RU2315297C1 - Method of determining hydrogen sulfide and mercaptans in hydrocarbon gases to perform process control - Google Patents

Abstract

FIELD: analytical methods.

SUBSTANCE: method of determining hydrogen sulfide and mercaptans in hydrocarbon gases in order to prevent corrosion of pipelines comprises sampling, consecutively introducing sample into reaction vessel with absorption solution of alkali metal carbonate to remove hydrogen sulfide and then into reaction vessel with absorption solution of alkali in ethyl alcohol to remove mercaptans, while simultaneously adding hydrochloric acid solution of p-phenylenediamine (photometric reagent) followed by adding auxiliary hydrochloric acid solution of ferric ions, stirring mixture with air flow to form uniform colored solution in each vessel, separately passing obtained solutions heated to 35-40°C to photometric detector, measuring signals from sample in photometric detector, and comparing measured signals with background signals.

EFFECT: simplified procedure and increased safety and information intensity of analysis.

4 cl, 3 tbl

Description

The invention relates to chemical methods of analysis and can be applied in analytical chemistry, in particular, for carrying out technological control of the quality of hydrocarbon gases in terms of the content of hydrogen sulfide and mercaptans.

Hydrocarbon, natural gases, gas condensate gases and associated petroleum gases include sulfur-containing compounds: hydrogen sulfide (H ₂ S), carbon disulfide, mercaptans (R-SH), thiophenes. The main percent by weight is H ₂ S and R-SH, which are acidic gases and can cause intense corrosion of pipelines, cause the appearance of toxic sulfur oxides during gas combustion, poison catalysts for chemical gas processing [1]. This shows the need to control the content of hydrogen sulfide and mercaptans in the gas, which is an indicator of its quality and should not exceed the standard value of 20 mg / m ³ for hydrogen sulfide and 36 mg / m ³ for mercaptan sulfur [2]. The control problem of hydrogen sulfide and mercaptans in a hydrocarbon gas is solved using several methods.

There is a method of gas chromatographic determination of hydrogen sulfide and mercaptans, based on the possibility of separation of the components of the analyzed samples in a chromatographic column and registration of H ₂ S and R-SH emerging from it by a flame photometric detector [3]. The disadvantages of this method are its low detection limit, high cost of equipment and the inability to continuously monitor gas quality according to these indicators.

A known method for the photometric determination of H ₂ S and R-SH in hydrocarbon gases [4], which is the closest prototype of the claimed method. The known method [4] is based on liquid absorption of analytes from a gas stream into aqueous solutions of chalcophilic element ions according to a traditional bubbling scheme with the subsequent formation of an analytical form - methylene blue and methylene red, respectively, as a result of the interaction of the substances with n-phenylenediamine or with its derivatives in an acidic environment in the presence of Fe (III) ions. To determine H ₂ S and R-SH by the above method, it takes a lot of labor and time to carry out routine procedures of manual analysis and reagents, which makes it ineffective in mass analysis to control the quality of hydrocarbon gas. In addition, concentrated analytes of chalcophilic ions of ions (lead, cadmium, mercury, zinc), which have toxic properties, are used to absorb analytes.

In the traditional orientation of industrial laboratories to the aforementioned methodology, the optimal solution is to search for instrumental solutions that would allow it to be automated, while maintaining users' confidence in the results obtained with its help. The most appropriate methodological solution to the formulated problem is the recently proposed new method of flow analysis - cyclic injection analysis (CIA) [5]. The CIA method, as well as the sequential injection analysis (SIA) [6], uses a single-channel reversible pump and a multi-way flow distributor valve. The difference is that the reaction and retention loops are replaced by a reaction vessel communicating with the atmosphere, which makes it possible to mechanically mix the sample and reagent solutions with a gas stream and maximize the unification of hydraulic circuits. When conducting analysis in the flow stopping mode, cyclic injection analysis, unlike SIA, allows optimizing the conditions for recording an analytical signal until the automated stationary analysis technique is fully reproduced using the lower limits of the range of determined concentrations that can be achieved with it.

The technical result of the claimed invention consists in lowering the detection limit, increasing the expressivity, reliability and safety of the determination of H ₂ S and R-SH in hydrocarbon gases for their continuous technical control, as well as for discrete laboratory analyzes.

In contrast to the prototype, instead of manually sampling and absorbing analytes in a series of bubblers in the claimed invention, the isolation occurs automatically directly in the reaction vessel of the analyzer, followed by the addition of the necessary reagent solutions and the formation of photometric forms. In addition, instead of salts of heavy metals for the absorption of H ₂ S and R-SH, it was proposed to use solutions of alkali in aliphatic alcohol and alkali metal carbonate in water, respectively, which makes the method safer for analysis. By varying the value of the studied parameter and observing the value of the analytical signal, it was found that the following conditions are optimal for determining analytes using the developed method: the concentration of Fe (III) ions is 0.2 g / l, the concentration of n-phenylenediamine is 1.5 g / l and the temperature range 35-40 ° C.

Taking into account the conditions found for the evolution of hydrogen sulfide and the formation of methylene blue, a method for the cyclic injection determination of H ₂ S in hydrocarbon gases has been developed, the main stages of which are presented in Table 1. In the first stage, an absorption solution is fed into the reaction vessel through the first position of the distribution valve - an aqueous solution of Na ₂ CO ₃ with a mass concentration of 10 g / l. At the second stage, using a reversible pump, when the dispensing valve is switched to the second position, a solution of a photometric reagent — 1.5 g / l n-phenylenediamine in a 4.5 M HCl solution — is fed into the reaction vessel. At the third stage, with the help of a reversible pump, when switching the dispensing valve to the third position, an auxiliary solution is fed into the reaction vessel — 0.2 g / l of a solution of Fe (III) ions in a 4.5 M HCl solution. In the fourth stage, using a reversible pump, when the dispensing valve is switched to the fourth position, air bubbles are introduced into the reaction vessel, and the solution is mixed in the reaction vessel. At the fifth stage, when changing the reverse of the pump and switching the dispensing valve to the fifth position, the solution enters the detector from the reaction vessel through a thermostatic spiral to heat the solution to the required temperature of 35 ° C. At the sixth stage, the pump stops and the background signal is measured in a photometric detector, the value of which corresponds to zero concentration of hydrogen sulfide in the sample. At the seventh stage, with the help of a reversible pump, through the first position of the dispensing valve, an absorption solution — an aqueous solution of Na ₂ CO ₃ with a mass concentration of 10 g / l — is fed into the reaction vessel. At the eighth stage, the pump stops, the tap valve takes the second position, and a gas sample enters the reaction vessel with the absorption solution, and hydrogen sulfide is absorbed by the absorption solution. At the ninth stage, the switch-tap returns to the first position, the distributor-tap takes the second position and, using a reverse pump, a solution of a photometric reagent — 1.5 g / l of n-phenylenediamine in a 4.5 M HCl solution — is fed into the reaction vessel. At the tenth stage, using a reversible pump, when the dispensing valve is switched to the third position, an auxiliary solution is fed into the reaction vessel — 0.2 g / l of a solution of Fe (III) ions in a 4.5 M HCl solution. At the eleventh stage, using a reversible pump, when the dispensing valve is switched to the fourth position, air bubbles are introduced into the reaction vessel, while the solution is mixed and a colored product forms in the reaction vessel. At the twelfth stage, when changing the reverse of the pump and switching the distributor valve to the fifth position from the reaction vessel, the solution enters the detector through a thermostatic spiral to heat the solution to the required temperature of 35 ° C. At the ten-th stage, the pump stops and the sample signal is measured in a photometric detector, the value of which corresponds to a certain concentration of hydrogen sulfide in the sample.

The method for determining hydrogen sulfide on a sample of domestic gas was tested, the experimental results are presented in table 2, and are also reflected in the first example.

Example 1. Determination of hydrogen sulfide content in household gas of the Faculty of Chemistry

Determination of the content of hydrogen sulfide in household gas of the Faculty of Chemistry was carried out according to the “input-found” method. To prepare a 0.01 M solution of S ^2–, 2.4 g of Na ₂ S · 9H ₂ O (Reagent, St. Petersburg, Russia), previously dried between sheets of filter paper, were dissolved in 1 l of a 0.01 N NaOH solution (Reagent, St. Petersburg, Russia) ), which was previously purged with nitrogen (flow rate of 0.1 l / min for 5 minutes). The resulting solution of S ² -ions was standardized by iodometric titration and stored at 5 ° C. To prepare working solutions of S ² -ions, a standard solution of S ² -ions was diluted with distilled water.

To generate H ₂ S in the gas phase, 0.5 ml of a working solution of S ² ions and 0.5 ml of a 0.2 M solution of H ₂ SO ₄ were placed in a glass bubbler (5 ml capacity), high purity nitrogen was passed at a rate of 0.3 l / min. The amount of H ₂ S (m _i ) dosed into the gas phase was calculated by the equation: m _i = C _i · V, where C _i is the concentration of sulfide ions in the working solution of S ² ions, g / l; V is the volume of the solution of S ² -ions placed in the bubbler (0.5 ml). So, with the introduction of 18 μg of hydrogen sulfide, 17 μg of hydrogen sulfide was found, therefore, the hydrogen sulfide content is below the detection limit of the procedure.

Taking into account the found conditions for the separation of R-SH and the formation of methylene red, a method for the cyclic injection determination of R-SH in hydrocarbon gases has been developed, the main stages of which are presented in table 1. In the first stage, an absorption tank is fed into the reaction vessel through the first position of the distribution valve the solution is an ethanol solution of KOH with a mass concentration of 11 g / l. At the second stage, using a reversible pump, when the dispensing valve is switched to the second position, a solution of a photometric reagent — 1.5 g / l n-phenylenediamine in a 4.5 M HCl solution — is fed into the reaction vessel. At the third stage, with the help of a reversible pump, when switching the dispensing valve to the third position, an auxiliary solution is fed into the reaction vessel — 0.2 g / l of a solution of Fe (III) ions in a 4.5 M HCl solution. In the fourth stage, using a reversible pump, when the dispensing valve is switched to the fourth position, air bubbles are introduced into the reaction vessel, and the solution is mixed in the reaction vessel. At the fifth stage, when changing the reverse of the pump and switching the dispensing valve to the fifth position, the solution enters the detector from the reaction vessel through a thermostatic spiral to heat the solution to the required temperature of 35 ° C. At the sixth stage, the pump stops and the background signal is measured in a photometric detector, the value of which corresponds to zero concentration of mercaptans in the sample. At the seventh stage, by means of a reversible pump, through the first position of the dispensing valve, an absorption solution — an ethanol solution of KOH with a mass concentration of 11 g / l — is fed into the reaction vessel. At the eighth stage, the pump stops, the switch tap takes the second position and a gas sample enters the reaction vessel through the bubbler with an aqueous solution of Na ₂ CO ₃ with a mass concentration of 10 g / l to eliminate the interfering effect of hydrogen sulfide, while in the reaction vessel absorption of mercaptans by absorbing solution. At the ninth stage, the switch-tap returns to the first position, the distributor-tap takes the second position and, using a reverse pump, a solution of a photometric reagent — 1.5 g / l of n-phenylenediamine in a 4.5 M HCl solution — is fed into the reaction vessel. At the tenth stage, using a reversible pump, when the dispensing valve is switched to the third position, an auxiliary solution is fed into the reaction vessel — 0.2 g / l of a solution of Fe (III) ions in a 4.5 M HCl solution. At the eleventh stage, using a reversible pump, when the dispensing valve is switched to the fourth position, air bubbles are introduced into the reaction vessel, while the solution is mixed and a colored product forms in the reaction vessel. At the twelfth stage, when changing the reverse of the pump and switching the distributor valve to the fifth position from the reaction vessel, the solution enters the detector through a thermostatic spiral to heat the solution to the required temperature of 35 ° C. At the ten stage, the pump stops and the sample signal is measured in a photometric detector, the value of which corresponds to a certain concentration of mercaptans in the sample.

The method for determining mercaptans on a domestic gas sample was tested, the experimental results are presented in table 3, and are also reflected in the second example.

Example 2. Determination of the content of R-SH in the household gas of the Faculty of Chemistry

Determination of the content of R-SH in the household gas of the Faculty of Chemistry was carried out according to the "input-found" method. An ethanol solution of mercaptoethanol in 0.2 M KOH was introduced into the reaction vessel. So, when 100 μg of R-SH was introduced into the reaction vessel, 100 μg of R-SH was found; therefore, the content of R-SH was below the detection limit of the procedure.

The proposed method for determining hydrogen sulfide and mercaptans in hydrocarbon gases for technological control in comparison with the prototype shows a lower detection limit (two times), higher reliability, safety analysis, and the ability to automate the process, which significantly reduces the analysis time by four times. All this allows you to successfully use the proposed method in analytical chemistry, in particular, for technological control of the quality of hydrocarbon gases in terms of the content of hydrogen sulfide and mercaptans.

Table 1.
The main stages of the method for determining hydrogen sulfide and mercaptans in hydrocarbon gases for technological control Stage Valve position Switch tap position Direction of rotation of the pump (-1; 0; 1) ^* Measurement (0; 1) ^** Comment one one one +1 0 Supply of the absorption solution to the reaction vessel 2 2 one +1 0 The supply of a solution of photometric reagent in the reaction vessel 3 2 one +1 0 Submission of auxiliary solutions to the reaction vessel four four one +1 0 Supply air to the reaction vessel to mix the solution 5 5 one -one 0 Supply of solution from the reaction vessel to the detector through a thermostatic spiral 6 5 one 0 one Background signal measurement 7 one one +1 0 Supply of the absorption solution to the reaction vessel 8 one 2 0 0 Gas sample supply to the reaction vessel 9 2 one +1 0 The supply of a solution of photometric reagent in the reaction vessel 10 2 one +1 0 Submission of auxiliary solutions to the reaction vessel eleven four one +1 0 Supply air to the reaction vessel to mix the solution 12 5 one -one 0 Supply of solution from the reaction vessel to the detector through a thermostatic spiral 13 5 one 0 one Sample Signal Measurement * -1 - pump rotation clockwise 0 - pump stop +1 - rotation of the pump counterclockwise ** 0 - no measurements are made 1 - registration of the signal from the detector

Table 2.
The results of the determination of hydrogen sulfide in hydrocarbon gas Introduced H ₂ S, micrograms Found H ₂ S, micrograms 0 C <detection limit eighteen 17 28 26 38 37

Table 3.
The results of the determination of mercaptans in hydrocarbon gas Introduced mercaptaethanol, micrograms Found mercaptoethanol, micrograms 0 C <detection limit fifty 49 one hundred one hundred 150 148

BIBLIOGRAPHY

1. Chemistry of oil and gas. S.-Pb .: Publishing. Chemistry, 1995, 448 p.

2. GOST 5542-87. Combustible natural gases for industrial and domestic purposes. M .: Publishing. Standards, 1987, 12 pp.

3. A.I. Busev, L.N. Simonova. / Analytical chemistry of sulfur. M .: Nauka, 1975, 262 p.

4. GOST 22387.2-97. Combustible natural gases. Methods for the determination of hydrogen sulfide and mercaptan sulfur. Minsk: Publishing House standards, 1998, 22 pp.

5. Patent No. 48413, Russia, 2006.

6. J. Ruzicka, G. D. Marshall // Anal. Chim. Acta. 1990. V.329. P.237.

Claims (4)

1. A method for determining hydrogen sulfide and mercaptans in hydrocarbon gases for technological control, including sampling, sequentially supplying it to a reaction vessel with an absorbing solution of alkali metal carbonate to separate hydrogen sulfide and to a reaction vessel with an absorbing solution of alkali in ethyl alcohol to separate mercaptans, feeding the indicated capacity of the hydrochloric acid solution of the photometric reagent of n-phenylenediamine with the subsequent supply of an auxiliary hydrochloric acid solution of iron (III) ions, p remeshivaniem air stream to produce in each container a homogeneous colored solution, the separate feed of the solutions while heating to 35-40 ° C in a photometric detector, the measurement sample signals in photometric detector and compare each of them with the background signal 2. The method according to claim 1, characterized in that when determining hydrogen sulfide as an alkali metal carbonate solution, an aqueous sodium carbonate solution with a mass concentration of the latter of at least 10 g / l is used. 3. The method according to claim 1, characterized in that when determining mercaptans as a solution of alkali in ethanol, a KOH solution with a mass concentration of alkali of at least 11 g / l is used. 4. The method according to claim 1, characterized in that the sampling, the release of analyte gas from the sample in the absorbent solution, the addition of the photometric reagent and auxiliary reagents to the concentrate, mixing, formation and temperature control of the resulting solution occurs automatically.