Design and modeling of the process

The extraction process for solid substances primarily consists of two stages, the extraction and the separation of the product from the solvent. At the extraction stage, the solvent passes through a fixed bed of solid particles and dissolves the solute from the solid. The solvent moves from the extractor to the separator, where the solvent is recovered and the extracted product is obtained. The design and modeling of the process is centered on the extraction stage.

In order to analyze this stage in a generic manner, it can be considered that the extraction of solutes from a solid matrix occurs in five basic stages that can occur in series/parallel (Brunner, 1993):

• The solvent is placed in contact with the surface of the solid matrix.

• Intraparticular solvent diffusion occurs as the solvent accesses the interior of the matrix. The separation process begins in this way.

• The retained solutes are moved by simple drag or displacement from the active sites of the matrix due to the higher affinity and/or concentration of solvent molecules. Solubilization (solvation) occurs immediately in the solvent.

• Solutes are transported from inside the matrix to its surface, essentially by diffusion, which is the most important mechanism of transport at this stage.

• The dissolved solutes cross through the interstitial film fluid that surrounds the solid and they are transported to the bulk solvent and removed from the bed of solid particles.

Numerous parameters must be taken into account when modeling an extraction process and these include the solid matrix, the accessibility of the solute to the solvent in the bed, the chances of ascending and descending flow, and the flow distribution of the solute within the solid matrix. However, variation of the amount of extracted product with time is relatively simple.

The variation of the extraction yield with the extraction time for the extraction of carotenoids from marine microalgae is represented in Figure 4.5 (Mac´ıas-S´anchez et al., 2009a). This figure shows the typical variation of the extraction yield for this kind of process.

It can be observed in Figure 4.5 that two stages are clearly differentiated. In the first stage, the variation in the quantity of extracted product with time fits a straight line, whereas in the second stage the yield follows a curve that tends to approach asymptotically a value that corresponds to the maximum amount of product that can be extracted. This variation indicates that the largest amount of solute is removed from the matrix during a short period of time at the beginning of the extraction. In the second stage the rate

Table 4.1 Some examples companies that employ supercritical fluid extractions at different scales

Organization Location System and application

Agrisana (www.agrisana.it) Notaresco (Italy) Extracts from plants for cosmetics, food industries

ALTEX (www.altex.es) Valencia (Spain) Three extractors of 1500 L for the extraction of flavors, material processing, spices, etc.

Applied separations

(www.appliedseparations.com)

Allentown, PA (United States) R&D supercritical fluids process Eden Botanicals

(www.edenbotanicals.com)

Hyampom, CA (United States) Extracts of flavors and spices Flavex (www.flavex.com) Rehlingen (Germany) Cosmetics, perfumery, food additives Fuji Flavor Co. (www.fjf.co.jp) Tokyo, (Japan) High-quality extracts from natural

materials, such as flowers, leaves and roots.

India Glycols Limited (www.indiaglycols.com)

New Delhi (India) 3×300 L extractors in spices and natural products

Indo-global Spices Ltd.

(www.indoglobalspices.com)

Karnataka (India) Three extractor of 300 L in spices and natural products

Industrial research limited IRL (www.irl.cri.nz)

Wellington (New Zealand) R&D Supercritical Fluids Process Kraft products

(www.kraftfoodscompany.com)

Glattpark (Switzerland) Decaffeination of coffee (HAG Coffee) and flavors

NATEX (www.natex.at) Terniz (Austria) R&D Supercritical Fluids Process Organix South Inc.

(organixsouth.com)

Bowling Green, FL (United States)

Production of Neem Bark Supercritical Extract Phasex corporation

(www.phasex4scf.com)

Lawrence, MA (United States) R&D Supercritical Fluids Process Philip Morris (www.pmi.com) NY (United States) Production of Tobacco without

nicotine

Raps & Co. (www.raps.de) Kulmbach (Germany) Extracts of flavors and spices Separex (www.separex.fr) Champigneulles (France) R&D supercritical fluids process

SKW/Trotsberg Dusseldorf (Germany) Different capacities—food

technology applications

SMS Natural Products Indore (India) Two extractors of 1100 L for spices and natural products

SOLUTEX (www.solutex.es) Zaragoza (Spain) Two extractors of 3800 L for flavors and fragrances

Talent Natural Extract Co. Ltd (www.naturalcn.com)

Wuho (China) Decaffeination of tea, production of flavors

TharProcess (www.thartech.com) Pittsburg PA (United States) R&D supercritical fluids process The herbaria (www.theherbarie.com) Prosperity, SC (United States) Flavors, essential oils from plants Xspray (www.xspray.com) Stockholm (Sweden) Various high-pressure systems in

pharmaceutical application

Clean room

Heater Heater

Heater Process

tank Cold HE

Cosolvent tank Reception of carbon

dioxide Recuperation

solvent tank

BPR2 BPR1

Filter

Filter

Filter

Separator 1 Extractor 1 Extractor 2 Extractor 3

Separator 2

Figure 4.3 Typical diagram of a supercritical extraction plant for solid samples

of extraction decreases with time. This general behavior can be caused by different factors and these are discussed later. This phenomenon may present a problem in the design of a process and in the efficiency of an extraction processes on an industrial scale.

The slope of the first part of the graph can be defined by the solubility equilibrium, although this is not always the case as a straight line may appear merely due to the existence of a constant resistance to mass transfer. In fact, such constant resistance is common in the extraction process and therefore the solubility does not control the process, especially if the extraction is carried out in a dynamic way. In many cases, the solute is present in the matrix in small quantities and during the extraction process the concentration of the solute in the supercritical fluid is well below the limit of solubility.

The graph presented in Figure 4.5 is very limited for the comparison of extraction processes for different materials extracted using different equipment. There are numerous variables that can affect the overall process. However, the information provided by this curve is very useful for comparing the results of a series of extractions of a given substance using the same apparatus operating under different conditions.

CO₂

Separador

S1 BPR1

BPR2

HE2

HE1 MV-4

MX-1

MV-2

MV-3

T P

T T

T

T P P

T P

Raffinate

Extract CO₂ Exit

Cooling bath

Sample

High pressure

pump

High pressure

pump

Control

Flowrate control

Figure 4.4 Diagram of a supercritical extraction plant using a countercurrent column

0 0.0

μg carotenoids/mg dry microalgae

1.0 2.0 3.0 4.0 5.0

60 120

Controlled by mass transfer Controlled

by solubility

Time (min)

180 240

Figure 4.5 Variation of the extraction yield with time for the SFE of carotenoids from a marine microalga Nanochlorposis gaditana

A range of theories have been published in the bibliography to describe extraction processes (Al-Jabari, 2002). All of these approaches can be classified into two types of theory: film theory and penetration theory. The Biot number makes it possible to select the most appropriate mass transfer model:

Bi =Ke2L

Di (4.1)

If the Biot number is greater than 10, then internal diffusion is the controlling stage of the extraction process (Perez Galindoet al. 2000). In these cases, the application of a penetration model is more appro-priate. On the other hand, if Bi<10, the controlling stage is the mass transfer in the interstitial fluid and in this case the most appropriate model is the film model.

4.4.1 Film theory

A wide variety of approaches can be used to develop mass transfer equations. On the one hand, it must be borne in mind that the double-layer theory uses mass transfer coefficients and the driving forces of the process are expressed as units proportional to concentration (usually in molar fraction). The flow rate of a solute from the solid phase (which exists as a liquid) can be expressed in the following way:

Flux=k_aA x–x_i

(4.2) On the other hand, the flow rate of the solute from the interface to the bulk of the solvent is given by the expression:

Flux=k_bA y_i–y

(4.3) where k_a and k_b are transfer coefficients, x_i y_i are the concentrations of solute in the interface, x is the concentration of solute in the inert phase, y is the concentration of solute within the solvent, and A is the transfer area.

If the interface reaches the steady state, these fluxes are equivalent and the following equation can be obtained:

y_i −y

x_i −x = −Ak_a

AK_b = −K_a

K_b (4.4)

The numerical value of this relationship is the slope of a line on anx-ydiagram and depends on the con-ditions of the main stage and is called the resistance phase relationship. In the development of this equation it is assumed that the interface does not offer any resistance, which has proven to be true in most cases.

4.4.2 Penetration theory

These kinds of models are the most commonly used in systems to describe extraction processes using supercritical fluids. These models usually involve a series of initial considerations that can simplify the resolution of the system. These theories considerer a transport system that is composed of fluid elements with the following assumptions:

• the fluid is completely mixed and at isothermal conditions;

• the fluid is in contact with the other phase, maintaining a determinate concentration in the surface;

• there is no pressure drop along the bed of particles;

• the particles are packed with a constant porosity and apparent density along the whole bed;

• finally, if we suppose that the concentration of the solute in the supercritical phase is low, then the density of the fluids, the axial dispersion, and the flow-rate of the fluid are constant throughout the process.

The following equation represents the global balance for the fluid phase in the supercritical extraction of solids (Oliveiraet al., 2011):

εc

∂C_i

∂t = −u_G∂Ci

∂Z +εcD_ax,i∂²C_i

∂Z² + 1−εc

J_fa_p (4.5)

where C_i is the concentration of component i in the supercritical phase,D_ax,i is the coefficient of axial dispersion of componenti,J_f is the flux density of material from the solid to the fluid phase, u_G is the interstitial fluid velocity anda_p is the specific surface area of the particle. Join to this equation, some initial and boundary conditions are required. This conditions and different assumption depend on the definition realized in the model.

The solution to the equation of balance is treated differently according to different authors. Oliveira et al. (2011) classified the models according to the following criteria:

• Linear driving force model. In this model it is assumed that the mass transfer flux is proportional to the difference between the mean concentration of the solute in the particle and the concentration of the solute in equilibrium with the fluid phase. In many cases, the particle is porous and the solute is present in the solid phase and in the fluid inside the pores—in this instance, two linear driving-force approximations are employed, one between the fluid phase and the fluid in the pores of the solid, and the other between the fluid phase in the pores and the solute in the solid phase.

• The shrinking core model: In this model it is assumed that there is a sharp boundary between the extracted and non-extracted parts of the particle. As the extraction proceeds, the boundary recedes until it reaches the center of the particle and all the solutes are extracted.

• The broken plus intact cell model. This model was initially proposed by Sovova (1994), Sovovaet al.

(1994) and Stastovaet al. (1996) and it has the advantage of providing a reasonably simple analytical solution to the mass balance equation and a good physical description of the process. In this model a similar physical representation is proposed in which the particles are composed of cells that are broken up during grinding and cells that remain intact. The existence of two mass transfer resistances during SFE was hypothesized. The first resistance is located in the supercritical mixture and controls the extraction process until all the essential oil in the broken cells is exhausted. The second resistance is in the walls of the intact cells and controls the remaining part of the process. Reverchon and Marrone (2001) proposed a modification of this model, with the existence of a “parallel resistances” mechanism where both broken and intact cells transfer solute to the fluid with different kinetics.

• Finally, the same authors also worked with a combined model: broken plus intact cells and a shrinking core. A new model was proposed by Fiori et al. (2009) and this combined the concepts of the two previous models. It is assumed that, in a particle obtained from milled grape seed, there are N concentric layers. The cells can either be broken by the milling process or remain intact. It is assumed that the broken cells are located in the outer layer of the particle.

In document Separation and Purification Technologies in Biorefineries (Pldal 105-110)