Sulfuric acid is one of the most highly produced and important chemicals. It finds widespread applications across in various industries and processes such as industrial chemical production (fertilizers, detergents, pigments, dyers), battery manufacturing, metallurgical processes, petroleum refining, dying and textile industry, production of explosives, semiconductor industry, laboratory use, and analytical chemistry, among others. There are different processes of sulfuric acid production: chamber process, single contact, and double contact process. The double contact process is currently the most preferred and widely used due to its high conversion ratio of sulfur trioxide to sulfuric acid. High conversion also means less SO2 residues in the emission gas, helping to reduce SO2 emission into the atmosphere and mitigate ambient air pollution.
The production of sulfuric acid involves a multi-step process, widely known as the Contact Process or the sulfuric acid manufacturing process. This process converts sulfur dioxide (SO2) into sulfuric acid (H2SO4). The raw materials used for the sulfuric acid production include sulfur containing gases obtained by burning of elemental sulfur, pyrites or off gases from metallurgical industry.
There are several steps in the production of sulfuric acid explained below:
Step 1: Sulfur Combustion
In cases where sulfuric acid is manufactured from elemental sulfur, pyrite or metal sulfide ore as raw material, the process begins with the combustion of sulfur (S) to produce sulfur dioxide (SO2).
S + O2 → SO2
Step 2: Sulfur Dioxide Conversion
Before sending sulfur dioxide to the converter, it undergoes pretreatment to remove impurities. First, sulfur dioxide is treated in a washing tower to reduce its temperature, then it is passed through a venturi scrubber to remove fine particles. Subsequently, it is routed to the cooling tower to further lower its temperature and remove water. Afterward, it passes through mist precipitators to eliminate any remaining fine particles and mist.
The conversion of sulfur dioxide and oxygen into sulfur trioxide is highly dependent on the amount of excess oxygen. Therefore, sulfuric acid is diluted with dry air to provide the necessary quantity of air for catalytic reaction. The last treatment step before entering the contact process at the converter is the drying tower, where any remaining water is removed from the sulfur dioxide gas.
Finally, the purified sulfur dioxide is sent to the converter for the contact process, during which it is converted into sulfur trioxide (SO3) in the presence of a catalyst. Typically, the catalyst used is either vanadium pentoxide (V2O5) or alumina (Al2O3).
2SO2 + O2 ⇌ 2SO3
The equilibrium mentioned above is highly sensitive to temperature and the excess of oxygen. Therefore, controlling the concentrations of SO2 and O2 after treatment process is crucial to establish optimal conditions before the conversion process.
Step 3: Absorption in Sulfuric Acid
Sulfur trioxide (SO3) is dissolved in concentrated sulfuric acid (H2SO4), typically at a concentration of 98-99%, to produce fuming sulfuric acid, also known as oleum (H2S2O7). Oleum is a mixture of sulfuric acid and sulfur trioxide. It is essential to maintain precise control over the concentration of sulfuric acid in the absorption tower to prevent upset conditions. Under upset conditions, the absorption process may not proceed correctly, underscoring the importance of maintaining optimal operational conditions. Depending on the manufacturing process (single contact single absorption or double contact double absorption), absorption can occur is a single tower or multiple absorption towers.
SO3 + H2SO4 → H2S2O7 (oleum)
Step 4: Dilution
The oleum produced in the absorption tower is diluted with water to achieve the desired concentration of sulfuric acid. Dilution is a critical step in producing commercial-grade sulfuric acid, which typically ranges in concentration from about 93% to 98%.
H2S2O7 + H2O → 2H2SO4
Step 5: Cooling and Storage
The diluted sulfuric acid is then cooled and transferred to storage tanks.
In the case of double contact double absorption process, SO3 is taken into concentrated sulfuric acid and creates oleum in the first (intermediate) absorption tower. Meanwhile, non-oxidized SO2 passes to the converter for the second contact. After contact, SO2 and O2 are converted to SO3 and then directed to the second (final) absorbing tower, where they are absorbed into concentrated sulfuric acid (H2SO4) to create fuming sulfuric acid, or oleum (H2S2O7).
The Contact Process is highly efficient and widely used in industry for the large-scale production of sulfuric acid. The double contact process has largely been replaced by the single contact process, which offers increased production yields of sulfuric acid and is more environmentally friendly due to its higher SO2 absorption ratio and reduced SO2 emissions. Due to the corrosive and potentially hazardous nature of sulfuric acid production, safety measures and environmental controls are crucial.
The sulfuric acid production process must be optimally controlled to attain a production yields of 98% or higher. HORIBA contributes to the sulfuric acid production process through its continuous gas analyzer, the ENDA-5000 series. The custom-designed sample gas pretreatment system, backed by over 50 years of experience and expertise in emission gas monitoring, as well as the in-house development and production of core components, contribute to accurate measurement of high-corrosive SO2 in the sulfuric acid production process.
The ENDA-5000 is offered and adopted in many sulfuric acid plants in Japan and overseas for monitoring of SO2, O2 and other components through the process. Highly important points to monitor for process optimization include (1) SO2 and O2 concentration before converter to maintain optimal conditions for conversion to SO3, (2) non-oxidized SO2 concentration after intermediate absorbing tower, and (3) after final absorbing tower for controlling absorbing efficiency and final regulated by local environmental regulations (4) stack emission monitoring.
■ The ENDA-5000 analyzer contains a custom-made sample gas treatment system which ensures minimal dissolution of SO2 in the drain and provides accurate measurement results.
■ With over 50 years of experience in designing continuous gas analyzers and stack gas analyzers, HORIBA has accumulated know-how in various sampling parts for different sample gas conditions. Among these parts is treatment for the removal of corrosive SO3 mist and other sampling parts for the pretreatment of difficult-to-handle sample gases. Such a pretreatment system allows the detector to serve a longer time without corrosion and major troubles.
■HORIBA's proprietary cross-modulation NDIR technology, applied in this analyzer, ensures the cleaning of the measurement cell with the purified ambient air after each measurement. This contributes to the long lifetime of analyzer and maintains clean conditions inside the sample gas cell over the long time, which is highly important for low zero drift and precise measurement.
Stack Gas Analysis System
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