In this work, the indirect solar drier (IDSD) is used for drying Thymus by using four techniques. First, the whole leaves of Thymus are dried using the IDSD without using phase change material (PCM). The obtained results of this experiment show that the moisture content of Thymus leaves decreases slowly in the first three days, which is considered abnormal behavior compared with most studied dried plants in the literature. In order to reduce the drying time, the Thymus leaves are cut before drying without using the PCM. The results indicated that cutting Thymus leaves reduces the drying time by 55.6%. To increase the operating time of the IDSD and control the drying temperature as well, the IDSD is integrated with paraffin wax as a PCM. This reduces the drying time of cut leaves by 50% compared with the system without using the PCM. Moreover, eleven mathematical models are tested in order to select the best one describing the drying behavior of whole and cut leaves in the IDSD without and with using the PCM. As a result of the abnormal behavior of drying Thymus, most of the examined mathematical models failed to describe the drying behavior of Thymus satisfactory. Therefore, a new mathematical model (four-parameter logistic model) is introduced aimed to well describe the drying behavior of Thymus.

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