Computational fluid dynamic design of spent coffee ground cabinet dryer using recycled heat from air compressor

Abstract

This study designed a cabinet dryer assembled to an air compressor to utilize a free-of-charge waste heat for drying. Spent coffee ground (SCG), a residue from coffee brewing, was the tested material. Applying computational fluid dynamics (CFD), an appropriate cabinet dryer configuration could be achieved. As varying the hot air inlet/outlet position, the best drying characteristics providing uniform air flow occurred when hot air entered the chamber at the top and left the chamber at the bottom, opposite side to the inlet. A model dryer was then developed and used to validate the CFD prediction of dryer capacity which was dependent on the maximum numbers of mounted trays and the thickness of SCG layer. The computational prediction agreed well with the experimental results, showing that the dryer loading SCG at 3cm thickness performed best with two trays in the chamber. Adding another tray would result in an obstruction of the air flow, leaving more than 20% of the moisture content in the final product. Experimental determination of drying duration for drying SCG with varying thickness indicated that each centimeter of the increase in SCG bed thickness extended the drying duration by 3 fold. Particularly at the SCG layer thickness of 4cm, this fitted well with the simulation results at 97.2% accuracy. Mounted with 3 trays, each loaded 300g SCG (1cm bed thickness), the dryer could complete 6 cycles within regular operation hours of a coffee shop (12h) and thus accounted for 5400g of SCG as daily capacity of a dryer.