How Sulfuric Acid Is Made: The Contact Process

Sulfuric acid is manufactured almost exclusively through the contact process, a method that converts elemental sulfur (or sulfur-containing gases) into sulfuric acid in three broad stages: burning sulfur to produce sulfur dioxide, catalytically oxidizing that sulfur dioxide into sulfur trioxide, and absorbing the sulfur trioxide into existing sulfuric acid to form a more concentrated product. The process earns its name from the “contact” between sulfur dioxide gas and a solid catalyst, and the chemistry involved is deceptively straightforward on paper but fiendishly tricky to manage in a real plant.

Starting with Sulfur Dioxide

Every contact process plant needs a reliable stream of sulfur dioxide (SO₂). The simplest route is to burn elemental sulfur in air. Liquid sulfur is sprayed into a furnace at roughly 1,000 °C, where it reacts with oxygen to form SO₂. This combustion is strongly exothermic, and the heat it throws off is the first of several energy-recovery opportunities the plant will exploit.

Elemental sulfur is not the only feedstock. Copper and zinc smelters generate SO₂-rich off-gases as a by-product of roasting metal sulfide ores. Some plants also crack spent sulfuric acid from petroleum refineries, breaking the old acid down thermally to liberate SO₂ for recycling. Each feedstock produces a gas stream with different impurity profiles, which has big implications for the purification steps that come before the catalyst bed.

The Catalyst and How It Works

The heart of the contact process is oxidizing SO₂ into sulfur trioxide (SO₃). In open air this reaction barely happens at a useful rate, so it needs a catalyst. Modern plants use vanadium pentoxide (V₂O₅) promoted with potassium sulfate, spread across ceramic pellets or rings arranged in a series of beds inside a large converter vessel. Older textbooks sometimes mention platinum as the original contact process catalyst, and it does work, but it is expensive and easily poisoned by trace impurities, so it was abandoned commercially over a century ago.

The vanadium catalyst operates through a redox shuttle. Vanadium cycles between its +5 and +4 oxidation states: it hands oxygen to SO₂, gets reduced, then picks up fresh oxygen from the air stream to regenerate itself. Research on the catalyst structure has shown that roughly half to three-quarters of the vanadium sits in a particular reduced form (VO²⁺), while the remainder forms larger vanadium-oxygen networks within the molten salt layer that coats the support pellets at operating temperatures.¹ Both the reduction of vanadium by SO₂ and its re-oxidation by oxygen reach equilibrium quickly. The slowest step turns out to be getting the newly formed SO₃ to leave the molten catalyst layer and enter the gas phase, a finding that surprised researchers because it means the bottleneck is physical desorption rather than the chemical reaction itself.¹

Why Temperature Is a Balancing Act

The oxidation of SO₂ to SO₃ is reversible and releases heat. That creates a fundamental tension. Higher temperatures speed up the reaction, which is good for throughput, but they also shift the equilibrium backward, favoring the reactants and limiting how much SO₃ you can ultimately produce. At around 650 K (roughly 375 °C), the equilibrium strongly favors SO₃ formation. Raise the temperature and the equilibrium constant drops steadily, meaning less SO₃ at the endpoint for any given pass through the catalyst bed.²

Plant designers solve this by using multiple catalyst beds with heat exchangers between them. The gas enters the first bed hot enough for the reaction to proceed quickly, and the exothermic reaction heats it further. Before it enters the next bed, the gas passes through a heat exchanger that cools it back down, pushing the equilibrium forward again. A typical converter has four or five beds, each one running slightly cooler than the last. By the time the gas leaves the final bed, overall conversion of SO₂ to SO₃ can exceed 99.5 percent in a well-designed plant.

Absorbing SO₃ into Sulfuric Acid

You might expect the next step to be dissolving SO₃ in water, which on paper gives you sulfuric acid directly. In practice, adding SO₃ to water is violently exothermic and produces a choking mist of tiny sulfuric acid droplets that is almost impossible to contain. Instead, the SO₃ is absorbed into concentrated sulfuric acid (around 98 percent strength) flowing down through packed towers. The SO₃ dissolves readily into the acid, forming what is called oleum, or fuming sulfuric acid, which is essentially sulfuric acid with extra dissolved SO₃. Adding water to the oleum in a controlled way then brings the concentration back to the desired product grade, typically 93 to 98 percent acid.

Controlling acid mist is still one of the trickier operational challenges. If any moisture is present in the gas stream or if temperatures are not right, fine droplets of sulfuric acid form and escape the absorber. Packed bed towers are the standard mist-removal equipment. Research on these towers has found that increasing the gas flow rate through the packing significantly improves the capture of acid mist, likely because faster-moving gas has more contact opportunities with the packing surface.³ Interestingly, pumping more liquid through the tower does not necessarily help and can actually reduce efficiency, probably because excess liquid floods the packing and creates channels the gas slips through.³

Double Contact Double Absorption

The single-absorption layout described above can convert around 97 to 98 percent of the SO₂ to SO₃. That sounds high, but sulfuric acid plants process such enormous volumes of gas that even a 2 percent slip represents a large amount of SO₂ escaping from the stack. Because sulfur dioxide is a major air pollutant and a precursor to acid rain, modern environmental regulations demand conversion rates above 99.5 percent.

The industry’s answer is the double contact double absorption (DCDA) process. After two or three catalyst beds, the partially converted gas is pulled out of the converter and sent through an intermediate absorption tower, where the SO₃ formed so far is removed. Stripping SO₃ from the gas shifts the equilibrium of the remaining SO₂-to-SO₃ reaction forward, so when the gas returns to the final catalyst beds it converts much more completely. A second absorption tower catches the last of the SO₃ after the final beds. The result is overall conversion comfortably above 99.5 percent and dramatically lower SO₂ emissions.⁴ Nearly all new sulfuric acid plants built in the last few decades use the DCDA layout.

Cleaning Dirty Feed Gas

When the sulfur source is clean elemental sulfur, the gas entering the converter is relatively pure SO₂ and air. Plants that process smelter off-gas or crack spent acid face a very different situation. Those gas streams carry heavy-metal fumes, dust, acid droplets, and other particulates that would foul the catalyst, produce low-quality acid, and accelerate corrosion throughout the plant.⁵

The gas cleaning train for a smelter-based plant is substantial. It typically includes humidification (spraying water into the hot gas to cool it), scrubbing towers that wash out particulates and soluble impurities, electrostatic precipitators that strip out remaining mist and fine particles, and drying towers that remove moisture before the gas reaches the converter. Fiber-reinforced plastic (FRP) has become a common construction material in these cleaning sections because it resists the dilute acid environment far better than carbon steel and costs less than exotic alloys.⁵ The whole point of this cleaning section is to protect the expensive catalyst downstream: vanadium pentoxide does not tolerate arsenic, fluorine, or chlorine, and once poisoned it loses activity permanently.

Harvesting the Heat

The contact process is a net energy producer. Burning sulfur is exothermic, oxidizing SO₂ is exothermic, and absorbing SO₃ into acid is exothermic. A modern sulfuric acid plant generates far more heat than it needs for its own operation, and recovering that energy as high-pressure steam is a major part of the plant’s economics. Many facilities export steam to neighboring chemical plants or use it to generate electricity.

Engineers have studied how to squeeze more useful work from the heat the process throws off. One analysis found that raising the steam pressure from 42 to 60 bar reduced wasted energy by about 2,400 kW and boosted potential power generation by roughly 16 percent.⁶ That same study found that tweaking heat-exchanger outlet temperatures produced only marginal further gains, and that the biggest energy losses in the furnace and heat recovery section are fundamentally unavoidable given the chemistry involved.⁶ In other words, the low-hanging fruit in energy optimization is generating higher-pressure steam, not redesigning the heat exchangers.

Surviving the Acid

Concentrated sulfuric acid is paradoxically gentler on metals than dilute acid. At 98 percent concentration and moderate temperature, steel forms a protective iron sulfate film and corrodes slowly. But at intermediate concentrations, at elevated temperatures, or anywhere moisture condenses, the acid attacks metals aggressively. This means the choice of construction material varies dramatically from one section of the plant to another.

Absorption towers and piping that handle hot concentrated acid often use cast iron, carbon steel lined with acid-resistant brick, or specialty alloys. Research into advanced stainless steels has shown that adding about 3 percent copper to a high-chromium, high-nickel austenitic stainless steel dramatically improves its resistance to concentrated sulfuric acid by forming a protective copper-rich surface film.⁷ The copper-containing steel performed on par with some nickel-based superalloys that cost substantially more, making it attractive for the most demanding spots in the plant, such as heat exchangers that handle acid at elevated temperature.⁷ Materials selection is one of the less glamorous aspects of sulfuric acid production, but getting it wrong means unplanned shutdowns and expensive repairs.

Where All That Acid Goes

Sulfuric acid is often called a bellwether of industrial activity because it touches so many sectors. Global production is on the order of 250 to 300 million tonnes per year, making it the most-produced industrial chemical by mass. The dominant end use is fertilizer manufacturing. Roughly half of the world’s sulfuric acid goes into producing phosphoric acid, which in turn becomes phosphate fertilizers, and about 95 percent of the world’s fertilizers depend on sulfuric acid at some stage of their production chain.⁸

Beyond fertilizer, sulfuric acid is used in petroleum refining (as an alkylation catalyst), in metal processing (for pickling steel and leaching copper and uranium ores), in the production of other chemicals like hydrochloric acid and detergent precursors, and in lead-acid battery manufacturing. Its sheer versatility explains why economies that industrialize tend to rapidly scale up sulfuric acid capacity. If a country wants to feed its population, refine metals, and run an automotive sector, it needs sulfuric acid, and lots of it.

What Happens to Spent Catalyst

Vanadium pentoxide catalyst lasts years in a well-maintained plant, but it gradually loses activity as pores clog with dust, as trace poisons accumulate, and as the support structure degrades. When a catalyst charge is finally spent, it becomes hazardous waste because of its vanadium content. Landfilling it is expensive and wasteful, given that vanadium is a valuable metal used in high-strength steel alloys and vanadium redox flow batteries.

Recovering vanadium from spent catalyst is an active area of applied research. One study examined leaching spent catalyst from a Jordanian sulfuric acid plant with oxalic acid and found that under optimized bench conditions, about two-thirds of the vanadium could be recovered. Scaling up to a column leaching setup with longer contact time pushed recovery above 94 percent.⁹ The recovered vanadium can be purified and reused in fresh catalyst or sold to other industries. This kind of circular-economy approach is increasingly important as environmental regulations tighten around hazardous waste disposal and as demand for vanadium grows outside the sulfuric acid industry.

Regenerating Spent Sulfuric Acid

Sulfuric acid does not always end its life as a dilute waste stream. In petroleum refineries, large volumes of acid are used as an alkylation catalyst and become contaminated with hydrocarbons over time. Rather than disposing of this spent acid, refineries send it to regeneration plants. The spent acid is heated to high temperatures where it decomposes, releasing SO₂ that is then fed right back into a contact process converter. This closes the loop and reduces both the demand for fresh sulfur and the volume of acid waste.

The thermal decomposition of spent alkylation acid is more complicated than burning clean sulfur. The acid droplets go through multiple stages as they are injected into a hot furnace: the water boils off, the organic contaminants pyrolyze, and the remaining sulfuric acid cracks to SO₂ and water. Researchers studying this process have found that SO₂ bubbles can form inside the droplets, causing them to distort and fragment, which actually helps the decomposition proceed faster by increasing surface area.¹⁰ Understanding this fragmentation behavior matters for furnace design: if droplets break up too violently they can hit furnace walls before fully decomposing, but if they do not break up enough the reaction is slow and incomplete.

Spent acid regeneration is a good example of why the contact process is not just a single recipe but a flexible platform. The core chemistry stays the same: make SO₂, oxidize it to SO₃ over vanadium catalyst, absorb it into acid. What changes from plant to plant is everything surrounding that core: the feedstock preparation, the gas cleaning requirements, the materials of construction, and how tightly the plant is integrated with the industries it serves. A modern sulfuric acid facility is as much an exercise in chemical engineering logistics as it is in chemistry.

1
Bulletin of the Chemical Society of Japan. Structure of the Vanadium-Potassium Catalyst during the Oxidation of Sulfur Dioxide and the Oxidation Mechanism
2
Scientific Research Journal of Engineering and Computer Science. Thermodynamics, Kinetics and Equilibrium Analysis of Sulphur dioxide Oxidation in a Catalytic Reactor
3
Journal of Environmental Health Science and Engineering. Influence of liquid and gas flow rates on sulfuric acid mist removal from air by packed bed tower
4
Journal of Cleaner Production. Multi-objective optimisation of a double contact double absorption sulphuric acid plant for cleaner operation
5
Corrosion. Recent FRP Applications in Gas Cleaning Section of Sulfuric Acid Plants
6
PubMed Central. Exergy Analysis of Sulfuric Acid Production: A Systematic Framework Using UniSim Design
7
Corrosion. Effect of Copper Addition on Corrosion Behavior of High-Performance Austenitic Stainless Steel in Highly Concentrated Sulfuric Acid Solution—Part 1
8
PubMed Central. The Future of Phosphoric Acid Production –Why We Have to Leave Trodden Paths
9
PubMed Central. The Recovery of Vanadium Pentoxide (V2O5) from Spent Catalyst Utilized in a Sulfuric Acid Production Plant in Jordan
10
AIChE Journal. Gasification of spent alkylation sulfuric acid droplets in hot environments