Chemical Waste Gas Collection and RTO Treatment Process Requirements
In the treatment of VOCs (waste gas) in the chemical industry, the Regenerative Thermal Oxidizer (RTO) is widely used as the core treatment equipment for medium- and high-concentration organic waste gas. An efficient and stable Regenerative Thermal Oxidizer System depends not only on the equipment itself but also on upstream waste-gas collection methods, gas-composition analysis, and supporting process design. Therefore, scientific and reasonable waste gas collection and targeted Regenerative Thermal Oxidizer treatment processes are the key to achieving compliant emissions and safe operation.
I. Chemical Waste Gas Collection
Chemical waste gas refers to toxic and harmful gases discharged from chemical plants during production. Before entering the Regenerative Thermal Oxidizer system, the waste-gas collection method directly affects treatment efficiency, energy consumption, and system stability. VOC emissions in the chemical industry mainly include hydrocarbons, alcohols, aldehydes, acids, esters, ketones, hydrogen sulfide, mercaptans, ammonia, amines, nitro compounds, and organic halogen derivatives.
Sources of chemical odors include both organized emissions and unorganized emissions. Unorganized emissions can be converted into organized emissions after collection, fundamentally reducing emissions and improving the operational stability of the Regenerative Thermal Oxidizer system.
There are three common collection methods:
- Direct collection at the equipment exhaust outlet, allowing waste gas to enter the RTO system directly;
- Overall enclosure of equipment to increase the concentration of waste gas entering the RTO;
- Overall enclosure of the production area, with centralized conveyance to the RTO system.
Organized emissions mainly come from storage tanks, structures, and production equipment. Storage tank emissions refer to VOCs released through breathing valves or nitrogen blanketing systems during material filling or pressure increase. Structural emissions mainly come from wastewater tanks and waste liquid tanks. Emissions from production equipment include those from reaction equipment, transmission equipment, separation equipment, containers, heat exchange equipment, and drying equipment, including vacuum exhaust, atmospheric exhaust, and high-pressure exhaust.
For unorganized emissions, common collection methods include hoods such as top hoods, side hoods, bottom exhaust, semi-enclosed collection, and fully enclosed collection. These methods directly affect the airflow volume and concentration stability entering the Regenerative Thermal Oxidizer system.
A collection hood is a device used in flue gas purification systems to collect pollutants, guiding dust and gaseous pollutants into the purification system while preventing their dispersion into the workshop and atmosphere. Its performance directly impacts the treatment efficiency and energy consumption of the RTO system. Due to differences in equipment structure and production processes, hood designs vary widely. Air curtain systems can be used to suppress pollutant diffusion, featuring low air volume, strong resistance to interference, no impact on process operations, and good performance.
An enclosure hood isolates part or all of the pollution source, maintaining negative pressure inside to prevent uncontrolled diffusion. It features the lowest required exhaust volume, the best control effect, and is not affected by indoor airflow. It includes full enclosures and partial enclosures. Full enclosures have large volume and good sealing, suitable for equipment with multiple dust sources, high airflow velocity, or vibration, such as centrifuges and wastewater tanks. Partial enclosures are smaller, consume less material, and are easier to operate and maintain, suitable for fixed dust sources with low airflow velocity and continuous emissions, such as solid material feeding stations and filters.
(1) External capture hoods collect pollutants near the source through suction. They have simple structures and are easy to manufacture; however, they require large air volumes and are susceptible to cross-airflow interference, resulting in lower capture efficiency. These include top hoods, side hoods, bottom hoods, and slot hoods. They are suitable for reactor feeding ports, raw material barrels, and similar sources.
II. Special Waste Gas Treatment Process Requirements in the Chemical Industry (Key RTO Applications)
1. Chlorine-Containing Waste Gas Treatment
Principle: Inorganic chlorine (HCl, Cl₂) should be pretreated and absorbed first (water scrubbing + alkaline scrubbing, single or multi-stage depending on concentration). Organic chlorine should preferably be collected and treated separately. If separate collection is not feasible, corrosion protection and post-treatment must be considered in the Regenerative Thermal Oxidizer system.
(1) After pretreatment, determine whether dehumidification is required based on moisture content (relative humidity ≥100% requires dehumidification).
(2) Corrosion protection should be considered for: rotary valves and gas distribution chambers, low-temperature pipelines, ceramic media support structures, chimneys, valves, and main fans.
(3) Anti-corrosion materials:
Metal materials such as duplex stainless steel 2205, 2507, and super stainless steel 904L can significantly extend the service life of the RTO system.
(4) Post-treatment: cooling + alkaline scrubbing (to absorb HCl and trace phosgene).
(5) Dioxins
① Formation conditions: at 300–600°C, under oxygen conditions with insufficient residence time, aromatic compounds and their derivatives react with chlorine to form dioxins, catalyzed by metal oxides.
② Control measures: dioxins begin to decompose at 700°C and are completely decomposed at 800°C within 2 seconds. Control measures include maintaining combustion chamber temperature at 850–950°C and ensuring residence time ≥2 seconds.
(6) Phosgene
Formation condition: produced during the combustion of aliphatic chlorinated organic compounds.
Treatment: decomposes into Cl₂ at high temperature; hydrolyzes into HCl and CO₂ in water; can be absorbed by alkaline solutions.
2. Sulfur-Containing Waste Gas Treatment
Principle: Inorganic sulfur (SO₂, H₂S) should be pretreated and absorbed first (water scrubbing + alkaline scrubbing or alkaline scrubbing alone). Organic sulfur is oxidized in the RTO to form SO₂ and SO₃, which require post-treatment (cooling + alkaline scrubbing).
(1) After pretreatment, determine whether dehumidification is required (relative humidity ≥100%).
(2) Corrosion protection requirements: determined based on sulfur content and acid dew point. For low sulfur and low dew point, 316L is sufficient; for high sulfur and high dew point, duplex stainless steel 2205 is recommended.
3. Nitrogen-Containing Waste Gas Treatment
Principle: Inorganic nitrogen compounds such as NO₂ should be pretreated (water scrubbing + alkaline scrubbing). NO entering the RTO requires post-treatment (cooling + alkaline scrubbing). Organic nitrogen compounds generate NOx after combustion and require post-treatment (SCR or SNCR).
(1) Determine whether dehumidification is required after pretreatment (relative humidity ≥100%).
(2) Rotary valves should be made of 316L material.
(3) Combustion products:
① NH₃ combustion produces N₂ and small amounts of NOx.
② Organic nitrogen compounds produce a mixture of N₂ and NOx. NOx formation depends on temperature, nitrogen and oxygen concentrations, fuel characteristics, and residence time.
4. Key Operational Risks and RTO Design Optimization
The following side reactions are critical in RTO systems:
(1) NH₃ + HCl → NH₄Cl (causes clogging of ceramic media)
(2) NH₃ + SO₃ → (NH₄)₂SO₄ (causes clogging of ceramic media)
(3) NH₃ + NO₂ → NH₄NO₃ (causes clogging of ceramic media)
Engineering measures (key RTO optimization points):
- Use anti-clogging ceramic media
- Provide maintenance access doors
- Regular cleaning of the regenerative beds
These measures are essential for the long-term stable operation of the RTO system.
Summary
In Vocs Treatment within the chemical industry, the Regenerative Thermal Oxidizer is not only the core equipment but also the central element of the entire system design. From waste gas collection methods to composition analysis, as well as corrosion protection, pretreatment, and post-treatment, every step directly affects the performance and lifespan of the Regenerative Thermal Oxidizer system.
Only by achieving:
- Proper collection (improving waste gas quality entering the RTO)
- Accurate pretreatment (protecting the RTO system)
- Scientific design (enhancing RTO efficiency)
Can a safe, efficient, stable, and compliant RTO waste gas treatment system be realized.