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Extraction of Natural Compounds From Plants
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The separation quality depends strongly on the solute that is being used.
Charcoal is used for low-pressure separations. Here, an adsorbent (nonpolar) is used with a mobile phase (non-polar). The compounds having a
higher refractive index are absorbed more strongly, this is because adsorption
depends greatly on the polarizability of the molecule (Jackson and Line,
1997; Otsuka, 2006).
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2.5.1.2. Partition Chromatography
We can divide partition chromatography into chemically bonded phases and
liquid-liquid chromatography (LLC). Two immiscible liquids are involved
in this type of chromatography, one acts as the stationary phase and is
fixed on a solid support, while the other serves as the mobile phase. The
principle upon which this separation takes place is an extraction process. By
the means of physical adsorption (LLC), it is possible to fix the stationary
liquid on the solid. This is easy but has the disadvantage that it results in a
continuous loss of stationary phase. We can also fix the stationary liquid on
the solid by chemical bonds, and nowadays, it is the most applied method
(Dou et al., 2012).
In the case of a non-polar mobile phase and a polar stationary phase
(mostly on an alumina or silica support), the process is termed normal phase
(NP) liquid chromatography. While, if the case is opposite, it is termed
reversed-phase (RP) liquid chromatography.
Ion pair chromatography and chiral liquid chromatography are special
cases of partition chromatography. Chiral liquid chromatography is of
great economic importance as it is used in drug generation for separating
racemates. Ion pair chromatography is used for separating ionic substances;
hence it is a type of RP chromatography. Here, on the stationary phase¸, the
ions to be separated or analyzed are retained along with their counter-ions.
Later, they are delivered with the forthcoming mobile phase. For performing
desorption, water-soluble organic solvent (such as methanol) is used as an
aqueous solution. Ion exchange chromatography often competes with ion-
pair chromatography (Gil-Ramírez and Rodriguez-Meizoso, 2021).
2.5.1.3. Ion Exchange Chromatography
We can make a two-phase system by making the particles (swollen) of an
ion exchanger come in contact with a mixture of an aqueous solution. If
ions are formed in the solution by the components, then with the ionogenic
functional groups, electrostatic interactions take place and ion exchange
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accompanies it. The ionogenic component’s charge influences the strength
of the interaction. Moreover, it is a diffusion-limited process and hence,
the ion’s diffusion coefficient influences it. The hydrated ion’s effective
size also influences it. We can classify different types of ion exchangers as
follows (Hunter and Nash, 1935; Marr and Gamse, 2000):
have anions or cations bound to a matrix, and with ionic sample components,
these anions or cations can be exchanged. Amphoteric ion exchangers have
both anions or cations exchanging groups. These ion exchangers can form
internal salts that may dissociate when it encounters the sample electrolytes
and hence can bind both cationic and anionic components. Dipolar ion
exchangers fall under a special type of amphoteric ion exchangers, here
amino acids are attached with the matrix, forming dipoles in an aqueous
solution. These dipoles interact selectively with biopolymers. Functional
groups are carried by the chelating ion exchangers. With metal ions, these
groups can form a complex bond. They bind alkaline earth metals and heavy
alkaline earth metals preferentially. A limited binding ability is possessed
by the selective ion exchangers, so they are capable of binding some ions
only (Wei et al., 2006; He et al., 2012). A stricter limitation is possessed by
specic ion exchangers, they react only with a specic type (only one) of
ion.
Natural Compounds: An Introduction
• Anion exchangers;
• Cation exchangers;
• Dipolar and amphoteric ion exchangers;
• Chelating ion exchangers;
• Specic or selective ion exchangers.
The simplest ion exchange resins are anion and cation exchangers. They
2.5.1.4. Gel Chromatography
Size exclusion chromatography (SEC) is known as Gel chromatography. In
this process, the mixtures are separated because of their unequal size. Swollen
gel particles are present in the stationary phase. They have a pore size that
helps in enabling the small molecules for penetrating the pores. Whereas the
medium size molecules can partly enter the pores. The pore size completely
excludes the greatest molecules from the pores. With a hydrophobic mobile
phase, we can use gel chromatography, it is often termed gel permeation.
When in the hydrophilic mobile phase, it is called gel filtration.
Extraction of Natural Compounds From Plants
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2.5.1.5. (Bio-) Afnity Chromatography
The latest variant of liquid chromatography is affinity chromatography.
The biological specificity of the interaction between protein and ligand is
exploited by this technique. Binding the ligand on support (insoluble support)
elaborates this concept. The technique consists of enzymes, nucleic acids, or
many other compounds, and may have high or low molecular weight. For
the ligand, certain proteins will be retained with an appreciable affinity but
only if the prepared substance is used as the column packing. Altering the
pH or composition of the mobile phase, these can be eluted to weaken the
ligand binding and to favor dissociation.
2.5.2. Continuous Techniques
The mechanical techniques are with continuous, semi-continuous, and
batch chromatographic apparatuses. Batch chromatography is discussed in
various articles with special emphasis on continuous techniques (Ladisch,
2001; Schulte et al., 2007).
2.5.2.1. True Moving Bed Chromatography (TMB)
The first attempt for achieving a counter-current (continuous)
chromatographic process was true moving bed chromatography (TMB).
Due to gravity, the solid phase moves downward and exits the system. While
exiting, it does not contain any adsorbed products and hence, is recycled to
the top again. Opposite direction (upwards) is followed by the liquid stream,
and its recycling is performed from the column’s top to the bottom. In the
binary case, the components A and B are contained in the feed, and in the
middle, it is injected.
The ow rates can be adjusted so that, with the uid stream, B can be
moved upward and with the solid, A moves down (considering different
afnities for both A and B. Out of these two, A should be more retained).
This leads to spatial separation. Two outlets and one inlet line is required.
The principle upon which TMB-process works is shown in Figure 2.12.
However, the following things can make moving bed systems suffer:
• It may become difcult to achieve packing ow control;
• an uneven column packing may result in low mass transfer
efciencies;
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Natural Compounds: An Introduction
• the increased shear forces may cause packing attrition;
• low mobile phase velocities (relatively) for preventing the
chromatographic bed’s uidization.
Due to these disadvantages, this process has never faced success.
Nonetheless, the underlying idea has helped in leading towards the
development of the simulated moving bed (SMB) process, which is now
being widely used.
Figure 2.12: True moving bed chromatography.
Source: https://www.researchgate.net/gure/True-moving-bed-process_
g2_251818935.
2.5.2.2. Simulated Moving Bed Chromatography (SMB)
We can achieve a lot of advantages of a true counter-current operation by
the appropriate shift of the collection and injection points and using various
fixed-bed columns in series; it is the SMB concept.
In an SMB plant, the outlet and inlet lines move gradually among xedbed columns. Shifting the external lines simulate its ow and the solid, no
longer moves. The rate of simulated solid ow is associated with the shift
period. The solid still moves in the direction of the liquid concerning the
external lines; this is because of the shifting of the outlet/inlet ports.
There reaches a quasi-stationary state after the initial start-up period.
At this point, the concentration proles proceed periodically regarding the
outlets and inlets. For an innitely low shifting time and an innite number
of columns, the proles are similar to the TMB process. The principle of this
process (SMB) is shown in Figure 2.13.
Extraction of Natural Compounds From Plants
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Figure 2.13: Simulated moving bed chromatography (SMB).
Source: https://www.azom.com/equipment-details.aspx?EquipID=4736.
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The feed is continuously pumped into the mobile phase and four-column
continuous system. On the other hand, two streams of products, the extract,
and the rafnate, are also continuously recovered. The components that enter
the feed has to exit with any of the product. If we focus from the feed port,
the view depicts that it becomes apparent for one fraction to go upstream to
the extracted port from the feed, while the other has to go downstream to the
other (rafnate) port. For keeping the concentration proles between these
columns, switching the outlet ports and feed altogether to the left is necessary
to get a quasi-stationary operation. For obtaining a stable operation and pure
components, the triangular theory of Morbidelli discusses the calculations
needed to obtain the ow ratios (Wu and Clausen, 2007).
At high concentrations, the separation operation is continued. The
usage of non-linear effects is allowed by this. It also allows the use of the
displacement effect for enhancing production and separation rate. In contrast
to conventional batch processes, the mass transfer between solid and liquid
phase is enhanced, which leads to better adsorbent usage and it results in
higher productivity because of the counter-current operation. Moreover,
we can drastically reduce adsorbent consumption. Among all continuous
chromatographic processes, it is one of the most widely used processes, it
is because of these discussed advantages. However, certain drawbacks are
discussed below:
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Natural Compounds: An Introduction
• High complexity results in difculties in making the design and
in operations and is time-consuming;
• We can collect only two fractions with a single plant;
• It consumes a long start-up period.
2.5.2.3. Annular Chromatography
Here the adsorbent is present in between concentric cylinders. These
cylinders are slightly rotated in the direction of their vertical axis. At the
top stationary region, the feed is introduced in the device. At the top, the
eluent is also introduced, and then, it is distributed uniformly along the
circumference excluding the region through which the feed is introduced
(Storti et al., 1993).
The separated components are affected by the rotations so that they
appear as helical bands, who have a stationary exit point. From the xed
feed entry, the angular displacement of each component and its retention
time remains constant, but only till the point when conditions are the same.
Due to this, we can collect the separated components at xed stationary
outlets. Thus, this process (separation) is truly continuous. The principle of
this process is shown in Figure 2.14.
In the inverse principle, the annulus, which is lled with the stationary
phase, the outlet ports, and the feed inlet, all rotate. This principle is quite
similar to the single-column classical batch chromatography. In batch
chromatography, separation is time-dependent, and here, it is transformed as
separation is now dependent on a spatial angle. However, the productivity of
these two processes is comparable. In annular chromatography, the modes
of batch chromatography, such as frontal analysis, recycle chromatography,
gradient elution, and displacement chromatography can be applied.
The major advantage of this process is its feasibility of multicomponent
separation (De Carli et al., 1990; Deckert and Arlt, 1997).
2.5.2.4. Carrousel Adsorbers
On a carousel, various fixed columns are present in a circle. The origin is
with chromatographic separations (CSEP) or liquid adsorption and ionic
separations (ISEP) (Reissner et al., 1997). With up to 30 columns, the
carousel rotates stepwise, while the withdrawal, as well as introduction, is
done via multiport-valves at fixed positions. The connection of the columns
to the outlets and inlets and with each other is very flexible. The carousel
Extraction of Natural Compounds From Plants
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75
principle is shown in Figure 2.15 with a volume of resin from 1 liter to 300
3
. In the food industry, these plants are used in large dimensions (they have
m
carrousel diameters in meters).
Figure 2.14: Annular chromatography (a) principle; (b) photo.
Source: http://www.sinerji-ltd.com/cac.htm.
Figure 2.15: Carrousel absorbers (CSEP ®).
Source: https://www.researchgate.net/gure/Schematics-of-the-Advanced-Separation-Technologies-system-CSEP-R-for-continuous_g2_225943262.
Nonetheless, other niche techniques also exist, such as annular
electrochromatography or circular chromatography. High numbers of
separation stages are offered by these techniques, but they have limited
throughputs and apply only to high-value pharmaceutical applications
(Wang et al., 2013; Dreisewerd et al., 2015).
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Natural Compounds: An Introduction
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