A Sticky Subject, Part 3
Continuing my discussion on how to use reverse-pulsing dust collectors to control solid particulate emission and recover final product, in this issue, I’ll conclude the series by looking at potential trouble spots.
Implicitly, exhaust streams in dryers contain moisture. If this moisture is permitted to condense, it will create a “sticky situation” and lead to blinding, buildup and bridging. One occurrence of condensation can potentially take down the plant for an extended period of time. Condensation problems frequently occur during or soon after a plant shutdown. This is because exhaust air is allowed to cool down inside the dust collector, which permits moisture to condense, forming moisture droplets on the surfaces.
To avoid this potential trouble spot, be sure that the dryer goes through a shutdown cycle that will stop the feed, purge the moisture-laden process gas and allow the gas to slowly cool under atmospheric humidity conditions.
More commonly, the moisture that affects the efficiency and operation of a baghouse is introduced by the compressed air used for pulsing. If wet compressed air is introduced, it will cause the bag to become wet, allowing the dust to stick to it.
To prevent this from occurring, the compressor needs a system to dry and filter the compressed air so that it supplies dry air for pulsing. The compressed air lines also need to be designed so that condensation is not trapped in the line, potentially finding its way inside the bags.
Then, there is acid. Sulfur as sulfur dioxide will form sulfuric acid that, if permitted to fall below its dewpoint, will allow a chemical attack on both the filter housing and the media. The acid dewpoint commonly is higher than the moisture dewpoint.
To overcome acid condensation issues, you need only provide sufficient insulation and maintain the exhaust temperature at a suitable setpoint. Also, you may want to select materials of construction (steel components as well as the fabric) so that the acid will not cause significant chemical attack.
Many drying systems employ reverse-pulsing (also referred to as pulse-jet or pulsing) dust collectors to control the solid particulate emission and recover the final product. Reverse-pulsing dust collectors rely on media filtration to clean the air. In using media filtration technology, a number of factors will influence the separation efficiency. Take all of these tips into account, and the specifics of your process, to ensure successful implementation.