3.1. General process steps

of bioactive compounds. The curves represent the boundaries (phase transition or phase equilibrium) between the different states, known as saturation curves. The curve between the solid and liquid phases is called fusion; the curve between solid and vapor phases is called sublimation and that one between liquid and vapor phases is called vapor pressure (also known as

The behavior of the thermodynamic diagrams of pure substances culminates in the determination of the reference equilibrium points that has great importance in the development of thermodynamic models for different processes applications. In the P-T diagram, there are two points: the triple point, where the three phases are in equilibrium and the critical point, which is particularly

The critical point of a pure substance is the maximum thermodynamic state reached by the saturation curve between liquid and vapor phases. When the substance is in the state above the critical temperature (Tc) and the critical pressure (Pc), it is called supercritical fluid, and when the pressure is above Pc and the temperature below Tc, the thermodynamic state is called subcritical liquid. The technology with fluids at high pressures consists in the use of substances that act like solvent when they are in the thermodynamic state near or above the critical point. The triple point of carbon dioxide is at pressure of 5.18 bar and at temperature of 216.58 K (56.57C), and the critical point is at pressure of 73.7 bar and at temperature of 304.15 K (31C) [10].

Density (r) is the most important thermodynamic property to define the solvating power of a solvent at high pressures, increasing the density of the solvent increases its solvating power. To better understand the influence of density on the solvating power to increase or decrease the solubility of an extract within a solvent at high pressures, one needs information concerning

Figure 3 shows the schematic representation of the density behavior (r = 1/V) of a pure substance with temperature and pressure variations, where V is the specific volume (volume per mass unit). In Figure 3, the density versus pressure isotherms are presented in descending order from T1 to T9. The red line represents the saturation curve between the liquid and vapor phases. The highest point of the saturation curve is the critical point. The dotted line within the saturation curve is the two-phase region. In the saturation curve, there is a sudden difference

The behavior of the P-r-T diagram shows that the density at constant temperature increases with the increasing pressure and at constant pressure increases with the decreasing temperature. In the region near the critical point, small variations of pressure and/or temperature cause great variations in density. For carbon dioxide, the critical point is at the pressure of 73.7 bar and at the temperature of 304.15 K (31C); it makes carbon dioxide the most applied solvent to

Below the critical temperature, in the subcritical region, the isotherms present two types of behavior: for the vapor region, at constant pressure, the density increases with the decreasing temperature and for the liquid region, the density varies very little with the temperature.

the density as a function of system pressure and temperature.

in the density between the liquid and vapor phases.

extract thermo-sensible substances.

of fundamental interest for applications in processes that use solvents at high pressures.

boiling curve).

214 Carbon Dioxide Chemistry, Capture and Oil Recovery

2.2. P-r-T diagram

Generally, the supercritical fluid extraction applied to a natural solid matrix consists of three steps: the system supply of solvent/co-solvent, the extraction unit, and the extract separation system from solvent/co-solvent. Figure 4 presents a general scheme of the supercritical fluid extraction unit without solvent recycle. The system supply of solvent/co-solvent consists by a booster air-driven fluid pump, a cooling bath, a co-solvent recipient, a co-solvent pump, and a mixer. The extract separation system from solvent/co-solvent consists by a control valve for extraction pressure reduction and a separation vessel to collect the extract.

Regarding the extraction, the supercritical solvent continuously flows through a fixed bed of solid particles and dissolves the extractable components of the solid. The solvent is fed into the extractor and evenly distributed at the inlet of the fixed bed. The system solvent and soluble components leave the extractor and feed the precipitator/separator, where the solvent products

[19] and Kiran et al. [21] described the most important parameters, and among the variables that determine the process, operating conditions (pressure and temperature), amount of solvent, conditions of solvent removal from extract (precipitation), pretreatment of solid matrix, and other

Carbon Dioxide Use in High-Pressure Extraction Processes

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In general, the parameters that define the behavior of the mass transfer at processes at high pressures are related to the configuration of the bed: particle size, height, and diameter, preparation of the raw material, solvent flow, among others, which contribute to define the shape of the kinetic extraction curves. The phenomenological discussions about supercritical fluid extraction mass transfer applied to solid matrices have been discussed in the literature [19, 22–24].

The experimental strategy used for the supercritical carbon dioxide extraction process of bioactive compounds is based on the previous results collected by our research group in

Açaí is a dark purple, berry-like fruit from typical Amazon palm tree Euterpe oleracea Mart.,

Recently, many studies have suggested its use as a functional food or food ingredient due to its antioxidant activity, explained by the high content of phenolic compounds, such as anthocyanins, specially cyanidin-3-glucoside and cyanidin-3-rutinoside, flavones, and phenolic acids [26–28]. Phenolic constituents are generally associated with health-promoting properties and prevention of diseases [29–33]. Anthocyanins constitute a group of pigments, also important in

The supercritical extraction experiments of the lyophilized açaí pulp under development were carried out in a Spe-ed™ SFE commercial unit (Allentown, PA, USA: model 7071 from Applied Separations) which is coupled to the solvent + co-solvent delivery system of Laboratory of Supercritical Extraction (LABEX), Faculty of Food Engineering-UFPA. The schematic repre-

The first step consisted of the extraction with supercritical CO2 (pure) to obtain extracts rich in fatty acids and byproducts of the residual solid matrix (defatted pulp). Analyses of the content of bioactive compounds (anthocyanins and total phenolic compounds) were performed. The second stage that is under development consists of the extraction with supercritical CO2 combined with water as co-solvent applied to the residual solid matrix to obtain extracts

In the first stage, Batista et al. [25] subjected samples of lyophilized açaí pulp to the supercritical carbon dioxide extraction process. Among the results, the study of the process variables (temperature, pressure, and solvent density) that maximize the extraction yield of açaí oil, the quantification of the total anthocyanins content and total phenolic compounds content, and the

Figure 6 shows the experimental results of the 50, 60, and 70C isotherms on dry basis and their standard deviations. In this study, the highest global yield was equal to 45.4 0.58%,

evaluation of the allelopathic potential of the extracts obtained can be highlighted.

3.2. Supercritical carbon dioxide extraction of bioactive compounds: a case study

mass transfer parameters can be highlighted.

integrated in the daily dietary habit of the native people.

the food industry, for the replacement of artificial colors [34–36].

sentation of the supercritical extraction system is shown in Figure 5.

obtaining açai extracts [25].

concentrated in anthocyanins.

Figure 4. Scheme of a supercritical fluid extraction plant applying solvent/co-solvent. CO2 cylinder (1); cooling bath (2); booster (CO2 pump-3 and Compressor-4); mixer (5); CO-solvent pump (6); co-solvent recipient (7); extraction unit (8); control valve (V-5); separation vessel (9); flow meter (10).

are separated by expansion (depressurizing), since at low pressures the density of the solvent sharply decreases, therefore it decreases the solubilizing power of the solvent as well and the products precipitate.

The choice of the operating condition (P and T) is a determining factor that contributes to the maximization of the extracts solubility in the supercritical solvent, and consequently the extraction yields. Thus, increasing the density of the supercritical fluid, the solubility of the solvent maximizes. The solubility increasing can also occur when a co-solvent is added, which changes the solvent power and, in this way, the new solvent is a mixture [19, 20].

To design a high-pressure fluid extraction process of valuable compounds from new natural solid matrices, it is necessary to define the size of the extraction unit and some important parameters have to be determined to obtain the optimum process conditions for each application. Brunner [19] and Kiran et al. [21] described the most important parameters, and among the variables that determine the process, operating conditions (pressure and temperature), amount of solvent, conditions of solvent removal from extract (precipitation), pretreatment of solid matrix, and other mass transfer parameters can be highlighted.

In general, the parameters that define the behavior of the mass transfer at processes at high pressures are related to the configuration of the bed: particle size, height, and diameter, preparation of the raw material, solvent flow, among others, which contribute to define the shape of the kinetic extraction curves. The phenomenological discussions about supercritical fluid extraction mass transfer applied to solid matrices have been discussed in the literature [19, 22–24].
