Biological processes accelerate the degradation of algae, resulting in an increase in H2S production. The question is, what to do with the hydrogen sulfide once it is produced ?
By now, you should have understood that we will transform it into liquid sulfuric acid by a chemical transformation. Industries use these chemical processes in particular to produce sulfuric acid from elemental sulfur, but they do not use gaseous hydrogen sulfide of biological origin.
The combination of these steps represents a real connexion between the fields of biology and chemistry in the outcome of the project.
Characteristics of chemical compounds
Characteristic of H2S :
Hydrogen sulfide, H2S, is a naturally occurring colorless gas with a foul odor. This gas results from the decomposition of organic matter by bacteria, which explains its massive presence in sewers and treatment plants. It is widely used in the chemical industry to produce inorganic sulfur and sulfuric acid.
Sulphuric acid does not exist naturally. It is a major industrial product. Its preparation can be considered as elementary in the industry of mineral chemistry and it is the strongest of simple acids.
It is a colorless, odorless, hygroscopic oily liquid, which turns yellow-brown in presence of impurities.
Physico-chemical property according to INRS :
Melting temperature
10.5 °C pure acid
3-5 °C 98 % acid
Boiling temperature
290 °C pure acid
335 °C 98% acid
Solubility
In water and alcohol
Density
1.830 pure acid
1.836 98% acid
Vapor pressure at 20°C
< 0,001 hPa
Conversion factor
1 ppm = 4.01 mg/m3
LogP
-0.84
The reactions
Claus process :
First reaction :
H2S has reducing properties. In presence of O2 it oxidizes SO2 as follows:
2H2S (g)+ 3O2(g) → 2SO2(g)+ 2H2O (g)
→ The combustion is exothermic
The combustion of H2S is carried out in a furnace where the oxygen supply is provided by atmospheric air.
Contact process
Second reaction :
Fig 1. Operation of the contact process
Oxidation of SO2 to SO3 :
2SO2(g)+ O2(g) → 2SO3(g)
→ The oxidation is strongly exothermic.
The oxidation occurs in a converter containing 4 catalytic beds whose active principle is vanadium pentoxide, or V2O5.The reaction starts at 440°C and quickly reaches 600°C. Before moving on to the next bed, the gas already formed is cooled to 430°C. 60% of this energy is used to produce water vapor.
Gas temperatures at the different beds of the converter :
Catalytic beds
Temperature range
Gas inlet (°C)
Gas outlet (°C)
I
440
600
II
430
520
III
430
460
IV
430
435
Fig.2 Diagram of the converter
The SO3 leaving the 4th bed is sent to the economizer, the “saving device” on the above diagram, where it is cooled down by the boiler water supply. The ratio between the number of SO3 molecules formed and the number of SO2 molecules present in the converter feed gas is measured by calculating the conversion rate :
To promote the reaction, some settings can be changed :
Oxygen concentration increase
Temperature drop
Pressure increase
Third reaction:
Hydratation of SO3 to form sulfuric acid H2SO4:
2SO3(g) + 2H2O(l) → 2H2SO4(l)
However, SO3 is hardly absorbed in water. We can break down this reaction into 2 steps:
SO3 is dissolved in concentrated sulfuric acid to produce oleum at a temperature of 70°C to 100°C:
H2SO4 (l) + SO3 (g) → H2S2O7 (l)
The oleum is then added to water to form sulfuric acid about 98% concentrated :
H2S2O7 (l) + H2O(l) → 2 H2SO4 (l)
H2SO4 decomposition
Under standard temperature conditions (25 °C) sulfuric acid is not volatile. Above 30°C, it emits vapors. Thus at the boiling temperature of 335°C, SO3 is released as white and opaque vapors :
H2SO4(g) → SO3(g) + H2O(l)
Detection methods
Monogas detection : H2S and SO2
Pac 6500 : Portable gas detector
According to SafetyGas websites The H2S electrochemical sensor measures concentrations between 0.4 and 100 ppm.
Principle :The electrochemical sensor has three electrodes :
Working electrode
Reference electrode
Counter electrode
The gas enters into the sensor where it interacts with the working electrode. The electrochemical reaction that occurs, depending on the gas, is oxidation or reduction. The reaction generates a current between the working electrode and the counter electrode, proportional to the quantity of gas.
Quantification
H2S, SO2 and SO3 quantification : Standard measurements like retention time, mass spectrum, will be needed. To determine them, standard samples can be used to fix the values with our equipment.
Gas Chromatography (GC) : Gas chromatography is an analytical technique used to detect chemical components to determine their presence, absence and quantity. Gas chromatographs are frequently hyphenated to mass spectrometers (GC-MS) to enable the identification of the chemical components
How does gas chromatography work ? Working principle : The sample is vaporized before passing through a long column (scaled in meters) before reaching a detector. The length of the column gives a slight separation of each component of a mix. For better results, the temperature program could be adjusted to obtain a better elution.
A carrier gas must be used in the separation such as H2, which plays a role in the mobile phase. The carrier gas transports the sample molecules through the GC system without reacting with the sample or damaging the instrument components.
Operating conditions before injection :
Adjust column flow
Set the oven temperature
Choose isocratic or gradient elution mode
Fig 3. Diagram of gas chromatograph
Legend :
1-3 : The sample is first introduced either via a gas valve inlet through a septum.
4 : The analytical column is a long (10 – 150 m), narrow (0.1 – 0.53 mm internal diameter) fused silica or metal tube containing the stationary phase. The analytical column is held in the column oven which is heated during the analysis to elute the less volatile components.
5 : The outlet of the column is inserted into the detector which responds to the chemical components eluting from the column to produce a signal.
6 : The signal is recorded by the acquisition software on a computer to produce a chromatogram.
According to Dr Diane Turner, Anthias Consulting Ltd. from Technology NETWORKS
Chromatogram :
The x-axis is the retention time. It is taken from the time the sample was injected into the GC at t0 to the end of the run.
Each analyte peak has a retention time measured from the apex of the peak.
The y-axis is the measured response of the analyte peak in the detector. Various measurements can be taken from the peak, such as baseline width, mid-height width, total height and area.
The latter two are proportional to the concentration, it is the area that is used for quantitation.
In this example, the H2S retention time is 6.5 min
According to Vincent Varlet and al, 2014. Journal of Analytical Technology.
Fig. 4 Diagram of retention obtained with a mixed air which contains H2S
→ We can quantify H2S, SO2 et SO3
To verify that the quantification is successful, the compounds can be identified by performing mass spectroscopy. This gives us access to the mass spectrum.
H2SO4 Quantification :
Dosage of H2S could be made with potassium hydrogen carbonate KHCO3
Fig. 5 Dosage of H2SO4
A sulfuric acid sample is first diluted and then dosed with a basic solution in presence of a colored pH-revelator.
The dosage of the solution is realized using a buret and an erlenmeyer flask, to which water, KHCO3 and a few drops of helianthine are added under agitation.
Reaction equation :
H2SO4 + 2HCO3- → 2 H2CO3 + H2SO4-2
The gap of pH during the dosage is indicated by a change from orange-yellow to red.
Conclusion and what comes next :
Most recently, researcher Alain Lafond guided us in an interview for our next year's project. The steps necessary to achieve the production of liquid sulfuric acid require sophisticated industrial equipment. Our labs don’t have adequate material and safety equipment. Moreover, yields would be very weak. Thus, next year, we will begin with the idea of fixing the H2S gas mix into a liquid. This separation process is carried out with an aqueous amine solution.
Isolated H2S does have commercial value, as does sulfuric acid. Industries that have the necessary equipments could carry out the described processes to produce sulfuric acid in large quantities.
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