Scale-Free Alkalinity in 45% Caustic for Propylene Oxide Saponification

How 45 wt% NaOH Feedstock Quality Dictates Saponifier Fouling Rates

In propylene oxide (PO) production via the chlorohydrin route, the saponification step—dehydrochlorination of propylene chlorohydrin (PCH) with excess sodium hydroxide—represents a persistent bottleneck when scale deposition restricts heat transfer and narrows flow paths. The reaction is typically carried out at 80–95 °C and 1.5–2.5 bar(g) in a continuous stirred-tank reactor (CSTR) cascade or a plug-flow tubular reactor equipped with static mixers, using 45 wt% NaOH diluted in-process to approximately 5–10 wt% active alkalinity. The choice of 45% membrane- or diaphragm-grade caustic, rather than lower concentrations, is driven by logistics and alkali inventory minimisation; however, the concentrated solution introduces a severe scaling risk driven by its extremely low tolerance for polyvalent cations. Calcium and magnesium contaminants, even at sub-ppm levels, precipitate instantly upon dilution and temperature elevation as Ca(OH)₂ (portlandite) and Mg(OH)₂ (brucite), while atmospheric CO₂ ingress promotes CaCO₃ (calcite) formation at the vapour-liquid interface. Production experience at a 150 kt/a PO facility documented a decline in overall heat transfer coefficient from 850 W/m²·K to 410 W/m²·K within a 90-day campaign when the average total hardness of the 45% NaOH feed exceeded 1.8 mg/L as CaCO₃, despite maintaining a stoichiometric excess of 5–7% NaOH. The corresponding increase in shell-side steam demand for the reboiler of the PO recovery column reached 12%, reflecting the energy penalty directly attributable to scale resistance. Analytical characterisation of the deposit by X-ray diffraction (XRD) in accordance with ASTM D934-13 confirmed a layered structure: a dense inner layer of Ca(OH)₂ intermixed with NaCl crystallites embedded in a brucite matrix, overgrown by a porous calcite outer layer. The presence of calcite points to atmospheric CO₂ absorption in open caustic day tanks, a pathway that can be suppressed by blanketing the 45% NaOH storage with nitrogen (0.5 barg pad pressure) and by sparging the dilution water with steam to remove dissolved CO₂, reducing the total inorganic carbon content to < 2 mg/L as C. In continuous stirred-tank saponifiers where the residence time distribution broadens, the zone of highest supersaturation emerges at the impeller discharge where rapid micromixing of the 45% caustic with the aqueous PCH phase occurs. Computational fluid dynamics (CFD) simulations of a 4-blade pitched-blade turbine in a 12 m³ vessel, validated by tracer studies, indicate that localised pH spikes exceeding 13.8 persist for 0.3–0.5 s before bulk neutralisation by the organic hydrochloride. In this transient alkaline plume, the ion product [Ca²⁺][OH⁻]² readily surpasses the solubility product Ksp of 5.5 × 10⁻⁶ at 90°C for portlandite, so that a calcium concentration of merely 0.5 mg/L in the combined feed yields an instantaneous saturation index above 10. The resultant colloidal precipitate adheres strongly to stainless steel surfaces, particularly when the surface roughness Ra exceeds 0.8 µm. Operational data from a twin-screw continuous reactor (L/D 25:1, 150 mm diameter) showed that after 600 h of run time, scale thickness on the screw flights reached 0.7 mm, raising the motor load by 18% and necessitating a turn-around for high-pressure water jetting at 700 bar. The mechanical cleaning frequency was reduced from every 4 weeks to 16 weeks after retrofitting the caustic feed system with a duplex strainer and a polishing column packed with iminodiacetic acid chelating resin, which lowered the total hardness of the 45% NaOH to < 0.05 mg/L as CaCO₃, as measured by inductively coupled plasma optical emission spectrometry per ISO 11885:2007. This modification also eliminated the need for periodic acid circulation (inhibited 5 wt% HCl at 50°C) that had previously been required to restore heat transfer surfaces in the shell-and-tube saponifier effluent cooler. A parallel concern arises from the silica content of diaphragm-cell caustic, which can reach 15–30 mg/L as SiO₂. In the presence of Ca²⁺ and Mg²⁺, amorphous magnesium silicate and calcium silicate hydrate gels form at the heated exchanger surface, creating a tenacious scale that is resistant to standard acid cleaning and requires mechanical removal or alternating alkaline-EDTA treatment. Plants sourcing caustic from mercury cells typically observe silica levels below 5 mg/L, but membrane-grade caustic now predominates for environmental reasons and routinely contains 3–8 mg/L SiO₂, a range that does not induce gel formation unless the magnesium hardness concurrently exceeds 0.2 mg/L. A monitoring programme that combines daily sampling of the recirculating alkaline brine for silica and magnesium by ISO 9964-1:1993 and ISO 11885 with quarterly borescope inspection of the saponifier vapour-space walls has proven effective in forecasting silica-based scale onset 7–10 days before hydraulic symptoms appear.

Threshold Concentrations for Alkaline Earth Metals in Diaphragm-Cell and Membrane-Cell Caustic

The scaling propensity of a given caustic soda shipment is not a single-valued function of total hardness but depends on the interplay of calcium, magnesium, carbonate, and silica concentrations under the dynamic pH and temperature conditions of the saponification loop. The table below summarises typical impurity ranges for three commercial 45 wt% NaOH grades and the corresponding Langelier Saturation Index (LSI) calculated for the diluted alkaline brine at 90°C and pH 13.5, using the Stiff-Davis stability index adapted for high-salinity brines per ASTM D3739-18. An LSI value exceeding +1.5 is associated with a high probability of rapid calcium carbonate scaling, while a value above +2.5 indicates portlandite deposition is thermodynamically favoured even in the absence of carbonate.

Caustic Grade	Ca (mg/L)	Mg (mg/L)	SiO₂ (mg/L)	Na₂CO₃ (wt%)	Calculated LSI at 90°C	Observed Scale Type
Membrane (high-purity)	0.1–0.5	0.02–0.1	3–8	0.05–0.1	+0.8 to +1.4	Thin calcite film after 6 months
Diaphragm (rayon-grade)	2–8	0.3–1.2	10–25	0.15–0.3	+2.2 to +3.5	Portlandite + calcite; rapid deposition
Diaphragm (evaporated, softened)	0.5–1.5	0.1–0.3	8–15	0.1–0.2	+1.7 to +2.3	Mixed Ca/Mg silicate if silica > 12 mg/L

The operational boundary for un-softened diaphragm caustic is therefore extremely narrow: the calcium mass flow must be reduced by at least 80% through upstream brine polishing before the saponifier to achieve a run length of 12 months between chemical cleanings. Several large-scale PO units operate a side-stream softening loop on the diluted caustic, employing cold-lime soda ash softening followed by a pressure sand filter and a polishing strong-acid cation exchanger in sodium form, which yields a brine with residual hardness consistently below 0.2 mg/L as CaCO₃, verified by on-line hardness analysers using colorimetric titration with Eriochrome Black T indicator compliant with ASTM D1126-17. The capital expenditure for such a polishing system is often recovered within 18–24 months through reduced steam consumption and avoidance of production downtime.

When Caustic Circulation Loops Drop Below 18°C

Temperature control of the 45% NaOH feed and return lines is critical not only for scaling chemistry but for fluid rheology. The dynamic viscosity of 45 wt% sodium hydroxide exhibits a steep non-linear increase as temperature falls: at 20°C the viscosity is approximately 25 mPa·s, rising to 50 mPa·s at 10°C and surpassing 100 mPa·s at 0°C, as published in the Dow Caustic Soda Handbook. In saponification plants located in cold climates, unjacketed carbon steel caustic storage tanks and transfer piping that lack heat tracing can cool below 18°C during winter shutdowns or turndown, causing a transition from turbulent to transitional or laminar flow in the feed lines when the line velocity drops below 1.2 m/s in DN50 schedule 80 piping. Under laminar conditions (Reynolds number < 2100), the wall shear stress is insufficient to dislodge loosely adherent calcium carbonate nuclei, and the residence time in the thermal boundary layer adjacent to any steam-jacketed segment increases sufficiently to promote complete dehydration of portlandite to a hard, cementitious deposit. Facilities that have installed electrical heat tracing (self-regulating, 15 W/m output) and 50 mm mineral wool insulation with vapour barrier cladding on all 45% caustic piping report that maintaining the feed temperature above 25°C eliminates cold-related scaling episodes entirely, even when using diaphragm-grade caustic with hardness up to 1.5 mg/L. The minimum circulation velocity of 2.0 m/s in the main saponifier loop is verified by a clamp-on ultrasonic flowmeter calibrated for high-alkalinity service per ISO 12242:2012, and any prolonged period below 1.8 m/s triggers an automated flush cycle with softened water. The corresponding temperature window on the hot side is bounded by the exothermic saponification reaction, which releases ~120 kJ/mol of PCH. Where localised overheating above 105°C occurs at the tube wall of a vertical thermosiphon reboiler, PO oligomerisation accelerates, forming polypropylene glycols that act as a sticky matrix entraining precipitated scale particles and carbonised organic residues. This composite foulant layer reduces the tube-side velocity further, leading to a self-accelerating deposition loop. Preventative measures include thermocouple placement within 5 mm of the tube wall in the hottest channel of each pass and a hard interlock that trims steam flow when any measured temperature exceeds 103°C.

Phosphonates Outperform Polyacrylates at Temperatures Exceeding 90°C in 10% NaOH

Chemical antiscalant programmes, borrowed from cooling water and boiler water technology, are applicable to PO saponification only after careful selection for thermal and hydrolytic stability in high-pH oxidative environments. Polyacrylic acid (PAA, MW 2000–5000 Da) dispersants lose threshold inhibition efficacy rapidly above 60°C in 10% NaOH due to chain scission and decarboxylation, as confirmed by gel permeation chromatography analysis of hot caustic loop samples after 24 h of exposure. Organophosphonates such as 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) retain measurable activity up to 110°C, provided the free calcium concentration in the brine is kept below 1 mg/L to avoid forming calcium phosphonate precipitates that themselves contribute to scale. A pilot-scale evaluation of PBTC at 2 mg/L active in the saponifier feed recorded an induction time for CaCO₃ precipitation of 145 min compared to 22 min for the untreated blank at 95°C and a Ca²⁺ challenge level of 5 mg/L, using a continuous stirred precipitation vessel with turbidity monitoring per ISO 7027-1:2016. The table below presents comparative performance data extrapolated from a 12-week plant trial on a 20 kt/a PO saponifier unit.

Antiscalant	Dosage (mg/L active)	Ca²⁺ in loop brine (mg/L)	Heat exchanger U-coefficient retention (%) after 12 weeks	Minimum brine temperature for efficacy (°C)
None (baseline)	—	0.4	42%	—
PAA (MW 4500)	5	0.4	61%	65
PBTC	2	0.4	88%	50
HEDP + polycarboxylate blend	3	0.4	84%	55
Polyaspartic acid (MW 3000)	4	0.4	79%	75

Despite the promising threshold inhibition, phosphonate use carries a critical operational boundary: the recirculating brine must be continuously analysed for orthophosphate (ISO 15681-1:2005) because any residual chlorine carried over from the chlorohydrin synthesis will oxidise PBTC and HEDP, releasing phosphate that coprecipitates with calcium as hydroxyapatite, a scale that cannot be removed by conventional sulfamic acid cleaning. Plants operating with chlorohydrin containing more than 2 mg/L free chlorine must either dechlorinate the organic feed with sodium bisulfite or select a non-phosphorus antiscalant, accepting a reduction in maximum permitted operating temperature to 85°C. Combining the polishing resin and antiscalant strategies produces an interesting interdependence: chelating resins that leak sub-ppb levels of iron (from corrosion of upstream steel piping) inadvertently introduce Fe³⁺ ions that deactivate phosphonate inhibitors through complexation and precipitation of iron phosphonate sludge. Consequently, the cation polishing column must be followed by a 5 µm absolute-rated polypropylene cartridge filter to trap any resin fines and particulate iron oxides, and the iron content in the polished caustic must be verified by ISO 6332:1988 (1,10-phenanthroline method) to remain below 0.05 mg/L. Installing clamp-on ultrasonic transit-time transducers on the saponifier circulation piping, at locations where scale accumulation has historically been observed, provides a continuous, non-invasive measurement of scale thickness based on changes in the acoustic velocity ratio between the pipe wall and the process fluid. In a DN200 Schedule 40 316L stainless steel pipe, a scale layer of 0.2 mm of calcite-brucite composite produces a measurable shift in the second harmonic signal of approximately 3.5%, which can be calibrated against coupon weight gain coupons inserted in a retractable bypass rack per ASTM D4410-16. The signal is transmitted to the distributed control system (DCS), where a fouling resistance algorithm calculates the corrected heat transfer coefficient of the downstream exchanger. When the calculated fouling factor exceeds 3.5 × 10⁻⁴ m²·K/W, an advisory alarm prompts the operations team to raise the antiscalant dosage by 30% and schedule off-line cleaning. In a documented application, this predictive strategy extended the interval between full mechanical decoking of the saponifier bundle from a reactive 8-month cycle to a condition-based average of 14 months, without any unplanned downtime attributed to scale blockage. The methodology is only viable if the ultrasonic transducers are temperature-compensated for the 80–100°C operating range and if the pipe outer surface is grit-blasted to a consistent sound coupling finish conforming to ISO 8501-1 preparation grade Sa 2½. Published data for this specific configuration in high-alkalinity turbulent multiphase flow is limited, but operational heuristics derived from plate-type saponifier coolers indicate that fouling resistance rises exponentially once the smooth calcite film transforms into a dendritic aragonite growth above 0.4 mm thickness, a morphological shift that can be detected by the onset of a low-frequency (50–200 Hz) vibration signature in the piping accelerometer data.