In palm-oil mills, every extra degree and minute in the clarifier can raise free fatty acids and drag down color. Newer separation lines cut hot-hold times from hours to minutes.
Industry: Palm_Oil | Process: Crude_Oil_Clarification_&_Purification
Crude palm oil leaves the press as a hot, viscous emulsion of oil, water, and solids. Mills dilute it with hot water at roughly a 3:1 ratio and heat the mixture—often to near boiling—to break emulsions (FAO guide). Small mills boil the oil/water slurry for 1–2 hours and then leave it 3–4 hours to clarify by gravity (FAO guide).
Large mills run multistage “continuous” clarifiers—“three compartments in series” (a feed compartment, a drying compartment, and an oil‑holding compartment)—with steam coils and heat‑exchange shells to keep flow warm (FAQ) (FAO guide). In facilities still using tanks, supplier offerings such as a clarifier reflect the sector’s reliance on gravity separation.
After settling, the decanted oil typically holds about 0.3–0.5% moisture and is dried—often by reheating or in a vacuum dryer—to around 0.15–0.25% (FAO guide). That drying step is essential to prevent autocatalytic hydrolysis—free fatty acids (FFA, acidity formed when water hydrolyzes triglycerides) rise rapidly if water remains (FAO guide). In large refineries, clarified oil is then polished in centrifuges called “purifiers” to remove fines and residual water.
Temperature–time quality trade‑off
Heating is necessary for separation, but higher temperature and longer residence time accelerate degradation. A laboratory study by Ruswanto et al. (2024) varied clarifier setpoints at 55–60, 65–70, and 75–80 °C, and settling times at 1.5, 3.0, and 4.5 hours. The result: higher temperature or longer time increased FFA and decreased DOBI (Deterioration of Bleachability Index, a proxy for carotene oxidation) (study) (study).
The best quality in that work—lowest FFA and highest DOBI—was at 55–60 °C and 1.5 hours settling (study) (study). At that condition, FFA was about 5.1% and DOBI about 3.58 (study). At 75–80 °C for 4.5 hours, FFA was well above 5% and DOBI fell to roughly 3.00 (study) (study).
In all trials, FFA exceeded the 5% maximum specified by Indonesian standards (SNI 01‑2901‑2006), underscoring FFA’s sensitivity to heat and time (study). Longer exposure allows residual moisture to hydrolyze the oil. DOBI also dropped at longer times: 4.5 hours at high temperature produced DOBI ≈ 3.00 (still “good” but at the low end of 2.93–3.24), whereas shorter/heater oil saw higher DOBI (study). Industry observations align: excessive heating steps—sterilization or clarification—oxidize pigments and drive DOBI down (study) (review).
One Indonesian review found average CPO DOBI values below 2.0—lower than global norms—largely due to “overheating during processing” (review). In practice, mills target FFA ≤ 5% (SNI 01‑2901‑2006), while many refiners demand DOBI ≥ 2.2–3.0 for premium oil (review) (study).
Quantitative impact on FFA and DOBI
FAO guidance notes that boiling the dilution and a 3–4 hour settle are “required” for gravity clarification (FAO guide), but also warns that residual moisture drives FFA and that reheating to lower moisture must be done carefully (FAO guide).
In the Ruswanto et al. work, extending settling from 1.5 to 4.5 hours at around 80 °C raised FFA above 5% and cut DOBI by roughly 0.2–0.5 units (study) (study). A clarifier held at about 90 °C for 4 hours—a common practice—can easily double FFA compared with a minimal settle. Reducing settling from 4 to 1.5 hours (at moderate temperature) lowered FFA from approximately 7–8% to about 5% in that experiment (study).
The commercial stakes are clear: every percentage point of FFA (~1% = 2°A acidity) needs more neutralization and erodes value; higher DOBI (>3.0 versus <3.0) can command a premium.
Decanter centrifuges and purifiers
To cut thermal load, mills are turning to mechanical separation. Modern decanter centrifuges (three‑phase machines that split oil, water, and solids) shrink hot‑hold from hours to minutes (Alfa Laval PANX) (Huading note). Alfa Laval cites PANX units in mills processing 30–90 t FFB/h (fresh fruit bunches per hour), enabling continuous clarification and recovery of virtually all oil from press effluent (Alfa Laval PANX).
In one Indonesian mill, replacing manual clarifiers with a three‑phase decanter cut oil losses in the sludge underflow and produced much cleaner effluent (IOPRI case). Decanters also deliver a drier cake, easing downstream drying. A disc‑stack “purifier” (a polishing centrifuge) typically follows, removing fine sludge and remaining water in tens of minutes.
This sequence lets mills feed hot press effluent directly at roughly 70–80 °C, move the oil quickly, and dry it promptly. By comparison, a passive tank clarifier may hold oil at 85–90 °C for 4 hours; a decanter‑plus‑purifier line achieves equivalent or better separation in under 15–20 minutes. Suppliers say decanters “improve separation of palm oil” with “maximum oil recovery” and “high‑quality” output (Alfa Laval PANX) (Huading note).
Heat integration and monitoring
Beyond hardware, mills use heat integration and tighter control. Continuous clarifiers often preheat incoming sludge with condensate coils, then hand off to cooler water in later stages to avoid superheating (FAQ). Some designs adopt two‑chamber clarifiers or flashing dryers to shorten the boiling step. A small‑scale prototype that combined clarification and drying extracted oil at 85 °C with about 74% efficiency (prototype).
At a minimum, mills monitor clarifier temperature and level, stopping heat when separation starts. Real‑time turbidity monitoring can trigger earlier overflow to cut holding time. In tank‑based flowsheets, a modern clarifier remains the thermal bottleneck—and therefore the control focus.
Cool storage and finish
Temperature control downstream matters, too. FAO notes storage tanks are kept near 50 °C with warm‑water coils because oxidation rates climb with temperature (FAO guide). Cooling as soon as processing ends helps avoid further quality loss. “Purified” CPO should be only warm enough for handling—50–60 °C—and not held at high temperature (FAO guide).
Netting it out: rigorous temperature control—using 80–90 °C only as needed—and minimizing hot‑hold time via faster clarification, centrifuges, and smarter heat exchange preserve FFA, DOBI, and color. Data‑driven experience shows mills that adopt these steps sustain FFA ≤ 5% and robust DOBI while maintaining yield (study) (Alfa Laval PANX).
Process references and notes
Process descriptions and small‑mill practices: FAO guide (FAO guide). Clarifier tank design and coils: FAQ. Experimental data on temperature/time effects and standards context: study (study) (study) (review) (review). Decanter and purifier equipment and case reports: Alfa Laval PANX (Alfa Laval PANX) (Huading note) (IOPRI case). Small‑scale prototype: prototype. Storage and temperature control: FAO guide.
