Inside fatty‑acid cleanup: vacuum columns, bleaching clays, activated carbon — and a regulatory curveball
Oleochemical producers lean on high‑vacuum distillation to split chain lengths, then deploy bleaching clays and activated carbon to strip color and odor — with exacting temperatures, pressures, and removal rates documented across industry literature. Indonesia’s reclassification of spent bleaching earth adds a pragmatic waste‑handling twist.
After fat‑splitting (hydrolysis), the crude free fatty acid (FFA) mix — typically still carrying glycerides, oligomers and oxidation products — doesn’t go straight to market. It goes under vacuum. Fractional columns at 160–250 °C and heavy vacuum (~2–20 mbar) segregate acids by boiling point, according to Food and Agriculture Organization guidance (www.fao.org).
In practice, thin‑film or molecular distillation (short residence, high surface renewal) is the workhorse for high‑purity cuts. One short‑path unit running ~110 °C at <0.4 mbar pushed decanoic (C10) acid from 96.0 to 99.8 wt% in two passes (www.mdpi.com).
Vacuum distillation and deodorization parameters
Thin‑film distillation is not just for neat acids. On high‑FFA palm oil (initial FFA ~8.7%), thin‑film operation cut FFA to ~0.9% and trimmed unsaponifiables (non‑fatty components like pigments and sterols) from 0.31% to 0.20% (www.researchgate.net). Falling‑film molecular setups have driven FFA in edible oils to near zero, with one study reporting rapeseed ≈0.05% and sunflower ≈0.04% (www.mdpi.com).
Final deodorization (vacuum steam distillation — steam stripping under vacuum to remove volatiles such as aldehydes and ketones) typically runs ~2–6 mbar (2–6 Torr) and 180–220 °C (www.fao.org). Straight (single‑phase) distillation leaves a heavy residue (~5–10% of feed) that can be reprocessed or downgraded; in one design, light ends are condensed and discarded while high‑molecular‑weight acids are recycled or re‑distilled (fostechnos.com).
Adsorptive bleaching with clays
Color and odor bodies are addressed via adsorptive bleaching either before or after distillation. Industrial practice uses 2–5% (w/w) acid‑activated bleaching clay (bentonite/fuller’s earth) at ~90–125 °C for ~15–30 minutes, often under slight vacuum (50–125 mmHg) (www.mdpi.com). These clays selectively adsorb carotenoids, chlorophylls, trace metals, soaps and peroxides (www.mdpi.com) (see also discussion of further compounds in the same review: www.mdpi.com).
Lab data are consistent: acid‑activated bentonite removed ~76% of chlorophyll pigments (www.researchgate.net) and is generally reported to adsorb >90% of β‑carotene and chlorophyll in oils (www.researchgate.net). Regenerated spent earth has removed 92.8–95% of these pigments in one report (www.researchgate.net). A test with 4% fuller’s earth reduced Lovibond red units (a visual oil color scale) from ~4–5 down to ~3–4 (patents.google.com). Clay bleaching also cuts oxygenated impurities, with reductions in peroxide values and polyaromatics noted.
There’s a waste‑side reality: spent bleaching earth (SBE) becomes loaded with organics, often ~20–40% residual oil, underscoring the need for regeneration or controlled disposal (news.mongabay.com).
Activated carbon co‑adsorption
Activated carbon is often added at <1% (w/w) to capture remaining chromogens and low‑threshold odorants; its large surface area targets dark oxidation products. Blended adsorbents can outperform clay alone: bleaching crude fish oil with 2% total adsorbent (0.2% activated carbon) cut oil color by ~87.5% and TBARS (thiobarbituric acid reactive substances, a proxy for oxidation) by ~79.7%, though ~44% of carotenoids were lost (www.researchgate.net). In practice, clay/carbon mixes are tuned to maximize color removal while minimizing loss of desirable lipophiles. For carbon supply and odor control duties, operators turn to activated‑carbon media such as /products/activated-carbon.
Final steam stripping and product targets
Post‑bleaching, a mild vacuum steam strip (deodorizer) at ~2–6 mbar and 180–220 °C is recommended, mirroring edible‑oil practice (www.fao.org). The outcome is a color‑light, odorless fatty‑acid product meeting typical commercial purity markers (monoglycerides <0.1–0.5 wt%, pigments ≲5 Lovibond units, etc.).
Across implementations, vacuum distillation routinely delivers 95–99% pure FFA fractions (www.mdpi.com; www.researchgate.net), and standard bleaches nearly clear color/odor compounds (www.researchgate.net; www.mdpi.com). These data inform practical engineering choices — for example, sizing a vacuum system for ~6 mbar at 200 °C — and set quality targets (Lovibond, FFA, unsaponifiables) for implementation.
Indonesia’s SBE rule change and risk controls
In Indonesia’s palm‑oil sector, SBE management is regulated. Because SBE can carry very high residual oil — up to ≈40% by weight — it poses fire and pollution risks if mismanaged (news.mongabay.com). Until 2021 it was classified as hazardous (B3) waste; PP No. 22/2021 (Feb 2021) reclassified palm‑oil SBE as non‑hazardous (non‑B3) if its oil content is ≤3% (news.mongabay.com).
The practical implication: refineries must recover most oil (e.g., by solvent extraction or thermal treatment) from spent clay before disposal or sale. While the delisting eases disposal costs, operators still need to monitor SBE oil content and meet emission/effluent standards (e.g., BOD/COD limits). Environmental NGOs have warned that weak enforcement could enable illegal dumping of oily SBE (news.mongabay.com).
Sources: International fat/oil processing literature and industry reports (www.fao.org) (www.mdpi.com) (www.researchgate.net) (www.mdpi.com) (www.researchgate.net) (news.mongabay.com). These include FAO oil‑processing guidelines, peer‑reviewed studies (MDPI Processes, Foods, Eur. J. Lipid Sci.), and Indonesian regulatory news (Mongabay, PP 22/2021) (news.mongabay.com).