(304d) How to Avoid Caking and Moisture Migration in Process Equipment

Caking is the significant increase in bulk unconfined yield strength with storage time. It is caused by the formation of crystal bonds between particles. In general, the steps to caking are: first, the formation of zone or regions where the local temperature and humidity are high enough for the bulk material to pick up significant surface moisture. Second, coalescence of surface moisture on the particle forms pendules between particle surfaces. Third, dissolution of soluble components. Fourth, a change in local equilibrium condition (temperature or humidity) results in moisture migration away from the particle surface. A thermal-gravimetric method can be used to measure equilibrium moisture isotherms for the components in a mixture. This information is used to determine the locations in the mixture that will attract moisture when the humidity is high. Sorption isotherms determine the potential amount of moisture at these particle junctions. The rate at which moisture will adsorb onto the particle surface from ambient air is measured. Rate constants allow computation of moisture transfer rates on particle surfaces as a function of the local temperature and humidity. A differential mass and temperature balance over the volume occupied by the material in the process equipment allows calculation of the temperature and relative humidity driving force causing moisture migration in the bulk material. Finite element calculation techniques can be employed to compute the local humidity, local temperature, and local solids moisture content in the process equipment. These calculations are combined with structure calculations of the bulk to estimate the zone and areas subject to caking. Finally, caking can be estimated from contact force models. The net result is a model that can be used to determine zones and regions where caking is problematic. This mode is then used to optimize process equipment design to avoid caking problems. This paper presents the general methodology to accomplish this task and relates it to real world situations.