Horizontal centrifuge with screw conveyor (decanter): optimization of oil/water levels and differential speed during olive oil extraction

Olive oil extraction using a horizontal centrifuge with a screw conveyor is an essential operation to reduce production costs. However, proper control of the plant is required to maintain a high level of extraction yield and olive oil quality. Rheological characteristics of the olive paste, which change in relation to water content, fruit variety, maturity level and seasonal temperature variations, greatly affect the efficiency of centrifugal extraction. If olive paste is fed to a decanter without automatic control, then non-optimal extraction is performed. After successfully testing a suitable flow mass sensor in order to set up an automatic system to control the olive paste mass flow rate fed to a decanter centrifuge during olive oil extraction, a feedback control system was tested in the laboratory and built in-line in an industrial processing plant. This allowed trials to be carried out at constant mass flow rate for both the paste and the added water, on a decanter centrifuge with variable differential speed between bowl and screw conveyor (i.e. variable DN) and with regulation capability of oil-water ring levels. Constant quality olives (cv. Coratina) were used for the trials; the correlation was evaluated between oil/water ring levels and DN with respect to the extraction yield, husk fat content and vegetable water fat content. The “oil recovery efficiency” (o) and “separation coefficient” S peaked with maximum residence time, using DN=13.3 and RING=284.8, while o minimum was achieved at higher DN. More specifically, for DN values higher than about 18 or lesser than 12, o becomes independent of RING. Furthermore, o and S values are strictly related to the geometry of the decanter centrifuge, whereas the paste dilution ratio determines the preservation quality of the oil extracted, and the maximum oil recovery efficiency represents the remaining objective related to the quantity of extracted oil. From experimental data some interesting relations were found linking decanter centrifuge parameters; their relationship depends on a complete quadratic interaction model constituted by the input variables that leads to highly correlated polynomials between DN vs. Uopdb ("mass fraction of oil in the fed olive paste" (dry basis)) and o optimal vs. Uopdb. These relations allow optimization of DN and RING values such that maximum oil recovery efficiency is achieved. Indeed, knowledge of Uopdb by an on-line method allows the decanter DN and RING operating parameters, to be calculated and modified at once, obtaining optimal maximum oil recovery efficiency.