Oil removal from waste coffee grounds using two-phase solvent extraction enhanced with ultrasonication
Highlights
► Waste/used coffee ground has 13% (db) oil and is a potential biodiesel feedstock. ► Ultrasound assisted two-phase oil extraction eliminates drying before extraction. ► Properties of the coffee oil biodiesel are similar to the soybean oil biodiesel. ► Ready-to-drink coffee producers can benefit from the results of this research.
Introduction
The residue after brewing the coffee grounds is an oil-containing waste material and has a potential to be used as a biodiesel feedstock. Waste coffee grounds contains 12–16% db oil and >50% moisture [1], [2]. Biodiesel production from waste coffee ground oil involves collection and transportation of coffee residue, drying, oil extraction and biodiesel production. Traditionally, coffee is brewed and consumed fresh. Collection and transportation of waste coffee grounds before oil extraction would be the major challenges of biodiesel production from the waste coffee oil. However, emergence of a world market for ready-to-drink or packaged coffee drinks during the last decade does not require the collection and transportation of waste coffee grounds. Beverage companies produce the packaged coffee products at their relatively large production facilities and use their logistics and distribution networks to market the coffee products. Processing the waste coffee ground at a nearby location where the packaged coffee drink products are produced would eliminate the requirement for collection and transportation of this waste material for biodiesel production. In addition to collection and transportation, drying before oil extraction from waste coffee grounds using conventional processes is another time and energy consuming process. Using wet extraction techniques such as two-phase oil extraction eliminates requirement of drying wet materials before oil extraction.
Several processes such as Soxhlet oil extraction and supercritical oil extraction are used for extracting oil from coffee grounds. Both of these methods usually require dried materials and drying wet coffee grounds may take long time depending on the drying process and conditions. Soxhlet oil extraction also takes several hours and therefore requires large quantities of solvent, time and energy [3], [4]. Supercritical carbon dioxide extraction requires various pressure and temperature conditions and necessitates relatively dry coffee grounds before oil extraction [5].
Two-phase solvent extraction is a process utilized for oil extraction from aqueous biological materials. Two-phase oil extraction process involves dispersing oil-containing particles in a single phase water-miscible organic solvent and adding an immiscible solvent to break the mixture into two phases [6]. This method was applied for oil extraction from canola and rapeseed [7]. Two-phase solvent extraction was also used for biodiesel production from rapeseed and Jatropha curcas seed and reduced the extraction time and cost of biodiesel [8], [9].
Ultrasound-assisted solvent extraction is an efficient process and was applied for extraction of oil from different seeds such as flaxseed and soybeans [10], [11]. Integrating high intensity ultrasound with two-phase oil extraction processes can enhance the oil extraction processes from oil-containing materials with high moisture contents. Kinetic mechanisms of oil extraction can be explained mathematically by using different models. A two-exponential model is one of the proposed models that was applied to conventional oil extraction processes [12]. This model assumes that the oil extraction mechanism involves washing and diffusion stages. The first stage refers to the extraction of free oil on the particle surfaces and cracks and the oil is recovered by solvent washing. The second stage refers to the diffusion process which involves solvent penetration to the solid particles to recover the entrapped oil [13]. Two-exponential kinetic model of solid–liquid oil extraction was applied for the oil extraction from olive cake, sunflower, rapeseed and canola [14], [15], [16]. Some of the parameters affecting the kinetics of oil extraction include solvent type and volume, particle size, convection in the surrounding solvent–oil mixture and the temperature during oil extraction [17]. Ultrasound cavitation, which enhances the diffusion of the solvent to the cell due to the mechanical activity of the sound waves, is another important factor for oil transfer from the cell to the solvent in ultrasound-assisted oil extraction processes [10], [15].
The objective of this study was to investigate ultrasound-assisted two-phase oil extraction from waste coffee grounds and examine the capability of two-exponential model to describe the oil extraction kinetics.
Section snippets
Sample collection
Waste coffee ground samples were obtained from a local café shop (Columbia, MO). The samples were a mixture of Arabica beans (Coffea Arabica) and Robusta beans (Coffea Canephora), which are two of the most commonly consumed coffee species in the world [18]. The initial moisture content of the coffee ground samples was 67% and this was determined using oven drying method. Approximately, 5000 g of waste coffee ground samples were used in this study.
Soxhlet extraction
Soxhlet extraction was carried out to determine
Results and discussions
Kinetics of ultrasound-assisted two-phase oil extraction from waste coffee grounds was investigated in this study. Ultrasonication enhanced the two-phase solvent extraction from waste coffee grounds. The effects of ultrasonication on oil yield was statistically significant at p < 0.05 level. The influence of ultrasound waves at the aqueous mixture of methanol and wet waste coffee grounds resulted from the ultrasonic cavitations, which destroyed the particle cell walls assisting the hexane
Conclusions
Ultrasound-assisted two-phase oil extraction method was applied for the extraction of oil from waste coffee grounds with high moisture content. Methanol and hexane were used as the first phase and second phase solvents. Ultrasonication increased the oil yield and reduced the amount of hexane required for oil extraction from waste coffee grounds. A two-exponential equation was used to model the kinetics of oil extraction from the waste coffee grounds. The model assumed that the extraction took
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