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Ask an Expert
How to avoid liquid and solid bridging, plastic deformation with hygroscopic materials
Q: We experience significant problems with the sugar we store and use in our manufacturing process. Sugar granules cake, making this hygroscopic ingredient difficult to work with in our process. What suggestions can you give us for the proper bulk handling and storage of sugar granules?
 
imageCaking of sugar granules during handling and storage is a major problem for manufacturers using this ingredient. Sugar is highly hygroscopic in nature, meaning it absorbs moisture based on temperature and relative humidity. Common locations in a bulk solids handling system where the caking problem occurs are storage silos, hoppers, and transfer chutes in the storage area as well as filter separation units or filter receivers, elbows, and rotary valves in a pneumatic conveying system.
Common caking mechanisms that occur with a hygroscopic material like sugar granules include liquid bridging, solid bridging, and plastic deformation.
imageLiquid bridging occurs because of moisture condensation that forms on the sugar granules and binds between particles, as shown in Figure 1. The forces that bind the particles together are surface tension and capillary forces. Solid bridging occurs because of the dissolved sugar granule crystallization that occurs when moisture is dried from the sugar granule surfaces, as shown in Figure 2. Plastic deformation occurs because of the material's own weight of consolidation stress acting over a period of time. When the sugar granules are in a rubbery state temperature above the glass transition temperature, sugar granule caking is more predominant because of plastic deformation. Glass transition temperature is the temperature at which the material is between a glassy and rubbery state. A glassy state is when the material is hard and brittle, and a rubbery state is when the material is soft and flexible.
imageThe glass transition temperature of sugar (sucrose) at various water activity ratios is shown in Figure 3. The water activity in a material is the ratio of partial vapor pressure of water to the partial vapor pressure of pure water and can also be expressed as the material's equilibrium relative humidity (ERH) (which is a percentage). We can see in Figure 3 that as the water activity of sugar increases, the glass transition temperature of sugar decreases. There are two common scenarios that will lead to sugar caking based on glass transition temperatures.
Scenario I:
At constant water activity (0.25) in the sugar, the temperature of sugar increases from 40°F to 80°F, causing the sugar state to move from glassy to rubbery, which leads to caking tendency. This commonly occurs in sugar silo storage. Sugar's water activity is only 0.25, which means the relative humidity of the storage air is only 25 percent. Even at this low relative humidity, when the sugar temperature increases from 40°F to 80°F, it leads to caking.
Scenario II:
At a constant temperature of 80°F as the water activity in the sugar increases from 0.15 to 0.25, this means the relative humidity of air increases from 15 to 25 percent, causing a shift from a glassy state to a rubbery state. This change in state leads to sugar caking.
Table I
Sugar's caking time related to relative humidity
Relative humidity at 68°F Time to cake
23% < 30 days
33% 11 days
54% > 48 hours
Source: Roe and Labuza. 2005
The time taken to achieve sugar caking depends on the air's relative humidity. When the air's relative humidity increases, the time taken to cause sugar caking decreases. Table I shows the time taken to cake sugar subject to different relative humidity at 68°F. These time periods change when there's a change in the sugar's temperature, which is very common when sugar is stored.
The most common caking phenomenon in the storage of sugar is moisture migration due to temperature gradient. When the sugar is loaded into the silo, the sugar's temperature is normally higher than the ambient temperature. However, the silo can experience a wind effect where one side is cooler than the other side. In the silo's cooler side, the sugar gets cooled and the air's relative humidity increases, making the sugar absorb moisture to maintain equilibrium with the surrounding air, which leads to liquid bridging and when this liquid bridge dries that leads to a solid bridge and sugar caking in the silo.
How to avoid sugar caking
  • Keep the sugar temperature constant throughout the storage period.
  • Send dehydrated air periodically through the stored sugar to remove high-humidity air as well as to reduce the temperature gradient.
  • Keep the sugar temperature and air relative humidity below glass transition temperature.
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