Commercial sulfuric acid is most frequently handled as a clear, heavy liquid with a concentration of 93–98 wt% H₂SO₄; the 98 wt% grade is assigned UN 1830 when the acid content exceeds 51 wt%, while dilute grades not exceeding 51 wt% are assigned UN 2796. Oleum is a separate product form consisting of sulfur trioxide dissolved in sulfuric acid and is transported under UN 1831. The double-contact/double-absorption process yields acid in the 98.0–98.5 wt% range after interpass absorption and final tail gas scrubbing; the 98.3 wt% azeotrope boils at 338°C, and pure 100 wt% acid freezes at 10.4°C, making unheated storage of near-100% acid vulnerable to solidification. The product is registered under CAS 7664-93-9 and EC 231-639-5, and the harmonised classification under CLP includes Skin Corr. 1A, H314. The density of 98 wt% acid at 20°C is 1.84 g/cm³.

Industrial consumption is dominated by wet-process phosphoric acid, sulfate-process titanium dioxide, petroleum alkylation, metal pickling, and microelectronics etching. In each downstream operation the concentration window, temperature profile, and alloy selection are more restrictive than the generic safety data sheet suggests. Small excursions generate viscosity changes, phase-separation failures, or soluble metal contamination.

Do You Need a Dual-Use Export Licence for Sulfuric Acid?

Under Regulation (EU) 2021/821, a dual-use export licence is not required for sulfuric acid as a single chemical because sulfuric acid is absent from Annex I and Annex IV. The U.S. Export Administration Regulations produce the same outcome for ordinary commercial shipments: sulfuric acid is not listed in the Commerce Control List, and the export classification is EAR99 absent an embargoed destination, denied party, or prohibited end use under Part 744. The absence of a dual-use listing does not release the exporter from precursor chemical controls. Sulfuric acid is scheduled as a Category 3 precursor in Annex I of Regulation (EC) No 273/2004 for intra-Union trade and under Regulation (EC) No 111/2005 for external trade. Depending on destination and consignment size, the competent national authority may require a precursor export authorisation, an end-user declaration, or a transaction record before release. In the United States, sulfuric acid is a List II chemical under 21 CFR 1310.02(b); import and export declarations are administered under 21 CFR Part 1313, and the applicable threshold is set by the current list in 21 CFR 1310.02.

The operational difficulty is that a shipment may be described as technical acid, battery electrolyte, or spent acid, while the legal schedule uses concentration and end-use rather than the commercial name. Oleum and spent acid are not separately listed in the EU dual-use annex, but their sulfur trioxide content and recoverable sulfuric acid content may bring them under precursor recordkeeping if the threshold is met. Mixtures in which sulfuric acid is an electrolyte, such as lead-acid batteries under ADR UN 2794, are classified by the article and transport designation; the battery does not trigger a dual-use licence solely because of the acid content. For automated export screening, the key data fields are ECCN/EAR99 status, EU dual-use Annex I absence, the precursor schedule category, and the UN number.

Wet-process phosphoric acid reactors using 93–98 wt% sulfuric acid at 70–80°C produce a slurry of calcium sulfate dihydrate. The free sulfate concentration in the reaction liquor is held at 15–30 mg SO₃/mL to keep the calcium sulfate in the dihydrate phase. Below approximately 10 mg SO₃/mL, the precipitation shifts toward hemihydrate, filter cake permeability drops, and the tilting-pan filter may blind; above 40 mg SO₃/mL, coarse gypsum crystals form but the cake compresses and residual phosphorus losses increase. Sulfuric acid feed is set by the calcium oxide content of the phosphate rock and the desired product acid strength; modern plants use online sulfate titration and ratio control rather than fixed acid-to-rock feed. The reaction slurry is cooled by vacuum flash evaporation, and the off-gas is scrubbed before fluoride recovery.

Mixed-acid nitration of toluene to mononitrotoluene operates with the sulfuric acid phase functioning as the dehydration agent rather than the oxidant. The spent acid strength at the end of nitration is maintained above 66 wt% H₂SO₄; at lower strength the reaction mass becomes less selective toward the mono-isomer and the formation of dinitro compounds increases. The mixed-acid ratio is set on the basis of the water generated during reaction and the water entering with nitric acid. A typical plant circulates spent acid through a concentrator, returns 90–96 wt% acid, and uses calibrated mass-flow meters to maintain the nitric acid-to-sulfuric acid ratio within the process-safety basis. The calorimetric thermal stability of the nitration mixture is measured in the laboratory and controlled by limiting the dosing rate; the reactor is cooled by a double envelope or external plate heat exchanger with a high-integrity trip on loss of agitation.

Alkylation Acid Strength Limits and the Acid-Soluble Oil Cliff Edge

In sulfuric acid alkylation, the reactor acid strength is held between 88 and 93 wt% H₂SO₄ in the emulsion zone of a horizontal contactor or vertical packed reactor. Water balance is the controlling variable: water enters with fresh acid and olefin feed, and spent acid withdrawal removes water and acid-soluble oils. When the spent-acid bleed is set too low, the circulating acid strength falls toward 85 wt%; below this threshold the selectivity to trimethylpentanes decreases and the acid-soluble oil fraction increases, producing a denser acid phase and a smaller density difference between acid and hydrocarbon. This weak-acid condition increases acid carryover into the downstream settler and can overload the reactor effluent treating system.

The contactor is designed for an acid-to-hydrocarbon volume ratio typically in the range 0.9:1 to 1.2:1; the discharge is a tight emulsion that is separated in an acid settler with a residence time of 30–60 minutes. On production-scale units the acid phase is recycled to the contactor suction, and a continuous spent-acid stream is sent to a sulfuric acid regeneration unit or a spent-acid storage tank. Acid strength is monitored by continuous density measurement compensated to 15.6°C and by periodic sodium hydroxide titration; an alarm is typically set at 87 wt% to allow operator response before the 85 wt% operational boundary is reached. The target is not arbitrary: at 85 wt% H₂SO₄ the water content is approximately 15 wt%, which is sufficient to change acid-phase viscosity and hydrocarbon solubility enough to affect reaction rate.

37–50 wt% sulfuric acid is metered into a mixed flash-mix channel through a diffuser; a pH setpoint of 6.5–8.5 is common, but local pH below 4.0 in the first seconds must be avoided because acid shock can disaggregate biological floc and release bound metals. Injection quill and static mixer placement are designed for a dilution ratio of at least 20:1.

When Free Sulfur Trioxide Condenses in Vent Lines During Oleum Storage

Oleum storage tanks handle free SO₃ levels from 20 wt% to 65 wt% and are maintained at ambient temperature because heating can raise the vapour pressure and increase free SO₃ release. Vent lines are designed with heat tracing and a condensation trap because SO₃ can combine with water vapour to form sulfuric acid mist and crystalline sulfate deposits. NACE SP0294-2006 provides design guidance for carbon steel storage tanks in concentrated fresh acid and oleum service; the standard limits air-moisture ingress, permits anodic protection for selected conditions, and requires velocity control in recirculation piping. For cylindrical tanks in continuous service, a bottom connection with a standpipe rather than a bottom nozzle is often used; frozen plugs are more likely at the tank bottom if the concentration is near the freezing point. The freezing point of concentrated sulfuric acid varies sharply with concentration: 100 wt% acid freezes at 10.4°C, while certain diluted grades freeze below -30°C. The temperature alarm in a tank storing 98 wt% acid is therefore set with enough margin to grade-specific solidification.

At transfer locations, unloading pumps are specified for sulfuric acid service with a maximum velocity below 0.9 m/s to limit erosion-corrosion; a Schedule 80 carbon steel line with a magnetic flow meter and a high-point air eliminator is a common configuration. Flexible connections are PTFE-lined braided or composite hoses with excess-flow valves; the fill line discharges to the tank bottom through a dip pipe to prevent static accumulation and acid mist generation.

Circulating acid in a single-contact drying tower is maintained at 93–96 wt% H₂SO₄ and 45–70°C; the acid absorbs moisture from the cleaned air stream before sulfur dioxide conversion. The brick-lined tower contains ceramic packing and a mist eliminator. Carryover of acid mist above 20 mg/Nm³ at the converter inlet accelerates converter catalyst deterioration and can cause visible stack emissions. A drop in drying tower density below 92 wt% indicates water breakthrough from the air-preheater or excessive dilution; the operator responds by diversion to the intermediate absorber or by adding concentrated acid.