Fig. 3: Extraction curve of Irvingia gabonensis kernel oil yield at different particle size diameters, temperatures and time
Temperature dependence and temperature effects
Fig. 1 (a) to (e) shows the effects of temperature on the oil yield ofIrvingia gabonensis kernel at particle sizes of 0.5, 1.0, 1.5, 2.0, and 2.5 mm, respectively. Similar to the effect of time, oil yield increased with increase in temperature from 35 °C to 55 °C. Thus, increasing the temperature from 35 to 55 °C favors the extraction yield. This is because of the ease of penetration of the IGK matrix by the already energized n-hexane solvent molecules [58]. This was also due to the increase in the diffusivity of the IGK oil and decrease in solvent viscosity at increased temperature. In general, increase in temperature enhances softening of IGK, thus, improves the mass transfer coefficient of extraction leading to improved extraction oil yield [59-60].
From the plots in Fig 1 (a – e), it is evident that the extraction process was very fast at the inception, between 30 and 90 min. Afterwards, it gradually slowed between 90 and 150 min. This phenomenon was due to internal diffusion. However, the rapid extraction process at the inception was due to free oil present on the surface of the milled IGK that was exposed to fresh solvent. Thus, there was easy solubility of the oil in the solvent, which lead to fast extraction of the oil [26,59]. In this present study, like the previous works in the literature, the oil yield of IGK increased with temperature and time. The highest oil yield of 68.8 % was obtained at 55 °C, 150 min and 0.5 mm particle size.
It is important to state at this point that rate of extraction at the very beginning, C1, and the constant related to maximum extraction yield, C2, were determined at different temperatures. They were dependent on temperature as could be seen in Figs. 4 and 5, respectively. On the other hand, the extraction capacity, Cs, the second order extraction rate constant, k, and the initial extraction rate, h, were also determined at different temperatures. They were also dependent on temperature as evident in Figs. 6, 7 and 8, respectively.
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