1. Chemical Reaction for DME Synthesis
The syngas from gasification mainly contains carbon monoxide
(CO) and hydrogen (H2). Overall DME synthesis reactions consist
of 3-step reactions: methanol synthesis, dehydration and water gas
shift (WGS) reaction as following reactions. The formation of methanol is considered by two reaction mechanisms as illustrated in both
reaction (1) and (2); reaction (1) is the formation of methanol participating CO and H2 as major reactants. This reaction emits high
reaction enthalpy. While reaction (2) is led to by participating CO2
and H
2, it has less reaction enthalpy than reaction (1). It is assumed
that both reactions can proceed to produce methanol simultaneously
in this study.
CO+2H
2↔CH3OH −43.4 kcal/mol-DME (1)
CO
2+3H2↔CH3OH+H2O −13.5 kcal/mol-DME (2)
Methanol passes through the following dehydration reaction with
DME synthesis catalyst as in reaction (3). DME catalyst is used to
Cu/ZnO/Al
2O3+γ-Al2O3 in this process. Reaction (4) is shown as a
WGS reaction, and it has a role to supply CO2 and H2 for DME synthesis.
2CH
3OH↔CH3OCH3+H2O −5.6 kcal/mol-DME (3)
CO+H
2O↔CO2+H2 −9.8 kcal/mol-DME (4)
By relative reaction rate among reactants, the overall DME reaction
shows two mechanisms: reaction (5) and reaction (6) by integrating reactions (1)-(4). The combination of reaction (1) and (3) gives
overall reaction (6); the combination of reactions (1), (3) and (4)
gives overall reaction (5) [2]. And if reaction (4) proceeds slowly,
the overall DME reaction leads to reaction (6) [3].
3CO+3H
2↔CH3OCH3+CO2 −58.8 kcal/mol-DME (5)
2CO+4H
2↔CH3OCH3+H2O −49.0 kcal/mol-DME (6)
According to DME overall reactions by reaction (5) and (6), the
DME reaction heat is exothermal reaction which generates reaction
heat highly. If DME reactor temperature is increased by reaction heat
of DME synthesis, thermodynamic equilibrium conversion is decreased. And for DME catalyst it is noted that its performance is decreased in the DME synthesis reaction. So its reaction heat is should
be cooled DME reactor temperature at the range of 240-280 oC by
heat exchanger in the DME synthesis [4].