Chapter3 - Ideal Gas

7/30/2019 Chapter3 - Ideal Gas

1/19

Interpolation Method

(kaedah tentu dalaman)

if required data is in between 2 given data.

Using linear function y=f(x)

Example:Properties 1 Properties 2

x1 y1xt yt

x2 y2(yt y1) (xt x1)

(y2 y1) (x2 x1)=

yt(xt x1)

(x2 x1) (y2 y1)= + y1

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Thermodynamics I (EMM2503)

Chapter 3 Properties of Pure Substances


2/19

What is gas?

Properties which can compressible.

ie oxygen, nitrogen, hydrogen

Pgas lower pressure

Tgas higher temp. ( 2 time critical point)

gasgas

containergas

PTVV

,

gas molecules move about random, continually colliding with

each other and the walls of container they are in.

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Chapter 3 Properties of Pure Substances (Ideal gas)


3/19

Differences between

Fluids and gasses

A liquid will take the

shape of its containerbut exhibits a free

surface.

A gas will fill its containercompletely and does not

exhibit a free surface.

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4/19

Ideal Gas (gas unggul) a properties that can easily compressed

in the gas phase.

Such as nitrogen, oxygen & hydrogen

Note:

oxygen - 164.4K

Hydrogen

33.3K

Nitrogen 126 K

Di anggap gas unggul pada suhu dantekanan atmosphere.

Wap air >748 C pada tekanan rendah boleh di anggap

sebagai gas unggul

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5/19

The ideal gas equation of state a state that related with pressure, temp., and specific volume of a

substance.

TRvp

This implies: (1) very little molecular interaction (p


6/19

R - gas constant (pemalar gas tentu)

Universal gas constant

Molecular weight

Note: the ideal gas equation of state can be derived from basic principle if one assume

i. Intermolecular force are small

ii. Volume occupied by the particles is small

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7/19

Jisim molekul dan pemalar gas tentu beberapa gas

pada tekanan atmosfera dan suhu 23 C.

Refer Table A-1 page 910.

Gas Formula

Kimia

Berat Molekul, M

(kg/kmol)

Pemalar gas tentu

R, (kJ/kgK)

Argon

Helium

Hidrogen

Karbon

dioksidaNitrogen

Oksigen

Udara

Wap Air

A

He

H2

CO2

N2O2

-

H2O

39.948

4.003

2.016

44.010

8.01331.999

28.970

18.015

0.2081

2.0770

4.1242

0.1889

0.29680.2598

0.2870

0.4615

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8/19

Is water vapor an ideal gas?

Under pressure below 10kPa water vapor called ideal gas.

However, at high pressure gas assumption yields unacceptableerror, particularly at critical point and at saturated vapor line (over100%).

Example: air conditioning water vapor is treated as ideal gas since water vapor is very low.

But steam power plant involved higher pressure ideal gasrelation should not be used.

Answer

cannot say Yes or No

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9/19

FIGURE 2-49

Percentage

of error

involved inassuming

steam to be

an ideal gas,

and the

region where

steam can betreated as an

ideal gas

with less

than 1

percent error.

2-12

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10/19

Enthalpy of an ideal gas

RTTuTh

pvuh

)()(

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11/19

Compressibility Factor(Faktor kebolehmampatan)

-Ideal gas equation use to analysis simple and easy gas state and

suitable to be used if the gas are at low density or at low pressure at

high temperature.

-We also known that gases deviate from ideal-gas and have the

problem to derive the limit of low density states and to interfered how

far behaviors of the gases can go.

-To avoid the uncertain situation and to get the accurate

measurements, the compressibility factor is introduced.

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12/19

Compressibility Factor(Faktor kebolehmampatan)

TRZvp

- Used to identify ideal gas behavior or a measure of deviation from gas behavior.

Where if

Z = 1 for ideal gas

Z > 1 or Z < 1 for real gas

TRZ

vp

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13/19

FIGURE 2-49

Percentage

of error

involved inassuming

steam to be

an ideal gas,

and the

region where

steam can betreated as an

ideal gas

with less

than 1

percent error.

2-12

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14/19

To measure Z value, we need :

1. Generalized compressibility Factor Chart2. Reduce pressure ratio Pr = P/Pcritical

3. Reduce temperature ratio Tr = T/Tcritical

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15/192-13

FIGURE 2-51

Comparison of Z factors for various gases. Page 141

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16/19

Reduced pressure and temperature

Generalized Compressibility Chart

Tr1Tr2

Z Pr

Pr = P/Pcritical Tr = T/Tcritical

(14.6)

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17/19

Real Gases

P

1

Z

T1

T2

T3

T1< T

2< T

3I deal Gas

TR

vp

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18/19

From the generalized compressibility

chart, we can observed that:

low pressure PR < 1 ideal gas regardless of thetemperature.

High temperature TR > 2

good ideal gas behaviorregardless of pressure.

The deviation of a gas from the ideal gas behavior isgreater in the vicinity of the critical point.

VR =V actual

RTcr / Pcr

Pseudo reduce

specific volume

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19/19

Other equation of state

Van der waals equation of state

Beattie-bridgeman equation of state Benedict-webb-rubin equation of state

Virial equation of state.

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Chapter3 - Ideal Gas