India
South Africa
Japan
DME production
Reference:
“The future of DME”, Nitrogen+Syngas 308 | November - December 2010
Dimethyl ether ("DME") is the simplest ether produced from methanol, non-toxic and currently used as a chemical for aerosol propellants as well as other former applications of chlorofluorocarbons. DME for chemical use is currently produced by the dehydration of methanol. However, the main market for DME in the future is likely to be as a fuel. The physical properties of DME make it very attractive as a substitute for LPG and diesel oil as a clean fuel producing neither SOx nor particulate matter (PM). See the table below.
Methanol Dehydration
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2 CH3OH = CH3OCH3 + H2O ΔH = -23.4 MJ/kmol (D1)
The boiling point of DME is close to that of LPG, so it is easily liquefied and stored, and it is not corrosive towards conventional materials of construction. Existing LPG infrastructure, such as tanks and refrigerated tankers, could therefore be used. The cetane number of DME is very similar to
diesel oil, so DME can be used in diesel engines. On the other hand, DME has a lower heating value, lower viscosity, lower lubricity and a tendency to cause swelling of specific rubber and plastics. Nonetheless, only minor modifications would be required to existing LPG infrastructures and diesel engines for DME substitute.
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The following Reaction (D1) represents the main chemical reaction entailed in DME production by the methanol dehydration route. The exit composition of DME reactor is also approaching to the equilibrium of methanol synthesis Reactions (M1) and (M2).
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DME synthesis based on methanol dehydration is very simple. The dehydration of methanol is an exothermic gas-phase reaction. The heat of reaction is considerably less than that of methanol synthesis itself. The selectivity of DME in methanol dehydration is very high, approximately 99.9%. The dehydration catalyst is based on commercially available γ-alumina. A typical flow diagram for DME synthesis is shown in the following figure.
Feed methanol is fed to a DME reactor after vaporisation. The synthesis pressure is 1.0-2.0 MPaG. The inlet temperature is 220-250°C and the outlet is 300-350 °C. Methanol one pass conversion to DME is 70-80 % in the reactor. The product DME with by-product water and unconverted methanol is fed to a DME column after heat recovery and cooling. In the DME column, DME is separated from the top as a product. Water and methanol are discharged from the bottom and fed to a methanol column for methanol recovery. The purified methanol from the column is recycled to the reactor after mixing with feedstock methanol. The methanol consumption for DME production is approximately 1.4 t methanol/t DME.
DME direct production from syngas
Reference:
Inokoshi O., Y. Ohno, T. Ogawa, N. Inoue & N. Tokoeda.: “A New DME Production Technology – 100tons/day DME Direct Synthesis Demonstration Plant Project”, Proceedings of The 15th International Offshore and Polar Engineering Conference, pp50-53, Seoul, Korea, 19-24 June (2005)
JFE Corporation made remarkable progresses in a development of DME direct synthesis
process (JFE process) with 5 ton/day DME pilot plant project followed by a100 tons/day DME direct synthesis demonstration plant project completed at the end of November 2003 with the total plant operation period reached more than 150 days by the middle of December 2004, and the total amount of produced DME was almost 8,000 tons. Regrettably, any commercial plant baed on the JFE process has not on stream yet.
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Followings are the reaction formulas relating with DME synthesis. The exit composition of DME reactor is also approaching to the equilibrium of methanol synthesis Reactions (M1) and (M2) and the above methanol dehydration Reaction (D1) at the same time.
Direct DME synthesis from syngas
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3 CO + 3 H2 = CH3OCH3 + CO2 ΔH = -23.4 MJ/kmol (D2)
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3 CO + 4 H2 = CH3OCH3 + H2O ΔH = 17.8 MJ/kmol (D3)
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Natural gas is reformed with oxygen and CO2 recycled from DME synthesis to H2/CO=1 syngas in ATR. DME is synthesized from syngas in the slurry phase reactor. By-produced CO2, methanol and water are separated from the product DME in the distillation columns. Methanol is recycled to DME synthesis reactor to be converted into DME. Total reaction is formulated as follows;
CH4 + 0.601 O2 → 0.435 DME + 0.621 H2O + Purge gas
All required utilities in the process are covered by the purge gas as fuel and the recuperated heat.
The composition of the product DME is assumed to be 99.5% of DME, 0.2% of methanol and 0.3% of water in this estimation. The cold gas efficiency is of 71.0 %.( 86.0% for ideal efficiency of 82.6%). CO2 emission of fuel is 0.112 g-C/10 kcal in the production and 0.752 in the combustion of DME, in total, 0.864 g-C/10 kcal.
