Fluid Separation

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MAS-600

Fluid Separation

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Process Engineering Guide:

Fluid Separation

CONTENTS Page
0
1
2
3
4
5

INTRODUCTION/PURPOSE
SCOPE
FIELD OF APPLICATION
DEFINITIONS
A SEPARATION LOGIC TREE
METHODS OF DISTILLATION
5.1 Fractional Distillation
5.2 Azeotropic Distillation
5.3 Extractive Distillation

3
3
3
3
3
4
4
7
8

6

LIQUID-LIQUID EXTRACTION

9

7

OTHER COMMERCIAL METHODS OF SEPARATION

11

7.1 Adsorption
7.2 Fractional Crystallization
7.3 Ion Exchange
7.4 Membrane Processes
7.4.1 Ultrafiltration
7.4.2 Reverse Osmosis
7.4.3 Pervaporation
7.4.4 Liquid Membranes
7.4.5 Gas Permeation
7.4.6 Dialysis
7.4.7 Electrodialysis

11
12
12
13
13
13
14
15
15
16
16

7.5 Supercritical Fluid Extraction
7.6 Dissociation Extraction
7.7 Foam Fractionation
7.8 Clathration
7.9 Chromatography

16
17
18
18
19


8

OTHER METHODS OF SEPARATION
8.1 Precipitation
8.2 Paper Chromatography
8.3 Ligand Specific Chromatography
8.4 Electrophoresis
8.5 Isoelectric Focusing
8.6 Thermal Diffusion
8.7 Sedimentation Ultracentrifugation
8.8 Isopycnic Ultracentrifugation
8.9 Molecular Distillation
8.10 Gel Filtration

19
19
19
19
19
20
20
20
20
20
20


APPENDICES
A

AT A GLANCE CHART BASED ON FENSKE, UNDERWOOD

21

B

A GENERALIZED y - x DIAGRAM

22

C

TEMPERATURE - COMPOSITION DIAGRAMS FOR
AZEOTROPIC MIXTURES

23

A TYPICAL y - x DIAGRAM FOR EXTRACTIVE DISTILLATION
(SOLVENT FREE BASIS)

24

RAPID ESTIMATION OF LIQUID-LIQUID EXTRACTION
REQUIREMENTS

25

D

E

F

LIQUID - LIQUID EXTRACTION - THE USE OF EXTRACT
REFLUX

26

G

SELECTIVITIES REQUIRED FOR EQUAL PLANT COSTS

27

FIGURE
1

SEPARATION LOGIC TREE

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

4

28


0 INTRODUCTION / PURPOSE
A beginner in the field of fluid separation can be overwhelmed by the apparent
wealth of choice available. In reality that choice is limited. This Engineering
Guide presents the options available with the intention of giving an overview of
methods of separation. Knowledge of the basic concepts is assumed.
1 SCOPE
This Engineering Guide describes each method of separation and outlines the
basic theory involved. For distillation and solvent extraction (the most widely
used fluid separation techniques) a short cut method of design is given. One or
two commercial processes for gas separation are arbitrarily included.
A conscious effort is made, however unapparent to the casual reader, to call on
experience to highlight important points and areas where caution should be
exercised. Where appropriate, the implication of the separation technique on the
total flowsheet is discussed.
A bibliography of further useful reading material is given at the end of each
Clause for the serious advocate.
This Engineering Guide does not cover the process engineering design of fluid
separation equipment.
2 FIELD OF APPLICATION
This Guide applies to the process engineering community in GBH Enterprises
worldwide.
3 DEFINITIONS
For the purposes of this Guide no specific definitions apply.


4 A SEPARATION LOGIC TREE
Based on an appreciation that simple is best and what is best understood usually
prevails, a separation logic tree can be proposed. This presents a stepwise
procedure for selecting the separation method most likely to be accepted and is
shown in Figure 1.

Realistically, if distillation can be used then this is the preferred technique; the
simpler the type of distillation the better. After distillation think of liquid-liquid
extraction. Any other method is specialized and should be considered with an
GBHE expert or the company offering the system.
The purist will maintain that the separation method chosen will depend on
feasibility and cost. However, a large monetary carrot is required to change from
a conventional, totally satisfactory, established technique to any other method.
Fractional distillation is favored because it is tried and trusted. It can be applied
over a wide range of conditions (i.e. where vapor and liquid co-exist) provided
that there is a difference in volatilities.
Azeotropic distillation, extractive distillation and liquid-liquid extraction are more
complex: another component is added to enhance the non-ideality of the mixture
to be separated. These methods are usually employed when classes of
components (e.g. paraffins from aromatics) have to be separated, or the system
is heat sensitive, or maybe the operating pressure for fractional distillation would
be very high or very low.
The other commercial methods of separation have often been developed for a
specific application. This has necessarily involved a large expenditure in
development time and money. Not unreasonably their propagators try to widen
their scope and applicability.
In line with the aforementioned, this document concerns itself mainly with
distillation in terms of how to reach the most appropriate system, with attention
also being given to liquid-liquid extraction. An outline of other methods of
separation, together with their general areas of applicability, is also given.


5 METHODS OF DISTILLATION
5.1.1 Fractional Distillation
Distillation involves the separation of the components of a liquid mixture by
partial vaporization of the mixture and separate recovery of vapor and residue.
To refresh memories relative volatility (a) is a direct measure of the ease of
separation by a distillation procedure. Using normal nomenclature for a binary
mixture A-B:


where K = vapor-liquid equilibrium constant
x = liquid mole fraction
y = vapor mole fraction
and for ideal solutions:

where P° = vapor pressure of pure components
The closer the value of a is to unity the more difficult the separation. In a simple
system a knowledge of the boiling points shows whether the mixture would be
easily separable.
Boiling Point Difference
°C
2
5
10
20
30
50
100

Approximate
Relative Volatility
1.05
1.11
1.25
1.6
2.0
3.1
8.7

Knowing α, a feel for still requirements can be readily obtained using Fenske,
Underwood, Gilliland or for α = 1.2 to 2.0 by use of Appendix A. For the operating
optimum, remember to take Nmin x 2 (and Rmin x 1.3).
This approach can be adopted for mixtures containing more than two
components by using the key components concept.
These only apply strictly to ideal systems (i.e. relative volatility does not change
with composition, constant molal overflow). In practice many systems are nonideal. To allow for deviations from ideality in the liquid phase the concept of
activity coefficients was introduced, thus:

, where ‫ = ﻻ‬activity coefficient


The majority of non-ideal systems (greater than 90%) exhibit positive deviations
from Raoult’s Law. Systems in which the components are strongly associated,
e.g. mixtures containing basic and acidic components, may exhibit negative
deviations. Qualitatively the differences displayed between ideal systems and
those exhibiting positive and negative deviations are apparent on
considering a generalized y-x diagram, see Appendix B.
The curves in Appendix B show that in non-ideal mixtures the greatest deviations
from ideal behavior occur at high dilution. In practice it will, for example, be more
difficult to obtain pure A, (the more volatile component) and easier to obtain pure
B (the less volatile component), for a system exhibiting positive deviations than
would be the case if the system behaved ideally. As on most occasions the
concern is to produce a pure tops product, care should be taken with the
design of columns embracing non-ideal systems. The Nmin requirement can often
be 1.5 to 2 times that calculated assuming ideal behavior, and on occasions very
much higher.
Vapor phase non-idealities can usually be neglected at atmospheric and subatmospheric pressures.
The aforementioned considers a single fractionation column, in real life multicomponent systems and separation trains have to be considered. In general
terms the distillation train should be designed to give the lowest total vapor boilup rate. Two rules of thumb are of help in sequencing columns to arrive at this
desired state of affairs, viz:
(a) Favor the scheme in which 25 to 50% of the feed is removed as distillate;
and, less importantly:
(b) Do difficult separations last.
In practice, another possibility which should be considered when addressing
multi-component systems is the suitability of including side-stream operation.
This is a very useful way of minimizing the total vapor rate required, and hence
saving capital and energy, providing a pure product is not required. Normally, if it
is more important to minimize heavy-ends content in the side stream product a
liquid side stream would be removed above the feed. If it is more desirable to
minimize light-ends in the side stream product a vapor (or liquid) side stream
would be removed below the feed.


Another choice that has to be made is whether to use batch or continuous
fractionation columns. Separation by batch distillation is widely used, especially
in the Fine Chemicals area. Continuous distillation is the accepted operation in
the petrochemicals and other large tonnage areas. Batch operation is usually
confined to low rates of production (say, 3000 te/annum), where adjacent
processing stages are batch operation and where there is the need for
operational flexibility in a multi product unit.
Attention may have to be given to components undergoing chemical reaction
during the distillation operation. This can be allowed for if the reaction is an
intention of the process. Difficulties can occur if reaction occurs at fractionation
conditions and this likelihood was not recognized at the design stage. Other
system properties which may detract from the performance of distillation
columns are the deposition of solids or the tendency to foam. The addition of an
anti-foam agent may offer a solution to the latter problem, though it is usually
possible to design for a foaming system so that anti-foam is not needed.
Behavior such as foaming is often not evident or is difficult to recognize in a
laboratory or semi-technical simulation of the system.
Although not strictly within the scope of this Engineering Guide, absorption is a
technique widely used in separation trains. It is akin to distillation in that the
absorption column is similar to that used in distillation, although not usually
including a condenser or reboiler.
Absorption is the removal of one or more selected components from a mixture of
gases by absorption into a suitable liquid. The process is dependent on the
differential solubility of the gas phase components in the liquid. It is usually
necessary to remove the gas from the solvent by stripping in another column,
either by pressure swing and/or increasing temperature.
The following documentation may prove to be of further value:
(1) GBHE-PEG-MAS-607
(4) Distillation systems design procedure, GBHE Engineering Group.


5.1.2 Azeotropic Distillation
An azeotrope is a mixture of two or more liquid components which boils at
constant temperature and distils over completely without change of composition.
The ease of formation of a binary azeotrope is determined by:
(a) the magnitude of the deviations from Raoult’s Law;
and
(b) the difference in boiling points of the two components.
The smaller the difference in boiling points the smaller the deviations from
Raoult’s Law (i.e. from ideality) required for azeotrope formation.
Positive deviations from Raoult’s Law (‫ ,)1 > ﻻ‬by far the most common, can give
rise to minimum boiling azeotropes. Negative deviations (‫ )1< ﻻ‬can result in the
formation of maximum boiling azeotropes. Azeotropes can be classified as
homogeneous (those which exist in one liquid phase and include minimum and
maximum boiling azeotropes) and heterogeneous (those which exist as
two liquid phases in equilibrium and are always minimum boiling).
Heterogeneous azeotropes are characterized by large positive deviations from
Raoult’s Law. Most azeotropic systems are of the minimum boiling type.
Typical temperature-composition diagrams are given in Appendix C. By
definition, Azeotropic systems are non-ideal and should from a design viewpoint
be treated with care. However, there is nothing mystical about these systems and
the azeotrope can in the most simple interpretation be considered as a pseudocomponent. For example, there may be a requirement to separate a
complex mixture, two of the components of which form an azeotrope. The
azeotrope may be a key component in the system. Considering the azeotrope as
a pseudo-component would allow Fenske-Underwood-Gilliland to be used. This
would at least be a safe approach giving an over design (although sometimes a
grossly over designed system). This approach can be significantly refined if
account is taken of the azeotrope composition (see reference asterisked!*).
The principal applications of azeotropic distillation, ie where an azeotropic agent
is added to the system, are:


(1)

In the separation of close boiling components, where the azeotropic
agent forms a minimum boiling azeotrope with only one component,
or if it forms binary azeotropes with two of the components in the
system, one of the binary azeotropes boils sufficiently lower than
the other;

(2)

To facilitate separation of the two components in an already
existing binary azeotrope by formation of a ternary azeotrope. The
ternary azeotrope should boil sufficiently below any binary
azeotrope, and the ratio of the original components in the ternary
azeotrope should be different from their ratio before the azeotropic
agent was added.

The introduction of an azeotropic agent to facilitate separation often necessitates
additional equipment and as a result a more complex separation train. Thus, if an
ester is added to make the separation of water from acetic acid easier the
condensed overheads will split into two phases. The ester-rich phase will be
returned to the column as reflux. The water phase will contain solvent
ester which should be recovered in an additional distillation column.
Remember the composition of an azeotrope can be changed with increase or
decrease in pressure.
In general, the added azeotroping agent is best returned as reflux to the top of
the column, usually in about 30 to 50% excess over that required to form the
azeotrope.
Swietoslawski, Azeotropy and Polyazeotropy, Pergamon Press, 1963.
Kirk-Othmer, Encyclopedia of Chemical Technology, Wiley-Interscience, 3rd
Edition, Vol 3, 352 (1978).
Horsley, For azeotropic data, see: Azeotropic Data -III, Amer Chem. Soc.,
Advances in Chemistry Series 116 (1973).


5.1.3 Extractive Distillation
Extractive distillation is based upon the addition of an extractive agent to a
mixture preferably containing two components of different chemical structure.
The added agent enhances the deviation from ideality or activity coefficient of
one component relative to the other, ie. it enhances the relative volatility. The
resulting difference in volatility permits fractionation which may not have
been economically attractive using fractional distillation. In some cases extractive
distillation is used where classes of components are to be separated (eg
paraffins from aromatics) where the boiling point spread could be such that
fractional distillation in one column would be impossible.
The extractive agent is chosen such that its volatility is low relative to those of the
feed components. The solvent is always introduced above the fresh feed stage in
order to maintain a high solvent concentration, which can be assumed to be
constant throughout most of the column. Usually in extractive distillation, 40 to 90
mole % solvent is required in the liquid phase to maximize the difference in
volatilities between the feed components. A typical y-x diagram, on a
solvent free basis, is given in Appendix D. Vapor-liquid equilibria data are
essential for design of an extractive distillation column.
Solvent is not fed to the top stage because a few plates should be provided
above the solvent entry point to reduce the concentration of solvent in the
overheads to an acceptable level. Feed is usually introduced to the column in
vapor form as liquid feed dilutes the descending solvent and reduces the solvent
concentration in the bottom section. Reflux at the top of the column also
dilutes the solvent, and increased reflux is not always synonymous with
increased separation.
In addition to being easily separable from the feed components the solvent
should be:
(a) Completely miscible with the top product under top plate conditions, ie the
solvent should not be too selective. The appearance of a second liquid
phase gives an unwanted decrease in relative volatility.
(b) Incapable of forming azeotropes with the feed components in the
extractive distillation zone.


The extractive distillation system requires at least two columns - the main
extractive column and a second column to separate the extracted components
from the solvent. The overall separation train may be even more complex if
solvent has to be recovered from the overheads leaving the extractive column.
This may require a further distillation column or a water wash system.
Plate efficiency is often low in extractive distillation columns (25 to 35%).
Perry, Chemical Engineers Handbook, 6th Edition, McGraw-Hill Book Company,
13-53 (1984).
6 LIQUID-LIQUID EXTRACTION
Liquid-liquid (or solvent) extraction involves the addition of an extractive solvent
to the mixture to be separated, the solvent being partially miscible with at least
one component, or class of components, in the mixture. The solvent is such that
it is selective toward one component, that is it enhances the deviation from
ideality or the activity coefficient of one component relative to another.
In general the solvent is required to have a selectivity factor β, greater than 2 and
a capacity or solubility for the component(s) to be extracted of not less than 10%.

Where

‫ﻻ‬A, xA = mole fraction of component A in co-existing equilibrium
phases.

‫ﻻ‬B, xB = mole fraction of component B in co-existing equilibrium
phases.
As β decreases, the number of extraction stages required for a given separation
increases; as capacity decreases, the amount of solvent required increases. In
practice, the choice of solvent is always a compromise, as β increases capacity
normally decreases. Solvent selection is a critical design step that depends on
the properties of the solutes to be recovered; there is no universally
applicable solvent. A common approach in solvent selection is to carry out a
literature survey of solvents used in similar applications. More erudite
approaches may be based on hydrogen bonding tendencies or the determination
of activity coefficients at infinite dilution by gas-liquid chromatography.

In liquid-liquid extraction, the partition (or distribution) coefficient gives an
indication of the ease of separation. The partition coefficient is defined as:

where C is the concentration of solute in phase I and phase II, respectively.
Ideally, K is independent of the concentration of solute and of the ratio of the two
immiscible phases.
If the required separation cannot be achieved using one extraction stage the
generally favored mode of operation is in a counter-current system, where the
solvent and feed travel in opposite directions. For such a system Appendix E
allows a rapid means of estimating the number of theoretical extraction stages at
a given solvent to feed ratio to achieve a required separation.
However, even with an infinite number of stages the richest extract layer is that in
equilibrium with feed. As a rule the richer the feed in extractable components the
richer will be the equilibrium extract layer in these substances and the sharper
the separation. The shortcomings of liquid-liquid extraction without reflux are
therefore obvious, this applies particularly to a feed lean in extractable
components.
What cannot be achieved by increasing the number of stages can be
accomplished by means of reflux. In a feed lean in extractable components the
use of extract reflux would give a sharper separation. Extract reflux involves
returning part of the extract phase from which the solvent has been completely
removed (the solute) or partly removed. The concentration of solute in the extract
layer is then greater than that in equilibrium with the feed. For example, Appendix
F illustrates a solvent extraction process using a high boiling solvent for the
separation of aromatics from non-aromatics. Part of the aromatic extract phase is
returned to the extractor as reflux.
In extractors operating with reflux the feed enters an intermediate point in the
system. Reflux return should not give a completely miscible system. The use of
reflux results in an increased energy requirement.


Three basic types of unit are available.
(a)

Mixer-settlers:
This is the name given to a type of extractor made up of a number of
mixing and settling chambers connected alternately in series. These are
normally only used when a few extraction stages are required. Scale up is
good and they have a good turn down ratio.

(b)

Packed and plate columns:
Probably the most widely used for simple systems requiring a small
number of stages and at low throughput. The system is good in that there
are no moving parts. Some caution should be exercised in the scale up
process.

(c)

Mechanically agitated columns:
As throughput and a larger number of stages becomes important,
mechanically agitated equipment should be used. A variety of proprietary
devices are available, including rotating disc contactors (RDC’s), the
Kuhni extractor etc. These systems offer a lower height per theoretical
extraction stage and also flexibility in terms of throughput and phase ratio.

In practice, liquid-liquid extraction is often used in conjunction with extractive
distillation; for example in the recovery of aromatics from hydrocarbon mixtures.
In this system the overheads from the extractive distillation column, consisting
mainly of light paraffins and naphthenes, are returned as reflux to the solvent
extraction column. They act as a backwash to remove heavier non-aromatics,
which would be more difficult to remove in the extractive distillation stage.
As in azeotropic and extractive distillation, the use of liquid-liquid extraction leads
to a more complex separation train. A distillation column is required to remove
the extracted components from the solvent. The raffinate may require water
washing to recover small amounts of solvent present. Quite often a further small
distillation column is required for clean up of a solvent purge.
Ideally, the system would use as solvent a component already present in the
process.
The presence of minor contaminants, in particular surfactants, can have a major
influence on the process. Before establishing final design a laboratory or semitechnical scale simulation should be carried out using the selected equipment,
actual process streams and at the proposed operating conditions.


Treybal, Liquid Extraction, McGraw-Hill, New York, 1963.
Reissinger, Schroeter, Modern Liquid-Liquid Extractors: Review and Selection
Criteria, I Chem E Symposium Series No 54.
The following Separation Processes Service reports (Harwell, Warren Spring):
SAR1 Liquid-liquid extraction, June 1974.
DR6 Selection of solvents for liquid-liquid extraction processes. Part 1 Organic
systems, October 1978.
An interesting paper of general applicability compares the selectivities in
fractional distillation, extractive distillation and solvent extraction which are
required to give equal plant cost. (Souders, Mott, Chem Eng Prog, 60, No 2, 75
(1964)).
Although published in 1964 the findings presented in Appendix G are still of
interest. The comparison assumes 67% solvent concentration and four times as
much liquid in extractive distillation and in solvent extraction as in fractional
distillation. Thus, for example, for the cost of separation to be the same the
relative volatility in fractional distillation would be 1.5, that in extractive distillation
2.0 and the selectivity factor for liquid-liquid extraction would be 6.0.
7 OTHER COMMERCIAL METHODS OF SEPARATION
The following sub clauses 7.1 to 7.9 (inclusive) outline other methods of
separation that have been used, or have obvious potential to be used,
commercially. They may be worth considering for specialized purposes. The
methods are not listed in any order of merit.
7.1 Adsorption
Adsorption is a physical phenomenon which occurs when gas or liquid molecules
are brought into contact with a solid surface. There are two main categories of
adsorption. The first type, and generally of primary interest, is known as physical
or van der Waals adsorption where the interaction between the solid and the
condensed molecule is relatively weak.


In this process the equilibrium between solid and condensed molecule is
reversible and is rapidly attained when the temperature and pressure are
changed. The second type is called activated adsorption or chemisorption where
interaction is strong, the bonds formed being almost as strong as those in
chemical compounds. This type of adsorption is often irreversible.
Important adsorbents include activated carbon, aluminium oxide, silica gel and
molecular sieves. The latter are widely used in commercial applications.
Molecular sieves function as physical adsorbents, they are highly efficient, easily
regenerable, crystalline silica-aluminas. They can facilitate separation by two
mechanisms :
(a) conventional adsorption where the sieve shows a strong preference for
polar compounds, particularly water;
and
(b) separation by size where only those molecules able to migrate through the
sieve’s pore or window opening are retained. The pore size of a particular
molecular sieve can be controlled accurately within a small range of
molecular dimensions.
The adsorption process requires cyclic operation, adsorption being followed by
desorption to recover the adsorbed species. The length of operating cycle
determines the number of beds which should be used to allow continuous
operation. Desorption is usually the most inefficient step in the cycle and can be
accomplished by means of thermal swing, pressure swing, purge gas stripping or
displacement cycles.
Adsorption is a technique widely used to remove impurities from various process
streams, for example in drying, sweetening and color removing operations.
However, in the last quarter of a century, largely due to the availability of
synthetic molecular sieves, adsorption has become established for specific bulk
separations difficult to achieve by other means. In particular the separation of
normal paraffins from admixture with other hydrocarbons (the vapor phase IsoSiv
process and the liquid phase Molex process), the separation of p-xylene from
other C8 aromatics (the Parex process) and the recovery of hydrogen from
process gas streams.


Edition, Vol 1, 531, 563 (1978).
Perry, Chemical Engineers Handbook, 6th Edition, McGraw-Hill Book Company,
16-5 (1984).
7.1.1 Fractional Crystallization
Crystallization processes make use of solid-liquid equilibria to effect a separation.
A liquid mixture cooled past its freezing point produces a solid phase, different in
composition from the mother liquor. Separating the two phases and remelting the
solid phase gives a purified product. Numerous types of solid-liquid equilibria are
known, the eutectic type is the most important industrially.
In a eutectic system, if the solid could be perfectly separated from the liquid
phase, 100% pure product could be produced in one stage. This is not achieved
in practice because of mother liquor adhering to the crystal surface, or held in the
crystal mass by surface tension and capillary forces, or being occluded in crystal
imperfections.
A conventional crystallization process would involve three stages:
(a) A crystallization stage where crystals are formed either in cooling tanks
with agitation for long periods or in scraped surface chillers.
(b) A separation stage, usually effected by centrifuges or filters.

(c) A purification stage.
Purification is usually carried out by using a wash liquor (chosen to be easily
separable from the required component) or by continuous countercurrent
treatment of the impure crystal mass with some of the melted crystal product (eg
the Phillips pulsed column).
Various continuous fractional crystallization devices have been proposed which
claim to produce high purity product in a single piece of equipment. Although
apparently proven on the laboratory and pilot plant scale they have not been
used on large tonnage commercial plant.


Although heats of fusion are much lower than heats of vaporization,
crystallization is an expensive technique and its use is consequently limited. It is
applied where components having very close boiling points and similar chemical
structure (usually isomers) are to be separated.
Edition, Vol 7, 243 (1978).
Separation Processes Service report (Harwell, Warren Spring), SAR2
(Reviewed), Industrial crystallization, March 1986.
7.1.2 Ion Exchange
Ion exchange is exactly what the name implies, an exchange of one ion for
another. Ion exchange resins are insoluble electrolytes, consisting of a high
concentration of polar or functional groups incorporated in a synthetic, resinous
polymer. The polar groups may be acidic (cation exchange resins) or basic
(anion exchange resins).
The strongly acidic cation exchange resins contain, for example, the sulphonic
group -S03H; the weakly acidic resins contain the carboxyl group -COOH. The
latter only has a useful capacity in neutral or alkaline solutions. The strongly
basic anion exchange resins contain the quaternary ammonium group - NR+30H; the weakly basic resins contain amino (NH2), mono – and di-substituted amino
groups. The latter can only usefully be used in neutral or acid solutions.
Important factors which influence the rate and extent of an ion exchange process
are the nature of the resin and the molecular size, valency, and concentration of
the ions to be absorbed. Reduction in the particle size of the resin results in an
increased rate of exchange, consistent with a diffusion controlled process.
Ion exchange allows the removal of one or more ionic species from a liquid
phase by means of an exchange, or transfer, for another ion. This transfer may
be required to purify or modify the liquid phase, to concentrate, isolate and/or
purify one or more of the ionic components, or to separate mixed ionic species
into two or more fractions. The use of ion exchange resins for water treatment
is widely recognized. Provided ions are present ion exchange processes can be
carried out in aqueous-organic systems and in non-aqueous solvents.


Schweitzer, Handbook of Separation Techniques for Chemical Engineers,
McGraw-Hill Book Company, 1979.
Li (Editor), Recent Advances in Separation Techniques -II, A.I.Ch.E Symposium
Series, Vol 76, No 192, 60 (1980).
7.4

Membrane Processes

The use of membrane separation processes has been mooted for some
considerable time. Advances have been made in some areas (noticeably gas
separation) but wider application still awaits the development of membranes
capable of giving high throughputs at high selectivities, whilst maintaining good
chemical and physical stability.
7.4.1 Ultrafiltration
Molecular filtration or ultrafiltration involves a sieving mechanism. The solvent
permeates the membrane by Poiseuille type flow down microcapillaries within the
membrane, and solute molecules are rejected because they are larger than the
pores through which the solvent molecules can pass.
Ultrafiltration can reject materials of molecular weight down to about 500. A
range of membranes is available with molecular weight retentions from 500 to
300,000. Because of the high molecular weights osmotic pressures are low and
operating pressures of only 5 to 60 psi g are required.
In general, this technique is excellent for concentration and may also be used to
separate two solutes (one able to pass through, the other held back by the
appropriate membrane). The major problem with ultrafiltration is membrane
fouling.
Ultrafiltration is widely used commercially. For example, in biological applications,
in the food industry (dewatering), the dairy product industry (cheese whey
treatment), the pharmaceutical industry (concentration and separation on
molecular weight basis) and in the purification and recovery of electrophoretic
paints.


7.4.2 Reverse Osmosis
Osmosis occurs whenever a solution is separated from its solvent, or from a
more dilute solution, by a membrane and is the phenomenon of spontaneous
flow of solvent through the membrane and into the more concentrated solution. In
osmosis the membrane is normally considered to be semipermeable, that is it
allows the passage of solvent but not the solute. The osmotic pressure of a
solution is that pressure which has to be applied to the solution to stop the
osmotic flow of solvent.
Application of a hydrostatic pressure in excess of the osmotic pressure results in
flow of solvent from the solution into the pure solvent. This process is called
reverse osmosis (and on occasions hyperfiltration).
Transport of material through membranes under reverse osmosis conditions is
via a sorption diffusion process. The criterion for solubility is that like dissolves
like. Rejection of solute occurs largely because of its low solubility in the
membrane. Thus, it is the chemical similarity between diffusing species and
membrane that determine the direction of separation. The rate of permeation
is inversely proportional to the membrane thickness and is directly proportional to
the difference between applied and osmotic pressure. As pressure has little
effect on the solute rate, selectivity appears to improve as the applied pressure is
increased. The process is usually operated at 400 to 600 psi (25 to 40 bar)
pressure.
Reverse osmosis is used commercially for desalination and for brackish water
treatment (75% to 85% treated water recovery is normal in the latter). Salt
rejection is high at 95 to 99% in water treatment. However, selectivity factors for,
say, isopropanol-water separation using a cellulose acetate membrane under
reverse osmosis conditions tumble to less than 2. In practice, there is
still not a suitable membrane for the separation of low molecular weight organic
mixtures. The technique is used by Organics Division for the concentration of
Procion T-dyes.
In the long term this technique may be developed such that it will find application
in:
(a) Removing the bulk of a material away from trace impurities which may
have unwanted effects, eg odor, color.
(b) Removing solvent from a homogeneous catalyst stream to allow catalyst
recycle.

(c) Processing industrial aqueous effluents to renovate the water.

7.4.3 Pervaporation
In the pervaporation process a liquid mixture is brought into contact with one side
of a polymeric film, the downstream side being maintained at a low pressure.
Separation is affected if one component permeates through the membrane at a
relatively faster rate. The temperature at which the process can be operated is
limited by the nature of the membrane material. It is preferred to heat the liquid
feed mixture to increase the vapor pressure and thus the rate of permeation. The
permeate is removed as a vapor.
A variation on this technique is Perstraction in which the permeate is dissolved
and carried away in a fluid diluent which does not interact with the membrane.
Like reverse osmosis the transport mechanism in pervaporation is a sorptiondiffusion process.
Very simply it proceeds via:
(a) Solution of the permeating molecules in the membrane;
(b) Diffusion through the membrane;
(c) Evaporation from the downstream surface of the membrane.

Selectivity is, therefore, due principally to preferential sorption. The rate of
permeation is inversely proportional to membrane thickness and is also
dependent on temperature. Pervaporation can be considered complementary to
reverse osmosis in that it is best suited to permeation of the component present
at low levels in a mixture rather than the major component, as in reverse
osmosis.
The use of pervaporation has been proposed for more than a quarter of a century
and small, 6m3/day, commercial units are now in operation. The main reported
usages of pervaporation are in the dehydration of alcohols, ketones and ethers.


7.4.4 Liquid Membranes
A liquid membrane, or more accurately a liquid surfactant membrane, is a film
formed at an oil-water interface by a surfactant solution. Such films are formed by
dispersing the solution to be separated in the form of very small droplets in a
surfactant solution. The droplets covered with liquid membrane are then
contacted with an organic solvent. One of the components of the mixture
permeates through the liquid membrane at a faster rate than the other. The
solvent therefore becomes richer in this component whilst the droplets become
richer in the less permeable component. To increase drop stability and
permeation rate the drop diameter is normally reduced by emulsifying the feed in
the surfactant solution. The emulsion is then mixed with the solvent which is the
continuous phase.
Separation is achieved by selective diffusion of one component through the liquid
membrane into the liquid of lower concentration. Once separation is effected, the
three phases can be separated by first settling the emulsion and continuous
phase and then breaking the emulsion.
Liquid membranes are purported to have several advantages over solid
polymeric membranes. They do not have pin holes, do not have to be replaced or
repaired and require no mechanical support. However, as yet, there is no
commercial exploitation of this technique. This is also true for the proposed
supported liquid membrane configuration. In this system the liquid membrane is
incorporated within an inert microporous support. Possible applications may be
found in:
(a) Separation of species which are chemically different;
(b) Recovery of products from low conversion processes by permeation of
components present in least amount;
(c) Removal of trace impurities, especially in waste water treatment.
7.4.5 Gas Permeation
One area in which membrane processes are competitive at the present time is in
the separation of gases. The most widely reported use is that of the Monsanto
Prism separator for hydrogen purification. High pressure feed gas is supplied to
one side of the membrane. Permeate accumulates on the membrane low
pressure side.

Again mechanisms describing gas transport generally involve solubilization and
diffusion. The diffusion rate depends on the size of the gas molecule and the gas
solubility in the polymer, with gas partial pressure as the mass transfer driving
force.
For the Monsanto system where the active membrane is a polysulfone, the
following is a list of 'fast’ and ’slow’ permeating gases:
Fast gas
Hydrogen
Helium
Hydrogen sulfide
Carbon dioxide
Water vapor

Slow gas
Oxygen
Methane, ethane etc
Carbon monoxide
Nitrogen
Ethylene, propylene etc

In general, gas separation processes are good for enriching; say from 50% to
90%. The feed pressure should be greater than 150 psi (10 bar) and temperature
in the range 10 to 50°C. They are not so good for obtaining a high purity product,
say 99%, or at feed pressures below 150 psi (10 bar) and temperatures greater
than 100°C. The polysulfone membrane is susceptible to certain aggressive
gases, eg, methanol, ammonia, acid gases and aromatics.
Gas permeation for hydrogen purification applications has successfully competed
with established processes - cryogenics, pressure swing adsorption. The latter
still has the edge where pure hydrogen is required. With industry gaining
confidence from the commercial ventures, gas permeation technology could
develop rapidly and other separations may become a reality on the large scale.
7.4.6 Dialysis
Dialysis involves the use of a membrane which selectively separates the solutes
in a solution by allowing the low molecular weight solutes to permeate through
the membrane into the pure solvent. The membrane restricts the passage of high
molecular weight solutes. At the same time, solvent will diffuse by osmosis in the
opposite direction. By periodically replacing the fresh solvent, complete
extraction of the diffusing solute can be achieved.
Concentration gradients provide the driving force and the nature of the
membrane establishes the selectivity in this diffusion process. The type of
membrane determines whether dialysis, a two way flow, or osmosis, flow of
solvent only into the concentrated solution, occurs.


The most widely known use of dialysis is in artificial kidney machines. The
relative slowness of the dialysis process gives it little scope for industrial usage.

7.4.7 Electrodialysis
Electrodialysis involves the transport of ionic species across a permselective
membrane under the driving force of an electric gradient. Normally alternating
anion and cation selective membranes are used. This allows the separation of
ionic substances present in the feed stream.
The largest commercial application for electrodialysis is in the treatment of
brackish water to produce a potable water and in desalination generally. It is
more economic than reverse osmosis at high salt concentrations. It is also used
in the dairy and pharmaceutical industries.
Torrey, Membrane and Ultrafiltration Technology, Noyes Data Corporation, 1984.
Hwang, Kammermeyer, Membranes in Separation, Vol VII, Techniques of
Chemistry, Wiley and Sons, 1984.

7.5

Supercritical Fluid Extraction

Supercritical fluid extraction refers to the use of fluids which are gases at ambient
temperature and pressure, but which become good solvents when compressed
to supercritical fluids (at pressures above the critical pressure). The supercritical
fluid region is loosely defined as being in the range of reduced temperatures
(actual temperature divided by critical temperature) of 0.9<Tr<1.4, and
reduced pressures of 1.0<Pr<5.0.
The properties of a supercritical fluid are between those of a liquid and a gas.
Fluids possess high solubilities similar to liquid solvents because of high
densities (specific gravities of 0.2 to 0.9). Viscosities and diffusivities of fluids are
intermediate to those properties for liquids and gases, this enables highly
efficient penetration and rapid mass transfer compared to liquid solvents. In the
supercritical fluid region relatively small changes in temperature and pressure
produce large changes in density and hence in solvent power.


Carbon dioxide has been widely used as a supercritical fluid extractant as it is
non-toxic, nonflammable, inexpensive and has a conveniently low critical
temperature of 31°C. Other gases, or combination of gases, can be used
depending on the extraction requirement.
Supercritical fluid extraction is similar to liquid-liquid extraction and can be carried
out using a countercurrent extraction system. The highly volatile solvent can be
recovered by letting down the pressure and/or by distillation before
recompressing and returning to the extraction system. The process requires
capital-intensive, high pressure equipment which should be evaluated against
any potential energy savings.
Probably the two most quoted commercial applications of supercritical fluid
extraction are:
(a) The Residual Oil Supercritical Extraction (ROSE) process used for
deasphalting oil residues with pentane in the 1950’s;
And
(b) The decaffeination of green coffee beans using carbon dioxide introduced
in the late seventies.
Other uses have been claimed including the separation of organic chemicals
from water, oils from natural products and in polymer processing.
Separation Processes Service report (Harwell, Warren Spring), SAR48,
Supercritical extraction and other high pressure extraction processes, September
1983.
Paulaitis et al, MIT Industrial Liaison Program, Report 9-33-82, Supercritical Fluid
Extraction, April 1982.

7.6

Dissociation Extraction

Dissociation extraction exploits the differences in the dissociation constants of
the components of a mixture in order to effect a separation.


Typically, if a mixture of two weak organic bases (differing in their dissociation
constants) in an organic solvent is contacted with an aqueous phase containing a
stoichiometric deficiency of a strong acid, relative to the bases, then the bases
will compete for the available acid. The stronger base, ie that with the higher
dissociation constant, will react preferentially with the strong acid forming a salt in
the aqueous phase. The weaker base will be consequently enriched in the
organic phase. Products of high purity can be obtained if a multi-stage countercurrent process is adopted, as in liquid-liquid extraction. A mixture of weak acids
can be separated in similar fashion using a strong base.
This separation technique has been used to separate isomeric mixtures which
could not be practically achieved using distillation or crystallization methods.
Such isomers often exhibit considerable differences in their dissociation
constants, eg 3- and 4-picoline and meta- and paracresol.
Hanson (Editor), Recent Advances in Liquid-Liquid Extraction, Pergamon, New
York, Chapter 4 (1971).

7.7

Foam Fractionation

Foam fractionation is dependent on the preferential concentration at the liquidgas interface of a naturally surface-active molecule. This species can be
separated from the bulk simply by providing sufficient interface and collecting the
resultant foam. A surface-inactive material can be removed by complex formation
with a suitable surfactant.
Separation at the normal air-water foam interface is affected by numerous factors
including:
(a) Concentration of the surfactant.
(b) pH.

(c) Temperature.
(d) Viscosity.

(e) Flow rates.

(f) Bubble size etc.

Foam fractionation is particularly useful when the concentration of the material to
be removed is low. The technique has been considered for the removal/recovery
of detergents, alcohols and phenol from waste water streams.
Schweitzer, Handbook of Separation Techniques for Chemical Engineers,
McGraw-Hill Book Company, 1979.

7.8

Clathration

Clathrates (or adducts) are inclusion or cage-like compounds which can be
considered as organic molecular sieves. Organic clathrates can trap other
molecules in the cavities of their regular geometric structure. The resultant
crystalline molecular complex is stable although normal chemical bonding is not
present. The formation and dissociation of the complex can therefore be
achieved with small changes in temperature and pressure.
Different types of clathration agents have been identified. They exist in several
forms ranging from spherical cavities, layer complexes, crystals with
interconnecting chambers and tubular structures.
The ability of clathrates to separate molecules on the basis of their shape has
been used commercially. Urea adducts were used to separate normal from
branched-chain paraffins. This separation is now carried out using synthetic
zeolites (inorganic molecular sieves) which afford a cleaner, simpler to operate
total system.
Bhatnagar, Clathrate Compounds, Chand and Company (1968).


7.9

Chromatography

Separations by chromatographic methods depend upon the different attraction
which two alternative phases have for the components in a feed mixture. The two
phases can be gas-solid, liquid-solid, liquid-liquid or gas-liquid. The latter is the
system commonly used in analytical applications.
In gas-liquid chromatography the mixture to be separated is vaporized and
passed together with a continuous stream of nitrogen or other inert gas through
the column. The column is packed with a solid impregnated with a non-volatile
liquid. The stronger the attraction between this stationary liquid phase and the
feed components, the more slowly is a given component swept through the
column by the inert gas (or carrier gas). The mixture therefore separates into
pure components as discrete bands within the column. These bands are eluted
from the column by the carrier gas.
Although widely used in the laboratory for analytical and preparative purposes,
chromatography has found limited commercial application. This is largely due to
the difficulty of obtaining good resolution in large diameter columns. The use of
chromatographic techniques on an industrial scale has been proposed for the
separation of close boiling mixtures (particularly isomers), the fractionation of
natural products and the purification of pharmaceutical intermediates and
products.
Schupp, Gas Chromatography, Technique of Organic Chemistry, Vol XIII,
Interscience Publishers (1968).

8 OTHER METHODS OF SEPARATION
The details contained in sub clauses 8.1 to 8.10 (inclusive) are included more for
completeness than practical usefulness. Some of the methods mentioned are,
and are likely to remain, laboratory oddities. Others have extremely limited usage
and are not strictly within the scope of this Engineering Guide.


8.1.1 Precipitation
A chemical reactant is added to a liquid mixture and reacts with one of the
components to form an insoluble precipitate.

8.1.2 Paper Chromatography
A liquid mixture is separated by differences in solubilities and adsorption
potentials on paper (or gel phase).

8.1.3 Ligand Specific Chromatography
A liquid mixture is contacted with an immobilized ligand which forms a reversible
chemical interaction with one of the components.
8.4

Electrophoresis

An electrical potential applied to colloidal systems dispersed in buffered solutions
in a cell causes the colloidal particles to migrate toward the electrodes according
to their charge.
8.5

Isoelectric Focusing

Carrier ampholyte mixtures are electrophoresed to establish a stable pH
gradient. The amphoteric macromolecules eg proteins, to be separated migrate
until they reach their isoelectric point; the pH at which the positive and negative
charges balance. The separation is therefore on the basis of composition rather
than size.

8.6

Thermal Diffusion

Components of a homogeneous solution (gas or liquid) are separated by means
of a temperature gradient. In a gas mixture the heavier molecules concentrate in
the cold region, in liquids molecular shape determines the separation.


8.7

Sedimentation Ultracentrifugation

The use of centrifuges rotated at high speeds that cause the rapid sedimentation
of macromolecules and allow, for example, the separation of large polymeric
substances according to molecular weight.
8.8

Isopycnic Ultracentrifugation

Biological substances are separated in high rotation centrifuges in which a
density gradient has been established. Use of the proper gradient material allows
particulates to be banded together isopycnically in the density gradient.

8.9

Molecular Distillation

In molecular distillation, the distance between evaporating and condensing
surfaces is less than the mean free path of the molecules involved at the
pressure used, normally high vacuum.
8.10 Gel Filtration
The separation of components is effected by the difference in their molecular size
and hence their ability to penetrate a swollen gel matrix.


DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents:
ENGINEERING GUIDES
GBHE-PEG-MAS-601 VLE Data : Selection and Use (referred to in 5.1)
GBHE-PEG-MAS-603 Shortcut Methods of Distillation Design (referred to in 5.1)
GBHE-PEG-MAS-607 Batch Distillation (referred to in 5.1)
.


Fluid Separation

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