Set-up and Mass Balances on Soxhlet Extraction Tests in “Learning by Doing” of Process Engineering Students

A group of bachelor students undertaking a training in a chemical engineering lab class has been involved by the lecturer in the Soxhlet extraction of compounds of interest from agri-food residues. Beyond pursuing experimental practice and investigating solid-liquid extraction, the lab training received added value by the engineering objectives to which the students were oriented: 1) setting up and verifying the closure of the macroscopic mass balances – never reported in the literature according to the authors’ investigation – on the Soxhlet apparatus; 2) quantifying the heat duty of a Soxhlet test as a function of the solvent choice. Roasted hazelnut cuticles and tomato pomace from industrial food transformation processes were used for testing. Two cases were raised to the students’ attention: I. the solvent and the solid to be extracted are perfectly anhydrous; II. water is present in both the tested solids and the extraction solvent. A scheme of the material flows in the Soxhlet experimental procedure was developed together with the trained students. Based on this, the mass balances were written, yielding three simple equations for dry solids, solvent and water. Their solution, after some simplifying assumptions, was quite straightforward and intuitive for the students. All in all, the Soxhlet experimental activity proved to be not only reliable and accurate, but also attractive and mind-opening.