Application of Regenerative Thermal Oxidizer in Film Coating Industry
Author: Process & Environmental Engineering Team
Category: VOCs Control | Film Coating Industry
Keywords: Film Coating, VOCs, RTO, Rotary Concentrator, Silicone-Containing Exhaust Gas
Table of Contents
1. Overview of VOCs Emissions in Film Coating Processes
2. Typical VOC Emission Characteristics in Film Coating
2.1 Common Solvents Used in Film Coating
2.2 Characteristics of Silicone-Containing Exhaust Gas
2.3 Safe VOC Concentration Limits
3. Selection of Vocs Treatment Equipment for Film Coating Plants
3.1 Treatment of Organized VOCs from Coating Ovens
3.2 Treatment of Large-Volume, Low-Concentration VOCs
4. VOCs Control Solutions for Film Coating Plants
4.1 VOCs Waste Gas Treatment Process:
4.2 Waste Heat Recovery Methods and Selection
1. Overview of VOCs Emissions in Film Coating Processes
During film coating production, a large amount of organized exhaust gas and high-airflow, low-concentration VOC emissions are generated.
These emissions have become one of the key factors restricting enterprises from achieving environmental compliance and green, sustainable development.
Therefore, the question arises: How can VOC emissions from film coating processes be effectively controlled?
2. Typical VOC Emission Characteristics in Film Coating
2.1 Common Solvents Used in Film Coating Processes
The following solvents are commonly used in film coating production, along with their typical physical and combustion properties:
| Substance | Molecular Formula | Molecular Weight | Lower Explosion Limit (%) | Density (kg/L) | Calorific Value (kcal/kg) |
| Ethyl Acetate (EAC) | C₄H₈O₂ | 88 | 2.2 | 0.895 | 6101 |
| Toluene | C₆H₅CH₃ | 92 | 1.2 | 0.870 | 10138 |
| Butanone (MEK) | CH₃COC₂H₅ | 72 | 1.7 | 0.810 | 8098 |
| n-Propyl Acetate | C₅H₁₀O₂ | 102 | 2.0 | 0.888 | 6770 |
| Xylene | C₈H₁₀ | 106 | 1.2 | 0.870 | 10295 |
| Methanol | CH₃OH | 32 | 6.0 | 0.792 | 5688 |
| Acetone | CH₃COCH₃ | 58 | 2.5 | 0.785 | 7363 |
| Cyclohexane | C₆H₁₂ | 84 | 1.1 | 0.947 | 10012 |
| Cyclohexanone | C₆H₁₀O | 98 | 1.1 | 0.950 | 8584 |
| Ethanol | C₂H₆O | 46 | 3.3 | 0.790 | 7098 |
| Isopropanol | C₃H₈O | 60 | 2.0 | 0.786 | 7902 |
| 120# Solvent Oil | Mixture of n-heptane, isoheptane, cycloheptane, etc. |
| 1.1 | 0.766 | 10695 |
2.2 Characteristics of Silicone-Containing Exhaust Gas
In the production of silicone release films, release papers, and silicone protective films, the exhaust gas contains organosilicon compounds.
The adhesives used for these products generally contain siloxanes and silicone resins as coating components, and typically use 120# solvent oil, toluene, xylene, or ethyl acetate as solvents.
During production, particularly in high-temperature oven zones, a small portion of siloxanes and silicone resins (approximately 0.7% of the coating mass) volatilizes and is discharged with the oven exhaust gas.
When such silicone-containing exhaust gas is treated in an RTO, the organosilicon compounds are oxidized under high temperature into silicon dioxide (SiO₂), commonly referred to as silica dust.
Therefore, when selecting VOC treatment equipment, the impact of silica formation on environmental control equipment must be fully considered.
2.3 Safe VOCs Concentration Limits
During film coating production, it is essential to ensure that the VOC concentration in oven exhaust gas remains below 25% of the Lower Explosion Limit (LEL) to ensure operational safety.
When VOCs concentration exceeds 25% LEL, there is a significant risk of explosion.
For this reason, LEL detectors should be installed at:
* Coating ovens
* Main exhaust ducts
* Total exhaust outlets
These detectors should be integrated with safety interlock systems and regularly calibrated to ensure safe operation.
3. Selection of VOCs Treatment Equipment for Film Coating Plants
3.1 Selection of Equipment for Organized VOCs from Coating Ovens
RTO is commonly selected as the core treatment equipment for organized VOCs emitted from coating ovens, due to the following advantages:
1. Capable of treating almost all types of organic compounds
2. Able to adapt to fluctuations in VOC composition and concentration
3. Insensitive to small amounts of dust or solid particulates in the exhaust gas
4. Highest thermal efficiency among all thermal oxidation technologies
5. Self-sustaining operation without auxiliary fuel under suitable VOC concentrations
6. Destruction efficiency up to 99.5%
7. Low maintenance workload, safe and reliable operation; organic deposits can be periodically removed, regenerative media can be replaced, overall system pressure loss is low, pressure fluctuation is minimal, and service life is long
8. Although the initial investment of an RTO is relatively high, the benefits of waste heat recovery are significant, and the investment can typically be recovered within 3–5 years.
3.2 Treatment of Large-Volume, Low-Concentration VOCs
Under typical conditions, VOCs emitted from coating heads, adhesive preparation rooms, and raw adhesive storage areas are characterized by large air volume and low concentration, usually below 500 mg/m³.
If such exhaust gas is treated directly by an RTO, the operating energy consumption will be extremely high, requiring a very large RTO capacity and resulting in high capital and operating costs.
Therefore, an adsorption–desorption concentration process is commonly adopted.
The low-concentration VOC exhaust gas is first concentrated by a factor of 10–12, and the concentrated stream is then sent to the RTO for thermal oxidation.
Comparison of Waste Gas Adsorption-Desorption Concentration Equipment
| No. | Item | Zeolite Molecular Sieve Rotor Adsorption Concentration | Fixed Bed Adsorption Concentration |
|---|---|---|---|
| 1 | Adsorption Material | Zeolite molecular sieve | Activated carbon |
| 2 | Desorption Temperature | 200°C, a higher desorption temperature enables more thorough desorption | 100°C, lower desorption temperature results in incomplete desorption |
| 3 | Desorption Speed | 1.5 m/s, approximately half the adsorption speed, ensuring thorough desorption | 0.45 m/s, limited by structural constraints, the desorption speed is 20% of the original adsorption speed, prone to uneven flow and incomplete desorption |
| 4 | Desorption Capability | High-boiling-point VOCs (boiling point above 200°C) can be desorbed through high-temperature regeneration. The concentrated VOC output remains highly stable. | High-boiling-point VOCs (boiling point around 200°C) may desorb partially, but complete desorption is difficult (leading to performance degradation). The concentrated VOC concentration is highly unstable (stable at the start but gradually declines over time). |
| 5 | Performance Retention | Highly stable performance (can maintain stable performance for about 5 years) | Performance continuously declines from the start of use, and stable performance is generally not guaranteed |
| 6 | Lifespan | Rotor lifespan is approximately 8 years | Activated carbon lifespan is approximately 1 year |
| 7 | Performance Variation | None | Continuous decline |
| 8 | Safety | High – thorough desorption, no residual solvents, and inorganic material eliminates fire risks | Low – incomplete desorption, localized residual solvents, and combustible carbon material may lead to fire if local temperatures reach the ignition point |
| 9 | Desorption Concentration Curve | At an inlet concentration of 100 mg/m³, the desorption concentration from honeycomb activated carbon shows large fluctuations, with an integrated average concentration of about 930 mg/m³, significantly lower than the 1921 mg/m³ from the molecular sieve rotor, leading to higher energy consumption for desorption in honeycomb activated carbon systems | |
| 10 | Footprint | Small (approximately 30% of the activated carbon system footprint) | Large |
| 11 | Post-treatment | Adsorption material is classified as general waste | Adsorption material is classified as hazardous waste, requiring specialized disposal; market disposal cost is approximately ¥3,000–5,000 per cubic meter |
| 12 | Material Replacement Cost | Low | High |
Precautions for Rotary Concentrator Selection:
| Status | Substance/Component | Reason |
|---|---|---|
| Difficult to absorb substances | Methanol | High polarity makes adsorption difficult |
| Cyclohexane | Structural properties hinder adsorption | |
| Formaldehyde, acetaldehyde, other low-boiling substances (<C4 alkanes, alkenes, halogenated hydrocarbons, etc.) | Low boiling point reduces adsorption effectiveness | |
| Oil mist/tar mist | Poor adsorption capability | |
| Difficult to desorb substances | Plasticizers (e.g., DEP, DOP) | High boiling point impedes desorption |
| Terpineol | Reacts and accumulates within micropores | |
| Monomeric vinyl chloride, acrylonitrile, isocyanates, other polymerizable substances | Tendency to polymerize | |
| Monoethanolamine (MEA) | Low vapor pressure hinders desorption | |
| Other amines | Chemical transformation during adsorption reduces desorption efficiency | |
| High-boiling substances (>200°C) | Difficult to desorb due to high boiling point | |
| Substances causing molecular sieve degradation | Substances with vapor pressure <20 Pa (at 20°C) | Low volatility impedes desorption |
| Acidic or alkaline substances | Degrade zeolite structure | |
| Paints/coatings | Coat molecular sieve surface, leading to deactivation |
4. VOCs Control Solutions for Film Coating Plants
4.1 VOCs Waste Gas Treatment Process:
Having reviewed equipment selection, the following section presents practical VOC control solutions and optimal process configurations used in the film coating industry.
* Large-volume, low-concentration VOCs from coating heads, adhesive preparation rooms, and raw adhesive warehouses are first concentrated by 10–12 times, and then treated in an RTO.
* High-concentration organized exhaust gas from coating oven outlets is directly introduced into the RTO, where VOCs are oxidized at approximately 800 °C into carbon dioxide and water.
The heat released during oxidation is recovered and reused to supply thermal energy to the coating process.
4.2 Waste Heat Recovery Methods and Selection
As one of the most effective VOC treatment technologies, the rotary valve RTO not only ensures the complete decomposition of organic components and compliant emissions but also generates excess recoverable heat when VOC concentration exceeds 1.5–2.0 g/m³.
By integrating waste heat recovery systems, high-temperature flue gas heat can be recovered, improving overall system energy efficiency and economic performance.
At present, waste heat recovery methods commonly applied in the film coating industry include:
* Hot air recovery
* Thermal oil systems
* Steam generation
The specific method can be selected according to the heating mode of the coating line.
