Energy Management in Plastics Processing - Part 8 Drying
A series of energy efficiency worksheets by Dr. Robin Kent for the Carbon Trust to help the plastics industry reduce costs through efficient use of energy. UK Government Environment and Energy Helpline 0800 585 794

Drying

Drying uses large amounts of energy but is necessary for processing hygroscopic polymers (i.e. those that absorb water) and for repeatable processing of non-hygroscopic polymers. If a polymer is not dried, any moisture present will be converted to steam during processing and create surface marks or even weaken the moulding. However, simple measures can achieve significant energy and carbon savings during drying.

• Tip: Drying is a hidden cost. Find out the time taken, the optimum temperature required and the energy used.

• Tip: Good storage of materials will reduce their moisture content before drying.

• Tip: Reducing cycle times in warm dry weather will reduce energy use.

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Drying is a hidden cost and new technologies offer improved energy performance and reduced costs.

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Desiccant drying

Desiccant dryers pass moisture-laden air through a desiccant (a moisture-removing material) to produce warm dry air, which is then passed through the polymer. The air then removes moisture from the granules and is recycled back to the dryer for further drying and use.

The desiccant canister has to be regenerated using a high heat cycle to remove the moisture that it has adsorbed. A typical dryer uses continuously rotating desiccant canisters or valve arrangements to cycle the desiccant through the drying and regeneration stages. The typical time for drying using a desiccant is 4–6 hours. The most common desiccant used for plastics processing is crystalline alumino-silicate molecular sieve, which has a high affinity for moisture at very low dew points.

Heat exchangers and heat recovery

Conventional desiccant machines do not recover the heat lost from the dryer during the process and often incur cooling costs. The latest machines use integral heat exchangers to recover heat from the exhaust air and recycle this back to pre-heat the cooler dried air from the desiccant dryer. This process can improve the heat balance such that up to 56% of the input energy is used to actually dry the polymer. This almost doubles the efficiency of the system and significantly reduces energy use and costs.

• Tip: Units that have automatic desiccant regeneration controlled by dew point sensors or preferably by material moisture content are more consistent.
• Tip: The lower the dew point of the air supplied, the quicker the drying time – but this needs to be balanced against the frequency of regeneration and the energy used for this.
• Tip: Small spherical desiccant sieves give faster drying, better reactivation and greater adsorption.
• Tip: High reactivation temperatures improve reactivation and give greater adsorption in use.
• Tip: Optimise cycle times for the desiccant during drying to avoid overloading the desiccant and thus reducing process efficiency.
• Tip: Design desiccant drying systems to be ‘closed loop’ to exclude ambient air and obtain the lowest dew point.

The heat balance for a conventional dryer is shown below; only 34% of the total input energy is used to heat and dry the polymer. The rest of the input energy is lost before or after drying the polymer.

Energy flow in a conventional drying system

New technologies

Carousel drying

This is a lightweight rotating wheel carrying a fibreglass substrate that is impregnated with desiccant crystals. The wheel continuously rotates and passes the desiccant through the adsorption, regeneration and cooling cycles every 4.5 minutes. Control of the drying process is by adjusting the dryer speed and other variables.

The wheel has a low thermal mass that allows the use of lower regeneration temperatures than conventional systems whilst still achieving the necessary overall temperature for regeneration. The wheel also produces a lower pressure drop and this allows the use of smaller, energy efficient blowers.

Low pressure drying

Low pressure drying (LPD) uses a vacuum applied to the dryer cabinet to accelerate drying. The vacuum reduces the boiling point of water from 100°C to around 56°C, and water vapour is driven out of the granules even at low temperatures. LPD reduces drying times by up to 85%, reducing energy use by 50–80%. It also simplifies the process plant needed for effective drying, as desiccants are eliminated and no longer need to be regenerated and replaced. This provides a further opportunity to save money and energy.

The system is suited to machine-side drying of materials and rapid material changes. The short drying time enables a rapid start-up and the smaller batches of material reduce the cleandown and changeover times. LPD also reduces the risk of thermal degradation of the polymer by reducing both the heating cycle and the temperatures used.

Infrared rapid drying

Infrared drying uses infrared radiation to heat the polymer granules directly. The energy applied to the granules creates internal heating though molecular oscillation. This internal heat drives moisture out of the material into a stream of cool ambient air that removes it from the process.

The system uses a drum with an internal spiral feed to transport and agitate the material as it is carried along underneath the infrared heaters. The final moisture content of the polymer is controlled by a combination of the power of the infrared heaters and its residence time in the system.

Infrared drying is particularly suitable for drying reprocessed PET material because it can combine the processes of recrystallisation and drying in a single pass. Drying and recrystallisation times for PET can be reduced to less than 10 minutes, with an energy consumption as low as 120 watts/kg/hour for drying to a final moisture content of less than 0.005%.

The "Energy Management" series is designed to give plastics processors an insight into how to manage a valuable resource.

Download the complete series as an Adobe Acrobat file.

Last edited: 22/10/07  

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