Sulfuric Acid Production Line Process Flow

The sulfur-to-acid unit consists of the following sections: sulfur melting, sulfur combustion and conversion, drying and absorption, and final product handling.

1. Sulfur Melting Section

Solid sulfur from the raw material section is conveyed via a belt conveyor into a rapid sulfur melting tank for melting. The molten sulfur flows by gravity through the overflow port into a combined filtration tank, from where it is pumped into the molten sulfur filter for filtration and then stored in a molten sulfur storage tank.
Before filtration, a certain amount of filter aid is added to the pre-coating tank containing molten sulfur. Under the action of the agitator, the filter aid and molten sulfur are thoroughly mixed. The pre-coating pump transfers the mixture to the molten sulfur filter, where the filter leaves are covered with the filter aid. After the pre-coating operation is completed, formal filtration begins. The purified molten sulfur is then sent to the refined sulfur storage tank for storage. During production, molten sulfur flows by gravity from the refined sulfur storage tank to the underground refined sulfur tank and is pumped into the sulfur combustion furnace.

The rapid sulfur melting tank, filtration tank, and pre-coating tank are made of integrated concrete structures and are installed underground. All sulfur storage tanks, including the refined sulfur storage tanks, are equipped with steam heating coils using 0.5~0.6 MPa (absolute pressure) saturated steam for indirect heating. The molten sulfur temperature is maintained between 135°C and 145°C.

Other equipment, such as molten sulfur filters, sulfur pumps, molten sulfur pipelines, fittings, and valves, are steam-jacketed and insulated with additional insulating materials.

Steam condensate and discharged exhaust steam (1.2 tons/hour) are collected in a condensate recovery tank. Once the water quality meets requirements, the condensate is sent to the boiler deaerator.

The sulfur melting unit is designed for a sulfur consumption rate corresponding to a ***-ton/year sulfuric acid plant.

The sulfur warehouse is designed and constructed by the owner.
The molten sulfur storage tank is designed with a capacity of 400 tons.

2. Sulfur Combustion and Conversion Section

Molten sulfur is pressurized by a refined sulfur pump and sprayed into the sulfur combustion furnace through a sulfur gun. Air required for sulfur combustion is filtered through an air filter, pressurized by an air blower, dried in a drying tower with 98% sulfuric acid, and then supplied to the sulfur combustion furnace. The moisture content of the dried air is ≤0.1 g/Nm³.

The dried air mixes with sulfur in the sulfur combustion furnace, generating furnace gas containing 10.0% SO₂. The furnace gas is cooled in a fire-tube waste heat boiler from approximately 970°C to 420°C before entering the first stage of the gas converter for conversion. The inlet temperature to the first stage is adjusted using high-temperature bypass lines and cold air quenching.

The furnace gas exiting the waste heat boiler at 420°C enters the first catalyst layer of the converter, where its temperature rises to approximately 606°C after the reaction. The gas then enters the high-temperature superheater for heat exchange, producing superheated steam for the main plant. The gas is cooled to 450°C and enters the second catalyst layer for further oxidation, raising the temperature to approximately 514°C. It then passes through heat exchangers, cooling to 440°C, before entering the third catalyst layer for oxidation. After the third layer, the temperature rises to about 457°C. The gas is cooled through a series of heat exchangers and economizers to approximately 170°C and sent to the intermediate absorption tower. Here, SO₃ is absorbed using 98.5% concentrated sulfuric acid. The gas exits through a demister, is reheated in the heat exchangers, and then enters the fourth catalyst layer at 420°C for final oxidation.

The gas leaving the fourth layer enters low-temperature superheaters and economizers to recover heat, cooling to approximately 150°C before entering the second absorption tower. SO₃ is absorbed using 98% sulfuric acid, and the residual gas is vented through the tail gas stack.

The total SO₂ conversion rate is ≥99.8%.
Temperature adjustment for each catalyst layer is achieved using bypass lines and valves.

A startup alkali scrubber system is installed after the second absorption tower to remove unconverted SO₂ from the tail gas during startup.

Heat exchange equipment in the conversion process is equipped with necessary bypass lines for temperature control.

For startup and heating, a regenerative method is used. A startup blower and heating lines are installed for the first and fourth stages of the converter. A short-circuit line is provided for initial heating, allowing the system to operate in a single-conversion-single-absorption mode, shortening heating time and reducing costs. Once heating is complete and sulfur injection begins, the short-circuit line is closed, and the system transitions to dual-conversion-dual-absorption mode.

3. Drying and Absorption Section

The drying system uses 98% sulfuric acid for drying, and the absorption system uses 98% sulfuric acid for absorption.

The drying tower, first absorption tower, and second absorption tower are packed towers.
The concentrated acid circulation tank is a horizontal tank, serving both drying and absorption systems.

Moist air passes through an air filter to remove dust, enters the drying tower, and is dried by 98% sulfuric acid. The acid absorbs moisture from the air and drains into the concentrated acid circulation tank, where it is pumped back into the system after concentration adjustments.

SO₃ generated during absorption combines with 98% H₂SO₄ to form 98% H₂SO₄, which overflows into the circulation tank.

The acid cooling circulation system operates as: tank → pump → acid cooler → tower → tank.

Excess 98% sulfuric acid becomes the final product, cooled by the product acid cooler, measured by an electronic flow meter, and transferred to the sulfuric acid storage tank or loading tank in the final product section.

The process employs a three-tower, two-tank, three-pump, and three-acid cooler configuration, simplifying the process, reducing investment, and facilitating operations. Dissolved SO₂ in the tail gas is minimal, and high-temperature absorption further minimizes SO₂ in the product acid.

Pipelines for concentrated acid are made of 304 stainless steel with anodic protection to ensure long-term stable operation. Acid return, product acid, and acid discharge pipelines are made of acid-resistant cast iron.

An alkali scrubber is installed after the second absorption tower to remove traces of SO2 during startup, ensuring no environmental pollution.

4. Final Product Section

The 98% sulfuric acid from the product acid cooler is stored in sulfuric acid storage tanks.

Startup mother acid is supplied from the sulfuric acid storage tanks to the concentrated acid circulation tank via the acid supply pump.

Two sulfuric acid storage tanks, each with a capacity of 1,000 tons, are provided.