Uniform steam, not brute force, is determining yield and energy bills. Newer horizontal designs and PLC/SCADA control are setting the benchmark on pressure–time precision, oil quality, and steam efficiency.
In palm oil milling, the sterilizer is where fresh fruit bunches (FFB, whole bunches harvested from the palm) meet high-pressure steam. Get that step right, and oil flows; get it wrong, and enzymes and cold spots inflate free fatty acids (FFA) and steam usage. The dominant contenders — horizontal and vertical batch vessels — treat the same fruit very differently, and the gap widens further when automation enters the picture.
Across designs, mills drive saturated steam to set pressures and temperatures for ~60 minutes to inactivate lipases, soften mesocarp (the oily flesh), and prep the bunches for threshing and pressing. But how uniformly that heat penetrates — and how tightly the cycle follows a programmed pressure–time curve — is what ultimately moves the needle on oil extraction ratio (OER), cost, and quality (Doingoilmachine Industry News).
Batch vessel design: horizontal vs. vertical
Horizontal sterilizers are long cylindrical vessels loaded with low-height cages so fruit layers stay shallow (typically ≤0.5–1 m). The low stacking height spreads fruit evenly, allowing steam to penetrate uniformly from multiple directions; condensate drains freely through the shallow layer, improving heat transfer and minimizing compression (which otherwise forces oil out prematurely) (Doingoilmachine). Horizontal systems typically use saturated steam at moderate pressure (often ~3–4 bar absolute), targeting ~120–130 °C for about 60 min, following a programmable single- or multi-peak curve; well-operated units report ~200–300 kg of steam per tonne FFB (model- and schedule-dependent) (Doingoilmachine).
Vertical sterilizers load FFB into tall pressure vessels (often without cages), creating several meters of stacking height. Heat penetrates slowly through the deep pile as trapped air resists steam, and the fruit column impedes drainage; engineers often resort to “triple‑peak” cycles (raise pressure, cut back to vent air/condensate, reheats) at higher pressure (~4 bar, ~143 °C) to compensate. Reported steam use runs ~305–355 kg/tonne FFB with ~60‑minute cycles (Doingoilmachine) (Doingoilmachine). The heavy fruit column and slower drainage mean EFB (empty fruit bunches) and condensate can soak up oil, lowering extraction efficiency and often necessitating extra fiber reprocessing (pressing or solvent extraction) at added energy and quality cost (Doingoilmachine).
Overall, horizontal sterilizers dominate modern mills due to energy/yield advantages and more favorable space‑speed trade‑offs (Doingoilmachine) (ResearchGate). One Indonesian case study of a new 45 t/h mill using a horizontal‑indexer sterilizer cited positive NPV and 26.6% IRR as evidence of feasibility and preference over vertical or continuous designs (ResearchGate).
Steam and condensate handling in these high‑pressure cycles places a premium on clean, stable loops; engineers commonly specify high‑pressure housings when filtration is required, for example steel filter housings rated for industrial service.
Continuous sterilizer trade‑offs and boiler load
A third variant, the continuous sterilizer (including crush‑and‑steam designs), was developed to avoid batch swings. Here, pre‑crushed/flattened FFB move through a steam‑heated chamber at near‑atmospheric pressure. The approach smooths boiler load because it avoids large pressure peaks; however, systems still must thoroughly destroy enzymes, often with a second high‑temperature post‑heating stage (Doingoilmachine).
In practice, continuous lines run at lower pressure (~1–2 bar) and temperature (~98 °C) but very long residence times, making them the least steam‑efficient: prolonged heating plus mechanical power (e.g., a 60 t/h unit might need ~200 kW for handling) drives consumption higher than batch types (Doingoilmachine) (Doingoilmachine). Since most mills run intermittently (start/stop each day), the buffer storage in batch cage systems (as used with horizontals) helps even out supply — an advantage continuous chains lack (Doingoilmachine).
Complete and uniform sterilization outcomes
Uniform sterilization — complete enzyme deactivation and consistent cooking across the entire batch — is essential for yield and oil quality. Air pockets must be removed in the deaeration phase because trapped air acts as a barrier to heat transfer, creating cold spots. Over‑sterilization (too hot or long) also degrades quality, increasing oxidation, FFA, and poorer bleachability; kernels can discolor at high temperature (FAO) (FAO).
The payoff is measurable. In a lab study optimizing steam cycles (100–120 °C for 20–80 min) at different ripeness, crude oil FFA ranged between 0.62% (underripe fruit) and 3.16% (loose fruits), all below common trading specs (often ≤5% FFA) (ResearchGate). Field data show FFA in FFB rises with handling delays and high ripeness; detached loose fruits can reach >3% FFA even with good processing (ResearchGate).
On yield, OER (oil extraction ratio; share of oil as a percentage of FFB weight) is typically ~23–24% from good Tenera palms (FAO), but well‑run mills optimizing sterilization have reported oil yield approaching 25–30%. One synchronized medium‑scale mill study achieved as high as 29.3% with optimized operations (ResearchGate). Uniformity also supports throughput: under‑heated fruit forces longer or harder digestion, while over‑heated masses leak oil into EFB, raising losses and muddying press liquor. Industry sources call sterilization “critical,” with “intelligent” cycles maximizing capacity and profits (FAO) (DPSTAR).
Automated control: PLC/SCADA and valve sequencing
Automation underpins repeatable, efficient sterilization. PLCs (programmable logic controllers) and SCADA (supervisory control and data acquisition) systems run programmable pressure–time curves — including multi‑peak or triple‑peak profiles — by comparing live pressure against a setpoint curve (via PID, proportional–integral–derivative control) and modulating the main steam valve accordingly. The same PLC sequences exhaust, vent, and condensate valves by time or pressure, replacing manual toggling with digital precision (pdfcoffee.com).
Malaysian suppliers such as DPSTAR report delivering Omron PLCs with touch‑screen SCADA for sterilizer lines; the systems integrate weight sensors (to detect full cages and trigger cycles), multiple pressure/temperature transducers, real‑time logging, and alarms for abnormal conditions. Plant‑wide SCADA can monitor all sterilizers and the on/off/trip status of motors, alerting if loads exceed setpoints (DPSTAR) (DPSTAR).
Benefits are both qualitative and quantitative: tighter regulation reduces cycle time variability and steam wastage, and it frees operators from manual valve control. A related case showed that automating digester valves via current‑monitor feedback cut manual errors and improved oil recovery — an analogous gain to expected sterilizer automation outcomes (Ultech Engineering). Analysts emphasize that integrated PLC/SCADA “improves end‑product quality, reduces wastage and keeps production cost low” (DPSTAR).
Because these cycles depend on stable steam, mills also scrutinize steam purity and condensate return. Where condensate quality must be maintained after heat exchange, operators specify condensate polishers to protect the loop.
Key operational metrics and targets
Sterilization typically consumes 30–60% of a mill’s steam, making it a major lever on energy cost (Doingoilmachine). Horizontal batch sterilizers often use ~200–300 kg steam per tonne FFB, whereas vertical triple‑peak cycles can reach ~305–355 kg/t at ~4 bar and ~143 °C (Doingoilmachine). Continuous designs run ~1–2 bar and ~98 °C with long residence times, requiring prolonged heating and extra power (e.g., ~200 kW for a 60 t/h line), and have been reported as the least steam‑efficient among the three (Doingoilmachine) (Doingoilmachine).
On yields and quality, OER targets ~23–24% for Tenera under good conditions, with modern optimized operations reporting up to ~29% in specific studies (FAO) (ResearchGate). Post‑sterilization FFA should stay below ~5%; lab optimizations report ~0.6–3.2% depending on fruit ripeness and timing (ResearchGate).
Programmed cycles, repeatability, and economics
Well‑run horizontal systems typically require ~3–4 bar and ~120–130 °C for ~60 min to achieve uniform heating with lower compaction; vertical designs generally exceed this pressure (reported ~4 bar, 143 °C with triple peaks) to compensate for poorer heat transfer, raising steam use and reducing oil recovery efficiency (Doingoilmachine) (Doingoilmachine). PLC‑based sequencing helps every batch follow the same optimized pressure/time program, and plant SCADA pinpoints leaks or inefficient cycles quickly, which industry analysts link to improved end‑product quality, lower wastage, and lower cost (DPSTAR).
Sources and references
Claims and numbers above are supported by: peer‑reviewed studies quantifying time/temperature effects and yields; industry analyses on steam usage and design trade‑offs; FAO processing guidance; and supplier documentation of PLC/SCADA controls. All URLs have been embedded inline where used.
- Doingoilmachine Industry News (multiple sections cited, including steam shares, pressure/temperature, and cycle types)
- FAO: Palm Oil Processing (uniformity, oxidation, OER benchmarks)
- Jusoh et al., 2015 (ResearchGate) (FFA outcomes by ripeness and timing)
- Morakinyo & Bamgboye, 2016 (ResearchGate) (vertical sterilizer optimization; ~29.3% yield in synchronized operations)
- Automation of Palm Oil Mills (pdfcoffee.com) (PLC/PID setpoint tracking and valve sequencing)
- DPSTAR control systems and BPV/SCADA case (PLCs, SCADA integration, alarms)
- Additional references noted in the source paper: Sivasothy, K. & Lim, N.B.H. (1985). Automation of Palm Oil Mills. J. Am. Oil Chem. Soc. 62(2): 268–.
