Conversion of Linear Alkylbenzene Sulfonic Acid to LAS-Na with Continuous Neutralization

In industrial sulfonation plants producing linear alkylbenzene sulfonic acid (HLAS) via SO₃ thin-film or jet reactor technology, the subsequent continuous neutralization step is critical to converting the corrosive, metastable sulfonic acid into a shelf-stable, high-molecular-weight anionic surfactant paste suitable for detergent powder agglomeration, liquid laundry formulations, and industrial cleaners. HLAS exiting the sulfonation unit typically contains 96–98 wt% active sulfonic acid, 0.5–2.0 wt% free sulfuric acid (from SO₃ overstoichiometry), 0.2–1.0 wt% unsulfonated linear alkylbenzene (LAB), and trace moisture. Upon reaction with aqueous sodium hydroxide (50 wt% NaOH), the sulfonic acid group is neutralized exothermically, releasing 55–60 kJ per mole of HLAS, thereby necessitating precise thermal and stoichiometric control to avoid gel-phase formation, localized hot spots, and byproduct generation such as dialkyl sulfones. Continuous neutralization systems have been deployed at capacities from 500 kg/h to over 10,000 kg/h of LAS-Na paste, employing recirculating loop reactors with inline high-shear rotor-stator mixers operating at tip speeds exceeding 15 m/s. The product LAS-Na, designated INCI: Sodium Lauryl Sulfate (for C12–C14 chain lengths) or more correctly Sodium C10-13 Alkylbenzene Sulfonate, is manufactured as a viscous paste of 70 ± 1 wt% active matter, with the remainder being water, 0.5–1.5 wt% sodium sulfate, and <0.3 wt% free oil. This document examines the process chemistry, equipment configuration, thermal management, and quality control frameworks essential for continuous LAS-Na neutralization, drawing on established industrial practices and published data from operating plants.

Why does the neutralization stoichiometry dictate a narrow processing window?

The neutralization reaction between HLAS and NaOH proceeds with a stoichiometric coefficient of 1:1 on a molar basis for the sulfonic acid group, yet the presence of free sulfuric acid in the feed stream—originating from the sulfonation reactor’s SO₃ excess—introduces a secondary acid-consuming species that shifts the effective alkali demand. Therefore, the actual NaOH:HLAS molar feed ratio must be dynamically adjusted to 1.00:1.00 ± 0.005 relative to total acid equivalents (sulfonic + sulfuric). If the ratio falls below the equivalence point, residual HLAS remains in the paste, lowering the apparent active matter on a free-acid basis and causing corrosion in mild steel storage vessels; the residual acid also catalyzes intramolecular cyclization of the alkylbenzene sulfonic acid to form p-alkylbenzene sulfone, a poorly biodegradable compound that raises the unsulfated matter content beyond the 1.5 wt% maximum permitted by ISO 894. Conversely, an NaOH overfeed increases the free alkalinity, which in the concentrated paste environment can reach localized pH values above 12 and promote saponification of trace methyl esters or glyceride impurities present in the LAB feedstock, generating fatty acid soaps that alter foam characteristics and cause off-odors during storage. The target free alkalinity window, measured as NaOH on an as-is paste basis per ISO 4314, lies between 0.02 wt% and 0.05 wt%, corresponding to a pH of 7.5–9.0 when the paste is diluted to 1 wt% active matter in deionized water and measured with a temperature-compensated glass electrode at 25 °C. Direct pH measurement in the undiluted paste is unreliable owing to low water activity, junction potential drift, and the formation of a gel layer on the electrode surface. For inline control, a slipstream of partially diluted neutralization mass is diverted through a low-dead-volume flow cell housing a flat-membrane pH electrode (e.g., Mettler Toledo InPro 3250i) with automatic temperature compensation and continuous KCl electrolyte replenishment; the measured millivolt signal is fed to a PID controller that drives a variable-stroke diaphragm metering pump for the NaOH solution. The extreme viscosity sensitivity near stoichiometric balance—where even a 0.5% molar excess of NaOH can elevate the paste viscosity by 50–100% due to electroviscous effects in the hexagonal liquid crystalline phase—underscores the need for stoichiometric precision.

Loop Reactor Configuration and High-Shear Homogenization

A continuous loop neutralization skid comprises a recirculation pump (typically a multistage centrifugal or progressive cavity pump capable of 5–15 m³/h circulation against a head of 3–6 bar), an inline high-shear mixer, a shell-and-tube or plate heat exchanger, injection quills for HLAS and caustic feeds, and instrumentation for pH, temperature, and pressure. The HLAS stream, preheated to 40–50 °C to reduce its viscosity to 200–500 mPa·s, and the 50 wt% NaOH solution at 20–30 °C are introduced immediately upstream of the rotor-stator mixer through concentric or opposed-nozzle injection quills designed to prevent pre-reaction before the high-shear zone. The rotor-stator mixer, operating at a tip speed of 15–25 m/s (corresponding to 3,000–6,000 rpm for a 50 mm rotor diameter), imposes a shear rate exceeding 50,000 s⁻¹ within a radial gap of 0.3–0.5 mm, rupturing the gel nuclei that form instantaneously upon contact of acid and alkali and dispersing the nascent LAS-Na into the recirculating paste mass. Without this intense mechanical energy input, the neutralization reaction would generate a persistent hexagonal liquid crystalline gel phase with an apparent yield stress above 500 Pa, which adheres to pipe walls, stalls the recirculation pump, and catastrophically fouls heat transfer surfaces. The loop residence time is kept extremely short, on the order of 10–30 seconds, to minimize the thermal history of the product before a side-stream is withdrawn through a pressure control valve to a downstream static mixer for final water adjustment. Operating experience across multiple 5,000–8,000 kg/h lines indicates that the loop circulation rate should be sized to achieve a recirculation ratio (circulation flow : withdrawal flow) of 20:1 to 50:1 to dampen disturbances from feed flow variations and to ensure that the temperature rise across the heat exchanger does not exceed 3–5 K per pass.

Comparative Process Parameters: Batch versus Continuous LAS-Na Neutralization

Parameter	Batch Neutralization (Stirred Vessel)	Continuous Loop Neutralization
Active matter consistency (lot-to-lot ±1σ)	±1.5 wt%	±0.3 wt%
Free alkalinity variability (as NaOH)	0.02–0.15 wt%	0.02–0.05 wt%
Color (Klett, 40% AM solution, 5 cm cell, ASTM D1209)	15–40	5–15
Residence time	30–120 minutes	10–30 seconds
Specific mechanical energy input	2–5 kWh/tonne	8–15 kWh/tonne
Cooling duty per tonne product	40–60 MJ	30–40 MJ (due to efficient heat exchange)
Capital cost factor (relative scale >5,000 tpa)	1.0	0.6–0.8
Inline quality feedback capability	Limited (grab samples)	Full (pH, NIR, conductivity, temperature)

Once neutralized and adjusted to the target active matter concentration of 70 wt%, the LAS-Na paste exhibits pronounced non-Newtonian, shear-thinning behavior characterized by a power-law flow index n of 0.2–0.5 and a consistency index K of 500–2,000 Pa·sⁿ at 40 °C. The apparent viscosity, measured with a Brookfield RVT viscometer using Spindle No. 7 at 20 rpm, falls within 8,000–25,000 mPa·s for a typical C10–C13 alkyl chain distribution, with the viscosity minimum occurring near the fully neutralized condition and increasing sharply as free alkalinity or free acidity moves off-spec. Paste viscosity is also a strong function of temperature; cooling below 25 °C transforms the lamellar phase into a gel-like hexagonal phase, raising the apparent viscosity by an order of magnitude and requiring storage vessels and transfer piping to be electrically traced or hot-water jacketed to maintain a bulk temperature of 35–45 °C. Transfer from the neutralization loop to storage or tanker loading is accomplished with positive-displacement pumps—typically progressing cavity pumps (e.g., Netzsch NEMO series) or double-diaphragm pumps—operating at low speeds (<100 rpm) and equipped with pressure-relief valves set at 3–4 bar to prevent pipe rupture. Overheating of the paste above 80 °C must be avoided because it initiates thermal dehydration reactions that form dialkyl sulfones and alkyl thiophenes, leading to irreversible darkening (Gardner color > 5 via ASTM D6166) and the release of organosulfur odor compounds detectable at sub-ppm levels.

When feed impurity profiles shift, how does the neutralization curve deviate?

Commercial HLAS streams derived from linear alkylbenzene with C10–C13 alkyl chains invariably contain variable proportions of free sulfuric acid, unsulfonated oil, and trace sulfonation byproducts such as sulfones and anhydrides, and these impurities collectively alter the shape and position of the neutralization titration curve. Free sulfuric acid, present at 0.5–2.0 wt% of the HLAS mass according to the sulfonation reactor’s SO₃:LAB ratio and the efficiency of the post-sulfonation gas-liquid separator, consumes NaOH in a 1:2 molar ratio (H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O), effectively introducing an offset in the apparent acid value that must be compensated by the NaOH feedforward controller. A shift from 1.0 wt% to 2.0 wt% free H₂SO₄ increases the NaOH requirement by approximately 1.1% relative to the sulfonic acid mass flow, which, if uncompensated, results in a free acid excursion of 0.1–0.2 wt% as HLAS in the paste, easily exceeding the 0.05 wt% upper specification limit. Inline near-infrared (NIR) spectroscopy probes installed in the HLAS feed line (e.g., Bruker Matrix-F with a transflectance immersion probe) can measure the active acid concentration and free sulfuric acid content in real time by multi-component partial least squares regression models built against labor-intensive titration reference data (ISO 2271 and ISO 6844). The NIR-predicted acid value is cascaded to a feedforward control loop that adjusts the NaOH metering pump stroke every 5–10 seconds. Unsulfonated oil (USO) does not participate in the neutralization stoichiometry but dilutes the active matter; therefore, when USO rises from a typical 0.5 wt% to 1.5 wt% due to temperature excursions in the sulfonator or LAB quality variation, the neutralization outlet water addition setpoint must be reduced to maintain the 70 wt% active specification, which in turn raises paste viscosity and demands higher heat exchanger pump energy. Some operators supplement the NIR loop with inline conductivity measurement on the diluted paste slipstream; because sodium sulfate generation from free sulfuric acid increases the ionic background, a deviation in conductivity at constant pH signals a change in the sulfuric acid/sulfonate ratio and triggers a cascade correction to the NaOH ratio.

Managing the Thermal Profile Across the Neutralization Loop

Maintaining the thermal equilibrium within the neutralization loop requires continuous removal of the exothermic heat of reaction, which for a HLAS throughput of 5,000 kg/h (average molecular weight 326 g/mol) amounts to 850–920 MJ/h—corresponding to a cooling duty of 236–256 kW. This duty is typically handled by a plate-and-frame heat exchanger with 0.5 mm plate gaps and 316L stainless steel or Hastelloy C-276 plates to resist chloride-induced pitting and acid corrosion. The cooling water circuit operates with an inlet temperature of 20–25 °C and a return temperature of 30–35 °C, yielding a log-mean temperature difference of 8–12 K. Under the laminar flow conditions prevailing in the high-viscosity paste (Reynolds number <10), the overall heat transfer coefficient falls to 100–200 W/m²·K, forcing a total installed heat transfer area of 150–300 m² for the stated throughput. To minimize the thermal degradation risk, the temperature rise across the mixer and heat exchanger is limited to 3–5 K per pass by the high recirculation ratio; a typical loop operating at 45 °C will see temperatures of 48–50 °C at the mixer discharge and 44–45 °C downstream of the heat exchanger. Fouling of the plate surfaces by tenacious organic films is a known operational problem: paste-side fouling resistances can increase by 0.0002–0.0005 m²·K/W over 200–400 operating hours, requiring the implementation of clean-in-place (CIP) cycles using hot water at 70 °C followed by 5 wt% NaOH solution recirculated at 1.5–2.0 m/s to restore heat transfer performance. A safety interlock is programmed into the distributed control system (DCS) to trip the HLAS feed valve and divert the product stream to an emergency dump tank if the loop outlet temperature exceeds 60 °C, because sustained operation above this threshold rapidly generates gel phase in downstream static mixers and can cause pressure spikes exceeding the 10 bar design limit of the paste transfer piping. Quality assurance protocols for continuous LAS-Na production rely on a suite of standardized analytical methods that define the product’s fitness for use in detergent formulations. The primary specification, active matter content (anionic surfactant plus neutral oil), is determined by the two-phase mixed indicator titration method ISO 2271 (methylene blue active substance, MBAS) using cetyltrimethylammonium bromide (CTAB) as titrant; typical lot release values fall within 69.5–70.5 wt% for the standard paste grade. Free alkalinity or free acidity is measured by potentiometric titration of a 1 wt% aqueous dilution per ISO 4314, with acceptance criteria of 0.02–0.05 wt% as NaOH or <0.10 wt% as HLAS. Unsulfated matter (free oil plus dialkyl sulfones) is quantified via ISO 894 extraction with cyclohexane, and the specification ceiling is 1.5 wt% on a 100% active matter basis. Sodium sulfate content, a byproduct of free sulfuric acid neutralization, is controlled below 2.0 wt% (as-is basis) via ISO 6844 or ion chromatography. Sodium chloride, introduced with the 50 wt% NaOH solution, is monitored to remain under 0.5 wt% (ISO 457). Regarding environmental and regulatory compliance, LAS-Na manufactured by this route meets the ready biodegradability criteria of OECD 301B (≥ 60% degradation within 28 days), with typical CO₂ evolution exceeding 90% of the theoretical value, thereby satisfying the requirements of the EU Detergents Regulation (EC No 648/2004) and the US TSCA inventory. Toxicological classification under the Globally Harmonized System (GHS) assigns the substance as Skin Irrit. 2 (H315) and Eye Irrit. 2 (H319) based on in vitro testing following OECD 439 and OECD 492; the typical 50% active raw paste is not classified as acutely toxic by the oral route (LD₅₀ > 2,000 mg/kg). The finished product is transported in insulated stainless steel tankers and stored in heated, nitrogen-blanketed tanks to prevent moisture gain and microbial contamination, because undiluted LAS-Na paste has a water activity below 0.75, which inhibits vegetative bacterial growth but permits osmophilic yeast proliferation at the air-paste interface if vent filters are compromised.

LAS-Na Paste: Quality Control Matrix and Standardized Test Protocols

Property	Test Method	Specification Range
Active matter (anionic surfactant)	ISO 2271 (two-phase MBAS titration)	69.0–71.0 wt% as-is
Unsulfated matter (free oil + sulfones)	ISO 894 (cyclohexane extraction)	≤ 1.5 wt% on 100% active
Free alkalinity (as NaOH)	ISO 4314 (potentiometric, 1% dilution)	0.02–0.05 wt%
Free acidity (as HLAS)	ISO 4314	<0.10 wt%
Sodium sulfate	ISO 6844 or IC	<2.0 wt% as-is
Sodium chloride	ISO 457	<0.5 wt%
Color (Klett, 5 cm cell)	ASTM D1209 (40% AM solution)	<15
pH (1 wt% aqueous solution)	ISO 4316	7.5–9.5
Viscosity (40 °C, 20 rpm, Spindle 7)	Brookfield RVT (internal)	8,000–25,000 mPa·s
Biodegradability (ready)	OECD 301B (CO₂ evolution)	≥ 60% in 28 days (typically > 90%)

Operation of the continuous neutralization line imposes specific material constraints on wetted components downstream of the high-shear mixer, because the combination of a moderately alkaline (pH 9–10 in the bulk), high-conductivity (10–20 mS/cm) paste phase and residual chloride ions from the NaOH feedstock creates conditions for chloride-induced stress corrosion cracking (Cl-SCC) in austenitic stainless steels at temperatures exceeding 50 °C. The loop piping, heat exchanger plates, and pump internals are therefore specified as 316L (UNS S31603) with controlled ferrite content below 5% or, for extended service life, as duplex 2205 (UNS S32205). Elastomeric seals in the rotor-stator mixer and transfer pumps must be composed of ethylene propylene diene monomer (EPDM) or perfluoroelastomer (FFKM), because nitrile butadiene rubber (NBR) swells and loses mechanical integrity upon continuous exposure to the high-electrolyte, medium-polarity paste. Shaft seals on the recirculation pump utilize double mechanical cartridge seals with a pressurized barrier fluid (water-glycol at 1–2 bar above loop pressure) to prevent paste leakage into the atmosphere and to lubricate the seal faces. These material choices, combined with rigorous monitoring of the chloride content in the 50 wt% NaOH feedstock (specified at <500 ppm NaCl), prevent the type of catastrophic corrosion failures that were documented in early-generation batch units constructed from 304L stainless steel, which suffered through-wall pitting within 12–18 months of commissioning. The entire assembly is designed for 24/7 operation with an on-stream factor exceeding 95% when integrated with upstream sulfonation, and the process automation includes asset management software that tracks pump vibration spectra, heat exchanger pressure drop trends, and pH electrode slope degradation to schedule predictive maintenance during planned shutdown windows.