Chemical Recycling: Pyrolysis, Depolymerisation & Solvolysis Pathways

Mechanical recycling is limited by degradation (loss of properties after repeated extrusions) and cannot easily process mixed, contaminated, or multi-layer plastics. Chemical recycling (tertiary recycling) overcomes these limits by cracking polymers back into their monomer or chemical feedstocks. These feedstocks are purified and repolymerized into virgin-quality resins safe for food contact. Understanding pyrolysis, depolymerisation, and solvolysis reactions is essential for circular economy engineers.

• Compare the primary chemical recycling pathways (pyrolysis, depolymerisation, solvolysis).
• Explain the reaction kinetics and catalysts used in PET solvolysis (glycolysis, methanolysis).
• Analyze pyrolysis products (naphtha, gas, char) as a function of temperature and residence time.
• Solve mass balance and conversion calculations for a depolymerisation reactor.
• Reference chemical recycling validation and certification standards.

3.1 Chemical Recycling Pathways

• Thermal Pyrolysis: Heating mixed polyolefin waste (PE, PP) to $400 - 600^\circ$C in the absence of oxygen to break C–C bonds, producing liquid pyrolysis oil (crude naphtha analogue), hydrocarbon gases, and carbon char. The oil is co-fed into steam crackers.
• Solvolysis: Using solvents and catalysts to cleave ester or amide bonds in condensation polymers (PET, Nylon, Polyurethane):
  • Glycolysis: Reacting PET with Ethylene Glycol (EG) to yield BHET monomer.
  • Methanolysis: Reacting PET with methanol to yield Dimethyl Terephthalate (DMT) and EG.
• Depolymerisation: Thermally or catalytically unzipping polymers back to monomers (e.g., PMMA to MMA).

3.2 PET Glycolysis Reaction

The glycolysis of PET is a transesterification reaction catalyzed by metal acetates or ionic liquids:

$$\text{PET} + \text{EG (excess)} \xrightarrow{\text{Catalyst, } 180 - 220^\circ\text{C}} \text{BHET } (\text{bis(2-hydroxyethyl) terephthalate})$$

The BHET monomer is purified and repolymerized to food-grade PET.

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Problem: A chemical recycling plant depolymerises post-consumer PET flakes via glycolysis. The feed is $5.0$ metric tonnes (MT) of PET flakes (monomer molecular weight equivalent $M_{PET} = 192.2$ g/mol). The reactor uses excess Ethylene Glycol and a zinc acetate catalyst. The reaction yields $5.50$ MT of purified BHET monomer ($M_{BHET} = 254.2$ g/mol). Calculate:

1. The theoretical yield of BHET monomer (in MT) from 5.0 MT of PET.
2. The chemical conversion yield (%) of the glycolysis reactor.

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Solution:

1. Calculate the theoretical yield of BHET monomer: One mole of PET repeating unit ($192.2$ g/mol) reacts with one mole of EG to yield one mole of BHET ($254.2$ g/mol):

$$\text{Theoretical BHET} = \text{PET Feed Mass} \times \frac{M_{BHET}}{M_{PET}}$$ $$\text{Theoretical BHET} = 5.0 \text{ MT} \times \frac{254.2 \text{ g/mol}}{192.2 \text{ g/mol}} = 5.0 \times 1.3226 = \textbf{6.613 MT of BHET}$$

1. Calculate the chemical conversion yield (%):

$$\text{Conversion Yield} = \frac{\text{Actual BHET Yield}}{\text{Theoretical BHET Yield}} \times 100\% = \frac{5.50 \text{ MT}}{6.613 \text{ MT}} \times 100\% = \textbf{83.17\%}$$

Interpretation: The glycolysis reactor achieves an 83.17% chemical conversion yield, producing 5.50 MT of purified BHET monomer. The remaining fraction consists of unreacted oligomers and process losses, which are separated and recycled back to the reactor.

Indian polyester manufacturers (such as Reliance Industries and recycling ventures) run chemical recycling setups. They depolymerise waste PET bottles and spinning waste back into DMT/TPA and EG, repolymerizing them into circular polyester yarns.

• Pyrolysis: Anaerobic thermal cracking of polyolefins into liquid fuels and feedstocks.
• Glycolysis: Chemical recycling of PET using ethylene glycol to yield BHET monomer.
• BHET: Bis(2-hydroxyethyl) terephthalate; the monomer intermediate produced by PET glycolysis.
• Solvolysis: Solvent-assisted depolymerisation of condensation polymers.
• ISCC PLUS: International certification verifying mass balance claims for chemically recycled polymers.

1. ISCC PLUS — International Sustainability and Carbon Certification guidelines for circular feedstocks
2. ASTM D8058 — Standard test methods for evaluating chemical recycling pyrolysis oils

1. Write the balanced transesterification reaction of PET methanolysis, displaying the inputs, catalyst, and outputs.
2. Contrast thermal pyrolysis with catalytic pyrolysis of mixed PE/PP waste. Focus on processing temperatures and product selectivity.
3. Design a process separation schematic to purify BHET monomer from a glycolysis reactor, removing unreacted glycol, catalyst residues, and colorants.

Q1. Which chemical recycling pathway involves the anaerobic thermal cracking of mixed polyolefin waste into liquid oil at 500°C?

• B) Pyrolysis

Q2. The solvolysis of PET using Ethylene Glycol to yield BHET monomer is called:

• C) Glycolysis

Q3. What is the theoretical yield of BHET monomer (MW = 254.2) from 10.0 tonnes of PET flakes (MW = 192.2)?

• B) 13.22 tonnes

Q4. Which certification standard verifies the mass balance traceability of chemically recycled polymers?

• B) ISCC PLUS

Q5. Condensation polymers like Nylon and Polyurethanes are recycled primarily through which process?

• C) Solvolysis