Modeling, Simulation, and Optimization of Supercritical and Subcritical Fluid Extraction Processes E-Book

Zainuddin A. Manan

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 385

Veröffentlichungsjahr: 2021

Beschreibung

This book provides a complete guide on tools and techniques for modeling of supercritical and subcritical fluid extraction (SSFE) processes and phenomena. It provides details for SSFE from managing the experiments to modeling and optimization. It includes the fundamentals of SSFE as well as the necessary experimental techniques to validate the models. The optimization section includes the use of process simulators, conventional optimization techniques and state-of-the-art genetic algorithm methods. Numerous practical examples and case studies on the application of the modeling and optimization techniques on the SSFE processes are also provided. Detailed thermodynamic modeling with and without co-solvent and non equilibrium system modeling is another feature of the book.

Leseprobe

Modeling, Simulation, and Optimization of Supercritical and Subcritical Fluid Extraction Processes

Zainuddin A. Manan

Universiti Teknologi Malaysia (UTM)

Johor Bahru, Malaysia

Gholamreza Zahedi

Universiti Teknologi Malaysia (UTM)

Johor Bahru, Malaysia

and

Ana Najwa Mustapa

College of Engineering

Universiti Teknologi MARA (UiTM)

Shah Alam, Malaysia

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Library of Congress Cataloging‐in‐Publication Data Applied for:

ISBN: 9781118460177

Cover Design: WileyCover Image: Courtesy of Zainuddin A Manan

Preface

Supercritical and subcritical fluid extraction (SFE/SCFE) technologies have become increasingly popular methods for extraction and purification of food ingredients, cosmetics, and pharmaceuticals over the last 30 years due to their unique advantages over conventional processing methods. These include low‐temperature operation, inert solvent, selective separation, and the extraction of high‐value product or new product with improved functional or nutritional characteristics. SFE/SCFE are also environmentally benign technologies since the processes typically generate no waste.

Supercritical fluid exhibits high‐density like liquids, which contributes to greater potential for solubilization of materials, and low viscosity similar to gases, which enables its penetration into the solid. Subcritical fluid, which is also known as a high‐pressure liquid, exhibits similar behavior to and can be exploited in the same manner as, supercritical fluids albeit at much lower pressure and temperature. SCFE is therefore typically classified under SFE technology. Nowadays, SFE technology is used to process hundreds of millions of pounds of coffee, tea, and hops annually, and is increasingly becoming of common use in the pharmaceuticals industry for purification and nanoparticle formation. Supercritical fluid processing is also gaining in the botanicals, vitamins, and supplements industries, where they are becoming synonymous with the highest purity and quality.

Commercial application of SFE technology has been relegated to only special applications due to high‐equipment capital cost required. Besides that, conceptual development of extraction processes for natural oils, fats, cosmetic, and pharmaceuticals based on supercritical fluid technology is hindered by the lack of suitable design tools and reliable thermodynamic data and models at high pressure. Optimization of SFE processes involves a search for the optimum conditions above the critical point that is in a narrow limit and needs special care. Process simulation is used for the design, development, analysis, and optimization of technical processes and is mainly applied to chemical plants and chemical processes. Integration of optimization techniques into simulation practice, specifically into commercial software, has become nearly ubiquitous, as most discrete‐event simulation packages now include some form of optimization routines. Even though modeling, simulation, and optimization tools have been widely used for design of chemical processes, their application has been very limited in vegetable oil processing particularly at supercritical conditions. In line with this limitation, there is a dearth need for literature on this topic. So far there has been no book written on modeling, simulation optimization of SFE processes in the market.

This book provides a complete guideline on tools and techniques for modeling of SFE as well as SCFE processes and phenomena and provides details for both SFE and SCFE from managing the experiments to modeling and simulation optimization. The book also includes the fundamentals of SFE as well as the necessary experimental techniques to validate the models.

The simulation optimization section includes the use of process simulators, conventional optimization techniques, and state‐of‐the‐art genetic algorithm methods. Numerous practical examples and case studies on the application of the modeling and optimization techniques on the SFE processes are also provided. Detailed thermodynamic modeling with and without cosolvent and nonequilibrium system modeling is another feature of the book.

The book consists of seven chapters. Chapter 1 presents an overview of the field of supercritical and subcritical fluid extraction (SSFE) and their importance to food, cosmetic and pharmaceutical industries. Chapter 2 describes the concepts and methodologies for modeling, simulation, and optimization. It presents conservation laws related to SFE traditional first principle modeling and optimization techniques, as well as advanced artificial intelligence (AI) techniques such as genetic algorithm, fuzzy logic, and artificial neural network. The characteristics and physical properties of palm oil as the most referred solute in the book, and descriptions of some existing palm oil industrial processes are presented in Chapter 3. In Chapter 4, the first principle methodology is applied for modeling of properties of palm oil components, mixtures, and for the SFE of palm oil components. Modeling applications involving advanced techniques such as AI, ANN, and fuzzy logics and ANFIS are discussed in Chapter 5. Next, Chapter 6 describes experimental design concepts and procedures as well as statistical optimization techniques involving SSFE processes. Finally, optimization of SSFE using first principle modeling and other advanced techniques are presented in Chapter 7. The colored version of few figures from this chapter can be viewed on the product's page of the following website, https://www.wiley.com

Zainuddin Abdul Manan, Gholamreza Zahedi,School of Chemical and Energy Engineering,Faculty of Engineering, Universiti Teknologi Malaysia,UTM Johor Bahru 81310Johor MalaysiaAna Najwa MustapaUniversiti Teknologi MARA

Nomenclature

Symbol

Definition

AAD:

Average Absolute Deviation

ANFIS:

Adaptive Neuro Fuzzy Inference System

ANN:

Artificial Neural Network

Bi

:

Biot number, (

k

f

R

p

)/

D

s

C

(kg/m

3

):

Oil concentration in the supercritical fluid phase

:

Oil concentration in the supercritical phase

COG:

Centre of Gravity

:

Oil concentration at the surface of the vetiver particle

D

ext

(m):

Diameter of extraction column

,

,

:

Axial dispersion coefficient, Molecular diffusion coefficient, Diffusivity of oil in the vetiver particle

F

:

Percent of extract

FIS:

Fuzzy Inference System

FPM:

First Principle Model

GA:

Genetic Algorithm

GB:

Gray Box

K

:

Extract equilibrium constant between solid and fluid phase (‐)

k

:

Equilibrium constant

:

Mass transfer coefficient

L

(m):

Length of extractor

MLP:

Multilayer Perceptron

n0 (kmol):

Initial mole of solute in the bed

NF:

Neuro‐Fuzzy

OECs:

Overall Extraction Curves

P

(MPa):

Pressure

PDE:

Partial differential equation

Pe

b

,

Pe

p

:

Peclet number for the bed, (L

ν

)/

D

l

, Peclet number for the vetiver particle, (

R

p

ν

)/

D

s

Q

:

Degree of Membership

Q

:

Flow rate of supercritical fluid, (

ν

A

ερ

f

)

q

(kg/m

3

):

Oil concentration in the solid phase

Re

:

Reynolds number

RMSE:

Root Mean Square Error

R

p

(m):

Vetiver particle radius

r

(m):

Axial coordinate in the vetiver particle

SFE:

Supercritical Fluid Extraction

Sc

:

Schmidt number,

μ

f

/(

ρ

f

D

m

)

Sh

:

Sherwood number, (2

R

p

k

f

)/

D

m

T

:

Temperature (K)

t

:

Time (s)

V

(m/s):

Velocity of the fluid

WB:

White Box

x

(m):

Distance of a point in bed from place of input fluid

x

o

:

Initial mass fraction of extractable oil in solid phase

z

:

Dimensionless axial coordinate along the bed, x/L

Subscript

a

:

Apparent

b

:

Bed

c

:

Critical

ext:

Fluid

i

:

Inter phase

p

:

Particle

s

:

Surface of particle

0:

At time zero

Greek Letter

:

Supercritical fluid viscosity

μ

:

Membership function

ρ

:

Dimensionless radial coordinate in the vetiver particle, r/Rp

ρ

(kg/m

3

):

Density

:

Supercritical fluid density

ε

:

Void fraction of packed bed

:

Interstitial fluid velocity

τ

:

Dimensionless time, (

t

ν

)/

L

:

Rate of increase of mass per unit volume

−(∇.

ρv

):

1Fundamentals of Supercritical and Subcritical Fluid Extraction

1.1 Introduction

The behavior of supercritical fluids (SCF) was first observed and reported by Charles Cagniard de la Tour in 1822. From his early experiments, the critical point of a fluid was first discovered, and the unique properties of the SCF phase were observed. Many decades later, the power of SCFs to act as solvents for substances in solid matrices was demonstrated by Hannay and Hogarth in 1879. The group of scientists found that increasing pressure tended to dissolve solutes, while decreasing pressure caused the dissolved materials to precipitate like snow. These observations were fundamental to the understanding of the SCF extraction (SFE) technology. Zosel (1974) first demonstrated the decaffeination of coffee using SC fluids. Since then, numerous scientific works have emerged from a wide range of applications including food industry, polymers, and pharmaceuticals. The use of a SCF as a solvent to selectively extract a substance is known as SFE. The extracted material is typically recovered simply by reducing the temperature or pressure of the SCF, thereby allowing the fluid to evaporate.

SFE technology has received much attention over the last two decades. Factors like the high cost of organic solvent, increasingly stringent environmental regulations along with new requirements from the medical and food industries for ultrapure and high‐quality products have driven the development of new and clean technologies for the recovery of substances. These special needs have resulted in SFE technology applications in the food, aroma, and waste treatment industries during the 1980s. In the food and pharmaceutical industries, for example, SFE has been applied for the extraction of flavors from hops, cholesterol, and fat from eggs, nicotine from tobacco, acetone from antibiotics, and for coffee and tea decaffeination.

Knowledge and understanding of the properties of SCF are vital prerequisites for engineers and scientists to capitalize on SFE as a specialized technique for the recovery of valuable components. This chapter presents the fundamental principles and the applications of SFE technology.

1.2 Supercritical Fluid Properties

A SCF is a substance that exists above its critical temperature (Tc) and critical pressure (Pc). Figure 1.1 shows the phase diagram for a pure compound illustrating the supercritical region where an SCF may exist. The fluid above the Tc and Pc cannot be liquefied regardless of the applied pressure and demonstrates unique properties that are different from those of gases or liquids under standard conditions. The fluid also exhibits higher densities that resemble liquids, and lower viscosities that resemble gases. SCF has unique and desirable properties that make it suitable for performing a challenging extraction process. This includes the ability of the fluid properties to change with a slight variation in pressure and temperature near the critical point. High densities for SCFs contribute to greater solubilization of compounds, while lower viscosities allow SCFs to penetrate solids better and to flow with less friction. Surface tension and heat of vaporization are relatively very low for SCFs (Castro et al. 1994). Since solute diffusivities in SCFs are typically higher than those of liquid solvents by an order of magnitude, and their viscosities are lower also by an order of magnitude, their mass transfer properties are much more favorable.

Figure 1.1 A typical P‐T diagram for a pure component.

Table 1.1 shows the properties of supercritical CO2 compared with typical liquid and gas properties. These unique properties become a major advantage of the SFE particularly for the recovery of thermally sensitive compounds as well as the production of high‐purity products. By employing the SFE method, compounds can be extracted at low operating temperature, and highly pure solutes can be easily recovered from the solvent.

Table 1.1 Comparison of the properties of supercritical CO2 and those of ordinary gases and liquids.

Substance

Density (g/mL)

Viscosity (g/cm·s)

Diffusion (cm

2

/s)

Gases

10

–3

10

–4

10

–1

Supercritical CO

2

0.3–1

10

–3

to 10

–4

10

–4

to 10

–6

Liquidlike

Gaslike

Liquidlike

Liquids

1

10

–2

10

–5

1.3 Subcritical Condition

“Near‐critical” or “critical region” is defined as the area around the critical point of a solvent. It comprises sub‐ and supercritical conditions of state for the solvent (Brunner 1994). The critical temperature is the highest temperature at which a gas can be converted to a liquid by the increase of pressure, while the critical pressure is the highest pressure at which a liquid can be converted to a gas by an increase in the liquid temperature. A fluid heated to above the critical temperature and compressed to above the critical pressure is called a SCF. Meanwhile, a pure component is considered to be in a subcritical state if its pressure is higher than the critical pressure and its temperature is lower than the critical temperature. The regions of both fluids are shown in Figure 1.2 as an example of a substance’s phase diagram. The subcritical region is located above the critical pressure and below the critical temperature (Brunner 1994). Behavior of phase boundaries on the left of the supercritical region is comparable and does not change dramatically. Moreover, the high‐pressure liquid region (subcritical) has many of the characteristics of SCF and is exploited in similar ways (Brunner 1994 ).

Figure 1.2 An example of phase diagram of pure substance indicating a point of supercritical and subcritical region.

A subcritical fluid is simply a substance that resembles a gas but exists as a compressible fluid that takes the shape of its container. It is liquid but the density and solvating power of the fluid is greater than a conventional liquid since the fluid compressed higher than its critical pressure. Their low viscosity and low‐surface tension allow them to penetrate solute matrix and enable rapid wetting. Their higher liquid densities enable them to dissolve solutes from a solid or liquid matrix (Castro et al. 1994 )

Brunner (2005) reported that the solubility of analytes in subcritical (liquid) fluid (solvent) increases at constant pressure up to temperatures slightly below the Tc of the solvent. Therefore, it is preferred to operate at or near the critical temperature of the SCF and adjust the pressure in order to obtain optimal fluid density for the extraction to be carried out. In addition, in many cases, the enhancement of the solubility is especially significant near the critical temperature of the solvent (Walas 1985). According to Catchpole and Proells (2001), the use of new near‐critical solvents, with the same benefits of CO2 but substantially lower operating pressures, could increase the applicability of near‐critical fluids as solvent. In fact, Illés et al. (2000) in the investigation on extraction of essential oils using SC‐CO2 and propane at super‐ and subcritical conditions concluded that oil with high biological and commercial value could be produced with compressed gases at super‐ or subcritical conditions.

Extraction at subcritical conditions has become an increasingly popular alternative method in the extraction of bioactive compounds from natural products, food waste, herbal plants, and in environmental applications. Subcritical water extraction (SWE) is one of the most studied field in the application of environmental analysis, plants, food by‐product such as recovered polyphenols from potato peel (Singh and Saldaña 2011), pomegranate (He et al. 2012) algae, and microalgae (Herrero et al. 2006). Subcritical water is also known as superheated water, pressurized hot water, or hot liquid water at temperatures between 100 and 320 °C, and pressure below the critical pressure (220 bar) that maintains water in liquid state. In the extraction of polyphenols, subcritical water offered better extraction yield as compared to extraction using methanol or ethanol. In addition, by using subcritical water, it is possible to extract a different type of free and bound form of phenolic compounds (Singh and Saldaña 2011; Zeković et al. 2014 ).

1.4 Physical Properties of Subcritical Fluid

As mentioned in the previous section, subcritical fluids have many similar characteristics to SCFs. In fact, subcritical fluid behaviors are exploited in the same manner. Discussions in this book may refer mostly to CO2 as a supercritical solvent primarily because CO2 is the most well established and most commonly used solvent in SFE applications. However, the SFE principles also apply to solvents other than CO2 because the principal behaviors and the characteristics of subcritical and supercritical regions are similar.

The key property that contributes to the sub and SCF characteristics is solvent density. Density of the critical fluids depends on pressure and temperature (Turner et al. 2001). In addition, the density is extremely sensitive to minor changes in temperature and pressure near the critical point. The typical density of both critical fluid changes in a similar way where the density in both regions increases sharply with increasing pressure at a constant temperature, and also decreases with increasing temperature at a constant pressure. The density of SCF is similar to that of a liquid, whereas the subcritical fluid has a higher density since the phase exists as a high‐pressure liquid. The solvent power of a fluid increases with density at a given temperature and could increase with temperature at a given density. However, polar solvents exhibit more marked changes in their dissolving power with density increase compared to less‐polar solvents (Castro et al. 1994 ).

The properties of SCFs are frequently expressed in terms of reduced, rather than absolute values (Castro et al. 1994). Figure 1.3 is a phase diagram of both the supercritical and subcritical regions. In the subcritical liquid region, the greatest density changes can occur, and is therefore most effective for changing density when minimal temperature and/or pressure changes are introduced. To achieve subcritical liquid condition, the pressure of a solvent should be applied above the critical pressure, while the temperature is maintained below the critical temperature.

Figure 1.3 Reduced density–reduced pressure including several reduced temperature for supercritical and subcritical region.

Source: Castro (1994).

As in the case for density, diffusivity, and viscosity of a sub‐ or SCF are also related to temperature and pressure. The diffusivity and viscosity of a SCF approach a liquid behavior as pressure is increased. The diffusivity of a solute in a subcritical fluid decreases with pressure increase at a constant temperature and always exceeds the solute diffusivity in an ordinary liquid solvent. On the other hand, viscosity increases with pressure at a given temperature (Castro et al. 1994). In addition, the transport properties of near critical or subcritical fluid are almost similar to those of SCFs. On the other hand, the solvating power of near critical fluid is stronger than that of SCF (see Figure 1.4).

Figure 1.4 Relative solvent power and diffusion characteristics of liquids, near‐critical and SCFs as well as gases.

Source: Griffith (2001). The reference: Griffith, K.N. (2001). Environmentally Benign Chemical Processing in Near- and Supercritical Fluids and Gases Expanded Liquids. Thesis for Degree Doctor of Philosophy in Chemistry. Georgia Institute Technology.

Polarity is one of the key properties that have strong influence on solubility. It can be altered in order to modify the selectivity of an extraction process. A pressure rise or the presence of a modifier does not increase the polarity of a fluid as this is determined by the dipole moment of the fluid molecules. CO2 is a nonpolar solvent, and this limits the solubility of several polar compounds in CO2. Therefore, the addition of an entrainer is needed to enhance the solubility (not increase the polarity of solvent) of polar compounds. Hansen et al. (2001) has found that polar halocarbons such as 1,1,1,2‐tetrafluoroethane should be more able to dissolve polar groups such as capsaicin molecules.

1.5 Principles of Sub‐ and Supercritical Extraction Process

Subcritical fluid is also known as a high‐pressure liquid. Figure 1.5 shows a typical phase diagram for a pure material and the thermodynamic states of various separation processes. Even though the terminology is different from SCF; however, the characteristics and principles of extraction are identical. The basic element required for conducting SFE includes a fluid source, a compressor to pressurize and pump the fluid, an extraction vessel to hold the material to be extracted, a temperature/pressure control system, a collection device, and a backpressure regulator. The backpressure regulator is used to allow a drop in pressure which will enable separation of extract from solvent (Brunner 1994). Additionally, other equipment such as valves, flow meters, and heater/coolers for temperature control of the fluid are needed for proper operation of the process (Rivizi et al. 1986). In industrial applications, the consumption of CO2 is high, thus a recycle system is needed to control and minimize the cost of the material consumption (Herrero et al. 2006 ).

Figure 1.5 Phase diagram of a pure material and the thermodynamic states of various separation processes.

Source: Rizvi, S.S.H., Yu, Z. R., Bhaskar, A. R. and Chidambara Raj, C. B. (1994). Fundamental of Processing with Supercritical Fluids. In: Rizvi, S. S. H. (Ed) Supercritical Fluid Processing of Food and Biomaterials, Blackie Academic and Professional. Glasgow NZ, Chapman & Hall, (pp. 1-26).

1.5.1 Solid Sample Extraction

Extraction of solutes from solids can be represented as a two‐stage process comprising extraction and separation of the extract from the solvent (see Figure 1.6). During the extraction step, the solvent is first compressed to above critical pressure before it flows through a fixed bed of solid particles in the extractor and dissolves the extractable components of the solid. The loaded solvent is removed from the extractor and fed to a precipitator. In a large‐scale SFE process, the solvent is typically recycled to the separation stage. Solvent losses are compensated by a make‐up stream (Brunner 1994 ).

Figure 1.6 Flow diagram of a separation process.

Collection of the extract yield is a key step in an SFE process. In order to maximize sample collection efficiency, techniques such as cooled collectors and liquid traps should be included. Sample collection can be accomplished by simply allowing the solvent/extract mixture to depressurize completely to atmospheric pressure while the solvent is evaporated and dissipated, leaving only the extracted material in a collection vial. Otherwise, pressure could be reduced to the level which is enough to decrease the solubility of the extract in the solvent. This can be done without complete depressurization in a separation vessel held above atmospheric pressure. In this way, the solvent can be recycled and the energy costs associated with pressurizing the solvent can be saved.

1.5.2 Liquid Sample Extraction

The process equipment for carrying out separation by SCF consists of mainly two columns as shown in Figure 1.7. The first column acts as a separation column for the removal of less‐volatile substances, while the other serves as a precipitation column for the separation of the extracted less‐volatile components from the SCF cycle.

Figure 1.7 Schematic of a SFE process.

The mixture to be separated is normally introduced into the middle or on top of the separation column, while the compressed supercritical solvent flows upward through the column; thus, creating a counter current flow. Separation of low‐volatility components takes place in this column. The component in the mixture which is more soluble in the SCF phase is removed from the mixture by supercritical solvent and becomes enriched in the up‐flowing supercritical solvent. Operating conditions of the separation column are at a constant temperature above the critical temperature of the SCF in the range of 1.1–1.5 times the reduced temperature of the SCF; and a pressure of about 2–3 times the reduced pressure of the SCF (Brunner and Peter 1982 ).

The second column (precipitation column), that is mainly used for solvent recovery, can be operated on different schemes: (i) operates at the same pressure as the separation column except with increased temperature; (ii) operates at the same temperature as the separation column, but the pressure in the precipitation column is reduced. Both techniques drastically reduce the solubility of the low‐volatility components in the SCF phase. The liquefied substances flowing downward in the column and are divided into the top product (also known as the extract) and the reflux for the separation column. The regenerated supercritical solvent leaves the second column at the top and is recycled via a circulating pump into the separation column.

The liquid reflux flows downward through the separation column in a counter‐current direction to the SCF stream. The reflux changes its composition on account of mass transfer in the column between liquid and SCF. The liquid stream that is withdrawn at the bottom of the separation column is called raffinate. It contains components that are less soluble in the supercritical solvent.

Pressure and temperature are the main operating parameters in controlling an SFE process. Generally, the solubilities of fats and oils are increased by increasing both pressure and temperature (Stahl et al. 1988). Therefore, if a maximum extractability is required, higher operating pressure and temperature are applied. Application of SFE at low pressure and temperature can remove undesirable odors from products such as vegetable oils (Koseoglu et al. 1996 ).

1.6 Applications of SCF Extraction