Exergy Analysis of Ultrasonic-Assisted ex-situ Biodiesel Production from Spent Coffee Grounds

Waste management has become a global menace due to increasing waste generation caused by high population growth as well as the lack of efficient management technologies. Spent coffee grounds (SCGs), which are the wastes generated from coffee brewing, are generated globally at an annual rate of about 6 million tons and contain significant amounts of oil (about 10 – 21 wt%) which can be harnessed for biodiesel production. Exergy analysis is a sustainability assessment tool which can pinpoint stages within a manufacturing process where the quality portion of resources (available to do work) is destroyed hence helping engineers to make informed decisions on sustainability. In this study, exergy analysis of ultrasonic-assisted ex-situ (where SCG oil extraction and transesterification are carried out separately) biodiesel production technology (including the ultrasonic SCG oil extraction, ultrasonic transesterification, SCG biodiesel purification steps) is carried out to determine its sustainability hence suggesting process improvement options. For 1000 kg SCG as wet feedstock with a total exergy content of 28,261 MJ processed in an ultrasonic-assisted transesterification unit, an overall process exergy efficiency of about 45.5% was recorded in generating biodiesel and glycerin. Both the ultrasonic extractor and reactor recorded the highest exergy efficiencies of about 72.99% and 86.92% respectively. It was found in this study that ultrasonic-assisted ex-situ transesterification of SCG significantly improved the exergy efficiency of the system due to improved product yield and significant reduction of reaction time. Thus, SCG represents a potentially sustainable source of biofuel in the long term using ultrasonic cavitation.