*3.1. Technical Performance Evaluation*

3.1.1. Temperature Variation during Drying

Temperature variation in the plenum and the drying chamber, compared to the ambient during the drying process, is shown in Figure 5. The plenum temperature increased steadily from 38 ◦C to a maximum of 58 ◦C within the 5 h drying period. This resulted in a corresponding increase in the air temperature in the drying chamber from 35 ◦C to a maximum of 44 ◦C during the same period. Average temperatures of 51.5 ± 4.8 ◦C and 38.5 ± 2.8 ◦C were recorded at the plenum and drying chamber of the column dryer, respectively. The mean temperature in the drying chamber was 9 ◦C higher than the ambient temperature.

As a common phenomenon with column drying systems, there was not much difference in the drying air temperatures at different levels, L1, L2, and L3, as shown in Figure 6. In the drying chamber, average temperatures of 39.4 ± 2.3, 36.1 ± 1.2 and 39.9 ± 2.5 ◦C were recorded at L1, L2, and L3, respectively. A similar observation was made by Alam et al. [20] and Kumar et al. [21], who worked on the performance of a column drying system where there were no substantial differences in temperatures between the top, middle, and bottom sections of the drying chamber.

**Figure 5.** Temperature variations in the plenum and drying chamber during the experiment.

**Figure 6.** Temperature variations at L1, L2, and L3 in the drying chamber.

3.1.2. Moisture Content Variation During Drying

The variation in moisture content at the inner and the outer sections of the column dryer during the drying experiment is shown in Figure 7. The grain moisture content decreased with drying time, with grains closer to the plenum (inner section) reaching a lower moisture content after 5h of drying compared to grains close to the outer edge of the drying column (outer section). The drying process occurred in the falling rate period, as shown in Figure 7, where the moisture content of maize decreased from 22.3% wet basis (w.b.) to 11.6 ± 0.3% and 15.4 ± 0.3% for grains at the inner and outer sections, respectively, within the 5 h drying period.

Variations in the moisture content of grains sampled transversally across the drying chamber could be attributed to variation in the drying air temperature. It is forced through the drying bed with all the grains not fully exposed to the same drying air condition which is established by plenum. As demonstrated in Figure 8, the grain mass at the inner section of the dryer is exposed to drying air of high temperature compared to the grain mass at the outer section. As drying air moves from the plenum across the mass of maize, moisture is lost from the grains to the drying air, thereby increasing the humidity of the drying air along the transversal depth of the maize grains towards the outer section. According to Chakraverty and Singh [22], this is a common phenomenon in deep bed dryers, which leads to grains at the inner section drying faster compared to grains at the outer section.

**Figure 7.** Moisture content variation with time at the inner and outer sections of the dryer.

**Figure 8.** Schematics of the deep bed drying principle.

The findings of this study are corroborated by Kumar et al. [21]. They made similar observations in moisture content variations of wheat and maize grains dried in a similar deep bed dryer. The final moisture content of dried samples was 10.76% and 10.84% for the inner and outer sections, respectively.

The moisture contents of maize grains at different levels L1, L2, and L3, as shown in Figure 2, along the longitudinal depth at the inner and outer sections of the dryer was analyzed. The analysis of MC of grains across the longitudinal depth at both the inner and outer section of the dryer did not vary significantly, as shown in Figure 9. Grains at different levels at both the inner and outer sections for the drying chamber reached 11.6 ± 0.3% and 15.4 ± 0.3% moisture contents, respectively.

**Figure 9.** Variation of moisture contents at L1, L2, and L3 at the (**a**) inner section of the drying chamber; and (**b**) outer sections of the drying chamber.

To ensure that maize dried in a column dryer reaches the safe moisture content of about 13% (w.b.) before storage in tropical weather conditions like Ghana, Kaaya and Kyamuhangire [11] recommend that thorough mixing of grains close to the inner and outer sections during unloading should be encouraged.

#### 3.1.3. Dryer Performance Specification

Table 1 shows the column dryer's technical performance, which satisfies the drying needs of smallholder maize farmers. The average temperature distribution of 38.5 ± 2.8 ◦C in the drying chamber was not too much of a drying temperature that can result in the loss of seed viability [2,23,24]. This is an essential consideration for adopting grain dryers as about 80% of smallholder grain farmers rely on their seed stock from their previous harvest [2,25]. Hence, using dryers that tend to reduce the seed viability of their harvest, usually due to high drying temperatures of 70–100 ◦C, should be avoided. More so, the designed capacity of the grain dryer matches the harvesting rate of grain farmers, making the column dryer suitable for adoption by grain farmers [12].

**Table 1.** Summary of dryer technical performance.

