📖 About 3 min read
Dense and dilute phase conveying differ in gas demand, solids loading, velocity and operating behavior. Neither mode should be assigned to every product of a given polymer name. DYPNF Co., Ltd. (formerly Dong Yang P&F) reviews each line against its material, capacity, route and product quality target.
| Category | Pneumatic Conveying |
|---|---|
| Primary cause | Gas demand, solids loading and conveying velocity differ by mode, changing pressure demand and the stress placed on the product |
| Key variables | Material properties, minimum, normal and maximum rates, route and elevation, allowable pressure, quality limits |
| Fix level | Conveying mode selected line by line, with the conveying gas chosen alongside it |
| Verification method | Design calculations for the new line and, where behavior or quality is uncertain, tests with representative material |
| Reference base | Dense phase and dilute phase lines supplied for polymer fluff and pellet duties, including both modes within one plant (Hanwha Total ADL Project, HDPE/LLDPE, 400 KTA) |
| Related standard | Project- and material-specific; conveying gas selected with the process requirement |
Understanding the Flow Modes
In dilute phase conveying, particles are generally transported in a gas stream at a velocity sufficient to maintain reliable flow. Dense phase generally uses a higher solids-to-gas ratio and lower conveying velocity. Dense phase flow patterns vary with the material and system design.
| Design issue | Dilute phase | Dense phase |
|---|---|---|
| Gas and solids loading | Generally more gas per unit of solids | Generally higher solids-to-gas ratio |
| Conveying velocity | Generally higher | Generally lower |
| Pressure | Depends on duty and route | May need a higher differential and rated equipment |
| Product quality | Control velocity, bends and product impact | Lower velocity may reduce damage; confirm stable transport |
| Operation | Maintain reliable transport at all rates | Review start-up, stopping, clearing and restart |
Why the Material and Duty Matter
Pellets vary in size, shape, temperature and surface condition. Fluff can have low bulk density or cohesive behavior that changes feeding and transport. Even grades in the same polymer family can behave differently. Lower pellet velocity may reduce impact, fines and streamer formation, but dense phase is not automatically preferable. A suitably designed dilute phase line can also protect sensitive pellets.
Different lines in the same plant can therefore call for different modes. In the Hanwha Total ADL Project (HDPE/LLDPE, 400 KTA), dense phase and dilute phase conveying were both applied within the same plant, with each line designed for the material and duty it carries.
Inputs to the Selection
Define representative material properties; minimum, normal and maximum rates; route and elevation; allowable pressure; receiver and filtration; quality limits; and start-up, shutdown and restart cases. Where behavior or quality is uncertain, tests with representative material can establish feasible operating conditions. Reference projects support comparison but do not replace the new line’s design calculations or tests.
Choosing the Conveying Gas
Gas selection follows process requirements. Nitrogen may be specified for oxygen control or inerting. Moisture control also requires an appropriate gas condition and system design: nitrogen by itself does not establish a dry environment. Select the gas and conveying mode together, then define the proposed velocity range, pressure demand and quality verification method.
Frequently Asked Questions
Is dense phase conveying always better for pellet quality?
No. Lower pellet velocity may reduce impact, fines and streamer formation, but dense phase is not automatically preferable. A suitably designed dilute phase line can also protect sensitive pellets.
Can the conveying mode be decided from the polymer name alone?
No. Pellets vary in size, shape, temperature and surface condition, and fluff can have low bulk density or cohesive behavior. Even grades in the same polymer family can behave differently, so each line is reviewed against its own material, capacity, route and quality target.
When is nitrogen specified instead of air?
Nitrogen may be specified for oxygen control or inerting. Moisture control also requires an appropriate gas condition and system design, because nitrogen by itself does not establish a dry environment.
Can one plant use both dense and dilute phase conveying?
Yes. Different lines in the same plant carry different materials and serve different duties. In the Hanwha Total ADL Project (HDPE/LLDPE, 400 KTA), dense phase and dilute phase conveying were both applied within the same plant.
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Engineering teams defining the conveying mode for a polymer line are welcome to review material properties, rates, route and quality limits with DYPNF.