Sulfuric acid (CAS 7664-93-9, EC 231-639-5) is dispatched as technical-grade 98.0 ± 0.5 wt% H₂SO₄, oleum with 20–30% free SO₃, and battery-grade electrolyte at 37.5 wt%. The product is assigned UN 1830, Class 8, Packing Group II for transport. A specific lot is sampled from the production header only after a 2-hour minimum recirculation period and after the acid temperature at the closed-loop sample cooler has stabilized within 20–30°C. Total acidity is determined by titration in accordance with ASTM E223-23; specific gravity is measured with a glass hydrometer at 15.6/15.6°C. A provisional certificate of analysis is attached to the loading record before the bulk transfer starts.

What Happens Between Quote and Delivery: Our Pre-Shipment Routine for Acid Cargo

Purchase order acceptance initiates a technical review under ISO 9001:2015 clause 8.2.3. The review verifies the requested grade, the transport classification, and the receiving vessel compatibility. The material is assigned hazardous goods identification UN 1830, Class 8, Packing Group II; the proper shipping name remains “Sulfuric acid” for virgin material and “Sulfuric acid, spent” for recovered alkylation streams. The loading schedule assigns a tank that has passed an internal inspection for scale, weld pitting, and relief-device calibration. For 93.0–98.5 wt% acid, carbon steel tanks with a corrosion allowance of 3–6 mm are accepted only when the product temperature is maintained below 40°C. Before transfer, the header is purged with dry air at a dew point below -40°C to reduce surface dilution of the acid film. Loading pumps are started against a closed discharge until manifold pressure reaches 1.5 bar; only then is the discharge valve opened. The linear velocity in carbon steel piping is kept below 1.2 m/s to preserve the iron sulfate passivation film.

The product is sampled at the loading header after 15 minutes of continuous flow using a closed-loop cooler rather than an open thief hatch. A 500 mL retention sample is stored at 20–25°C for 90 days. The certificate of analysis is released after both the provisional sample and the retention sample conform. If the total acidity drops below 96.0 wt%, the batch is either re-blended with higher-strength acid or downgraded; it is not loaded against a 98.0 wt% contract. The post-loading sample is drawn from the top of the road tanker through the same closed-loop sampling device after a 10-minute settling period. If the post-loading total acidity varies from the provisional result by more than 0.2 wt%, the cargo is quarantined and the receiving vessel is inspected for heel contamination or unsealed manway gaskets. During this hold, the tank is blanketed with dry air at 20 mbar positive pressure and the fill level is re-measured by radar gauge. The radar reading is compensated for acid density at 1.836 g/cm³; a discrepancy above 0.5% against the mass flow meter total is investigated before departure.

For deliveries supplied under Incoterms 2020 DAP, the risk transfer occurs at the named destination, but the loading inspection is completed before the vehicle leaves the terminal. For CIF maritime cargoes, the surveyor’s report is signed at the loading port and the cargo record book entry is completed under MARPOL Annex II for Category Y substances. Batch-to-batch variance observed on production lines includes a pump discharge pressure rise of 0.3–0.7 bar during winter campaigns when acid viscosity increases. Concentrated acid viscosity at 20°C ranges from approximately 18 mPa·s at 93 wt% to 26 mPa·s at 98 wt%; unheated lines can exceed the pump motor service factor if the fill rate is not reduced. The terminal procedure therefore reduces the loading rate by 15% when the product temperature falls below 10°C. At a 120 m³ bulk terminal, transfer time for 98 wt% acid at 25°C through an 80 mm loading arm is typically 2.5–3.0 h. When ambient temperature drops below 5°C, the same transfer extends by 30–45 minutes unless the trace heating on the manifold is operating at 15 W/m. The Coriolis mass flow meter is calibrated for density 1.80–1.85 g/cm³ and verified against the shore tank dip before and after loading; a deviation above 0.3% triggers meter re-calibration before the next cargo.

At 98.0 wt%, carbon steel corrosion control depends on the stability of the FeSO₄ film. Published data for this specific configuration is limited, but terminal observations show that a water-washed segment that has not been re-passivated can release iron into the cargo at 3–8 mg/kg during first contact. Water flushing is therefore prohibited immediately before acid transfer. Lines are brought into service by circulating product acid for 30 minutes and checking the iron concentration by ICP-OES at 259.940 nm. A rise above 25 mg/kg in the circulating acid triggers a re-passivation hold, not continued loading. Sulfur dioxide outgassing in virgin 98 wt% acid is controlled below 40 mg/kg when the vent line is unobstructed. In spent acid recovered from cumene or alkylation units, SO₂ rises to 120–180 mg/kg and the material is segregated by a dedicated manifold. Commingling of spent and virgin acid is permitted only after a re-certified blend meets the export specification for iron, residue on ignition, and SO₂. In cases where the acid has been stored in unlined carbon steel for more than 30 days, iron pickup is measured before shipment. If iron exceeds 20 mg/kg, the cargo is discharged to a settling tank and filtered through a 10 µm PTFE membrane before re-certification. This filter bypass is activated only when the pressure drop across the polishing filter exceeds 0.8 bar; otherwise the main filter remains in-line.

How Are Sampling Ports Validated for Homogeneous Acid Cargoes?

Sampling ports are checked by comparing top, middle, and bottom density readings after a 4-hour settling period. A cargo is considered stratified if the bottom density exceeds the top density by more than 0.0015 g/cm³. Recirculation through a 150 mm PTFE-lined return line is continued until the density gradient is below 0.0005 g/cm³. The closed-loop sample cooler is flushed with product acid for 5 minutes before collection to remove dead-leg contamination. All samples are collected in glass bottles with PTFE-lined caps. Total acidity is measured by sodium hydroxide titration to a pH endpoint of 5.2 ± 0.1 as described in ASTM E223-23. Specific gravity is determined at 15.6/15.6°C using a hydrometer calibrated to ASTM E100-19. Iron is measured after 1:10 mass dilution by ICP-OES at 259.940 nm. A sample point is rejected if the conductivity of the rinse blank exceeds 10 µS/cm after a distilled-water wash, indicating chloride or sulfate residue from an earlier batch.

When Free SO₃ in Oleum Exceeds 20% in Bulk Transfers

For oleum grades, free SO₃ is determined by ISO 910:1977. The cargo remains under UN 1830, but the transfer hazard changes because oleum releases SO₃ vapour and reacts violently with water. The loading line is kept under a nitrogen pad with a dew point below -50°C. The shipping tank is limited to 85% fill volume to allow for SO₃ vapour expansion. Loading is suspended when ambient relative humidity exceeds 80% because a leak of oleum in humid air forms sulfuric acid mist. The transfer pump is a canned magnetic-drive unit rated for a 1.5 bar differential and interlocked with a pH sensor in the bund sump. The bund is filled with limestone chips, not water, to neutralize any spill without generating a violent exotherm.

During oleum transfers, the vapour return line is routed to a scrubber charged with 96 wt% sulfuric acid. The scrubber exit gas is monitored for SO₂ with an electrochemical cell. If the outlet concentration exceeds 10 ppm, the transfer is stopped and the loading valve closes automatically. The loading arm gaskets are specified as PTFE envelope gaskets only; no FKM or EPDM is permitted in contact with oleum because the free SO₃ attacks the polymer backbone and causes swelling above 10% volume.

Transport documentation for a bulk road tank includes the ADR transport document with the UN number and packing group, the SDS compiled under REACH Regulation (EC) 1907/2006 Annex II, the signed COA, and an emergency response sheet. Under CLP Regulation (EC) 1272/2008, the label carries Skin Corr. 1A and H314. Maritime consignments under the IMDG Code are segregated from alkalis, cyanides, sulfides, powdered metals, and oxidizing agents. Sulfuric acid is listed as a Category Y noxious liquid substance under MARPOL Annex II; the cargo record book entry is signed by the surveyor before sailing. The bill of lading must show the exact concentration range because a difference of more than 0.5 wt% between the COA and the transport document is a nonconformity under ISO 9001:2015 clause 8.6.

Tank Lining and Gasket Compatibility Matrix

Gasket selection for 98 wt% sulfuric acid changes with temperature. At 20°C, EPDM is not used; plain PTFE or flexible graphite with a PTFE insert covers loading-arm joints. At 50°C, fluoroelastomer FKM swells above 10% volume and is replaced by PTFE envelope gaskets. The tank internal lining is not required for carbon steel when the acid concentration remains within 93–98.5 wt% and the product temperature is below 40°C. Below 90 wt%, the carbon steel corrosion rate increases rapidly, and either high-density polyethylene lining or glass-fibre-reinforced plastic with a vinyl ester barrier is specified. Lining integrity is checked by a 5 kV holiday spark at 3 mm thickness. The relief valve is set at 2.5 bar and functionally tested at the start of each loading campaign.

The same material boundary applies to tank containers. A tank previously used for sodium hypochlorite is not accepted until chloride residue is below 2 µg/cm² by a distilled-water rinse measured with ion chromatography. Chloride in concentrated sulfuric acid produces hydrogen chloride vapour and can initiate pitting at weld seams. In tanks with a rubber lining, the lining must be spark-tested at 3 kV/mm; any field repair is documented and the tank is re-inspected after 24 hours of set time.

Tank cleaning certificates from previous cargo are reviewed for residues of methanol, ethylene glycol, or caustic soda. Any residue in the tank heel above 0.1 wt% is rejected because exothermic dilution or neutralization can elevate the vapour space temperature above 60°C. The last rinse water is drained to a pH above 5 before the acid fill is permitted. The terminal’s load authorization is then recorded in the batch log with the time, operator stamp, and calibration status of the mass flow meter.

Loading manifold interlocks are set so that the acid flow stops if the pump discharge temperature exceeds 45°C or if the pH in the process area sump falls below 2.0. These interlocks are tested with a simulated signal before each shift. The terminal’s telemetry system records pressure, temperature, and flow at 1-second intervals; records are retained for 3 years under the site’s operating permit.