Key takeaway: Palm kernels are dense, hard, and high in saturated oil; the crushing process must minimize the generation of excessive fines. In contrast, soybeans, peanuts, and rapeseed are relatively soft and can tolerate some fines; the primary goal here is to flake the material.
1. Palm Kernels
Texture: Dense and hard (like hard nuts) with high compressive strength; oil content is approximately 48%–52%, with high saturated fatty acid levels, making them prone to stickiness at low temperatures.
Moisture Control: Must be dried to 6%–8%. High moisture softens the kernels, causing them to stick to rollers and clog equipment during crushing; excessively low moisture causes them to shatter into excessive fines.
Crushing Objective: To break the kernels into 4–8 fragments while minimizing the production of fines. Fine particles can clog the press cage and wash away with the oil, leading to reduced oil yield and cloudy crude oil.
Preferred equipment: Double-roll crusher (toothed rolls); utilizes compression and shearing, resulting in minimal fines. Exercise caution with hammer mills, as they produce excessive fines.
2. Soybeans
Texture: Tough yet relatively soft; low oil content (17%–20%) and high protein content.
Objective: Break into small particles; the subsequent key step is flaking to disrupt protein cells. Fine particles are permissible.
Equipment: Toothed roll crusher; proceed directly to flaking after crushing. Usually requires a conditioning stage (adjusting temperature and moisture to enhance plasticity).
3. Peanut kernels
Texture: Soft; high oil content (45%–50%); oil tends to be liquid at room temperature.
Objective: Break into 4–6 segments; suitable for direct steaming/roasting and pressing. Higher tolerance for fines compared to palm kernels.
Note: Peanuts tend to release oil and stick to rollers, but they do not cake as severely as palm kernels.
4. Rapeseed (Canola)
Very small seeds; brittle texture.
Generally not crushed separately; proceeds directly to flaking. Crushing tends to produce powder, causing dust issues and oil loss.
Palm kernel pretreatment:
Cleaning/impurity removal → Double-roll crushing → Conditioning (mild temperature adjustment) → Steaming/roasting → Screw pressing.
Palm kernels are rarely flaked thinly; they are usually crushed into chunks and pressed after steaming/roasting. Key control parameter: Fines content.
Soybean route:
Cleaning → Crushing → Conditioning → Flaking (rolling into thin flakes) → Steaming/roasting → Pressing/solvent extraction.
Crushing prepares the beans for flaking; flaking is the core process step.
Peanut route:
Cleaning/shelling → Crushing → Steaming/roasting → Pressing.
Simple process; the conditioning stage can be omitted.
1. Palm kernels: High hardness, requiring superior roll wear resistance; roll gap settings must be precise, as a gap that is too narrow results in immediate pulverization. Hammer mills are not recommended for primary crushing; they are suitable only for biomass pulverization.
2. Soybeans / Peanuts: Soft materials resulting in low roll wear; wider gap adjustment range and higher tolerance for operational variance.
Palm kernels: Extremely narrow moisture content window—moisture >9% causes sticking to the rolls, while <5% produces excessive fine powder; residual shell fragments in the raw material cause severe roll wear, necessitating thorough pre-processing to remove shells.
Soybeans: Higher moisture content merely increases material toughness rather than causing sudden, massive powder output; plasticity can be adjusted through specialized softening processes.
Peanuts: Oil is released after high-temperature roasting; the main crushing challenge is sticking to the rolls, without the abrasive wear issues caused by the hard particles found in palm kernels.
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