How is Plastic Recycled: Explained
Post consumer recycled PET plastics (PCR PET) bottles are normally recycled for textiles or garments, carpets, sheets and injection molded goods. However, FDA approved bottle-grade rPET is of a higher quality, has lower contaminants, and has better performance characteristics relative to other grades of rPET. Therefore, in order to capture the value created by bottle-grade rPET it is important to understand the processing of rPET flakes.
Currently textile fibers and filaments comprise about 70% of rPET flake demand where lower quality rPET flakes are adequate. Further growth in this application is limited due to low profit margins. Improved recycling profit margins are possible when alternate applications such as food and nonfood containers, industrial yarn, mono filaments, strapping, film, sheet, and building materials such as foam boards are the targeted end markets for the recycled PET flakes.
These above mentioned higher profit margin end markets require higher quality rPET flakes or pellets (higher IV and lower contaminant contents). Bepex provides a robust continuous hot gas wash process with low conversion costs and capable of integration into large scale recycling plants to add value by improving the rPET flake quality – the BePET™ process.
The BePET™ process for processing of rPET flakes in the production of bottle-grade rPET is cost effective, energy efficient, suitable for large scale recycling of multiple grades of rPET flakes and capable of increasing the quality and value of the processed flakes into FDA approved bottle-grade rPET.
PET Flake Recycling Process: Overview
A generalized flow sheet linking the various unit operations in the traditional recycling process value chain is presented in Fig. 1.Commercial recycling operations vary in the equipment and process details such as batch, semi-batch and continuous. The flake washing steps also vary. Some utilize a hot caustic wash followed by a detergent wash while others may omit either the hot caustic wash or the detergent wash. The clean, dewatered rPET flakes typically suitable for further processing on the BePET™ system produced by other upstream operations have the following general characteristics:
- Viscosity (IV) [dl/g] 0.65-0.76
- Melt temperature [°C] 240-250
- Colored flakes content [ppm] 10,000-100
- Polyolefin content [ppm] 100-10
- PVC content [ppm] 50-10
- Glue content [ppm] 20-5
- Metal content [ppm] 10-3
- Moisture content [%] < 0.4
- Color number: "b" value of 5-2
- Bulk density [lb./ft3] 20-22
- Nominal flake size [inch] 3/8 to 1/2
These rPET flakes typically have poor material flow characteristics and may contain other volatile organic contaminants and particulate matter that would limit their utility. In order to enhance the utility and performance characteristics of the rPET flakes and thereby add value, the following property modifications are required:
- Viscosity (IV) [dl/g] increase to 0.80-1.05
- Colored flakes content [ppm] decrease to < 50
- Polyolefin content [ppm] decrease to < 5
- PVC content [ppm] decrease to < 5
- Glue content [ppm] decrease to < 3
- Solids content [ppm] decrease to < 5
- Moisture content [ppm] decrease to < 50
- Color number decrease "b" value to < 2
- Bulk density [lb./ft3] increase to > 40
The processes of extrusion, melt filtration, solidification and pelletizing are normally used to improve rPET flake material flow characteristics along with its bulk density while decreasing its solid contaminant content. However, this additional melt history adversely affects the IV, acetaldehyde content and color of the rPET flakes not to mention the associated additional capital requirements (fixed costs), utility costs and operational costs (variable costs) for these additional unit operations.
Bepex has developed and demonstrated a simplified continuous hot gas wash decontamination SSP process – known as BePET™ - which is effective in dynamically controlling the IV (up or down) and bulk density of rPET flakes while simultaneously reducing the rPET flake PVC content, glue content and volatile organic contaminant contents with low capital, utility and operating costs. The Bepex rPET process, or BePET™, shown in Fig. 2 effectively replaces the traditional extrusion, melt filtration, solidification and pelletizing downstream of the rPET flake washing process.
After the rPET flakes are rinsed and dewatered in the upstream process, they are continuously fed to the BePET™ Preheater (200) to be crystallized and heated to reaction temperature. A photograph of the BePET™ Preheater is shown in Fig. 3. The high surface temperature of the rotating discs carbonizes any PVC and PE contaminants in the feed material while removing part of the glue and volatile contaminants. The preheated rPET flakes exit the Preheater through an overflow weir and enter the continuous BePET™ Reactor (300). 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, volatile organic contaminants and SSP reaction by-products.
The efficiency of the hot gas wash process in removing the volatile organic contaminants is a key feature of the process. The residence time in the BePET™ Reactor is short compared to pellets due to the high SSP rate of the rPET flakes and IV as high as 1.2 can be easily achieved. The bed temperature of the rPET flakes in the Reactor can be as high as 220 °C. The lack of agitation and the high temperature in the Reactor would typically result in poor flow properties of the rPET flakes. However, Bepex has effectively addressed this problem by incorporating its proprietary mechanical discharge (310) at the bottom of the Reactor as shown in Fig. 4.
The rotating arm of the mechanical discharge at the bottom of the Reactor meters the solid rPET flakes through a gap in the circumference of a circular table. This arrangement prevents material bridging and effectively breaks any flow-blocking lumps formed due to the high temperature and the bulk static pressure. It also assists with better gas distribution across the bed cross-section. Furthermore, the mechanical discharge is a metering device providing uniform residence time for the rPET flakes thereby assuring uniform product quality.
Economics of Plastics Recycling: Considerations of the BePET™ Process
The rPET flakes exiting the BePET™ process are hot and dry and can be directly extruded into the finished article or into pellets. No additional drying step is required thereby reducing the capital and utility costs associated with solids drying systems. With a bulk density of around 35 lb./ft3, standard single screw extruders are adequate thereby further reducing the capital and utility costs compared to twin screw extruders or ring extruders.
The utility consumption and the conversion costs due to economies of scale have an impact on further reducing the costs for larger plant capacities. Capacities of up to 150 tons/day of rPET flakes are possible with the BePET™ system. Capital and conversion costs for other SSP processes such as the batch and the semi batch would be higher than that of the BePET™ system.
The heat energy requirements are the lowest when the rPET flakes are directly extruded into the final product after the BePET™ process when a streamlined and integrated system is installed downstream of the rPET flake washing system. A similar analysis would indicate a significant savings in the capital cost and labor costs when rPET flake from a BePET™ system is directly coupled to product extrusion.
PET Plastics Recycling: 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. There are several technologies available to recycle and reuse post consumer PET containers. Those technologies that add the most 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.