(41e) Application of Process Intensification and Advanced Heat Integration to Plastic Chemical Recycling Processes

Plastic waste is an important global issue, and the recycling of plastic waste is an important part of addressing this issue. There is growing interest from manufacturers and governments to incorporate certain amounts of recycled materials into products. Chemical recycling, or the depolymerization of polymers to monomers, is one way to recycle plastics. Some plastics, particularly step growth polymers, can be easily converted to monomers, purified, and then converted back to polymers. These processes offer a way to recover monomers from lower quality plastic wastes that are otherwise difficult to mechanically recycle, increasing the scope of waste plastics that can potentially be recycled.

The economics of plastic depolymerization processes dictate the process must be profitable with the recycled monomers selling at similar prices to virgin monomers. It is thus critical to the development and implementation of these processes, that capital and operating costs are minimized. Application of process intensification is important for these processes to be economically viable. This work investigates different cases where we model the application of process intensification and advanced heat integration for plastic recycling processes.

One example of process intensification in plastic recycling is the vapor methanolysis process originally developed by Eastman Chemical, which has its first commercial scale process starting up in 2024. This process uses methanol in the vapor phase to depolymerize poly(ethylene terephthalate) (PET) into dimethyl terephthalate (DMT) and ethylene glycol. The methanol vapor strips the products into the vapor phase, which overcomes the reaction equilibrium limitation and simultaneously separates the products from heavy impurities and byproducts. We apply additional improvements to the heat integration of the process by implementing a compression loop for the vapor methanol to avoid condensing and pumping liquid methanol to recycle to the reactor. We consider the effect of light gas accumulation in the compression loop and include a purge outlet. Overall, the implementation of the compression loop can reduce thermal duty of the process by ~40%. We apply other heat integrations where applicable such as using liquid methanol as cooling fluid in high temperature condensers and combining reboilers and condensers of distillation columns in the process.

We demonstrate the applicability of process intensification to a version of the glycolysis process that converts PET to bis(2-hydroxyethyl) terephthalate by reacting with ethylene glycol. One innovation to this process is generating vapor in the reactor to strip volatile impurities such as water and acetaldehyde that form undesirable byproducts and make downstream separations more challenging. The heat used to generate the vapor is mostly recovered with generated vapor entering a rectifying column where most of the ethylene glycol in the vapor is condensed and returned to the reactor. The heat from this rectifier condenser is used elsewhere in the process. We also implement a heat integration for a column where ethylene glycol is recovered from a side draw where a heat transfer element is installed above the side draw to take advantage of the high temperature at which ethylene glycol condenses. Generating steam from this heat transfer element allows a net export of low-pressure steam or higher solvent rates for BHET crystallization without impacting the total process energy demand.

We demonstrate the application of process intensification to the alkaline hydrolysis process that ultimately converts PET to terephthalic acid (TPA) and ethylene glycol using water. This process utilizes large amounts of water that must be evaporated to concentrate a salt byproduct. We demonstrate how using 4 stages of multi-effect evaporation with an evaporator reduces the total evaporation energy by over 75%. We also demonstrate in this process an option for a dividing wall column configuration to separate water, ethylene glycol, and diethylene glycol that is equivalent to a conventional two column configuration and can reduce medium pressure steam duty by ~30%.

Finally, we demonstrate the application of process intensification for the nylon 6 depolymerization process where nylon 6 is depolymerized in the presence of water to caprolactam. This process is similar to the vapor methanolysis process where steam is used to strip caprolactam into the vapor phase to overcome equilibrium and separate caprolactam from heavy impurities and byproducts. We also apply a compression loop to circulate the reaction steam and consider various reactor configurations to further reduce process energy demand, including a configuration that operates without the use of steam and operating under vacuum.

Overall, we demonstrate that there is tremendous potential to apply process intensification and heat integrations to plastic depolymerization processes. Developing a depolymerization process capable of producing products that can be sold at the same or lower price than virgin feedstocks is challenging. It is important that in the development of these processes that all available heat integrations are taken advantage of to minimize the energy demand and cost of these processes, for them to achieve commercial implementation.