Thema: 3er / 4er allg. M B T E !!!
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Alt 25.06.2002, 19:56     #3
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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 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 and of gasoline are found in the table below:

Motor Octane Number Research Octane Number Reid Vapour Pressure (kPa)* Boiling point (°C) Water tolerance
Ethers
MTBE
ETBE
TAME
101
102
99
118
118
109
55
28
10
55
72
86
Excellent
Excellent
Excellent
Alcohols
Methanol
Ethanol
TBA
92
96
95
107
130
105
32 1
124 1
48 1
65
78
71
Poor
Very poor
Poor
Gasoline 82-88 92-98 70-100 26-230
*Typical blending (effective) vapour pressure at 10 vol-% concentration in gasoline blend
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