Pairing multi‑stage air winnowing with water‑based hydrocyclones routinely pushes kernel recovery above 90% while driving shell contamination down to a few percent, meeting industrial specs. The payoff is kernel quality that protects value and standards compliance.
Palm kernels make up roughly 45–48% of the fresh fruit bunch (FFB) weight and contain about 45–50% oil (www.mdpi.com). After the nuts are cracked, the result is a mixture of kernel pieces and shell fragments that must be separated (www.mdpi.com).
Efficient separation maximizes kernel yield: one five‑stage winnowing system achieved kernel recovery of about 5.7–6.9 wt% of FFB with only ~0.11–0.30 wt% kernel loss, and the residual shell “dirt” in the kernels was about 4.56–6.03 wt% (www.mdpi.com). More generally, multi‑stage pneumatic (dry) separators routinely recover over 90% of kernels with only a few percent shell contamination (www.researchgate.net) (www.mdpi.com).
Figure: Cracked palm nuts yielding kernels (dark brown seeds) and shells (light fragments) before separation (www.mdpi.com).
Pneumatic winnowing columns
Winnowing (pneumatic air separation) exploits the weight difference between kernels and shell. In a vertical winnowing column, air is blown upward through the cracked mix: since kernels have higher bulk density and mass, they fall out of the airflow while lighter shell fragments are carried up and discharged (www.mdpi.com).
Modern systems use multiple cascading columns at different flow rates so that smaller, lighter kernels lost in the airflow can be recaptured downstream (www.mdpi.com). This multi‑stage approach significantly boosts purity: one study found pneumatic separation limited shell/fibre in the product to only ~2 wt% (compared to ~6–8% for hand or clay‑bath methods) (www.researchgate.net). The dry‑separated kernels were also low in moisture and free fatty acids (FFA, a quality indicator): kernels from the air‑separation stage had only ~3% FFA compared to 8% FFA from a clay bath (www.researchgate.net).
In large mills, initial winnowing often removes ~20% of the shell mass (www.mdpi.com), substantially reducing the load for downstream separation. In practice, such dry separation schemes have achieved up to ~97.5% raw kernel recovery from the cracked mix, yielding kernel purity around 92–95% (i.e., only 5–8% residual shell) (www.mdpi.com).
Hydrocyclone density separation
After pneumatic cleaning, the remaining heavy mixture (kernels plus shell fragments) is separated by density in water using hydrocyclones (accelerated gravity separators that create a tangential swirl). The heavier shell fragments (specific gravity, SG, ~1.15–1.20) are flung outward to the cyclone’s underflow, while the lighter kernels (SG ~1.07) migrate to the overflow (sawitindonesia.com) (www.mdpi.com). In practice, liquid density is often raised with clay/kaolin to about 1.12 so that kernels float (sawitindonesia.com).
Hydrocyclones are essentially an accelerated gravity separator requiring only clean water—no chemicals. They achieve very high separation efficiency: one study reported hydrocyclones alone bearing ~96% kernel recovery efficiency (www.researchgate.net). Because clean feed water matters, some facilities specify continuous debris removal ahead of the cyclone with an automatic screen.
Where clay/kaolin is used to raise liquid density to about 1.12, accurate chemical dosing can be handled by a dedicated dosing pump. After water separation, the kernels must be dried before sale; Indonesian standards limit kernel moisture to ≤8% (id.scribd.com). Between separation loops, clarifying recirculated water can be supported by compact equipment such as a lamella settler.
The three‑stage process—primary winnowing, secondary winnowing, then hydrocyclone—is common practice (news.kharisma-sawit.com). Removing shells in advance means the hydrocyclone handles a smaller fraction of material, reducing water use and enabling very clean separations in the final stage.
Combined stages and product quality
In practice, mills combine both methods to maximize yield and quality. A typical throughput might divert ~20% of light shells via winnowing and send the remaining 80% of nuts through hydrocyclones (www.mdpi.com) (news.kharisma-sawit.com).
The result is a kernel product with minimal contamination: pneumatic plus hydraulic methods routinely produce kernels with well under the 6% impurity limit specified by Indonesian SNI, which also demands ≤3% FFA and ≤8% moisture (id.scribd.com). The strongest separation stages reported shell content as low as ~2% (www.researchgate.net).
Clean separation is critical because residual shell or fiber dilutes oil content and can drive up measured FFA, damaging oil quality. Every percentage point of extra kernel recovered translates directly into more saleable oil or cocoa replacement (palm kernel cake). Figure: Cleanly separated palm kernels after drying—nearly free of shell and fiber, ready for oil extraction (meets standards of ≤6% impurities) (id.scribd.com) (www.researchgate.net).
Sources: Rohaya M. Halim et al., “Dry Separation of Palm Kernel and Palm Shell Using a Novel Five‑Stage Winnowing Column System,” Technologies 4 (2016) (www.mdpi.com) (www.mdpi.com); O.M. Akusu et al., “Palm Kernel Separation Efficiency and Kernel Quality from Different Methods…” J. Food Technol. Res. 4(2) (2017) (www.researchgate.net) (www.researchgate.net); Indonesian Standard SNI 01‑0002‑1987 Inti Kelapa Sawit (quality specs) (id.scribd.com); S. Luthfi, Optimalisasi Efisiensi Proses Pengolahan Inti Sawit, SawitIndonesia.com, 2018 (sawitindonesia.com); PT Kharismapratama Abadisejatindo, Kenali Lebih Dalam Hydrocyclone untuk Pabrik Kelapa Sawit, Jan 2024 (news.kharisma-sawit.com).
