State

In general, a gas can be changed into a liquid by reducing its temperature, and a liquid to a solid by reducing its temperature further. To some extent, an increase in pressure will cause a substance to liquefy and solidify at higher temperature than would otherwise be required. The transition from liquid to gas is known as boiling and the transition from liquid to solid as freezing. Accordingly, each substance has a characteristic boiling temperature and freezing temperature (at a given pressure). The opposite transitions, from gas to liquid and solid to liquid, are known as condensation and melting respectively. The condensation temperature is the same as the boiling temperature and the melting temperature is the same as the freezing temperature. The process of condensation is also known as liquefaction and the process of freezing is also known as solidification. Boiling and freezing temperatures are most meaningfully compared relative to "absolute zero". Absolute zero (-273.15 °C) is the lowest temperature in the universe at which all molecular motion stops [46].

Hydrogen has the second lowest boiling point and melting points of all substances, second only to helium. Hydrogen is a liquid below its boiling point of 20K (-253 °C) and a solid below its melting point of 14K (-259 °C) and atmospheric pressure. Obviously, these temperatures are extremely low. Temperatures below 200K (-73 °C) are collectively known as cryogenic temperatures, and liquids at these temperatures are known as cryogenic liquids. The boiling point of a fuel is a critical parameter since it defines the temperature to which it must be cooled in order to store and use it as a liquid. Liquid fuels take up less storage space than gaseous fuels, and are generally easier to transport and to handle. For this reason, fuels that are liquid at atmospheric conditions (such as gasoline, diesel, methanol and ethanol) are particularly convenient. Conversely, fuels that are gases at atmospheric conditions (such as hydrogen and natural gas) are less convenient as they must be stored as a pressurized gas or as a cryogenic liquid.

The boiling point of a pure substance increases with applied pressure up to a point. Propane with a boiling point of -42 °C, can be stored as a liquid under moderate pressure, although it is a gas at atmospheric pressure. Unfortunately, hydrogen's boiling point can only be increased to a maximum of -240 °C through the application of approximately 1.3 MPa, beyond which additional pressure has no beneficial effect.

Table A-1 Physical Properties of H2, methane, gasoline, and propane [12, 36, 46]

Property

Hydrogen

Methane

Gasoline

Propane

Units

Molecular weight

2.016

16.043

107

44.097

amu

Triple point pressure

0.007042

0.01174

-

-

MPa

Triple point temperature

13.803

90.68

180 - 220

85.5

K

Normal boiling point (NBP) temp.

20.268

11.632

310-478

231.1

K

Critical pressure

1.293

4.5988

2.48 - 2.7

4.3

MPa

Critical temperature

32.976

190.56

540-569

369.8

K

Density at triple point

31.4

160.4

230

kg/m3

Density of liquid at triple point

77

451.6

-

kg/m3

Density of solid at triple point

68.65

487.2

-

kg/m3

Density of liquid at NBP

70.8

422.6

700

575

kg/m3

Density of vapour at NBP

1.34

1.82

4.5

2.40

kg/m3

Density of gas at NTP

0.083764

0.65119

4.4

1.69

kg/m3

Density ratio: NBP liquid to NTP gas

845.2

649

159

340

-

Heat of fusion

58230

58470

161000

J/kg

Heat of vaporization

445590

509880

309000

J/kg

Heat of sublimation

507390

602440

-

J/kg

Heat of combustion (low)

119.9

50.0

44.5

46.3

MJ/kg

Heat of combustion (high)

141.9

55.5

48.0

50.4

MJ/kg

Energy density

8.49

21.14

31.15

23.1

MJ/liter

Specific heat (Cp) of NTP gas

14890

2220

1620

J/kg-K

Specific heat (Cp) of NBP liquid

9690

3500

2200

J/kg-K

Specific heat ratio (Cp/Cv) of NTP gas

1.383

1.308

1.05

1.14

-

Specific heat ratio (Cp/Cv) of NBP liquid

1.7

1.7

-

-

Viscosity of NTP gas

0.8

1.1

0.52

8.0

10-5 kg/m-s

Viscosity of NBP liquid

0.000013

0.000113

0.0002

kg/m-s

Thermal conductivity of NTP gas

0.1897

0.033

0.0112

W/m-K

Thermal conductivity of NBP liquid

0.1

0.186

0.131

W/m-K

Surface Tension

0.00193

0.01294

0.0122

N/m

Dielectric constant of NTP gas

1.00026

1.00079

1.0035

-

Dielectric constant of NBP liquid

1.233

1.6227

1.93

Index of refraction of NTP gas

1.00012

1.0004

1.0017

Index of refraction of NBP liquid

1.11

1.2739

1.39

Adiabatic sound velocity in NTP gas

1294

448

154

m/s

Adiabatic sound velocity in NBP liquid

1093

1331

1155

m/s

Compressibility factor (Z) of NTP gas

1.0006

1.0243

1.0069

-

Compressibility factor (Z) of NBP liquid

0.01712

0.004145

0.00643

-

Gas constant (R)

4.123

518.27

78.02

m3-Pa-/kg-K

Isothermal bulk modulus of NBP liquid

50.13

456.16

763

MN/m2

Volume expansivity (b) of NBP liquid

0.01658

0.00346

0.0012

/K

percentage of thermal energy radiated from diffusion flame to surroundings

17-25

23-32

30-42

percent

NTP= 1 atm and 20°C (293.15K) Normal temperature and Pressure; NBP= Normal Boiling Point

NTP= 1 atm and 20°C (293.15K) Normal temperature and Pressure; NBP= Normal Boiling Point

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