Soap Noodle Production Saponification Stoichiometry and Free Alkali Control

Industrial soap noodle production integrates saponification stoichiometry with downstream vacuum drying and mechanical finishing, where the free alkali content is not a secondary quality parameter but a direct mass-balance residual of the reaction between caustic soda and triglyceride feedstocks. For a triacylglycerol of average molecular mass 860 g/mol, complete saponification consumes 3 mol NaOH per mole of glyceride, equivalent to 120 g NaOH per 860 g oil or 0.1395 g NaOH per gram of oil. This theoretical demand corresponds to a saponification value of 195.7 mg KOH/g because the saponification value is expressed as milligrams of potassium hydroxide per gram of sample and the conversion to sodium hydroxide uses the factor 40.00/56.11. Commodity soap noodle feedstocks typically range from palm oil, with a saponification value of approximately 190–205 mg KOH/g, to coconut oil at approximately 250–264 mg KOH/g, while palm kernel oil falls between 240–255 mg KOH/g. A batch-to-batch shift of ±2 mg KOH/g in saponification value alters the stoichiometric caustic requirement by approximately 1.4–1.5 g NaOH per kilogram of oil when the 40.00/56.11 factor is applied. The free alkali target in the finished noodle is therefore managed as a controlled over-stoichiometric excess that is subsequently trimmed, not as a simple pH reading, because hot concentrated soap phases do not provide a thermodynamically meaningful hydrogen-ion activity. In continuous saponification lines, the caustic dosing set point is derived from the measured saponification value and acid value of the oil blend, with a narrow deliberate excess to force the ester hydrolysis toward completion and to compensate for saponification reversion in the vacuum drying stage. The residual sodium hydroxide is then partially removed through glycerine-brine phase separation and partially neutralised with controlled fatty acid addition. Saponification value alone does not capture the free alkali behaviour of the process; the water content of the caustic feed, the intensity of mechanical shear, and the electrolyte composition of the neat soap phase jointly determine whether excess sodium hydroxide remains chemically available for reaction or becomes occluded inside viscous soap lamellae.

What Drives Free Alkali Excursions in Continuous High-Shear Saponification?

Free alkali excursions in continuous high-shear saponification originate primarily from mass-transfer limitations rather than from an incorrect equilibrium condition. A high-shear recycle reactor operating at 130–150 °C and 2.5–4.0 bar gauge typically achieves more than 98% conversion of the reactive glycerides within 8–12 min when the emulsion droplet diameter is maintained below 20 µm. The remaining conversion occurs in a plug-flow finishing reactor with a residence time of 20–40 min, where the free caustic value is deliberately held in the range of 0.05–0.15% NaOH on a wet neat soap basis to avoid precipitation of unsaponified matter at the dryer feed. When the emulsion droplet diameter exceeds 50 µm, interfacial area collapses, conversion drops, and the free alkali value at the discharge of the finishing reactor becomes unreliable because unreacted caustic is retained in the continuous soap matrix rather than being titrated as immediately available alkalinity. The dilution of the caustic stream directly controls both the water load and the phase continuity of the saponifying mass. At 50% w/w NaOH, the aqueous phase viscosity is higher and the interfacial renewal rate is reduced, creating localised glycerol-rich zones that retain free caustic. At 32% w/w NaOH, the additional water reduces neat soap viscosity but increases the vacuum dryer load and can alter the partition of free caustic between the neat soap phase and the aqueous glycerine phase during centrifugal separation. Sodium chloride is commonly maintained between 0.5–1.0% in the neat soap phase to break the emulsion and to control the phase inversion point; insufficient electrolyte causes free alkali to remain entrained in the soap layer, while excessive electrolyte can create a separate brine phase that interferes with the saponification mass balance. The free alkali excursion in such systems is therefore a combined function of droplet size, caustic feed strength, finishing reactor temperature, and electrolyte concentration, and it must be interpreted against the specific mechanical configuration of the saponification plant.

Analytical quantification of free alkali in hot neat soap is performed by titration according to ISO 684:1983, which reports total free alkali after separation with ethanol and titration with hydrochloric acid in the presence of barium chloride to exclude carbonate interference. In production practice, the sampled neat soap must be cooled to 60–70 °C before dissolution in ethanol to avoid saponification continuing during the analytical interval. The result is expressed as % NaOH on the sample mass; when some purchase specifications require % Na₂O, the conversion factor is 0.775. A common toilet soap noodle specification requires free alkali expressed as NaOH not to exceed 0.10%, while chloride content tested according to ISO 457:1983 is commonly limited to 0.5% to avoid corrosion and efflorescence in downstream bar finishing. The analytical lag between sampling and titration in a continuous plant is typically 15–25 min, which means that the free alkali reading must be interpreted as a trailing indicator. Feed-forward correction is therefore used where the saponification value of each oil batch is determined before discharge from the feedstock tank, and the caustic mass flow is adjusted using the 40.00/56.11 conversion factor. Published data for inline near-infrared measurement of free alkali in dried soap noodles is limited; plant calibrations are usually validated against ISO 684:1983 on a day-by-day basis rather than treated as absolute primary methods.

Neat Soap Viscosity, Caustic Partitioning, and Phase Inversion Control

Neat soap viscosity is not a single-variable function of moisture content; it responds to the chain-length distribution of the fatty acid feed, the electrolyte concentration, the degree of saponification, and the temperature of the finishing reactor. A principally C16–C18 palm oil soap with a moisture content of 30–35% exhibits a substantially higher plastic viscosity than a mixed palm kernel/coconut soap of the same moisture content. That viscosity difference alters the measurable free alkali because high-viscosity neat soap can occlude water-rich caustic domains and delay their release to the titration solvent. In a continuous centrifugal separator, the free caustic partitions preferentially into the aqueous glycerine phase when the soap phase remains above 80 °C and the sodium chloride content is held between 0.5–1.0%; if the separator feed temperature drops below 70 °C, the soap phase thickens and the free caustic partition coefficient shifts toward the soap layer. The phase inversion point in saponification is also sensitive to the ratio of soap to water. When the soap concentration exceeds 65% in the reactor, the system may invert from an oil-in-water emulsion to a water-in-oil dispersion, and the apparent free alkali measured after neutralisation no longer corresponds to the residual caustic because a fraction of the caustic resides in dispersed water droplets that are not immediately available for titration. Mechanical finishing of the neat soap after separation is performed in a vacuum spray dryer followed by a single-screw plodder equipped with screen packs of 80–200 mesh; the shear generated in the plodder redistributes the aqueous phase and can liberate occluded caustic, increasing the free alkali of the extruded noodle relative to the dryer feed. The control strategy therefore specifies a lower free alkali target in the neat soap before drying when the downstream plodder operates at high backpressure, because the additional mechanical work raises the temperature and can expose previously occluded caustic at the noodle surface.

When Palm Stearin Substitution Shifts the Stoichiometric Baseline

Substitution of palm stearin into a palm oil or palm kernel oil blend changes more than the saponification value: it also changes the melting range, the rate of dissolution in the hot caustic phase, and the phase continuity of the reacting mass. A blend of 80% palm oil with a saponification value of 200 mg KOH/g and 20% palm kernel oil with a saponification value of 245 mg KOH/g has a calculated blended saponification value of 209 mg KOH/g, corresponding to a theoretical NaOH demand of approximately 149 g/kg. If the control system incorrectly applies the saponification value of palm oil alone, the caustic underdose is approximately 6.4 g/kg, which produces residual free fatty acid rather than elevated free alkali; however, if the operator overcorrects without titration confirmation, the resulting free alkali excursion can reach 0.2% NaOH or higher in the neat soap. Palm stearin substitution above 15% of the oil blend raises the clear point of the partially saponified mass and increases the risk of gel-phase channeling in a plug-flow finishing reactor. The channeling effect means that portions of the reaction mass pass through the reactor with short effective residence time, leaving unsaponified glycerides and variable free caustic domains. The corrective response is not merely an increase in caustic feed but an increase in reactor temperature to 140–150 °C and a reduction in oil droplet size by raising the rotor-stator tip speed to 20–40 m/s. When palm stearin is used at levels above 25%, the higher titer of the final soap shifts the noodle hardness and slows the release of free alkali during the plodding step; the free alkali specification at the noodle exit may still pass ISO 684:1983, but the distribution of free alkali across the noodle cross-section becomes less uniform. Published data for this specific configuration is limited; however, plant observations on twin-screw vacuum plodders with an L/D ratio of 10:1 have shown that surface free alkali can exceed the core value by 0.01–0.03% NaOH when the outlet moisture falls below 10% and the plodder jacket temperature exceeds 30 °C. The practical control limit is therefore not a single value but a moisture-dependent boundary: at moisture above 12%, free alkali migration is slower, while at moisture below 8%, the surface concentration of free alkali becomes analytically and commercially significant.

Vacuum drying and extruder finishing impose additional constraints on free alkali because the removal of water from the neat soap changes the ionic strength and the solubility of sodium hydroxide in the remaining aqueous phase. A vacuum spray dryer operating at a product temperature of 85–95 °C and an absolute pressure of 10–20 kPa reduces the moisture content from 30–35% to 10–12% in the dried soap particles. During this drying step, a portion of the free caustic reacts with dissolved carbon dioxide to form sodium carbonate; the resulting carbonate content is not measured as free caustic by the barium chloride precipitation method in ISO 684:1983 but may still contribute to the total alkalinity specification if the purchasing standard requires total alkali as Na₂O. The tendency for free alkali to migrate to the surface of the drying soap particle is accelerated when the outlet moisture is below 10%, because the remaining water is concentrated in the outer region of the particle. In a falling-film dryer with a nozzle atomisation pressure of 3–6 bar, the surface free alkali of the resulting noodles has been observed to exceed the core value by 0.02–0.03% NaOH when the drying chamber outlet air temperature exceeds 100 °C. The corrective measure is to maintain the outlet moisture above 10% and to reduce the plodder backpressure by adjusting the screen pack from 200 mesh to 80 mesh, which decreases the mechanical heating and limits the release of previously occluded caustic. The interaction between free alkali and downstream perfume or fatty acid addition is also governed by the stoichiometric balance of the noodle; excessive free alkali in the noodle consumes added fatty acid during the final amalgamation step, shifting the finished bar formulation and reducing the expected free fatty acid neutralisation effect.

The free alkali value measured on a soap noodle cannot be interpreted without specifying the sampling location, the drying moisture, and the time elapsed between extrusion and analysis. A noodle sampled immediately after the vacuum plodder may exhibit a lower apparent free alkali than a noodle sampled after 24 h of conditioning at 25 °C and 50% relative humidity, because the post-crystallisation release of the aqueous phase alters the availability of residual caustic at the surface. In production audits, the free alkali result is therefore paired with moisture, chloride, and saponification value determinations, and the acceptance criterion is applied to the homogenised core sample rather than to the surface scrape. This integrated measurement strategy prevents the false conclusion that a low surface titration represents low residual caustic in the bulk noodle and ensures that the stoichiometric excess in the saponification reactor is controlled within the narrow window required for complete saponification without creating a downstream neutralisation burden.