Alcohols vs. ethers

When added to gasoline, alcohols by themselves tend to be very volatile - due to the formation of an azeotrope with light hydrocarbons - and water soluble, which can create problems in the fuel distribution system and vehicle engine as well as in the environment. These problems of volatility and water solubility can be overcome by "stabilising" the alcohols with various petroleum-derived components through a process known as etherification. Ethers retain all the benefits of their alcohol feedstocks, without their shortcomings.

MTBE is still the most widely used fuel oxygenate, due to its combination of technical advantages and supply availability. As an octane enhancer, MTBE is an effective replacement for aromatics, because it delivers high octane numbers at relatively low cost. By comparison with alcohols, MTBE offers low water solubility, low reactivity and low volatility - characteristics that enable refiners to avoid the handling problems associated with alcohol oxygenates.

The key properties of the most commonly used oxygenates, when blended at 5% in gasoline, are found in the table below:

Substance
Motor Octane Number

Research Octane Number

Reid Vapour Pressure (kPa)

Boiling point (°C)

Water tolerance

Ethers

MTBE
101

118

55

55

Excellent

ETBE
103

119

28

72

Excellent

TAME
99

109

10

86

Excellent

Alcohols

Methanol
92

125

522

65

Very poor

Ethanol
96

130

200

78

Very poor

TBA
95

105

62

71

Poor

Gasoline
82-88

92-98

70-100

26-230