Polymer Drying

Niro has supplied over 500 industrial dryers for drying of polymers since 1950. The polymer industry therefore benefits from our product expertise as well as from the transfer of value added technology from other drying applications.

When polymer drying involves wet powders (particles already created as a result of slurry polymerization or precipitation), water or organic volatiles are removed in flash and / or fluid bed dryers (figures 1 and 2).

Figure 1 Fluid bed dryer with contact heating. Contact Fluidizer™, CFD

Figure 2 Flash Dryer with Fluid Bed post Dryer, FLD + FBD

Applications: ABS, HDPE, MBS, PAN, POM, s-PVC, c-PVC, c-PE

Characteristics

• Particulate feeds
• Low specific energy consumption
• Low product processing temperature
• Long product residence time, gentle drying
• Average particle size range 50-800 μm
• Two-stage drying, cross-current flow mode

Applications: ABS, MBS, PAN, s-PVC

Characteristics

• Low specific energy consumption
• Short product residence time, rapid drying (FLD)
• Long product residence time, gentle drying (FBD)
• Average particle size range 50-800 μm
• Two-stage drying, co-current + cross current flow modes

When polymer drying involves formation and drying of particles of specific size and structure (polymers from emulsion and solution processes), spray dryers are used, often combined with fluid bed post-drying and cooling (figures 3, 4, and 5). When processing involves molten polymers, spray congealing is adopted in plant designs that resemble spray dryers.

Figure 3 Spray Dryer with Rotary Atomizer, SD-R

Figure 4 Spray dryer with pressure or two-fluid Nozzle atomizer(s), TALL FORM DRYER™, TFD-N or -2N

Applications: e-PVC, EVA, MF, PF, UF, PMMA, PVAc

Figure 5 Fluidized Spray Dryer (FSD™) with Rotary or Nozzle atomizer, Fsd™, FSD-R or -N

Characteristics

• Fluid feeds
• Single atomizer unit
• Average particle size range 30-125 μm
• Co-current flow mode
• Bag filter preferred powder collector

Applications: e-PVC, PMMA, PVP

Characteristics

• Fluid feeds
• Multiple atomizer assembly
• Average particle size range 15-250 μm
• Co-current flow mode
• Bag filter preferred powder collector

Applications: PMMA, specialties

Characteristics

• Agglomerated products, no fines fraction
• Fines recycled within dryer
• Average particle size range 100-300 μm
• Two drying stages
• Mixed flow mode
• Bag filter preferred powder collector

State-of-the-Industry

Today dry product rates already exceed 35 tons per hour for some of the wet powders and 5 tons per hour for some of the emulsions and solutions. Niro is able to supply single continuous-running drying units for these capacities. For the wet powders the drying equipment is fluid bed dryers or flash dryers and in some cases a combination. A special high-energy efficient fluid bed is the Niro Contact Fluidizer™, where 80-85% of the evaporative energy is supplied from heating panels submerged in the fluidized product layer. The overall energy saving compared to other types of dryers, for instance for s-PVC drying, is 10-45% depending on the grade in question. For emulsions and solutions the drying equipment is a Spray Dryer, often followed by fluid bed post drying or cooling.

When the polymers are water wet, which is the case for most of the polymers, open drying plants are often applied, meaning that ambient air is taken in, heated up, performing drying, cleaned of particles and sent back to the atmosphere. When the polymers are solvent wet (for example hexane/pentane wet HDPE or PP), the concentration of oxygen has to be very low in order to avoid explosion/fire in the dryer. Consequently, Nitrogen is often used as drying gas. In order to save cost, the Nitrogen is reused in a closed loop: it is heated up, performing drying, cleaned of particles, the evaporated organic solvent is condensed and pumped away, after which the Nitrogen gas is re-circulated to the inlet heater and used for drying once more.

Dryer Cleaning

Dryers are designed for continuous operation, with scheduled shutdowns for cleaning and grade changes. Automatic Cleaning-In-Place (CIP) systems can be incorporated in all dryers when required.

Dryer Capacities

Scale-up of polymer dryers presents no problems. Both present day and expected future process stream rates can be handled in single dryer units. Current dry product rates already exceed 35 tons an hour for slurry polymers, and 5 tons an hour for emulsion polymers.

Residual VOCs

Stripping of Volatile Organic Compounds from dry polymers is a growing requirement, and could well become a general processing standard due to concern about the release of VOCs during handling, storage and fabrication. Both integrated and separate stripping stages can be included in the dryer design.

Energy Savings

Contact heating (heating elements) can be incorporated in fluid beds to great advantage.

Heat recovery and partial recirculation of the drying medium reduce energy consumption in spray and flash dryers.

Operational Safety

Polymers in powder form are combustible and can form explosive mixtures in air. The selection of the safety concept is product specific and involves risk analysis procedures.

Water wet polymers can be dried using air as the drying medium. Plants are designed to eliminate possible ignition sources and operate under conditions that keep air-powder mixture concentrations low. Installation of explosion relief or suppression systems operating in combination with automatic fire extinguishing arrangements provides additional safety protection.

If a product grade presents a high powder explosion risk, drying can be safely carried out in self-inertizing recycle layouts. Inertizing is created by use of a special direct fired heater with venting of small volumes of exhaust drying air to maintain system equilibrium. This layout is ideal for preventing VOC emissions, since the small vent volume can be economically incinerated in an integrated heater system.

Solvent wet polymers are dried in closed cycle dryers operating with inert gas (Nitrogen) as the drying medium.

Environmental ProtectionDust Emissions

Cyclones operating alone or in combination with wet scrubbers meet emission requirements from flash and fluid bed dryers. Bag filters are required for spray dryers, since they are normally producing a smaller particle size.

VOC Emissions

Compliance with environmental regulations is often achieved by stripping of the wet feed. When this is insufficient or impractical, the exhaust air can be purified by adsorption or by catalytic or thermal incineration. If self-inertizing layouts are used for operational safety, incineration can be carried out in the direct fired heater incorporated in the layout (see above).