The pH 10.5 play: How lime and soda ash turn crude glycerine into higher‑value product

Crude glycerine, the viscous by‑product that splashes out of biodiesel and soap plants, is both a liability and an opportunity. Typical waste streams carry only about 50–60% glycerol by weight—though the range stretches from ~40% to nearly 100% depending on how the upstream transesterification or saponification was run (pmc.ncbi.nlm.nih.gov). On average, roughly 10 kg of crude glycerine is generated per 100 L of biodiesel produced (mdpi.com).

What’s in the rest? Water, methanol, free fatty acids (FFAs, unesterified organic acids), soaps, salts (Na/K chloride, sulfate), and non‑glycerol organic matter (MONG, the catch‑all for organics that aren’t glycerol) (pmc.ncbi.nlm.nih.gov). A palm‑oil case study clocked “recovered crude glycerine” at ~51.4% glycerol with 13.8% ash (inorganics) and 25.9% MONG (doczz.net).

Refined glycerine (≥99.5% purity) fetches far more in markets as diverse as pharmaceutical and food (USP specifications are common), placing a premium on purification (mdpi.com) (oleochemicals.in). With ~2,000 known end‑uses for glycerol (mdpi.com), even modest purity gains can move revenue noticeably.

Chemical precipitation system design

The core of the system is a stirred chemical reactor that doses lime (Ca(OH)2) and soda ash (Na2CO3) to precipitate impurities—an approach analogous to precipitation softening in water treatment (water.mecc.edu). Lime supplies Ca2+ and OH–; soda ash supplies CO3^2–. In the alkaline slurry, hardness ions and free acids form insoluble solids that drop out.

The working chemistry includes: Ca(OH)2 + Na2CO3 → CaCO3(s) + 2 NaOH; Mg2+ + 2 OH– → Mg(OH)2(s); and 2 RCOO–Na+ + Ca2+ → Ca(RCOO)2(s) + 2 Na+ (RCOO– denotes fatty acid soaps). The mixed precipitates—CaCO3, Mg(OH)2, and calcium fatty acid salts (Ca(RCOO)2)—also enmesh fine solids and organic matter, improving “scrubbing” efficiency (water.mecc.edu).

Residence time in the stirred reactor typically runs ~15–30 minutes (web.deu.edu.tr). Dosing often follows stoichiometry from softening practice—about 1 mole Ca(OH)2 per mole calcium present and 1 mole Na2CO3 per mole of non‑carbonate calcium (water.mecc.edu). As a reference point, removing 100 ppm (as CaCO3) of hardness in water typically consumes ~40–75 mg/L Ca(OH)2 plus ~30–50 mg/L Na2CO3 (water.mecc.edu). To hit those targets repeatably, plants pair the reactor with metering equipment such as a dosing pump for accurate chemical dosing.

Performance mirrors softening benchmarks. Lime–soda treatment can remove ~90–99% of Ca/Mg hardness, and when pH is raised to 10.3–10.6, calcium removal is essentially complete, leaving about 50–85 mg/L (as CaCO3) residual in water analogues (water.mecc.edu). In crude glycerine service, one study using chemical neutralization (strong acid + NaOH base) recovered ~91% of soluble salts as precipitate (doczz.net) (doczz.net). The design aim here is to cut ash (inorganics) from ~10–15% to well under 1% and to remove >90% of soap/FFA.

pH control as critical parameter

pH drives the equilibrium. Each precipitate’s solubility swings with pH, and many metal hydroxides show a solubility minimum at a characteristic point; push past it and amphoteric behavior makes them more soluble again (web.deu.edu.tr). In practice, CaCO3 precipitation is optimal around pH ~10.2–10.6. Lower pH leaves Ca2+ unprecipitated; higher pH can redissolve some solids. Water‑treatment data show targeting pH 10.3–10.6 is required for nearly complete Ca/Mg removal; even a 0.1–0.2 pH unit offset can reduce efficiency sharply (water.mecc.edu).

Crude glycerine behaves the same way. One study observed that higher pH actually increased dissolved salt in the product—overshoot can reverse precipitation and re‑dissolve NaCl/Na2SO4 (doczz.net). Conversely, acidifying to pH 1–2 reduced ash but increased MONG, and overly acidic conditions risk glycerol degradation via acid‑catalyzed dehydration to acrolein (doczz.net). The sweet spot is a controlled alkaline band, usually 9.5–10.5.

That demands tight feedback. Real‑time pH sensors modulate lime or carbonate addition, and conventional lime–soda plants monitor caustic alkalinity at the phenolphthalein endpoint (~pH 10.5) to stay within ±0.1–0.2 of setpoint (watertechnologies.com). In this glycerine system, the same control logic applies: add lime/soda until pH ≈10.5, as tuned by pilot tests (web.deu.edu.tr) (doczz.net). As a quantitative marker, boosting pH from 9.5 to 10.5 in hard water can raise calcium removal from ~60% to ~95% (water.mecc.edu); in crude glycerine, data showed that at pH 6–7 the recovered glycerine still carried >50% ash, while pushing pH to extreme values (acid or base) markedly shifted composition (doczz.net). The practical target is to reduce ash from ~10% to <1% in the final product, which typically requires control within about ±0.1; even a 1% residual of alkali or acid can fix tens of ppm of Ca/Mg in solution.

Clarification and filtration train

After precipitation, the slurry heads to clarification and filtration. Settling tanks or flocculation with polymer—often paired with dissolved air flotation (DAF, a process that floats fine solids on microbubbles)—knock down fines before final filters (water.mecc.edu). Plants commonly deploy a clarifier for the bulk solids, and in tighter footprints, a DAF unit can raise capture at 1–3 hours detention. Where polymers augment settling, commodity flocculants improve clarifier efficiency by 30–50%.

Final solids capture is handled by pressure or vacuum filters. Industrial service favors robust housings; steel filter housings rated for high pressure are typical in these applications. For downstream polishing to very low suspended solids, plants often specify sanitary housings—an example being 316L stainless cartridge housings used in pharmaceutical and food‑grade lines.

Typical designs (plate‑and‑frame presses or continuous rotary filters) deliver a dewatered cake at ~10–30% solids and a clear glycerol liquor. Filtration yields are high because the precipitates are very insoluble; water‑softening analogues consistently achieve >99% clarity (water.mecc.edu). One field report on palm‑ester glycerol processing recovered ~33.9% of the waste mass as purified glycerine (containing 51.4% glycerol) and captured ~65.2% as salt cake (doczz.net). By mass of glycerol residue, more than half was recoverable as solid salt. For sizing, if 1 tonne of crude glycerine (~500 kg actual glycerol) is treated, it is reasonable to anticipate on the order of 50–100 kg of dry cake (depending on initial ash), indicating high glycerol yield (>90%).

Evaporation, distillation, and residues

The filtrate glycerine—largely stripped of inorganic and soap impurities—may still contain water and methanol. Vacuum evaporation or distillation then boosts concentration; distillation can yield glycerine >98–99% purity. Upstream precipitation helps keep salts out of the still and out of the product.

By analogy to lime‑softening operations, filter cake moisture tends to run 80–90%, and the filtrate represents >95% of the liquid volume with minimal suspended solids (<50 mg/L). Example results after acid/base treatment and filtration reached nearly 50% glycerol with 9.8% ash (vs 13.8% originally) (doczz.net); with optimized lime/soda treatment at pH ~10.5, similar or better impurity removal is expected.

All solid waste (the filter cake of CaCO3, Mg(OH)2, and calcium soaps) can be handled as nonhazardous solid. In Indonesia, lime sludge might be used as soil amendment or disposed in controlled landfills per regulations; heavy metals should be monitored. The clarified glycerine, if further purified to >99.5% glycerol, can meet Indonesian pharmacopeia or food‑additive grade (oleochemicals.in) (mdpi.com).

Expected outcomes and yields

With controlled dosing of Ca(OH)2 and Na2CO3 and rigorous pH control, >90% of inorganic salts and soaps can be precipitated (water.mecc.edu) (web.deu.edu.tr). After filtration, glycerine yield is expected to exceed 95% of the input glycerol mass, with ash/residue down to 1% or lower (from initial ~10–15%).

As an example calculation: starting from 1 tonne of crude glycerine (~500 kg glycerol and ~100 kg ash), the process can recover ~480 kg glycerol in purified form, leaving <20 kg dissolved inorganic salts. One palm‑ester case recovered ~33.9% of the waste mass as purified glycerine and ~65.2% as salt cake (the glycerine fraction in that recovered stream was 51.4%) (doczz.net) (doczz.net).

The economic rationale follows: refined glycerine (≥99.5% purity) sells at a premium relative to crude (mdpi.com) (oleochemicals.in). The process leverages established precipitation chemistry and water‑treatment know‑how, where raising pH to 10.3–10.6 and holding it tightly unlocks high removal rates and clean filtrate streams (water.mecc.edu) (web.deu.edu.tr).