The ubiquitous water bottle, which seemingly accompanies consumers everywhere, creates a challenge for recyclers. One heat processing equipment supplier suggests environmentally friendly, cost-effective recovery technologies that promise acceptable returns to the recycler.
Recycling post-consumer plastic containers such as beverage and water bottles has not kept pace with the growth in bottle consumption. One report estimated that 2.3 million tons of PET containers were consumed in 2005, of which 0.530 million tons were recycled. The balance -- 1.773 million tons -- were wasted, predominantly discarded in landfills. This compares with 0.588 million tons wasted in 1995. The Container Recycling Institute in Washington, D.C., has expressed concern, noting that growth in container usage far out paces any gains made in recovery.
There is an urgent need to reverse this wasting trend, in part through environmentally friendly, cost-effective recovery technologies that promise acceptable returns to the recycler. The process technology and equipment must be cost effective, energy-efficient, adaptable to large-scale recycling, and capable of increasing the quality and value of the processed flakes.
Among the options for recyclers is a simplified continuous thermal treatment process that enhances the value of the recycled polyethylene terephthalate (rPET) flakes. The hot gas wash process adds value to the rPET flakes by:
Increasing the molecular weight.
Improving material-handling characteristics.
Reducing PVC, glue and volatile organic contaminant (VOC) contents.
Integrated processing systems such as the flake-processing system shown in figure 1 can effectively replace the extrusion, melt filtration, solidification and pelletizing systems typically used downstream of the rPET flake-washing process. Integrated systems can help reduce capital costs.
With an integrated system, after the rPET flakes are rinsed and dewatered in the upstream process, they are continuously fed to an indirect thermal processor to be crystallized and heated to reaction temperature. Examples of commercially available indirect solids heaters are rotary dryers and agitated dryers.
Heated discs mounted on a tubular rotor, which is housed in a jacketed U-shaped vessel, contact the product; the heat transfer media does not. The rotor supplies 65 percent to 90 percent of the total heat transfer surface. The rotor discs are not pitched, and transport of the product from inlet to overflow discharge is both from hydraulic movement from incoming material and a small amount of push from small paddles on the discs.
These type of heaters have a relatively slow rotor speed and can process materials continuously or in batches. In addition, vacuum operation can be accomplished in a batch process.
During operation, a high relative velocity between the rotating discs and the product contributes to a high heat transfer coefficient. Agitator plow or scraper bars can be added to increase exposure of new surfaces and prevent material buildup on surfaces. These features allow the dryer to achieve higher heat transmission coefficients than many other indirect heaters. The rotor speed can be set for optimum heat exchange and thorough mixing, independent of residence time. Residence time ranges from minutes to several hours, ensuring that the disc dryers are versatile enough for any rPET product throughput rate.
During production, the high surface temperature of the rotating discs carbonizes any PVC and PE contaminants in the feed material while removing a part of the glue and volatile contaminants. The preheated rPET flakes exit the disc dryer through an overflow weir and enter a continuous decontamination reactor. The rPET flake bed moves from the top to the bottom of the reactor by gravity while a countercurrent stream of hot nitrogen gas washes the flakes, removing surface glue, VOCs and solid-state polymerization (SSP) reaction byproducts.
In summary, there is an urgent need to increase PET recovery and to reverse the wasting trend as the explosive growth in PET packaging consumption continues. Several technologies are available to recycle and reuse the post-consumer PET containers. Those technologies that add value to the rPET flakes at the lowest capital and operating costs are particularly well suited to expand the recycling market and thereby reduce the amount of PET bottles that reach the landfill.
Currently, textile fibers and filaments comprise about 80 percent of the rPET demand where low quality rPET flakes are adequate. Further growth in demand for low-quality rPET flakes is limited due to overseas competition and a low profit margin. By contrast, improved margins are possible with alternate applications such as food and nonfood containers, industrial yarn, monofilaments, strapping, film, sheet and building materials such as foam boards. These applications require high-quality rPET flakes or pellets (higher intrinsic viscosity [IV] and lower contaminant contents). Using indirect solids-heating equipment can provide a robust, continuous hot-gas wash process with low conversion costs that is capable of integration into large-scale recycling plants to add value by improving the rPET flake quality.