Dimethyl ether synthesis

Dimethyl ether (DME) is an alternative fuel that could potentially replace petroleum-based fuels. Dimethyl ether is the simplest ether (CH₃OCH₃). The physical properties of DME are similar to those of liquefied petroleum gases (propane and butane). It burns with a visible blue flame and is non-peroxide forming in the pure state or in aerosol formulations. Unlike methane, DME does not require an odorant because it has a sweet ether-like odor. It is a volatile organic compound, but is non-carcinogenic, non-teratogenic, non-mutagenic and non-toxic.

Currently, the major usage of DME is as a propellant in the aerosols industry. In addition, it can be used as a clean-burning fuel in diesel engines, as a household fuel (LPG alternative) for heating and cooking, as a fuel for gas and turbines in power generation, as a fuel for fuel cells, and as a chemical feedstock for higher ethers and oxygenates.

Traditionally, DME has been produced in a two steps process where syngas is first converted to methanol, followed by methanol dehydration to dimethyl ether.

Natural gas is not the only resource that can be used to generate syngas, coal and biomass can also be used. Hence, DME production is not limited to one feedstock and the price of DME synthesis process is directly related to the price of the feedstock. New processes are being commercialized to produce DME in a single step via autothermal reactors and slurry phase reactors.

DME can be introduced and exploited with existing technologies, and enable the eventual implementation of advanced technologies, such as fuel cells. Because DME is produced from natural gas, coal or biomass, it can increase the energy security by displacing petroleum derived fuels.

The prominent advantages of DME as a fuel and energy carrier are:

-DME, due to its high cetane number, can be used in the most efficient engine technology currently produced. DME demonstrated lower NO_x and SO_x than conventional diesel, is sootless.

-Using exiting engine technology, DME produces the least amount of well-to-wheel greenhouse gas emissions compared to FT diesel, FT naptha, bio-diesel, bio-naptha, methanol, methane and ethanol.

-Excluding natural gas, DME has the highest well-to-wheel efficiency of all non-petroleum based fuels using conventional, hybrid and fuel processor fuel cell vehicle technologies.

-DME can be used as a residential fuel for heating and cooking.

-On-board automotive fuel processors using methanol and DME exhibit the lowest start-up energies and the lowest fuel processor volumes-correlating to higher overall efficiencies as compared to ethanol, methane and gasoline fuel processor fuel cell vehicles.

-The infrastructure of DME is less cost intensive than that for hydrogen because DME can use the existing LPG and natural gas infrastructures for transport and storage.

Synthesis of DME from syngas in a single step is more favorable in thermodynamic and economical. Single-stage DME synthesis in the vapor phase suffers from low per pass conversions, due, in part, by the effects of high temperature on the catalysts. Gas-phase DME low hydrogen and synthesis processes, in general, suffer from the drawbacks of CO conversions per pass, along with low yield and selectivity of DME, coupled with a high yield of carbon dioxide. These processes are typically expensive due to high capital costs for reactors and heat exchangers, and high operating costs due to inefficient CO utilization and high recycle rates. Using an inert liquid as a heat sink for highly exothermic reactions offers a number opportunities in syngas processing. Heat generated by the exothermic reactions is readily accommodated by the inert liquid medium. This enables the reaction to be run isothermally; minimizing catalyst deactivation commonly associated with the more adiabatic gas phase technologies. The single stage, liquid phase DME synthesis process, investigated in detail, incorporates the sequential reaction of methanol synthesis and methanol dehydration in a slurry phase reactor system. Combining these reversible reactions in a single step makes each reaction thermodynamically more favorable by utilizing its inhibiting products as reactants in the subsequent reaction. In addition to the superior heat management allowed by the liquid phase operation, the synergistic effect of these reactions occurring together yields higher quantities of DME than that could be obtained from sequential processing.

The process is based on dual-catalytic synthesis in a single reactor stage, and based on a combination of an equilibrium limited reaction (methanol synthesis) and an equilibrium unlimited reaction (methanol dehydration). The methanol synthesis and the water gas shift reaction take place over the coprecipitated Cu/Zn/Al₂O₃ catalyst and the methanol dehydration takes place over γ- Al₂O₃ or zeolite catalysts. Moreover, by varying the mass ratios of methanol synthesis catalyst, it is possible to co-proportion DME and methanol in any fixed proportion, from 5% DME to 95% DME.