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AZ9RBAYCAN KIMYA JURNALI № 2 2017
UDC 661.2
OXIDATION OF HYDROGEN SULPHIDE WITH NITRIC ACID TILL SULPHUR AND IT REPRODUCTIVITY AT REACTION MEDIUM
M.M.Ahmadov, A.A.Ibrahimov, R.M.Vakilova, N.I.Abbasova
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
Ali.Ibrahimov_i@mail.ru Received 25.01.2017
Possibilities of oxidation of hydrogen sulphide with nitric acid till sulphur and at the same time repro-ductivity of nitric acid at reaction medium were studied. 100% oxidation of H2S to sulphur and full re-productivity conditions of HNO3 were determined.
Keywords: hydrogen sulphide, oxidation, nitric acid, reproductivity HNO3, suspension.
Introduction
In modern world detoxication of sulphuric gases is not only conducted for solution of ecological problems, but also for these gases are the source obtaining of free sulphur. Thus, natural sulphur sources are being exhausted and in most part of required sulphur was spent at the expense of these gazes [1]. Recently to obtained sulphur from industrial gases H2S is mainly captured by solutions of amines. This method proves its value for the purification of high density gases. When the amount of H2S in gas mixture is less than 2% this method is not economically profitable [2]. After capturing the H2S with amines, its desorption and the use of Claus process conversion to free sulphur are required. Claus process is multistage, in each stage catalyst and high energy consumption are required. On the other hand, Claus process is reversible and gases cannot be fully purified and the left gases must be purified again and even in all existing methods [3] gases are relatively brought to high purity, big investment is required and when technical disorder occurs and the ratio of SO2:H2S is distorted many difficulties occur. That's, the purification of such gases, mainly less density gases must be conducted in liquid phase with adsorbents. The demand is that in liquid phase the oxidation of H2S in absorbing solution must be conducted at low temperature, with high speed. Most of existing oxidation methods are performed in alkali medium and with the use of catalyst. Oxidation of H2S in neutral and alkali media occurs within several days [4]. Therefore, oxidation of H2S acidic medium is more expedient.
The main aim in the present work is to purify H2S fully without using catalyst, in more less time, without getting intermediate product, with less consumption of additional reagents and reproducing absorbing solution. To achieve this aim for the first time we propose nitric acid as an oxidizer of hydrogen sulphide in industrial waste gases. Since H2S is a strong reducer, and HNO3 is an oxidizer the reaction goes very fast. One of advantages of this process is that at an ordinary condition it is possible to reduce oxidizer (HNO3) in reaction medium by giving air oxygen or pure oxygen. Thus, as it is known from literature materials the oxidation of NO in liquid phase which is the interaction product of H2S and HNO3, has many advantages [5]. The reaction is conducted in fewer capacities. It is possible to circulate oxygen in closed contour. Existence of HNO3 in medium increases the oxidation rate of NO. Oxygen giving activities of liquid oxidizers in this process was experimentally tested and the following result has been obtained [6]:
KBrO3 >HNO3>KMnO4>H2O2>Na2CrO4> K2CrO4>K2Cr2O7.
As it seen in the oxidation of NO with HNO3 shows enough high activity.
Experiment part
Heterogeneous oxidation of NO in HNO3 is related to the following reactions:
NO+ 2HN03 3N02+H20 -73.6 kJ (1) 2N02+ H20 HN03+ HN02. (2)
To obtain initial gas mixture after purifying H2S obtained from the interaction of 100 g FeS and 500 ml 36% HCl in Kipp's apparatus, is mixed with air (in thick gases with pure oxygen) into obtain such ratio the necessary density of mixture. Gas container is filled by measuring with rheometer according to defined density of air and H2S in gas. In gas container saturated solution of NaCl is used as a repulsive liquid is used. Kerosene is added into this solution to prevent gas loss. To pass gas mixture into the reactor we used the pressure of N2 tank. The liquid is pushed into gas container and released gas is passed from down to up with counter flow through column prepared from quartz glass with 5 cm diameter and 50 cm height filled with liquid and glass particles (Raschig rings) and HNO3, to increase touching time of gas. The reactor consists of internal and casing part. Casing part is for circulation of a liquid coming from thermostat. 10% Cd(CH3COO)2 solution was added into drechsel in outlet of the reactor that if H2S is not enter into the reaction, obtaining of yellow colored CdS will show it.
Experiments are conducted by passing 1100% (vol.) H2S containing (the rest is air or pure oxygen) gas mixture through the column containing 10-60% (mas.) HNO3 solution at 25-700C with counter flow and 50-200 ml/min speed.
In primary experiments at 250C temperature 20% (vol.) H2S containing gas mixture is given to 30% HNO3 solution with 100 ml/min speed. Each 100 ml gas contains 20 ml H2S and 80 ml air and the following reaction occurs: 2HNO3+3H2S^3S+2NO+4H2O. (3)
O2 of initial mixture reacts with obtained
NO:
2NO+O2 ^ 2NO2. (4)
Oxidation reaction of NO also occurs more easily.
NO2 in its turn is absorbed with water and converted into HNO3 and thus, oxidizer is reproduced:
4NO2+2 H2O+ O2 ^ 4HNO3 . (5)
Results and discussion
According to theoretical calculations in (4) and (5) reactions 8.33 ml O2 is required for
oxidation. This exists in 40 ml air. So, in 80 ml air is two times more than the amount of O2 which is used in (4) and (5) reactions and according to (1) and (2) reactions HNO3 is in the medium, these increase the speed of oxidation reaction of NO.
Before and after the reaction density of HNO3 is determined with NaOH solution the density of which was defined with HCl fixanal. According to densities of HNO3 solutions before and after the experiment the degree of re-productivity of nitric acid is calculated. After the reactor Cd(CH3COO)2 solution in drechsel does not turn yellow, this shows that H2S fully enters into the reaction and is fully converted to sulphur. Before and after the experiment the densities of HNO3 solutions are the same, so nitrogen gases are fully absorbed and are converted into HNO3 acid.
As it is seen in the Table 1 of the dependence of oxidation of H2S and reproductivi-ty of HNO3 on gas density and given speed in lower densities and relatively lower values of given rates of gas both processes (oxidation and reproductivity HNO3) finish with 100% yield. It is possible to convert H2S with any density into sulphur in given condition. But, when the density of H2S is higher than 20%, since the amount of NO obtained at unique time is high it cannot be fully oxidized. When giving rate of gas increases up to 150 l/min reproductivity percent of HNO3 decreases. The reason of it is less time for full oxidation of gases due to high rate.
Studying the dependence of oxidation of H2S and reproductivity of HNO3 on temperature (Table 2) show that when temperature increases one part of H2S is oxidized till sulphate. Thus, at the end of experiments solution samples in the reactor were analyzed. Existence of barium chloride and sulphate ions were defined and there amount was determined by the known method [7]. The reason of conversion of free sulphur to sulphate by the increase of temperature is obtaining of dispersive sulphur with small particles during the (3) reaction.
It is known that sulphur reacts with thick hot nitric acid. According to experiments dispersive sulphur after 500C temperature reacts with HNO3.
Table 1. The dependence of H2S oxidation and repro-ductivity of HNO3 on gas density and given rate (reaction time - 30 min, temperature - 250C).
ml/min (H2S), % Amount of obtained Conversion degree of H2S to S , % Reproductivity of HNO3,
Ь.1 (vol.) sulphur, g %
1 0.021 100 100
10 0.21 100 100
50 20 0.42 100 100
50 1.07 100 97
80 1.71 100 87
100 2.14 100 82
1 0.042 100 100
10 0.42 100 100
100 20 0.85 100 100
50 2.125 100 89
80 3.42 100 80
100 4.28 100 68
1 0.063 100 100
10 0.63 100 97
150 20 1.26 100 92
50 3.21 100 81
80 5.04 100 73
100 6.42 100 62
1 0.048 100 93
10 0.84 100 89
200 20 1.68 100 88
50 4.25 100 75
80 6.72 100 64
100 8.56 100 53
Table 2. The dependence of oxidation of H2S and repro-ductivity of HNO3 on the reaction temperature [Vgas - 50
ml/min, (H2S) - 1% (vol.)]
T, Conversion Mass of ob- Reproduc-
0C degree of H2S, tained BaSO4, tivity of
% g HNO3, %
25 100 - 100
40 100 - 100
50 100 0.02 100
60 100 0.035 100
70 100 0.048 100
When the temperature is increased up to 700C, it does not negatively influence on repro-ductivity of HNO3 at given temperature and density of gas. It can be explained by the increase of endothermic (1) reaction rate with the increase of temperature.
In the following experiments we studied the influence of density of nitric acid on the process. When the density of HNO3 increases,
the absorbance of NO and NO2 in reaction medium and the conversion to nitric acid occur more easily. But, due to high density of HNO3 and fine structure of obtained sulphur, sulphur is oxidized till sulphate. This case is observed when the density of HNO3 is higher than 30% (Table 3).
Table 3. The dependence of oxidation of H2S and repro-
% ) ,3 s. SI H( tzf 1= H2 ( % Conversion degree of H2S to S, % (vol.) Reproductivity degree of HNO3, % (mas.)
20 1 100 100
30 1 100 100
40 1 99.8 100
50 1 99.2 100
60 1 98.4 100
The increase of reaction rate was observed when the reaction product sulphur suspension increased the touching surface of gas and liquid. To clarify this as in previous experiments H2S containing gas is passed through 30% nitric acid (t=250C, giving rate of gas 100 ml/min), amount of sulphur in obtained solution is reached 10 g/l. The following works are conducted with sulphur suspension containing nitric acid and at different rates of gas mixture. As is seen in Table 1 when given rate of gas is higher than 150 ml/min the full reproductivity of HNO3 is not possible. It was determined that existence of suspension accelerates both oxidation of H2S and reproductivity of HNO3. Thus, at the same time and condition amount of sulphur obtained in the solution is more than sulphur obtained by the oxidation of H2S with pure HNO3. And it allows conducting the reproductivity of HNO3 at relatively higher temperatures (Figure 1).
One of the problems of obtaining free sulphur from waste gases in liquid phase is separation of dispersive sulphur which is formed during the process, from suspension. Usually, in such suspensions amount of sulphur may vary from 2-3 to 20-30 g/l according to the processes it was determined that when pH of solution and amount of sulphur increase free sulphur particles in suspension grow bigger and are separated from the solution. In abovemen-
tioned experiments when adding H2S into acid at 250C within 30 min 20% H2S is passed
solution, over dispersiveness of sulphur which through 30% acidic solution. Obtained sulphur
is obtained at first 15 min, the solution becomes suspension containing solutions is neutralized
white and gradually turns yellow. Depending on with 0.5 M NaOH solution up to corresponding
given rate of gas and density amount of sulphur pH and kept for full separation of sulphur. As it
reaches 8-10 g/l within 30 min. Viscosity of is seen from Figure 2 when pH of solution de-
solution increases, floatation of definite part of creases, less time is required for separation of
sulphur occurs. sulphur. Value of pH - 0.75 corresponds to
Depending on pH of solution and amount 30% solution of HNO3 and as it was shown it is
of sulphur separation time of sulphur on solu- considered to be optimal density for the pro-
tion was studied (Figure 2). In all experiments cess.
Ms, g 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
50
100
150
200
Vgas, ml/min
Fig. 1. The dependence of amount of sulphur (Ms) on given rate of gas: 1 - for solution containing suspension, 2 - for solution without suspension.
References
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HÍDROGEN SULFÍDÍN NÍTRAT TUR§USU ÍLO KÜKÜRDO QODOR OKSÍDLO§MOSÍ VO REAKSÍYA MÜHÍTÍNDO OKSÍDLO§DÍRlCÍNÍN BORPASI
M.M.Ohm3dov, e.e.ibrahimov, RM.Vakilova, N.Í.Abbasova
Hidrogen sulfidin nitrat tur§usu ila kükürda qadar oksidla§masi va eyni zamanda reaksiya mühitinda nitrat tur§usunun barpa olma imkanlari óyranilmiíjdir. H2S-in kükürda 100% oksidla§masi va HNO3-ün tam barpa olundugu §arait müayyanbíjdirilmiíjdir.
Agar sozlar: hidrogen sulfid, oksidh§ma, nitrat tur§usu, HNO3-ün tam barpasi, suspenziya.
ОКИСЛЕНИЕ СЕРОВОДОРОДА АЗОТНОЙ КИСЛОТОЙ ДО СЕРЫ И ВОСПРОИЗВОДИМОСТЬ
ОКИСЛИТЕЛЯ В РЕАКЦИОННОЙ СРЕДЕ
М.М.Ахмедов, А.А.Ибрагимов, Р.М.Векилова, Н.И.Аббасова
Изучены окисление сероводорода азотной кислотой до серы и возможность воспроизводимости азотной кислоты в реакционной среде. Определены условия 100%-ного окисления H2S до серы и воспроизводимости HNO3 в реакционной среде.
Ключевые слова: сероводород, окисление, азотная кислота, воспроизводимость HNO3, суспензия.
