Phase behaviour of sterols and vitamins in supercritical CO2

Abstract

Extraction with supercritical solvents has been used in different areas, such as petroleum desasphaltation, descaffeination of coffee and tea and in the separation of other types of natural products. The supercritical solvent most frequently utilized in the extraction of natural products is carbon dioxide (CO2) due to its several advantages over other solvents such as low cost, atoxicity and volatility. The design, evaluation and optimization of a supercritical extraction that is based on phase equilibrium require phase equilibrium data. This type of data is very scarce for natural compounds like sterols and vitamins. These natural compounds are produced synthetically, but nowadays interest in their extraction from natural sources is increasing. Therefore, the objective of this work is to study the thermodynamic modelling equilibrium of systems containing vitamins A, D, E and K, using the predictive LCVM model. The sensitivity of critical properties in the calculation of the phase behavior was also studied. This study proved that the choice of a group contribution method to calculate thermodynamic properties is very important for obtaining good results in the phase equilibrium calculations.

Phase behavior; Vitamins; Sterols; Solubility data; CO2; LCVM


PHASE BEHAVIOUR OF STEROLS AND VITAMINS IN SUPERCRITICAL CO2

R. Gerszt 1, F.L.P. Pessoa 1 and M.F. Mendes 2* * To whom correspondence should be addressed

1 Escola de Química (UFRJ), Centro de Tecnologia, Bloco E, Sala 209,

CEP 21949-000, Cidade Universitária, Rio de Janeiro (RJ), Brazil

2 Programa de Engenharia Química/COPPE/UFRJ, Centro de Tecnologia, Bloco G,

Sala 115, CEP 21945-970, Cidade Universitária, Rio de Janeiro - RJ, Brazil

E-mail: marisa@peq.coppe.ufrj.br

(Received: February 10, 2000 ; Accepted: April 17, 2000)

Abstract - Extraction with supercritical solvents has been used in different areas, such as petroleum desasphaltation, descaffeination of coffee and tea and in the separation of other types of natural products. The supercritical solvent most frequently utilized in the extraction of natural products is carbon dioxide (CO2) due to its several advantages over other solvents such as low cost, atoxicity and volatility. The design, evaluation and optimization of a supercritical extraction that is based on phase equilibrium require phase equilibrium data. This type of data is very scarce for natural compounds like sterols and vitamins. These natural compounds are produced synthetically, but nowadays interest in their extraction from natural sources is increasing. Therefore, the objective of this work is to study the thermodynamic modelling equilibrium of systems containing vitamins A, D, E and K, using the predictive LCVM model. The sensitivity of critical properties in the calculation of the phase behavior was also studied. This study proved that the choice of a group contribution method to calculate thermodynamic properties is very important for obtaining good results in the phase equilibrium calculations.

Keywords: Phase behavior, Vitamins, Sterols, Solubility data, CO2, LCVM.

INTRODUCTION

During the past 30 years, interest in the use of supercritical fluids as solvents in separation processes has been increasing. This is due to its important applications in the extraction of natural products and to the many advantages it has over conventional processes. The principal advantages are low critical temperatures, absence of residues of organic solvents and easy separation of the solute from the solvent.

Many solvents can be used as supercritical fluids, but carbon dioxide is the most frequently utilized to extract natural products due to its characteristics such as low cost, high volatility, atoxicity and low critical temperature. High volatility is the most important characteristic of the solvent, which makes possible its use in the extraction of natural products with a high degree of purity in the pharmaceutical and food industries.

Interest in the extraction of vitamins from natural sources has increased in spite of the fact that these vitamins are produced synthetically. So the motivation of this work was to study phase equilibrium between the most important vitamins and supercritical carbon dioxide.

The most important vitamins studied are A, D, E and K. Vitamin A, known as retinol, is a primary polyethylenic alcohol and has a high reactive capacity. It is present in its esterified form in animal tissues, mainly the liver. It is often used in the composition of medicines, due to its anti-inflamatory actions and it is found in liver, cheeses, fishs, eggs and carrots. Vitamin D has two active forms, D2 and D3 which are also known as ergosterol and calecalciferol, respectively. These are derived from sterols and because of this present the same characteristics. They are found at high concentrations in fish liver oils, with the highest concentrations found in shark and tuna fish liver oils.

Vitamin E, known as tocopherol, is an antioxidant agent and is present in concentrations from 13 up to 18% in soybean sludge. Soybean sludge, or deodorized distillated is a by-product of the refining of the soybean oil. This raw material is composed of fatty acids, tocopherols and sterols. The tocopherols are present in the form of four isomers, a , b , d and g , where a is the most powerful as vitamin E and isomer d is present in soybean sludge at higher concentration than the others. Vitamin E is used as an additive in the food industries and is found in eggs, butter, soybeans, peanuts and wheat germ. Vitamin K1 was first isolated from alfalfa and has phytyl side chain consisting of four isoprene units, three of which are fully reduced. Vitamin K1 was first isolated from a plant source, and plants remain a good source of this vitamin.

Due to the importance of the use of vitamins, the motivation of this work is to study a thermodynamic model with the LCVM mixing rule to predict the phase behaviour of these vitamins with supercritical carbon dioxide under different operational conditions. Knowledge of equilibrium is an essential factor in designing an extraction process like the supercritical extraction that is based on phase equilibrium.

LITERATURE REVIEW

Vitamins are organic compounds required in small amounts in the diets of animals in order to ensure healthy growth and reproduction. Vitamins are essential nutrients because the animal cannot synthesize such compounds in amounts adequate for its daily needs. The absence of a vitamin in the diet, or its poor absorption in the digestive tract, usually produces a disease with characteristic symptons (Conn et al., 1987).

Vitamins can be classified by solubility into two groups: the water-soluble vitamins and the fat-soluble vitamins. Vitamins A, D, E and K are known as fat-soluble vitamins. They are all derived from isoprene units, and are therefore classified as isoprenoid compounds. In contrast to the other group, these fat-soluble vitamins can be stored in the body, especially the liver. Vitamin K deficiencies in healthy humans are rare since the intestinal bacteria produce the daily amounts required. Man produces vitamin D with the aid of sunlight from a compound normally found in his skin. Vitamin E deficiency has never be observed in humans; this vitamin in ubiquitous in plant products normally found in the human diet. Vitamin A is therefore the only fat-soluble vitamin that might be defficient in the diet. About 1 mg/day is required by the adult human, and this can be obtained easily by eating yellow plant products (carrots, pumpkin, squash) containing carotenoids. The structural formulas of the vitamins can be observed in Figure 1.

Only a small amount of solubility data on tocopherols in supercritical fluids has been published (Chrastil, 1982; Ohgaki et al., 1989; Meier et al., 1994; Pereira et al, 1993; Birtigh et al., 1995). No solubility data for the other compounds in supercritical fluid were found in the literature.

LCVM THERMODYNAMIC MODEL

The predictive thermodynamic model, chosen to study the phase behaviour of the sterols and vitamins and the supercritical carbon dioxide, is the model that uses the Peng-Robinson (1976) equation of state with the LCVM (Boukouvalas et al.,1994) mixing rule.

The Peng-Robinson (1976) cubic equation of state used to calculate liquid-vapor equilibrium and solubilities is expressed as

(1)

where P is the total pressure of the system, R is the gas constant, T is the absolute temperature of the system, v is the molar volume, a is the attractive parameter and b is the repulsive parameter.

The LCVM mixing rule, proposed by Boukouvalas et al. (1994), was created by the linear combination of the Huron-Vidal (1979) (P® ¥ ) and MHV1 (Michelsen, 1990) (P® 0) mixing rules.

It is expressed in terms of a as follows:

(2)

where a v is given by Huron-Vidal, a M by MHV1 and l is the parameter that ranges from zero to one, weighing the contribution of the Huron-Vidal model and MHV1. The original value of l is 0.36. The expression of a is represented as

(3)

where AV and AM are 0.623 and –0.52, respectively, for the Peng-Robinson equation of state. Parameter "b" is a linear mixing rule.

Solubility calculations for solid-vapor equilibrium were made with the LCVM thermodynamic model. The solubility (y) is given by

(4)

where the exponential term is known as the Poyting factor, Vi is the molar volume of the solid, P is the absolute pressure, Pisat is the saturation pressure, f sat is the fugacity coefficient at the saturation conditions and y2 is the solubility of the solute.

METHODOLOGY

Phase equilibrium calculation using a thermodynamic model requires thermodynamic properties that were not found in the literature and must therefore be predicted. These properties are critical temperature, critical pressure, the acentric factor, critical volume and the parameters presented in the equations that calculate the saturation pressure of the compounds.

Melo et al. (1996) studied heuristic rules to estimate the critical properties for different groups of families, using the best group contribution method, but the family groups found in the structural formulas of the vitamins used in this work were not easily identified in this previous work. Because of that, the critical properties of the vitamins were predicted using all the group contribution methods studied by Melo et al. (1996), Joback, Pretel, Lydersen, Ambrose and Jalowka, and the sensitivity of these properties in the solubility calculations was also studied. Variations found for each estimated property for all the vitamins can be observed in Table 1.

The solubility data used for all the components were collected at 40, 60 and 80 0C and from 20 to 35 MPa (Johannsen & Brunner, 1997).

Solubilities were calculated using the different estimated critical properties and by varying parameter l , present in the LCVM thermodynamic model. This variation on parameter l occurred because the parameter value used by the authors was only valid for hydrocarbons.

RESULTS AND DISCUSSION

A comparison between the experimental and calculated solubilities of each vitamin in supercritical carbon dioxide is shown in Figures 1 to 6, varying the value of the critical properties and parameter l . Results presented for all the vitamins are for 600C which make is possible the comparison between them. The experimental solubilities were compared to the calculated ones, using the different thermodynamic properties obtained by the Joback, Pretel, Lydersen, Ambrose and Jalowka group contribution methods.

CONCLUSIONS

Calculation of the solubilities was very sensitive to the critical properties of the components, principally in relation to calculation of the saturation pressures.

The behaviour of the predictive model used in this work was sensitive to parameter l , which suffered several variations from the estimated value found in the literature, 0.36. This is relevant because 0.36 is appropriate for hydrocarbon systems. Results obtained when was adopted 0.36 as the parameter value were worse than the results presented in the Figures 2 to 7.

The behaviour of the LCVM model varied for each vitamin in phase equilibrium, where the worst results were presented for the solubilities of a and d -tocopherols. A different best group contribution method and a different value of l were obtained for each vitamin with the phase equilibrium calculations; for example, for vitamin K1 the properties obtained from the Pretel group gave better results than the others, while for vitamins D2 and D3 the best methods were those of Jalowka and Ambrose, respectively.

This work showed that the choice of a group contribution method is very important to obtain good results in phase equilibrium calculations, and before equilibrium conditions are studied, the physico chemical properties must be obtained.

ACKNOWLEDGEMENTS

The authors would like to thank CNPq and FAPERJ for their financial support.

NOMENCLATURE

a parameter of the attractive term of the cubic equations of state

b parameter of the repulsive term of the cubic equations of state

P pressure

T temperature

xi molar fraction of component i in the liquid phase

yi molar fraction of component i in the vapor phase

v molar volume

R universal constant of the gases

fugacity coefficient of the component i in the mixture

l liquid

v vapour

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  • *
    To whom correspondence should be addressed

Publication Dates

  • Publication in this collection
    18 Oct 2000
  • Date of issue
    Sept 2000

History

  • Received
    10 Feb 2000
  • Accepted
    17 Apr 2000
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