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Supercritical CO2 extraction of hiprose seed oil: experiments and mathematical modelling
Chemical Engineering Science 55 (2000) 2195}2201
Supercritical CO extraction of hiprose seed oil: 2 experiments and mathematical modelling E. Reverchon!,*, A. Kaziunas", C. Marrone! !Dipartimento di Ingegneria Chimica e Alimentare, Universita% di Salerno Via Ponte Don Melillo, I-84084 Fisciano (SA), Italy "Applied Separations, 930 Hamilton Street, Allentown, PA, 18101, USA Received 19 May 1999; received in revised form 22 September 1999; accepted 30 September 1999
Abstract Hiprose seed oil has been extracted by supercritical CO operating at pressures of 1500, 3000, 6000 and 10 000 psi, temperatures of 2 40, 50 and 703C, CO #ow rates of 1, 2, 4 and 6 g/min using mean particle sizes of 0.42, 0.79 and 1.03 mm. The obtained oil yield data 2 has been used to validate a mathematical model of the extraction process, based in part on the information obtained from the microscopic structure of the hiprose seed particles. The di!erential mass balances forming the model have been numerically integrated. The best "t between model curves and data has been obtained when a linearly variable internal mass transfer coe$cient was adopted, with an initial value of 1.9]10~5 m/s. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: Supercritical extraction; Hiprose oil; Modelling
1. Introduction Hiprose seeds contain a relatively low percentage of oil (between 7 and 15% by hexane extraction) when compared to other seeds; for example, almonds contain up to about 50% by weight of oil. The main component of the oil is linoleic acid; however, in spite of the low oil content, the characteristic composition of the hiprose oil makes it a valuable product for cosmetic industry. Therefore, an improved process for its extraction would be of industrial interest. Supercritical #uid extraction (SFE) has been applied to the extraction of vegetable oils from several seeds as recently reviewed by Eggers (1996). Despite the large number of species processed, only some papers on the modelling of the SFE of seeds have been published. All of the published reports agree that at least the "rst part of the SFE process is governed by the solubility equilibrium between the oil and the #uid phase. The equilibrium relationship has been generally supposed to be linear since more precise information is not available in such complex systems.
* Corresponding author. Tel.: #39-089-964116; fax: #39-089964057. E-mail address: [email protected] (E. Reverchon)
From the mathematical point of view, the models previously proposed are generally based on di!erential mass balances integration. Bulley, Fattori, Meisen and Moyls (1984), Lee, Bulley, Fattori and Meisen (1986) and Fattori, Bulley and Meisen (1988) assumed that mass transfer resistance occurred only in the solvent phase. In other models an internal mass transfer resistance was considered (Roy, Goto, Hirose, Navaro & Hortacsu, 1994). A sigmoidal equilibrium curve was adopted by Perrut, Clavier, Poletto and Reverchon (1997) to "t the experimental data. King and Catchpole (1993) used a shrinking core model to describe a variable external resistance where the solute balance on the solid phase determines the thickness of the mass transfer layer in the external part of the particles. However, more reliable models can be produced when the vegetable structure of the seed is considered in the equations. From this respect, SovovaH (1994) proposed a model based on the hypothesis that the internal part of seeds is formed by specialized structures that contain the oil. Therefore, as a result of grinding, the seed structures located on the surface of the particles are broken, whereas the internal structures remain intact. The broken and intact `cellsa model was applied to the extraction of caraway seed oil (SovovaH , Kucera & Jez, 1994). This model was thus based on sound physical hypotheses and represented the "rst attempt to introduce a description of
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the structure of the vegetable matter into the mathematical model. However, the model required the knowledge of several parameters related to physical properties of the seed which were di$cult to measure or calculate. Therefore, the authors adjusted these parameters to "t the experimental results. A large number of parameters require an even larger number of data sets for model validation. Better results can be obtained when some of the model parameters are independently measured. Marrone, Poletto, Reverchon and Stassi (1998) in the modelling of the supercritical extraction of almond oil, demonstrated that it is possible to "x all parameters but the internal mass transfer resistance, one, in the Sovova` model, since they measured by scanning electronic microscope (SEM) the number and the dimensions of the spherical oil bearing structures in almond seed particles. This approach was successfully adopted also in the case of fennel oil supercritical extraction where the oil bearing structures were hexagonal (Reverchon, Daghero, Marrone, Mattea & Poletto (1999)). In this case the authors "rst selectively extracted the fennel essential oil at low pressure (90 bar; 503C) to avoid vegetable oil contamination with essential oil and then they extracted the vegetable oil at a higher pressure. However, as a rule, the models proposed until now in the literature were applied to a small set of experimental data and as such explored the in#uence of only some process parameters. In addition, other varieties of seeds may show di!erent behavior when dissimilar internal structures characterize the seeds. In this work, supercritical extraction of hiprose seed oil has been performed at various pressures, temperatures, particle sizes and CO #ow rates with the aim of high2 lighting the in#uence of all process parameters on the extraction process. In addition, we evaluated the possibility of applying the broken and intact cells model equations. The hiprose seed microstructure data was obtained by SEM and was of fundamental importance in the modelling of the extraction process.
particle sizes of 0.42, 0.79 and 1.03 mm. The mean particle size was measured by mechanical sieving of seed particles and applying the Sauter formula. The e!ect of pressure was studied "rst; then, the pressure was "xed at 6000 and 10 000 psi and all the other variables were varied one at a time, thus obtaining several data sets on hiprose oil yield against the extraction time. Some crucial experiments were repeated twice. We de"ned the oil extraction yield (% by weight) as the quantity of oil extracted divided by the quantity of vegetable matrix loaded in the extractor multiplied by 100.
2.1. Ewect of pressure As expected, the rate of extraction of hiprose oil largely increased with pressure as shown in Fig. 1 where oil yield data obtained at 1500, 3000, 6000 and 10 000 psi are reported for experiments performed at 403C temperature, 6 g/min CO #ow rate and 0.42 mm mean particle size. 2 The e!ect of the extraction pressure can be explained taking into account the large increase in the solubility of the oil constituents (mainly triglycerides) with pressure. Moreover, a large part of the extraction process is controlled by this thermodynamic parameter, as suggested by the experimental data: the "rst part of yield data could be "tted using a straight line. At 6000 and 10 000 psi an asymptotic yield of about 7.4% was obtained over very short extraction times (see Fig. 1). This is a combined result of the large solubility of triglycerides at these pressures and of the relatively small quantity of oil contained in this kind of seed.
2. Experimental results and discussion Supercritical CO extraction tests were performed us2 ing a 25 cm3 extraction vessel (¸"15 cm) in which approximately 13 g of hiprose seed particles was loaded. The vessel was placed in a temperature-controlled oven (Applied separations SFE-2, Allentown, PA, USA) and liquid CO was pressurized by a reciprocating pump, 2 heated and then pumped to the extractor. At the end of the extraction, the supercritical #uid was depressurized across a #ow control valve to atmospheric pressure and the oil was collected in a pre-weighed collection vial. Extractions were performed at temperatures of 40, 50 and 703C, pressures of 1500, 3000, 6000 and 10 000 psi, CO #ow rates of 1, 2, 4 and 6 g/min and hiprose seed 2
Fig. 1. E!ect of pressure on the hiprose oil yield at 403C, 6 g/min CO #ow rate and 0.42 mm particles diameter; continuous curves 2 are generated by the model (h: 1500 psi, C "0.0005 g/g; L: 3000 psi, 0 C "0.005 g/g; n: 6000 psi; C "0.017 g/g; £: 10 000 psi, 0 0 C "0.04 g/g). 0
E. Reverchon et al. / Chemical Engineering Science 55 (2000) 2195}2201
2.2. Ewect of temperature The e!ect of temperature has been studied for experiments performed at 6000 and 10 000 psi pressures, 0.42 and 0.79 mm particle size and a 6 g/min CO #ow rate. 2 Temperatures of 40, 50 and 703C were explored for each pressure. The scienti"c literature (Klein & Schulz, 1989; Eggers, 1996; Goodrum, Kilgo & Santerre, 1996) signals an increase of the seed oil solubility with the extraction temperature that can be signi"cant when the process is performed at pressures higher than 6000 psi. Our experimental results at a pressure of 6000 psi show a negligible e!ect of the temperature increase on the hiprose extraction rate; i.e. the experimental points at di!erent temperature practically overlap as shown in Fig. 2, in which the experimental results obtained by operating at 6 g/min CO #ow rate, 0.79 mm mean par2 ticle diameter and at 40, 50 and 703C are reported. Instead, the experiments performed at 10 000 psi give extraction rates at di!erent temperatures that could be in agreement with the previous cited literature. However, at this pressure the extraction is very fast and the number of experimental points collected in the "rst part of the extraction process was not su$cient to obtain conclusive information about the e!ect of temperature.
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approximately reduces the extraction time to one-half. These results are immediately explained when we consider, as in the previous paragraphs, that the equilibrium solubility of hiprose oil controls at least the "rst part of extraction process. 2.4. Ewect of particle size These experiments were performed using 0.42, 0.79 and 1.03 mm mean particle sizes at a pressure of 10 000 psi, CO #ow rate of 6 g/min and temperatures of 40, 50 and 2 703C. The asymptotic hiprose oil yield obtained for 0.42 mm particles was about 7.4% at all temperatures studied (as in the previously discussed experiments) whereas, the asymptotic yield was 5.2 and 4.9% for 0.79 and 1.03 mm particles, respectively. In Fig. 4 these results
2.3. Ewect of yow rate The in#uence of the CO #ow rate has been studied at 2 a temperature of 703C, pressure of 10 000 psi and 0.42 mm particle size for CO #ow rates of 1, 2, 4 and 2 6 g/min. The corresponding yield data are reported in Fig. 3. The extraction time required to obtain the asymptotic oil yield is inversely proportional to CO #ow rate; 2 i.e., an increase of the CO #ow rate by a factor of two, 2
Fig. 2. E!ect of temperature on the hiprose oil yield at 6000 psi, 6 g/min CO #ow rate and 0.79 mm particles diameter; continuous 2 curve is generated by the model (h: 403C; L: 503C; n: 703C).
Fig. 3. E!ect of CO #ow rate on the hiprose oil yield at 10 000 psi, 2 703C and 0.42 mm particle diameter; continuous curves are generated by the model (h: 1 g/min; L: 2 g/min; n: 4 g/min; £: 6 g/min).
Fig. 4. E!ect of particle size on hiprose oil yield at 10 000 psi, 403C and 6 g/min CO #ow rate; continuous curves are generated by the model 2 (h: 0.42 mm; L: 0.79 mm; n: 1.03 mm).
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are illustrated for the experiments performed at 403C. The large decrease in oil yield observed when the particle size was increased was not the result of a single extraction test: various experiments were performed at several temperatures and some of them were also repeated obtaining the reported yields. A similar behavior has been reported in the literature in some cases of supercritical #uid extraction of oil from seeds when large particle sizes were used (Roy et al., 1994; Goodrum et al., 1996). In these cases part of the oil was not extracted due to the very long di!usion times of the solvent in the vegetable particles. In our case, the hiprose seed particles are not particularly large (maximum mean diameter 1.03 mm); therefore, this result could be related to the characteristics of the hiprose seed particles. We observed various hiprose seed particles by scanning electron microscope (SEM). A typical SEM image is shown in Fig. 5. In this image, the seed is distinguished by a lignin structure crossed by very long microscopic channels. In this case we did not "nd speci"c oil bearing structures; therefore, we supposed that the channels could contain the hiprose seed oil. The channels have an approximate diameter of 20}30 lm and a variable length of several hundreds of microns. Thus, the microscopic structure of hiprose seed is very much di!erent from the other seed structures we have studied by SEM in previous papers devoted to seed oil supercritical extraction (almond and fennel) (Marrone et al., 1998;
Reverchon et al., 1999). Indeed, the microscopic internal structures of almond and fennel seeds showed spherical and elongated hexagonal structures, respectively, that contained the oil. Moreover, the intact `cellsa were easily accessible by di!usion into the cellulosic structure. In the present case, the unbroken channels are protected by a lignin structure that is probably too compact to allow an e!ective di!usion of the supercritical solvent in a reasonable extraction time. This microscopic evidence, thus, explains the reduction of hiprose oil yield when larger particles were used: i.e., larger particles contain a nonnegligible fraction of oil that cannot be extracted since it is blocked in intact channels.
3. Mathematical modelling, results and discussion As stated previously, the SEM image of hiprose seed particle in Fig. 5 allowed us to explain the di!erences in oil yield due to particle size di!erences. This image also suggested that, in this case, the standard model of the broken and intact cells cannot be applied. In the case of hiprose seeds, we have broken and intact channels but, the oil contained in the unbroken channels is not accessible. Moreover, the channels structure suggests that only the oil contained near the opening of the channel is readily available for extraction. To extract hiprose oil
Fig. 5. SEM image of hiprose seed particle. The vegetable structure contains very long microscopic channels that can contain the seed oil.
E. Reverchon et al. / Chemical Engineering Science 55 (2000) 2195}2201
located inside the channels, the supercritical solvent faces an increasing resistance due to the di!usion in the channels at an increasing depth, as long as the extraction lasts. These considerations are also supported by the fact that oil yield data in the second part of the extraction process do not lie on a straight line; the curvature of yield data can be correlated to the presence of a mass transfer resistance. For these reasons, we inserted in the model an internal mass transfer resistance that increases linearly till the oil contained in the channels is extracted. 3.1. General hypotheses The modelling of the extraction process was also based on the following hypotheses. (a) Several components are generally involved in the extraction of a seed oil. However, we supposed that their behavior with respect to the mass transfer phenomena is similar and can be described by a single pseudo-component. (b) The commonly accepted continuous description of the extraction bed has been assumed with the implicit hypothesis that concentration gradients in the #uid phase develop at scales larger than the particle size. The solute concentration in the #uid phase depends only on time, t, and on the axial coordinate, z. Its value, C, is given in terms of solid mass per unit of solvent mass. (c) The solvent #ow rate, with super"cial velocity u, is uniformly distributed in every section of the extractor. The pressure drop as well as temperature gradients within the column can be neglected. The axial dispersion is negligible. (d) The volume fraction of the #uid, e, is not a!ected by the reduction of the solid mass during the extraction, in other words, the solids do not change their volume during the extraction process. Moreover, we supposed that the loading time of the solvent into the extractor was long enough to enable the #uid to reach the equilibrium concentration before the extraction started. The standard model of broken and intact cells is formed by three partial di!erential equations: one for oil in broken cells, one for oil in the intact cells and one for oil in #uid phase. As a result of the previous considerations, the di!erential balance for oil contained in intact cells can be omitted in this case. Therefore, the model equations and the initial and boundary condition are those obtainable by two di!erential mass balances (overall and solid phase): LC u LC 1!e o Lq s # # "0, Lt e Lz e o Lt f
(1)
Lq "!K (>)a(q!qH), i Lt qH"K C, %2 C"C at time t"0 for each z, 0 q"q at time t"0 for each z, 0 C"0 at z"0 for each t,
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(2) (3) (4) (5) (6)
where u (cm/min) is the super"cial velocity of supercritical CO , e is the void fraction of the extractor, o (g/cm3) 2 f is the density of supercritical CO , o (g/cm3) is the 2 s density of hiprose seed, > is the yield of oil seed, a (cm~1) is the speci"c surface of vegetable matter, K is the linear %2 equilibrium constant, C and q are the concentrations of oil in #uid phase and solid phase, respectively, expressed as a mass ratio (g/g) and qH is the concentration of oil in solid phase, at the solid}#uid interface. The only one adjustable parameter contained in this model is the internal mass transfer coe$cient K that we i assumed to be linearly variable (see the previous discussion) as K (>)"K !(K !K )(>/> ), (7) i io io if f where > is the asymptotic value of the oil yield, K and f io K are the initial and "nal value of the mass transfer if coe$cient, respectively. The proposed model was numerically integrated using a "nite di!erences method in which an explicit numerical cell was implemented. The proposed mathematical model was very sensitive to the values of K used for the "tting, whereas, it was io not in#uenced by K values lower than approximately if 1]10~8 m/s. Therefore, we can put K "0; therefore, if Eq. (7) becomes K (>)"K (1!>/> ) (8) i io f and K was the only adjustable parameter that remained io in the model. The best "t between the model and all the sets of experimental data was obtained when we set K "1.9]10~5 m/s. This value of K was used to proio io duce all the model curves reported in Figs. 1}4. In Fig. 1 the comparison between the extraction results obtained at various pressures for 0.42 mm particles at 403C and 6 g/min CO #ow rate and the best "t model 2 curves is given. In this case we also used di!erent values of C to take into account the increase of oil solubility 0 with extraction pressure. The di!erent values of C were 0 calculated from the "rst linear part of the extraction data and are reported in Table 1. The overlap between experimental data and model curves is fairly good for all the pressures studied. In Fig. 2 the comparison between experimental results and the model curve for the extraction test performed for 0.79 mm particles at 40, 50, and 703C, a pressure of 6000 psi and 6 g/min CO #ow rate is shown. As in the 2
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Table 1 Values of C (g/g) calculated from the extraction data at di!erent 0 pressures Pressure (psi)
C (g/g) 0
1500 3000 6000 10 000
0.0005 0.005 0.017 0.04
previous case the model behaves well when compared to the oil yield data. In Fig. 3 the in#uence of the CO #ow rate at 2 10 000 psi, 703C and 0.42 mm particle size is shown. The comparison between model curves and experimental data was also successful in this case. This result con"rms that the extraction process is mainly controlled by equilibrium in the "rst part and then the linearly increasing mass transfer resistance controls the behavior of the extraction process. The comparison between the model curves and the experimental data obtained by varying the particle size at 10 000 psi, 403C and 6 g/min CO #ow rate is reported in 2 Fig. 4. In this case, the di!erent asymptotic yield for di!erent particle sizes has also been taken into account in the model. The model curve obtained for 0.42 mm particles compares well with the experimental data, whereas, the other two model curves gave less satisfactorily results, especially for the description of the "rst part of the extraction process. At this point in the modelling, we collected su$cient results to a$rm that the proposed model is able to "t the overall experimental evidences and that the SEM observations on hiprose seed particles had a relevant role in the successful "tting. But, is it still possible to improve the "tting between model curves and data for di!erent particle sizes? The model proposed relies on the assumption that there is su$cient oil on the surface of the particles to reach the equilibrium conditions at the beginning of the extraction process. But, this assumption cannot be true when 0.79 and 1.03 mm particles are used. Indeed, (a) the quantity of oil available on the surface of hiprose seed particles is not large, due to the channels structure; (b) at 10 000 psi the solubility of oil in supercritical CO is very 2 large and (c) larger particles expose a reduced oil surface to the extraction when compared to smaller particles. We can take into account all these facts considering that the quantity of oil available for immediate extraction in supercritical CO (equal to C for 0.42 mm particles), 2 0 depends on the ratio of the speci"c surfaces a , a and 0 1 a for the various particle sizes. Thus, 2 C "C (a /a ) 1 0 1 0
for 0.79 mm particles,
(9)
C "C (a /a ) 2 0 2 0
for 1.03 mm particles,
(10)
Fig. 6. E!ect of particle size on hiprose oil yield at 10 000 psi, 403C and 6 g/min CO #ow rate. The model curves obtained in this case take into 2 account the speci"c surface ratios in the calculation of C ; continuous 0 curves are generated by the model (h: 0.42 mm, C "0.04 g/g; L: 0 0.79 mm, C "0.021 g/g; n: 1.03 mm, C "0.016 g/g). 0 0
where a , a and a can be readily calculated from the 0 1 2 experimental data. When we repeated the "tting of the experimental data using the model Eqs. (1)}(6) and (8) and the same value of K as in previous modelling tests, we obtained the io curves reported in Fig. 6. These new model curves show a good "t with the experimental data for the larger particle sizes too. This result con"rms that the ratio of speci"c surfaces of particles also plays a role in this extraction process. 4. Conclusions The mathematical modelling of hiprose seed oil supercritical extraction has been successfully performed using di!erential mass balances. The SEM analysis of the vegetable structure characteristic of this kind of seed allowed the formulation of a mathematical model that produced a fairly good "t for all the experimental data and is applicable to all seeds having a similar vegetable structure. We also considered that: (a) it is possible to have an excluded volume when large particles are used, i.e., a volume non-accessible in reasonable extraction times to the supercritical solvent; (b) the ratio of the speci"c surface of particles can be used to take into account deviations from the equilibrium extraction during the "rst part of the process. References Bulley, N. R., Fattori, M., Meisen, A., & Moyls, L. (1984). Supercritical #uid extraction of vegetable seeds. JAOCS, 61, 1362}1365.
E. Reverchon et al. / Chemical Engineering Science 55 (2000) 2195}2201 Eggers, R. (1996). Supercritical extraction of oilseed/lipids in natural products, In: J. W. King & G. R. List, Supercritical yuid technology in oil and lipid chemistry (pp. 35}64). Champaign, IL: AOCS Press. Fattori, M., Bulley, N. R., & Meisen, A. (1988). Carbon dioxide extraction of canola seed: Oil solubility and e!ect of seed treatment. JAOCS, 65, 968}974. Goodrum, J. W., Kilgo, M. K., & Santerre, C. R. (1996). Oil seed solubility and extraction modeling. In: J. W. King & G. R. List, Supercritical yuid technology in oil and lipid chemistry (pp. 101}131). Champaign, IL: AOCS Press. King, M. B., & Catchpole, O. (1993). Physico-chemical data required for the design of near critical #uid extraction process. In: M. B. King & T. R. Bott, Extraction of natural products using near-critical solvents (pp. 184}228). Glasgow: Blackie Academic Professional. Klein, T., & Schulz, S. (1989). Measurement and model prediction of vapor}liquid equilibria of mixtures of rapeseed oil and supercritical carbon dioxide. Industrial and Engineering Chemistry Research, 28, 1073}1081. Lee, A. K. K., Bulley, N. R., Fattori, M., & Meisen, A. (1986). Modelling of supercritical carbon dioxide extraction of canola oilseed in "xed beds. JAOCS, 63, 921}925.
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Marrone, C., Poletto, M., Reverchon, E., & Stassi, A. (1998). Almond oil extraction by supercritical CO : Experiments and modelling. Chem2 ical Engineering Science, 53, 3711}3718. Perrut, M., Clavier, J. Y., Poletto, M., & Reverchon, E. (1997). Mathematical modelling of sun#ower seed extraction by supercritical CO . Industrial and Engineering Chemistry Research, 36, 2 430}435. Reverchon, E., Daghero, J., Marrone, C., Mattea, M., & Poletto, M. (1999). Supercritical fractional extraction of fennel seed oil and essential oil: Experiments and modelling. Industrial and Engineering Chemistry Research, 38, 3069}3075. Roy, B. C., Goto, M., Hirose, T., Navaro, O., & Hortacsu, O. (1994). Extraction rates of oil from tomato seeds with supercritical carbon dioxide. Japanese Journal of Chemical Engineering, 27, 768}772. SovovaH , H. (1994). Rate of the vegetable oil extraction with supercritical CO * I. Modelling of extraction curves. Chemical Engineering 2 Science, 49, 409}414. SovovaH , H., Kucera, J., & Jez, J. (1994). Rate of the vegetable oil extraction with supercritical CO * II. Chemical Engineering 2 Science, 49, 415}420.
Supercritical CO extraction of hiprose seed oil: 2 experiments and mathematical modelling E. Reverchon!,*, A. Kaziunas", C. Marrone! !Dipartimento di Ingegneria Chimica e Alimentare, Universita% di Salerno Via Ponte Don Melillo, I-84084 Fisciano (SA), Italy "Applied Separations, 930 Hamilton Street, Allentown, PA, 18101, USA Received 19 May 1999; received in revised form 22 September 1999; accepted 30 September 1999
Abstract Hiprose seed oil has been extracted by supercritical CO operating at pressures of 1500, 3000, 6000 and 10 000 psi, temperatures of 2 40, 50 and 703C, CO #ow rates of 1, 2, 4 and 6 g/min using mean particle sizes of 0.42, 0.79 and 1.03 mm. The obtained oil yield data 2 has been used to validate a mathematical model of the extraction process, based in part on the information obtained from the microscopic structure of the hiprose seed particles. The di!erential mass balances forming the model have been numerically integrated. The best "t between model curves and data has been obtained when a linearly variable internal mass transfer coe$cient was adopted, with an initial value of 1.9]10~5 m/s. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: Supercritical extraction; Hiprose oil; Modelling
1. Introduction Hiprose seeds contain a relatively low percentage of oil (between 7 and 15% by hexane extraction) when compared to other seeds; for example, almonds contain up to about 50% by weight of oil. The main component of the oil is linoleic acid; however, in spite of the low oil content, the characteristic composition of the hiprose oil makes it a valuable product for cosmetic industry. Therefore, an improved process for its extraction would be of industrial interest. Supercritical #uid extraction (SFE) has been applied to the extraction of vegetable oils from several seeds as recently reviewed by Eggers (1996). Despite the large number of species processed, only some papers on the modelling of the SFE of seeds have been published. All of the published reports agree that at least the "rst part of the SFE process is governed by the solubility equilibrium between the oil and the #uid phase. The equilibrium relationship has been generally supposed to be linear since more precise information is not available in such complex systems.
* Corresponding author. Tel.: #39-089-964116; fax: #39-089964057. E-mail address: [email protected] (E. Reverchon)
From the mathematical point of view, the models previously proposed are generally based on di!erential mass balances integration. Bulley, Fattori, Meisen and Moyls (1984), Lee, Bulley, Fattori and Meisen (1986) and Fattori, Bulley and Meisen (1988) assumed that mass transfer resistance occurred only in the solvent phase. In other models an internal mass transfer resistance was considered (Roy, Goto, Hirose, Navaro & Hortacsu, 1994). A sigmoidal equilibrium curve was adopted by Perrut, Clavier, Poletto and Reverchon (1997) to "t the experimental data. King and Catchpole (1993) used a shrinking core model to describe a variable external resistance where the solute balance on the solid phase determines the thickness of the mass transfer layer in the external part of the particles. However, more reliable models can be produced when the vegetable structure of the seed is considered in the equations. From this respect, SovovaH (1994) proposed a model based on the hypothesis that the internal part of seeds is formed by specialized structures that contain the oil. Therefore, as a result of grinding, the seed structures located on the surface of the particles are broken, whereas the internal structures remain intact. The broken and intact `cellsa model was applied to the extraction of caraway seed oil (SovovaH , Kucera & Jez, 1994). This model was thus based on sound physical hypotheses and represented the "rst attempt to introduce a description of
0009-2509/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 5 1 9 - 9
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the structure of the vegetable matter into the mathematical model. However, the model required the knowledge of several parameters related to physical properties of the seed which were di$cult to measure or calculate. Therefore, the authors adjusted these parameters to "t the experimental results. A large number of parameters require an even larger number of data sets for model validation. Better results can be obtained when some of the model parameters are independently measured. Marrone, Poletto, Reverchon and Stassi (1998) in the modelling of the supercritical extraction of almond oil, demonstrated that it is possible to "x all parameters but the internal mass transfer resistance, one, in the Sovova` model, since they measured by scanning electronic microscope (SEM) the number and the dimensions of the spherical oil bearing structures in almond seed particles. This approach was successfully adopted also in the case of fennel oil supercritical extraction where the oil bearing structures were hexagonal (Reverchon, Daghero, Marrone, Mattea & Poletto (1999)). In this case the authors "rst selectively extracted the fennel essential oil at low pressure (90 bar; 503C) to avoid vegetable oil contamination with essential oil and then they extracted the vegetable oil at a higher pressure. However, as a rule, the models proposed until now in the literature were applied to a small set of experimental data and as such explored the in#uence of only some process parameters. In addition, other varieties of seeds may show di!erent behavior when dissimilar internal structures characterize the seeds. In this work, supercritical extraction of hiprose seed oil has been performed at various pressures, temperatures, particle sizes and CO #ow rates with the aim of high2 lighting the in#uence of all process parameters on the extraction process. In addition, we evaluated the possibility of applying the broken and intact cells model equations. The hiprose seed microstructure data was obtained by SEM and was of fundamental importance in the modelling of the extraction process.
particle sizes of 0.42, 0.79 and 1.03 mm. The mean particle size was measured by mechanical sieving of seed particles and applying the Sauter formula. The e!ect of pressure was studied "rst; then, the pressure was "xed at 6000 and 10 000 psi and all the other variables were varied one at a time, thus obtaining several data sets on hiprose oil yield against the extraction time. Some crucial experiments were repeated twice. We de"ned the oil extraction yield (% by weight) as the quantity of oil extracted divided by the quantity of vegetable matrix loaded in the extractor multiplied by 100.
2.1. Ewect of pressure As expected, the rate of extraction of hiprose oil largely increased with pressure as shown in Fig. 1 where oil yield data obtained at 1500, 3000, 6000 and 10 000 psi are reported for experiments performed at 403C temperature, 6 g/min CO #ow rate and 0.42 mm mean particle size. 2 The e!ect of the extraction pressure can be explained taking into account the large increase in the solubility of the oil constituents (mainly triglycerides) with pressure. Moreover, a large part of the extraction process is controlled by this thermodynamic parameter, as suggested by the experimental data: the "rst part of yield data could be "tted using a straight line. At 6000 and 10 000 psi an asymptotic yield of about 7.4% was obtained over very short extraction times (see Fig. 1). This is a combined result of the large solubility of triglycerides at these pressures and of the relatively small quantity of oil contained in this kind of seed.
2. Experimental results and discussion Supercritical CO extraction tests were performed us2 ing a 25 cm3 extraction vessel (¸"15 cm) in which approximately 13 g of hiprose seed particles was loaded. The vessel was placed in a temperature-controlled oven (Applied separations SFE-2, Allentown, PA, USA) and liquid CO was pressurized by a reciprocating pump, 2 heated and then pumped to the extractor. At the end of the extraction, the supercritical #uid was depressurized across a #ow control valve to atmospheric pressure and the oil was collected in a pre-weighed collection vial. Extractions were performed at temperatures of 40, 50 and 703C, pressures of 1500, 3000, 6000 and 10 000 psi, CO #ow rates of 1, 2, 4 and 6 g/min and hiprose seed 2
Fig. 1. E!ect of pressure on the hiprose oil yield at 403C, 6 g/min CO #ow rate and 0.42 mm particles diameter; continuous curves 2 are generated by the model (h: 1500 psi, C "0.0005 g/g; L: 3000 psi, 0 C "0.005 g/g; n: 6000 psi; C "0.017 g/g; £: 10 000 psi, 0 0 C "0.04 g/g). 0
E. Reverchon et al. / Chemical Engineering Science 55 (2000) 2195}2201
2.2. Ewect of temperature The e!ect of temperature has been studied for experiments performed at 6000 and 10 000 psi pressures, 0.42 and 0.79 mm particle size and a 6 g/min CO #ow rate. 2 Temperatures of 40, 50 and 703C were explored for each pressure. The scienti"c literature (Klein & Schulz, 1989; Eggers, 1996; Goodrum, Kilgo & Santerre, 1996) signals an increase of the seed oil solubility with the extraction temperature that can be signi"cant when the process is performed at pressures higher than 6000 psi. Our experimental results at a pressure of 6000 psi show a negligible e!ect of the temperature increase on the hiprose extraction rate; i.e. the experimental points at di!erent temperature practically overlap as shown in Fig. 2, in which the experimental results obtained by operating at 6 g/min CO #ow rate, 0.79 mm mean par2 ticle diameter and at 40, 50 and 703C are reported. Instead, the experiments performed at 10 000 psi give extraction rates at di!erent temperatures that could be in agreement with the previous cited literature. However, at this pressure the extraction is very fast and the number of experimental points collected in the "rst part of the extraction process was not su$cient to obtain conclusive information about the e!ect of temperature.
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approximately reduces the extraction time to one-half. These results are immediately explained when we consider, as in the previous paragraphs, that the equilibrium solubility of hiprose oil controls at least the "rst part of extraction process. 2.4. Ewect of particle size These experiments were performed using 0.42, 0.79 and 1.03 mm mean particle sizes at a pressure of 10 000 psi, CO #ow rate of 6 g/min and temperatures of 40, 50 and 2 703C. The asymptotic hiprose oil yield obtained for 0.42 mm particles was about 7.4% at all temperatures studied (as in the previously discussed experiments) whereas, the asymptotic yield was 5.2 and 4.9% for 0.79 and 1.03 mm particles, respectively. In Fig. 4 these results
2.3. Ewect of yow rate The in#uence of the CO #ow rate has been studied at 2 a temperature of 703C, pressure of 10 000 psi and 0.42 mm particle size for CO #ow rates of 1, 2, 4 and 2 6 g/min. The corresponding yield data are reported in Fig. 3. The extraction time required to obtain the asymptotic oil yield is inversely proportional to CO #ow rate; 2 i.e., an increase of the CO #ow rate by a factor of two, 2
Fig. 2. E!ect of temperature on the hiprose oil yield at 6000 psi, 6 g/min CO #ow rate and 0.79 mm particles diameter; continuous 2 curve is generated by the model (h: 403C; L: 503C; n: 703C).
Fig. 3. E!ect of CO #ow rate on the hiprose oil yield at 10 000 psi, 2 703C and 0.42 mm particle diameter; continuous curves are generated by the model (h: 1 g/min; L: 2 g/min; n: 4 g/min; £: 6 g/min).
Fig. 4. E!ect of particle size on hiprose oil yield at 10 000 psi, 403C and 6 g/min CO #ow rate; continuous curves are generated by the model 2 (h: 0.42 mm; L: 0.79 mm; n: 1.03 mm).
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are illustrated for the experiments performed at 403C. The large decrease in oil yield observed when the particle size was increased was not the result of a single extraction test: various experiments were performed at several temperatures and some of them were also repeated obtaining the reported yields. A similar behavior has been reported in the literature in some cases of supercritical #uid extraction of oil from seeds when large particle sizes were used (Roy et al., 1994; Goodrum et al., 1996). In these cases part of the oil was not extracted due to the very long di!usion times of the solvent in the vegetable particles. In our case, the hiprose seed particles are not particularly large (maximum mean diameter 1.03 mm); therefore, this result could be related to the characteristics of the hiprose seed particles. We observed various hiprose seed particles by scanning electron microscope (SEM). A typical SEM image is shown in Fig. 5. In this image, the seed is distinguished by a lignin structure crossed by very long microscopic channels. In this case we did not "nd speci"c oil bearing structures; therefore, we supposed that the channels could contain the hiprose seed oil. The channels have an approximate diameter of 20}30 lm and a variable length of several hundreds of microns. Thus, the microscopic structure of hiprose seed is very much di!erent from the other seed structures we have studied by SEM in previous papers devoted to seed oil supercritical extraction (almond and fennel) (Marrone et al., 1998;
Reverchon et al., 1999). Indeed, the microscopic internal structures of almond and fennel seeds showed spherical and elongated hexagonal structures, respectively, that contained the oil. Moreover, the intact `cellsa were easily accessible by di!usion into the cellulosic structure. In the present case, the unbroken channels are protected by a lignin structure that is probably too compact to allow an e!ective di!usion of the supercritical solvent in a reasonable extraction time. This microscopic evidence, thus, explains the reduction of hiprose oil yield when larger particles were used: i.e., larger particles contain a nonnegligible fraction of oil that cannot be extracted since it is blocked in intact channels.
3. Mathematical modelling, results and discussion As stated previously, the SEM image of hiprose seed particle in Fig. 5 allowed us to explain the di!erences in oil yield due to particle size di!erences. This image also suggested that, in this case, the standard model of the broken and intact cells cannot be applied. In the case of hiprose seeds, we have broken and intact channels but, the oil contained in the unbroken channels is not accessible. Moreover, the channels structure suggests that only the oil contained near the opening of the channel is readily available for extraction. To extract hiprose oil
Fig. 5. SEM image of hiprose seed particle. The vegetable structure contains very long microscopic channels that can contain the seed oil.
E. Reverchon et al. / Chemical Engineering Science 55 (2000) 2195}2201
located inside the channels, the supercritical solvent faces an increasing resistance due to the di!usion in the channels at an increasing depth, as long as the extraction lasts. These considerations are also supported by the fact that oil yield data in the second part of the extraction process do not lie on a straight line; the curvature of yield data can be correlated to the presence of a mass transfer resistance. For these reasons, we inserted in the model an internal mass transfer resistance that increases linearly till the oil contained in the channels is extracted. 3.1. General hypotheses The modelling of the extraction process was also based on the following hypotheses. (a) Several components are generally involved in the extraction of a seed oil. However, we supposed that their behavior with respect to the mass transfer phenomena is similar and can be described by a single pseudo-component. (b) The commonly accepted continuous description of the extraction bed has been assumed with the implicit hypothesis that concentration gradients in the #uid phase develop at scales larger than the particle size. The solute concentration in the #uid phase depends only on time, t, and on the axial coordinate, z. Its value, C, is given in terms of solid mass per unit of solvent mass. (c) The solvent #ow rate, with super"cial velocity u, is uniformly distributed in every section of the extractor. The pressure drop as well as temperature gradients within the column can be neglected. The axial dispersion is negligible. (d) The volume fraction of the #uid, e, is not a!ected by the reduction of the solid mass during the extraction, in other words, the solids do not change their volume during the extraction process. Moreover, we supposed that the loading time of the solvent into the extractor was long enough to enable the #uid to reach the equilibrium concentration before the extraction started. The standard model of broken and intact cells is formed by three partial di!erential equations: one for oil in broken cells, one for oil in the intact cells and one for oil in #uid phase. As a result of the previous considerations, the di!erential balance for oil contained in intact cells can be omitted in this case. Therefore, the model equations and the initial and boundary condition are those obtainable by two di!erential mass balances (overall and solid phase): LC u LC 1!e o Lq s # # "0, Lt e Lz e o Lt f
(1)
Lq "!K (>)a(q!qH), i Lt qH"K C, %2 C"C at time t"0 for each z, 0 q"q at time t"0 for each z, 0 C"0 at z"0 for each t,
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(2) (3) (4) (5) (6)
where u (cm/min) is the super"cial velocity of supercritical CO , e is the void fraction of the extractor, o (g/cm3) 2 f is the density of supercritical CO , o (g/cm3) is the 2 s density of hiprose seed, > is the yield of oil seed, a (cm~1) is the speci"c surface of vegetable matter, K is the linear %2 equilibrium constant, C and q are the concentrations of oil in #uid phase and solid phase, respectively, expressed as a mass ratio (g/g) and qH is the concentration of oil in solid phase, at the solid}#uid interface. The only one adjustable parameter contained in this model is the internal mass transfer coe$cient K that we i assumed to be linearly variable (see the previous discussion) as K (>)"K !(K !K )(>/> ), (7) i io io if f where > is the asymptotic value of the oil yield, K and f io K are the initial and "nal value of the mass transfer if coe$cient, respectively. The proposed model was numerically integrated using a "nite di!erences method in which an explicit numerical cell was implemented. The proposed mathematical model was very sensitive to the values of K used for the "tting, whereas, it was io not in#uenced by K values lower than approximately if 1]10~8 m/s. Therefore, we can put K "0; therefore, if Eq. (7) becomes K (>)"K (1!>/> ) (8) i io f and K was the only adjustable parameter that remained io in the model. The best "t between the model and all the sets of experimental data was obtained when we set K "1.9]10~5 m/s. This value of K was used to proio io duce all the model curves reported in Figs. 1}4. In Fig. 1 the comparison between the extraction results obtained at various pressures for 0.42 mm particles at 403C and 6 g/min CO #ow rate and the best "t model 2 curves is given. In this case we also used di!erent values of C to take into account the increase of oil solubility 0 with extraction pressure. The di!erent values of C were 0 calculated from the "rst linear part of the extraction data and are reported in Table 1. The overlap between experimental data and model curves is fairly good for all the pressures studied. In Fig. 2 the comparison between experimental results and the model curve for the extraction test performed for 0.79 mm particles at 40, 50, and 703C, a pressure of 6000 psi and 6 g/min CO #ow rate is shown. As in the 2
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Table 1 Values of C (g/g) calculated from the extraction data at di!erent 0 pressures Pressure (psi)
C (g/g) 0
1500 3000 6000 10 000
0.0005 0.005 0.017 0.04
previous case the model behaves well when compared to the oil yield data. In Fig. 3 the in#uence of the CO #ow rate at 2 10 000 psi, 703C and 0.42 mm particle size is shown. The comparison between model curves and experimental data was also successful in this case. This result con"rms that the extraction process is mainly controlled by equilibrium in the "rst part and then the linearly increasing mass transfer resistance controls the behavior of the extraction process. The comparison between the model curves and the experimental data obtained by varying the particle size at 10 000 psi, 403C and 6 g/min CO #ow rate is reported in 2 Fig. 4. In this case, the di!erent asymptotic yield for di!erent particle sizes has also been taken into account in the model. The model curve obtained for 0.42 mm particles compares well with the experimental data, whereas, the other two model curves gave less satisfactorily results, especially for the description of the "rst part of the extraction process. At this point in the modelling, we collected su$cient results to a$rm that the proposed model is able to "t the overall experimental evidences and that the SEM observations on hiprose seed particles had a relevant role in the successful "tting. But, is it still possible to improve the "tting between model curves and data for di!erent particle sizes? The model proposed relies on the assumption that there is su$cient oil on the surface of the particles to reach the equilibrium conditions at the beginning of the extraction process. But, this assumption cannot be true when 0.79 and 1.03 mm particles are used. Indeed, (a) the quantity of oil available on the surface of hiprose seed particles is not large, due to the channels structure; (b) at 10 000 psi the solubility of oil in supercritical CO is very 2 large and (c) larger particles expose a reduced oil surface to the extraction when compared to smaller particles. We can take into account all these facts considering that the quantity of oil available for immediate extraction in supercritical CO (equal to C for 0.42 mm particles), 2 0 depends on the ratio of the speci"c surfaces a , a and 0 1 a for the various particle sizes. Thus, 2 C "C (a /a ) 1 0 1 0
for 0.79 mm particles,
(9)
C "C (a /a ) 2 0 2 0
for 1.03 mm particles,
(10)
Fig. 6. E!ect of particle size on hiprose oil yield at 10 000 psi, 403C and 6 g/min CO #ow rate. The model curves obtained in this case take into 2 account the speci"c surface ratios in the calculation of C ; continuous 0 curves are generated by the model (h: 0.42 mm, C "0.04 g/g; L: 0 0.79 mm, C "0.021 g/g; n: 1.03 mm, C "0.016 g/g). 0 0
where a , a and a can be readily calculated from the 0 1 2 experimental data. When we repeated the "tting of the experimental data using the model Eqs. (1)}(6) and (8) and the same value of K as in previous modelling tests, we obtained the io curves reported in Fig. 6. These new model curves show a good "t with the experimental data for the larger particle sizes too. This result con"rms that the ratio of speci"c surfaces of particles also plays a role in this extraction process. 4. Conclusions The mathematical modelling of hiprose seed oil supercritical extraction has been successfully performed using di!erential mass balances. The SEM analysis of the vegetable structure characteristic of this kind of seed allowed the formulation of a mathematical model that produced a fairly good "t for all the experimental data and is applicable to all seeds having a similar vegetable structure. We also considered that: (a) it is possible to have an excluded volume when large particles are used, i.e., a volume non-accessible in reasonable extraction times to the supercritical solvent; (b) the ratio of the speci"c surface of particles can be used to take into account deviations from the equilibrium extraction during the "rst part of the process. References Bulley, N. R., Fattori, M., Meisen, A., & Moyls, L. (1984). Supercritical #uid extraction of vegetable seeds. JAOCS, 61, 1362}1365.
E. Reverchon et al. / Chemical Engineering Science 55 (2000) 2195}2201 Eggers, R. (1996). Supercritical extraction of oilseed/lipids in natural products, In: J. W. King & G. R. List, Supercritical yuid technology in oil and lipid chemistry (pp. 35}64). Champaign, IL: AOCS Press. Fattori, M., Bulley, N. R., & Meisen, A. (1988). Carbon dioxide extraction of canola seed: Oil solubility and e!ect of seed treatment. JAOCS, 65, 968}974. Goodrum, J. W., Kilgo, M. K., & Santerre, C. R. (1996). Oil seed solubility and extraction modeling. In: J. W. King & G. R. List, Supercritical yuid technology in oil and lipid chemistry (pp. 101}131). Champaign, IL: AOCS Press. King, M. B., & Catchpole, O. (1993). Physico-chemical data required for the design of near critical #uid extraction process. In: M. B. King & T. R. Bott, Extraction of natural products using near-critical solvents (pp. 184}228). Glasgow: Blackie Academic Professional. Klein, T., & Schulz, S. (1989). Measurement and model prediction of vapor}liquid equilibria of mixtures of rapeseed oil and supercritical carbon dioxide. Industrial and Engineering Chemistry Research, 28, 1073}1081. Lee, A. K. K., Bulley, N. R., Fattori, M., & Meisen, A. (1986). Modelling of supercritical carbon dioxide extraction of canola oilseed in "xed beds. JAOCS, 63, 921}925.
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Marrone, C., Poletto, M., Reverchon, E., & Stassi, A. (1998). Almond oil extraction by supercritical CO : Experiments and modelling. Chem2 ical Engineering Science, 53, 3711}3718. Perrut, M., Clavier, J. Y., Poletto, M., & Reverchon, E. (1997). Mathematical modelling of sun#ower seed extraction by supercritical CO . Industrial and Engineering Chemistry Research, 36, 2 430}435. Reverchon, E., Daghero, J., Marrone, C., Mattea, M., & Poletto, M. (1999). Supercritical fractional extraction of fennel seed oil and essential oil: Experiments and modelling. Industrial and Engineering Chemistry Research, 38, 3069}3075. Roy, B. C., Goto, M., Hirose, T., Navaro, O., & Hortacsu, O. (1994). Extraction rates of oil from tomato seeds with supercritical carbon dioxide. Japanese Journal of Chemical Engineering, 27, 768}772. SovovaH , H. (1994). Rate of the vegetable oil extraction with supercritical CO * I. Modelling of extraction curves. Chemical Engineering 2 Science, 49, 409}414. SovovaH , H., Kucera, J., & Jez, J. (1994). Rate of the vegetable oil extraction with supercritical CO * II. Chemical Engineering 2 Science, 49, 415}420.