175
Dimethyl Ether
then, natural gas is a preferred raw material, also because it is cleaner and
the gasifier construction is simpler and cheaper. However, other issues such
as resource availability and environmental sustainability also affect the
choice of the feedstock.
6.3.2 DMe/Methanol Production
The production of DME and methanol are closely related. Basically to make
DME, methanol is formed from syngas, and then DME is formed from the
methanol by a dehydration process. These processes can occur in separate
reactors or in a common reactor. The former method is called the indirect method, and the latter method the direct method of DME production.
Though it is called the direct method, the same reaction steps occur, they just
take place in one reactor instead of two. That is to say, DME is not directly
formed. Methanol is formed first and the dehydration reaction occurs thereafter in the same reactor.
The following reactions are involved in the production of DME from
syngas. The first is the formation of methanol from syngas over a suitable
catalyst.
CO + 2H → CH OH ∆H = −89.4 kJ/kMol Methanol .
(6.5)
−
2
3
R
The water gas reaction also occurs:
CO + H O → CO + H ∆H =− 41.7 kJ/kMol − CO.
(6.6)
2
2
2
R
These two reactions form the basis of commercial methanol production, a
process that has been used for many years.
In the production of DME, the final reaction is the catalytic dehydration of
the methanol to form the DME.
2CH OH → CH OCH + H O ∆H = −25.5 kJ/kMol − DME. (6.7)
3
3
3
2
R
Combining Reactions 6.5, 6.6, and 6.7, the overall process for the conversion
of syngas to DME is obtained.
3CO 3H
+
→ CH OCH + CO ∆H =− 245.7 kJ/kMol DME.
−
(6.8)
2
3
3
2
R
Combining Reaction 6.4 with Reaction 6.8, the overall formation of DME
from natural gas is obtained:
2CH + O → CH OCH + H O ∆H =− 276.7 kJ/kMol − DME. (6.9)
4
2
3
3
2
R
The same result is obtained by combining Reactions 6.3, 6.5, and 6.7.