Module 2 - Extraction Note - DR Akinsiku PDF

COVENANT UNIVERSITY
COLLEGE OF SCIENCE AND TECHNOLOGY

DEPARTMENT OF CHEMISTRY
COURSE TITLE: PROCESS SCIENCE II
COURSE CODE: CHM240 UNIT: 3
SESSION: 2019/2020
LECTURER: Dr A. A. Akinsiku
EXTRACTION
Extraction and gas absorption are separation processes used in food, chemical industry,
and for environmental control. These two operations are the mechanism of mass
transfer, as both take place by unidirectional mass transfer, i.e. transfer operations
involving diffusion of the component to be separated through a stationary phase like gas
absorption, liquid extraction, adsorption, and ion exchange.
Extraction as a unit operation means separating the components of a mixture. Separation
is based on the difference of solubility of components when a certain solvent is used.
Notice the analogy between distillation and extraction especially when recycle flows are
included.
The transfer of the components from the feed to the solvent is controlled by the solubility
behaviour of each component in the corresponding phase. Two phases result from the
extraction step: one enriched (EXTRACT Phase) and the other depleted (RAFFINATE
Phase) in the components to be separated, respectively. Afterwards in order to
regenerate the solvent, another separation step (e.g. distillation) is finally required.
Liquid-liquid extraction is most widely used. It is applied to remove heavy metals or acids
from waste water or for the production of aromatic compounds from mixtures of
hydrocarbons. Another application is gas-liquid extraction which is also called
absorption.
THEORY
The following notions are assumed to be familiar:
• Ideal stage, ideal stage model, calculation of the amount of ideal stages
• Mass transfer ratio, equimolar, equimass
• Phase equilibrium
• Saturated flow
In order to achieve a high degree of solute removal from the feed mixture, multiple stages
are used (as in absorption and other separations driven by phase equilibrium effects). Such
stages can be individual physical units, or they can be combined in a column configuration. In
either case, the phases flow from stage to stage counter-currently as in absorption. The solute
transfer rate depends strongly on the interfacial area available for mass transfer. To
encourage a high transfer rate, liquid extraction devices are designed to mix the phases
intimately, leading to a high interfacial area.
1|Dr Akinsiku
Figure 1: Schematic diagram of solvent extraction stage
Liquid Extraction Processes

1. Single Stage
Given a quantity of solution F containing solute B, one adds a quantity of solvent S (not
necessarily pure). Assume that M is the resulting mixture point colinear with F and S, and
that it is inside zone (2). Mixture M separates to form extract E1 and raffinate R1 (see Fig. 2).
Fig. 2: Single stage extraction
A single liquid extraction stage involves three steps, as illustrated in Figure 1 for the case of
one transferring solute :
1. Mixing of the liquids, one of which (the feed mixture) contains the solute. The other
(the solvent) has a higher affinity for the solute.
2. Solute transfer from the feed mixture to the solvent. The direction of transfer is such
that the phases approach an equilibrium condition. In an ideal equilibrium stage, the
phases achieve equilibrium.
3. Phase separation or disengagement, usually driven by a density difference.
The solvent phase leaves as the extract, which is now richer in solute. Ideally, the extract
would contain all of the feed solute, but phase equilibrium conditions are almost never that
favourable. Thus, some solute remains in the raffinate product, but the raffinate is leaner in
solute than the feed.
2. Multi-stage, cross-flow
If a single extraction stage doesn’t recover enough of the solute, raffinate R1 can be contacted
with additional solvent, forming another extract E2 and raffinate R2. This can be repeated for
n stages, as shown in Fig. 3. The n extracts, E1 to En, can be combined to form a single product.
2|Dr Akinsiku
Fig. 3: Multi-stage extraction, cross flow of solvents
3. Multi-stage, counter-current extraction

The process as shown in Fig. 4 can achieve a high recovery of B from mixture F, but at the cost
of a high solvent consumption and a combined extract product that is dilute in B. Although
the solvent, S, is chosen to make a subsequent S/B separation easy (e.g., by distillation), it is
economically advantageous to minimize solvent use. A counter-current arrangement does
this. See Figure 7. If these are equilibrium stages, the streams exiting stage i, Ei and Ri, are in
equilibrium, and must correspond to a tie-line pair on the LLE diagram.
Fig. 4: Counter-current, multi-stage liquid extraction process
3|Dr Akinsiku
Mass Balance (steady state)
For a system boundary around the entire column, the steady-state conservation of total
mass and solute gives:
Total mass: mF + mS = mE + mR = mM
Solute B: xFmF + xSmS = y1`mE + xnmR = xMmM
where mi is the mass flow rate of stream i, and xi or yi is the solute mass fraction in stream
i. These equations have 8 unknowns. They can be solved for 2 unknowns if the other 6 are
specified. In the experiment, the mass rates and compositions of the feed and solvent will
be
specified. Setting two other variables, e.g., xn and mR , allows the equations to be solved.
In general, extraction is preferred to distillation for the following applications:

1. In the case of dissolved or complexed inorganic substances in organic or aqueous
solutions, liquid-liquid extraction, flotation, adsorption, ion exchange, membrane
separations, or chemical precipitation become the processes of choice.
2. For the removal of a component present in small concentrations, such as a colour
former in tallow, or hormones in animal oil, extraction is preferred.
3. When a high-boiling component is present in relatively small quantities in a waste
stream, as in the recovery of acetic acid from cellulose acetate, extraction becomes
competitive with distillation.
4. It is nontoxic, inexpensive, and easily recoverable, a good solvent should be relatively
immiscible with feed components(s) other than the solute and have a different density
Bibliography
1. N. L. Ricker, 2005. Solvent Extraction. ATELIERATELIER INTER UNIVERSITAIRE DE
GENIE DES PROCEDES.
University of Washington.
2. Aalto University 1(12) Chemical Engineering
4|Dr Akinsiku

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COVENANT UNIVERSITY

COLLEGE OF SCIENCE AND TECHNOLOGY

Liquid Extraction Processes

Fig. 2: Single stage extraction

3. Multi-stage, counter-current extraction

Fig. 4: Counter-current, multi-stage liquid extraction process

In general, extraction is preferred to distillation for the following applications:

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