ТЕОРЕТИЧЕСКИЕ ОСНОВЫ ХИМИЧЕСКОЙ ТЕХНОЛОГИИ, 2013, том 47, № 4, с. 447-453
УДК 541.123:663.8
SOLUBILITY OF SOME PHENOLIC ACIDS CONTAINED IN CITRUS SEEDS IN SUPERCRITICAL CARBON DIOXIDE: COMPARISON OF MIXING RULES, INFLUENCE OF MULTICOMPONENT MIXTURE AND MODEL VALIDATION © 2013 г. J. Moncada", C. A. Cardona", Yu. A. Pisarenko*
aInstituto de Biotecnología y Agroindustria, Departamento de Ingeniería Química, Universidad Nacional de Colombia sede Manizales, Manizales, Colombia bMoscow State University of Fine Chemical Technology, Moscow, Russia ccardonaal@unal.edu.co Received 22.05.2012
The solubility prediction of any compound in supercritical carbon dioxide is obtained using the dense gas formulation to calculate the phase equilibria. To achieve this, the Peng—Robinson equation of state with the Stryjek—Vera modification is used. Then, this is coupled with the Wong—Sandler and Van der Waals mixing rules. The latter was included in order to evaluate the influence of the mixing rules in the calculation of the solubility. Therefore, the obtained results from the model are compared with experimental data reported in literature for ferulic acid, p-coumaric acid and caffeic acid contained in citrus seeds. Good agreement was obtained between the model and the experimental data for the phenolic acids, when the Wong—Sandler mixing rules are used. Also the influence of the multicomponent mixture is considered for the prediction of the solubility of phenolic acids in supercritical carbon dioxide.
DOI: 10.7868/S0040357113040064
INTRODUCTION
The replacement of synthetic antioxidants by those of natural origin has benefits on health because the implications and broad functionality that many of them can have. For instance, the solubility in both oil and water, which in turn is relevant to emulsions industry [1]. However, some such as those from spices and herbs (like oregano, thyme and rosemary) have limited applications in the field of food because they can cause an upset in the flavor given to their characteristics. Nevertheless these may have high antioxidant activity, which makes them interesting for another use. Many materials from plant sources contain compounds with antioxidant activity. Many plants have been studied, as potential sources of antioxidants and several compounds have been isolated. Polyphenols are the compounds that most are present as an antioxidant in plant materials. A wide range of polyphenols from plant sources with high and low molecular weight, contains different antioxidant properties. These properties have been studied and proposed for different purposes depending on the source from which they come [2—5].
Citrus fruits are planted around 80 million tons per year. The percentage of citrus fruit processed in juice is about 34%, but in countries like Brazil and the United States, this percentage can reach 96% [6]. Peel and seeds of citrus are the main fruit residues. Seeds are rich in unsaturated fatty acids. Both peel and seeds are a useful source of polyphenolic compounds, including phenolic acids and flavonoids [6]. Flavonoids in citrus
fruits are represented by two classes of compounds: polymethoxylated flavones and glycosylated fla-vones. These compounds are found only in citrus fruits, which makes citrus waste an interesting commercial reference [7]. It has been found that flavonoids from citrus have health-related properties, including anticancer, antiviral and anti-inflammatory activities [8, 9]. For the case of citrus seeds, many hydroxycin-namic acids have been isolated [10]. For instance ferulic, p-coumaric and caffeic acids are concentrated in larger amounts in citrus seed, than other phenolic compounds [10].
On the other hand, supercritical fluid extraction with carbon dioxide is fully implemented on a commercial scale for obtaining hops for brewing, extracting aromas and flavors of spices and herbs and coffee and tea without caffeine [1, 11—13]. In addition, several processes are in expansion, such as obtaining soft drinks, cholesterol-free animal products and seed oils (it can be considered the case of antioxidants) [1, 12—15]. Supercritical fluid extraction is a technique of separation, which consists in the dissolution and extraction of substances contained in a solid matrix. The latter is based primarily on the ability of certain fluids in a supercritical phase to amend its solvent power [16]. The solvent power of a supercritical fluid can be high, depending on the conditions of temperature and pressure applied that allow the selective dissolution of certain substances in the supercritical fluid. The extracted substances are easily separated from the supercritical fluid. The
Table 1. Solute properties of phenolic compounds used for the calculation of sublimation pressure and solubility correlations
Solute Tb, Ka Tc, Ka Pc, bara Vc, cm3/mola wb
Caffeic acid 658.527 856.988 44.723 460.260 1.347
p-Coumaric acid 627.844 804.144 38.006 451.880 1.411
Ferulic acid 648.121 838.183 31.726 532.020 1.194
a Calculated from [28].
b Calculated from the Clapeyron equation [29].
extraction is performed without phase changes, simply by varying the conditions of pressure and/or temperature of the supercritical fluid, because the solvent power of a pure substance depends largely on its density, which depends on pressure and temperature [16— 18]. In this way, in an effort to optimize supercritical fluid extraction processes, it is a requirement of knowledge of the solubility of the compounds to be extracted. Due to the limited experimental data of solubility of many phenolic compounds, which is needed to analyze its behavior in supercritical fluid, there are many thermodynamic models that are able to predict the phase behavior in such systems [19].
In this work, the formulation of dense gas is used to predict the phase equilibria of ferulic, p-coumaric and caffeic acids in supercritical CO2. The latter is achieved by using the Peng—Robinson equation of state with the Stryjek—Vera modification. This equation is applied using the Wong—Sandler and Van der Waals mixing rules, which in previous works have been reported to work well at high pressures [20—26]. After this the UNIFAC DORDTMUND model [27] was employed to predict the excess Helmholtz free energy needed in the Wong—Sandler mixing rules.
In this sense, the aim of this paper is to evaluate and validate the thermodynamic model for dense gas using the Peng—Robinson equation of state with the Stryjek—Vera modification. To do this, the prediction of the model is compared with experimental data reported by Murga et al. [20] for ferulic, p-coumaric and caffeic acids. This analysis is done using both Wong— Sandler and Van der Waals mixing rules. Also the influence of a multicomponent mixture in the prediction of solubility in supercritical carbon dioxide is evaluated.
THEORETICAL ANALYSIS
In this work, the Peng—Robinson equation of state with the Stryjek—Vera modification (PRSV equation of state) with the Van der Waals (VDW) and Wong— Sandler (WS) mixing rules have been used.
To estimate the critical properties of each phenolic acid (i.e. critical temperature, critical pressure, critical volume), normal boiling point (necessary to estimate the acentric factor), the method proposed by Marrero and Gani have been used [28]. This method is based on group contribution at three different levels, including specific group interaction common in the phenolic
compounds [28]. The acentric factor has been calculated using the Clapeyron equation [29]. The estimated properties are shown in Table 1.
The fugacity method for calculating the phase equilibria is based on the equality of chemical potentials, for each component in each phase, at fixed temperature and pressure [12, 30, 31].
In the supercritical fluid phase, the fugacity can be represented by an expression for expanded liquid or dense gas [9]. Given this, a solid—gas formulation is proposed, where the solid phase fugacity is equal to the fugacity in the vapor phase is presented. It is considered a binary fluid mixture consisting of component 1 (carbon dioxide) and the /th component (phenolic acid):
-nsubli isubli
P2 ф2
y2 = 2 ^T2—exp
P Ф2
VL (p
RTK
-bsubli\
(1)
The calculation of the solubility y also requires knowledge of the solid sublimation pressure p.subh, solid molar volume V- , sublimation fugacity coefficient
faubh (near to unity) and fugacity coefficient fa in the supercritical phase which is calculated by the equation of state with its associated mixing rule [24].
Because the substances of interest shown above have not been widely studied, and therefore the data reported in the literature for these are extremely rare, to obtain the properties necessary for the development of equation (1), it is necessary to propose a solid-vapor equilibrium for the pure substance where the vapor phase fugacity is equal to the fugacity in the solid phase, at fixed temperature [30, 31]. In this way when the equality of fugacities is reached, automatically the necessary properties for calculation of the solubility of the solid in supercritical CO2 are obtained [30, 31].
For determining the fluid-solid phase equilibria using an equation of state approach, the equation of state must be able to describe both the fluid-phase and solid-phase behavior reliably [12, 30, 32]. In this paper, the PRSV equation of state is used, which contains two parameters to describe the properties of the pure fluids and it can be shown that it is better to predict phase equilibria at high pressures [12, 22, 30]. In this work, the mixture parameters are giving by the VDW and WS mixing rules.
The PRSV equation of state has the following form: RT a
P =
v - b v2 + 2bv - b2 '
(2)
where P is the absolute pressure, T is the absolute temperature, R is the ideal gas constant, and a and b are the energy and size parameters.
The parameters a and b are calculated by
a = 0.477235
R 2T2
P.c
-a
PR,n
b = 0.07779
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