CSD - PSY 301 - Seed Moisture Migration

Overview

It is generally understood that in the ear corn drying process the moisture content internal to the kernels and cobs is removed by the “process of evaporation” by means of forcing heated air through the seed pile.

While this is observed and understood by every ear corn dryer operator and probably by most people in general, the “inner workings” of the drying process is not as commonly understood.

However, a better understanding of these “inner workings” will amplify the dryer operators understanding of the complexities and difficulties of the ear corn drying process and cause them to excel in ear corn dryer management. A more in-depth understanding of the science “driving” the drying process provides insight to the dryer operator that aids in the overall management of the dryers as well as the added knowledge to troubleshoot and solve drying irregularities and anomalies.

The objective of this article is threefold:

First, we need to identify the “force” that causes the moisture of the kernels and cobs to move, or to “migrate”, from the interior of the kernels and cob to their exterior.

Second, to reveal the phenomenon that causes the evaporation of the moisture into the moving air once moisture reaches the exterior of the kernel and cob.

Third, we will discuss the role that moisture migration and evaporation of the kernel moisture has in establishing the seed Dry Rate.

The Drying “Force”

In order for the moisture to evaporate, it must first be “forced” to move from the interior of the kernel and cob to the exterior. What exactly is the physical “force” that causes this “migration” of moisture content from inside the kernel and cob needed to initiate the drying process?

Fundamentally, this unseen “force” is Vapor Pressure. Vapor Pressure exists because of differences in water vapor concentrations between two independent permeable entities. In the case of ear corn drying, it is the moisture concentration within the ears kernels and cob verses the moisture concentration of the surrounding air that establishes a Vapor Pressure difference.

This is illustrated in Figure 1 above where the Vapor Pressure internal to the kernel (P1) is much greater than the external Vapor Pressure of the surrounding air (P2).

This is due to a higher concentration of water vapor inside the seed. The concentration of water vapor in the surrounding air volume is quantified as the Humidity Ratio (Lbv/Lba). “Lbv/Lba” means “Pounds Of Vapor (water) Per Pound Of Air”.

It can be seen in the Psychrometric Chart of Figure 2 below that the higher the temperature of the air, the more water vapor it is able to keep in “suspension” within the air volume. It can also be seen that this effect is not linear. With linear increase in Dry Bulb Temperature there is a corresponding geometric increase in Humidity Ratio (HR.). This being the case, we further see that a smaller change in temperature at higher temperatures results in a much larger change in the HR. A 20°F change from 60°F to 80°F has essentially the same effect on HR as an 8°F change from 90°F to 98°F.

Humidity Ratio And Vapor Pressure

The Humidity Ratio (HR) value represents the “quantity” of water vapor that the associated air volume is capable of holding at a given temperature. The Vapor Pressure (VP) value represents the “pressure” the water vapor “quantity” at a given temperature would exert on a volume of Dry Air.

As illustrated by the Psychrometric Chart in Figure 2, the HR and the VP have an approximately linear relationship. The VP values (In. Of Hg.) are displayed on the far right vertical axis of the chart.

In Table 1 below we can see the numerical relationship between the DBT Increase, the HR Increase and the VP Increase.

Vapor Pressure Deficit

In reality there is no place on earth where actual Dry Air (0% RH) naturally exists. In some arid regions such as deserts, the RH “dips” to single digits and there have been some instances where ambient Relative Humidity of less than 1% has been recorded. However, in regions where cereal crops are grown, harvested and dried, there is always a significant moisture content held by the air volume that is utilized in the drying process.

Consequently the existing moisture content of the air volume decreases the effective “pressure” it exerts. Therefore the “effective pressure” exerted is the difference between the Vapor Pressure at air volume saturation and the corresponding Vapor Pressure at the ambient moisture content. This pressure value difference is referred to as the Vapor Pressure Deficit (VPD).

We see that it is the “effective” Vapor Pressure, technically VPD, which is the “force” behind the migration of moisture content from inside the kernel and cob to the outside. A larger VPD will always increase the Seed Dry Rate while a smaller VPD will always impeded the drying process and decrease the Seed Dry Rate.

Humidity Ratio Deficit

Once the moisture reaches the exterior of the kernel and cob, there needs to be sufficient “energy” and “space” provided by the surrounding air volume to facilitate evaporation of the moisture at the desired Seed Dry Rate. The Dry Bulb Temperature (DBT) and Humidity Ratio (HR.) of the surrounding air work in concert to provide this needed “energy” and “space”.

Increasing the air temperature is typically the most cost effective method of increasing the HR of the air. This is illustrated in the Psychrometric Chart in Figure 2 and the values listed in Table 1.

Like Vapor Pressure, the “effective” Humidity Ratio of the air is the difference between the Humidity Ratio at air volume saturation and the corresponding Humidity Ratio at the ambient moisture content. This humidity difference is referred to as the Humidity Ratio Deficit (HRD).

Seed Resistance To Moisture Migration

The physiology of the seed is another factor that has both a significant and dynamic effect on Seed Dry rate. First, the composition of the seed serves to “impede” the migration of seed moisture from the interior to the exterior of the seed. Different varieties of seeds with significantly different physiology can, and do, exhibit different fundamental drying characteristics which translate into different “impedances” to the seed moisture migration phenomenon.

Additionally, as the seed dries the seed’s moisture migration “impedance” tends to increase. Because of this, greater VPD is required to maintain the desired Seed Dry Rate.

Seed Dry Rate

We begin to see that managing Seed Dry Rate is not as simple as it first appears. It is not just forcing heated air through a seed pile. Seed Dry Rate is a formulation of numerous interrelated factors working together to methodically accomplish the removal of moisture content from the kernel and cob of ear corn. So far we have discussed the following contributors:

• Vapor Pressure (VP)
• Humidity Ration (HR)
• Dry Bulb Temperature (DBT)
• Seed Physiology And Impedance (Z) To Moisture Migration (MM)

These are the “fundamental” factors. They may are also considered the “fixed” factors that predictively work together to determine Seed Dry Rate.

There are other factors that are considered “variable” factors such as seed set, unusual or extreme weather conditions, process factors such as bin loading and the list goes on. We begin to see that what may have first appeared to be a “game of checkers” is in reality a “game of chess”.

As we continue to discuss ear corn drying, we will address these and other Seed Dry Rate influencing factors. We will also review different ear corn dryer management strategies both traditional and advanced including JHC’s Profile Drying.

Summary

The science of drying Psychrometric provides valuable insight into the “inner workings” of the ear corn drying process. “Unveiling” the relationships between the science and the process of ear corn drying provides a practical understanding of the ear corn drying that can be “translated” into dryer management strategies and procedures.

As our understanding of the ear corn drying process advances, it will help us to develop dryer management strategies that will empower the dryer manager to achieve the goals of methodical and consistent removal of kernel moisture. This in turn will result in optimized drying performance where both seed quality is preserved and dryer throughput is maximized.

Jacobsen Holz Corporation provides the seed drying “tools and knowledge” to equip your dryer operators with the information needed to maintain quality while maximizing dryer throughput.