Commercial sulfonation of linear alkylbenzene, toluene, and naphthalene for detergent and dye intermediate production shifts from 98 % sulfuric acid to oleum when the equilibrium water generated by the sulfonating agent dilutes the reaction medium below the acid strength required for acceptable conversion. Oleum (Fuming Sulfuric Acid) for Sulfonation: When You Need Free SO3, the product is specified by weight percent free SO3, most commonly 20 %, 30 %, and 65 %, with total acidity and free SO3 determined by ISO 910:1977 or ASTM E223-23e1. Free SO3 acts as both the active electrophile and an internal desiccant that consumes water released during sulfonation, maintaining anhydrous sulfonating conditions. Grade selection is governed by freezing point, SO3 vapor pressure, reaction exotherm, and the allowable concentration of sulfone and polysulfonic acid byproducts in the downstream application.

What Limits the Use of 98 % Sulfuric Acid in High-Conversion Sulfonation?

Sulfonation of benzene or alkylbenzene with 98 % H2SO4 proceeds through a reversible pathway in which one mole of water is liberated per mole of sulfonic acid formed. The water reduces the effective H2SO4 concentration and shifts the equilibrium toward the unsulfonated substrate. Substitution of oleum introduces free SO3 that reacts with this water: 18 g of water consumes 80 g of SO3 to form 98 g of H2SO4. This stoichiometric relationship is the basis for specifying oleum in direct sulfonation of aromatics where water removal by azeotropic distillation or vacuum stripping is not practical. At free SO3 contents above 30 %, the acidity function becomes sufficiently negative to promote sulfone formation and oxidative degradation of alkyl side chains when the reaction temperature exceeds 60 °C. The processing window for mono-sulfonation of linear alkylbenzene with oleum is therefore bounded on the lower side by water dilution and on the upper side by side-product formation; typical direct sulfonation setpoints use 20 % oleum and a maximum bulk temperature of 50 °C unless the plant is designed with intensive cooling and short residence time.

In continuous falling-film sulfonation, SO3 gas generated from sulfur burning is preferred for detergent-grade linear alkylbenzene sulfonic acid because the short gas-liquid contact time, typically 0.5–2 s, limits sulfone formation. Oleum 65 % is used in some sites as a dry SO3 source by thermal stripping at 120–180 °C; the released SO3 is then diluted with air to 2–6 vol % before entering the falling-film reactor. The narrow processing window is controlled by two interlocks: high-temperature shutdown at 80 °C in the reactor gas inlet and low-flow shutdown at 10 % of design air flow to prevent hydrocarbon carryover into the SO3 generation train. Published data for this specific configuration is limited because most detergent sulfonation units use sulfur burning rather than oleum stripping.

When Free SO3 Must Remain Available After Water Formation

In dye intermediate production, naphthalene sulfonation with 30 % oleum is operated at 40–60 °C to favor mono-sulfonation while limiting disulfonic acid formation. The free SO3 concentration must remain sufficient to react with water generated from the sulfonating acid, otherwise the acid strength falls below 90 % and the reaction stalls before the target conversion is reached. With 20 % oleum at the lower end of the temperature window, naphthalene-1-sulfonic acid is the dominant species; raising the temperature or free SO3 content to 30 % increases the proportion of naphthalene-2-sulfonic acid and naphthalene disulfonic acids. Process control therefore requires simultaneous management of temperature, free SO3 availability, and mixing intensity. A 100 kg sulfonation charge initially at 98 % H2SO4 that generates 18 kg water requires 80 kg free SO3 to regenerate 98 kg H2SO4; this stoichiometry explains why oleum charge is calculated from the expected water of reaction rather than from the substrate weight alone.

A 5000 L glass-lined batch reactor with turbine agitation and tempered-water jacket cooling is charged with 20 % oleum at 20–25 °C. Alkylbenzene is metered below the liquid surface at a rate that holds bulk temperature between 35 °C and 50 °C; a free SO3-to-substrate molar ratio of 1.00:1 to 1.15:1 is maintained for mono-sulfonation. Feedstock moisture is controlled to 0.01 % water because each 18 g water consumes 80 g free SO3; in humid plants, pre-drying at RH above 60 % is required to prevent oleum demand drift. The agitator tip speed is held at 3–5 m/s to disperse the feed and prevent localized free SO3 depletion at the point of injection. The sulfonation exotherm for alkylbenzene substrates is approximately 150–200 kJ/mol, and jacket cooling uses tempered water at 5–10 °C to hold the reactor within ±2 °C of setpoint. Production-scale failure modes include fouling of internal cooling coils with sulfone-rich tar when the local free SO3 content exceeds 30 %, causing temperature excursions above 70 °C and darkening of the product. After the aromatic feed is exhausted, the batch is held for 2–4 h, and residual free SO3 is titrated by ISO 910:1977; a residual value above 1.5 wt % indicates incomplete conversion or excess oleum charge requiring correction before neutralization.

Storage, Metering, and Off-Gas Controls for 20–30 % Free SO3 Oleum

Oleum storage for sulfonation units uses horizontal carbon steel tanks with internal steam coils or external heat tracing to keep 20 % oleum at 15–25 °C and 30 % oleum at 30–40 °C because the 30 % grade solidifies near 21 °C. Tanks are blanketed with dried nitrogen at a dew point below -40 °C to prevent atmospheric moisture from diluting the free SO3 and increasing corrosion. Transfer piping conforms to ASME B31.3; joints are minimized and flanges are shielded because SO3 vapor condenses to corrosive liquid films at ambient temperatures. Carbon steel is acceptable for oleum because the free SO3 maintains a passive iron sulfate film; however, flow velocity is limited to 1 m/s to avoid erosion of the protective film. Metering is performed with PTFE-lined magnetic flow meters or sealless canned pumps with PTFE or ETFE wetted parts. Pump seals and valve packings are the primary leak sources; mechanical seals, where present, are purged with dry nitrogen at 0.3–0.5 bar above stuffing box pressure. Storage vents and pump recovery lines discharge to a packed scrubber irrigated with 10 % sodium hydroxide solution, which neutralizes SO3 mist and prevents emissions above the threshold set by the site air permit.

Spent acid from batch oleum sulfonation typically leaves the reactor at 75–85 wt % H2SO4, containing dissolved sulfonic acids, sulfone byproducts, and trace metal impurities. Neutralization with 32 % sodium hydroxide solution is carried out in a continuous neutralizer at 40–60 °C; the exotherm is removed by chilled water to prevent foaming from water vaporized during acid-base neutralization. Sulfone and polysulfonic acid impurities are measured by HPLC with UV detection at 254 nm. A sulfone content above 2.0 wt % in crude sulfonated alkylbenzene is typically rejected for detergent intermediate use because it imparts odor and reduces solubility in aqueous formulations. The lower acid concentration boundary for spent acid transfer is 70 %; below this value, the corrosion rate of unprotected carbon steel increases sharply, so transfer to neutralization must occur before that threshold or the piping must be PTFE-lined.

Water contact with oleum in open systems generates a corrosive acidic aerosol and rapid local boiling; therefore dilution is carried out by adding oleum to chilled water or to 70 % sulfuric acid in a dedicated quench vessel, never by adding water directly to the oleum charge. The quench vessel is a glass-lined, jacketed reactor with a high-level interlock and a reflux condenser connected to the caustic scrubber. The maximum adiabatic temperature rise from diluting 20 % oleum to 70 % acid is approximately 60 °C; close-loop temperature control at 30–40 °C prevents vaporization of SO3 and protects the glass lining from thermal shock.

Incoming oleum is tested for free SO3 by the two-stage titration procedure of ISO 910:1977; the same method is used on reactor samples to determine residual free SO3. Total acidity is determined by titration with 1 N sodium hydroxide using a pH or colorimetric endpoint; free SO3 is then calculated from the difference between total acidity and the sulfuric acid content. Sampling requires sealed glass ampoules or PTFE-lined syringes to prevent moisture uptake; atmospheric exposure for more than 30 s can alter the free SO3 result by more than 0.2 % absolute. Iron content is controlled below 50 mg/kg because dissolved iron accelerates color body formation in sulfonated aromatic intermediates. Color is measured with a 50 mm cell according to ASTM D1209-05(2019); the release limit for surfactant-grade sulfonated alkylbenzene after neutralization is 50 APHA. For product shipped within the European Union, the substance and its sulfonated derivatives are registered under REACH, and ready biodegradability of the detergent intermediate is assessed by OECD 301B with a 28-day pass level of 60 % ThCO₂. Transport of oleum is classified under UN1831, packaging group I, with corrosion-resistant vented caps and no free liquid in secondary packaging.