Temperature and Moisture Gradients in Sugar Silos

Abstract

Danisco Sugar is the main sugar producer in Denmark. The sugar is extracted from sugar beets which are harvested during the beet campaign September -- December. The refined sugar is stored in huge silos and withdrawn on a regular, but not constant, basis for distribution until the next campaign. Danisco Sugar utilizes several types of silos for the storage of crystalline sugar. They differ in construction material e.g. concrete or steel plate silos, dimension, design with or without a central tower and isolation. Various methods of filling and emptying are utilized, that is first in - last out, or first in - first out. The silos are equipped with one or more facilities for sugar conditioning. Those includes temperature control of walls, control of the condition of the air space above sugar, ability to blow conditioned air through the bulk sugar etc.

A number of problems arise when storing sugar for long periods. Due to distribution of heat and moisture the sugar can `cake' or harden and moist sugar is difficult to empty from the silos. The chemical and microbial stability of the sugar is influenced by the temperature and moisture distribution. On the other hand under very dry conditions there is a risk of dust explosions. The object is to control the temperature and moisture content within a lower and an upper limit. Danisco Development Centre wishes to employ mathematical modelling as a tool for understanding and predicting temperature and moisture distribution in sugar stored in silos.

In moisture migration the physical processes which have to be taken into account are diffusion in the porous bulk sugar, natural convection due to temperature gradients, forced convection due to bulk sugar air conditioning, conditioning of the air above the bulk sugar and migration of moisture through concrete silo walls. The temperature distribution is influenced by seasonal variations, the temperature of the air space above the bulk sugar, remixing processes when filling and emptying silos, air conditioning and others. The following physical properties are assumed of importance when formulating models to describe the formation of temperature and moisture gradients: bulk sugar heat capacity, heat conductivity, density, porosity, tortuosity and sorption isotherms. Most will vary with the sugar quality (that is: crystal size, reducing sugar content and ash content), some with temperature and moisture.

Mathematical models for moisture and heat conduction in stored cereal grains have previously been developed. In modelling moisture and heat conduction in sugar our starting point is to use the approach applied in modelling moisture and heat conduction in grain and adopt the model to the case of stored sugar, including the possibility of air conditioning.

In our modelling carried out during the week of the Study Group, we used a number of simplifying assumptions, such as air flow through the sugar being dominated by forced, and constant, convection, and having processes happening fast enough, so that local equilibrium holds. (Estimates obtained at the time indicated that the temperature varies very little over the length scale of the sugar grains and their interstices.)