Top Banner
Chemical Chemical Thermodynamics Thermodynamics 2013/2014 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia
12

Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

Dec 26, 2015

Download

Documents

Bathsheba York
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Page 1: Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

ChemicalChemical ThermodynamicsThermodynamics2013/20142013/2014

12th Lecture: Mixtures of Volatile LiquidsValentim M B Nunes, UD de Engenharia

Page 2: Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

2

IntroductioIntroductionnThe concept of ideal solution cam now be extended to solutions of several volatile components. In an ideal solution of two volatile liquids we have:

** and BBBAAA pxppxp

The total vapor pressure of mixture is then:

ABAB

BBAABA

xpppp

pxpxppp

***

**

or

This clearly show that the total vapor pressure (at constant T) changes linearly with the composition of mixture.

Some mixtures with resemblance in molecular structure

Page 3: Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

3

Vapor pressure diagramsVapor pressure diagrams

Coexistence curve or “bubble line”

Page 4: Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

4

The composition of vaporThe composition of vapor

The composition of liquid and vapor is not necessarily the same. It seems obvious that the vapor should be more richer in the more volatile component. Using Dalton`s law:

ppyp

py BB

AA and

It follows that: AB

ABAB

AAA yy

xppp

pxy

1 and

***

*

Inverting this expression we obtain: AABA

BAA yppp

pyx

***

*

Now, combining this two results:

AABA

BA

yppp

ppp

***

**

Page 5: Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

5

The composition of vaporThe composition of vapor

p

yA

pB*

pA*

Coexistence curve or “dew line”

Page 6: Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

6

Total phase diagram Total phase diagram

Combining both diagrams into one plot allow us to see the composition of both liquid and gas phase:

Above the liquid line the liquid phase is more stable. Bellow the vapor line the gas phase is more stable. In the middle region of the diagram we have the liquid vapor equilibrium, with two phases coexisting. At a given pressure we have a liquid with composition a in equilibrium with vapor of composition b.

Page 7: Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

7

Interpretation of the diagram Interpretation of the diagram

If we know the composition of one phase at a given temperature we can determine the composition of another phase from the diagram.

The line a2a2’’a2’, for instance, is a “tie linetie line”. It describes a state at constant pressure, and since T is already fixed, the Gibbs phase rule allow us to conclude that the compositions of both phases are completely defined!

Remember: g+F = C+2

Page 8: Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

8

The lever ruleThe lever rule

To calculate the amounts of both phases in equlibria we use the lever rulelever rule: we measure the distances l and l’ along the horizontal tie line.

l

l

n

n

Page 9: Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

9

T-x DiagramsT-x Diagrams

Instead of T being fixed as in previous diagrams, we can keep constant pressure, and generate T-x diagrams. These diagrams have great importance in practical separation processes like distillationdistillation.

TB*

TA*

hea

t

vaporization

condensation

Page 10: Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

10

Real Real Solutions Solutions Some mixture behave almost ideally. But, in many other cases, real solutions display marked deviations. In some systems with strong non-ideality, the mixtures form an azeotropeazeotrope (coming from the Greek word for “boiling without changing”)

Page 11: Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

11

Azeotropic Azeotropic MixturesMixtures

Component A: tb/°C Component B: t/°C Azeotrope: %A and tb/°C

H2O 100 HCl -80 20.2 108.6

C6H5OH 182.2 C6H5NH2 184.3 42 186.2

CHCl3 61.2 CH3COCH3 56.1 78.5 64.4

H2O 100 C2H5OH 78.3 4.0 78.2

CCl4 76.8 CH3OH 64.7 79.4 55.7

CHCl3 61.2 CH3OH 64.7 87.4 53.4

Suppose we try to distillate an ethanol/water mixture in a fractionating column. The fractionation shifts the vapor towards the azeotropic composition and vapor emerges on the top of the column. Then it boils unchanged when water content is 4% and temperature around 78 °C (commercial ethanol)

Page 12: Chemical Thermodynamics 2013/2014 12 th Lecture: Mixtures of Volatile Liquids Valentim M B Nunes, UD de Engenharia.

12

Fractionating column (oil)Fractionating column (oil)