Mixed Plastics Waste to Ethylene and Propylene Feedstocks

Circular recycle of waste plastic holds significant environmental benefit in reducing the need for crude oil feed to produce plastic monomers and in addressing massive global accumulation of plastic waste. A two-stage cracking process is here explored for the reduction of long-chain polyethylene (PE), polypropylene (PP), and polystyrene (PS) to ethylene and propylene. The reaction yields useful byproducts, such as liquid fuel used to sustain the high energy demands of the process, and pressurized steam. A feed of 70 MT/day of PE, PP, and PS is assumed to be treated first in a rotary kiln pyrolysis reactor and secondly in a steam-cracking unit for the formation of short-chain unsaturated hydrocarbons. 41% of the feedstock by weight is converted to either ethylene or propylene. Due to the random nature of cracking, a pilot plant is deemed necessary to better understand this conversion. Heat integration is explored extensively throughout the cracking to employ other process products as fuel sources. A novel separation train and refrigeration cycle are then designed to isolate the two products of interest. The process is found not to be profitable, with an Internal Rate of Return of -4.74%, Net Present Value of - $18.8MM, and Return on Investment (ROI) in the third year of operation of -2.12%. However, a circular monomers facility holds significant value environmentally, and options are thus explored to potentially reduce the cost or make the process profitable.