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The alkylation of benzene with linear internal olefins—produced via dehydrogenation of C10–C13 n-paraffins or via the Shell Higher Olefin Process—yields linear alkylbenzene (LAB) conforming to specifications such as ASTM D3673-89. The sulfonation of this feedstock is conducted almost exclusively with gaseous sulfur trioxide (SO₃) in continuous falling-film reactors, a process that has displaced oleum and chlorosulfonic acid routes due to minimal waste-acid generation and superior active-matter yields. A typical LAB cut exhibits a bromine index below 10 mg Br/100 g, total aromatics content exceeding 99 wt%, and a linear isomer fraction above 92%; these parameters directly influence the color and biodegradability of the resultant linear alkylbenzene sulfonic acid (LABSA). The carbon distribution is tightly controlled, with the C12 homologue predominating at 30–35% to balance detergency and solubility. Feedstock preheating to 45–50°C reduces viscosity to 10–15 mPa·s, ensuring uniform distribution across the reactor’s film-forming weir. Any deviation in LAB monoalkylate purity—specifically the presence of dialkyltetralins or branched isomers—increases the unsulfonated organic matter after reaction, raising the free oil content of the finished acid beyond the accepted ceiling of 1.5 wt% determined by ISO 4323:2018.
Sulfur trioxide generation for large-scale sulfonation employs the sulfur-burning route, where molten sulfur (135–145°C) is atomized into a stream of dried combustion air inside a refractory-lined furnace. The air must be dehumidified by dual-bed silica gel or molecular-sieve dryers to a pressure dew point of -60°C or better, as measured at the dryer outlet per ISO 7183:2007. If residual moisture exceeds 20 ppmv, the SO₂/SO₃ gas mixture forms sulfuric acid mist at the converter exit, leading to severe corrosion of the 316L stainless-steel interconnecting ductwork and fouling of the downstream electrostatic mist precipitator. Sulfur dioxide oxidation proceeds over V₂O₅ catalyst rings arranged in four-pass adiabatic beds; inlet gas SO₂ concentrations are maintained between 10.5 vol% and 11.5 vol% to balance conversion efficiency and hotspot risk. The converter beds operate with an inlet temperature of 420°C and a maximum bed temperature not exceeding 610°C, as catalyst sintering accelerates beyond 630°C based on manufacturer’s technical bulletins. Conversion efficiency must exceed 97.5%—continuously monitored via in-line UV photometry—because unconverted SO₂ passed to the sulfonation reactor dilutes the reactive SO₃ partial pressure and depresses the reaction rate, requiring a compensatory increase in the SO₃:LAB molar ratio that elevates free sulfuric acid formation. When the SO₃ gas is tempered with dry air to a safe film-reactor inlet concentration of 4–6 vol%, the dew point of the mixed stream remains below -20°C to avoid acid condensation in the distribution plenum.
The multi-tube falling-film reactor, typified by Chemithon or Ballestra designs, comprises 30–80 vertically oriented 316L tubes with an inner diameter of 12–25 mm and a length of 3.5–6 m. Liquid LAB is fed to an upper weir chamber and distributed as a thin film flowing down the internal tube walls under gravity. The SO₃–air mixture enters co-currently at a gas velocity of 20–40 m/s, creating intense heat and mass transfer at the gas–liquid interface. A minimum liquid loading of 0.15 m³/h per metre of tube circumference must be sustained to guarantee a continuous wetted film; falling below this threshold produces dry patches where localized SO₃ absorption triggers temperatures exceeding 120°C in the metal wall, as inferred from cooling-water jacket thermocouple excursions. Published data on the precise onset of runaway for a given tube geometry is limited, but production experience shows that tube-side heat transfer coefficients collapse when the film Reynolds number drops below 200, accelerating sulfone and color-body generation. The cooling-water system—operating at an inlet temperature of 28–32°C and a maximum outlet temperature of 45°C—removes the exothermic heat of sulfonation, which ranges between 170–210 kJ/mol LAB. Tube-wall temperature is held below 70°C to suppress side reactions; if local skin temperature exceeds 85°C, the unsulfonated oil reacts further to form diphenylsulfone derivatives that plasticize gaskets downstream and raise the Klett color of the acid by 30–80 units. The SO₃:LAB molar ratio is tightly controlled at 1.03:1 to 1.05:1 through mass-flow metering; a ratio above 1.08:1 pushes the free sulfuric acid content beyond 2.0 wt% and promotes disalt (Na₂SO₄) formation after neutralization, causing viscosity instability in the finished sulfonate.
The aged acid leaving the reactor bottom holds approximately 96–98 wt% active sulfonic species, but roughly 5–10% of the sulfonated material exists initially as pyrosulfonic acid anhydrides that must be hydrolyzed to the monosulfonic acid before neutralization. Acid from the reactor is routed to a continuously stirred aging vessel, where a residence time of 30–60 minutes at 50–65°C is maintained. The vessel’s length-to-diameter ratio of at least 2.5:1 is essential to approximate plug-flow behavior; field measurements using lithium chloride tracer tests on a 15 m³ aging tank at a West European detergent alcohol sulfonation plant revealed that short-circuiting reduced the effective mean residence time by 22% when the L/D ratio was 1.8:1, causing intermittent free oil spikes above 2.0%. During aging, dissolved SO₃ and entrained acid mist also hydrolyze, while excess SO₂ (carryover from incomplete conversion) desorbs and is vented to a caustic scrubber. If the aging temperature falls below 45°C, the hydrolysis of pyrosulfonic acids slows to an extent that the degree of conversion to monosulfonic acid may not reach 99%, leading to foaming during neutralization due to CO₂ evolution from residual anhydride reacting with carbonate impurities in caustic soda. The aged acid is then fed to the neutralization step via a positive-displacement pump against a back-pressure of 2–3 bar(g), ensuring no vapor breakout in the metering section.
Neutralization of LABSA with sodium hydroxide is carried out in a continuous high-shear loop reactor or in-line rotor-stator mixer. Aqueous caustic soda at 32–50 wt% concentration—supplied from a membrane cell plant—is injected co-currently with the aged acid at a carefully maintained stoichiometric excess of 0.1–0.5 wt% NaOH to target a finished pH of 7.0–8.0 when measured as a 1% active solution per ISO 4316:1977. The neutralization heat effect, ≈55 kJ/mol, raises the product temperature to 65–85°C under adiabatic conditions; a downstream plate-and-frame heat exchanger using cooling tower water is sized to maintain a discharge temperature below 40°C to avoid accelerated darkening at elevated pH. If the NaOH concentration falls below 28 wt%, the water introduced into the sulfonate matrix drives the active matter below 70 wt% and disrupts the lamellar phase structuring that stabilizes the liquid detergent intermediate. The result is a sudden reduction in dynamic viscosity from 2,500–5,000 mPa·s to below 500 mPa·s at 25°C (Brookfield LV, spindle 4, 20 rpm), accompanied by phase separation of a low-viscosity aqueous layer. This phenomenon is irreversible without removal of excess water by vacuum evaporation, a process that introduces thermal stress and increases sulfonate color by 10–30 Hazen units. Conversely, highly concentrated caustic soda above 50 wt% reduces neutralization rate at the acid droplet interface and can produce localized alkali-rich domains where the sulfonate undergoes Hoffmann degradation, generating volatile amines detectable by odor panel testing. Industrial practice standardizes on a caustic feed concentration of 48–50 wt%, with on-line refractive index analyzers monitoring diluent water ingress. A ratio controller locks the acid-to-caustic mass-flow setpoint based on the feed-forward signal from the acid’s acid value determination according to ASTM D2357-14 two-phase titration; automatic feedback trims the ratio by referencing a continuous pH electrode placed in a side-stream loop.
The sulfonated product at this stage typically contains 95–97 wt% active matter, 0.5–1.5 wt% free oil (unsulfonated LAB), and 0.8–2.0 wt% free sulfuric acid, all reported on an anhydrous basis. Color is 30–80 Klett for standard-grade material and below 20 Klett for cosmetic-grade commodity produced via post-sulfonation hydrogen peroxide bleaching. A downstream storage tank equipped with stainless-steel steam coils maintains the material at 30–35°C to avoid gelation, as the neutralized sodium salt can form a high-viscosity mesophase below 25°C when active matter exceeds 70%. Published data for this specific configuration is limited, but tests conducted on a 10,000-litre storage vessel at a Brazilian sulfonation plant showed that a temperature drop to 22°C over an unheated weekend caused stratification and a 15% variation in active matter between top and bottom sampling ports, delaying discharge by 8 hours while recirculation heating was applied.
| Parameter | Method | Unit | Specification Range |
|---|---|---|---|
| Active matter (MW 348 average) | ASTM D2357-14 / ISO 2271:1989 | wt% | 95.0–97.0 |
| Free oil (unsulfonated organic) | ISO 4323:2018 (petroleum ether extraction) | wt% | ≤1.5 |
| Free sulfuric acid (as H₂SO₄) | ISO 4323:2018 / ASTM D3673-89 | wt% | ≤2.0 |
| Sodium sulfate (Na₂SO₄) | Gravimetric after ethanol dissolution | wt% | ≤1.5 |
| Water (Karl Fischer) | ISO 4317:2011 | wt% | 1.0–3.0 |
| Color (10% active in water) | ISO 6271:2015 (Hazen/APHA) | — | ≤50 |
| pH (1% aqueous) | ISO 4316:1977 | — | 7.0–8.5 |
Off-gases from the aging tank and sulfonation reactor—carrying unconverted SO₂ and entrained SO₃ mist—are passed through a venturi scrubber followed by a packed-bed caustic tower circulating 5–10 wt% sodium hydroxide solution. The scrubber liquor pH must be held between 9.5 and 12.0 to ensure complete neutralization of acid gases while preventing crystallization of sodium sulfite/bisulfite eutectics that block the packing. If the pH falls below 8.0, SO₂ breakthrough occurs, and the stripped gas can cause corrosion in the downstream exhaust blower (typically constructed of Hastelloy C-276 or rubber-lined carbon steel). Field failure records from a South African LABSA facility document that a pH probe fouling incident allowed the scrubber sump to drop to 6.2 for 45 minutes, resulting in visible stack opacity and a mandatory production hold until the packing was cleared of solidified sodium sulfite scales by hot-water washing at 70°C. The scrubber is also designed to handle a peak SO₃ load of 5 kg/h per 1000 m³/h of vent gas, a figure derived from worst-case reactor start-up conditions when the film is not yet established and excess trioxide escapes. Scrubber blowdown containing 8–12 wt% sodium sulfate and sulfite is treated in the plant’s wastewater oxidation basin before discharge, ensuring compliance with the sulfite discharge limit of 2 mg/L according to local permits referencing EU BAT Conclusions for Waste Gas Treatment in Chemical Sector (2016/902).
Storage and handling of the finished LABSA paste (70–96% active matter) must account for its corrosive nature toward carbon steel. Tanks are fabricated from 304L or 316L stainless steel with a design pressure rating of Full vacuum to withstand the collapse risk during cooling of warm product after loading. Unlined carbon steel is incompatible; mild steel coupons exposed to LABSA with 1.5% free sulfuric acid at 40°C exhibit a corrosion rate exceeding 0.5 mm/year, per ASTM G31-21 immersion testing. All unloading connections are specified with PTFE-lined flexible hoses and dry-disconnect couplings to minimize exposure. The product is classified as corrosive under UN 2586 (alkyl sulfonic acids, liquid) and requires appropriate ADR/IMDG labelling. Worker exposure to aerosol during drumming operations is controlled below the 8-hour TWA of 2 mg/m³ (respirable fraction) by local exhaust ventilation, with continuous monitoring using a real-time aerosol photometer calibrated for sulfuric acid mist.
| Process Variable | Operating Setpoint / Range | Consequence of Deviation |
|---|---|---|
| SO₃:LAB molar feed ratio | 1.03–1.05 | Below 1.01: free oil > 2%. Above 1.08: free H₂SO₄ > 2.5%, disalt formation. |
| Reactor cooling-water outlet temperature | 40–45°C | Exceed 50°C: tube-wall temperature approaches 85°C, sulfone/color rise. |
| Aging vessel temperature | 50–65°C | Below 45°C: incomplete hydrolysis, foaming. Exceed 70°C: color gain. |
| Caustic soda concentration to neutralizer | 48–50 wt% | Below 28%: product active matter falls below 70%, phase separation. |
| Finished product pH (1% sol.) | 7.0–8.5 | Below 6.5: corrosion risk for stainless tanks. Above 9.0: discoloration. |
| Scrubber liquor pH | 9.5–12.0 | Below 8.0: SO₂ emission, packing scaling. Above 12.5: excessive NaOH consumption. |
During formulation into liquid household laundry products, LABSA is post-dosed as the neutralized sodium salt or as the free acid with in-situ neutralization. The paste is metered via eccentric disc pumps into a water charge at 35–40°C under moderate agitation. When direct acid neutralization is performed, the addition of 50% caustic soda must be staged to keep local pH below 10 to minimize color development; a pH controller cascaded to the alkali dosing valve typically operates at a setpoint of 8.5 for the processing step. The sulfonate’s tolerance to calcium ions, expressed by its Kraft point, drops as the linear alkyl chain length increases; a LABSA derived from predominantly C13 feedstock exhibits a Kraft point around 15°C, acceptable for cold-water detergency only when combined with nonionic co-surfactants at a ratio of 4:1 anionic to nonionic. For heavy-duty powder detergents, the sulfonate is sprayed onto the base powder in a post-tower coating drum along with 2–5 wt% zeolite and perfume; the rate of migration of the anionic surfactant into the granule core depends on the mean pore diameter of the sodium carbonate/silicate matrix, and insufficient pre-drying of the granules to a moisture content below 3% causes rapid swelling and stickiness when the paste is applied at a loading of 12–18 wt% active on powder.