Regenerative Thermal Oxidizer for VOCs Control in Plastic Pelletizing

I. Overview

Plastics, due to their light weight, high strength, easy processability, and low cost, are widely used in packaging, electronics, automotive, and daily necessities industries. With the rise of plastic recycling, a large amount of Volatile Organic Compounds (VOCs) and odorous gases are generated during the processing of waste plastics into pellets, causing environmental pollution.

VOCs generated during plastic pelletizing, melting, injection molding, and drying processes mainly include benzene, toluene, ethylbenzene, styrene, acetone, butanol, and ethyl acetate, which are hydrocarbon compounds. To effectively control these emissions, the Regenerative Thermal Oxidizer (RTO) has become the preferred treatment equipment. Regenerative Thermal Oxidizer can oxidize and decompose VOCs at high temperatures while recovering heat through ceramic heat recovery media, achieving efficient and energy-saving waste gas treatment.

Currently, there are two main production methods in the plastic industry: recycling of waste plastics and production of new plastics and products. The process flows are largely similar, with waste plastic recycling mainly based on extrusion molding. The production process flow is shown in Figure 1:

Figure 1 Waste Plastic Recycling Production Process Flow

After recycling, waste plastics are processed with specialized pelletizing equipment through sorting and classification, crushing, washing, heating, plasticizing, extrusion molding, and pelletizing to produce recycled plastic pellets. These recycled pellets can further be processed into various plastic products through injection molding. The sources of waste plastics generally come from two aspects:

1. Offcuts from resin plants or plastic processing factories;
2. Post-consumer mixed plastic products.

During plastic production, small amounts of organic waste gas, mainly VOCs, are produced, primarily from heating, melting, and injection molding processes. Common plastics include polystyrene (PS), polypropylene (PP), low-density polyethylene (LDPE), high-density polyethylene (HDPE), polycarbonate (PC), polyvinyl chloride (PVC), polyamide (PA), and polyurethane (PU). Chemically, plastics have a carbon content between coal and petroleum, and a hydrogen content higher than both, making their hydrocarbon compounds prone to generating VOCs under high-temperature conditions, becoming an important source of industrial VOCs. VOC emissions from plastic melting, injection molding, and drying mainly include benzene, toluene, ethylbenzene, styrene, o-xylene, m-/p-xylene, acetone, butanol, isopropanol, ethyl acetate, butyl acetate, and n-undecane.

To ensure environmental compliance and safe production during plastic pelletizing, the integration of RTO (Regenerative Thermal Oxidizer) technology into the waste gas treatment system is essential. RTO not only efficiently treats VOCs but also mitigates odors generated during production, providing reliable support for green manufacturing.

II. Treatment Technology Route

During plastic melting, injection molding, and drying, organic waste gas is usually collected through closed pipelines and preliminarily filtered using activated carbon before discharge. However, the odors produced during production are often not effectively collected, making it necessary to capture and treat this portion of the waste gas. The overall treatment process flow is shown in Figure 2:

Figure 2 Waste Gas Treatment Process Flow

Process Description:
Considering that the waste gas may contain oil and sticky substances, these can adhere to the lower layers of the RTO (Regenerative Thermal Oxidizer) heat recovery media during operation. To prevent interference with the normal operation of the RTO, a high-temperature purification gas line is set up for periodic heating and desorption. When mixing occurs, waste gas enters the mixing chamber and combines with the high-temperature gas (~800°C) from the RTO, effectively removing sticky substances from the lower heat recovery media. To prevent fire hazards due to long-term accumulation of oil and sticky materials in the pipelines, a fire damper is installed at the fan inlet.

Waste gas is drawn through ducts into the rotary RTO (Regenerative Thermal Oxidizer) main fan and introduced into the rotary gas distribution chamber. After uniform distribution, the gas enters five heat recovery chambers for preheating. Preheated waste gas then enters the thermal oxidation chamber with full turbulence. Under the effect of auxiliary fuel or when the waste gas concentration is sufficient, the VOCs in the waste gas are fully oxidized and decomposed, maintaining an oxidation temperature above 800°C. The resulting high-temperature clean gas enters the other five heat recovery chambers to release heat, which is stored in the ceramic heat recovery media, and then discharged through the stack (temperature difference ≤25°C). Part of the clean gas is extracted via a purge fan to another heat recovery chamber for purging, returning unreacted organics in the media to the oxidation chamber for decomposition. In this case, the VOC concentration alone cannot maintain the oxidation chamber temperature at 800°C, so natural gas is supplemented as fuel to keep the RTO oxidation chamber temperature.

Since the waste gas may contain oil mist, the RTO heat-recovery media are at risk of blockage, which can cause exhaust obstruction and reduce RTO heat-recovery efficiency. The following measures are adopted to prevent blockage:

1. Large-pore ceramic heat recovery media at the RTO bottom;
2. Optimized layout and inspection port arrangement of RTO heat recovery media.

III. Equipment Selection Factors

1. Front-end Process Selection
During plastic production, VOCs and odors are both generated. The average odor concentration of plastic waste gas is around 5000 (dimensionless). For waste gas generated during melting, initial collection is typically through closed pipelines and activated carbon adsorption, which can continue to use existing pipelines. However, for odors emitted during production, collection is required. In this scheme, gas hoods are installed at odor generation points on each pelletizing line to capture plastic waste gas, which is drawn under negative pressure by fans. Collected waste gas is merged into the original VOC collection duct or directly sent to the mixing chamber. For example, a plastic pelletizing plant with nine pelletizing lines has an uncontrolled emission rate of 2,500 m³/h per line, converging to a total of 22,500 m³/h through the main duct.

2. Back-End Equipment Selection
Available purification methods in the plastic pelletizing industry include electrostatic, adsorption, and absorption methods. However, these approaches have low efficiency, incomplete odor removal, and may generate secondary pollution, making it difficult to meet emission standards. Waste gas contains complex VOCs, with large air volumes and low concentrations, but its main components are hydrocarbon combustibles that can fully oxidize when exposed to air at temperatures above 790°C for more than 0.5s. Current mainstream thermal treatment technologies include RCO and RTO. Due to generally low VOC concentration, complex composition, and potential catalyst poisoning or failure in RCO, along with high investment cost for large-volume RCO and catalyst lifespan limited to 8,000–100,000 hours, the back-end treatment method prefers energy-efficient rotary RTO.

IV. Emission Standards

Table 3: Odorous Pollutants Emission Standard (GB14554-1993)

Table 4: Integrated Emission Standard of Air Pollutants (GB16297-1996)

V. Social Benefits

Using rotary RTO equipment to treat VOCs from plastic pelletizing can achieve the following:

• Non-methane hydrocarbons and other indicators comply with national emission standards;
• Odors in the waste gas can be effectively removed;
• Energy-efficient, stable operation and environmentally friendly production are ensured.