Air‑based separation is the core of pulse dry fractionation. Two distinct physical mechanisms govern particle sorting in air streams: size‑based classification and density‑based classification.
The integrated air‑classifier mill (grinding unit) performs particle‑size classification: it separates particles primarily according to aerodynamic diameter, to control the PSD of milled pulse flour (D90 10‑65 μm). Downstream dry‑fractionation air separators (turbo classifiers / density separators) perform density‑based separation, to split milled pulse flour into protein‑rich fraction and starch‑rich fraction.
Many process misunderstandings arise from confusing these two mechanisms. Size separation sorts for particle dimension; density separation sorts for material composition. Both use air flow, but their equipment design, physical principles, targets and limitations differ greatly.
Fundamental Physical Background
In moving air, particle aerodynamic behaviour depends on aerodynamic diameter, which combines:
- Geometric particle size
- Particle true density
- Particle shape
Size‑dominated classification: Density differences are minor; particle size is the dominant variable determining whether a particle follows air flow or is rejected by centrifugal force.
Density‑dominated separation: Particles have overlapping size ranges; differences in true density drive separation, even for particles of similar geometric size.
For pulse flour:
- Protein‑body aggregates: higher true density ~1.35‑1.45 g/cm³
- Starch granules: lower true density ~1.25‑1.30 g/cm³
Protein‑rich and starch‑rich particles overlap widely in geometric size after milling. This is why you cannot separate protein and starch simply with a size‑only classifier.
Classifying by Particle Size (Dynamic Air Classifier inside the Air‑Classifier Mill)
Principle
Rotating classifier wheel creates centrifugal force. Particles are separated mainly by aerodynamic size. For particles of similar density, larger particles experience higher centrifugal force and are rejected for re‑grinding; smaller particles pass through as finished product. Density differences between protein and starch have negligible influence on the cut‑point.
Key characteristics
- Primary target: Control finished flour PSD (D10/D50/D90, span). Produce well‑liberated pulse flour within D90 10‑65 μm.
- Cut‑point is tuned by classifier‑wheel rotational speed and process‑air volume.
- Output: One single bulk flour stream; no separation of protein vs starch. Oversized particles go back to grinding zone.
- Limitation: A large‑sized low‑density starch agglomerate will be rejected as “coarse”, and a small‑sized high‑density protein particle will pass through as “fine”. Size‑based sorting cannot isolate protein from starch when their sizes overlap.
Application location: Inside the air‑classifier mill, before dry fractionation. Its job is milling‑control, not protein enrichment.
Classifying by Particle Density (Dry‑Fractionation Air Density Separator)
Principle
Feed flour has already been milled to a controlled narrow PSD. Protein‑rich and starch‑rich particles have overlapping geometric sizes but different true densities. Under controlled air flow and centrifugal / gravitational force, particles with same size but different density follow different trajectories:
- Higher‑density protein‑rich particles: higher inertia; separate into protein‑enriched fine stream.
- Lower‑density starch‑rich particles: follow air flow more readily; report to starch‑rich coarse stream.
For density separation to work well:
Protein bodies and starch granules must be physically liberated (broken apart by milling). If they remain bonded in composite agglomerates, the particle’s bulk density becomes intermediate, and separation fails.
Key characteristics
- Primary target: Fractionate the flour into two product streams: protein concentrate and starch‑rich fraction.
- Pre‑requisite: Tight‑PSD feed flour (D90 10‑65 μm). If PSD is too broad, size effects override density effects and destroy separation efficiency.
- Tuning parameters: air velocity, rotor speed, feed dispersion, particle de‑agglomeration.
- Limitation: If particles are not sufficiently liberated, composite intermediate‑density particles go to both streams, reducing protein purity and recovery yield.
Application location: Downstream of the air‑classifier mill, for dry fractionation.
Side‑by‑Side Comparison Table
| Parameter | Size‑based air classification (mill‑integrated classifier) | Density‑based air separation (dry‑fractionation separator) |
|---|---|---|
| Dominant sorting variable | Particle aerodynamic size; density effect minimal | Particle true density; requires tightly controlled narrow‑size feed |
| Main purpose | Control milling PSD; produce spec‑compliant pulse flour | Separate protein‑rich from starch‑rich fractions; protein enrichment |
| Input material | Dehulled pulse kernels for fine grinding | Pre‑milled pulse flour (D90 10‑65 μm, well‑liberated) |
| Output streams | Single finished flour; oversize recirculated internally | Two product streams: protein concentrate + starch‑rich fraction |
| Critical tuning variables | Classifier wheel speed, feed rate, process‑air volume | Air velocity, dispersion efficiency, rotor speed; feed PSD quality |
| Pulse‑specific limitation | Cannot separate protein vs starch; only controls fineness | Poor performance if feed has broad PSD or un‑liberated composite agglomerates |
| Equipment location | Built‑into air‑classifier mill | Stand‑alone downstream dry‑fractionation unit |
Critical Practical Takeaways for Pulse Dry‑Fractionation
- Size classification does NOT equal density separation. The mill‑integrated classifier only makes the correct flour PSD; it cannot enrich protein.
- Density‑based separation will fail if the incoming flour PSD is out‑of‑spec:
- Too coarse: many un‑liberated protein‑starch composites.
- Too fine: excessive ultrafines, size effects dominate over density differences.
- The complete workflow must follow sequence:
Dehulling → Fine grinding + size‑based classification (air‑classifier mill) → density‑based air separation (protein‑starch fractionation).
- Even ideal density‑separation hardware cannot compensate for poor milling outcomes (bad PSD, un‑liberated particles, heat‑damaged agglomerates).
Size‑based air classification sorts particles mainly by aerodynamic dimension and is implemented inside the air‑classifier mill to produce narrow‑PSD pulse flour in the D90 10‑65 μm range. Density‑based air separation sorts particles by true material density, and operates downstream to separate liberated protein‑rich particles from starch‑rich particles for dry fractionation.
Density‑driven separation relies entirely on high‑quality output from the preceding size‑controlled milling stage. Confusing these two mechanisms is a common root‑cause of low protein‑concentrate purity and low recovery in pulse dry‑fractionation plants.
