Determination of Recycled Chemical Allocation Losses and Yield Attribution Boundaries for Mixed Plastics

Boundary

Chain of custody verification tracks material flows from the sorting plant gate through intermediate chemical cracking units to final resin pelletization. Establishing physical attribution boundaries requires defining where mass balance accounting starts, where non-polymer losses are deducted, and how chemical co-products receive credit. When mixed plastic oil enters a steam cracker alongside conventional fossil feeds, physical tracing of individual recycled molecules becomes impossible.

Mass balance credit standards manage this co-feeding complexity by substituting inventory ledger balances for physical molecular tracking. ### Attribution Methodologies Across Mass Balance Accounting Standards Certification protocols like ISO 22095 establish formal inventory controls for attributing recycled properties to physical output batches. Under strict mass balance models, mass credit attribution follows one of three distinct approaches.

Proportional allocation distributes output credits based strictly on the chemical yield efficiency of the process. Roll-over or free allocation permits an operator to concentrate recycled credits onto specific polymer streams, such as food-grade polypropylene, while assigning zero recycled content to co-produced streams like heavy fuel oil or pyrolysis gasoline. Gasification produces synthesis gas streams.

High thermal cracking generates heavy tars. Uncredited fractions reduce net mass balances.

Annex B of the ISCC PLUS system specifies that energy recovery off-gases generated during thermochemical conversion remain excluded from recycled material allocation ledgers.

Attribution boundaries dictate the precise physical locations where mass measurements take place. Processors must establish four distinct measurement gates across the conversion cycle:

1. Dry waste input measurement at the chemical plant boundary gate following sorting and mechanical drying.
2. Crude oil yield determination directly after primary reactor condensation and solid ash separation.
3. Purified oil feedstock quantification after chemical hydrotreatment, desulfurization, and particulate filtration.
4. Monomer yield ledger entry at the cracker fractionator outlet valve prior to polymerization mixing.

Determining allocation losses involves calculating the yield factor (YF) across each transformation boundary. Consider a commercial operational sequence processing post-consumer mixed polyolefins. Assume a processing lot of 100.0 metric tonnes of mixed plastic waste containing 78.0% polyolefins (PE/PP), 10.0% polystyrene (PS), 5.0% PET, 3.0% PVC, and 4.0% non-polymeric dirt and moisture.

Primary thermal pyrolysis converts this 100.0-tonne input into three physical streams: 73.5 tonnes of crude pyrolysis oil, 16.5 tonnes of non-condensable fuel gas, and 10.0 tonnes of solid char and inorganic residue. The first conversion yield boundary (Y1) equals 0.735 (73.5%). The crude pyrolysis oil undergoes hydrotreatment and catalytic guard-bed filtration to lower chlorine content below 10 parts per million.

This purification step yields 68.2 tonnes of refined synthetic naphtha, generating 5.3 tonnes of heavy wax bottoms and spent filter cake. The secondary purification yield boundary (Y2) equals 0.928 (68.2 tonnes / 73.5 tonnes). The 68.2 tonnes of refined synthetic naphtha enters a steam cracker operating at high severity.

Steam cracking yields 21.8 tonnes of ethylene (32.0%), 12.3 tonnes of propylene (18.0%), 6.1 tonnes of butadiene and mixed C4s (9.0%), 15.0 tonnes of pyrolysis gasoline (22.0%), and 13.0 tonnes of methane and fuel gas (19.0%). The olefin chemical yield boundary (Y3) covering ethylene, propylene, and butadiene equals 0.590 (40.2 tonnes total monomer / 68.2 tonnes oil input). Under proportional allocation, the net allocatable recycled monomer credit (Mcredit) generated from the original 100.0 tonnes of mixed waste input follows the cumulative yield calculation: Mcredit = Input × Y1 × Y2 × Y3 Mcredit = 100.0 tonnes × 0.735 × 0.928 × 0.590 = 40.24 tonnes If the plant operates under a free-allocation framework that permits pyrolysis gasoline aromatics to count toward polymer resins via subsequent benzene-toluene-xylene (BTX) extraction, the added 15.0 tonnes increases total material credits to 55.24 tonnes.

However, allocating the 13.0 tonnes of methane cracker off-gas or the initial 16.5 tonnes of pyrolysis gas as recycled polymer credits breaches basic attribution rules across major European certification schemes. Section 4.2 of the ISCC PLUS system specifies the deduction of internally combusted fuel gas fractions from material credit balances prior to resin credit transfers.

Refinement

Steam cracking units process hydrotreated pyrolysis liquids alongside conventional fossil naphtha feeds under high-temperature thermal conditions.

Thermal cracking inside furnace coils breaks long-chain alkanes into ethylene, propylene, butylenes, and aromatic compounds. Co-feeding synthetic oils alters gas volume balances, furnace tube coking rates, and coil residence times. Calculating yield attributions across steam cracking requires precise mass spectrometry profiling of the hydrocarbon feed cuts.

### Olefin Yield Determination for Purified Pyrolysis Oils Synthetic naphtha derived from polyolefin pyrolysis exhibits higher paraffin content and lower naphthenic structures than standard straight-run fossil naphtha. Paraffinic feeds yield higher ethylene ratios under high cracking severity, but elevated diolefin content accelerates carbon deposition on furnace walls. Decoking cycles reduce furnace operational availability, introducing indirect process losses that alter plant energy requirements.

Polyethylene yields high alpha-olefin cuts. Contaminants lower liquid recovery fractions. Polymerization requires monomeric purity standards.

Higher inert ash content in waste inputs decreases liquid oil recovery while accelerating furnace tube fouling.

Refining efficiency depends on feedstock distillation cuts. Heavy wax fractions fed directly to cracking coils increase heavy fuel oil and pyrolysis tar production. These heavy bottoms cannot generate polymer mass credits under standard compliance systems.

Converting heavy fractions demands primary thermal visbreaking or secondary catalytic hydrocracking, adding process energy and carbon dioxide emissions that must be captured in broader lifecycle accounting ledgers. High aromatic content in pyrolysis feedstocks reduces light olefin yields while increasing furnace coking rates during continuous steam cracking.

Discrepancy

Laboratory scale thermogravimetric testing fails to replicate full-scale industrial continuous reactor hydraulics.

Small batch reactors achieve high heat transfer rates, rapid vapor removal, and controlled residence times, yielding liquid oil efficiencies exceeding 82% on clean polyolefin samples. Industrial plants processing heterogeneous bales encounter mass transfer limitations, temperature gradients, and feed fluctuations that expand gas and char formation. Mass balance audit files built solely on pilot-scale benchmarks create immediate compliance discrepancies during third-party site verification.

### Verification Protocols for Conversion Yield Audit Files Auditors inspecting mass balance claims examine operational logbooks against gas chromatography composition reports. Discrepancies between physical input mass and credited polymer output typically arise from unmeasured moisture content, uncredited side-stream purges, or incorrect characterization of off-gas combustion. Gas chromatographs measure hydrocarbon boiling ranges rather than raw elemental yield attributions across chemical reactors.

Chromatographic boiling point curves verify hydrocarbon cuts without measuring raw elemental yield losses inside reactor beds.

Auditing mass balance declarations requires checking physical evidence across specific verification checkpoints:

• Mass Flow Meter Calibration Logs inspect differential pressure and Coriolis meter certification records against ambient temperature density correction tables.
• Heteroatom Mass Balance Ledgers compare total incoming organic chlorine and oxygen against scrubbed wastewater effluent and hydrotreater spent catalyst receipts.
• Energy Recovery Side-Stream Manifests verify whether volatile pyrolysis gases routed to internal steam generation boilers were incorrectly booked as material recycling credits.
• Batch Stock Reconciliation Files cross-examine physical inventory held in intermediate storage tanks against digital credit transfers recorded on third-party chain of custody registries.

Third-party verifiers flag accounting adjustments where processing losses were shifted onto fossil co-products to preserve a 1:1 recycled credit allocation on prime resins. If a plant inputs 10 tonnes of mixed plastic oil and yields 6 tonnes of light olefins and 4 tonnes of heavy tars, assigning 10 tonnes of recycled resin credits to a customer purchasing the light olefin cut violates physical mass conservation principle ISO 22095. Mass balance audits require full reconciliations.

The analytical community leaves unresolved whether volatile C4 hydrocarbon streams routed to refinery fuel gas headers qualify as recycled chemical feedstocks or constitute process efficiency losses.

Obligation

National packaging mandates impose statutory fees on non-recycled plastic containers placed on commercial markets. Under regulations like the UK Plastic Packaging Tax and the European Union Packaging and Packaging Waste Regulation, producers must substantiate all recycled content claims with accredited audit dossiers.

Chemical recycling mass balance credits receive intense regulatory oversight to prevent double-counting and artificial credit inflation derived from energy recovery streams. ### Regulatory Frameworks Governing Recycled Content Claims European policy directives restrict chemical recycling credits to outputs directly converted into new chemical substances. Co-products combusted for process heat, electrical energy, or transport fuel generation fall outside statutory definitions of material recycling.

Importers and packaging converters placing resin products on European markets assume direct financial liability for inaccurate mass balance certificates issued by upstream chemical recyclers.

Failure to account for thermochemical conversion losses invalidates compliance declarations. Customs authorities and tax regulators conduct joint audits of imported plastic packaging, requesting granular yield calculations that trace raw mixed plastic waste inputs down to final monomer mass. Non-compliant declarations trigger retroactive packaging tax liabilities, customs demurrage fees, and civil administrative penalties.

Misallocating fuel fractions as polymer credits exposes brand owners to statutory tax adjustments, retroactively applied compliance fines, and immediate customs rejections of imported packaging.