Web coating or converting is a process of applying specialized coatings over a wide variety of substrates or webs. A web is a long, thin, and flexible material. Common webs include paper, foil, plastic, film, metal and wire.
Web coating or converting is used in many industrial applications including the production of pressure-sensitive tapes and labels, flexible packaging, photographic film, coated vinyl, wall coverings, sandpaper and other abrasives, paperboard boxes, vinyl flooring, industrial and decorative laminating, carbon paper and carbonless paper, circuit boards, and business forms.
The coating process consists of applying a coating material to a moving web of flexible substrate. Different types of coating lines may be found in each industry, but broadly speaking each system is based on a roll design and will consist of a metering system for applying the coating material to the substrate, followed by a drying tunnel to dry the coated web before it is rewound. The coatings provide the substrate with enhanced aesthetic and physical properties derived from the coating material. These specialized coatings contain volatile organic compounds (VOCs) such as: toluene, methanol, methyl ethyl chloride, ethylene glycol, xylenes, phenol, methylene chloride, glycol ethers, hexane, methyl isobutyl ketone, cresols, cresylic acid, dimethyl formamide, vinyl acetate, formaldehyde, and ethyl benzene which when evaporated during the coating process, react in the atmosphere to form ozone which can cause adverse effects on human health.
The following areas in web coating & converting operations may generate significant VOC emissions which are released to the environment:
Thermal and Catalytic Oxidation are the most widely accepted control technologies available to control the VOC emissions from the web coating manufacturing process. Oxidation of VOCs and other air pollutants works by simply oxidizing the air pollutants with oxygen and heat. In this environment the VOCs are converted to harmless inert byproducts like CO2, water vapor (H2O), and usable heat. These harmless by-products are released to the atmosphere or used within primary or secondary energy recovery techniques to further lower the operational costs.
A Regenerative Thermal Oxidizer (RTO) is recognized as the most effective and cost-efficient way to destroy VOCs from web coating processes. Each Regenerative Thermal Oxidizer (RTO) system incorporates specialized ceramic media in the regenerator (heat transfer bed) to allow thermal efficiencies up to 97%. RTOs operate by preheating the process air as it passes upward through the first ceramic heat exchange media. This pre-heated air then exits the heat exchange media and enters a combustion chamber where it is heated to the setpoint temperature and oxidization of the VOC’s is completed. Hot, clean air is then passed downward through the second ceramic heat exchange media bed where it is cooled. This regeneration of the heat exchanger allows for much higher thermal efficiencies than traditional recuperative style heat exchangers.
Silicone Dioxide (SiO2) Carryover
Many industries utilize silicone as a necessary additive for various coated products. Silicone is vaporized in the drying process along with the VOCs and is forced into the dryer exhaust. Silicone when heated above certain temperatures will form an inorganic particulate, Silicon Dioxide (SiO2), which clings to hot metallic surfaces and forms a destructive and efficiency robbing barrier layer. SiO2 will poison catalytic oxidizers and can affect Regenerative Thermal Oxidizer (RTO) valving and clog ceramic media. SiO2 will create boundary layers in typical shell and tube heat exchangers.
While the air pollution control equipment is installed to destroy the VOCs, any system must be designed to also process the silicone vapors. By utilizing a special silicone media design, providing reduced pressure drops and the highest thermal efficiencies, the RTO has the ability to destroy VOCs and tolerate small amounts of particulate carryover including silicone.
For processes that produce considerable amounts of particulate carryover or high SiO2 loadings, the QUADRANT SR Silicone Series Thermal Oxidizer provides an economical answer to the disastrous effects of SiO2. Special design features include:
To learn more about VOC abatement, click here.