How to recycle solvent exhaust gas from the printing workshop? Collection and condensation configuration of ink drying exhaust gas

The packaging and printing industry is a major user of solvents: the inks used in gravure printing and flexo printing are mainly composed of ethyl acetate, ethanol, and isopropanol. Drying ovens emit a large amount of solvent vapor, and there is also sporadic evaporation during cleaning and ink mixing in the workshop. The annual consumption of ethyl acetate for a medium-sized gravure printing production line is measured in hundreds of tons, and the economic benefits of recycling are very clear - what is recycled is solvent that can be directly reused, while what is released is an emission burden. The treatment of exhaust gas in printing workshops revolves around the main battlefield of oven exhaust.

Characteristics of exhaust gas from printing workshop

Firstly, high air volume and low concentration: To ensure the drying effect, the drying oven uses high air volume for purging, which dilutes the solvent concentration in the exhaust gas. This is the biggest difference between printing exhaust gas treatment and chemical workshops. Secondly, the components are relatively stable: After the ink formula is determined, the exhaust gas components change little from batch to batch, with ethyl acetate usually accounting for the majority. This is a favorable condition for the configuration of the recovery section. Thirdly, the rhythm is clear: it starts immediately after startup and stops immediately after shutdown, with the load following the production shifts.

Stable components and clear rhythms mean that the device can be designed according to typical operating conditions, without having to leave excessive margin for extreme operating conditions, resulting in higher investment efficiency.

How to collect exhaust from an oven

The exhaust air from the drying oven is centrally discharged through the exhaust vent of the oven, with each color group branch converging into the main pipe. An air valve is installed on the manifold, which is opened and closed in conjunction with the activated color groups to prevent dilution of the concentration in the idle color groups. The temperature of the exhaust air from the drying oven is usually above 70 to 80 degrees Celsius. The collection pipeline undergoes heat exchange and cooling first, and the waste heat is reused for preheating the fresh air. The cooled exhaust gas then enters the treatment section, where the load and selection of equipment are reduced.

For scattered emissions (such as ink mixing and cleaning) in the workshop, a separate small-volume collection branch is set up and merged into the main pipe for joint treatment. For instantaneous high-concentration operations such as printing plate cleaning, fixed points are arranged, and the branch is activated during the operation period.

How to proceed with the route of concentration and condensation

Direct condensation of high-volume, low-concentration exhaust gas is uneconomical. The mainstream approach is to first concentrate and then condense: zeolite rotors or activated carbon adsorption are used to enrich the solvents in the high-flow exhaust gas into a low-flow, high-concentration desorbed gas, which is then sent to the condensation recovery section. The concentration ratio determines the scale of the downstream condensation section - a higher concentration ratio results in a smaller condensation section, saving both investment and energy consumption. The condensed recovery liquid is a mixed liquid primarily consisting of ethyl acetate, which is dehydrated and reused according to purity levels: high-purity liquid is used to replenish ink, while slightly lower purity liquid is used as a cleaning agent.

The accounts for recovered liquid and ink requisition are reconciled monthly: the requisition quantity minus the loss should match the recovery quantity. For months where the accounts do not match, first check the collection seal and the efficiency of the rotary wheel.

Configuration key points reference

How to implement operation management

1. The oven branch is linked to the air valve according to the startup color group, and the no-load color group is closed;
2. Waste heat recovery from exhaust gas and preheating of fresh air reduce the energy consumption of the drying oven;
3. The concentration ratio of the concentration rotor should be periodically verified, and if the desorbed gas concentration decreases, the rotor should be inspected first;
4. The temperature level of the condensation section is set according to the concentration of desorbed gas, and the condensate is tested batch by batch;
5. Recycled liquid is reused according to its purity level, and the dehydration process is determined based on the intended use;
6. The quantity of recycled materials and the quantity of requisitioned materials are reconciled on a monthly basis, and the discrepancy is reviewed and adjusted month by month.

Frequently Asked Questions

Can the air volume of the exhaust from the drying oven be reduced?

Yes, but it is necessary to ensure the quality of drying: control the air volume by zones and sections, adjust the air volume of each section according to the solvent evaporation curve, and reduce the total air volume on the premise that the drying is qualified. As the air volume decreases, the loads of the collection, concentration, and condensation sections also decrease, which is a reduction at the source of the treatment end.

Can the recycled ethyl acetate be directly used to reconfigure ink?

Once the dehydration and purity meet the standards, it can be reused. Moisture has a significant impact on the printability of ink. The moisture and purity of the recycled liquid should be tested batch by batch. The ink that meets the standards can be reused, while the ink with slightly lower standards can be downgraded and used as a cleaning agent. The account for graded reuse is the clearest.

What is used to protect the concentrating rotor?

The filter and cooling system should be installed before the rotary wheel to protect it from damage caused by ink mist and high temperatures. The temperature and concentration monitoring of the high-concentration desorption gas system must be in place, and safety accessories should be regularly inspected according to regulations. The system safety design for this section should be overseen by a professional party.

This article is compiled by Luoyang Wohong Petrochemical Equipment Co., Ltd., which provides design, manufacturing, and operation and maintenance services for condensation recovery, membrane separation recovery, and complete solvent recovery equipment to chemical and pharmaceutical enterprises. If you need process evaluation or scheme configuration, please feel free to contact us for discussion.