Present Status of Mixed Plastic Waste Pyrolysis: Plant Simulation through Aspen Hysys

The widespread use of plastics results in plastic waste generation. After reuse, recycling is the preferred pathway to reduce the need for virgin feedstock. Concerning mechanical recycling, pyrolysis, consisting of heating the feedstock to promote the thermal degradation of polymers, allows to process Mixed Plastic Wastes (MPW) which cannot be easily sorted. Nowadays, chemical recycling through pyrolysis has reached the demonstration scale, but it is still challenging its further scale-up. In this work, a simulation of a MPW pyrolysis plant coherent with the present technology status is presented. The simulation was performed on Aspen Hysys v11. Unit operations composing the system were chosen according to literature and patent reviews. The process flow diagram of the system is composed of 4 lines in parallel, each one constituted by a reactor, one or more condensation (flash) steps, and a water scrubber (ex-situ dehalogenation) for incondensable gases before their combustion to sustain the reactor energy demand. Each line processes 5000 t/y, representing current scalability limits. The MPW feedstock mass composition assumed is the following: 45 % for both polypropylene (PP) and polyethylene (PE), 8 % for polystyrene (PS), and 1 % each for polymethylmethacrylate (PMMA) and polyvinylchloride (PVC). The reactor has been modeled as a conversion reactor which satisfies the conservation of atoms yielding gaseous, liquid, and solid products of 20, 70, and 10 % of the feedstock mass respectively. After the condensing units, a single distillation column collects the oil produced by each line. The influence of changing the number of condensation (i.e., flash) steps has been investigated. The maximum condensate production was observed for one flash unit. Employing fewer flash units allows to obtain more condensate, paying a slightly larger reboiler duty.