Gravity tanks, kept near 90–95 °C, and vibrating screens are doing the heavy lifting in palm oil mills—thinning viscous oil, settling out sludge, and protecting downstream equipment.
Industry: Palm_Oil | Process: Crude_Oil_Clarification_&_Purification
In palm oil, temperature is money. Mills that hold crude palm oil close to 90–95 °C inside continuous settling tanks (CSTs, continuous gravity clarifiers) see faster separation, lower oil losses—and fewer headaches for pumps and centrifuges. The design looks simple: a tall vessel with an inverted cone and adjustable skimmers. The operation is not. It depends on precise hydraulics, tight temperature control, and upstream screening that pulls coarse solids out before they can stall the process.
The hardware is recognizable to any process engineer, but the details are written in the numbers: a 5,000 L crude‑oil holding tank feeding a ~15,000 L/h line for roughly 20 minutes of retention; oil fed in below the oil layer so it rises to skimmers, while sludge drains just above the cone apex; and an oil layer tuned to around 1.5 m via a 15 cm differential between overflow lips—small adjustments with oversized consequences (www.scribd.com; cybex.id; cybex.id).
Clarifier tank geometry and flows
The CST is typically a vertical cylinder with an inverted cone at the bottom; diluted hot oil is piped in below the oil–water interface so the oil phase floats upward and decants over adjustable skimmer pipes, while the heavier water/sludge exits through an outlet just above the cone apex (cybex.id). Operators set overflow lips so the oil layer is thick—on the order of ~1.5 m. In one example, a 15 cm height difference between oil and water outlets produced that ~1.5 m “clean” oil zone (cybex.id).
Geometry and hydraulics matter. Clarifiers are often 2–3 m in diameter and ~8–10 m tall with conical bottoms, sized for residence time that lets gravity work. A cited setup used a 5 m³ tank at 15 m³/h—about 20 minutes retention—while modern mills target 15–30 minutes or more, since longer retention and larger area let more particulates settle (www.scribd.com). Oil skimmed at the top goes to an oil knockout tank; sludge collects in the cone and is periodically flushed.
Plants often standardize on purpose‑built clarifier vessels; the role mirrors industrial units such as clarifier tanks used to hold flow long enough for suspended solids to drop out.
Heating provisions and temperature targets
Oil cools quickly in large vessels, so mills install steam coils or jackets to keep the feed hot. In one three‑screw‑press line (~15,000 L/h), a 5,000 L crude‑oil surge tank is steam‑heated so oil reaches ≈95 °C before pumping into the CST; an open steam coil is specified because the ≈20 minute hold would not heat enough with a closed coil (www.scribd.com). Maintaining nearly 95 °C entering the CST counteracts cooling losses and ensures good separation (www.scribd.com; id.scribd.com).
Standard operating practice keeps oil around 90–95 °C across flotation filters, retention tanks, and the CST itself (id.scribd.com; repository.unsri.ac.id). An Indonesian mill SOP explicitly requires sand traps and the crude‑oil feed tank to be held at ~90 °C during operation (id.scribd.com). A process guide states that if oil entering the sand trap is below ≈95 °C it will not settle, whereas 95 °C yields maximum settling of heavy solids and separation of free oil (www.scribd.com).
Viscosity, settling rates, and separation
The physics are straightforward: palm oil viscosity drops sharply with heat, turning the flow closer to Newtonian behavior (viscosity remains constant with shear) at higher temperatures. One rheology study showed a non‑linear viscosity decrease as temperature rose even from 20–70 °C (www.researchgate.net). Practically, mills see this: palm oil viscosity at 95 °C can be in the single‑digit cP (centipoise) range vs ~100 cP around 80 °C (estimates), so even mild cooling degrades settling performance.
Data back it up. A sludge‑settling test reported maximum rates at 95 °C—0.2313 cm/s—compared with cooler runs (repository.unsri.ac.id). Trials in an Indonesian mill (feed 23 t/h, tonnes per hour) found that holding oil at 90 °C cut oil losses in the subsequent separator to ~0.6% of throughput (www.researchgate.net). Another report saw ~74% oil‑recovery efficiency in a prototype clarifier after raising feed temperature to 85–90 °C (www.researchgate.net).
Controllers and alarms often track CST temperature around 90–95 °C; some plants consider >90 °C “safe.” A recent Riau study maintained a CST at ~89 °C as an upper safe limit, using a microcontroller and DS18B20 sensor for remote monitoring (apic.id). Even 5–10 °C deviations visibly alter viscosity. Business impact follows: a 1% gain in separation efficiency from better heating can represent dozens of extra barrels per day in a 60 t/h mill.
In short, extended retention cannot compensate for high viscosity: a CST running at 70–80 °C will see much slower settling than one at 90–95 °C, and in practice “oil is never allowed to cool below ~85–90 °C” during clarification (www.scribd.com; id.scribd.com).
Vibrating screen pretreatment and mesh selection
Before oil ever touches the clarifier, mills install vibrating screens between the sand trap and the crude‑oil tank to remove coarse solids—fibers, empty shells, stray grit—and shield pumps and separators (id.scribd.com; cybex.id). Typical arrangements cascade apertures—say a 20‑mesh screen followed by 30–40 mesh—to trap everything bigger than roughly 0.5–1.0 mm, depending on the mesh (mesh denotes holes per inch) (id.scribd.com).
Operation is simple and relentless: diluted press oil flows from the screw press through a sand trap, then over vibrating decks oscillating ~1–3 mm and tipped so solids convey off the end. Evidence of sand or fiber buildup is checked routinely; screens are cleaned each shift to maintain flow, often with hot‑water sprays to keep oily fibers from clogging (id.scribd.com; noakmech.com). Many plants use automated devices comparable to an automatic screen for continuous solids removal. Others rely on robust coarse‑mesh deck systems akin to a manual screen where >1 mm debris is expected.
The sand trap step itself is pivotal (“crude oil dari screw press … untuk mengurangi jumlah [pasir]”), protecting the line from abrasive wear before the screens finish the job of catching fibers (“corpuscles”) (fr.scribd.com; cybex.id). The approach parallels physical separation trains in industrial water, such as wastewater screening and oil removal, adapted here for hot oil service.
Solids caught on the screen are periodically washed back—often by water injection—into the fiber slurry, recovering bound oil before it leaves the process (id.scribd.com; noakmech.com). By the time oil enters the clarifier, it is effectively “filtered” of grit and coarse fiber.
Performance outcomes across the line
Screening and hot settling combine for outsized yield gains. Case studies report that adding vibrasonic screens boosted oil recovery by ~5–10% of available oil (mff-oilfield.com). Properly maintained tanks—oil ≈90–95 °C, retention ~30 minutes—routinely achieve >90% oil recovery, with residual oil‑in‑water/sludge <0.5% (www.researchgate.net; id.scribd.com).
At the other end of the spectrum, “substandard” operation—cold oil, clogged screens—pushes oil to evaporation ponds and sludge, with losses often >1–2%. Indonesian mill guidelines therefore mandate continuous heating and robust screening in clarification to meet both process efficiency and environmental standards (id.scribd.com; cybex.id).
The downstream payoff is product quality. With upstream screening and hot settling in place—and after the full, multi‑stage clarification that includes centrifuges—final CPO (crude palm oil) impurities are cited at <0.002% (cybex.id). The pre‑clarifier screening role described here is consistent with automated industrial devices, as seen in an automatic screen, or simpler coarse options like a manual screen, depending on debris load and maintenance strategy.
