A new laboratory method to characterize the sticking properties of free-flowing solids

The flow properties of solids, both powders and granulated materials, depend on moisture and temperature. If a free-flowing solid with a given moisture content is heated beyond a certain temperature, its flow properties change abruptly, as the particles begin to stick to each other once this temperature is reached. This temperature is called the sticking point. It declines with increasing moisture content in a given solid. The sticking behavior of a solid in the presence of the solvent must be known, as must its thermal decomposition temperature and the solid–vapor sorption isotherm, in order to design a drying unit for the solid that operates in a safe and reliable way. The operational diagram of a dryer calculated from these system properties is explained in detail. The sticking behavior of solids is measured with a newly developed laboratory apparatus and may thus be determined significantly faster and more reliably than with previous methods. A rotating sample of the free-flowing solid with a known moisture content—either water or solvent—is subjected to a temperature program. The motion of the product is observed with an optical sensor and recorded. The sticking point is the temperature at which the product assumes a different flow behavior, thus causing significant change to the sensor signal.