*3.3. Drying Kinetics*

The drying kinetics were determined by measuring the dry basis moisture content, *X*, according to Equation (2) and varying with time, for both pineapple slice thicknesses of 6–8 mm and 12–14 mm. The total drying time of 480 min was chosen together with the solar radiation intensity of 1100 <sup>W</sup>/m2. Moisture content was measured every 30 min.

Figure 9 shows the drying kinetics for both slice thicknesses. The drying curve progressions can each be divided into two parts. In the case of the lower slice thickness of 6–8 mm (Figure 9a), the first part is characterized by a high constant drying rate, d *X*/d<sup>τ</sup>, and the moisture content decreasing nearly linearly until the critical time τc = 350 min is reached. Then, the drying process transitions from the external convective heat and mass transfer to the second part. where the drying rate is decreasing and is controlled by the internal di ffusive mass transfer. For the slice thickness of 12–14 mm (Figure 9b), the drying curve is similar with the critical time being τc = 100 min. In both cases, however, the lowest possible residual moisture content in the dried pineapple is limited by the equilibrium water content in the material at a given temperature characterized by the moisture sorption isotherm [64].

**Figure 9.** Drying kinetics of pineapple slices with the thickness of (**a**) 6–8 mm and (**b**) 12–14 mm.

### *3.4. Consumption of Fossil Fuels in the Post-Solar Drying Procedure*

As stated in Section 2.4, for a typical pineapple processing rate of approximately 900 kg/d and conventional drying time of 20 h, the butane consumption is around 49.5 kg/d. Should the pineapple fruit be pre-dried using the solar dryer to the moisture content of 29.4 wt %, then the necessary post-solar drying time to reach the target moisture content of 13.7 wt % would be approximately 6.7 h. This translates to the butane consumption of 16.6 kg and, consequently, significant daily savings of 32.9 kg of butane, or 66%.

### *3.5. Modification of the Solar Collector Design*

As can be seen from Table 9, nearly the same outlet temperatures of drying air are reached with both solar radiation intensities. However, higher absolute air humidity in Togo requires a higher air flow rate. The original solar thermal collector was therefore modified [65]. This entailed changes to the absorber and the thermal insulation to ensure a higher air temperature at the solar collector outlet as well as increased overall efficiency of the solar collector. The modified absorber was built from empty aluminum beverage cans with a commercial black acrylic varnish being applied (Figure 10). Mineral wool insulation (thermal conductivity: 0.040 W/(m K)) was used instead of expanded polystyrene due to higher air temperatures. The bottom insulation panel of the collector was twice as thick than before (40 mm) and the sides contained 10 mm thick insulation layers. In addition, the axial fan was equipped with a control system to regulate the outlet air temperature. The additional cost of all the modifications made amounted to 120 EUR.

**Figure 10.** Comparison of the (**left**) original and (**right**) improved designs of the solar thermal collector.

The modified solar thermal collector was tested under the same conditions as mentioned in Section 3.1. With the solar radiation intensity of 1100 <sup>W</sup>/m2, the resulting air temperature at the collector outlet was markedly higher (83.4 ◦C instead of the original 56.8 ◦C). The overall performance

and efficiency of the modified solar thermal collector were also improved. The corresponding main results are summarized in Table 10.


**Table 10.** Main technical parameters of the modified solar thermal collector.
