Saponification Process Phase Shift Below 0.05% Free Alkali

In the production of toilet soap base via full-boiled kettle saponification followed by vacuum spray drying, the control of residual free alkali determines whether the processed mass remains pumpable as a lamellar neat soap phase or undergoes a sharp transition to a highly viscous middle soap phase that disrupts downstream finishing operations. Saponified fats blends typically composed of 80:20 tallow and coconut oil are neutralized with 50% sodium hydroxide solution at 80–85°C in a continuous crutcher, after which the neat soap at a moisture content of approximately 30% is discharged into a vacuum spray drying tower of the Mazzoni LB-500 type operating at 2.7 kPa absolute pressure, reducing moisture to 12–14% while preserving the lamellar liquid crystalline structure. Free alkali is titrated potentiometrically according to ASTM D460-91 (Section 28, method for free caustic alkali) and a parallel check by the AOCS Da 4a-48 method with ethanol extraction yields values that must be held between 0.06% and 0.10% expressed as NaOH on an anhydrous basis; this electrolyte content stabilizes the neat soap Lα phase, which exhibits a characteristic small-angle X‑ray diffraction repeat spacing of 3.4–3.6 nm and a fluid-like rheological profile with a zero‑shear viscosity of 8,500–12,000 mPa·s at 70°C as measured on an Anton Paar MCR 302 with cone‑plate geometry (CP50‑1, gap 0.1 mm). When the free alkali drops below 0.05% — an event frequently triggered by variations in the fatty acid neutralization value of incoming tallow shipments or by fluctuations in the crutcher residence time distribution — the interlayer electrolyte concentration becomes insufficient to maintain the planar bilayer arrangement, and the soap spontaneously converts into a hexagonal liquid crystalline middle soap phase. This phase shift is detectable by a loss of birefringence flow lines under cross-polarized light microscopy and is accompanied by an abrupt rise in viscosity: at 0.045% free alkali the steady‑shear viscosity at 5 s⁻¹ jumps to 158,000 mPa·s, an increase of more than an order of magnitude that transforms the spray-dried powder downstream into a cohesive, non‑flowable paste inside the feed screw of the finishing plodder. On a Bühler twin‑screw plodder with an L/D ratio of 24:1 and screw diameter 150 mm, the motor draw at the constant‑torque setpoint of 2,100 Nm escalates from a baseline 22 kW to 37–42 kW when the middle soap phase enters the barrel, pushing the main drive into overload alarm and forcing an immediate cool‑down of barrel zones 3 and 4 to avoid thermal decomposition of the soap that generates rancid off‑odors and a rise in the iodine value. Published industrial data from processing lines producing 6 tonnes/h of noodles indicate that even a single batch with free alkali below 0.05% can leave residual high‑viscosity material in the plodder cone chamber, contaminating subsequent production for up to 45 minutes and elevating the scrap rate by approximately 7% for the affected campaign. Electrolyte supplementation with sodium chloride (0.3–0.5% on anhydrous soap) is a common countermeasure, but it narrows the processing window because chloride levels above 0.7% shift the phase boundary in the opposite direction and cause a premature transition to a waxy omega phase that yields brittle, fractured billets at the extruder die exit at pressures exceeding 12 MPa.

Operational experience on a 4,500 kg/h Meccaniche Moderne vacuum plodder, fitted with a Maag gear pump for precise throughput control, shows that the extruder head pressure sensor (Kistler 4021A, 0–25 MPa range) detects the phase transition as a sharp pressure instability with oscillation amplitudes exceeding ±1.8 MPa within 30 seconds of the middle phase entering the compression zone; this signature has been incorporated into an adaptive control loop that automatically injects a dilute sodium hydroxide solution (1.5°Bé) at the crutcher outlet to restore the free alkali to 0.065% within 90 seconds. The threshold sensitivity is further influenced by the sodium carbonate content, which contributes to the total alkalinity but does not substitute completely for hydroxyl ions in stabilizing the lamellar hydration layer — according to phase‑equilibrium data on the sodium stearate‑water‑electrolyte system obtained by small‑angle neutron scattering, the critical aggregate parameter (critical packing parameter, CPP) increases from approximately 0.77 to 0.89 when NaOH is replaced by Na₂CO₃ equivalent, lowering the free electrolyte osmotic pressure and triggering the middle phase at higher nominal “free alkali” as measured by ashing‑titration methods. Consequently, plants that rely on saponification with sodium carbonate followed by causticizing in a separate step must maintain a tighter NaOH specification of 0.08–0.10% to avoid phase instability, and in-line NIR probes calibrated against the ASTM D460 titration reference provide trending at 6-second intervals to detect downward drifts before the 0.055% alarm limit is breached. The particle size distribution after spraying and the plodder screw profile — specifically the position of the mixing pins in zone 2 — also alter the apparent viscosity onset because high‑shear work input can temporarily disorder the hexagonal packing; nevertheless, published material from Bühler’s soap processing handbook warns that sustained operation with free alkali below 0.05% at plodder sealing pressures above 8.5 MPa accelerates abrasive wear on the bronze‑alloy bushings by a factor of 2.4 due to the increased mechanical energy dissipation coefficient.

When Does Residual Sodium Hydroxide Below 500 mg/kg Alter EVOH Melt Rheology?

During the saponification of ethylene‑vinyl acetate (EVA) copolymers to produce ethylene‑vinyl alcohol (EVOH) barrier resins, the residual free alkali after precipitation and washing directly governs the thermal stability and melt processability of the final pelletized product. The EVA precursor, typically containing 32–44 mol% ethylene and a melt flow index of 2–25 g/10 min (ASTM D1238, Procedure A, 190°C/2.16 kg), is dissolved in a methanol‑water mixture (60:40 w/w) to a solids content of 18–22% and fed through a Sulzer SMR static mixer reactor together with a stoichiometric excess of 0.5–1.2% sodium hydroxide solution relative to the acetyl content determined by ASTM D1617-07. The saponification progresses to greater than 99.8% conversion at 65–80°C and 1.8 bar back‑pressure, after which the polymer solution is precipitated by injecting it into a steam-stripping column operating at 105°C. The resulting EVOH slurry undergoes counter‑current washing in a series of three disc‑nozzle centrifuges (Alfa Laval CHPX type) until the free sodium hydroxide content of the wet cake, measured by a Mettler Toledo T70 autotitrator using 0.1N HCl with endpoint at pH 8.3 and corrected for sodium acetate interference via ion chromatography (ASTM D4327-17), reaches less than 0.05 wt% on dry solids (equivalent to 500 mg/kg). This threshold is not arbitrary; it corresponds to the alkali concentration above which EVOH containing 32 mol% ethylene exhibits detectable crosslinking during melt processing, because at barrel temperatures above 230°C the nucleophilic hydroxyl side groups can undergo intermolecular etherification catalyzed by the residual base, forming non‑reversible gel networks that manifest as visible specks in cast film and cause catastrophic pressure fluctuations in the melt filtration system. On a Coperion ZSK‑40 Mc18 twin‑screw extruder with a screw diameter of 40 mm and an L/D of 42, processing EVOH dried to <0.01% moisture in a Piovan H‑B dryer at 105°C for 6 hours, the torque required to maintain a screw speed of 200 rpm under a throughput of 45 kg/h and a melt temperature of 238°C remains stable at 58–62 Nm when the free alkali is 0.02–0.04%, while at 0.07% free alkali the torque rises to 84 Nm and at 0.12% the process triggers an emergency shutdown due to estimated melt fracture pressures exceeding 280 bar at the screen changer. The accompanying table summarizes the quantitative relationship between free alkali content, melt flow rate, and gel content for a typical 32 mol% ethylene grade determined in accordance with DIN 53765‑1:1991 for gel particle counting on blown film samples of 50 µm thickness.

Production‑scale experience at a 12,000 tonne/year EVOH facility indicates that the washing centrifuge’s conductivity sensor — a Mettler Toledo InPro 4800 — provides a predictive signal for free alkali that correlates linearly (R² = 0.96) with the titrated value, and an alarm setpoint of 0.8 mS/cm in the centrifuge effluent triggers a diversion valve that recirculates insufficiently washed product until the signal drops below 0.5 mS/cm. However, reliance solely on conductivity is inadequate when sodium acetate residuals exceed 0.15% because acetate ions contribute a non‑negligible fraction of the ionic strength; therefore the plant operating procedure (referenced in the internal quality agreement aligned with ISO 9001:2015, clause 8.5.1) mandates a confirmatory potentiometric titration every 4 hours using a sample dried according to ISO 15512:2019 (method A). Once the dry EVOH powder reaches the silo prior to compounding, any undetected residual alkali above the 0.05% limit will also degrade the polymer’s color stability during extrusion lamination, raising the yellowness index (ASTM E313-20) from an acceptable 0.8 to >3.2 after a single pass — an issue that cannot be rectified by subsequent additive masterbatch addition because the chromophores are formed via conjugated carbonyl sequences that are chemically fixed to the polymer backbone. When EVOH is coextruded as a tie‑layer between polyethylene and polyamide in a 5-layer cast line operating at 25 m/min, the increased gel count caused by even moderate free alkali excursion to 0.08% produces die‑line deposits that require line stops every 18 hours for die cleaning, compared with a normal cleaning interval of 72 hours for material maintained below 0.05%. It is critical to note that the 0.05% threshold is specific to the 32 mol% ethylene grade; EVOH copolymers with 27 mol% ethylene, which possess a higher hydroxyl density and a correspondingly greater reactivity at melt temperatures, exhibit gel formation starting at free alkali as low as 0.03%, necessitating a post‑washing ion‑exchange polishing column charged with AmberLyst H‑form resin to achieve a product specification of <0.01% residual Na₂O.

Continuous Alkaline Hydrolysis Reactor Operation and Sodium Terephthalate Precipitation Thresholds

In the alkaline depolymerization of post‑consumer poly(ethylene terephthalate) (PET) for chemical recycling, the saponification reaction of the ester linkages proceeds through nucleophilic attack by hydroxide ions, yielding disodium terephthalate (Na₂TPA) and ethylene glycol. The reaction is carried out in a continuous tubular reactor equipped with Sulzer SMX static mixing elements that provide radial homogenization; the reactor feed consists of flake PET of 10 mm average chip size suspended in an aqueous sodium hydroxide solution at a 5 wt% NaOH concentration, preheated to 150°C and pressurized to 0.6 MPa using a diaphragm metering pump (Lewa ecoflow). The saponification consumes hydroxide stoichiometrically, reducing the free alkali concentration along the reactor length until, near the reactor exit at a residence time of 45–55 minutes, the NaOH content falls below 0.05% (approximately 0.016 mol/L in the mixed liquor). At this depletion level, the system crosses the liquidus line of the Na₂TPA ternary system, and the salt begins to precipitate out of solution as fine needle‑shaped crystals with a median particle size of 15 µm as measured by a Malvern Mastersizer 3000 using the ISO 13320:2020 standard for laser diffraction. The onset of crystallization marks a phase shift from a viscous but handling‑friendly Newtonian solution (viscosity 18 mPa·s at 160°C) to a shear‑thinning slurry that develops a measurable yield stress. On a TA Instruments DHR‑2 rheometer fitted with a pressure cell to suppress boiling, serial measurements at the reactor outlet temperature confirm that when free alkali drops to 0.04%, the dynamic yield stress derived from a controlled‑stress sweep stands at 35 Pa, which is sufficient to create a sediment layer in the downstream flash cooler if the velocity falls below the critical deposition velocity predicted by the Wasp model (0.9 m/s in a 150 mm diameter pipe).

Process equipment at a 5 kt/a demonstration plant (using a 152 mm internal diameter, 40 m‑long reactor) must therefore carefully balance throughput and alkali feed rate so that precipitation initiates only in the final 5 m of the tubular path, otherwise premature crystal formation in the static mixer sections causes plug flow with local hot spots exceeding 195°C, which accelerate glycol ether formation and lower the product purity (sodium terephthalate content below 98.5% by ASTM D5296-19). Free alkali is monitored continuously by an ion‑selective electrode (Mettler Toledo InLab Science Pro‑ISM) that has been factory‑calibrated against high‑purity NaOH standards and cross‑referenced with an automatic potentiometric titration in a sample quenched to 25°C and extracted with 20% sodium chloride solution to suppress glycol interferences. The operating protocol mandates a minimum free alkali target of 0.06% measured at the 85% reactor length position, allowing a buffer against feed fluctuations of up to ±12% in PET flake mass flow. When the free alkali approaches the 0.05% mark prematurely, the control system reduces the heating medium temperature by 7°C around the affected static mixer bay to decrease the reaction rate, bringing the caustic depletion point back toward the exit. This strategy, validated in accordance with the process control requirements of ISO 22514‑3:2020 for continuous processes, avoids the common failure mode of reactor fouling that can increase pressure drop by 0.8 bar over a 72‑hour campaign and require a hot water flush to restore performance. An additional limitation is that the glycol concentration must stay above 15 wt% throughout the hydrolysis zone because lower glycol levels decrease the solubility of Na₂TPA, causing the precipitation threshold to shift to a higher free alkali value of approximately 0.08% and leading to premature solidification that can warp the static mixer blades and reduce their mixing efficiency below the critical 80% degree of segregation (as evaluated by residence time distribution tests with a lithium tracer). Published simulation data on continuous PET saponification, based on kinetic constants derived from ASTM D4603-18 for intrinsic viscosity of PET feed and validated against pilot‑scale heat‑balance measurements, indicate that the optimal operating envelope that avoids the phase‑shift crystallization within the tubular reactor corresponds to an alkali‑to‑polymer molar ratio of 2.05–2.15:1 and a solid loading of 14–16 wt%, confirming the critical role of the 0.05% free alkali threshold in defining the safe processing window.