Hydrochloric acid is an aqueous solution of hydrogen chloride gas with CAS 7647-01-0, EC 231-595-7 and transport designation UN 1789. The molecular weight of HCl is 36.46 g/mol; commercial shipments in Europe are typically 30–36 % by weight HCl with a density at 20 °C between 1.149 and 1.179 g/cm³. The atmospheric-pressure azeotrope is approximately 20.2 % w/w HCl with a boiling point of 108.6 °C, which limits simple distillation and defines the practical concentration ceiling for low-cost bulk acid. Vapour pressure and fuming tendency increase sharply above 36 %, so most merchant acid is handled below that concentration unless pressurised or refrigerated storage is economically justified.

European production is split between by-product streams from isocyanate and chlorinated organic synthesis and on-purpose synthesis from chlorine and hydrogen in integrated chlor-alkali sites. The by-product route ties merchant HCl availability to polyurethane demand and to oxychlorination operating rates; a reduction in MDI or TDI output lowers by-product HCl supply even before any logistics disruption. The on-purpose route is energy-intensive and is normally operated to balance chlorine demand rather than to serve merchant acid markets. Consequently, hydrochloric acid supply in Europe is structurally inelastic in the short term and highly sensitive to simultaneous demand and logistics shocks.

Quality specifications depend on end use. Water treatment grade hydrochloric acid is covered by EN 939:2016, which sets maximum limits for arsenic, cadmium, chromium, mercury, nickel, lead, antimony, selenium and oxidising substances. Steel pickling grade tolerates higher iron and organic content but requires tight free-chlorine control to prevent inhibitor degradation. Food additive E507 compliance follows Commission Regulation (EU) No 231/2012, which specifies acidity, non-volatile residue, oxidising substances, arsenic and heavy metal limits. Technical grade acid for chemical synthesis may be sold with lower trace-metal guarantees but with controlled sulphate and fluoride levels to prevent catalyst poisoning or reactor fouling.

Material selection for 30–36 % hydrochloric acid excludes 316L stainless steel because chloride concentrations cause pitting and stress corrosion cracking. Rubber-lined carbon steel, fibre-reinforced vinyl ester, polypropylene, PVDF and PTFE are used for tanks, pipes and seals. Lined diaphragm valves are preferred over ball valves with stainless internals for flow control. Pump selection typically requires sealless magnet-drive or lined centrifugal units because the liquid has a specific gravity above 1.15 and releases hydrogen chloride vapour at the wet end.

Rhine Low Water and Force Majeure: Logistics Risk in European HCl Supply

The central logistics corridor for northwest European hydrochloric acid is the Rhine, where barge transport links the ARA chemical hub with the Ruhr and Upper Rhine production sites. The Kaub gauge, located in the Middle Rhine gorge, is used as the reference for navigable depth. The equalised water level at Kaub is 78 cm, corresponding to 1.9 m of fairway depth under German federal waterway definitions. When the gauge falls below 78 cm, the available depth declines and tank barge payload must be reduced in proportion to vessel draft. Published trade reports from the 2018 and 2022 Rhine low-water periods describe chemical tanker payload reductions of 30–50 % and sharp increases in spot barge freight rates.

Force majeure is declared when the seller cannot tender cargo at the agreed delivery point because barge logistics become impossible or commercially impracticable. In European chemical contracts, the clause normally requires a notice period of 24–72 h and evidence that the event was beyond the party's reasonable control. For HCl, a low-water event at Kaub may trigger force majeure at a supplier whose storage is full or whose nominated barge operator refuses to load at reduced payload. The supply interruption then propagates to water works, steel pickling lines and metal finishing plants that consume acid continuously.

At receiving terminals, bulk HCl is stored in rubber-lined carbon steel, fibre-reinforced vinyl ester or other corrosion-resistant tanks. Vent headers are routed to caustic scrubbers to control hydrogen chloride emissions; national permits under the Industrial Emissions Directive 2010/75/EU commonly reference a waste gas HCl emission limit of 10 mg/Nm³. During low-water barge disruption, terminal throughput drops below the minimum flow needed to keep unloading systems in continuous service. The bottleneck is not tank capacity at the terminal but the availability of road loading slots, rail siding windows and certified operators for transfer operations.

What Limits Continuous HCl Unloading During a Low-Water Event?

Downstream buffer stocks are the first operational boundary. A water treatment plant under EN 939-certified HCl dosing for pH correction may hold 15–30 m³ of concentrated acid, sufficient for 7–14 days at normal dose rates. During a prolonged low-water event, replenishment orders of 25 t truck deliveries cannot replace a 300–600 t barge parcel without increasing unloading frequency beyond staffed shifts. The limiting factor shifts from tank capacity to receiving slot availability, ADR vehicle scheduling and certified operator coverage.

Ion exchange regeneration for demineralised water production consumes 50–100 L of 30 % HCl per cubic metre of strong acid cation resin per cycle. Interruptions therefore delay demineralised water supply for steam generation and can force boiler load reductions. Steel pickling lines are less tolerant. Continuous pickling lines recirculate 18–22 % w/w HCl at 70–85 °C; if fresh acid supply is interrupted for more than 48–72 h, the line must reduce speed or shut the pickle section, which upstream cold rolling cannot easily buffer without piling intermediate strip inventory.

When Rail and Truck Substitution Becomes the Only Route After a Force Majeure Declaration

Rail and truck movements are governed by ADR/RID. Hydrochloric acid is UN 1789, Class 8, and assigned Packing Group II or III according to concentration. Bulk road tankers and rail tank wagons used for 30–33 % HCl are rubber-lined and fitted with top or bottom discharge. Tank codes and special provisions under RID Chapter 6.8 require periodic leakproofness testing and lining inspection. Tunnel code E forbids passage through many road tunnels for large HCl tanks, forcing detours and adding transit time.

Delivery cost increases are measurable. Published multimodal freight benchmarks for liquid chemicals in northwest Europe place inland barge below rail and road on a cost-per-tonne-kilometre basis. Road delivery can raise delivered HCl cost by a factor of three to six compared with barge, which erodes the already low margin of by-product HCl. The substitution is also capacity-limited because the European chemical tank truck and tank wagon fleet is not sized to replace sustained barge loss on the Rhine. Rail tank wagon availability is further constrained because the fleet is sized for base barge flows and has low idle redundancy.

Steel Pickling, Acid Regeneration, and the Circular HCl Balance

The largest single European HCl demand is carbon steel pickling. In continuous pickling, hot-rolled strip is passed through 18–22 % w/w HCl at 70–85 °C. Oxide scale dissolves by the following stoichiometric reactions: FeO + 2 HCl → FeCl₂ + H₂O and Fe₂O₃ + 6 HCl → 2 FeCl₃ + 3 H₂O. Acid consumption is approximately 10–25 kg HCl per tonne of steel, depending on scale thickness and line speed. Spent pickle liquor contains 150–250 g/L iron chloride and 10–50 g/L free HCl, and is regenerated rather than neutralised.

Regeneration uses spray roasting or fluidised bed hydrolysis. Spray roasting at 500–900 °C converts ferrous chloride back to hydrogen chloride gas and iron oxide: 2 FeCl₂ + 2 H₂O + 0.5 O₂ → Fe₂O₃ + 4 HCl. The HCl is absorbed into rinse water to make 18–20 % regenerated acid, which returns to the pickle line. The process closes 95–99 % of the acid loop in modern lines, but make-up acid is still required to replace losses from rinse drag-out, stack emissions and iron oxide residue. Low-water interruptions therefore hit both the make-up supply and the external merchant HCl supply to smaller picklers without regeneration.

Regenerated acid may require filtration through plate and frame or cartridge filters to prevent insoluble Fe₂O₃ fines from re-entering the pickling bath. Conductivity and redox potential are monitored online; bath HCl concentration is maintained between 8 % and 12 % at the exit of the pickle section, while iron concentration is limited to 120–160 g/L to avoid surface staining. Continuous pickling lines operate at strip speeds from 60 to 180 m/min; at the upper end of this range, even short acid supply interruptions create a large buffer of unfinished strip because hot-rolling and cold-rolling campaigns are tightly scheduled.

A second merchant HCl balance in Europe arises from isocyanate plants. In the phosgenation of methylenediphenyl diamine to MDI, hydrogen chloride is formed as a co-product; if not recycled to oxychlorination or sold as acid, the plant must reduce operating rate. The conversion of chlorine to HCl in toluene diisocyanate and MDI production means that a slowdown in polyurethanes can reduce by-product HCl supply simultaneously with low-water logistics. By-product acid can require purification by adsorption or air stripping to remove traces of aromatic compounds before it can be used in water treatment under EN 939.

In chlorinated solvents and vinyl chloride monomer production, HCl is produced during cracking of ethylene dichloride: C₂H₄Cl₂ → C₂H₃Cl + HCl. The HCl stream is typically recycled to oxychlorination. Merchant availability therefore depends on whether integrated oxychlorination units can absorb the stream. In periods of low Rhine water, even recycled HCl cannot easily be moved to external consumers because pipeline and barge corridors are saturated. Published data for site-specific offtakes during force majeure events is limited, but the operational coupling between inland waterway capacity and by-product acid clearance remains the central logistics constraint in the European HCl market.