Crude C10 aromatics recovered from steam-cracked heavy naphtha or FCC light cycle oil consist of a complex mixture in which naphthalene typically ranges from 15 to 45 wt% within a naphthalene-enriched heart cut, depending on upstream severity, cut points, and recycle loops. The co-boiling and near-boiling species include benzothiophene, indane, indene, 1-methylnaphthalene, 2-methylnaphthalene, biphenyl, acenaphthene, fluorene, and residual C9/C10 alkylbenzenes. Because naphthalene sulfonation is electrophilic, highly exothermic, and sensitive to impurities that terminate chain growth or generate color bodies, selection of the naphthalene cut must account not only for freezing point and boiling point but also for the concentration of basic nitrogen species, sulfur heterocycles, and readily polymerizable olefins such as indene. A naphthalene heart cut isolated by fractional distillation between 215 and 220 °C at atmospheric pressure, followed by crystallization to a purity of 95–98 wt%, is the common industrial feed for naphthalene sulfonate-formaldehyde condensate superplasticizers. Lower purity material containing more than 2 wt% methylnaphthalenes shifts condensation kinetics by monofunctional chain termination and reduces the average degree of polymerization to a point where concrete water reduction at 0.5–1.0 wt% admixture solids falls below the 12–15% threshold expected for high-range water reducers under ASTM C494/C494M. Gas chromatography with flame ionization detection, adapted from ASTM D5769 for aromatic distribution, is used to quantify naphthalene, methylnaphthalenes, biphenyl, indene, benzothiophene, and acenaphthene in the feed. Because published data for specific upstream C10 cut yields vary with refinery configuration, incoming feed is normally characterized batch-by-batch before charging to the sulfonation reactor, and the crystallization point of the naphthalene fraction is used as a rapid quality surrogate because it correlates with purity and occlusion of methylnaphthalene-rich mother liquor.

If the Crude C10 Heart Cut Contains More Than 2 wt% Indene, Sulfonation Produces Solvent-Insoluble Resin

Indene and its alkyl homologues undergo acid-catalyzed oligomerization at sulfonation temperatures between 160 and 165 °C when oleum is added. The resulting polyindene and indene-coumarone resins have low solubility in the subsequent aqueous formaldehyde condensation medium, forming a black-brown sludge that deposits on reactor walls, reduces heat transfer, and contaminates the final SNF. A maximum indene content of 0.5–1.0 wt% in the crystallized naphthalene feed is therefore specified for light-color superplasticizers used in architectural concrete where water-soluble color bodies are measured spectrophotometrically at 420 nm. Removal of indene is achieved by selective hydrogenation over a nickel-molybdenum catalyst at 180–220 °C and 2–4 MPa hydrogen partial pressure, or by sulfuric acid pre-treatment that converts indene to high-boiling polymer prior to final rectification. The hydrogenation route is preferred when the C10 stream also contains benzothiophene, because the same catalyst simultaneously reduces sulfur to below 50 mg/kg, thereby preventing the formation of sulfone by-products that can act as cement hydration retarders. Batch-to-batch variation from the same C10 storage tank can shift indene concentration by ±0.4 wt%, which is sufficient to move the product from an acceptable viscosity specification to a filter-clogging sludge when the sulfonation reactor has a narrow residence time distribution. Consequently, feed tanks for the sulfonation unit are equipped with external recirculation loops and in-line refractive index analyzers calibrated against the naphthalene crystallization point, and feed batches exceeding the indene limit are diverted to a hydrotreatment step before sulfonation.

Fractional distillation alone cannot achieve naphthalene purity above approximately 95 wt% when the crude C10 feed contains significant 2-methylnaphthalene because the relative volatility between naphthalene and 2-methylnaphthalene is only 1.12–1.15 at atmospheric pressure, requiring a column with more than 80 theoretical stages and a reflux ratio above 15:1. The associated reboiler duty for a 30,000 t/year naphthalene recovery unit typically exceeds 12–15 MW, and the bottom temperature approaches 260 °C, at which thermal polymerization of reactive olefins accelerates coking. Consequently, distillation is used only to obtain a naphthalene-enriched heart cut of 90–94 wt%, with final purification assigned to continuous fractional crystallization. Falling-film crystallizers operating with a scraping rate of 10–30 rpm and a cooling ramp of 0.1–0.3 °C/min separate naphthalene crystals from the mother liquor, which is rich in methylnaphthalenes and biphenyl. A sweating step at 80–81 °C removes occluded mother liquor, raising crystal purity to 98.5–99.5 wt% while sacrificing 10–20% of the feed as residue. The residue is not wasted; it is routed to a secondary crystallization stage or blended into low-grade naphthalene sulfonate for oilfield dispersants where color and molecular weight distribution are less tightly specified. The final crystallized naphthalene must show a crystallization point not lower than 79.8 °C and a sulfur content below 50 mg/kg to avoid poisoning of the sulfonation catalyst and discoloration of the SNF. Moisture content above 0.1 wt% is also rejected, because water hydrolyzes oleum and reduces the sulfonating strength of the mixed acid, while also introducing a safety hazard during molten naphthalene handling.

Sulfonation Reactor Temperature Windows and Oleum Strength

Molten naphthalene at 85–95 °C is charged to a glass-lined or cast-iron sulfonation reactor equipped with a jacket capable of removing 400–600 kJ per mol of naphthalene sulfonated. Sulfonation with 20–25% free SO₃ oleum is preferred over 98% sulfuric acid because the higher electrophile concentration increases the reaction rate at 160–165 °C and reduces the required reaction time from 8–12 h to 3–5 h. The temperature window is narrow: below 155 °C the reaction remains incomplete and the unconverted naphthalene forms a separate organic phase, while above 170 °C the formation of naphthalene-1,5-disulfonic acid and sulfone by-products increases sharply, and the product color shifts from light brown to dark red-brown. The ratio of oleum to naphthalene is controlled at 1.8–2.2 mol SO₃ per mol naphthalene to favor the monosulfonic acid isomers, particularly naphthalene-2-sulfonic acid, which yields a linear condensate with higher molecular weight than the 1-isomer. The isomer distribution is monitored by high-performance liquid chromatography using a C18 column and a 254 nm detector, with the target 2-sulfonic acid content above 85 area%. In a jacketed 10,000 L reactor, the heat transfer coefficient is typically 250–350 W/m²·K during the initial oleum addition if the agitator tip speed exceeds 3.0 m/s; lower agitation leads to localized hot spots and sulfone formation. After sulfonation, hydrolysis water is added to decompose excess SO₃ and dilute the sulfonic acid mixture to 40–45 wt% solids before formaldehyde condensation. The hydrolysis step itself releases heat of dilution and must be controlled below 90 °C to prevent premature condensation of naphthalene sulfonic acid with any residual formaldehyde from recycled wash water.

Condensation of naphthalene sulfonic acid with aqueous formaldehyde is carried out at 0.7–0.9 mol formaldehyde per mol naphthalene sulfonic acid. At ratios below 0.7, the average degree of polymerization remains below 6–8 naphthalene units, producing a product with insufficient molecular weight to disperse cement particles effectively; at ratios above 0.95, the condensation can form a crosslinked gel that is no longer water-soluble. The reaction is conducted at 100–120 °C for 4–8 h under atmospheric pressure, with the pH maintained between 0.5 and 1.5 to ensure acid-catalyzed methylene bridge formation. The condensation is terminated by neutralization with 30–50 wt% sodium hydroxide or calcium hydroxide, depending on the desired counterion. Sodium naphthalene sulfonate condensates typically have a weight-average molecular weight of 10,000–30,000 Da, measured by gel permeation chromatography using sulfonated polystyrene standards, and a polydispersity index between 2.0 and 3.5. Higher molecular weight fractions improve water reduction but can increase concrete air entrainment and retard setting; lower fractions improve slump retention but reduce early strength at 24 h. The selection of naphthalene feedstock purity influences this distribution because impurities with only one reactive position, such as 2-methylnaphthalene, terminate chain growth and shift the entire molecular weight curve toward lower degrees of polymerization.

What Limits the Formaldehyde-to-Naphthalene Sulfonic Acid Ratio in SNF Synthesis?

The formaldehyde-to-naphthalene sulfonic acid ratio is limited by the concentration of reactive positions para to the sulfonic acid group and by the competing tendency toward branching when higher ratios are used. With a naphthalene content of 95 wt%, impurities such as 1-methylnaphthalene and biphenyl act as monofunctional chain stoppers, reducing the effective branching functionality and shifting the gel point to a higher formaldehyde ratio, while also lowering the average molecular weight at a given ratio. When the naphthalene feed is refined to 99 wt%, the linear chain extension is more efficient and the practical maximum ratio is 0.85 before the polymerization mixture exhibits a rapid viscosity rise above 3,000 mPa·s at 110 °C. Reactor torque monitoring is therefore used on production-scale units; a torque increase of more than 20% within a 30-minute interval triggers automatic termination by neutralization. The critical ratio is also affected by the sulfonation isomer distribution, because naphthalene-1-sulfonic acid has lower reactivity at the para positions and can introduce branching rather than linear growth. Hence, feedstock selection must simultaneously control naphthalene purity, sulfur content, indene content, and crystallizer occlusion to maintain the required isomer distribution and gel-point margin. Published data for this specific configuration is limited, so gel-point behavior is normally confirmed on a 5 L pilot condensation reactor before a new naphthalene lot is approved for full-scale charging.

The SNF Molecular Weight Distribution Shifts with Formaldehyde Ratio and Reaction Time

The molecular weight distribution of sodium naphthalene sulfonate condensate is governed by the formaldehyde-to-naphthalene sulfonic acid ratio, the condensation time, the reaction temperature, and the purity of the naphthalene feedstock. As reaction time progresses from 2 h to 8 h at 110 °C, the weight-average molecular weight increases from approximately 8,000 to 25,000 Da, while the low-molecular-weight fraction below 5,000 Da declines from 25–30 wt% to 10–15 wt%. This redistribution is monitored by gel permeation chromatography with ultraviolet detection at 254 nm, and the target range for concrete superplasticizer applications is typically 12,000–20,000 Da for a balance between water reduction and slump retention. A feedstock containing 1.5 wt% 2-methylnaphthalene reduces the final weight-average molecular weight by 2,000–4,000 Da compared with a 99 wt% naphthalene feed under identical reaction conditions, which translates into a measurable loss of water reduction of 2–4 percentage points at the same admixture dosage. The relationship between molecular weight and performance is not linear; below 8,000 Da the polymer behaves mainly as a dispersant with poor slump retention, while above 30,000 Da the product can cause excessive retardation and air entrainment in concrete. Therefore, the condensation reactor is not operated to complete conversion but is stopped at a predetermined conversion corresponding to the target molecular weight, and the reaction mass is immediately neutralized and cooled below 50 °C to prevent further chain growth.

Neutralization of the acid condensation mixture with 50 wt% sodium hydroxide produces a sodium naphthalene sulfonate condensate solution with a solids content of 40–45 wt%, a viscosity of 50–150 mPa·s at 25 °C, and a pH of 7.0–9.0. Calcium hydroxide neutralization is used when the superplasticizer is intended for concrete containing reactive aggregates where sodium ion loading must be limited; however, the calcium salt has lower solubility and increases the risk of precipitation at storage temperatures below 5 °C. The final solution is evaluated for water reduction, setting time, and compressive strength using a standard concrete mixture at a cement content of 350 kg/m³ and a slump of 200 ± 20 mm before admixture addition. The admixture dosage is expressed as weight percent solids on cement, typically 0.5–1.0 wt% for high-range water reduction. Performance is benchmarked against ASTM C494/C494M Type F or EN 934-2:2009+A1:2012 high-range water-reducing admixtures, and the batch is rejected if water reduction falls below 12% or if the compressive strength ratio at 7 days and 28 days fails the standard limits. Chloride ion content is controlled below 0.1 wt% of the liquid admixture to avoid corrosion risk in reinforced concrete, and sulfate content is reported on the certificate of analysis. The compatibility of the SNF with the specific cement lot is tested by flow table spread using DIN EN 1015-3 or a rotational rheometer, because cement C₃A content and sulfate availability can shift the dispersion response and cause rapid slump loss even when the SNF molecular weight distribution is within specification.

At storage temperatures above 35 °C, sodium naphthalene sulfonate condensate solutions can undergo slow oxidative darkening and microbial growth, while below 5 °C the viscosity rises and calcium-neutralized products may precipitate. The final SNF solution is therefore stored in stainless steel or high-density polyethylene tanks at 5–35 °C, protected from direct sunlight, and recirculated at low shear before use to eliminate concentration gradients. Storage time beyond 12 months is not recommended without retesting for pH, solids, water reduction, and color, because condensation by-products can continue to form slowly even after neutralization. Incompatibility is observed with amine-based polycarboxylate ether superplasticizers when the two are mixed undiluted, as the anionic sulfonate groups can form insoluble complexes with protonated amine groups; simultaneous addition to concrete is acceptable only when each admixture is dispersed into the mixing water separately. The production unit is therefore stopped at the point where the final admixture batch is released by quality control, without extrapolating beyond the tested storage conditions and concrete mix designs.