Liquid–Liquid Extraction
ECKART M€ULLER, Frankfurt a. M., Germany
ROSEMARIE BERGER, Ludwigshafen, Germany
ECKHART BLASS, Technische Universit€at M€unchen, M€unchen, Germany
DOMIEN SLUYTS, Bayer AG, Antwerpen, Belgium
ANDREAS PFENNIG, RWTH Aachen, Aachen, Germany
Liquid – liquid extraction or solvent extraction
is a separation process which is based on the
different distribution of the components to be
separated between two liquid phases. It depends
on the mass transfer of the component to be
extracted from a first liquid phase to a second
one.
Liquid – liquid extraction is the separation
method of choice where distillation fails, e.g.,
for azeotropic mixtures or temperature-sensitive components. Separation is achieved by
adding a liquid solvent phase to the original
liquid carrying the component(s) to be
extracted. One of the phases must be dispersed
into droplets in the other, continuous phase to
achieve a sufficiently large mass-transfer interface. Extraction is performed in mixer–settler
equipment or extraction columns, which are
frequently equipped with rotating internals or
pulsators for energy input to positively influence droplet size.
Definitions. The phases form definite contact areas and cannot or can only to a limited
extent mix with one another on account of their
properties. The phases are composed of individual substances, so-called components, and
flow as liquid streams through the extractor (see
Fig. 1).
Generally, more than three components are
involved in an extraction. To make the description of the problem simpler, key components
are defined. The feed to a liquid – liquid
extraction is the solution that contains the
components to be separated. The key component of this liquid phase (the phase from which
the solute is extracted) is called A, the
# 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10.1002/14356007
transferred substance (the solute) C. The
(extraction) solvent is the liquid added to
the process to extract the substance C from
the feed. The solvent can be the pure component, B, but usually contains small quantities
of C and A, because the solvent is usually
recycled from a recovery system. The solvent
phase leaving the extractor is the extract. The
extract contains mainly B and the extracted
component C but also small quantities of A.
The raffinate is the liquid phase left from the
feed after being contacted by the extraction
solvent; it is composed mainly of A, but
generally small quantities of the extracted
component C and solvent B.
Comparison with Distillation. Liquid –
liquid extraction has similarities with distillation. In both cases the substances to be separated have concentration ratios 6¼ 1 in the two
phases. In the case of distillation the second
phase (the vapor phase) is formed exclusively
from the components of the initial (liquid)
phase, with extraction a new substance, the
(extraction) solvent, must be added to form
the second (liquid) phase. The solvent enters
into the material balance of the process and
its behavior towards the substances to be
separated is decisive for the separation
process.
Generally, the extracted substance is separated from the solvent by distillation and
the solvent B is recycled to the extractor.
Sometimes the selective action of the solvent
is used in the distillation which generates the
reflux for the extraction, e.g., extractive distillation (extraction of aromatic compounds) or
2
Liquid–Liquid Extraction
Liquid – liquid extraction is used in industry
for the following purposes:
Figure 1. Principles of extraction — streams, phases and
components
azeotropic distillation (extraction of acetic
acid). In the extraction of metal salts the purity
is often achieved by adjusting the pH. Extraction is an isothermal process, normally carried
out at ambient temperature and pressure.
Comparison with Absorption. In both
absorption and extraction solvents are used
which are recycled and thereby take up the
required components selectively and then
release them again. Selection of the most
suitable solvents therefore plays an important
role in the design of the two separation
processes.
Applications of Liquid – Liquid Extraction. Liquid – liquid extraction is primarily
applied where direct separation methods such
as distillation and crystallization cannot be
used or are too costly. Liquid – liquid extraction is also employed when the components to
be separated are heat-sensitive (e.g., antibiotics) or relatively nonvolatile (e.g., mineral
salts).
Separation of systems with similar boiling
points (e.g., separation of aromatics from
aliphatic hydrocarbons)
Separation of high boilers and low-concentration solutes from aqueous solutions
(e.g., phenol)
Separation of mixtures with high boiling
points (e.g., vitamins)
Separation of temperature-sensitive compounds (e.g., acrylates, biotechnology)
Separation of azeotropic mixtures (extraction of acetic or formic acid from aqueous
media using, e.g., MTBE as solvent)
Extraction of organic compounds from
salt solutions (e.g., caprolactam)
Extraction of salts from polymer solutions
(e.g., ketone resins, polyols)
Extraction of metal salts from low-grade
ores (e.g., copper)
Extraction of metal salts from wastewater
(e.g., copper)
Recovery of nuclear fuels (Purex process).
It is expected that the importance of solvent
extraction will increase as the feedstock of
chemical processes changes from crude oil to
biomass. The components obtained in the first
processing steps from biomass have more functional groups and are less volatile than the key
components obtained from crude oil. Thus, it
may be economically more attractive to use
extraction with suitable solvents (including
extractants like ionic liquids) than to use distillation at low pressure.