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Pneumatically conveying hygroscopic materials
What are some tips for handling my hygroscopic material in a pneumatic conveyor?
Eric Maynard, Jenike & Johanson, says: A hygroscopic material "readily takes up and retains moisture," according to Merriam-Webster's Collegiate Dictionary. Common hygroscopic bulk solids include sugar, salts, phosphates, and acids. Often the finer the powder, the greater its hygroscopicity because of its higher surface-area-to-volume ratio.
The effects of moisture uptake can include mold, rot, or fermentation with food powders; caking or self-heating with chemicals; and stickiness, buildup, and cohesion. In many cases, moisture influx to a pneumatic conveying system handling a hygroscopic powder results in reduced solids conveying capacity due to gradual buildup, and in severe situations, complete line plugging.
A common response to moisture problems is to use a dehumidifying or drying system for the conveying air or other gas. But before implementing such a system, run moisture sorption isotherms to quantify your material's hygroscopicity. By doing this, you can discover, for instance, that sugar can be conveyed reliably with air provided it's below 50 percent relative humidity and 90ºF; exceeding these levels will induce buildup, deliquescence, and possibly melting.
Knowing your material's hygroscopicity will help you properly select air drying equipment so you can avoid unnecessary use of bone-dry air, which is very costly and increases electrostatic sensitivity and explosivity.
Besides moisture sorption testing, cohesive strength tests per ASTM Standard D6128 can help determine how sticky a bulk solid will become with moisture pickup. You can also use the test data to calculate bridging and ratholing potential in a bin.
Once you determine the effects of moisture sorption, you can implement proper strategies for handling the hygroscopic material. For instance, selecting positive pressure instead of vacuum conveying will limit ambient (and humid air) ingress to the conveying line since leakage is always outward. Using a heat exchanger and then an in-line desiccant dryer can effectively reduce the air's temperature and moisture content.
Knowing how dry the air must be is vital to equipment selection – it's rare that 40ºF-dew-point air is required! But in some cases, a complete air-conditioning system is necessary to lower the air's relative humidity to a safe operating level.
Eric Maynard is senior consultant and director of education at Jenike & Johanson.
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