What is the difference between dimethyl ether, diesel and liquefied petroleum gas?

Physical Properties	Unit	Dimethyl Ether	Diesel Fuel	Liquefied Petroleum Gas
Molecular Formula	Â	Ch3och3	Cxhy	C3h8~ 4h10
Molecular Weight	Â	47	190~220	44~56
Boiling Point	℃	－24.9	180~360	－42.1
Liquid Density	G/Cm3	0.668	0.84	0.501
Stoichiometric Air/Fuel Ratio	Kg/ Kg	9.01	4.61	2.72
Octane Number	Â	55~60	40~55	Â
Latent Heat Of Vaporization	Kj/Kg	460(-20℃)	270	Â
Low Calorific Value	Mj/Kg	28.43	42.5	46.36
Ignition Point	℃	235	250	470
Liquid Viscosity	Â	0.15	4.4~5.4	Â
Carbon Content Percentage	%	52.2	86	81.8
Hydrogen Percentage	%	13.0	14	18.2~17.9
Oxygen Percentage	%	34.8	0	0
Gas Pressure At 20°C	Bar	5.1	0	8.4

 

 

Dimethyl ether production process:

At present, the production methods of DME at home and abroad mainly include one-step synthesis gas method and methanol method (two-step method). The methanol method is further divided into a methanol gas phase method and a methanol liquid phase method. The industrialization technology of the one-step process of syngas is not yet mature.

 

The reasons are:

â‘  The existing technology has not been tested by the device;

â‘¡ Even according to the existing technology, its production cost is higher than that of the methanol gas phase method.

 

One step method:

The one-step synthesis gas method takes synthesis gas (CO + H2) as the raw material, and the methanol synthesis reaction and methanol dehydration reaction are completed in one reactor, accompanied by the CO shift reaction.

The reaction formula is as follows: 2CO+4H2 = 2CH3OH CO+H2O =CO2 +H2

2CH3OH =CH3OCH3 +H2O

Total reaction: 3CO + 3H2 =H3COCH3 +CO2

 

The main feature of the one-step synthesis gas method lies in the advantages of the reaction, the methanol synthesis reaction and the methanol dehydration reaction are completed in one reactor. The reaction equilibrium constant is large, and the one-way conversion rate of synthesis gas is high, reaching 40.0% ~75.0%. Since the methanol generated by the reaction immediately undergoes dehydration reaction to generate dimethyl ether, the weakness of the low conversion rate of the synthetic methanol reaction is overcome.

 

Two-step method:

1. Methanol liquid phase method

Methanol dehydration reaction is carried out in liquid phase, under normal pressure or slight positive pressure, at 130°C. Its chemical reaction formula is as follows: 2CH3OH =H3COCH3 +H2O

The methanol liquid phase method is developed from the sulfuric acid method, and the production of dimethyl ether by the sulfuric acid method is the first half of the production process in the production process of dimethyl sulfate by the sulfuric acid method. Methanol dehydration reaction is carried out in liquid phase, under normal pressure or slight positive pressure, at 130°C. Methanol enters the reactor after preheating, and undergoes dehydration reaction under the catalysis of inorganic acid. By heating, the dimethyl ether, water and phase equilibrium methanol produced by the reaction are vaporized and sent out of the reactor. The reaction product is condensed and separated, and the uncondensed gas phase is compressed and liquefied to obtain the product dimethyl ether. The condensate is separated by rectification, water is discharged from the tower kettle, and methanol is returned as raw material.

 

2. Methanol gas phase method

The chemical reaction formula of methanol dehydration reaction is as follows.

Main reaction: 2CH3OH =H3COCH3 +H2O

Main side reactions: CH3OH =CO + 2H2 H3COCH3 =CH4 +H2 +CO CO +H2O =CO2 +H2

Gas-phase catalytic dehydration of methanol is the most widely used industrial production method of dimethyl ether at home and abroad. It is characterized by mature and reliable technology, low investment, flexible product adjustment, simple process and low production cost. The reaction pressure is 0.5~1.5MPa, and the temperature is 230~400°C. Methanol is vaporized in the heat exchanger to exchange heat with the reaction product from the reactor, and then enters the reactor for gas-phase catalytic dehydration reaction, and the reaction product is cooled and condensed by circulating water after heat exchange. Reactor structures include adiabatic fixed bed, heat exchange fixed bed, multi-stage chilled fixed bed and isothermal tubular fixed bed. The material after cooling and condensing is subjected to gas-liquid separation in the intermediate tank of crude methyl ether. The gas phase is the non-condensable gas produced by the side reaction and the saturated vapor of dimethyl ether and methanol, which is sent to the washing tower to absorb and recover the dimethyl ether in methanol or methanol-water solution. The absorption liquid is returned to the intermediate tank of crude methyl ether, and the absorption tail gas is sent out to the device. The crude dimethyl ether in the intermediate tank of crude methyl ether is separated by rectification with a rectification tower. The dimethyl ether vapor from the top of the rectification tower is condensed by the rectification tower condenser, and part of it flows back into the tower, and part of it is sent to the product storage tank as a product. . The methanol-water solution obtained from the dimethyl ether rectification tower is sent to the methanol concentration tower to rectify and concentrate the methanol, and the concentrated methanol is returned as the reaction raw material. Alcohol-containing wastewater is discharged from the tank of the methanol concentration tower.

 

3. Storage and transportation

 

Dimethyl ether application fields

1. In terms of civilian use:

Dimethyl ether is a colorless, non-toxic, non-carcinogenic, and less corrosive product. It has good combustion performance, high thermal efficiency, no residue, no black smoke during combustion, and low CO and NO emissions. It can also be mixed with liquefied petroleum gas, coal gas or natural gas to increase heat, and ≥95% dimethyl ether can be directly used as a fuel to replace liquefied gas. Therefore, it may be an ideal clean fuel to replace liquefied gas. DME can replace coal gas and liquefied petroleum gas for domestic fuel. The vapor pressure of dimethyl ether at normal temperature is 0.5MPa. At the same temperature, the saturated vapor pressure of dimethyl ether is lower than that of liquefied petroleum gas, and its storage and transportation are safer than that of liquefied petroleum gas. If dimethyl ether is used alone as a fuel, its pressure level is in line with that of liquefied petroleum gas. Requirements, existing liquefied gas tanks can be used for centralized and unified canning, and the cooker can also be used in common with the liquefied gas cooker. Dimethyl ether can also be mixed into city gas or natural gas in a certain proportion for peak regulation, and can improve gas quality and heat value. At the same temperature, the saturated vapor pressure of dimethyl ether is lower than that of liquefied petroleum gas, so its storage and transportation are safer than liquefied petroleum gas; the lower explosion limit of dimethyl ether in air is twice that of liquefied petroleum gas, so it is also Safer than liquefied petroleum gas; although the calorific value of dimethyl ether is lower than that of liquefied petroleum gas, because dimethyl ether itself contains oxygen, the air required in the combustion process is much lower than that of liquefied petroleum gas, so the calorific value of dimethyl ether premixed gas and its theoretical The combustion temperature is higher than that of liquefied petroleum gas. In addition to being used alone, the mixture of dimethyl ether, methanol, water (not added, from the reaction of raw material methanol and methanol to dimethyl ether) and other components can be formulated into a stable fuel-alcohol ether fuel.

 

2. As an alternative fuel to fuel oil:

Since petroleum resources are non-renewable, research and development of alternative fuels for future vehicles are being conducted worldwide. The biggest potential market for future DME applications is as an alternative fuel for diesel. In contrast, the cetane number of conventional engine alternative fuels such as liquefied petroleum gas, natural gas, methanol, etc. is less than 10, which is only suitable for ignition engines. The cetane number content is an important indicator of diesel combustion performance. The cetane number of dimethyl ether is higher than that of diesel oil, which has excellent compressibility and is very suitable for compression ignition engines. DME can reduce nitrogen oxide emissions by replacing diesel oil. Realizing smokeless combustion, it is an ideal clean fuel for diesel engines. Using dimethyl ether, the tail gas does not need catalytic conversion treatment, and the emission of nitrogen oxides and black smoke particles can meet the requirements of ultra-low emission tail gas of California fuel vehicles, and can reduce engine noise. Studies have shown that existing car engines can use DME fuel with only a slight modification. Although DME costs more than diesel, it is less expensive and less polluting than low-polluting alternatives such as liquid propane.

Using dimethyl ether as fuel requires only a slight improvement to the fuel system of the original diesel engine. Under the premise of maintaining the efficiency of the original diesel engine, the same output power, torque and fuel economy, without any exhaust gas recirculation system and exhaust gas treatment device, the nitrogen oxides can be greatly reduced to below 2.5g/(kW·h) , At the same time, the contradiction between controlling nitrogen oxides and particulate emissions no longer exists, soot emissions are zero, there is no accelerated smoke, and particulate emissions are also greatly reduced.

 

3. Dimethyl ether power generation:

DME can also be used as a fuel for combined cycle power plants. Power generation systems generally use syngas as fuel. When the power generation load is low, the syngas can be converted into DME products, which can be conveniently stored for re-use or export at high load. The effect is similar to using methanol as fuel for combined cycle power generation.

 

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