Odor Treatment for Recycled Plastics: Equipment Configuration and Process Improvement Methods

Odor in recycled plastics is a key obstacle restricting their high-value applications, especially in fields sensitive to olfactory experience such as packaging and home furnishing. Odors mainly originate from residual impurities, polymer degradation products, and volatile additives, presenting as pungent smells or excessive odor intensity. To solve this problem, a three-level treatment logic of "source control-in-process removal-end purification" should be adopted. Through equipment upgrading and process optimization, the odor removal rate can reach ≥90%, meeting industry application standards.

I. Analysis of Odor Sources and Characteristics (Premise for Treatment)

Odor components vary significantly in different recycled materials, requiring targeted treatment plans:

II. Core Treatment Equipment Configuration Plan

Odor treatment requires collaboration of multiple devices to achieve full-link purification from materials to exhaust gas:

(1) Pretreatment Stage: Reducing Odor Sources (Impurities and Microorganisms)

Goal: Remove over 70% of odor precursors to reduce subsequent treatment load.

1. Enhanced Cleaning Equipment

• High-temperature alkaline cleaning tank: For ink/pesticide residues, use 80-90℃, 2%-3% NaOH solution for soaking 20-30min.  Organic impurities are decomposed through saponification;

• Ultrasonic cleaner: Under 20-40kHz frequency, cavitation effect peels off residual additives in micro-cracks on film surfaces;

• Sterilization dryer: For moldy materials, use 120℃ hot air + ultraviolet combined treatment to kill microorganisms and decompose metabolites.

2. Sorting and Purification Equipment

• Near-infrared (NIR) sorter: Identifies and removes foreign plastics with strong odors to reduce mixed odors;

• Density sorting tank: Separates oil-contaminated waste through 1.05-1.1g/cm³ brine to reduce greasy odor sources.

(2) Processing Stage: Removing Volatile Odors (High-Temperature Devolatilization)

Goal: Force separation of small molecular odor substances through negative pressure and high temperature during plastic melting.

1. Multi-stage Vented Extruder

• Equipment configuration: Twin-screw extruder with 3-4 vent ports distributed gradiently from the melting section to the homogenizing section;

• Key parameters:

• Vacuum degree: Primary venting removes surface volatiles; deep venting forces out small molecules generated by polymer degradation;

• Temperature control: 5-10℃ higher than conventional processing temperature to promote volatilization of odor substances without causing excessive degradation;

• Application scenario: Treating recycled materials with odors from processing degradation.

2. Devolatilization Auxiliary Devices

• Thin-film evaporation devolatilizer: Added before extrusion, pre-treats crushed plastic flakes under vacuum and 120-150℃ for 10-15min, removing over 70% of low-boiling odor substances;

• Inert gas purging system: Introduces nitrogen at the vent port to carry volatiles away from the melt quickly, avoiding re-dissolution.

(3) End Purification: Treating Residual Odors in Exhaust Gas

Goal: Reduce VOCs concentration in emitted exhaust gas to ≤50mg/m³, with odor threshold ≤level 2.

1. Adsorption Purification Equipment

• Activated carbon adsorption tower: Uses modified activated carbon with ≥90% adsorption efficiency for acidic odors. Bed wind speed is controlled at 0.8-1.2m/s to ensure contact time ≥1.5s;

• Molecular sieve rotor: For low-concentration, high-airflow exhaust gas, 13X molecular sieve adsorbs, then regenerates through 200-220℃ hot air desorption, with total purification efficiency ≥95%.

2. Catalytic Oxidation Equipment

• Regenerative catalytic oxidation (RCO): For high-concentration odor exhaust gas, oxidizes organic odor substances to CO₂ and H₂O at 300-350℃ with catalyst , removal rate ≥99%, waste heat recovery rate >90%;

• Low-temperature plasma purifier: For intermittently emitted exhaust gas, generates hydroxyl radicals through high-frequency discharge to decompose small molecular odor substances, with equipment resistance <500Pa, suitable for small and medium-sized production lines.

3. Combined Process

• Recommended "activated carbon adsorption + RCO" combination: Front-end activated carbon adsorbs and concentrates low-concentration exhaust gas, while back-end RCO achieves thorough oxidation, balancing treatment efficiency and operating cost.

III. Process Improvement Methods: Reducing Odor Generation from the Source

(1) Raw Material Pretreatment Optimization

1. Classified storage: Separate agricultural films, packaging films, and industrial scraps for storage to avoid cross-contamination of odors;

2. Drying control: Control moisture content of recycled materials at ≤0.1% to avoid odor from hydrolysis during high-temperature processing.

(2) Refined Adjustment of Processing Parameters

1. Temperature gradient optimization: Adopt "low-temperature feeding - gradient heating" mode to avoid degradation from local overheating;

2. Screw shear control: Use low-shear screw combinations for high-fill recycled materials to reduce small molecular odor substances from mechanical degradation.

(3) Additives for Auxiliary Deodorization

1. Adding deodorants: Add 0.5%-2% composite deodorants during extrusion. Through physical adsorption and chemical neutralization, odor intensity is reduced by 60%-80%;

2. Introducing compatibilizers: For mixed recycled materials, add 5%-8% POE compatibilizers to reduce small molecule precipitation caused by interface defects.

IV. Case Studies of Treatment Plans for Typical Scenarios

(1) Agricultural Film Recycled Materials (Pesticide Residues + Soil Odor)

• Equipment configuration: High-temperature alkaline cleaning tank → ultrasonic cleaning → twin-screw extruder → activated carbon adsorption tower;

• Process parameters: Extrusion temperature 180-200℃, 1.5% zeolite-based deodorant added;

• Effect: Odor removal rate 92%, recycled PE can be used for construction protective films.

(2) Packaging Film Recycled Materials (Ink + Adhesive Odor)

• Equipment configuration: NIR sorting → three-tank deinking cleaning → thin-film evaporation devolatilizer → RCO catalytic combustion;

• Process parameters: Deinking temperature 60℃, devolatilization vacuum -0.09MPa, RCO temperature 320℃;

• Effect: VOCs emission concentration ≤30mg/m³, recycled PP can be used for stationery.

The core of odor treatment for recycled plastics is "prevention first, comprehensive management"—reducing odor sources through pretreatment, enhancing devolatilization during processing, and ensuring compliance through end purification. For recycled material enterprises, only about 5%-8% of treatment costs need to be added to upgrade products from low-value fillers to mid-to-high-end application raw materials, significantly improving market competitiveness while meeting stringent environmental regulations on exhaust emissions.