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Publication Detail
Effect of temperature and pressure on the efficiency of solvent extraction of lipids from spent coffee grounds
  • Publication Type:
    Conference presentation
  • Publication Sub Type:
    Presentation
  • Authors:
    Efthymiopoulos I, Hellier P, Ladommatos N, Eveleigh A, Mills-Lamptey B
  • Date:
    16/10/2018
  • Name of Conference:
    7TH INTERNATIONAL SYMPOSIUM ON ENERGY FROM BIOMASS AND WASTE
  • Conference place:
    Venice, Italy
  • Conference start date:
    15/10/2018
  • Conference finish date:
    18/10/2018
  • Keywords:
    waste valorisation, solvent extraction, lipid recovery, spent coffee grounds, biofuels
Abstract
Spent coffee grounds (SCG) are the main residues of the coffee beverage industry, and a potentially valuable source of energy-dense C16-C18 triglycerides [1]. The present study investigates the effect of process temperature and pressure on the solvent extraction of lipids from dried SCG in an effort to evaluate their impact on oil extraction efficiency, composition and energy content. The experimental part of this study consisted of solvent extraction of oil from SCG at elevated pressure and temperature in a prototype closed pressure vessel and an Accelerated Solvent Extraction (ASE) unit respectively. ASE experiments were performed with hexane and ethanol at temperatures up to 185 °C, with the high pressure inside the extraction cell (70-140 bar) allowing the solvent to remain in liquid state at temperatures above its atmospheric boiling point [2]. However, the ASE unit only allows independent selection of extraction temperature, with pressure as a dependent variable. Therefore, a closed pressure vessel that allowed independent control of both temperature and pressure was used to investigate the effect of pressure (up to 110 bar) on the solvent extraction of lipids from SCG. Three extraction temperatures (25 °C, 45 °C, 65 °C) and varying process durations were investigated with hexane as the extraction solvent. The composition of the ASE-extracted lipids in terms of mono-, di- and triglycerides and free fatty acids (FFA) was determined through 1H Nuclear Magnetic Resonance (NMR), whereas the FFA content of lipids obtained through solvent extraction in the pressure vessel was measured by titration. The higher heating value (HHV) of selected oil samples was determined by a Bomb Calorimeter. Experimentation with the number of ASE static cycles, with fresh solvent introduced in each extraction cycle, revealed an optimum setting of 3 cycles in terms of oil extraction efficiency, while prolongation of extraction duration was found to be counterproductive. Increase of ASE extraction temperature resulted in higher oil yields when polar ethanol was the solvent used, but revealed an optimum temperature for oil extraction with the non-polar hexane at 145 °C, suggesting that solvent polarity affects the oil extraction efficiency at elevated temperatures. Solvent selection and process temperature had a significant impact on the proportions of di- and triglycerides and FFAs present in the ASE-extracted oil, with increasing temperature generally leading to lower concentration of triglycerides and higher levels of 1,2- and 1,3-diglycerides, while there was no systematic effect of temperature on the FFA content of the recovered lipids. The experiments performed with the closed pressure vessel showed that prolongation of pressurized solvent extraction duration from 5 to 10 minutes resulted in improved oil extraction efficiency, while further increase of duration led to a slight oil yield decrease. Introduction of fresh solvent through subsequent washing and static extraction cycles also resulted in improved oil yields, and showed that addition of extraction cycles was more effective than increasing the duration of a single cycle in terms of oil extraction. An increase in extraction pressure up to 75 bar was found to improve the oil extraction efficiency regardless of the process temperature, however, extraction above 75 bar at temperatures significantly below the boiling point of hexane resulted in a decreased oil yield, while an extraction temperature close to the solvent boiling point led to a linear increase. Finally, there was no systematic effect of extraction temperature and pressure on the HHV of SCG lipid samples recovered by ASE and closed pressurized vessel extractions, which ranged between 38.53 MJ/kg and 39.12 MJ/kg, while the extraction pressure was found to have negligible effect on the FFA content of lipids extracted by pressure vessel experiments. 1. R. Campos-Vega, G. Loarca-Piña, H. Vergara-Castañeda, B.D. Oomah, Spent coffee grounds: A review on current research and future prospects, Trends Food Sci. Technol. 45 (2015) 24–36. doi:10.1016/j.tifs.2015.04.012. 2. Camel, V., 2001. Recent extraction techniques for solid matrices—supercritical fluid extraction, pressurized fluid extraction and microwave-assisted extraction: their potential and pitfalls. Analyst 126, 1182–1193. https://doi.org/10.1039/b008243k.
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