How is industrial acid made?
Sulfuric Acid: Manufacturing Process
Sulfuric acid is the most widely produced industrial chemical globally, with an output of 200 million tons per year. Concentrated sulfuric acid (93-98%) is essential in manufacturing fertilizers, explosives, dyes, and petroleum products.
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The primary material for sulfuric acid production is clean, dry sulfur dioxide (SO2) gas. This gas can be obtained by burning molten sulfur, utilizing metallurgical off-gases, or decomposing spent sulfuric acid. Over recent decades, the contact process has become the standard for sulfuric acid production, replacing the older "Lead Chamber" process from the 18th century.
In the contact process, SO2 is oxidized to sulfur trioxide (SO3) at high temperatures (around 450°C) in the presence of a vanadium catalyst. SO3 is then dissolved in concentrated sulfuric acid, creating fuming sulfuric acid (oleum), which can be safely reacted with water to yield concentrated sulfuric acid.
Significant advancements have been made in processing and plant design to enhance energy recovery. This recovered heat is used to generate high-pressure steam and/or electricity. The outdated "single absorption" process has been largely replaced by the "double absorption" process, improving acid yield and minimizing emissions. A typical flowchart for a double absorption sulfuric acid plant is illustrated in Figure 1.
Uses of Sulfuric Acid
Sulfuric acid is a crucial compound in the chemical industry, produced extensively for various applications. Primarily, it is utilized in the manufacturing of phosphoric acid, which is a key ingredient for phosphate fertilizers, including calcium dihydrogen phosphate and ammonium phosphates. It is also vital for producing ammonium sulfate, which serves as an important fertilizer in sulfur-deficient soils.
Its applications extend to metal processing, like copper and zinc production, as well as cleaning steel sheets, a process known as "pickling," before they are coated with a thin layer of tin for food can manufacturing.
Moreover, sulfuric acid is involved in the production of caprolactam, which is subsequently converted into polyamide 6, and in the manufacture of titanium dioxide, used as a pigment in various products.
Annual Production of Sulfuric Acid
Worldwide sulfuric acid production statistics are as follows:
- World: 231 million tonnes
- China: 74 million tonnes
- U.S.: 37 million tonnes
- India: 16 million tonnes
- Russia: 14 million tonnes
- Morocco: 7 million tonnes
Data sourced from Merchant Research & Consulting Ltd. Projections indicate that by the near future, global production may exceed 250 million tonnes.
Manufacture of Sulfuric Acid
The sulfuric acid production process comprises four stages:
- Extraction of sulfur
- Conversion of sulfur to sulfur dioxide
- Conversion of sulfur dioxide to sulfur trioxide
- Conversion of sulfur trioxide to sulfuric acid
(a) Extraction of Sulfur
The primary source of sulfur is extracted from natural gas and oil, where sulfur compounds, including organic sulfur and hydrogen sulfide, must be eliminated prior to use.
Another significant source of sulfur is derived from metal refining, where metal sulfide ores are roasted to produce oxides and sulfur dioxide, particularly in lead manufacturing. Other metals such as copper, nickel, and zinc are also sourced from their sulfide ores.
Globally, about 35% of sulfur is derived from the roasting of sulfide ore. This percentage continues to grow as facilities that previously released sulfur dioxide now recover it as sulfuric acid; notably, China produces a substantial portion of its sulfuric acid from pyrites, an iron sulfide ore.
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Sulfuric acid can also be obtained from ammonium sulfate, a by-product in poly(methyl 2-methylpropenoate) production, or recovered from 'spent' (i.e., used) sulfuric acid.
(b) Conversion of Sulfur to Sulfur Dioxide
When sulfur is the feedstock, it must first be converted to sulfur dioxide. Molten sulfur is injected into a furnace, where it burns in a blast of dry air at about 700 K, producing a characteristic blue flame.
Due to the excess air used, the resultant gas comprises 10-12% sulfur dioxide and 10% oxygen by volume. These hot gases are then passed through heat exchangers to cool them to approximately 700 K, generating steam in the process. Upon producing one tonne of sulfuric acid, one tonne of high-pressure steam is also produced.
(c) Conversion of Sulfur Dioxide to Sulfur Trioxide (The Contact Process)
The standard plant configuration includes a cylindrical vessel acting as a fixed bed reactor with multiple catalyst beds. Sulfur dioxide and air pass through these beds, heated to 700 K.
The catalyst employed is vanadium(V) oxide on silica, typically formed into small pellets with cesium sulfate added to lower the melting point, ensuring it's molten at 700 K.
Figure 2 illustrates the vanadium(V) oxide catalyst used for sulfuric acid production.
(d) Conversion of Sulfur Trioxide to Sulfuric Acid
Finally, sulfur trioxide produced from the third and residual from the fourth bed is converted into sulfuric acid. This reaction occurs with water; however, direct water usage is avoided due to a significant temperature increase and the formation of a sulfuric acid mist, which is challenging to handle. Instead, approximately 98% concentrated sulfuric acid is employed.
Heat exchangers maintain the operational temperature around 400 K during this phase. The non-absorbed gases, primarily nitrogen and oxygen, are filtered and released to the atmosphere via a high stack.
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