ANALYSIS OF VARIABLES AND MODELING OF GEVUINA AVELLANA OIL EXTRACTION WITH ETHANOL NEAR AZEOTROPE...

jfpe_237 664..681

ANALYSIS OF VARIABLES AND MODELING OFGEVUINA AVELLANA OIL EXTRACTION WITH ETHANOL

NEAR AZEOTROPE CONDITIONS

DANIEL FRANCO, JORGE SINEIRO1 and MARÍA JOSÉ NÚÑEZ

Department of Chemical EngineeringSchool of Engineering

University of Santiago de Compostela15782 Santiago de Compostela, Spain

Accepted for Publication December 5, 2007

ABSTRACT

Oil extraction from Gevuina avellana Mol. (Chilean hazelnut) withethanol, near the conditions of its azeotrope with water, was carried out in thiswork. The effects of solubility, liquid-to-solid ratio and moisture content ofethanol were studied using 92% ethanol, azeotropic (96%) and absoluteethanol (99.9%) as solvents. Water content had a high effect on oil solubility,which reached 140 g/L in 99.9% ethanol, whereas it was 40 g/L with azeotro-pic ethanol. Oil accounted for 93% of total extractable compounds withabsolute ethanol.

Kinetics studies of the extraction process were performed at 50C, givingas a result apparent diffusivity values near 10-11 m2/s, being the highest valuesobtained for ethanol 92% (7.5–16 ¥ 10-11). It was also found that the higherthe liquid-to-solid ratio, the higher the diffusivity. Simulation of four-stagecountercurrent extraction with azeotropic ethanol yielded 23.5% oil extrac-tion, whereas simulation of four-stage cross-flow extraction yielded 40.7%.Ethanol can be an alternative to batch cold pressing or hexane solvent extrac-tion, for G. savellana seeds or meal processing.

PRACTICAL APPLICATIONS

The results presented in this paper are applicable for obtaining oil fromoilseeds by extraction with ethanol. It includes relevant results for the optimi-zation of extraction conditions and particularly those regarding liquid-to-solidratio and percentage of water. Considering the more specific focus of this

1 Corresponding author. Escuela Técnica Superior de Ingeniería, University of Santiago de Com-postela, E-15782 Santiago de Compostela, Spain. TEL: +34981563100, ext. 16777; FAX:+34981528050; EMAIL: [email protected]

Journal of Food Process Engineering 32 (2009) 664–681. All Rights Reserved.© Copyright the AuthorsJournal Compilation © 2008 Wiley Periodicals, Inc.DOI: 10.1111/j.1745-4530.2007.00237.x

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research, the results are applicable to obtaining Gevuina avellana oil by usingan ethanol-based process, which will allow to avoid one of the cold-pressingprocess drawbacks: the high oil content of the meal, which is a factor limitingits lifetime.

INTRODUCTION

Nowadays, natural products are strongly demanded by consumers. Ben-eficial substances can be extracted from many vegetables, among them oils forfood and cosmetic applications. Gevuina avellana (commonly known as“Chilean hazelnut”) is a nonconventional seed, a source of high value-addedoil and a good quality meal used for baking. It owes its name to a similaritywith the European hazelnut (Corylus avellana), although this one belongs tothe Corylaceae family, whereas the former corresponds with the Proteaceaefamily.

Chilean hazelnut seeds are the edible part of this fruit, presenting a greatnutritional value, especially for its content of proteins and lipids, with noantinutritional compounds. The fatty acid profile was characterized by differ-ent authors (Bertoli et al. 1998; Aitzetmuller 2004; Romero et al. 2004),giving 93% of unsaturated fatty acids, with a high content in palmitoleic acid(C16:1), including the rare C16:1D11c or C16:1n-5. Total n-5 fatty acids (FAs)account for 51% of total FAs. Because the hazelnut oil is rich in 16-carbonunsaturated FAs, its absorption through the skin is easier than for conventionaloils, rich in 18-carbon FAs. This is why it is used in the cosmetic industry, forskin regeneration or as a way to introduce nutrients and other protectingsubstances into the skin.

Hazelnut oil captures radiation of the ultraviolet spectrum of light, soacting as a radiation filter and protecting the skin from erythemas and burns(Franco et al. 2001).

Substances extracted from hazelnut hulls with solvents show a highantioxidant activity (Moure et al. 2000), which adds a new attraction toChilean hazelnut, because the entire fruit can be profitable.

Usually, seeds are processed by batch pressing, rendering oil and a meal,which typically contains 3 to 7% oil. This meal is not stable and quicklybecomes oxidized, which is why it must be extracted with solvents for storageor nonimmediate processing. The solvent usually used for this task is hexane,but due to its toxicity and flammability (Johnson and Lusas 1983), alcoholicextraction can be an alternative to hexane extraction. It can also be used toperform the complete conventional process, including the main pressing stageand the defatting of the meal. The main benefit of this process with respect tothe conventional process would be taking advantage of the low oil solubility in

GEVUINA AVELLANA OIL EXTRACTION WITH ETHANOL 665

alcohols, which allows the separation of the solvent by cooling and not bydistilling (Abraham et al. 1988), so saving much energy. Another benefit isavoiding the production of hazardous contaminants in the process, whichimplies additional environmental advantage.

The main objective of this work is to evaluate the potential of ethanol assolvent, applied to oil extraction from a scarcely known oilseed. The effects ofseveral parameters on the extraction yield were analyzed: solubility, liquid-to-solid ratio (L/S) and moisture content of ethanol.

MATERIALS AND METHODS

Materials

Seeds. G. avellana seeds (“Chilean hazelnut”) were kindly supplied bythe School of Biochemical Engineering of Valparaiso (Chile) from a samplingof local cultivars. These seeds (about 4 kg) were stored at 4C until they wereprocessed for characterization or extraction studies. In that moment, they weremanually cracked with a nutcracker, and the seed (edible part) and cuticlewere taken away. Then, seeds were milled in a coffee grinder to achieve aparticle size lower than 0.6 mm. Finally, flour was stored in plastic bags, sealedand stored at 4C, and these samples were processed in the next 48 h, becausethe oil is easily oxidized.

Reagents. Absolute ethanol, petroleum ether and other analytical-grade reagents were obtained from Panreac (Barcelona, Spain). N-cetyltrimethylammonium bromide, sodium laurylsulfate and 2-ethoxyethanolwere purchased from Sigma (Tres Cantos, Madrid, Spain).

Analytical Methods

Moisture Content. Moisture percentage was calculated by weight lossexperimented by the flour of hazelnut, maintained in the oven at 105C, untilconstant weight (method 935.2; AOAC 1990). Samples used for kineticexperiments were dried at 50C to prevent oil denaturing, maintaining theirmoisture at a range between 2.5 and 3% (dry basis), to avoid significantmodifications of water in extraction solvent.

Oil. Oil percentage was determined by Soxhlet extraction with petro-leum ether (Am-2-93; AOCS 1990). After solvent extraction, the solvent wasevaporated and oil content was calculated by gravimetry.

666 D. FRANCO, J. SINEIRO and M.J. NÚÑEZ

Ash. Ash percentage was calculated according to AOAC 942.05 (AOAC1990) by gravimetry. Two grams of hazelnut flour was incinerated into aporcelain capsule at 600C until constant weight.

Fiber (Cellulose, Hemicellulose and Lignin). Detergent methods ofvan Soest were applied to characterize cellulose, hemicellulose and lignincontents (Goering and van Soest 1970; van Soest et al. 1991).

Acid Detergent Fiber Solution. The solution was prepared by dissolving20 g of N-hexadecyltrimethylammonium bromide (Sigma) and 2 mL ofdecahydronafthalene (Panreac) in 1 N sulfuric acid (Panreac) to a final volumeof 1 L.

Neutral Detergent Fiber Solution. The solution was prepared by dissolv-ing 18.61 g of ethylenediaminetetraacetic acid, and 6.81 g of Na2B4O7·10H2O(Panreac) in water to a volume of 150 mL, dissolving 30 g of sodium dodecylsulfate (Sigma) and 10 g of 2-ethoxyethanol (Sigma) in water to a volume of700 mL, dissolving 4.56 g of Na2HPO4 (Panreac) in water to a volume of150 mL, then incorporating these solutions and finally adjusting pH to 6.9–7.0with phosphoric acid (Panreac).

Neutral detergent fiber content (NDF) and acid detergent fiber content(ADF) were determined by adding 100 mL of either ADF solution or NDFsolution plus 2 mL of decahydronaphthalene (Panreac) and 0.5 g of sodiumsulfite (Panreac) to solid samples (1 g) in 250 mL Erlenmeyer flasks andheating in an autoclave at 121C for 15 min. The remaining solid was filteredthrough filtering crucibles (no. 2), washed with hot water (500 mL), acetone(10 mL), then removed and dried in an oven at 105C for 4 h. ADF and NDFwere determined as percentage of initial mass, in dry basis.

Acid detergent lignin content (ADL) was determined by mixing 1-gsamples with approximately 20 mL of 72% (w/w) sulfuric acid into filteringcrucibles. The content was periodically homogenized and the level of sulfuricacid restored every hour for a total period of 3 h. Then, the residue was washedwith water and dried at 105C until constant weight. ADL results wereexpressed as percentage of the initial weight, in dry basis, and correspondedwith the content in lignin.

The following expressions were used to estimate cellulose andhemicellulose percentages: hemicellulose = NDF-ADF; cellulose content =ADF-ADL.

Protein. Protein content was determined according to the Kjeldahl totalnitrogen method (method 976.06; AOAC 1990), multiplying the total nitrogencontent by 6.25.


Experimental Methods

Oil Solubility. Oil solubility was assayed using commercial G. avellanaoil from Forestal Casino Ltd. (Santiago, Chile). Experiments were performedbetween 20 and 70C, with ethanol concentrations between 92 and 99.9% toanalyze both the effects of temperature and moisture content near azeotropeconditions. Experiments were developed in glass capped tubes immersed in awater bath, which was heated by a Techne TE 8J heater (Techne, Staffordshire,U.K.). Each tube contained a 10-mm-length magnetic bar for stirring; enoughamount of solvent was added to the tubes to ensure that an oil excess wasmaintained. Tubes were so stirred for 24 h, until equilibrium state was reached.Aliquots from supernatants (approximately 2 mL) were obtained by usingmicropipettes with tips maintained at the same temperature as test tubes toavoid the instantaneous oil condensation, and the solvent was eliminated in aBüchi rotary evaporator (Flawil, Switzerland). Oil content was determinedgravimetrically, and analyses were done in triplicate.

Kinetic Studies. Samples were dried at 50C for 1 h. Ethanol extraction(92, 96 and 99.9%) was carried out in a G24 New Brunswick orbital shaker(New Brunswick Scientific, Edison, NJ) at 150 rpm and at a temperaturewhich will be fixed following the results of solubility assays. Extraction timewas fixed not to exceed 200 min, because more time would be excessive forpilot-plant scaling and industrial operations. The working volume was100 mL, fixing the sample’s weight as a function of the desired liquid-to-solidratio (L/S of 15:1, 25:1 or 50:1). Two-milliliter aliquots were taken every30 min. Due to fast cooling and the subsequent oil condensation on the tipsurface, the pipette’s tip was maintained to the same temperature asthe extraction process. Extractable compounds were gravimetricallydetermined.

Characterization in Both Oil and Total Extractable Compounds. Theprotocol of extraction was similar to that described earlier, although sampleswere not taken every 30 min, but only at the end of extraction (100 h; trying toguarantee the total extraction of all compounds and, therefore, equilibriumconditions). Besides, after ethanol was eliminated in the rotary evaporator, dryextract was extracted again with petroleum ether in Soxhlet to quantify theamount of oil into samples.

Simulation of Industrial Extraction. Simulations of industrial extrac-tion, such as cross-flow or countercurrent extraction, were carried out in thelaboratory (simple schemes appear in Fig. 1). The methodology of batchmixing used to simulate countercurrent extraction was adapted from Adu-


Peasah et al. (1993). It was not possible to obtain the value of O3 fraction withthis methodology because liquid-to-solid ratio would be modified (this fractionwas not discarded). Thus, only values for O1, O2 and O4 are shown. Theexperiments were performed in duplicate, using ethanol 96% at L/S = 15 in arotary shaker at 50C and 120 rpm.

MATHEMATICAL MODELING

Total mass transfer resistance in the extraction and/or washing of veg-etable matrix comes from the sum of two terms: the external resistance due toconvection and the internal resistance due to diffusion in solid and particlepores. These two resistances can be represented by Fick’s first and second law,applying a mass balance in the solid–liquid interphase. Because the mecha-nism of extraction depends mainly on the diffusion (Nieh and Snyder 1991),the experiments were carried out in stirred conditions, when external masstransfer resistance becomes negligible, and the problem can be solved apply-ing Fick’s second law. Crank (1975) formulated solutions to this equation fordifferent geometries. For spherical geometry, the equation takes the followingform:

EC C

C C

C C

C nn

D t

rn

a=−−

=−

= −⎛⎝⎜

⎞⎠⎟

∞

∞ =

∞

∑e

i e

M M

M e

6 12 2

1

22

2ππ

exp (1)

As this series converges quickly, only the first terms in Eq. (1) are significantlydifferent from zero at times long enough. If we only use the first three terms ofthe series, the expression becomes

FIG. 1. FOUR-STEP EXTRACTION PROCESSES(a) Countercurrent. (b) Cross-flow.


E a t a t a t= − ×[ ] + − × ×[ ] + − × ×[ ]⎛⎝

⎞⎠

6 1

44

1

99

2πexp exp exp (2)

where:

ar

Da= ×π 2

e2 (3)

Solving of Eq. (2) requires nonlinear data regression. The Curve Expert 1.2software from Hyams (1996) was used to perform these calculations.

RESULTS AND DISCUSSION

Characterization and Definition of Extraction Conditions

Hazelnuts used in this work had an average weight of 1.6 g; averagepercentages of seed, hull and cuticle were 61, 34 and 5%, respectively.Samples of flour were characterized (results are shown in Table 1). The highoil content in Gevuina avellana is remarkable, when compared with that inother oleaginous seeds like soybean (with oil content about 20% in weight) orrapeseed (about 40% oil), with its content similar to that of sunflower (about50% oil) (Sineiro et al. 1992). Oil percentage was higher than values found bymost other authors, who report values in the range 40–48% (Mella and Masson1984; Malec et al. 1986; Halloy et al. 1996), although Zúñiga et al. (2003)also obtained values as high as 52%. These differences are explained by thedifferent environmental, geographic and storage conditions. Fiber content wasalso relevant, about 25% of dry weight, making the defatted meals useful forfiber-enriched products.

TABLE 1.CHEMICAL COMPOSITION OF HAZELNUT FLOUR,

DRIED AT 50C, BEFORE EXTRACTION

Component Mean value (g/100 ghazelnut flour dry basis)

Moisture 3% � 1.1Oil 52% � 1.3Protein 10% � 1Ashes 3% � 0.1Lignin 14% � 0.7Cellulose 6% � 0.5Hemicelluloses 5% � 0.5Other (sugars) ª7%


The strong influence of moisture content in the ethanol extraction of oilymatrix (Abraham et al. 1988) made necessary a study of drying conditions toensure solid moisture to be 3% or lower. Besides, drying conditions must besmooth to not alter the oil and the properties of the solid, the selected tem-perature being 50C. The initial moisture of the samples was about 7%, andafter 1 h of drying, values became �3%, this value being almost stable evenwhen prolonging the time. Samples will be dried in these conditions, previousto the extraction process. Once hazelnut flour was characterized and theprotocol of drying was established, the effect of two variables was studied:liquid/solid ratio and percentage of ethanol near azeotrope.

Effect of Temperature on the Solubility of Hazelnut Oil

The effect of temperature on the solubility of hazelnut oil within the range20 to 70C and for different ethanol–water mixtures (92–99.9% ethanol) wasstudied. The results are shown in Fig. 2, allowing us to know the limits of oilextraction with ethanol and the influence of both variables, water content andtemperature. For the same temperature (30C), the solubilities of hazelnut oil in99.9 and 96% ethanol were 50 and 15 g/L of ethanol, respectively, indicatingthis difference as a high influence of water content. We can see in this figurethat the SD was higher when temperature exceeded 50C in the case of 99.9%ethanol. An explanation could be that the experiment was conducted close to

FIG. 2. SOLUBILITY OF HAZELNUT OIL IN ETHANOL AT DIFFERENT TEMPERATURES� 92% ethanol; � 96% ethanol; � 99.9% ethanol.


the boiling point of the solvent, whereas in the experiments with 92 and 96%ethanol, the presence of water increases the boiling point of the mixture, somaking the sampling easier, and increasing the reliability of the measures.Solubility in ethanol was considerably lower than in hexane, for which a valueof 175 g of hazelnut oil per liter at 20C was determined. Results in Fig. 2 showthat the influence of temperature was dependent on water content, in a noto-rious manner with 99.9% ethanol and very slightly with 92% ethanol. Theeffect of water content in ethanol on solubility must not be neglected because,by taking water from the solid, oil extraction from plant materials can becomesolubility limited. Different authors have obtained analogous results withrespect to the influence of moisture in alcohols working with oil of other seedslike soybean (Rao and Arnold 1958) or corn (Chien et al. 1990). Abrahamet al. (1988) worked in the extraction of oil from cottonseed with 96% ethanolat different temperatures into a wide range (78, 75 and 5C), reporting solubilityvalues of 12, 10 and 5%, solubility not being so dependent on temperature aswe found in this work with 99.9% ethanol.

Despite the best results obtained at the highest temperature, we did notchoose such a high temperature (70C) for the studies of extraction kinetics,because oil could become degraded together with proteins, which are a valu-able product and contribute to the economy of an industrial process, and wouldprobably be denatured. A high temperature also implies operational (losses ofsolvent) and economic (costs of heating) problems. For these reasons, 50C wasthe chosen temperature to accomplish the study of extraction kinetics, showingan acceptable value of solubility.

Determination of Oil from Extractable Compounds: Correlation

Because not only oil is extracted in ethanol, but also some sugars orproteins, the oil concentration in ethanol miscellas was determined asdescribed in Materials and Methods. A very good correlation between oil andextractable compound concentrations was found, with the results shown inFig. 3, where each symbol indicates different liquid-to-solid ratios. The use ofthese linear relationships allowed us to convert the data of extractable com-pounds to oil data.

We must note that the observed slope increases with ethanol purity, goingup to 1 for absolute ethanol. This observation indicates that, when usingabsolute ethanol, almost all extractable compounds consist of oil, and, incontrast, when using 92% ethanol, oil represents about 67% of the extractablecompounds.

By that, it can be stated that water content, even at low concentrations,influenced the extracted component profile in such a way that the relationshipbetween “total oil” and “total extractables” is perfectly correlated when


ethanol contains water (Fig. 4a) showing a regression coefficient as high as0.9979. However, when data for extraction with 99.9% ethanol were alsoincluded (Fig. 4b), R2 dramatically decreased to 0.9454. This result indicatedthat applying a unique factor to convert total extractables to oil is not possible;instead, a factor for each solvent must be used. It seems that water contentmodulates the extraction process in some way, so its absence slightly changedthe behavior relationship.

G. avellana Oil Extraction Kinetics: Estimation of Apparent Diffusivity

The effects of both liquid/solid ratios (15, 25 or 50 mL solvent/g drymass) and moisture content in solvent using ethanol of 92, 96 and 99.9% wereanalyzed.

We can see in Fig. 5 the results of hazelnut oil extraction at differenttimes; it can be observed that the extraction yields obtained with the threesolvents were improved by increasing liquid/solid ratio. The best result wasobtained with 99.9% ethanol (L/S = 50), with a value of 44%. In contrast, witheither 92 or 96% ethanol and in the same experimental conditions, the obtainedyields were 23 and 37%, respectively. For L/S 15 and 25, the observeddifferences of extraction yields were lower.

By applying the mathematical model described in the Materials andMethods section, the results summarized in Table 2 were obtained. It can beobserved that apparent diffusivity values increased with L/S ratio up to 25,with slight increases for higher L/S ratios, except for 99.9%.

FIG. 3. CORRELATION BETWEEN OIL AND EXTRACTABLES AT DIFFERENTLIQUID-TO-SOLID RATIOS

�: Ethanol 92%, �: Ethanol 96%, �: Ethanol 99.9%.


An explanation for this behavior can be the higher difficulty of ethanoldiffusion within dry particles, reported by Chien et al. (1990). These authorsextracted oil from corn with ethanol, and they verified that the extraction wasmore difficult if the corn flour was not previously soaked with ethanol. Clearly,the presence of water is not interesting, as it was demonstrated by the oilsolubility assays, but in this case we worked with the entire vegetable matrixand the presence of water in a minimum amount can be necessary for molecu-lar diffusion of ethanol within the seed not being limited. Capillary flow intoseed pores highly depends on viscosity, this one being higher for pure ethanolthan for ethanol–water mixtures. In fact, Abraham et al. (1988) evaluated anddefined the critical moisture for cottonseed as “that moisture that the seed hasbefore the extraction and which does not vary in the extraction process.” It was3 and 6%, respectively, in contact with 92% ethanol and 88% isopropanol. Inthis way, working on top or under this critical moisture we can dry the solventor humidify it. It is interesting to achieve a drying of the solvent in contact withthe solid, because, in this way, the seeds would act as a method to control water

FIG. 4. CORRELATION BETWEEN OIL CONTENT AND TOTALEXTRACTABLE COMPOUNDS

(a) Only water–ethanol mixtures. (b) All solvents.


accumulation during alcoholic extraction. Actually, Chien et al. (1988) sug-gested a potential method to dry alcohol and they propose the use of corn likedehydrating agent as a method of producing absolute ethanol, based on thisdrying phenomenon of adsorption/desorption. Karlovic et al. (1992) demon-strated that moisture of corn seed had an important effect on the speed ofextraction, because when these authors reduced the initial moisture contentfrom 12 to 8%, the time of extraction shortened from 240 to 90 min for thesame residual oil content in corn flour. Hojilla-Evangelista et al. (1992) aimedto dry ethanol from 5 to 1.3%, working at 75C.

FIG. 5. EFFECT OF LIQUID-TO-SOLID (L/S) RATIO IN THE CHILEAN HAZELNUT OILEXTRACTION WITH ETHANOL

(a) 92%; (b) 96%; (c) 99.9% (� L/S = 15; � L/S = 25; �L/S = 50). d.b., dry basis.


By working within a smaller range of particle sizes, the estimation ofapparent diffusivity probably could be improved. The size used to obtainapparent diffusivity was a mean value representative of the ground material.The softness of G. avellana makes difficult the control of particle size duringthe milling stage, because it becomes easily very small. The geometric variablefrom which we obtain diffusivity values could be controlled better by usingslices, but it would not be representative of milled G. avellana. Several authorshave also found that the diffusion of a solute caused changes of volumebecause of contraction and/or shrinkage phenomena (Kopelman et al. 1975),modifying the geometry during mass transfer. This phenomenon has beenextensively studied from drying operations (Aguerre et al. 1985), but not foroil extraction. Intracellular content is responsible for the cell consistency andcell walls can become broken because of osmosis phenomena during theextraction process. Voilley (1983) studied changes of size detected in somecoffee ground samples at different times and temperatures, which were pro-duced by the diffusion of extractables in the preparation of coffee. In this workwe considered spherical and rigid particles from 0 to 0.6 mm, and therefore didnot take into account volume changes that may be produced in particles.

Few diffusivity data of G. avellana oil are available in the literature.Villaroel et al. (1988) found diffusivity values in the interval of 4.8 ¥ 10-13 to7.8 ¥ 10-13 m2/s for the entire hazelnut and divided in four, respectively. Theseauthors did not work with flours, so values of apparent diffusivity becamearound 100 times lower than ours, as foreseeable. Other values of diffusivityfor oils were found some time ago by Osburn and Katz (1944) and theyproposed the existence of different diffusivities for the soybean, as a result ofa double structure, a more porous one and another portion of less permeablematerial. Values of apparent diffusivity calculated by these authors, for

TABLE 2.APPARENT DIFUSSIVITY VALUES OBTAINED BY

APPLYING NONLINEAR REGRESSION OF EXPERIMENTALDATA (NONEXTRACTED FRACTION) TO EQ. (2)

Solvent (%) Liquid-to-solid ratio

Da ¥ 10-11

(m2/s)R2

99.9 15 2.02 0.9999.9 25 2.19 0.9699.9 50 5.80 0.9596 15 3.33 0.9796 25 5.76 0.9796 50 5.93 0.9392 15 7.48 0.93292 25 15.0 0.9692 50 15.98 0.97


different thicknesses of soybean, were found to be in the range 0.15 ¥ 10-12 to0.11 ¥ 10-11 m2/s.

Simulation of Industrial Processes

Simulation of countercurrent and cross-flow extraction processes wascarried out by multiple single-contact steps (Adu-Peasah et al. 1993), theresults being shown in Fig. 6a,b. Extraction yields were higher than thoseobtained for one batch stage at equilibrium conditions (23.5 and 40% forcountercurrent and cross-flow, respectively, versus 15% [see Fig. 5] for onebatch). It must be noted that for countercurrent contact, the consumption ofsolvent is four times less. Conditions for experiments were chosen as the resultof previous assays (Ta of 50C), or by being compatible with industrial opera-tion (96% ethanol or a low volume of solvent, L/S = 15). Even so, the increaseover batch was substantial.

A higher extraction yield after four-stage extraction was found for cross-flow extraction; this is a logic result, because fresh solvent was added in eachstage, so maximizing the concentration gradients either in solid or in liquid.This type of contact allows an almost total exhaustion of the hazelnut flour if

FIG. 6. EXTRACTION YIELD OBTAINED IN EACH STAGE (�) AND ACCUMULATED (�)(a) Cross-flow contact. (b) Countercurrent contact.


enough stages are performed, but it requires a higher amount of solvent andproduces a less concentrated ethanol–oil mixture, which will require a higheramount of energy to recover the oil.

CONCLUSIONS

The best extraction results were obtained by working with 99.9% ethanol(absolute ethanol) and L/S ratio of 50, 44.1% oil extraction yield beingobtained. The increase of temperatures and the decrease of the amount in waterin the alcohols always increase the solubility of oil. Nevertheless, workingwith temperatures higher than 50C caused operational issues and/or instabilityof solvent. Good results were obtained from the extraction with 96% ethanol,which is the azeotrope obtained by distillation. For both the acceptable valuesof solubility and lower cost, 96% ethanol would be the advisable solvent.Dehydration of ethanol, as is performed in bioethanol plants, will be recom-mended always by the benefits of removing water. The disadvantage of thelower solubilizing power of ethanol with respect to nonpolar solvents canbecome an advantage in the stages of separation because it can be used torender a clean nonsoluble oil fraction by slightly decreasing temperature.

Kinetic data adjusted well to Fick’s second law, with high regressioncoefficients being obtained. Apparent diffusivity values were the same order ofmagnitude as those obtained by other authors. Apparent diffusivity (about10-11 m2/s) of oil increases with L/S ratio (especially up to L/S = 25) becauseof the high difference in concentration gradient solid–liquid interphase andbetween the solutions.

Simulation of an industrial process with 96% ethanol led to values of 40%extraction yield in cross-flow contact, thus increasing by more than 80% theresults of batch process.

NOMENCLATURE

E nonextracted fraction in solid, at time tC residual oil content in the solid, at time t (g/L)Ce residual oil content in the solid at equilibrium (value to 100 h) (g/L)Ci initial oil content in the solid (g/L)CM oil content in bulk liquid, at time t (g/L)CM• oil content in bulk liquid, at equilibrium, (value to 100 h) (g/L)re particle radius (m)t extraction time (s)Da oil diffusivity (m2/s)n 1,2, . . . •


ACKNOWLEDGMENTS

The authors acknowledge the European Community for the financialsupport through project ERBIC 18 CT970206 and the Spanish Ministry ofScience and Technology through project PPQ2000-0688-C05-01.

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ANALYSIS OF VARIABLES AND MODELING OF GEVUINA AVELLANA OIL EXTRACTION WITH ETHANOL NEAR AZEOTROPE...

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