phase, and proceeds according to the following equations [1]:

\[ \mathrm{Na_2CO_3 + H_2S \rightleftarrows NaHS + NaHCO_3,} \]

\[ \mathrm{Na_2CO_3 + CO_2 + H_2O \rightleftarrows 2NaHCO_3.} \]

The equilibrium under consideration represents a case of a complex system, when in the liquid phase $\mathrm{H_2S}$ and $\mathrm{CO_2}$ are simultaneously present in equilibrium with the absorbent solution.

The absorption of $\mathrm{H_2S}$ and $\mathrm{CO_2}$ from gas by a soda solution is substantially influenced by a large number of factors and, first of all, by the concentration of the absorbent, the absorption temperature, the irrigation density, and the residual concentration of $\mathrm{H_2S}$ in the absorbent solution. Therefore, in order to determine the optimum absorption conditions, the effect of the absorbent concentration was studied from 25 to 35 g/liter at a residual NaHS content in the solution of 2.5 to 3.5 g/liter; the irrigation density was varied from 11.3 to 18.2 m³/m² per h, which corresponded to a solution feed to the absorber of 5–8 m³/h. The absorption temperature was varied from 35 to 45°C.

The hydrogen sulfide content in the gas fluctuated from 600 to 14,000 ppm with the following average composition of the coke-oven gas: $\mathrm{CO_2}$—2.5%, $\mathrm{C_mH_n}$—2.4%, $\mathrm{O_2}$—0.5%, $\mathrm{CO}$—7.5%, $\mathrm{H_2}$—57.5%, $\mathrm{CH_4}$—26.5%, HCN—0.06%. Since the gas purification is carried out under a pressure of 12–13, this concentration of hydrogen sulfide in the compressed gas reaches almost 20 g/m³.

The results of the study are presented in Figs. 2–3.

It follows from the data obtained in Fig. 2 that, at an irrigation density of 14.7 m³/m²·h, the degree of purification of coke-oven gas from hydrogen sulfide increases substantially as the soda concentration in the solution changes from 25 g/liter to 35 g/liter, and reaches the final value for a temperature of 35°C—86.6%, 38°C—85.68%; 48°C—84.0%, 42°C—81.2%; 45°C—77.2%.

When the concentration of soda in the absorbing solution is increased from 25 to 30 g/l, the degree of extraction of hydrogen sulfide from the gas increases on average by 8.5%. When the concentration of soda is increased from 30 to 35 g/l, however, the degree of gas purification changes by only 1.7% for all the temperatures investigated. The residual hydrogen sulfide content in the gas at these soda concentrations is 140–170 ppm, which can be removed by the water-alkaline purification provided in the technological scheme.

Fig. 2. Dependence of the degree of purification of coke-oven gas on the concentration of the solution at an irrigation density of 14.7 m³/m²·h and a NaHS concentration in the solution after regeneration of 2.5 g/l.

The data in the graph in Fig. 2 show that the greatest degree of hydrogen sulfide capture is attained at an absorption temperature of 35°C. When the temperature changes from 35 to 38°C, the increase in the degree of gas purification is insignificant. Increasing the temperature above 38°C leads to a decrease in the degree of purification. It follows that the best capture of hydrogen sulfide occurs at 35°C. However, according to available literature data, it is known that lowering the absorption temperature to 35°C increases the absorption of carbon dioxide, which leads to a sharp decrease in the concentration of hydrogen sulfide in the gas after regeneration, to greater consumption of soda, to accumulation of bicarbonate in the solution, and also to an economically disadvantageous consumption of steam for regeneration. Therefore, the optimum absorption temperature should be considered 38–40°C, since at this temperature the greatest selectivity of absorption with respect to hydrogen sulfide is achieved.

As can be seen from Fig. 3, with an increase in the irrigation density of the absorber from 11.3 m³/m²·h to 14.7 m³/m²·h, the degree of purification of the gas from hydrogen sulfide increases (by 9–16%). A further increase in the irrigation density to 18.2 m³/m²·h leads to a smaller increase in gas purification, amounting to 2.9%. With an increase in the irrigation density, better contact is ensured between the gas and the absorbing solution. However, with an increase—

Fig. 3. Dependence of the degree of coke-oven-gas purification on the irrigation density at different NaHS concentrations in the regenerated solution: 1 — 2.5 g/l, 2 — 3.0 g/l, 3 — 3.5 g/l.

in the irrigation density, the total amount of absorbing solution increases, which leads to a significant increase in steam consumption for regeneration and excessive consumption of electric power for pumping the solution.

In order to ensure a sufficient degree of purification of coke-oven gas from hydrogen sulfide, and based on the operating experience of this installation, the most rational irrigation density should be considered to be 14.7 m³/m²·h, which corresponds to a solution feed to the absorber of 6.5 m³/h.

From these same graphs in Fig. 3 it can be seen that an increase in the concentration of bisulfide in the regenerated absorbing solution leads to a significant decrease in the degree of purification of coke-oven gas from hydrogen sulfide. With an increase in the residual concentration of bisulfide in the regenerated solution from 2.5 to 3.0 and 3.5 g/l, the degree of gas purification decreases.

approximately by 6:5 and 6.0%, respectively. Therefore, in order to improve the purification of gas from hydrogen sulfide, the concentration of bisulfide in the absorbing solution should be maintained at no higher than 3.0 g/l.

Study of the regeneration process

The process of regeneration of a soda solution saturated with hydrogen sulfide consists in shifting the equilibrium in the reversible reaction

\[ \mathrm{Na_2CO_3 + H_2S \rightleftarrows NaHS + NaHCO_3} \]

from right to left by lowering the partial pressure of hydrogen sulfide in the equilibrium system. The intensity of solution regeneration, as is known, depends on the degree to which the partial pressure of hydrogen sulfide above the solution is lowered. The latter will be greater, the more water vapor there is in the system, i.e., at the boiling temperatures of the solution. However, the rate of hydrogen sulfide desorption increases as the boiling temperature decreases

Fig. 4. Dependence of the degree of NaHS regeneration on the vacuum in the column at various solution-feed temperatures.

from 100 to 50–60°, i.e., with increasing vacuum. At one and the same temperature, the degree of hydrogen sulfide desorption is directly proportional to the concentration of sodium bicarbonate in the solution (1, 3, 4, 5, 6). It is known that, with increasing temperature, the equilibrium of the endothermic reaction

\[ \mathrm{2NaHCO_3 \rightleftarrows Na_2CO_3 + CO_2 + H_2O} \]

shifts from left to right, and the concentration of bicarbonate in the solution decreases, which leads to poorer regeneration. A solution boiling temperature of 50–60° corresponds to a vacuum in the column of 650–580 mm Hg.

We investigated the effect of vacuum on the degree of solution regeneration within the range 405–480 mm Hg; achieving a higher vacuum with the existing equipment was impossible. The temperature at the bottom of the column and the residence time of the solution in the column were kept constant, respectively 76°C and 116–118 sec.

The results of the investigation, presented in Fig. 4, indicate that as the vacuum in the column is increased from 405 to 480 mm Hg, the degree of regeneration of sodium bisulfide increases at all the investigated solution feed temperatures to the column.

The best temperature for feeding the solution to the column is 56–57°, since at these temperatures, evidently, the best conditions for bisulfide regeneration are established. When the temperature of feeding the solution to the column is lowered to 55°C, the temperature at the top of the regenerator decreases under vacuum conditions of 470–480 mm.

When the temperature of feeding the solution is increased from 58 to 70°C, there is a sharp decrease in the degree of regeneration of sodium bisulfide as a result of overheating of the solution above the optimum conditions. Regeneration of sodium hydrosulfide proceeds appreciably as long as there is a relatively sufficient amount of bicarbonate in the solution; with an increase in temperature in the column, the rate of decomposition increases.

Fig. 5. Dependence of the degree of regeneration of NaHS on the temperature at the bottom of the regenerator at different solution feed temperatures.

sodium bicarbonate, i.e., the conditions for regeneration of sodium hydrosulfide deteriorate.

Because, during regeneration of the absorbing solution by boiling under vacuum, the rate of desorption of hydrogen sulfide depends on the boiling temperature, the temperature in the circulation heater was varied within the range 74–80°; the residence time of the solution in the regenerator was maintained at 116–118 sec at a vacuum of 455 mm Hg.

Analysis of the results obtained, presented in Fig. 5, shows that when the temperature at the bottom of the regenerator is raised from 74 to 78°, the degree of regeneration of sodium hydrosulfide increases sharply (by 11–19%); with a further rise in temperature (above 78°), the average increase in the degree of regeneration of sodium bisulfide is only 1.5–3%. Therefore, in order to achieve the maximum possible regeneration of sodium hydrosulfide at all temperatures of solution feed to the column, the temperature at the bottom of the regenerator should be maintained at not less than 78–80°, and the temperature of solution feed to the regenerator should be within 56–61%.

Conclusions

1. It has been established that the vacuum-soda method can be used for removing hydrogen sulfide from coke-oven gas under pressure, as applied to the technological schemes of certain nitrogen-fertilizer plants.

2. The degree of removal of hydrogen sulfide from coke-oven gas was 75–85% at an initial concentration of 600–1400 g/m³, under conditions of a pressure of 13 atm, an absorption temperature of 38–40°, a soda concentration in the solution of 35 g/l, and an irrigation density of 14.7 m³/m²·h.

3. The residual concentration of hydrogen sulfide in the absorbing solution after regeneration can be reduced to 2.5–3.5 g/l under conditions of a vacuum in the column of 440–480 mm Hg, a temperature in the circulation heater of 78–70°, a temperature of solution feed to the regenerator of 58–61°, and a residence time of the solution in the column of not less than 120 sec.

Literature

1. M. S. Litvinenko. Purification of Coke-Oven Gas from Hydrogen Sulfide. Metallurgizdat, 1959.

2. N. N. Egorov, M. M. Dmitriev, D. D. Zykov. Removal of Sulfur from Coke-Oven and Other Combustible Gases. Metallurgizdat, 1960.

3. New Methods for Purifying Gases from Harmful Impurities, Information Bulletin, No. 10, Giproazotochistka, 1960.

4. A. P. Sergeev and A. P. Ilyichev. Regeneration of ballast salts from vacuum-potash sulfur-removal units. Coke and Chemistry, No. 8, 1958.