A common application for Venturi scrubbers is potash mining. Venturi scrubbers are used to remove particulate from industrial dryers that dry the potash material. A cyclonic separator or chevron style entrainment separatordownstream of the Venturi removes particulate–laden water droplets from the gas before it is exhausted into the atmosphere. The type of entrainment separator can impact the system performance and make a significant
difference in plant emissions. A well designed chevron style mist eliminator achieves significantly lower emissions than a cyclonic separator. This is because the chevron style mist eliminator allows the Venturi scrubber to operated at a higher pressure drop for the same system pressure drop. The chevron entrainment separator is also more efficient at removing water drops.
A chevron style mist eliminator removes more than 99% of the water droplets down to 25 microns with less than 0.25 inches W.C. of pressure drop. By comparison, a cyclonic mist eliminator requires 4 to 6 inches W.C. of pressure drop to remove 98% of 25 micron droplets. The additional pressure drop required by the cyclonic separator effectively reduces the pressure drop that can be applied to the Venturi throat and consequently lowers collection efficiency. The additional water droplets that escape the cyclonic separator (98% vs. 99% removal for 25 micron water droplets) contain particulate which will impact stack test results.
Consider an air permit for a potash facility located in the Southwestern United States. The permit allows for the operation of two 2 dryer systems, each with a Venturi scrubber. The dryers contribute significantly to the facility wide total suspended particulate (TSP) emissions. A typical potash dryer for this size of facility exhausts around 40,000 acfm with a particulate loading as high as 3 gr/dscf. This is a little more than half a ton per hour of particulate for both dryers.
Venturi scrubbers with cyclonic separators designed for 99% removal result in approximately 10.4 lb/hr of particulate emissions for both scrubbers or 46 tpy. However, an equivalent Venturi scrubber with a chevron style mist eliminator achieves 99.9% removal. This results in less than 1 lb/hr of particulate emissions for both dryers, or less than 5 tpy. The higher performing Venturi scrubber design reduces plant emissions by over 40 tpy. This performance increase can have a significant difference in air quality for the surrounding community.
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