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How temperature, humidity affect material flow behavior
Q: How do changes in temperature and humidity affect material flow behavior?
Powder and bulk solids such as chemical, mineral, plastic, pharmaceutical, or food materials absorb moisture based on the equilibrium moisture content (EMC) or the material's moisture sorption characteristics. The EMC is defined as the moisture content of a material that produces vapor pressure equal to the environment vapor pressure at the same temperature and relative humidity (RH). When a material is at EMC, there's no moisture adsorption or desorption.
The factors that affect the material's EMC are the surrounding environment's temperature and RH, which are considered separately from the material's properties. The environment's temperature and RH change during the course of a day, and the material absorbs or desorbs moisture based on the temperature and RH of the particular hour of the day. Materials commonly experience a higher temperature and lower RH during the day and a lower temperature and higher RH at night. During higher EMC periods (low temperature and high RH), material can absorb moisture from the environment. This increase in moisture content significantly affects the material's flow property. Likewise, during a low EMC period (high temperature and low RH), a material may desorb moisture. This decrease in moisture content eventually increases the material's static electricity.  Understanding the moisture sorption characteristics of your material under different temperatures and relative humidity levels is important as those characteristics relate to a material's flow property. The two types of moisture sorption characteristics are adsorption and desorption. Adsorption is the adhesion in an extremely thin layer of molecules to the surface of a solid body with which they come in contact. Desorption means "to remove" and is the reverse of adsorption. The desorption characteristic curve is always greater than the adsorption curve, and the difference is called hysteresis. Hysteresis occurs when there is a slowing effect when the forces acting on a body are changed. For example, the moisture sorption characteristics of the pharmaceutical powder magnesium stearate is shown in Figure 1. Figure 1 shows that the moisture released during desorption and the moisture added during adsorption weren't the same for this example material. This could be because the kinetics of filling moisture in the material's capillary pores is different than the kinetics of removing the same material's moisture.
So, how can moisture adsorption affect the flow properties of your material? When a material absorbs moisture from the environment that material's flowability decreases. This is because the increased thickness of the material's adsorbed liquid layer increases the strength of liquid (capillary) bridges developed between particles. This process continues during storage or at stagnation points in a pipeline, which can lead to caking or crust formation. Similarly, adsorbed moisture increases the material's cohesiveness because of the increase in surface tension. In some cases, interlocking due to surface roughness can be reduced by an increase in moisture. In this case, the particles' angle of internal friction increases with an increase in moisture content. In some other cases, interlocking due to surface roughness can be reduced by an increase in moisture.
Another important force that can affect a material's flow property is adhesion. The total force of adhesion is given by the formula (Stewart, 1986): Fab = Fv + Fc + Fe + Fes
Where:
Fab is the force of adhesion,
Fv is the Lifshitz-van der Waals' force,
Fc is the capillary force,
Fe is the electrical force, and
Fes is the electrostatic (Coulomb) force.
The percentage of water in the material determines the strength of capillary forces. When the ambient air's RH is above 65 percent, capillary force dominates over all other forces within the material. That is, moisture's effect on material flowability dominates. Therefore, by monitoring the environment's RH and temperature to keep the RH below 65 percent, you'll be able to control the capillary force, which can help prevent flow problems, including those caused by adhesion.
Here are two examples of how this can occur in your plant:
- Wheat flour can hold significant amounts of moisture. However, when it's conveyed into a silo, the pressure blower's high temperature may cause moisture to desorb, causing flow issues, wetness, and mold in the silo. The problem can be avoided by cooling the conveying air and using a vacuum instead of a pressure system because vacuum has a lower operating temperature. You could also add a blanket of dry air to the silo with a desiccant dryer.
- When pneumatically conveying hygroscopic material, the material absorbs moisture from the environment if the ambient air's RH is greater than the material's RH. This RH imbalance will change the material's flow property and chemical reactions and create line buildup. Attaching a desiccant or refrigerant dryer to the conveying line's air inlet is a common practice to reduce high-RH conveying air. The conveying lines are insulated to avoid condensation problems because of temperature differences between the conveying air and ambient air. The temperature change affects the material's EMC, and the EMC subsequently affects the material's flow properties.
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