A characterization study on oil palm decanter cakes (OPDC) was performed to gain an in-depth understanding of the material's characteristics to aid in potential residual oil recovery. The OPDC was characterized by a high moisture content, high biodegradability, high organic content, and a nutrient-rich composition. Microscopic observation showed that the oil attachments in OPDC, and a vast majority of the droplets, were less than 50 μm in size. Furthermore, contact angle measurement revealed the hydrophilic and oleophilic characteristics of OPDC. Specifically, the contact angles of water and crude palm oil (CPO) with OPDC were both less than 45° with absorption rates of 0.0265  0.003 µL/s and 0.1042  0.05 µL/s, respectively. The OPDC is a fibrous material, and the surface area and pore size measured were 7.103 m 2 /g and 481.7 Å, respectively. Fourier transform infrared spectroscopy (FTIR) and thermogravimetric (TG) analysis results showed the functional groups and degradation properties of OPDC, respectively.

In this study, d-limonene as green solvent was used as an alternative to n-hexane to recover the residual crude palm oil (RCPO) from the OPDC. The OPDC obtained from the palm oil mill contained about 12.55±3.15% (dry basis) of RCPO. Similar as n-hexane, the results proved that d-limonene as solvent was able to recover 100% of the RCPO from the OPDC successfully. The recyclable of d-limonene was 90%, higher than n-hexane (70%) and not much variation on fatty acids composition of extracted RCPO were observed. To ensure complete RCPO recovery, microscopic observation and FTIR spectra analysis on OPDC before and after the extraction were performed. This study concludes that d-limonene is comparable to n-hexane. However, the residual oil is classified as non-edible as it is not extracted via mechanical method. Therefore, further analysis is necessary in order to determine the potential use of the residual oils in other industries such as pharmaceutical, food packaging, fiberboard manufacturing and others.

This paper provides an overview of the use of green solvents for oil extraction from natural product using soxhlet extraction and supercritical fluid extraction (SFE) methods. In terms of qualitative and quantitative determinations on the extracted oils, the use of green solvents was comparable with n-hexane. For soxhlet extraction method, the integration of microwave application into the soxhlet at their optimum conditions has shown improvement in oil yield, oil quality, solvent consumption and extraction time. SFE using CO2also revealed promising results as soxhlet extraction but apparently it involved high operating cost and difficult to control. Despite of that, all the studies successfully demonstrated the suitability of green solvent as an alternative to petroleum driven solvents for extraction of oils from natural products.