Industrial Production of Sulfuric Acid

Overview of Industrial Sulfuric Acid Production

Sulfuric acid is one of the most important bulk chemicals worldwide (a “key indicator” of industrialization). Modern production almost exclusively uses the contact process. In this chapter, the focus is on:

• Typical raw materials and their treatment
• The individual process stages and conditions
• Process engineering aspects (heat management, gas purification, recycling)
• Environmental and safety considerations

Underlying concepts such as redox reactions, catalysts, or equilibrium are assumed to be known from other chapters.

Raw Materials and Feedstocks

Industrial production can start from different sulfur-containing raw materials, chosen according to local availability and economics:

• Elemental sulfur (often from natural gas and petroleum desulfurization)
• Metal sulfide ores (e.g., ZnS, CuFeS$_2$) in metallurgical plants
• Hydrogen sulfide-containing gases (e.g., from refineries, natural gas processing)

Elemental sulfur is the dominant feedstock in many regions due to high purity and ease of handling.

Elemental Sulfur as Feedstock

Elemental sulfur is typically:

• Delivered solid, often stored as molten sulfur in heated tanks
• Pumped and sprayed as fine droplets into sulfur burners
• Burned with dry air or oxygen-enriched air

Main combustion reaction:

$$
\text{S} + \text{O}_2 \rightarrow \text{SO}_2 \quad (\Delta H < 0)
$$

This strongly exothermic reaction is the primary heat source for the entire plant.

Sulfur-Containing Off-Gases and By-Products

In metallurgical or refining complexes, gases may contain:

• SO$_2$ from roasting (e.g., sulfide ores)
• H$_2$S from desulfurization of fuels or sour gas

These can be converted into SO$_2$ (e.g., by thermal or catalytic oxidation) and integrated into the contact process, enabling:

• Pollution control (reduced sulfur emissions)
• Resource efficiency (sulfur recovery as a valuable product)

The Contact Process: Overall Scheme

The contact process converts sulfur (or sulfur-containing gases) into concentrated sulfuric acid in several key steps:

1. Burning sulfur or sulfides to SO$_2$
2. Purification and drying of the gas
3. Catalytic oxidation of SO$_2$ to SO$_3$
4. Absorption of SO$_3$ in sulfuric acid to form more H$_2$SO$_4$

The overall chemistry (starting from sulfur) can be summarized conceptually as:

$$
\text{S} + \text{O}_2 + \text{H}_2\text{O} \longrightarrow \text{H}_2\text{SO}_4
$$

In practice, water is only indirectly involved; the process avoids direct contact of SO$_3$ with liquid water (see below).

Gas Treatment: Cleaning and Drying

Before catalytic oxidation, the SO$_2$-containing gas must be very clean and dry to avoid:

• Catalyst poisoning (e.g., by dust, arsenic compounds, heavy metals)
• Equipment corrosion
• Formation of problematic mists or deposits

Typical conditioning steps (details of unit operations belong to chemical engineering fundamentals, not repeated here):

1. Dust removal
• Cyclones, electrostatic precipitators, or bag filters
• Protect downstream equipment and catalyst beds
2. Gas cooling
• Via waste heat boilers or heat exchangers
• Allows heat recovery as steam or hot water
3. Washing/scrubbing (if needed)
• Removes soluble impurities (e.g., halides, certain metal compounds)
4. Drying
• Contact with concentrated sulfuric acid in a drying tower
• The acid absorbs water; the gas leaving is almost completely dry

Drying reaction (simplified):

$$
\text{H}_2\text{SO}_4 (\text{conc.}) + \text{H}_2\text{O}_\text{(gas)} \rightarrow \text{H}_2\text{SO}_4 \cdot \text{H}_2\text{O}
$$

The drying acid becomes slightly diluted and is later reconcentrated using process heat.

Catalytic Oxidation: SO₂ to SO₃

The core of the contact process is the catalytic oxidation:

$$
\text{SO}_2 + \frac{1}{2}\text{O}_2 \rightleftharpoons \text{SO}_3 \quad (\Delta H < 0)
$$

Key features:

• Reversible, exothermic gas-phase reaction
• Catalyst: typically vanadium(V) oxide (V$_2$O$_5$) supported on silica, often with promoters
• Operated in fixed-bed reactors with several catalyst layers (“beds”)
• Reaction conditions are a compromise between equilibrium yield and reaction rate

Reaction Conditions and Equilibrium Considerations

Because the reaction is exothermic, thermodynamics favor:

• Lower temperatures → higher SO$_3$ equilibrium concentration
• Higher pressures → slightly higher conversion (but effect is modest)

However, kinetics require:

• Sufficiently high temperature for an acceptable reaction rate

Typical industrial practice:

• Inlet gas temperatures: ~400–450 °C to “ignite” the catalyst
• Peak temperatures in catalyst beds often 500–600 °C
• Near-atmospheric pressure or slightly elevated pressure

The contact reactor is designed with multiple beds and inter-stage cooling to keep the temperature in an optimal range.

Reactor Design: Multi-Bed Contact Converters

A typical converter includes:

• Several catalyst beds arranged in series
• Heat exchangers between beds for inter-stage cooling
• Sometimes intermediate absorption stages in modern double-contact, double-absorption plants (see below)

Flow pattern:

1. SO$_2$/O$_2$ feed enters first bed (rapid partial conversion, temperature rises)
2. Gas cooled in a heat exchanger
3. Re-enters next bed (further conversion, again heating)
4. Steps repeated through multiple beds until high overall SO$_2$ conversion is achieved

This design balances:

• High equilibrium conversion at cooler entry temperatures
• Adequate reaction rates at higher temperatures inside each bed

Catalyst lifetime depends strongly on gas purity; trace poisons can permanently reduce activity.

Absorption: Formation of Sulfuric Acid

SO$_3$ cannot simply be bubbled directly into liquid water:

• Direct hydration forms fine droplets of sulfuric acid, creating a difficult-to-handle acid mist
• The mist is poorly absorbed and harmful to equipment and environment

Industrial solution: absorb SO$_3$ into existing concentrated sulfuric acid, forming oleum and then convert it to sulfuric acid.

Absorption in Concentrated Acid and Oleum Formation

Main absorption step:

$$
\text{SO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{S}_2\text{O}_7
$$

The product, disulfuric acid H$_2$S$_2$O$_7$, is effectively present as oleum: sulfur trioxide dissolved in sulfuric acid, often described as H$_2$SO$_4 \cdot \text{SO}_3$.

This absorption occurs in an absorption tower:

• Packed or tray column
• Incoming gas (containing SO$_3$) flows countercurrent to down-flowing concentrated H$_2$SO$_4$
• Temperature and concentration are carefully controlled to avoid excessive vapor formation or corrosion

Conversion of Oleum to Sulfuric Acid

Oleum is then “diluted” with water to form more sulfuric acid:

$$
\text{H}_2\text{S}_2\text{O}_7 + \text{H}_2\text{O} \rightarrow 2\,\text{H}_2\text{SO}_4
$$

Or conceptually:

$$
\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4
$$

By adding water to oleum rather than adding SO$_3$ to water, acid mist formation is avoided and heat release is better manageable.

The final commercial product typically has concentrations around 96–98 wt% H$_2$SO$_4$ for general industrial use; other concentrations are also produced as needed.

Double Contact, Double Absorption (DCDA) Process

To reach very high SO$_2$ conversion and low emissions, many modern plants use the DCDA process:

• “Double contact”: gases pass over catalyst in two stages
• “Double absorption”: SO$_3$ is absorbed in two separate absorption towers (intermediate and final)

Simplified flow:

1. SO$_2$ + O$_2$ → SO$_3$ in first series of catalyst beds
2. Gas enters first absorption tower: most SO$_3$ is absorbed
3. Remaining SO$_2$ is re-catalyzed in additional beds (second contact)
4. Gas enters second absorption tower: remaining SO$_3$ is absorbed

Advantages:

• Overall SO$_2$ conversion > 99.5%
• Stack SO$_2$ concentrations very low → reduced air pollution
• High yield of product per unit of sulfur

Heat Management and Energy Integration

The contact process is strongly exothermic at several points:

• Combustion of sulfur to SO$_2$
• Oxidation of SO$_2$ to SO$_3$
• Absorption and hydration of SO$_3$

Industrial plants exploit this heat to improve efficiency:

• Waste heat boilers: hot combustion gas generates high-pressure steam
• Heat exchangers: recover heat from reactor effluents to preheat incoming gas streams
• Steam turbines or process heating: produced steam may drive generators or supply other processes

Good heat integration:

• Lowers fuel and operating costs
• Helps maintain optimal temperatures in catalytic beds and absorption equipment
• Reduces thermal stress on materials

Environmental and Safety Aspects

Emission Control

Main environmental concerns:

• SO₂ and SO₃ emissions (acid rain precursors)
• Acid mist from stacks
• Local air quality impacts

Mitigation measures:

• High-conversion DCDA processes
• Tail-gas scrubbing (e.g., alkaline scrubbers, wet ESP for mist removal)
• Stringent leak prevention and monitoring

SO₂ emission regulations often drive plant design and upgrade decisions.

Handling of Sulfuric Acid and Oleum

Hazards:

• Strongly corrosive liquids and mists
• Highly exothermic reactions with water and many organic materials
• Potential for severe chemical burns

Operational precautions:

• Use of appropriate materials (high-alloy steels, special linings, PTFE, brick linings)
• Controlled dilution (add acid to water, not water to acid, in open systems)
• Ventilation and personal protective equipment (PPE)
• Strict temperature and concentration control in storage tanks and transfer lines

Oleum requires even greater care due to its high SO₃ content and strong fuming tendency.

Integration with Pollution Control from Other Industries

Sulfuric acid plants are often integrated with:

• Non-ferrous metal smelters: converting SO₂-rich off-gas to H₂SO₄ prevents direct SO₂ release
• Refineries and gas processing: recovery of sulfur via Claus process followed by sulfur combustion to SO₂

This integration transforms environmental liabilities (sulfur emissions) into valuable product streams.

Variants and Special Processes

While the contact process is standard, some variants exist for specific contexts:

• Wet gas processes for lower-concentration or impurity-rich gas streams
• Oxygen-enriched operation to reduce gas volumes and equipment size
• Integrated processes where produced sulfuric acid is directly used on-site (e.g., fertilizer plants producing phosphoric acid)

The choice of process variant depends on:

• Composition and volume of incoming sulfur-containing gases
• Required product specification (concentration, impurity limits)
• Local energy prices and environmental regulations

Economic and Industrial Importance

Sulfuric acid is consumed in large quantities in:

• Fertilizer production (especially phosphates)
• Mineral processing and leaching of ores
• Petroleum refining and petrochemicals
• Production of organic and inorganic chemicals
• Wastewater treatment and pH control

Because it is produced in massive tonnages and used in many sectors, the efficiency and environmental performance of sulfuric acid plants have significant economic and ecological impact, making the industrial contact process a central example in chemical engineering practice.