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Printing waste gas is mainly composed of volatile organic compounds (VOCs), including benzene, toluene, xylene, ethyl acetate, ethanol and other organic solvents. A small amount of dust and ink peculiar smell are usually accompanied. The treatment scheme shall be determined according to waste gas concentration, air volume, composition and local environmental protection requirements. The common treatment technologies are divided into two categories: pretreatment process and end-of-pipe treatment process. The detailed introduction is as follows:
Ⅰ. Pretreatment Process
The pretreatment is mainly used to remove impurities such as dust, mist and viscous substances in waste gas, protect subsequent end treatment equipment and improve the overall purification efficiency.
1. Dry Filtration Method
(1) Treatment Principle
Filter materials such as non-woven fabric, glass fiber cotton and activated carbon cotton are adopted to intercept particulate matter and viscous substances in printing waste gas.
(2) Applicable Scenarios
Suitable for pretreatment of powder spraying and ink particles in printing workshops. Advantages: Simple equipment, low system resistance and convenient maintenance. Disadvantages: The filter material needs regular replacement.
2. Wet Scrubbing Method
(1) Treatment Principle
Waste gas is introduced into the scrubbing tower for countercurrent contact with water, alkaline or acidic absorption liquid. Water-soluble impurities and partial soluble VOCs are dissolved and captured through spraying and filler adsorption.
(2) Applicable Scenarios
Suitable for printing waste gas containing high dust content and water-soluble organic matter. Advantages: Excellent dust removal performance and effective flue gas cooling. Disadvantages: Wastewater is generated, supporting wastewater treatment facilities are required.
Ⅱ. End-of-Pipe Treatment Process
As the core link of waste gas treatment, the end process is applied to degrade and remove VOCs to meet emission standards.
1. Activated Carbon Adsorption Method
(1) Treatment Principle
Depending on the porous structure of activated carbon, VOCs molecules in waste gas are physically adsorbed to realize purification. Saturated activated carbon can be regenerated by thermal desorption or disposed as hazardous waste by qualified institutions.
(2) Applicable Scenarios
Suitable for low-concentration and large-air-volume printing waste gas, such as offset printing and flexographic printing workshops. Advantages: Low investment cost and simple operation. Disadvantages: Activated carbon is easily saturated under high concentration conditions, not suitable for independent treatment of high-concentration waste gas.
2. Catalytic Oxidation (CO)
(1) Treatment Principle
After pretreatment, waste gas is heated to 200–350℃. Under the action of catalysts (precious metals such as platinum and palladium, or non-precious metal oxides), VOCs are completely oxidized and decomposed into CO₂ and H₂O.
(2) Applicable Scenarios
Suitable for medium and high-concentration printing waste gas (2000–8000mg/m³), such as solvent-based ink waste gas from gravure printing and flexographic printing. Advantages: Low energy consumption, high treatment efficiency (≥95%) and no secondary pollution. Disadvantages: High catalyst cost; catalysts are sensitive to sulfur and chlorine impurities, which may cause catalyst poisoning and deactivation.
3. Regenerative Thermal Oxidizer (RTO)
(1) Treatment Principle
Waste gas is heated above 760℃ for direct thermal oxidation decomposition of VOCs. Ceramic regenerators recover combustion waste heat to preheat the incoming flue gas and reduce operating energy consumption. Common types include two-chamber, three-chamber and rotary RTO.
(2) Applicable Scenarios
Suitable for high-concentration and large-air-volume printing waste gas, especially benzene and ester organic waste gas from gravure printing workshops.Advantages: High purification efficiency (≥99%), high heat recovery rate (≥95%) and stable operation. Disadvantages: High initial investment and large floor area.
4. Adsorption-Desorption-Catalytic Combustion Combined Process
(1) Treatment Principle
Activated carbon is used to adsorb low-concentration waste gas firstly. Hot air is adopted for desorption after carbon saturation. The desorbed high-concentration waste gas is sent to the catalytic combustion unit for oxidative decomposition. The heat generated by decomposition can be recycled for carbon desorption to realize energy-saving circulation.
(2) Applicable Scenarios
Suitable for low-concentration and large-air-volume printing waste gas. It is the most widely used combined process in the printing industry. Advantages: Balanced investment and operating cost, high treatment efficiency and no secondary pollution.
5. Photocatalytic Oxidation Method
(1) Treatment Principle
Ultraviolet rays excite photocatalysts such as TiO₂ to generate hydroxyl radicals for VOCs oxidative decomposition. It is usually combined with activated carbon adsorption and plasma technology.
(2) Applicable Scenarios
Suitable for low-concentration and odor-based printing waste gas. Advantages: Small equipment footprint and simple operation.Disadvantages: Limited treatment efficiency, susceptible to humidity, not applicable for high-concentration waste gas.
6. Plasma Method
(1) Treatment Principle
High-voltage electric field generates high-energy plasma to crack VOCs molecular chains and realize oxidative decomposition.
(2) Applicable Scenarios
Suitable for low-concentration and small-air-volume printing waste gas. Advantages: Fast reaction speed. Disadvantages: Secondary pollutants such as ozone may be generated; low efficiency for high-concentration waste gas.
Ⅲ. Selection Guidelines for Printing Waste Gas Treatment Scheme
Low-concentration & Large-air-volume Waste Gas: Adsorption-desorption-catalytic combustion combined process is preferred.
Medium & High-concentration Waste Gas: Select Catalytic Oxidation (CO) or Regenerative Thermal Oxidizer (RTO).
Waste Gas Containing Dust and Viscous Impurities: Adopt dry filtration + wet scrubbing for pretreatment, then connect with end treatment equipment.
Small Printing Workshop with Low-concentration Odor: Adopt combined process of photocatalytic oxidation + activated carbon adsorption.
