Measurement and simulation of transient moisture and heat diffusion in a potash layer

Abstract

Potash fertilizer is granular bulk product and is comprised almost entirely of potassium chloride. It is used widely, often with other fertilizers, for agricultural soils low in potassium, an essential nutrient for plant growth. It readily accumulates moisture when subjected to humid air for extended time periods during handling and storage.

In this thesis, the moisture accumulation near the surface of granular potash fertilizer products is studied. The coupled heat and moisture transfer during handling processes and warehouse storage is modeled as a coupled one-dimensional transient heat and moisture diffusion problem at the air-potash interface.

A one-dimensional transient numerical model of heat and moisture transfer, including adsorption/dissolution and condensation, is developed. Temperature and moisture accumulation profiles within potash bed are simulated and compared to measured data under a wide range of operating conditions for two types of potash. The numerical model is validated using experimental data from potash layer in a test facility subject to a wide range oftest conditions.

For humid airflow over the top of a potash layer with three different supply air humidities (i.e.45%, 65% and 80% RH), moisture accumulation and temperature distribution have been investigated under the constant room temperature (i.e.22°C) with and without a cold temperature at the bottom boundary. When the local relative humidities are less than 52% RH, water vapor accumulates on the particle surface by adsorption which causes a very small moisture accumulation. When local relative humidities exceed 53% RH, water vapor accumulates on potash by dissolution which creates a thin layer of electrolytic solution on particle surfaces and causes significant moisture accumulation. A 20°C temperature difference across the potash layer results in moisture contents 10 time larger than the case with no temperature difference, implying that the moisture transfer in potash is strongly coupled with temperature distribution in the heat and moisture diffusion processes.

To illustrate the application of the simulation model, simulations were done for the problems of moisture redistribution within potash after mixing and the rapid moisture gains for cold potash placed on a conveyor belts exposed to humid air. It is found that the process of moisture redistribution in potash will continue as long as temperature and moisture content differences persist anywhere within potash bed. The rapid moisture accumulation is only significant for a narrow surface layer which is about 10% of potash mass placed on a conveyor belts. Heating the potash on a conveyor belt near the surface layer to that of ambient air temperature could reduce the moisture accumulation by 4 to 8 times.

It is concluded from this research that the complex physical and chemical processes for water vapor accumulation in exposed granular potash can be modeled and simulated.