2. Diagrams of pure substances

### 2.1. P-T diagram

of solid matrices (leaves, seeds, pulps, etc.) or by extraction/fractionation of liquid mixtures (aqueous solutions, fish oils, microalgae oils, vegetable oil, deodorize distillates, etc.) [1–5]. In processes at high pressures, which are near or above the critical point (pressure and temperature), the solvent density increases drastically and this is the most important parameter associated to the solvent power. As illustrated in Figure 1, carbon dioxide, a non-toxic substance, acting as solvent, co-solvent, or anti-solvent, is the most important fluid used in the supercritical fluid technology in extraction, separation, fractionation, micronization, and encapsulation processes applied to obtain extracts concentrated with bioactive compounds for food, pharmaceuti-

Carbon dioxide has a critical temperature near to room temperature, contributing to the operating conditions (pressure and temperature) to extract thermolabile substances, such as bioactive compounds. In addition, this substance is non-polar and to enlarge the application spectrum to extract bioactive compounds, ethanol, water, or both are usually used as cosolvents. Moreover, carbon dioxide acts as co-solvent when in the mixture it is used more than 60% of ethanol or water, and as anti-solvent, when the solute extract is not soluble in carbon

The information accuracy related to the physical (pressure, temperature, and density) and transport properties (diffusivity, viscosity) and the accuracy of thermodynamic and mass transfer relations used for the solvent, co-solvent, and solute reach directly the costs of investment in extraction/separation units in supercritical conditions. The thermodynamic phase equilibrium determines the limits for the mass transfer among different phases, which are

cal, and cosmetic applications [6–9].

212 Carbon Dioxide Chemistry, Capture and Oil Recovery

dioxide during the depressurizing step.

Figure 1. Carbon dioxide applications.

The pressure versus temperature (P-T) diagram describes the different aggregation states of pure substances called solid, liquid, and vapor/gas.

Figure 2 is a schematic representation of the P-T diagrams for carbon dioxide and the substances most commonly employed as co-solvent, ethanol, and water, in high-pressure extraction processes

Figure 2. Solid-liquid-gas-supercritical fluid phase diagram. TP = triple point. CP = critical point. Pc = critical pressure. Tc = critical temperature. Tt = triple point temperature. Pt = triple point pressure.

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 boiling curve).

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

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].

## 2.2. P-r-T diagram

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 the density as a function of system pressure and temperature.

3. Supercritical fluid extraction

3.1. General process steps

(Pc, Tc, and rc).

The extraction/separation processes applied to solid matrix using carbon dioxide as solvent are the most studied supercritical processes in the search for new natural products that have biological activity, according to numerous applications described in the literature [1, 4–6, 11–18].

Figure 3. Pressure-density (P-r) phase diagram for carbon dioxide. CP = critical point (Pc, Tc, and rc). CP = critical point

Carbon Dioxide Use in High-Pressure Extraction Processes

http://dx.doi.org/10.5772/intechopen.71151

215

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

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

extraction pressure reduction and a separation vessel to collect the extract.

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 in the density between the liquid and vapor phases.

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 extract thermo-sensible substances.

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.

Figure 3. Pressure-density (P-r) phase diagram for carbon dioxide. CP = critical point (Pc, Tc, and rc). CP = critical point (Pc, Tc, and rc).
