Desiccant dryers move hot, dry air over the resin to remove moisture from the pellets. Drying systems are typically a closed loop in which the moist air is recirculated through a desiccant that removes the moisture from the air. The desiccant is then regenerated by heating it to remove the moisture. Moisture level is measured by dewpoint; a lower dewpoint indicates less moisture. While the industry standard for very dry air has been a dewpoint of -40°C (-40°F), some technologies achieve even lower dewpoints. Desiccant dryer designs include the traditional multiple-bed or twin-tower bed arrangements, and the newer desiccant rotor or wheel.

 

Traditional desiccant bed systems are considered to be very reliable and user-friendly. Manufacturers of desiccant bed dryers have introduced improvements in efficiency and consistency. Dri-Air's system uses temperature rather than time or dewpoint to control desiccant bed regeneration, and has an automatic temperature set back to eliminate over-drying. "Bed regeneration with temperature control is very efficient because it looks at what is actually happening inside the beds and prevents temperature spikes caused by too frequent switches," explains Manny Salinas, regional sales manager at Dri-Air Industries, Inc. Universal Dynamics PCT-2 twin-tower drying system uses a patented airflow design to allow seamless bed changeovers, which is an improvement over typical twin-tower systems, comments Bob Crawford, vice-president of engineering at Universal Dynamics, Inc. AEC's twin-tower HiCore™ (heater in core) technology places the electric heating element inside the hollow core of the desiccant canister. This design uses energy more efficiently, which lowers energy costs by 10 to 15% and allows regeneration and cooling within one hour instead of the four to eight hours needed in traditional twin tower designs, says Kaihan Tavakoli, product manager for materials handling at AEC. Novatec's new NovaTouch™ touch panel PLC controller uses thermocouples on the hopper to monitor air inlet and outlet temperatures constantly. When a pre-set, optimum drying temperature is reached, the ‘Energy Saver’ feature shuts down the process heater, but not the blower, until a temperature drop is detected on either the inlet or outlet air duct, explains Joe Rich, national sales manager at Novatec.

 

Continuous desiccant rotors, also called wheels or honeycomb rotors, were first introduced more than ten years ago by Matsui America, and dryer manufacturers note an increase in use of this technology in the last three to four years. "We see increased use of desiccant wheel technology. The wheel saves physical space, has better consistency, and has fewer moving parts, resulting in less downtime," says Graydon Griesse, vice-president of Bry-Air Systems, which introduced its Brysorb Plus Desiccant Media wheel in 1995 and gradually phased out its multi-bed driers. Honeycomb driers dry in less than half the time of desiccant bed driers and are popular globally, says Sam Rajkovich, dryer manager at Comet Automation Sytems, Inc., which introduced its Honeycomb Matrix Dryer two years ago. The desiccant in a honeycomb rotor or wheel can be a molecular sieve or a silica gel, which does not generate dust like granular desiccants and can last between 5 and 15 years, depending on the type of desiccant, say dryer manufacturers. The desiccant in a wheel is more concentrated and takes less time to heat to process drying or regeneration temperature than the desiccant in a bed system. Because there are no bed changes, a rotor design gives consistency in temperature and dew point. Avoiding dew point spikes reduces the chance of over-drying and degrading the resin. "The rotor is the most efficient at delivering a specific dewpoint. While -40°C (-40°F) is typical, you can reduce costs if you only need -23 or -29°C (-10 or -20°F)," comments Mr. Griesse. "Customers using honeycomb rotors are seeing cost savings in using less electricity, less frequent maintenance and less frequent desiccant media replacement," adds Mr. Rajkovich. John Smith, technical sales manager at Conair, reports that Conair's new Carousel Plus wheel has an energy saving of 25% over Conair's indexing carousel desiccant bed system and 40% over twin tower systems. Conair expects rapid acceptance of the wheel in the marketplace and plans to transition all its dryers to this technology. Wittmann offers both DryMax twin bed and CDD continuous desiccant wheel dryers.

While compressed air dryers are finding use primarily in smaller throughput operations, large volume compounding operations may use them as ‘maintenance’ dryers mounted on the throat of the extruder or as dryers for additives. Compressed air dryers have a small footprint and are virtually maintenance-free, says Novatec, which markets a patented compressed air/membrane NovaDrier™. The largest NovaDrier runs 68 kg/hour (150 lb/hour). The cost of compressed air can limit the practical size of compressed air driers, although more efficient units are being planned, explains Mr. Rich.

 

Vacuum dryers, which use a vacuum to pull moisture out of the resin, are the most recent technology to be promoted for resin drying and are expected to have high demand globally as processors see their advantages, say suppliers. While vacuum drying has been used in the past in plastics and other manufacturing processes, Maguire says that its technology, introduced five years ago, is the first affordable, reliable, and user-friendly system for resin drying. Maguire's LPD™ (Low Pressure Dryer) has three indexing stations for heating and filling, vacuum drying, and discharging. Creating a vacuum reduces the boiling point of water, turning moisture in the pellets into water vapour that is evacuated out of the dryer canister, explains Pat Smith, vice president of marketing and sales at Maguire. The primary benefit of vacuum technology is drying speed in one-sixth the time of desiccant bed dryers. "While desiccant dryers may take four hours to dry material, the LPD only takes 40 minutes," says Mr. Smith. In addition to the obvious manufacturing efficiency advantages, shorter drying times also reduce the risk of degrading materials by reducing the time that they are exposed to heat. Vacuum dryers can also use lower drying temperatures, with PET drying at 149°C (300°F) rather than 177°C (350°F), reports Mr. Smith. Vacuum dryers completely eliminate the risk of over-drying and embrittling nylon. The system runs at 93 to 94% of full vacuum, and the moisture level inside and outside the pellets reaches an equilibrium at a fraction of a percent that is above the level where there is a risk of embrittlement, explains Mr. Smith. Vacuum dryers offer huge energy savings because no energy goes into desiccant regeneration. The LPD used up to 80% less electricity in customer comparisons to their standard desiccant beds, reports Maguire. Maguire introduced its smaller LPDs five years ago, and this summer is commercializing the LPD-1000, for rates up to 454 kg/hour (1000 lb/hour). Units targeted for 227 kg/hour (500 lb/hour) and 707 kg/hour (2000 lb/hour) are planned. Maguire is also developing a horizontal vacuum dryer to handle powders, such as additives or wood flour, that need to be agitated inside the dryer. "We see a huge potential in dryers for wood flour, which is currently dried using augers with hot oil at temperatures above the flour's flash point. Vacuum dryers can operate below the flash point, reducing the risk of explosion," explains Mr. Smith.

 

AEC is developing a vacuum dryer to be commercialized in the first half of 2006, says Mr. Tavakoli. AEC's design will include heaters inside the vacuum canisters for drying of difficult materials such as PET. Vacuum dryers will eventually replace desiccant dryers, predicts Mr. Tavakoli. "The vacuum dryer is simpler, uses less floor space, dries faster, and uses 30 to 40% less energy. Desiccants eventually break down and have risk of contaminating the resin. As customers begin to see these advantages over the next few years, they will choose vacuum technology over desiccant," he concludes.

 

The continuing trend in the plastics industry is towards more fully automated material handling and using bulk silo storage to gain cost savings from larger material purchases. However, compounders are typically already using silos and automated conveying. Equipment suppliers see a demand for complete ‘packages’ of upstream equipment. They see major growth opportunities for material handling equipment in Asia and the Mid-East following growth in resin production, compounding and masterbatching installations. New developments include improved control systems and equipment designed to improve operator safety.

In material handling system design it is important to consider the overall physical plant layout, resin types and throughput rate, comments Ron Yap, systems project manager at Wittmann, which offers the full range of upstream equipment from railcar unloaders and silos to blenders and hoppers. Today's lean companies are relying more on equipment suppliers for engineering design and installation, and are interested in going to one supplier for everything upstream of the extruder, agrees Bob Barnett, global vice president of sales and marketing at K-Tron, which supplies complete upstream packages. "Our surveys have shown that customers like having an integrated package. Dealing with one vendor simplifies troubleshooting," he adds. An emerging trend in the compounding industry is the growth of shorter speciality runs and the need for quick changeovers. "All upstream auxiliary equipment must also be easy to clean and changeover," adds Mr. Barnett.

 

Material handling system controllers continue to become more sophisticated, say industry experts. While vacuum loaders often have dedicated control systems that need special interfaces to connect to feeder controllers, there is a trend towards having one controller to cover both the feeder and vacuum loader, comments Mr. Barnett. More companies are using load cells on their silos for more accurate inventory control, says Mr. Rich. Load cells or ‘yo-yo’ sensing devices measure how much remains in the silo, and automated delivery can be triggered when the silo level falls below a set point. Manufacturing control systems have evolved to provide not only timely raw data about the process but also useful data manipulations such as charts to show whether a process is within specification. Universal Dynamics' Factory Automation and Control System (FACS) uses distributed two-wire technology at low voltage, which allows users to conveniently add control point anywhere in the factory, comments Mr. Crawford.

 

In handling various materials in bulk bags, operator safety is becoming a major issue globally, comments Todd Reed, president at National Bulk Equipment (NBE). Primary concerns are dust-free environments and equipment that is designed for safe use. NBE's new Bulk Bag Filler design, based on ergonomic study results, gives the operator unobstructed access within arms length to the fill spout and bag loop hooks for attaching and detaching bags, eliminating the need for an operator to lean into or climb on the equipment, explains the company. For unloading bulk bags, NBE's new Bulk Bag Unloader's framework is designed to meet or exceed the ANSI and ASME specifications for under-hung cranes and monorail systems. It is also designed so that the operator is never positioned under the crane. The bag spout interface has a dust seal, is designed to be dust free, and to completely discharge material from the bag with minimal operator involvement, says the company.