CH3OCH3 is a compound called dimethyl ether can be used as propellant and as fuel additive for diesel, it has quite unique advantages when compared to other fuel obtained from petroleum one of them is that it produces less particulate matter and nitrogen oxides. It is non-toxic, non carcinogenic and is suitable to gasoline engines also emitting less smoke with less engine noise when compared to petrol, although it can be used as feedstock to produce olefins like ethylene, dimethyl sulfate and acetic acid.
By dehydration of methanol dimethyl ether can be produced, methanol nowadays is produced from biomass so dimethyl which is produced from this methanol becomes an alternative fuel to fossil fuel because DME matches to the specification of the fuel requirement of new designed hybrid and fuel cell cars. Dimethyl ether boiling point is about -24.9oC at ambient conditions it can be stored in pressure vessel in liquid state.
Process description of DME manufacturing by methanol dehydration:
Indirect process: Methanol is fed to the packed bed reactor (packed with zeolite catalyst), through a heat exchanger to gain the temperature up to 250oC and pressure is build to 16.8 atm. Due to the exothermic reaction products attain a temperature approximately up to 360oC. Vapour outlet is cooled and depressurised to 10 atm. whatever DME is formed in the reactor is distilled out and un-reacted fraction of methanol is recycled to the inlet of the reactor.
Chemical reaction take place in the reactor would be:
Methanol (2CH3OH) <==> dimethyl ether (CH3OCH3) + water (H2O)
There are other processes which can convert methanol to dimethyl ether so that the production cost is reducing to make the plant process commercially stable. For example the reaction can be carried out in liquid phase in a slurry reactor or by application of reactive catalytic distillation equipment to reduce the operation cost. Lot of research work is under progress to find the utmost process with optimum condition so that they can provide a green fuel to run the transportation vehicles.
Process flow sheet of methanol dehydration:
Engineering and Technology Aspects of Methanol Dehydration
1) Methanol dehydration done on aluminum oxide (Al2O3) catalyst in slurry reactor:
Aluminum oxide, commercial catalyst used for dehydration operations. In methanol dehydration process Al2O3 play important role to produce dimethyl ether (used as refrigerant and LPG substitute) . In large-scale operation, methanol in vapor state converted to dimethyl ether on the surface of Al2O3. The surface reaction is explained by Langmuir – Hinshelwood dissociation adsorption that generates the rate equation of dehydration. Studies on the kinetics of methanol dehydration are required with more accuracy so that an industrial reactor of fixed bed type can be developed. Research programs are initiated to find out the best reactor and design condition so that a fail safe industrial reactor can produce dimethyl ether with high conversion of methanol at low temperature and pressure (operating parameters).
Even methanol with lowest purity can be converted to dimethyl ether at satisfying rate with the help of aluminum oxide. The intrinsic kinetics and global rate kinetics developed for this catalyst provide the facility to determine the reaction extent and control it with high accuracy even at high temperatures. Its global rates involve heat and mass transfer effects that influence the rate of reaction in a designed reactor. A standard sample of aluminum oxide available in market has a surface area about 237 sqm/g, total volume of 0.48 cm3 /g and average pore diameter of 6.9 nm. However, the water formed in reaction will affect the activity of Al2O3 to overcome this kind of problems varieties of catalysts like zeolites and alpha-Al2O3 are developed.
Aluminum oxide works with same efficiency whether it is a slurry reactor, fixed bed or reactive distillation equipment that carries out dehydration. Based on its particle size methanol conversion varies indirectly.
2) Reactors preferred for dehydration of methanol:
- Slurry reactor converts methanol to desired products in liquid phase. Methanol with boiling point 64oC at 1 atm is fed to a slurry reactor, which contains hydrocarbon oils along with catalyst dispersed in it. Vapors are generated as the reaction proceeds to produce dimethyl ether. DME exists at vapor state at atmospheric pressure so as and when it forms at 1 atm pressure slurry reactor it bubbles out from the slurry and collected from the outlet of the autoclave stirred vessel (slurry reactor) by the compressors.
- Another type of reactor model called fixed bed reactor allows the methanol vapours to pass over a catalyst bed. Methanol molecules absorb on the catalyst surface at basic site and on the proton. By condensation DME is formed its mole fraction increases along the bed of the reactor and so the temperature of the gas increases due to exothermic reaction.
- Catalytic distillation reduces overall capital cost of methanol dehydration process. It is the combination of reaction and distillation operations in single equipment. Reaction of methanol to DME and separation of DME from the reaction phase is done simultaneously in one unit. Heat utilization in this method reaches minimum optimum value. Catalytic reactive distillation, know as CD process can handle all varieties of catalysts and Amberlyst 35 (is an acidic ion exchange resin catalyst) best fits for this model. Rate determining step of dehydration reaction depends on the surface reaction which have Eley-Rideal mechanism. Minimum 13 stages should be fixed as reaction zone in a 30-stage distillation column containing catalyst. Top and bottom stages of CD column functions as separation systems and middle stage as reactor system when methanol is fed to it middle feed plate.
3) Catalyst arrangement in fixed bed reactors:
Catalyst of spatial patterned arrangement effect on productivity of reaction system other than regular physical mixture catalyst types. In the field of single step product synthesis, the available raw materials are converted to desired product in single step operation and in single reactor itself. This benefits in reducing the equipment and operation cost. To accomplish this catalysts are mixed in physical proposition that handles each reaction from raw material to product.
For example, synthesis gas is converted to methanol in one reactor and then methanol is fed to other reactor to produce dimethyl ether. Two reactors are used with separate catalyst that does the job. For effective utilization of the respective catalyst the two catalysts are physically mixed so that two reactions can be carried out in a single rector. The single reactor itself handles methanol synthesis and dehydration of methanol. However, physical mixture of catalyst is somewhat a raw idea. If the individual catalyst are layered in a systematic way than the reactor will sure reduce the operation cost for high productivity. In a fixed bed reactor, by arranging the two catalysts in spatial (layer by layer) form each layer provide effective control of reactions.
Layering of catalyst with different functions helps to drive the equilibrium towards product side. Equilibrium limitation of the processes that operate at low temperature and pressure can be solved.