Food Biochemistry and Food Processing (2 edition)

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770 Part 7: Food Processing

Table 40.1.Comparison of Physical Properties of Supercritical CO 2 at 20 MPa and 55◦C with Some Selected Liquid

Solvent at 25◦C

Properties CO 2 n-Hexane Methylene Chloride Methanol

Density (g/mL) 0.75 0.66 1.33 0.79

Kinematic viscosity (m^2 /s) 1.0 4.45 3.09 6.91

Diffusivity (m^2 /s) 6.0× 109 4.0× 109 2.9× 109 1.8× 109

Cohesive energy density (δ(cal/cm^3 )) 10.8 7.24 9.93 14.28

Source: Modified from King et al. 1993.

other raw or waste agricultural materials (Kasamma et al. 2008,

Shi et al. 2009a, Yi et al. 2009, Huang et al. 2010, Shi et al.

2010a, 2010b, Xiao et al. 2010). The scale up of some supercrit-

ical extraction processes have already proven to be possible and

are in industrial use.

The process requires intimate contact between the packed beds

formed by a ground solid substratum (fixed-bed of extractable

material) with supercritical CO 2 fluid. During the supercritical

extraction process, the solid phase comprising of the solute and

the insoluble residuum (matrix) is brought into contact with the

fluid phase, which is the solution of the solute in the supercritical

CO 2 fluid (solvent). The extracted material is then conveyed to

a separation unit.

Process Concept Schemes

The physicochemical properties of the supercritical fluids are

crucial to the understanding of the process design calculation

and modeling of the extraction process. Therefore, selectivity of

solvents to discriminate solutes is a key property for the process

engineer. Physical characteristics such as density and interfa-

cial tension are important for separation to proceed; the density

of the extract phase must be different from that of the raffi-

nate phase and the interfacial properties influence coalescence,

a step that must occur if the extract and raffinate phase are to

separate. The supercritical state of the fluid is influenced by tem-

perature and pressure above the critical point. The critical point

is the end of the vapor–liquid coexistence curve as shown on

the pressure–temperature curve in Figure 40.1, where a single

gaseous phase is generated. When pressure and temperature are

further increased beyond this critical point, it enters a supercrit-

ical state. At this state, no phase transition will occur, regardless

of any increase in pressure or temperature, nor will it transit to a

liquid phase. Hence, diffusion and mass transfer rates during su-

percritical extraction are about two orders of magnitude greater

than those with solvents in the liquid state.

Substances that have similar polarities will be soluble in each

other, but increasing deviations in polarity will make solubility

increasingly difficult. Intermolecular polarities exist as a result of

van der Waals forces, and although solubility behaviors depend

on the degree of intermolecular attraction between molecules,

the discriminations between different types of polarities are also

important. Substances dissolve in each other if their intermolec-

ular forces are similar or if the composite forces are made up

in the same way. Properties such as the density, diffusivity, di-

electrical constant, viscosity, and solubility are paramount to

supercritical extraction process design. The dissolving power of

SCF depends on its density and the mass transfer characteristic,

and is superior due to its higher diffusivity and lower viscosity

and interfacial tension than liquid solvents.

Although many different types of supercritical fluids exist and

have many industrial applications, CO 2 is the most desirable for

SCE of bioactive components. Table 40.1 shows some physical

properties of compressed (20 MPa) supercritical CO 2 at 55◦Cas

compared to some condensed liquids that are commonly used as

extraction solvents at 25◦C. It should be noticed that supercritical

CO 2 exhibits similar density as those of the liquid solvents, while

being less viscous and more highly diffusive. This fluid-like

attribute of CO 2 coupled with its ideal transport properties and

other quality attributes outlined above make it a better choice

over other solvents.

The specific heat capacity (Cp)ofCO 2 rapidly increases as

the critical point (31.1◦C temperature, 7.37 MPa pressure, and

467.7 g/L flow rate) is approached. Like enthalpy and entropy,

the heat capacity is a function of temperature, pressure, and den-

sity (Mukhopadhyay 2000). Under constant temperature, both

the enthalpy and entropy of supercritical CO 2 decreases with

increased pressure and increases with temperature at constant

pressure. The change in specific heat as a result of varying the

pressure and temperature is also dependent on density. For ex-

ample, under constant temperature, specific heat increases with

increasing density up to a certain critical level. Above this crit-

ical level, any further increase in density reduces the specific

heat.

Process System

The supercritical CO 2 fluid extraction process is governed by

four key steps: extraction, expansion, separation, and solvent

conditioning. The steps are accompanied by four generic pri-

mary components: extractor (high pressure vessel), pressure

and temperature control system, separator, and pressure inten-

sifier (pump; Fig. 40.2). Raw materials are usually ground and

charged into a temperature-controlled extractor forming a fixed

bed, which is usually the case for a batch and single-stage mode

(Shi et al. 2007a, 2007b, Kasamma et al. 2008).

The supercritical CO 2 fluid is fed at high pressure by means

of a pump, which pressurizes the extraction tank and also