Introduction to Mechanical and Chemical Recycling of Polymers

Managing post-consumer and post-industrial plastic waste is a critical environmental and economic priority. Mechanical recycling (sorting, shredding, washing, re-extrusion) reprocesses thermoplastics with minimal chemical change, while chemical recycling (pyrolysis, solvolysis, depolymerisation) breaks down polymer chains into monomers or synthetic crude oil. Mastering both technologies enables circular economy engineering and compliance with global recycled content mandates.

By completing this lesson, you will be able to:

• Differentiate mechanical recycling steps from chemical depolymerisation/pyrolysis.
• Calculate complete multi-stream mass balance yields in pyrolysis oil recovery.
• Compare mechanical flake quality degradation vs chemical feedstock virgin-grade equivalence.
• Diagnose cross-contamination in mixed polyolefin waste streams.

graph TD
    A["Post-Consumer Plastic Waste Input"] --> B["Automated NIR Optical Sorting & Flake Washing"]
    B --> C{"Recycling Process Selection"}
    C -->|"Mechanical Recycling"| D["Shredding, Wash-Float Tank & Compounding Extrusion"]
    C -->|"Chemical Recycling"| E["Pyrolysis Reactor (500°C in N2 Atmosphere)"]
    D --> F["Recycled Pellets (rHDPE / rPET Flakes)"]
    E --> G["Synthetic Pyrolysis Oil & Naphtha Feedstock"]

Full Mass Balance Equation

The mass flow of incoming plastic waste feed ($dot{m}_{feed}$) equals the sum of all 5 output streams:

$$\dot{m}_{feed} = \dot{m}_{oil} + \dot{m}_{gas} + \dot{m}_{char} + \dot{m}_{water} + \dot{m}_{contaminants}$$

Worked Numerical Example:

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Problem: A commercial pyrolysis plant processes $\dot{m}_{feed} = 1000\text{ kg/h}$ of polyolefin waste. The measured output stream mass rates are:

• Synthetic crude oil: $\dot{m}_{oil} = 780\text{ kg/h}$
• Non-condensable syngas ($\text{CH}_4, \text{C}_2\text{H}_4$): $\dot{m}_{gas} = 140\text{ kg/h}$
• Solid carbonaceous char: $\dot{m}_{char} = 60\text{ kg/h}$
• Process moisture / aqueous phase: $\dot{m}_{water} = 15\text{ kg/h}$
• Filtered particulate contaminants / inorganic residue: $\dot{m}_{contaminants} = 5\text{ kg/h}$

Calculate:

1. Liquid oil mass yield percentage
2. Verify total mass balance closure

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Solution:

1. Liquid Oil Mass Yield:

$$\text{Oil Yield}\% = \left( \frac{780\text{ kg/h}}{1000\text{ kg/h}} \right) \times 100\% = 78.0\%$$

1. Total Mass Balance Closure:

$$\text{Total Output} = 780 + 140 + 60 + 15 + 5 = 1000\text{ kg/h} \quad (100.0\% \text{ Closure})$$

Engineering Note: The 78.0% oil yield represents an illustrative engineering scenario for clean, low-moisture polyolefin feedstock. Real-world municipal waste feeds with higher PVC or PET contamination yield lower oil fractions and higher char/gas streams.

• rPET Bottle-to-Bottle Lines: Super-cleaning mechanical recycling for food contact PET. (Illustrative Indian industry scenario based on rPET plant operations in Wada, Maharashtra).

• Mechanical Recycling: Thermal re-extrusion; chain scission reduces MFI/viscosity.
• Pyrolysis: Thermal cracking ($450-550^circ ext{C}$) in oxygen-free atmosphere yielding liquid hydrocarbon fuel.

1. ISO 15270:2008 — Plastics — Guidelines for the recovery and recycling of plastics waste, ISO.
2. IS 14534:1998 — Guidelines for Recycling of Plastics, Bureau of Indian Standards.