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Handouts
Lecture: 1
Distillation is defined as: A process in which a liquid or vapor mixture of two or more substances is
separated into its component fractions of desired purity, by the application and removal of heat.
Distillation is based on the fact that the vapor of a boiling mixture will be richer in the components
that have lower boiling points.
Therefore, when this vapor is cooled and condensed, the condensate will contain more volatile
components. At the same time, the original mixture will contain more of the less volatile material.
Distillation columns are designed to achieve this separation efficiently.
Although many people have a fair idea what “distillation” means, the important aspects that seem to
be missed from the manufacturing point of view are that:
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Distillation is the most common separation technique
It consumes enormous amounts of energy, both in terms of cooling and heating
requirements
It can contribute to more than 50% of plant operating costs
The best way to reduce operating costs of existing units, is to improve their efficiency and operation
via process optimization and control. To achieve this improvement, a thorough understanding of
distillation principles and how distillation systems are designed is essential.
The purpose of this set of notes is to expose you to the terminology used in distillation practice and
to give a very basic introduction to:
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Types of columns
Basic distillation equipment and operation
Column internals
Reboilers
Distillation principles
Vapor liquid equilibrium
Distillation column design and
Factors that affect distillation column operation
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Lecture: 2,3
TYPES OF DISTILLATION COLUMNS
There are many types of distillation columns, each designed to perform specific types of
separations, and each design differs in terms of complexity.
Batch and Continuous Columns
One way of classifying distillation column type is to look at how they are operated. Thus we have:
batch and continuous columns.
Batch Columns
In batch operation, the feed to the column is introduced batch-wise. That is, the column is charged
with a 'batch' and then the distillation process is carried out. Whe the desired task is achieved, a next
batch of feed is introduced.
Continuous Columns
In contrast, continuous columns process a continuous feed stream. No interruptions occur unless
there is a problem with the column or surrounding process units. They are capable of handling high
throughputs and are the most common of the two types. We shall concentrate only on this class of
columns.
Types of Continuous Columns
Continuous columns can be further classified according to: the nature of the feed that they are
processing,
 binary column - feed contains only two components
 multi-component column - feed contains more than two components
the number of product streams they have multi-product column - column has more than two
product streams where the extra feed exits when it is used to help with the separation,
extractive distillation - where the extra feed appears in the bottom product stream
azeotropic distillation - where the extra feed appears at the top product stream
The type of column internals
Tray column - where trays of various designs are used to hold up the liquid to provide better contact
between vapor and liquid, hence better separation
Packed column - where instead of trays, 'packings' are used to enhance contact between vapor and
liquid
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Lecture: 4
BASIC DISTILLATION EQUIPMENT AND OPERATION
Main Components of Distillation Columns
Distillation columns are made up of several components, each of which is used either to transfer
heat energy or enhance material transfer. A typical distillation contains several major components:
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Vertical shell where the separation of liquid components is carried out
Column internals such as trays/plates and/or packings which are used to enhance component
separations
Reboiler to provide the necessary vaporization for the distillation process
Condenser to cool and condense the vapor leaving the top of the column
Reflux drum to hold the condensed vapor from the top of the column so that liquid (reflux)
can be recycled back to the column
The vertical shell houses the column internals and together with the condenser and reboiler,
constitute a distillation column. A schematic of a typical distillation unit with a single feed and two
product streams is shown below:
Basic Operation and Terminology
The liquid mixture that is to be processed is known as the feed and this is introduced usually
somewhere near the middle of the column to a tray known as the feed tray. The feed tray divides the
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column into a top (enriching or rectification) section and a bottom (stripping) section. The feed
flows down the column where it is collected at the bottom in the reboiler.
Heat is supplied to the reboiler to generate vapor. The source of heat input can be any suitable fluid,
although in most chemical plants this is normally steam. In refineries, the heating source may be the
output streams of other columns. The vapor raised in the reboiler is re-introduced into the unit at
the bottom of the column. The liquid removed from the reboiler is known as the bottoms product or
simply, bottoms.
The vapor moves up the column, and as it exits the top of the unit, it is cooled by a condenser. The
condensed liquid is stored in a holding vessel known as the reflux drum. Some of this liquid is
recycled back to the top of the column and this is called the reflux. The condensed liquid that is
removed from the system is known as the distillate or top product.
Thus, there are internal flows of vapor and liquid within the column as well as external flows of
feeds and product streams, into and out of the column.
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Lecture: 5,6
DISTILLATION PRINCIPLES
Separation of components from a liquid mixture via distillation depends on the differences in
boiling points of the individual components. Also, depending on the concentrations of the
components present, the liquid mixture will have different boiling point characteristics. Therefore,
distillation processes depends on the vapour pressure characteristics of liquid mixtures.
Vapor Pressure and Boiling
The vapour pressure of a liquid at a particular temperature is the equilibrium pressure exerted by
molecules leaving and entering the liquid surface. Here are some important points regarding vapor
pressure:
 Energy input raises vapor pressure
 Vapor pressure is related to boiling
 Liquid is said to ‘boil’ when its vapor pressure equals the surrounding pressure
 The ease with which a liquid boils depends on its volatility
 Liquids with high vapor pressures (volatile liquids) will boil at lower temperatures
 the vapor pressure and hence the boiling point of a liquid mixture depends on the relative
amounts of the components in the mixture
 distillation occurs because of the differences in the volatility of the components in the liquid
mixture
The Boiling Point Diagram
The boiling point diagram shows how the equilibrium compositions of the components in a liquid
mixture vary with temperature at a fixed pressure. Consider an example of a liquid mixture
containing 2 components (A and B) - a binary mixture. This has the following boiling point
diagram.
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The boiling point of A is that at which the mole fraction of A is 1. The boiling point of B is that at
which the mole fraction of A is 0. In this example, A is the more volatile component and therefore
has a lower boiling point than B. The upper curve in the diagram is called the dew-point curve while
the lower one is called the bubble-point curve.
The dew-point is the temperature at which the saturated vapour starts to condense.
The bubble-point is the temperature at which the liquid starts to boil.
The region above the dew-point curve shows the equilibrium composition of the superheated vapor
while the region below the bubble-point curve shows the equilibrium composition of the subcooled
liquid.
For example, when a subcooled liquid with mole fraction of A=0.4 (point A) is heated, its
concentration remains constant until it reaches the bubble-point (point B), when it starts to boil. The
vapors evolved during the boiling has the equilibrium composition given by point C, approximately
0.8 mole fraction A. This is approximately 50% richer in A than the original liquid.
This difference between liquid and vapor compositions is the basis for distillation operations.
Relative Volatility
Relative volatility is a measure of the differences in volatility between 2 components, and hence
their boiling points. It indicates how easy or difficult a particular separation will be. The relative
volatility of component ‘i’ with respect to component ‘j’ is defined as yi = mole fraction of
component ‘i’ in the vapour , xi = mole fraction of component ‘i’ in the liquid.Thus if the relative
volatility between 2 components is very close to one, it is an indication that they have very similar
vapour pressure characteristics. This means that they have very similar boiling points and therefore,
it will be difficult to separate the two components via distillation.
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Lecture: 7,8
VAPOUR LIQUID EQUILIBRIA
Distillation columns are designed based on the boiling point properties of the components in the
mixtures being separated. Thus the sizes, particularly the height, of distillation columns are
determined by the vapour liquid equilibrium (VLE) data for the mixtures.
Lecture : 9,10
DISTILLATION COLUMN DESIGN
As mentioned, distillation columns are designed using VLE data for the mixtures to be separated.
The vapour-liquid equilibrium characteristics (indicated by the shape of the equilibrium curve) of
the mixture will determine the number of stages, and hence the number of trays, required for the
separation. This is illustrated clearly by applying the McCabe-Thiele method to design a binary
column.
McCABE-THIELE DESIGN METHOD
The McCabe-Thiele approach is a graphical one, and uses the VLE plot to determine the theoretical
number of stages required to effect the separation of a binary mixture. It assumes constant molar
overflow and this implies that:
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molal heats of vaporisation of the components are roughly the same
heat effects (heats of solution, heat losses to and from column, etc.) are negligible
for every mole of vapour condensed, 1 mole of liquid is vaporised
The design procedure is simple. Given the VLE diagram of the binary mixture, operating
lines are drawn first.
Operating lines define the mass balance relationships between the liquid and vapour phases
in the column.
There is one operating line for the bottom (stripping) section of the column, and on for the
top (rectification or enriching) section of the column.
Use of the constant molar overflow assumption also ensures the the operating lines are
straight lines.
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Operating Line for the Rectification Section
The operating line for the rectification section is constructed as follows. First the desired top
product composition is located on the VLE diagram, and a vertical line produced until it intersects
the diagonal line that splits the VLE plot in half. A line with slope R/(R+1) is then drawn from this
instersection point as shown in the diagram below.
R is the ratio of reflux flow (L) to distillate flow (D) and is called the reflux ratio and is a measure
of how much of the material going up the top of the column is returned back to the column as
reflux.
Operating Line for the Stripping Section
The operating line for the stripping section is constructed in a similar manner. However, the
starting point is the desired bottom product composition. A vertical line is drawn from this point to
the diagonal line, and a line of slope Ls/Vs is drawn as illustrated in the diagram below.
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Ls is the liquid rate down the stripping section of the column, while Vs is the vapour rate up the
stripping section of the column. Thus the slope of the operating line for the stripping section is a
ratio between the liquid and vapour flows in that part of the column.
Lecture : 11,12
Equilibrium and Operating Lines
The McCabe-Thiele method assumes that the liquid on a tray and the vapour above it are in
equilibrium. How this is related to the VLE plot and the operating lines is depicted graphically in
the diagram on the right.
A magnified section of the operating line for the stripping section is shown in relation to the
corresponding n'th stage in the column. L's are the liquid flows while V's are the vapour flows. x
and y denote liquid and vapour compositions and the subscripts denote the origin of the flows or
compositions. That is 'n-1' will mean from the stage below stage 'n' while 'n+1' will mean from
the stage above stage 'n'. The liquid in stage 'n' and the vapour above it are in equilibrium,
therefore, xn and yn lie on the equilibrium line. Since the vapour is carried to the tray above without
changing composition, this is depicted as a horizontal line on the VLE plot. Its intersection with the
operating line will give the composition of the liquid on tray 'n+1' as the operating line defines the
material balance on the trays. The composition of the vapour above the 'n+1' tray is obtained from
the intersection of the vertical line from this point to the equilibrium line.
Number of Stages and Trays
Doing the graphical construction repeatedly will give rise to a number of 'corner' sections, and each
section will be equivalent to a stage of the distillation. This is the basis of sizing distillation columns
using the McCabe-Thiele graphical design methodology as shown in the following example.
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Given the operating lines for both stripping and rectification sections, the graphical construction
described above was applied. This particular example shows that 7 theoretical stages are required
to achieve the desired separation. The required number of trays (as opposed to stages) is one less
than the number of stages since the graphical construction includes the contribution of the reboiler
in carrying out the separation.
The actual number of trays required is given by the formula:
(number of theoretical trays)/(tray efficiency)
Typical values for tray efficiency ranges from 0.5 to 0.7 and depends on a number of factors, such
as the type of trays being used, and internal liquid and vapour flow conditions. Sometimes,
additional trays are added (up to 10%) to accomodate the possibility that the column may be underdesigned.
Lecture : 13,14
The Feed Line (q-line)
The diagram above also shows that the binary feed should be introduced at the 4'th stage. However,
if the feed composition is such that it does not coincide with the intersection of the operating lines,
this means that the feed is not a saturated liquid. The condition of the feed can be deduced by the
slope of the feed line or q-line. The q-line is that drawn between the intersection of the operating
lines, and where the feed composition lies on the diagonal line.
Depending on the state of the feed, the feed lines will have different slopes. For example,
q = 0 (saturated vapour)
q = 1 (saturated liquid)
0 < q < 1 (mix of liquid and vapour)
q > 1 (subcooled liquid)
q < 0 (superheated vapour)
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The q-lines for the various feed conditions are shown in the diagram on the left.
Using Operating Lines and the Feed Line in McCabe-Thiele Design
If we have information about the condition of the feed mixture, then we can construct the q-line and
use it in the McCabe-Thiele design. However, excluding the equilibrium line, only two other pairs
of lines can be used in the McCabe-Thiele procedure. These are:
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feed-line and rectification section operating line
feed-line and stripping section operating line
stripping and rectification operating lines
This is because these pairs of lines determine the third.
OVERALL COLUMN DESIGN
Determining the number of stages required for the desired degree of separation and the location of
the feed tray is merely the first steps in producing an overall distillation column design. Other things
that need to be considered are tray spacings; column diameter; internal configurations; heating and
cooling duties. All of these can lead to conflicting design parameters. Thus, distillation column
design is often an iterative procedure. If the conflicts are not resolved at the design stage, then the
column will not perform well in practice. The next set of notes will discuss the factors that can
affect distillation column performance.
Lecture : 15
EFFECTS OF THE NUMBER OF TRAYS OR STAGES
Here we will expand on the design of columns by looking briefly at the effects of
the number of trays, and the position of the feed tray, and on the performances of distillation
columns.
Effects of the Number of Trays
It can be deduced from the previous section on distillation column design that the number of trays
will influence the degree of separation. This is illustrated by the following example.
Consider as a base case, a 10 stage column. The feed is a binary mixture that has a composition of
0.5 mole fraction in terms of the more volatile component, and introduced at stage 5. The steadystate terminal compositions of about 0.65 at the top (stage 1) and 0.1 at the bottom (stage 10) are
shown below:
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Composition Profile: 10 stages, feed at stage 5
Suppose we decrease the number of stages to 8, and keep the feed at the middle stage, i.e. stage 4.
The resulting composition profile is:
Composition Profile: 8 stages, feed at stage 4
We can see that the top composition has decreased while the bottom composition has increased.
That is, the separation is poorer.
Now, if we increase the number of stages to 12, and again introduce the feed at mid-column, i.e.
stage 6, the composition profile we get is:
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Composition Profile: 12 stages, feed at stage 6
Again, the composition has changed. This time the distillate is much richer in the more volatile
component, while the bottoms has less, indicating better separation.
Thus, increasing the number of stages will improve separation.
Effect of Feed Tray Position
Here we look at how the position of the feed tray affects separation efficiency. Suppose we have a
20 stage column, again separating a binary mixture that has a composition of 0.5 mole fraction in
terms of the more volatile component. The terminal compositions obtained when the feed is
introduced at stages 5, 10 and 15 (at fixed reflux and reboil rates) are shown in the following plots.
Composition profile: 20 stages, feed at stage 5
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Composition profile: 20 stages, feed at stage 10
Composition profile: 20 stages, feed at stage 15
As the feed stage is moved lower down the column, the top composition becomes less rich in the
more volatile component while the bottoms contains more of the more volatile component.
However, the changes in top composition is not as marked as the bottoms composition.
The preceding examples illustrate what can happen if the position of the feed tray is shifted for this
particular system. They should not be used to generalise to other distillation systems, as the effects
are not straightforward.
Lecture : 16
FACTORS AFFECTING DISTILLATION COLUMN OPERATION
The performance of a distillation column is determined by many factors, for example:
 feed conditions
o state of feed
o composition of feed
o trace elements that can severely affect the VLE of liquid mixtures
 internal liquid and fluid flow conditions
 state of trays (packings)
 weather conditions
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Some of these will be discussed below to give an idea of the complexity of the distillation process.
Feed Conditions
The state of the feed mixture and feed composition affects the operating lines and hence the number
of stages required for separation. It also affects the location of feed tray. During operation, if the
deviations from design specifications are excessive, then the column may no longer be able handle
the separation task. To overcome the problems associated with the feed, some column are designed
to have multiple feed points when the feed is expected to containing varying amounts of
components.
Reflux
Conditions
As the reflux ratio is increased, the gradient of operating line for the rectification section moves
towards a maximum value of 1. Physically, what this means is that more and more liquid that is rich
in the more volatile components are being recycled back into the column. Separation then becomes
better and thus less trays are needed to achieve the same degree of separation. Minimum trays are
required under total reflux conditions, i.e. there is no withdrawal of distillate.
On the other hand, as reflux is decreased, the operating line for the rectification section moves
towards the equilibrium line. The ‘pinch’ between operating and equilibrium lines becomes more
pronounced and more and more trays are required. This is easy to verify using the McCabe-Thiele
method.
The limiting condition occurs at minimum reflux ration, when an infinite number of trays will be
required to effect separation. Most columns are designed to operate between 1.2 to 1.5 times the
minimum reflux ratio because this is approximately the region of minimum operating costs (more
reflux means higher reboiler duty).
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Lecture:17
Vapour Flow Conditions
Adverse vapor flow conditions can cause
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foaming
entrainment
weeping/dumping
flooding
Foaming
Foaming refers to the expansion of liquid due to passage of vapour or gas. Although it provides
high interfacial liquid-vapour contact, excessive foaming often leads to liquid buildup on trays. In
some cases, foaming may be so bad that the foam mixes with liquid on the tray above. Whether
foaming will occur depends primarily on physical properties of the liquid mixtures, but is
sometimes due to tray designs and condition. Whatever the cause, separation efficiency is always
reduced.
Entrainment
Entrainment refers to the liquid carried by vapour up to the tray above and is again caused by high
vapour flow rates. It is detrimental because tray efficiency is reduced: lower volatile material is
carried to a plate holding liquid of higher volatility. It could also contaminate high purity distillate.
Excessive entrainment can lead to flooding.
Weeping/Dumping
This phenomenon is caused by low vapour flow. The pressure exerted by the vapour is insufficient
to hold up the liquid on the tray. Therefore, liquid starts to leak through perforations. Excessive
weeping will lead to dumping. That is the liquid on all trays will crash (dump) through to the base
of the column (via a domino effect) and the column will have to be re-started. Weeping is indicated
by a sharp pressure drop in the column and reduced separation efficiency.
Flooding
Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the vapour up
the column. The increased pressure from excessive vapour also backs up the liquid in the
downcomer, causing an increase in liquid holdup on the plate above. Depending on the degree of
flooding, the maximum capacity of the column may be severely reduced. Flooding is detected by
sharp increases in column differential pressure and significant decrease in separation efficiency.
Column Diameter
Most of the above factors that affect column operation is due to vapour flow conditions: either
excessive or too low. Vapour flow velocity is dependent on column diameter. Weeping determines
the minimum vapour flow required while flooding determines the maximum vapour flow allowed,
hence column capacity. Thus, if the column diameter is not sized properly, the column will not
perform well. Not only will operational problems occur, the desired separation duties may not be
achieved.
State of Trays and Packings
Remember that the actual number of trays required for a particular separation duty is determined by
the efficiency of the plate, and the packings if packings are used. Thus, any factors that cause a
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decrease in tray efficiency will also change the performance of the column. Tray efficiencies are
affected by fouling, wear and tear and corrosion, and the rates at which these occur depends on the
properties of the liquids being processed. Thus appropriate materials should be specified for tray
construction.
Weather Conditions
Most distillation columns are open to the atmosphere. Although many of the columns are insulated,
changing weather conditions can still affect column operation. Thus the reboiler must be
appropriately sized to ensure that enough vapour can be generated during cold and windy spells and
that it can be turned down sufficiently during hot seasons. The same applies to condensors.
These are some of the more important factors that can cause poor distillation column performance.
Other factors include changing operating conditions and throughputs, brought about by changes in
upstream conditions and changes in the demand for the products. All these factors, including the
associated control system, should be considered at the design stages because once a column is built
and installed, nothing much can be done to rectify the situation without incurring significant costs.
The control of distillation columns is a field in its own right, but that's another story.
Lecture: 18
Steam distillation
Steam distillation is a special type of distillation (a separation process) for temperature sensitive
materials like natural aromatic compounds.
Many organic compounds tend to decompose at high sustained temperatures. Separation by normal
distillation would then not be an option, so water or steam is introduced into the distillation
apparatus. By adding water or steam, the boiling points of the compounds are depressed, allowing
them to evaporate at lower temperatures, preferably below the temperatures at which the
deterioration of the material becomes appreciable. If the substances to be distilled are very sensitive
to heat, steam distillation can also be combined with vacuum distillation. After distillation the
vapors are condensed as usual, usually yielding a two-phase system of water and the organic
compounds, allowing for simple separation.
Principle
When a mixture of two practically immiscible liquids is heated while being agitated to expose the
surfaces of both the liquids to the vapor phase, each constituent independently exerts its own vapor
pressure as a function of temperature as if the other constituent were not present. Consequently, the
vapor pressure of the whole system increases. Boiling begins when the sum of the partial pressures
of the two immiscible liquids just exceeds the atmospheric pressure (approximately 101 kPa at sea
level). In this way, many organic compounds insoluble in water can be purified at a temperature
well below the point at which decomposition occurs. For example, the boiling point of
bromobenzene is 156 °C and the boiling point of water is 100 °C, but a mixture of the two boils at
95 °C. Thus, bromobenzene can be easily distilled at a temperature 61 C° below its normal boiling
point.
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Applications
Steam distillation is employed in the manufacture of essential oils, for instance, perfumes. In this
method, steam is passed through the plant material containing the desired oils. It is also employed in
the synthetic procedures of complex organic compounds. Eucalyptus oil and orange oil are obtained
by this method on the industrial scale.
Steam distillation is also widely used in petroleum refineries and petrochemical plants where it is
commonly referred to as "steam stripping.
Other industrial uses of steam distillation include the production of consumer food products such as
sprayable or aerosolized condiments such as sprayable mayonnaise.
Lecture: 19
Extractive distillation is defined as distillation in the presence of a miscible, high boiling,
relatively non-volatile component, the solvent, that forms no azeotrope with the other components
in the mixture. The method is used for mixtures having a low value of relative volatility, nearing
unity. Such mixtures cannot be separated by simple distillation, because the volatility of the two
components in the mixture is nearly the same, causing them to evaporate at nearly the same
temperature at a similar rate, making normal distillation impractical.
The method of extractive distillation uses a separation solvent, which is generally nonvolatile, has a
high boiling point and is miscible with the mixture, but doesn't form an azeotropic mixture. The
solvent interacts differently with the components of the mixture thereby causing their relative
volatilities to change. This enables the new three-part mixture to be separated by normal distillation.
The original component with the greatest volatility separates out as the top product. The bottom
product consists of a mixture of the solvent and the other component, which can again be separated
easily because the solvent doesn't form an azeotrope with it. The bottom product can be separated
by any of the methods available.
It is important to select a suitable separation solvent for this type of distillation. The solvent must
alter the relative volatility by a wide enough margin for a successful result. The quantity, cost and
availability of the solvent should be considered. The solvent should be easily separable from the
bottom product, and should not react chemically with the components or the mixture, or cause
corrosion in the equipment. A classic example to be cited here is the separation of an azeotropic
mixture of benzene and cyclohexane, where aniline is one suitable solvent.
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Lecture:20
Azeotropic distillation
In chemistry, azeotropic distillation is any of a range of techniques used to break an azeotrope in
distillation. In chemical engineering, azeotropic distillation usually refers to the specific technique
of adding another component to generate a new, lower-boiling azeotrope that is heterogeneous (e.g.
producing two, immiscible liquid phases), such as the example below with the addition of benzene
to water and ethanol. In actual fact, this practice of adding an entrainer which forms a separate
phase is a specific sub-set of (industrial) azeotropic distillation methods, or combination thereof. In
some senses, adding an entrainer is similar to extractive distillation.
Example - distillation of ethanol/water
A common distillation with an azeotrope is the distillation of ethanol and water. Using normal
distillation techniques, ethanol can only be purified to approximately 96% strength of some
commercially available grain alcohols).
Once at a 96.4% ethanol/water concentration, the vapor from the boiling mixture is also 96.4%,
therefore further distillation is ineffective. Some uses require a higher percentage of alcohol, for
example when used as a gasoline additive. The 96.4% azeotrope needs to be "broken" in order to
refine further.
Material separation agent
The addition of a Material Separation Agent, such as benzene, to the Ethanol/Water Mixture,
changes the molecular interactions and eliminates the azeotrope (i.e. "breaking the azeotrope") .
Unfortunately, another separation is needed to remove the benzene. It is simpler to remove the
benzene from water via dehydration than to separate Ethyl past 96.4% via distillation.
Lecture:21,22
Importance and application of Humidification and Dehumidification operation, Vapour liquid
equilibria
Vapor-liquid equilibrium (sometimes abbreviated as VLE) is a condition where a liquid and its
vapor (gas phase) are in equilibrium with each other, a condition or state where the rate of
evaporation (liquid changing to vapor) equals the rate of condensation (vapor changing to liquid) on
a molecular level such that there is no net (overall) vapor-liquid interconversion. Although in theory
equilibrium takes forever to reach, such an equilibrium is practically reached in a relatively closed
location if a liquid and its vapor are allowed to stand in contact with each other long enough with no
interference or only gradual interference from the outside.
The concentration of a vapor in contact with its liquid, especially at equilibrium, is often in terms of
vapor pressure, which could be a partial pressure (part of the total gas pressure) if any other gas(es)
are present with the vapor. The equilibrium vapor pressure of a liquid is usually very dependent on
temperature. At vapor-liquid equilibrium, a liquid with individual components (compounds) in
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certain concentrations will have an equilibrium vapor in which the concentrations or partial
pressures of the vapor components will have certain set values depending on all of the liquid
component concentrations and the temperature. This fact is true in reverse also; if a vapor with
components at certain concentrations or partial pressures is in vapor-liquid equilibrium with its
liquid, then the component concentrations in the liquid will be set dependent on the vapor
concentrations, again also depending on the temperature. The equilibrium concentration of each
component in the liquid phase is often different from its concentration (or vapor pressure) in the
vapor phase, but there is a correlation. Such VLE concentration data is often known or can be
determined experimentally for vapor-liquid mixtures with various components. In certain cases such
VLE data can be determined or approximated with the help of certain theories such as Raoult's Law,
Dalton's Law, and/or Henry's Law.
Such VLE information is useful in designing columns for distillation, especially fractional
distillation, which is a particular specialty of chemical engineers. Distillation is a process used to
separate or partially separate components in a mixture by boiling (vaporization) followed by
condensation. Distillation takes advantage of differences in concentrations of components in the
liquid and vapor phases.
In mixtures containing two or more components where their concentrations are compared in the
vapor and liquid phases, concentrations of each component are often expressed as mole fractions. A
mole fraction is number of moles of a given component in an amount of mixture in a phase (either
vapor or liquid phase) divided by the total number of moles of all components in that amount of
mixture in that phase.
Binary mixtures are those having two components. Three-component mixtures could be called
ternary mixtures. There can be VLE data for mixtures with even more components, but such data
becomes copious and is often hard to show graphically. VLE data is often shown at a certain overall
pressure, such as 1 atm or whatever pressure a process of interest is conducted at. When at a certain
temperature, the total of partial pressures of all the components becomes equal to the overall
pressure of the system such that vapors generated from the liquid displace any air or other gas
which maintained the overall pressure, the mixture is said to boil and the corresponding temperature
is the boiling point (This assumes excess pressure is relieved by letting out gases to maintain a
desired total pressure). A boiling point at an overall pressure of 1 atm is called the normal boiling
point.
Lecture:23
Vapour gas mixtures, air-water system
Psychrometrics or psychrometry are terms used to describe the field of engineering concerned
with the determination of physical and thermodynamic properties of gas-vapor mixtures.
Although the principles of psychrometry apply to any physical system consisting of gas-vapor
mixtures, the most common system of interest is the mixture of water vapor and air, because of its
application in heating, ventilating, and air-conditioning and meteorology. In human terms, our
comfort is in large part a consequence of, not just the temperature of the surrounding air, but
(because we cool ourselves via perspiration) the extent to which that air is saturated with water
vapor.
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Lecture:24,25
Adiabatic saturation curves, concept of wet bulb and dry bulb temp, Lewis relation
Wet-bulb temperature
The thermodynamic wet-bulb temperature is a thermodynamic property of a mixture of air and
water vapor. The value indicated by a simple wet-bulb thermometer often provides an adequate
approximation of the thermodynamic wet-bulb temperature.
A wet-bulb thermometer is an instrument which may be used to infer the amount of moisture in the
air. If a moist cloth wick is placed over a thermometer bulb the evaporation of moisture from the
wick will lower the thermometer reading (temperature). If the air surrounding a wet-bulb
thermometer is dry, evaporation from the moist wick will be more rapid than if the air is moist.
When the air is saturated no water will evaporate from the wick and the temperature of the wet-bulb
thermometer will be the same as the reading on the dry-bulb thermometer. However, if the air is not
saturated water will evaporate from the wick causing the temperature reading to be lower.
The accuracy of a simple wet-bulb thermometer depends on how fast air passes over the bulb and
how well the thermometer is shielded from the radiant temperature of its surroundings. Speeds up to
5,000 ft/min (60 mph) are best but dangerous to move a thermometer at that speed. Errors up to
15% can occur if the air movement is too slow or if there is too much radiant heat present (sunlight,
for example).
A wet bulb temperature taken with air moving at about 1-2 m/s is referred to as a screen
temperature, whereas a temperature taken with air moving about 3.5 m/s or more is referred to as
sling temperature.
A psychrometer is a device that includes both a dry-bulb and a wet-bulb thermometer. A sling
psychrometer requires manual operation to create the airflow over the bulbs, but a powered
psychrometer includes a fan for this function.
Lecture: 26, 27
Water cooling with air, Dehumidification of air-water vapor, Water cooling towers
Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere.
Cooling towers may either use the evaporation of water to remove process heat and cool the
working fluid to near the wet-bulb air temperature or rely solely on air to cool the working fluid to
near the dry-bulb air temperature. Common applications include cooling the circulating water used
in oil refineries, chemical plants, power stations and building cooling. The towers vary in size from
small roof-top units to very large hyperboloid structures (as in Image 1) that can be up to 200
metres tall and 100 metres in diameter, or rectangular structures (as in Image 2) that can be over 40
metres tall and 80 metres long. Smaller towers are normally factory-built, while larger ones are
constructed on site.
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Lecture:28
Importance and application of drying operation
In the drying of solids, the desirable end product is in solid form. Thus, even though the solid is
initially in solution, the problem of producing this solid in dry form is classed under this heading.
Final moisture contents of dry solids are usually less than 10%, and in many instances, less than
1%.
The mechanism of the drying of solids is reasonably simple in concept. When drying is done with
heated gases, in the most general case, a wet solid begins to dry as though the water were present
alone without any solid, and hence evaporation proceeds as it would from a so-called free water
surface, that is, as water standing in an open pan. The period or stage of drying during this initial
phase, therefore, is commonly referred to as the constant-rate period because evaporation occurs at
a constant rate and is independent of the solid present. The presence of any dissolved salts will
cause the evaporation rate to be less than that of pure water. Nevertheless, this lower rate can still be
constant during the first stages of drying.
A fundamental theory of drying depends on knowledge of the forces governing the flow of liquids
inside solids. Attempts have been made to develop a general theory of drying on the basis that
liquids move inside solids by a diffusional process. However, this is not true in all cases. In fact,
only in a limited number of types of solids does true diffusion of liquids occur. In most cases, the
internal flow mechanism results from a combination of forces which may include capillarity,
internal pressure gradients caused by shrinkage, a vapor-liquid flow sequence caused by
temperature gradients, diffusion, and osmosis. Because of the complexities of the internal flow
mechanism, it has not been possible to evolve a generalized theory of drying applicable to all
materials. Only in the drying of certain bulk objects such as wood, ceramics, and soap has a
significant understanding of the internal mechanism been gained which permits control of product
quality.
Most investigations of drying have been made from the so-called external viewpoint, wherein the
effects of the external drying medium such as air velocity, humidity, temperature, and wet material
shape and subdivision are studied with respect to their influence on the drying rate. The results of
such investigations are usually presented as drying rate curves, and the natures of these curves are
used to interpret the drying mechanism.
When materials are dried in contact with hot surfaces, termed indirect drying, the air humidity and
air velocity may no longer be significant factors controlling the rate. The “goodness” of the contact
between the wet material and the heated surfaces, plus the surface temperature, will be controlling.
This may involve agitation of the wet material in some cases.
Drying equipment for solids may be conveniently grouped into three classes on the basis of the
method of transferring heat for evaporation. The first class is termed direct dryers; the second class,
indirect dryers; and the third class, radiant heat dryers. Batch dryers are restricted to low capacities
and long drying times. Most industrial drying operations are performed in continuous dryers. The
large numbers of different types of dryers reflect the efforts to handle the larger numbers of wet
materials in ways which result in the most efficient contacting with the drying medium. Thus, filter
cakes, pastes, and similar materials, when preformed in small pieces, can be dried many times faster
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in continuous through-circulation dryers than in batch tray dryers. Similarly, materials which are
sprayed to form small drops, as in spray drying, dry much faster than in through-circulation drying.
Lecture:29,30
Equilibrium relationship, definition of types of moisture content, Batch drying, rate of drying curve,
time of drying under constant condition
Terms related to level/nature of moisture in drying materials
 Bone Dry Material: Any material, which has been dried at sufficiently high temperature for
a prolonged time by well-established methods till it is deviled of all traces of moisture, is
called ‘Bone Dry Material’.
 Moisture Content: The loss of moisture under standard prescribed drying condition till bonedry stale is reached is termed as the ‘moisture content’ of the material and is usually
expressed as a fraction of moisture per kg of wet material (wet basis) or expressed as
fraction of moisture per kg of bone-dry material (bone dry basis). Moisture refers to water,
although other liquids may follow the same testing techniques.
 Moisture Gradient: In the bulk of material like in a thick felt or in the tray dryer, moisture
may not be uniformly distributed in all portions of the solid at a given moment during the
process of drying. The actual distribution/content of the moisture in the solid is termed as
moisture gradient.
 Bound Moisture: Liquid bound in the solid in its capillaries, by solution in its cells/walls, by
solution and by chemical/physical adsorption. It is to be noted that this bound moisture
exerts less vapour pressure (i.e. the drying force for evaporation) than that of pure liquid in
free condition at the same temperature.
 Equilibrium moisture content: It is the level of bound moisture in a given material which is
attained on stabilization under specified conditions of temperature and humidity either by
loosing excess moisture by drying or by absorbing moisture from surroundings.
 Free moisture: In a hygroscopic material, it is the moisture in excess of the equilibrium
moisture content at existing humidity and temperature and includes unbound as well as
bound moisture which can be removed.
 Critical moisture: Is the level of moisture content of a material when the rate of drying
changes from a constant level to a gradually reducing level.
The Drying Curve
For each and every product, there is a representative curve that describes the drying characteristics
for that product at specific temperature, velocity and pressure conditions. This curve is referred to as
the drying curve for a specific product. Figure shows a typical drying curve. Variations in the curve
will occur principally in rate relative to carrier velocity and temperature.
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Drying occurs in three different periods, or phases, which can be clearly defined. The first phase,
or initial period, is where sensible heat is transferred to the product and the contained moisture.
This is the heating up of the product from the inlet condition to the process condition, which enables
the subsequent processes to take place. The rate of evaporation increases dramatically during this
period with mostly free moisture being removed. In some instances, pre-processing can reduce or
eliminate this phase. For example, if the feed material is coming from a reactor or if the feed is
preheated by a source of waste energy, the inlet condition of the material will already be at a raised
temperature. The second phase, or constant rate period, is when the free moisture persists on the
surfaces and the rate of evaporation alters very little as the moisture content reduces. During this
period, drying rates are high and higher inlet air temperatures than in subsequent drying stages can
be used without detrimental effect to the product. There is a gradual and relatively small increase in
the product temperature during this period.
Interestingly, a common occurrence is that the time scale of the constant rate period may determine
and affect the rate of drying in the next phase. The third phase, or falling rate period, is the phase
during which migration of moisture from the inner interstices of each particle to the outer surface
becomes the limiting factor that reduces the drying rate.
Terms related to drying process
 Periods of Drying: As drying proceeds, moisture content and rate of drying change with
respect to time as follows. Initially the moisture evaporates from the saturated surface of a
solid. In this phase, the rate of drying per unit drying area is CONSTANT. At the end of
this, there is a decrease in the area of saturated surface and a transition level called
CRITICAL MOISTURE CONTENT is reached. Finally, the water diffuses from the interior
and then evaporates. In this phase called FALLING RATE PERIOD of drying, the
instantaneous rate of drying continuously decreases, in falling rate period. During the
process of drying after the superficial moisture is evaporated there comes a state when
outside air starts getting sucked in to the pores by capillary action. Later as drying proceeds
further, capillary action also cannot occur because a continuous film of liquid no longer
exists between and around the discrete particles. The DRYING CURVE is a graphical
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representation of moisture content of the product vs. time during the process of drying and it
identifies the constant, critical and falling rate regimes of drying. The DRYING RATE is
measured as moisture lost in unit time and DRYING TIME is the time taken for reducing
the moisture in the product from higher to lower level. RESIDENCE TIME is the time taken
by the product to travel from the feed end to the discharge end.
Lecture:31
Fluidised Bed Dryers
Fluid bed dryers are found throughout all industries, from heavy mining through food, fine
chemicals and pharmaceuticals. They provide an effective method of drying relatively free flowing
particles with a reasonably narrow particle size distribution. In general, fluid bed dryers operate on a
through-the-bed flow pattern with the gas passing through the product perpendicular to the direction
of travel. The dry product is discharged from the same section.
 With a certain velocity of gas at the base of a bed of particles, the bed expands and particles
move within the bed.
 High rate of heat transfer is achieved with almost instant evaporation.
 Batch/continuous flow of materials is possible.
 The hot gas stream is introduced at the base of the bed through a dispersion/distribution
plate.
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Lecture:32
Rotary dryer, Spray dryer, fluidized bed dryer etc
The rotary dryer is a type of industrial dryer employed to reduce or minimize the liquid moisture
content of the material it is handling by bringing it into direct contact with a heated gas. The dryer is
made up of a large, rotating cylindrical tube, usually supported by concrete columns or steel beams.
The dryer slopes slightly so that the discharge end is lower than the material feed end in order to
convey the material through the dryer under gravity. Material to be dried enters the dryer, and as the
dryer rotates, the material is lifted up by a series of internal fins lining the inner wall of the dryer.
When the material gets high enough to roll back off the fins, it falls back down to the bottom of the
dryer, passing through the hot gas stream as it falls. This gas stream can either be moving toward
the discharge end from the feed end (known as co-current flow), or toward the feed end from the
discharge end (known as counter-current flow). The gas stream can be made up of a mixture of air
and combustion gases from a burner, in which case the dryer is called a direct heated dryer.
Alternatively, the gas stream may consist of air or another (sometimes inert) gas that is preheated.
When the gas stream is preheated by some means where burner combustion gases do not enter the
dryer, the dryer known as an indirect-heated type. Often, indirect heated dryers are used when
product contamination is a concern.
Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with
a hot gas. This is the preferred method of drying of many thermally-sensitive materials such as
foods and pharmaceuticals. A consistent particle size distribution is a reason for spray drying some
industrial products such as catalysts. Air is the heated drying media; however, if the liquid is a
flammable solvent, such as ethanol, or the product is oxygen sensitive nitrogen is used.
All spray dryers use some type of atomizer or spray nozzle to disperse the liquid or slurry into a
controlled drop size spray. The most common of these are rotary nozzles and single-fluid pressure
swirl nozzles. Alternatively, for some applications two-fluid or ultrasonic nozzle are used.
Depending on the process needs drop sizes from 10 to 500 micron can be achieved with the
appropriate choices. The most common applications are in the 100 to 200 micron diameter range.
The hot drying gas can be passed as a co-current or counter-current flow to the atomiser direction.
The co-current flow enables the particles to have a lower residence time within the system and the
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particle separator (typically a cyclone device) operates more efficiently. The counter-current flow
method enables a greater residence time of the particles in the chamber and usually is paired with a
fluidised bed system.
A spray dryer is a device used in spray drying. It takes a liquid stream and separates the solute or
suspension as a solid and the solvent into a vapor. The solid is usually collected in a drum or
cyclone. The liquid input stream is sprayed through a nozzle into a hot vapor stream and vaporised.
Solids form as moisture quickly leaves the droplets. A nozzle is usually used to make the droplets as
small as possible, maximising heat transfer and the rate of water vaporisation. Droplet sizes can
range from 20 to 180 μm depending on the nozzle.
Spray dryers can dry a product very quickly compared to other methods of drying. They also turn a
solution, or slurry into a dried powder in a single step, which can be advantageous for profit
maximization and process simplification.
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Lecture 33
In this lecture following topics would be covered
 Adsorption phenomena
 Types of Adsorption
 Nature of adsorbents
 Adsorption Equilibria
Adsorption occurs whenever a solid surface is exposed to a gas or a liquid: it is
defined as the enrichment of material or increase of density of the fluid in the vicinity
of an interface
Adsorbent
Adsorbate
(porous structure)
Adsorbent
(‘flat’ surface)
Physical (Physisorption)
- van der Waals interactions
(result in attractive forces between
adsorbent and adsorbate molecules)
Chemical (Chemisorption)
- Chemical bonds between
adsorbate and adsorbent formed
- Adsorbed molecules maintain their
identity
- Adsorbed molecules loose their
identity
- Multilayers
-No activation energy
- Always reversible
•
•
Natural or synthetic
Amorphous or microcrystalline structure
- Monolayers
- Often activation energy required
- Can be irreversible
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•
•
Very high specific surface area
Examples:
• Charcoal
• Kaolin
• Bentonite
• Activated carbon
• Silica gel
• Activated alumina
• Zeolite (molecular sieves),
Lecture 34-35
In this lecture following topics would be covered
 Adsorption Equilibria
 Adsorption of vapor and gas mixtures,
 Dilute and concentrated liquid solutions
In many respects the equilibrium adsorption characteristics of a gas or vapor on a solid
resemble the equilibrium solubility of a gas in a liquid.
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Fig: Equilibrium Adsorption of vapors on activated carbon
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Fig: Types of adsorption isotherms for vapors
Fig: System Oxygen- Nitrogen on activated carbon
Lecture 36-37
In this lecture following topics would be covered
 Single stage adsorption operation
 Multi stage operation
 Application of Freundlich equation
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X
Fig: Single stage adsorption operation
The solute removed from the liquid equals to that picked by solid.
For a multistage operation, a schematic flow sheet and operating diagram for a typical operation is
shown below
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Fig: Two stage crosscurrent adsorption operation
Following equation can be used to calculate the intermediate concentration Y1
Lecture 38-39
In this lecture following topics would be covered
 Numerical problem based on stage wise operation
 Unsteady state- Fixed bed absorbers,
 Thermal swing and Pressure swing absorbers
Unsteady State-fixed Bed Adsorbers
Consider a binary solution, either gas or liquid, containing a strongly adsorbed solute at
concentration c0. The fluid is to be passed continuously down through a relatively deep bed of
adsorbent initially free of adsorbate.
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Fig: The Adsorption Wave
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Fig: The Heatless Adsorber
Fig: Fixed bed Adsorber for vapors at high pressure
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Lecture 40
In this lecture following topics would be covered
 Chromatography
 Rate of adsorption in a fixed bed
 Ion exchange and its application
Chromotography
Consider a solution containing two sources A and B which are differently adsorbed at equilibrium,
A more strongly. A small quantity of this solution, insufficient to saturate all but a small quantity of
the adsorbent
Fig: Chromatographic separation of two solutes
Rate of adsorption in fixed Bed
The design of a fixed bed adsorber and prediction of the length of the adsorption cycle between
revivifications require knowledge of the percentage approach to saturation at the breakpoint
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Fig: Idealized Breakthrough curve
Lecture 41
In this lecture following topics would be covered
 Solubility curve
 Super-saturation,
 Methods of super-saturation,
 Mier’s super-saturation theory
Crystallization is the (natural or artificial) process of formation of solid crystals precipitating from
a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical
solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a
pure solid crystalline phase occurs.
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The crystallization process consists of two major events, nucleation and crystal growth. Nucleation
is the step where the solute molecules dispersed in the solvent start to gather into clusters, on the
nanometer scale (elevating solute concentration in a small region), that becomes stable under the
current operating conditions. These stable clusters constitute the nuclei. However when the clusters
are not stable, they redissolve. Therefore, the clusters need to reach a critical size in order to
become stable nuclei. Such critical size is dictated by the operating conditions (temperature, super
saturation, etc.). It is at the stage of nucleation that the atoms arrange in a defined and periodic
manner that defines the crystal structure — note that "crystal structure" is a special term that refers
to the relative arrangement of the atoms, not the macroscopic properties of the crystal (size and
shape), although those are a result of the internal crystal structure.
The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical
cluster size. Nucleation and growth continue to occur simultaneously while the supersaturation
exists. Supersaturation is the driving force of the crystallization, hence the rate of nucleation and
growth is driven by the existing supersaturation in the solution. Depending upon the conditions,
either nucleation or growth may be predominant over the other, and as a result, crystals with
different sizes and shapes are obtained (control of crystal size and shape constitutes one of the main
challenges in industrial manufacturing, such as for pharmaceuticals). Once the supersaturation is
exhausted, the solid-liquid system reaches equilibrium and the crystallization is complete, unless the
operating conditions are modified from equilibrium so as to supersaturate the solution again.
Many compounds have the ability to crystallize with different crystal structures, a phenomenon
called polymorphism. Each polymorph is in fact a different thermodynamic solid state and crystal
polymorphs of the same compound exhibit different physical properties, such as dissolution rate,
shape (angles between facets and facet growth rates), melting point, etc. For this reason,
polymorphism is of major importance in industrial manufacture of crystalline products.
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Fig: Solubility Curve
Lecture 42
In this lecture following topics would be covered
 Principles of Crystallization
 Enthalpy Balance of a crystallizer

Product Purity. A sound, well-formed crystal itself is nearly pure, but it retains mother
liquor when removed from the final magma (the two phase mixture of mother liquor and
crystals of all sizes which occupies the crystallizer and is withdrawn as product), and if the
crop contains crystalline aggregates, considerable amounts of mother liquor may be
occluded within the solid mass. When retained mother liquor of low purity is dried on the
product, contamination results, the extent of which depends on the amount and degree of
impurity of the mother liquor retained by the crystals.
In practice, much of the retained mother liquor is separated from the crystals by filtration or
centrifuging, and the balance is removed by washing with fresh solvent. The effectiveness of these
purification steps depends on the size and uniformity of the crystals.
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
Yields. In many industrial crystallization processes, the crystals and mother liquor are in
contact long enough to reach equilibrium (equilibrium in crystallization processes is reached
when the solution is saturated) and the mother liquor is saturated at the final temperature of
the process. The yield of the process can then be calculated from the concentration of the
original solution and the solubility at the final temperature. If appreciable evaporation occurs
during the process, this must be known or estimated.
When the rate of crystal growth is slow, considerable time is required to reach equilibrium. This is
especially true when the solution is viscous or where the crystals collect in the bottom of the
crystallizer so there is little crystal surface exposed to the supersaturated solution. In such situations,
the final mother liquor, may retain appreciable supersaturation , and the actual yield will be less
than that calculated from the solubility curve.
If the crystals are anhydrous, calculation of the yield is simple, as the solid phase contains no
solvent. When the crop contains water of crystallization (is water that occurs in crystals but is not
covalently bonded to a host molecule or ion), account must be taken of the water accompanying the
crystals, since this water is not available for retaining solute in solution. Solubility data are usually
given either in parts by mass of anhydrous material per hundred parts by mass of total solvent or in
mass fraction anhydrous solute. These data ignore water of crystallization. The key to calculations
of yields of hydrated solutes is to express all masses and concentrations in terms of hydrated salt
and free water. Since it is this latter quantity that remains in the liquid phase during the
crystallization, concentrations or amounts based on free water can be subtracted to give a correct
result.

Energy Requirement. In heat balance calculations for crystallizers, the heat of
crystallization is important. This is the latent heat evolved when solids form from a
solution. Ordinarily, crystallization is exothermic, and the heat of crystallization varies with
both temperature and concentration. The heat of crystallization is equal to the heat absorbed
by crystals dissolving in a saturated solution, which may be found from the heat of solution
in a very large amount of solvent and the heat of dilution of the solution from saturation to
high dilution. Data on heats of solution and of dilution are available, and these, together with
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data on the specific heats of the solutions and of the crystals, can be used to construct
enthalpy-concentration charts which are useful in calculating enthalpy balances for
crystallization processes.

Nucleation. Nucleation is the initiation of a phase change in a small region, such as the
formation of a solid crystal from a liquid solution. It is a consequence of rapid local
fluctuations on a molecular scale in a homogeneous phase that is in a state of metastable
equilibrium. It is the number of new particles formed per unit time per unit volume of
magma or solids-free mother liquor.

Primary Nucleation
Primary nucleation is the initial formation of a crystal where there are no other crystals present
or where, if there are crystals present in the system, they do not have any influence on the
process.

Homogeneous Nucleation
In crystallization from solution, homogeneous nucleation almost never happens, except perhaps in
some precipitation reactions.
Crystal nuclei may form from various kinds of particles: molecules, atoms, or ions. In aqueous
solutions, these may be hydrated. Because of their random motion, in any small volume several of
these particles may associate to form what is called a cluster – a rather loose aggregation which
usually disappears quickly. Occasionally, however, enough particles associate into what is known as
embryo, in which there are the beginnings of the lattice arrangement and the formation of a new and
separate phase. For the most part, embryos have short lives and revert to clusters or individual
particles, but if the saturation is large enough, an embryo may grow to such a size that it is in
thermodynamic equilibrium with the solution. It is then called a nucleus, which is the smallest
assemblage of particles that will not redissolve and can therefore grow to form a crystal.
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Lecture 43
In this lecture following topics would be covered
 Classification of crystallizer
 Various types of crystallizers
 Numerical problem based on rate of crystallization
Cooling crystallizers
The simplest cooling crystallizers are tanks provided with a mixer for internal circulation, where
temperature decrease is obtained by heat exchange with an intermediate fluid circulating in a jacket.
These simple machines are used in batch processes, as in processing of pharmaceuticals and are
prone to scaling. Batch processes normally provide a relatively variable quality of product along the
batch.
The Swenson-Walker crystallizer is a model, specifically conceived by Swenson Co. around 1920,
having a semicylindric horizontal hollow trough in which a hollow screw conveyor or some hollow
discs, in which a refrigerating fluid is circulated, plunge during rotation on a longitudinal axis. The
refrigerating fluid is sometimes also circulated in a jacket around the trough. Crystals precipitate on
the cold surfaces of the screw/discs, from which they are removed by scrapers and settle on the
bottom of the trough. The screw, if provided, pushes the slurry towards a discharge port.
A common practice is to cool the solutions by flash evaporation: when a liquid at a given T0
temperature is transferred in a chamber at a pressure P1 such that the liquid saturation temperature
T1 at P1 is lower than T0, the liquid will release heat according to the temperature difference and a
quantity of solvent, whose total latent heat of vaporization equals the difference in enthalpy. In
simple words, the liquid is cooled by evaporating a part of it.
Evaporative crystallization
Another option is to obtain, at an approximately constant temperature, the precipitation of the
crystals by increasing the solute concentration above the solubility threshold. To obtain this, the
solute/solvent mass ratio is increased using the technique of evaporation. This process is of course
insensitive to change in temperature (as long as hydration state remains unchanged).
All considerations on control of crystallization parameters are the same as for the cooling models.
Nirma University
Chemical Engineering Department
2CH309 - MASS TRANSFER OPERATIONS-II Handouts
Evaporative crystallizers
Most industrial crystallizers are of the evaporative type, such as the very large sodium chloride and
sucrose units, whose production accounts for more than 50% of the total world production of
crystals. The most common type is the forced circulation (FC) model (see evaporator). A pumping
device (a pump or an axial flow mixer) keeps the crystal slurry in homogeneous suspension
throughout the tank, including the exchange surfaces; by controlling pump flow, control of the
contact time of the crystal mass with the supersaturated solution is achieved, together with
reasonable velocities at the exchange surfaces. The Oslo, mentioned above, is a refining of the
evaporative forced circulation crystallizer, now equipped with a large crystals settling zone to
increase the retention time (usually low in the FC) and to roughly separate heavy slurry zones from
clear liquid.
Fig: Schematic of a DTB crystallizer