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Gas Mixtures 1 Adv. Thermo-fluids

Composition of gas mixtures

P-v-T behaviour of gas mixtures

Properties of gas mixtures

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Gas Mixtures 2 Adv. Thermo-fluidsIntroduction

composition of gas mixture

properties of components

Many important engineering applications involve gas mixtures

In this study, gas mixtures are considered non-reacting

Non-reacting gas mixture can be treated as pure

substance because its composition is homogeneous

Each gas in gas mixture => called component orconstituent

Properties of gas

mixture depends on:

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Gas Mixtures 3 Adv. Thermo-fluidsGas Mixture Composition

molar analysis Mole numberof each component

gravimetric analysis Mass of each component

=

=k

iim mm

1

=

=k

i

im NN1

Composition of gas mixture can be described by:

Mass of mixture = sum ofmasses of

components

Mole number of mixture = sum of

mole numbers of components

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Gas Mixtures 4 Adv. Thermo-fluidsGas Mixture Composition

Mass fraction = mass ratio

of component to mixturem

iim

mmf =

Mole fraction = mole ratio ofcomponent to mixturem

ii

NNy =

Sum of mass fractions or

mole fractions equal to 1 ==

==k

ii

k

ii ymf

11

1

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Gas Mixtures 5 Adv. Thermo-fluidsGas Mixture Composition

Relation between mass m

and mole number N: NMm =

Apparent or average molar

mass Mm of mixture: ==

k

iiim MyM 1

Apparent or average gasconstant of mixture Rmm

um

MRR =

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Gas Mixtures 6 Adv. Thermo-fluidsP-v-T Behaviour

For ideal gases:

RTPv=

For real gases: ZRTPv =

For gas mixtures,

P-v-T behaviour can

be predicted using:

Daltons law of additive pressures

Amagats law of additive volumes

Kays Rule

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Gas Mixtures 7 Adv. Thermo-fluids

Daltons law of additive pressure

P-v-T Behaviour

Pressure of gas mixture equal to sum of

pressures each gas would exertif it existed

alone at the mixture temperature and volume

=

=k

i

mmim VTPP

1

),(Exact for ideal gases,

Approximate for real gases

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Gas Mixtures 8 Adv. Thermo-fluids

Amagats law of additive volume

P-v-T Behaviour

Volume of gas mixture equal to sum of volumes

each gas would occupy if it existed alone at the

mixture temperature and pressure

=

=k

i

mmim PTVV

1

),(Exact for ideal gases,

Approximate for real gases

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Gas Mixtures 9 Adv. Thermo-fluidsP-v-T Behaviour

=

==

k

i

iimmummmm ZyZTRNZVP1

,

For ideal gas, pressure fraction andvolume fraction equal to mole fraction i

m

i

m

i

m

i yNN

VV

PP ===

For real gases, deviation from ideal gas behaviour can beapproximated by including compressibility factor

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Gas Mixtures 10 Adv. Thermo-fluidsP-v-T Behaviour

=

=k

i

icrimcr PyP1

,,

=

=k

i

icrimcr TyT1

,,

cr

mR

T

TT

=

cr

mR

P

PP

=

Kays Rule:Use of pseudocritical pressure Pcrand pseudocritical temperature Tcr

mZ

mmmmm TRZVP =

Compressibility

Chart

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Gas Mixtures 12 Adv. Thermo-fluidsP-v-T Behaviour

==

==k

i

iim

k

i

iim uyuumfu11

,

==

==

k

i

iim

k

i

iim hyhhmfh11

,

==

==

k

i

iim

k

i

iim syssmfs11

,

== ==k

i

ipimp

k

i

ivimv CmfCCmfC1

,,

1

,, ,

Intensive property of gas mixture equal to sum of

intensive property of components multiplied by

mass fraction or mole fraction

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Gas Mixtures 13 Adv. Thermo-fluids