Experimental Study on Catalytic Upgrading of Pyrolysis Products from Medical Waste Polyamide (PA)

Medical waste polyamide (PA), widely used in syringes, catheters, sutures, and sterile packaging, can be converted into valuable fuels and chemical feedstocks through pyrolysis followed by catalytic upgrading. This experimental study examines how zeolite and metal-based catalysts transform raw PA pyrolysis vapors—typically rich in caprolactam, nitriles, ketones, and nitrogen heterocycles—into higher-quality oil and hydrogen-rich gas. The article details sample preparation, testing items, analytical methods, and practical applications of catalytic reforming for medical waste PA recycling.

Technical Overview

Polyamide (PA), including nylon 6 (PA6) and nylon 66 (PA66), is one of the most common engineering plastics in healthcare settings due to its mechanical strength, chemical resistance, barrier properties, and sterilizability. Sterilized or contaminated PA items such as syringe components, intravenous catheters, surgical sutures, and blister packaging enter the medical waste stream in large volumes, and conventional disposal by incineration raises concerns over nitrogen oxide (NOx) emissions because PA contains roughly 10–12 wt% nitrogen.

Pyrolysis offers a chemical recycling pathway in which PA is thermally decomposed at 400–600 °C under an inert atmosphere into condensable vapors, non-condensable gases, and solid char. In uncatalyzed pyrolysis, PA6 primarily depolymerizes into ε-caprolactam, while PA66 yields cyclopentanone, pentenenitriles, adiponitrile, and a range of aliphatic nitriles. However, the raw pyrolysis oil suffers from high nitrogen and oxygen content, strong odor, poor storage stability, and corrosiveness, which limits its direct use as fuel or chemical feedstock.

Catalytic upgrading (catalytic reforming) addresses these deficiencies by passing the hot pyrolysis vapors over an active catalyst bed—typically HZSM-5, HY, or Hβ zeolites, nickel-supported catalysts, or basic metal oxides such as CaO—before condensation. The catalyst promotes deoxygenation, partial denitrogenation, cracking of heavy oligomers, decarbonylation, and aromatization, shifting the oil composition toward hydrocarbons and light aromatics (BTX) while generating a hydrogen-enriched gas fraction. This article summarizes the experimental framework used to evaluate catalytic upgrading performance, including mass balance, nitrogen distribution, oil quality assessment, and catalyst characterization.

Testing Samples

Nylon 6 (PA6) Medical Waste

Sterilized PA6 fractions recovered from syringe barrels, luer connectors, protective packaging trays, and non-woven sterilization wraps. These samples are autoclaved before laboratory handling, shredded to particles below 2 mm, and dried to a moisture content below 5 wt% to ensure reproducible feeding and pyrolysis behavior.

Nylon 66 (PA66) Medical Components

PA66 items such as multi-lumen catheters, surgical instrument handles, and infusion set housings. PA66 exhibits a different decomposition pathway than PA6, producing more cyclopentanone and aliphatic dinitriles, and is therefore evaluated as a separate sample group.

Mixed Medical Plastic Streams

Realistic mixed streams containing PA blended with polypropylene (PP), polyethylene (PE), and minor amounts of other polymers. Blended feeds are tested to study synergistic or antagonistic interactions during co-pyrolysis and catalytic reforming, and to assess chlorine interference from PVC traces when relevant.

Catalyst Samples

Fresh and spent catalysts submitted for performance and deactivation evaluation, including HZSM-5 with varying Si/Al ratios, Hβ and HY zeolites, Ni/SiO2 and Ni/Al2O3 catalysts, CaO-based sorbent catalysts, and equilibrium FCC-type catalysts. Catalysts are pelletized, sieved to 0.3–0.8 mm, and pre-calcined according to standardized activation procedures.

Testing Items

The experimental program covers feedstock characterization, product quantification, and quality assessment across all three pyrolysis product phases:

• Feedstock characterization: proximate analysis (moisture, volatile matter, fixed carbon, ash), ultimate analysis (C, H, N, S, O), higher heating value (HHV), and bulk density.
• Thermal decomposition behavior: onset degradation temperature, peak mass-loss temperature, and residual char yield by thermogravimetric analysis.
• Product distribution: mass balance of pyrolysis oil, aqueous phase, non-condensable gas, solid char, and coke deposited on the catalyst, reported in wt% of feed.
• Oil composition: quantitative identification of ε-caprolactam, cyclopentanone, aliphatic nitriles, benzonitrile, indoles, pyridines, quinolines, phenol derivatives, and light aromatic hydrocarbons (benzene, toluene, ethylbenzene, xylenes) by GC-MS.
• Oil quality: water content, acid number, density, kinematic viscosity, HHV, elemental composition, nitrogen content, and simulated distillation boiling-point distribution.
• Gas composition: H2, CO, CO2, CH4, C2–C4 hydrocarbons, and nitrogen-containing species such as HCN and NH3.
• Char characterization: HHV, ash content, and specific surface area.
• Catalyst evaluation: BET surface area and pore volume, crystalline phase by XRD, acidity profile by NH3-temperature-programmed desorption, Brønsted/Lewis acid ratio by pyridine-adsorbed FTIR, and coke quantification by temperature-programmed oxidation.
• Nitrogen balance: distribution of feed nitrogen among oil, gas, char, and coke to evaluate denitrogenation efficiency.

Testing Methods

Feedstock Preparation and Sterilization

Medical waste PA is first sterilized by autoclaving at 134 °C under saturated steam pressure in compliance with recognized medical waste treatment requirements. Sterilized material is then shredded, homogenized, sieved, and dried in a convection oven. Representative subsamples are obtained by coning and quartering to guarantee analytical reproducibility.

Thermogravimetric Analysis (TGA)

Thermal degradation profiles are recorded under nitrogen purge at heating rates of 5, 10, and 20 °C/min up to 800 °C, following the general framework of ASTM E1131. Derived kinetic parameters (activation energy via the Kissinger or Flynn–Wall–Ozawa methods) guide the selection of pyrolysis temperatures for the catalytic experiments.

Pyrolysis-GC/MS Screening

Micro-scale pyrolysis coupled online with GC-MS (Py-GC/MS) provides rapid screening of evolved products at 500–700 °C, both with and without an in-line catalyst layer. This method identifies catalytic effects on product pathways—such as caprolactam conversion to nitriles and aromatics—before larger-scale trials are conducted.

Fixed-Bed Pyrolysis with In-Line Catalytic Upgrading

The core experiment uses a two-zone quartz fixed-bed reactor inside a temperature-programmed furnace. PA feed (5–20 g per run) is pyrolyzed in the first zone at 450–600 °C, and the evolved vapors pass immediately through a catalyst bed maintained at 350–500 °C with a weight hourly space velocity (WHSV) of 2–6 h⁻¹ and configurable catalyst-to-plastic ratios. High-purity nitrogen serves as carrier gas at controlled flow rates. A blank run without catalyst establishes the baseline for comparison.

Product Separation and Quantification

Vapors exiting the reactor are directed through a series of condensers held at progressively lower temperatures (ice bath and dry-ice/acetone traps) to recover the oil and aqueous fractions, while permanent gases are collected in gas-tight bags. Each phase is gravimetrically quantified to close the mass balance within ±3 wt%.

Analytical Characterization

• Oil and aqueous phases: compositional analysis by GC-MS with internal standard calibration; water content by Karl Fischer titration; elemental analysis (C, H, N, S) following ASTM D5291 with oxygen by difference; HHV by bomb calorimetry in accordance with ASTM D240; acidity, density, and viscosity measured by standard laboratory protocols.
• Gas fraction: H2, CO, CO2, and CH4 by GC-TCD; light hydrocarbons by GC-FID; HCN and NH3 trapped in absorption solutions and quantified by spectrophotometric or ion chromatographic methods.
• Catalysts: textural properties by nitrogen physisorption (BET method), structure by X-ray diffraction, acidity by NH3-TPD and FTIR of adsorbed pyridine, and coke content by TGA under air.

Catalyst Stability and Regeneration Testing

Spent catalysts are regenerated by controlled combustion of coke in air at 550–600 °C, then subjected to repeated pyrolysis-upgrading cycles. Conversion, oil yield, and product selectivity are tracked over multiple cycles to assess deactivation resistance and industrial viability.

Applications

Catalytic upgrading of medical waste PA pyrolysis products serves multiple technical and commercial objectives:

• Monomer recovery: enriching ε-caprolactam in PA6 pyrolysis condensates as a feedstock for closed-loop nylon 6 repolymerization.
• Alternative fuel production: upgrading raw nitrogen-rich condensates into stabilized oil with elevated heating value and reduced heteroatom content suitable for boiler or industrial furnace applications.
• Light aromatics production: directing product distribution toward benzene, toluene, and xylene (BTX) as petrochemical substitutes via shape-selective zeolite catalysis.
• Hydrogen-rich gas generation: producing fuel gas streams for on-site energy recovery within medical waste treatment facilities.
• NOx emission reduction: converting fuel-bound nitrogen into manageable condensed or captured species rather than releasing it during incineration.
• Process scale-up support: providing mass balance, kinetic, and catalyst longevity data for the design of continuous pyrolysis and reforming units handling sterilized medical plastic streams.
• Research and regulatory compliance: supplying accredited third-party laboratory data packages for academic research, patent filings, environmental permitting, and technology qualification.

FAQ

Why is catalytic upgrading necessary for PA pyrolysis oil? Uncatalyzed PA pyrolysis oil contains high levels of caprolactam, nitriles, and nitrogen heterocycles that cause odor, instability, and elevated nitrogen emissions during combustion. Catalytic reforming cracks and reshapes these compounds into hydrocarbons and light aromatics, improving heating value, stability, and downstream usability.

Which catalysts are most effective for upgrading PA pyrolysis vapors? HZSM-5 zeolites with moderate Si/Al ratios are generally the most studied and effective for aromatization and heteroatom removal, owing to strong acidity and shape selectivity that limits coking. Ni-based catalysts are favored when hydrogen is co-fed for hydrodenitrogenation, while basic oxides such as CaO help capture acidic species and reduce oxygenated compounds.

What operating temperatures are typically used? Pyrolysis is usually conducted at 450–600 °C, while the downstream catalytic bed is maintained at 350–500 °C. Higher catalyst temperatures increase cracking and gas yield, whereas lower temperatures favor liquid production with partial upgrading; optimization depends on whether the target product is monomer-rich oil or aromatics-rich oil.

How is the nitrogen originally present in PA accounted for? A full nitrogen balance is performed across oil, gas, char, and catalyst coke. Zeolite catalysts tend to transfer nitrogen into coke and lighter nitriles or HCN/NH3 in the gas, so gas scrubbing and condensate polishing are incorporated into the experimental protocol.

Is pre-treatment of medical waste PA required before testing? Yes. Medical waste must be autoclave-sterilized in accordance with applicable medical waste management regulations before any laboratory handling. Subsequent shredding, sieving, and drying ensure homogeneous, reproducible samples.

Which analytical standards govern the testing? Methods are aligned with recognized ASTM, ISO, and GB frameworks, including ASTM E1131 for compositional analysis by TGA, ASTM D5291 for elemental analysis, ASTM D240 for heat of combustion, and corresponding national standards for gas chromatographic and calorimetric determination, as applied in accredited third-party laboratories.