Advanced Chemical Recycling: Pyrolysis & Solvolysis Depolymerisation Pathways

Mechanical recycling degrades plastic properties over multiple loops due to thermal history and contamination. Advanced chemical recycling (feedstock recycling) solves this by cracking polymer chains back into monomers or chemical feedstocks. Pyrolysis converts mixed polyolefins (PE, PP) into pyrolysis oil (crude oil substitute), while solvolysis depolymerizes polyesters (PET) and polyurethanes. With India's Plastic Waste Management Rules (EPR guidelines) requiring plastic packaging to contain PCR content, chemical recycling is expanding. Indian refining majors like Reliance Industries and Indian Oil are building commercial-scale pyrolysis units.

• Distinguish mechanical recycling from chemical recycling pathways (pyrolysis, solvolysis, gasification).
• Explain the thermal cracking mechanism of polyolefins during pyrolysis.
• Detail the solvolysis depolymerisation chemistry of PET (glycolysis, methanolysis, hydrolysis).
• Calculate mass yields and conversion efficiencies for plastic-to-fuel pyrolysis processes.
• Reference international recycling standards such as ISO 15270 and ASTM D7611.

3.1 Advanced Recycling Taxonomy

3.2 Polyolefin Pyrolysis Mechanics

Pyrolysis is the thermal degradation of polymers in the absence of oxygen. It occurs via a free-radical chain mechanism:

1. Initiation: Random homolytic cleavage of C–C bonds in the backbone to generate macroradicals.
2. Propagation: Hydrogen transfer and $\beta$-scission reactions cracking chains into shorter oligomers.
3. Termination: Radical recombination or disproportionation to yield stable hydrocarbons (alkanes and alkenes). The product composition is controlled by temperature: higher temperatures ($> 600^\circ$C) favor gas products (ethylene, propylene), while moderate temperatures (400–500°C) optimize liquid pyrolysis oil yield.

3.3 PET Solvolysis Depolymerisation

Polyesters can be chemically depolymerized by reversing the esterification reaction:

• Glycolysis: PET reacts with excess Ethylene Glycol (EG) at 190–220°C using metal catalysts (zinc acetate) to yield bis(2-hydroxyethyl) terephthalate (BHET) monomer.
• Methanolysis: PET reacts with methanol under pressure to yield Dimethyl Terephthalate (DMT) and EG.
• Hydrolysis: Acidic, basic, or neutral water reaction to yield Terephthalic Acid (PTA) and EG.

Problem: A pilot-scale pyrolysis reactor is charged with 500 kg of sorted post-consumer PE/PP waste. After a run at 450°C, the process yields:

• Pyrolysis oil = 380 kg
• Gaseous hydrocarbons (c1–c4) = 85 kg
• Solid char = 35 kg Calculate:

1. The mass yield (%) of pyrolysis oil.
2. The mass conservation closure check.
3. The energy conversion efficiency if the raw plastic feedstock has a calorific value of 45 MJ/kg and the recovered pyrolysis oil has a calorific value of 42 MJ/kg.

Solution:

1. Calculate oil mass yield:

1. Mass conservation check:

1. Calculate energy conversion efficiency (based on oil recovery):

Interpretation: The pyrolysis run converts 76.0% of the solid plastic waste into liquid pyrolysis oil with an energy recovery efficiency of 70.93%. The gas fraction (17.0%) is often combusted to supply the process heat for the pyrolysis reactor, improving carbon cycle efficiency.

Reliance Industries Limited (RIL) (Jamnagar) has launched chemical recycling operations. They process post-consumer plastic waste to produce certified circular polymers. RIL utilizes pyrolysis oil as a co-feedstock in their Jamnagar refinery crackers to manufacture circular PP and PE.

Indian Oil Corporation Limited (IOCL) is building a large-scale plastic-to-pyrolysis-oil plant in Panipat, integrating it with their petrochemical refining operations to satisfy EPR mandates.

• Pyrolysis: Anaerobic thermal cracking of polymers into liquid hydrocarbon feedstocks.
• Solvolysis: Solvent-assisted chemical depolymerisation of condensation polymers.
• BHET: Bis(2-hydroxyethyl) terephthalate; monomer intermediate from PET glycolysis.
• EPR: Extended Producer Responsibility; Indian regulations requiring plastic packaging producers to recycle and reuse post-consumer waste.
• Pyrolysis Oil: Complex mixture of liquid hydrocarbons chemically identical to petrochemical naphtha.

1. ISO 15270 — Plastics — Guidelines for the recovery and recycling of plastics waste
2. ASTM D7611 — Standard Practice for Coding Plastic Manufactured Articles for Recycling
3. IS 14534 — Bureau of Indian Standards (BIS) guidelines for recycling of plastics

1. Detail the chemical reaction of PET glycolysis using Zinc Acetate catalyst. Show the structure of the resulting BHET monomer.
2. Explain why mechanical recycling cannot be repeated indefinitely for PVC and PET, highlighting thermal degradation and structural defects.
3. Discuss the challenges of chlorine content (from PVC contamination) in pyrolysis oil feedstocks for steam crackers, and how it is removed.

Q1. Which advanced recycling process converts mixed polyolefins into liquid crude oil substitutes under anaerobic conditions?

• C) Pyrolysis

Q2. Glycolysis depolymerisation of PET yields which monomer intermediate?

• B) BHET

Q3. In pyrolysis, thermal cracking of polymer chains occurs via which mechanism?

• A) Free-radical chain reactions (homolytic cleavage)

Q4. What Indian regulation mandates that plastic packaging manufacturers incorporate post-consumer recycled content?

• B) EPR (Extended Producer Responsibility) Guidelines

Q5. Which Indian company uses pyrolysis oil as a co-feedstock in their steam crackers at Jamnagar?

• B) Reliance Industries Limited