Countercurrent packed column supercritical CO2 extraction of olive oil. Mass transfer evaluation

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Abstract

Experiments to determine the number of transfer units (NTU) and height of a transfer unit (HTU) of different random packings such as Raschig rings, Dixon rings, Fenske rings and glass beads in a countercurrent (CC) supercritical fluid extraction (SFE) column of 17.6 mm I.D. and 1.8-m length at 20 MPa and 313 K, with olive oil and CO2, were performed. The extracts were collected by means of two separators, the first one operated at 10 MPa and 313 K and the second at 3 MPa and 273 K. The performance of SFE was studied by analyzing the content of vitamin E and sterols in the raffinate and in the collected extracts by HPLC equipped with a photodiode array detector. The value of HTU and the NTU were evaluated for each random packing at solvent to feed ratios from 23.14 to 41.65 kg CO2/kg oil. Fenske rings showed the best results in terms of efficiency and performance of the CC system for the SFE of olive oil.

Introduction

At present there is a great interest in the extraction of high-value compounds from natural products. Vegetable oils and their residues from the oil refining process are an important source of high-value raw material for chemical and nutraceutical industries, such as squalene, sterols, tocopherols and fatty acids.

Supercritical fluid extraction (SFE) and fractionation (SFF) of vegetable oils using CO2 as solvent represent actually an interesting alternative to other separation processes. CO2 is the selected solvent due to its non-toxic character, non-flammable, low cost, low operating temperature, easy of solute–solvent separation and high selectivity.

There are several studies that demonstrate the feasibility and applications of this technique [1]. Some of the fields of interest of the SFE and SFF are the deacidification [2], [3], [4] and extraction of minor components of olive oil [5], palm oil [6] and deodorized distillates [7]. Rui Ruivo et al. have presented the fractionation of a model mixture methyl oleate/squalene by supercritical carbon dioxide in a packed column using structured Sulzer Ex Gauze as packing material [8]. Carmelo et al. studied the deacidification of olive oil in supercritical countercurrent (CC) extraction column equipped with Sulzer laboratory gauze packing by means of different mass transfer models [9].

The evaluation of packed columns in supercritical extraction processes has been studied by Rathkamp et al. [10], Raschig rings were used in two model systems: carbon dioxide/ethanol/water and carbon dioxide/isopropanol/water. Lim and Rizvi [11] studied the mass transfer efficiency described by the number of theoretical stages (N) and the height equivalent to a theoretical stage (HETS) in a packed column using wire spirals and knitted mesh for continuous supercritical fluid processing of anhydrous milk fat. Brunner [12] has studied the number and height of theoretical stages for the squalene–tocopherol/sterol system and the separation of fatty acid ethyl esters using wire mesh packings. At present, to our knowledge, there is no additional information about the performance of different random packings for SFE of olive oil.

The goal of the present work was to experimentally determine the efficiency, measured as number of transfer units (NTU) and height of a transfer unit (HTU), of four different types of random packings such as Raschig rings, Dixon rings, Fenske rings and glass beads in a CC SFE column using olive oil as a raw material. The efficiency has been tested for extraction of high-value products from olive oil, such as sterols and vitamin E. The values of HTU and NTU have been evaluated on each random packing using different solvent to feed ratios.

Section snippets

Raw materials and reagents

Spanish olive oil (grade 1°) purchased in a local market was utilized in the present work. Composition of the olive oil in terms of vitamin E and sterols were: 0.019% (w/w) of vitamin E and 0.334% (w/w) of sterols. Standards of vitamin E (97% α-tocopherol) and sterols (50% β-sitosterol) were obtained from Sigma-Aldrich, USA. The random packings evaluated in this work were glass beads and Raschig rings supplied by Symta Ltd, Spain, and Fenske rings and Dixon rings supplied by Afora, S.A., Spain,

Results and discussion

The mass transfer evaluation of CC SFE of olive oil was performed considering the concept of NTU and HTU [13], for the different random packings, i.e., glass beads, Dixon rings, Raschig rings and Fenske rings. The NTU and HTU were calculated by knowing the concentration of vitamin E or sterols in the gas phase at the top (a) and at the bottom (b), as indicated in Fig. 2; the distribution coefficients (Ki) of each of these components (vitamin E and sterols) in the olive oil, and the height of

Conclusions

In the present work, an experimental determination of efficiency (expressed as NTU and HTU) of different random packings is presented using a pilot plant CC SFE system equipped with a packed column and using olive oil as raw material. The experimental data obtained indicates that Fenske rings provided the best results in terms of efficiency an performance of the CC system when both, sterols and vitamin E are considered.

Acknowledgements

This research was supported by the Ministerio de Ciencia y Tecnologı́a, Project AGL2000-0448. A.M. Hurtado Benavides gratefully acknowledges the Ministerio de Educación, Cultura y Deporte for a grant supporting his stay at the Universidad Autónoma de Madrid. F.J.S. thanks the Comunidad Autónoma de Madrid for his post-doctoral grant. This work was supported by AL Air Liquide, S.A.

References (13)

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