Gravity vs pneumatic transfer for PE/PP granulate

The fundamental question

When PE or PP granulate moves from FIBC big-bag into a silo trailer, the engineering choice is binary:

• Gravity transfer — bag lifted over compartment inlet, granulate falls by gravity into the tank
• Pneumatic transfer — granulate vacuumed or blown through pipes from a hopper into the silo trailer

Both methods move the material. The difference is what happens to the material during the move.

Premium chemical producers (LG Chem, Borealis, Synthos, SABIC, INEOS) contractually require gravity transfer for their FIBC-to-silo step. This is not preference. This is documented in their quality manuals because the difference is measurable and the impact at downstream customers is real.

What pneumatic transfer does to PE/PP

Fractionation

PE and PP granulate ships with a specified pellet size distribution — typically 3–5 mm diameter spherical or cylindrical pellets, with under 0.5% fines (particles below 0.5 mm).

Pneumatic transfer accelerates particles to velocities of 15–25 m/s. At these speeds, pellet-on-pellet collision and pellet-on-pipe collision creates:

• Fines generation — fragments of pellets, typically 0.1–1 mm
• Surface degradation — outer skin of pellet eroded
• Heat generation — friction causes localized heating, especially at elbows

Result: fines content rises from <0.5% to 1–3% per pneumatic cycle. The downstream customer either accepts degraded material or invests in additional pre-process screening.

Angel hair and snake skin

A specific subset of degradation products from pneumatic conveying:

Angel hair — thin (0.1–1 mm) polymer filaments, 5–50 mm long. Formed when softened pellet surface in elbow contact “draws” into a fiber. Clogs filters and dosing equipment.

Snake skin — flat polymer flakes from pipe wall coating buildup that releases. Visually distinct from pellets, more disruptive to downstream processing.

Both are inevitable in pneumatic systems with elbows over a certain length. Mitigation (bend protectors, optimized routing) reduces but does not eliminate the issue.

Material loss

Industry-documented typical loss per pneumatic transfer: 0.5–2% by weight. Distribution:

• 60–70% in dust filters and bag filters
• 20–30% in cyclones
• 5–10% in pipe deposits (slowly accumulates, periodically released)

For a 24-ton silo trailer load: 120–480 kg of material lost per transfer. At PE/PP commodity prices (€1200–1800/ton range historically), that’s €144–864 of material lost per cycle. Multiplied by hundreds of transfers per month for a major importer: significant.

Contamination from previous materials

Pneumatic systems are difficult to clean completely. A pneumatic system that previously handled:

• A different polymer grade: residual particles can mix into new batch
• A pigmented grade: color contamination of subsequent clear grade
• A mineral material: dust contamination of subsequent polymer
• Any aged material: oxidized particles can release into fresh material

Industrial-standard cleaning of pneumatic systems involves blowing through with clean polymer for “purge” — typically 200–500 kg of material discarded between grade changes. Even then, complete decontamination is not always verifiable.

What gravity transfer does (and does not) do

What it does

• Granulate falls 1.5–3 meters from FIBC inlet to compartment bottom
• Pellet velocity at impact: low (3–5 m/s), well below threshold for fragmentation
• No air contact during transfer — granulate is in atmospheric air, not high-velocity airstream
• No pipe contact — material moves through open space, then directly into compartment

What it does not do

• Does not damage pellets — measurable fines content after gravity transfer typically <0.1% above incoming spec
• Does not generate angel hair or snake skin — there is no surface to draw fiber from
• Does not lose material — losses are limited to negligible spillage during operator handling
• Does not contaminate from system residue — no shared system between batches

What it requires

Gravity transfer requires infrastructure designed for the operation:

• Sufficient ceiling height to lift FIBC above silo trailer inlet (typically 4–5 meters clear above compartment)
• Lifting equipment capable of handling FIBC weight (typically 1000 kg, requires 5-ton forklift or crane minimum)
• Silo trailer compartment access (proper inlet positioning, hatch opening)
• Operator trained in FIBC handling — opening procedure, spout management, foreign matter detection

These requirements explain why pneumatic transfer is sometimes chosen: pneumatic systems can be installed in lower-ceiling facilities, with smaller forklifts, and require less operator skill per batch. The trade-off is material quality.

Quantified comparison

For a typical PE/PP commodity grade, comparing handling methods on a 24-ton batch:

For commodity applications where the next process step is energy-intensive (extrusion of plastic film, blow molding) — the downstream cost of degraded material exceeds the upstream cost saving of pneumatic transfer. This is why premium producers specify gravity.

When pneumatic transfer is acceptable

Pneumatic transfer makes sense for:

• Internal plant transfers — moving material between silos within a factory, where total distance is short and the customer accepts some degradation
• Recycled grades with high incoming fines — material already at the spec limit for fines; further pneumatic transfer marginal impact
• Commodity grades for thick-section molding — large injection-molded parts (crates, pallets) tolerate higher fines
• Bulk transfer at originating terminal — silo-to-silo at port, where the end customer is the silo trailer (not the factory)

For final-stage FIBC-to-silo handling for premium customers — gravity remains the specified method.

Industry context

The decision between gravity and pneumatic transfer is not a technology preference. It reflects supply chain economics:

• Asian PE/PP producers ship spec-compliant material in FIBC
• European end customers (extruders, molders) signed quality contracts with these producers
• The handling step in Europe is the variable — if it damages spec, end customer either rejects material or claims against producer
• Producers therefore specify which terminals can handle their material — typically gravity-only terminals

Chorula serves this niche specifically. The terminal exists because Asian producers and European end customers needed a documented, ISO-compliant gravity transloading point at the right geographic location.

What this means for buyers

If you are sourcing PE/PP from Asia for European delivery:

1. Check end customer spec — does it specify handling method, or only material quality? Premium end customers increasingly specify both.
2. Verify terminal capabilities — pneumatic systems are common at ports; dedicated gravity terminals are rarer. Ask for documentation.
3. Account for material loss — pneumatic handling adds 0.5–2% to material requirement. For long-term contracts, this is significant.
4. Audit QC procedures — terminal should provide documentation supporting downstream QC at end customer.

For shippers prioritizing speed and cost over quality: port terminal pneumatic transfer is faster and cheaper.

For shippers prioritizing quality and contractual compliance: gravity transloading at dedicated terminals like Chorula.

Contact

For inquiries about gravity transloading for PE/PP/PA imports:

• Phone: +48 664 135 005
• E-mail: biuro@magnumchorula.pl
• Address: ul. Kościelna 9, 47-316 Chorula, Poland

We provide:

• Standard transloading per ISO 9001:2015
• Documented quality procedures
• Site visit possibility for QC verification by end customer
• Custom protocols for major chemical producer specifications

Related: Big-bag to silo trailer — workflow · Container to silo trailer — for importers · Batch traceability · Visit our facility · Contact