1.Dew Point with non-condensable components 2.Flash with liquid vapor products 3.Condenser and Flash drum for ammonia synthesis 4.Azeotrope Ideal Solutions vs. Azeotropes Types of Azeotropes • Number of Constituents: • Heterogeneous or Homogeneous: • Positive or Negative: 5.Enthalpy change of mixing 6.Solutropes

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Chemical Engineering Department
Chemical Engineering
Thermodynamics

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Director:
Prof. Dr. Muhammad Shuaib
Shaikh
Coordinator:
Mujeeb-UR-Rahman 17CH106
Thermodynamics Assignment.

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1. Dew Point with non-condensable components:
Calculate the temperature and composition of a liquid in equilibrium with a gas mixture
containing 10% pentane (1), 10% hexane and 80% nitrogen (3) at 3 bar. Nitrogen is far
above its critical point and may be considered non-condensable.
ENTROPY AND EQUILIBRIUM
Solution. To find the dew-point we use Σi xi = 1. However, nitrogen is assumed non-condensable
so x3 = 0. Thus, this component should not be included in below equation, which becomes
∑𝑖 =
𝑦𝑖
𝑝𝑖
𝑠𝑎𝑡 ( 𝑇)
=
1
𝑝
𝑦1
𝑝1
𝑠𝑎𝑡 ( 𝑇)
+
𝑦2
𝑝2
𝑠𝑎𝑡 ( 𝑇)
=
1
𝑝
Solving this implicit equation in T numerically gives
T = 314.82K and from 𝑥𝑖 = 𝑦𝑖 𝑝 𝑝𝑖
𝑠𝑎𝑡 ( 𝑇)⁄ the liquid composition is
x1 = 0.245, x2 = 0.755, x3 = 0

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2. Flash with liquid vapor products:
Next, consider a flash where a feed F (with composition zi) is split into a vapor product V
(with composition yi) and a liquid product (with composition xi) see in figure. For each of the
Nc components, we can write a material balance
F zi = L xi + V yi
Flash calculations are used for processes with vapor/liquid-equilibrium (VLE). A typical process that
requires flash calculations, is when a feed stream (F) is separated into a vapor (V) and liquid (L)
product;
In addition, the vapor and liquid is assumed to be in equilibrium,
yi = Ki xi
The K-values Ki = Ki (T, P, xi, yi) are computed from the VLE model. In addition, we have the two
relationships Σi xi = 1 and Σi yi = 1. With a given feed (F, zi), we then have 3Nc +2 equations in 3Nc +4
unknowns (xi, yi, Ki, L, V, T, p). Thus, we need two additional specifications, and with these the
equation set should be solvable.

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3. Condenser and Flash drum for ammonia synthesis:
The exit gas from an ammonia reactor is at 250 bar and contains 61.5% H2, 20.5% N2 and
18% NH3. The gas is cooled to 25oC (partly condensed), and is then separated in a flash drum
into a recycled vapor stream V and a liquid product L containing most of the ammonia. We
want to calculate the product compositions (L and V) from the flash drum.
Data. In spite of the high pressure, we assume for simplicity ideal gas. Use vapor pressure
data for ammonia from and Henry’s law coefficients for N2 and H2. For ammonia, we assume
ideal liquid mixture, i.e., ɣNH3 = 1 (which is reasonable since the liquid phase is almost pure
ammonia).
Solution. The feed mixture of H2 (1), N2 (2) and NH3 (3) is
z1 = 0.615, z2 = 0.205, z3 = 0.18
For ammonia, we have at T = 298.15 K and p = 250 bar (Raoult’s law):
𝐾3 =
𝑃3
𝑠𝑎𝑡 ( 𝑇)
p
=
9.83 bar
250 𝑏𝑎𝑟
= 0.0393
ENTROPY AND EQUILIBRIUM
For H2 and N2, we have from the given data for Henry’s coefficient at 25oC (298.15 K):
𝐾1 =
𝐻1
( 𝑇)
p
=
15200 bar
250 bar
= 60.8
𝐾2 =
𝐻2
( 𝑇)
p
=
8900 bar
250 bar
= 35.6
Now, zi and Ki are known, and the Rachford-Rice equation is solved numerically to find the vapor split
V/F = 0.8500. The resulting liquid and vapor compositions of the products are
x1 = 0.0119, x2 = 0.0067, x3 = 0.9814
y1 = 0.7214, y2 = 0.2400, y3 = 0.0386
This agrees well with flow sheet data from a commercial ammonia plant.

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4. Azeotrope:
An azeotrope is a mixture that exhibits the same concentration in the vapour phase
and the liquid phase. This is in contrast to ideal solutions with one component
typically more volatile than the other; this is how we use distillation to separate
materials. If the mixture forms an azeotrope, the vapour and the liquid concentrati ons
are the same, which preventing separation via this approach.
Introduction
Azeotropes are a mixture of at least two different liquids. Their mixture can either have
a higher boiling point than either of the components or they can have a lower boiling
point. Azeotropes occur when fraction of the liquids cannot be altered by distillation.
Typically when dealing with mixtures, components can be extracted out of solutions by
means of Fractional Distillation, or essentially repeated distillation in stages (hence the
idea of 'fractional'). The more volatile component tends to vaporize and is collected
separately while the least volatile component remains in the distillation container and
ultimately, the result is two pure, separate solutions.
Ideal Solutions vs. Azeotropes
Ideal solutions are uniform mixtures of components that have physical properties connected to their
pure components. These solutions are supported by Raoult’s law stating that interactions between
molecules of solute and molecules of solvent are the same as those molecules each are by
themselves. An example of ideal solutions would be benzene and toluene. Azeotropes fail to conform
to this idea because, when boiling, the component ratio of vaporized solution is equal to that of the
vaporized solution. So an azeotrope can be defined as a solution whose vapour has the same
composition its liquid. As you can imagine, it is extremely difficult to distil this type of substance. In
fact, the most concentrated form of ethanol, an azeotrope, is around 95.6% ethanol by weight
because pure ethanol is basically non-existent.
Azeotropes exist in solution at a boiling point specific for that component. This is best represented
graphically and the phase diagram of a maximum-boiling point azeotrope can be seen in the following
figure. The point Z represents where the azeotrope exists at a certain boiling point. Imagine that at
point Z, the A-B solution is 64% B by mass while component A is water. If that same solution contained
any less than 64%, the solution would then be water + the azeotrope. Conversely, if it were to be
greater than 64% then the solution would be component B + the azeotrope. This demonstrates that
an azeotrope can only exist at one temperature because any higher or lower temperature would result
in a different concentration of component A or B.

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Types of Azeotropes
Azeotropes may be categorized according to their number of constituents, miscibility, or boiling
points:
 Number of Constituents:
If an azeotrope consists of two liquids, it is known as a binary azeotrope. An azeotrope
consisting of three liquids is a ternary azeotropes. There are also azeotropes made of more
than three constituents.
 Heterogeneous or Homogeneous:
Homogeneous azeotropes consist of liquids that are miscible. They form a solution.
Heterogeneous azeotropes are incompletely miscible and form two liquid phases.
 Positive or Negative:
A positive azeotrope or minimum-boiling azeotrope forms when the boiling point of the mixture
is lower than that of any of its constituents. A negative azeotrope or maximum-boiling
azeotrope forms when the boiling point of the mixture is higher than that of any of its
constituents.
Also, a maximum-boiling point azeotrope is said to be a negative azeotrope because the boiling
point of the azeotrope itself is higher than the boiling point of its components. As you can
imagine, a positive azeotrope would have a lower boiling point than any of its components.

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5. Enthalpy change of mixing:
Assuming ideal behavior, so that interactions between individual gas molecules are
unimportant, it is fairly easy to calculate (Delta H) for each gas, as it is simply an isothermal
expansion. The total enthalpy mixing is then given by
Δ Hmix = ΔHA + ΔHB
And since the enthalpy Change for an isothermal expansion of an ideal gas is zero,
Δ Hmix = 0
Is a straight-forward conclusion. This will be the criterion for an ideal mixture.
In general, real mixture will derivative from this limiting ideal behavior due to interaction
between molecules and other concern. Also, many substances undergo chemical change
when they mix with other substances. But for now, we will limit ourselves to discussing mixture
in which no chemical reactions take place.

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6. Solutropes:
 Invariance in composition that occur when a line on a triangular diagram lies parallel to
an edge of the triangle. This invariance takes on special significance when the parallel
line is a tie line across a two phase region. When this occurs, the two phases in Liquid-
liquid Equilibrium have the same composition in one component, and the mixture is
called a solutrope.
 Their practical significance arises from their ability to inhibit separations by liquid
extraction, because transfer of components between phases are often hindered when
a mole fraction becomes the same in both phases. Such inhibitions may be
compounded if the densities of the phases also become equal, as they may near
solutropes. Since liquid extractions exploit density difference, no separation occur in an
extractions process when the densities of the two phases become equal, even if their
composition differ.
C
A
B
The system methyl cyclohexane (A) + n-hexane (B) + methanol (C) at 1 atm, 30 oC. Phase diagram
showing the boundaries of the region of demixing and tie lines connecting the position of the two.

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Effect of Temperature on TLE
Effect of the temperature on LLE in mixtures of phenol, water and trimethylamine. At 10oC a concolute
point occurs in mixtures lean in trimethylamine. At 10oC pure phenol solidifies. Composition plotted
here as weight fractions. Adopted from walas.
Gamma-Gamma Method Applied to LLE Calculations
𝑥𝑖
𝛼
𝛾𝑖
𝛼
( 𝑇, { 𝑥 𝛼}) = 𝑥𝑖
𝛽
𝛾𝑖
𝛽
(𝑇, {𝑥 𝛽
})
 The gamma-gamma method is commonly applied to low-pressure, liquid-liquid equilibrium
calculations.