Palm oil mills turn a thick, fiber‑strewn emulsion into saleable crude oil by settling it in steam‑heated clarification tanks. The design is deceptively simple, but the setpoints are not: about 85–90 °C, several hours of retention, and a vibrating screen up front can make the difference between ~74% recovery and double‑digit losses.
Industry: Palm_Oil | Process: Clarification
The clarifier is the quiet center of a palm oil mill: a large settling vat where diluted crude oil (oil + water + solids) separates under gravity. Mills first dilute and heat the milled oil, feed it from the bottom of the tank, let heavy particles settle down a sloped, conical base, and skim oil that rises to the top overflow (FAO) (FAO). FAO guidance is blunt about the starting point: press liquor is “very thick (viscous)” and must be thinned—often by ~3 volumes of hot water per volume of oil—so droplets can “flow through the watery mixture to the top” (FAO).
Tank geometry and staged operation
Typical tanks are large steel vessels—often 30+ m long or 10+ m in diameter—with a bottom inlet and a top overflow; heavy solids settle to the conical base while lighter oil migrates upward (FAO). Baffles or baffle‑weirs separate zones for raw feed, decanting, and polishing; continuous designs often combine these three stages in one tank (FAO) (FAO). There is no vigorous mixing—at most a low‑speed stirrer maintains uniform temperature.
These multi‑compartment tanks hold several hours’ volume to reach full separation. One prototype clarifier, for example, operated at 85 °C with a 1:2 water:oil ratio and required on the order of 3–5 h retention to reach ~74% separation efficiency (J. Eng. Res. & Reports, 2025) (J. Eng. Res. & Reports, 2025). After settling, clean oil is skimmed off, sludge (bottom solids and entrained oil) is withdrawn to a pit, and the purified oil is dried rapidly—typically by further heating—to ≤0.25% water to prevent hydrolysis (FAO).
In practice, mills specify dedicated clarifier hardware; the function mirrors a gravity unit like a clarifier, with detention measured in hours to let suspended solids settle and the oil phase coalesce.
Dilution and upstream coarse screening
Immediately after pressing, mills dilute the viscous press‑liquor with hot water at about ~1–3:1 water:oil to thin the slurry (FAO). A coarse screen upstream—often a simple grid—removes large fiber and shell fragments before settling starts (FAO). This dilution step creates a “water barrier” that lets sand and grit sink while oil droplets rise.
Clarifier vessels are steam‑heated and kept hot—typically 80–95 °C—to break proteinous emulsions and keep oil fluid; one source notes the diluted slurry is “boiled from one or two hours” in the tank to break the emulsion (FAO). Design targets often sit around ~85–90 °C; for instance, a test clarifier run at 85 °C with steam heating achieved good separation (J. Eng. Res. & Reports, 2025) (J. Eng. Res. & Reports, 2025). Afterward, oil is usually moved to storage held ~50 °C to avoid fractionation (FAO).
Temperature–viscosity relationship
Maintaining high temperature is crucial because hot oil is less viscous and coalesces faster. FAO notes the raw press liquor is “very thick (viscous)” and requires hot water dilution (FAO). Quantitatively, palm oil viscosity drops by roughly an order of magnitude when heated from ambient to ~85–90 °C—e.g., from tens of mPa·s at 25 °C to single‑digit mPa·s at 90 °C (mPa·s, millipascal‑seconds, is a viscosity unit) (tubingchina.com). Heated tanks therefore allow droplets (tens of microns) to rise more rapidly.
Operating near 85–95 °C ensures a low‑viscosity oil phase; mills use steam jackets or direct injection to keep mixtures boiling, thinning the emulsion much like dilution does (FAO). Lower viscosity increases separation rates and helps prevent solids from re‑entraining oil droplets.
Temperature optimization and CFD evidence
Empirical results back the heat: in one experiment at a 1:2 water:oil ratio, raising the clarifier from 80 °C to 85 °C increased oil extraction from ~13.3% to 16.3%, but further heating to 90 °C reduced yield (likely due to emulsification or splashing), implying an optimum around 85 °C under those conditions (J. Eng. Res. & Reports, 2025). Computational fluid dynamics (CFD, a flow‑simulation method) also quantifies the effect: inlet temperature has a strong positive influence on the final oil fraction (high R²), meaning warmer feed consistently yields more recovered oil (IOPscience).
Processing notes remain pragmatic: after settling, FAO prescribes skimming until moisture is ~0.15–0.25% to avoid water‑catalyzed rises in free fatty acids (FFA, a quality parameter) (FAO). In sum, ~85–90 °C via a jacket or steam injection is standard to maximize separation speed and oil quality.
Vibrating screen pre‑filtration
Before the clarifier, mills deploy a vibrating mesh screen—also called a harvesting sieve or oil filter—to strip out coarse debris. It protects the tank, pumps, and filters from abrasive, fibrous material. In POME (palm oil mill effluent) systems, this follows the “sand trap” stage and is typically an inline rotary or pendulum vibrator (NoakMech). For continuous debris removal in this role, mills often specify an automatic screen ahead of the clarifier feed.
The screen separates large solids—sand, nutshells, fiber clumps—from hot crude oil; one description states it “serves to separate solid sand or solids contained in crude oil,” and is installed after the sand trap and before the settling tank (NoakMech). Typical units are large circular stainless‑steel screens (~60″ diameter) with two layers of mesh; many mills run 20 holes‑per‑inch on top and 40 holes‑per‑inch below (NoakMech). An eccentric motor provides vibration to move material, and operators often spray hot water to keep fibers from clogging the mesh (NoakMech); mills select mesh sizes knowing smaller openings capture more but clog faster (NoakMech).
Captured solids mixed with some oil are discharged as concentrated “dirty oil” or sludge; this waste stream—oil plus sand/fiber—can be recycled in boilers or treated separately. Critically, the screen removes almost all large particles so the clarifier sees a finer emulsion, reducing wear and clogging in pumps and downstream separators and improving uptime; mills report steadier clarifier flow and higher net oil recovered with pre‑filtering (NoakMech) (NoakMech).
Performance metrics and losses
Typical clarifier retention times are on the order of a few hours. In reported trials, a unit held ~5 h at 85 °C achieved ~74.2% clarifier efficiency (percent of total oil recovered as clear oil) from the diluted crude (J. Eng. Res. & Reports, 2025). Well‑designed mills generally exceed 70–80% clarifier efficiency, then dry remaining moist oil to meet quality specs (FFA <5%, moisture <0.5%). Efficient clarification directly cuts losses: one study observed small mills with poor clarification losing up to 15% of oil, noting the clarification step generates >50% of mill wastewater and attendant oil loss (J. Eng. Res. & Reports, 2025).
Field trials of new clarifiers—paired with proper heating and screening—show oil recovery on the order of 16–17% of fresh‑fruit‑bunch (FFB) weight, with clarifier efficiencies around ~74% (J. Eng. Res. & Reports, 2025). Design choices have quantifiable impact: with a 1:2 dilution at 85 °C, one study reported ~16.3% oil with 74.2% of that oil successfully clarified into the pure output (J. Eng. Res. & Reports, 2025) (J. Eng. Res. & Reports, 2025). By contrast, mills without effective screening/clarification can lose ~10–15% of oil in the effluent (J. Eng. Res. & Reports, 2025).
Source notes and equipment references
FAO processing guidelines detail hot dilution and multi‑compartment clarifier designs (FAO) (FAO) (FAO) (FAO) (FAO). Recent pilot‑scale data on throughput and efficiency appear in J. Eng. Res. & Reports (Jan. 2025) (ResearchGate) (ResearchGate) (ResearchGate). CFD work underscores temperature’s role (IOPscience). Industry equipment notes cover vibrating‑screen placement and sizing (NoakMech) (NoakMech).
