Steam cracking methanation

Vast quantities of hydrogen are produced commercially each year by a process called steam cracking. Methane reacts with steam at 830°C in the presence of a nickel catalyst. 

A high purity hydrogen and a low purity methane stream result. The 95% hydrogen may be used directly to hydrogenate steam cracked naphtha or directly consumed elsewhere in the refinery. The methane stream goes to fuel. 

Higher molecular weight hydrocarbons present in natural gases are important fuels as well as chemical feedstocks and are normally recovered as natural gas liquids. For example, ethane may be separated for use as a feedstock for steam cracking for the production of ethylene. Propane and butane are recovered from natural gas and sold as liquefied petroleum gas (LPG). Before natural gas is used it must be processed or treated to remove the impurities and to recover the heavier hydrocarbons (heavier than methane). The 1998 U.S. gas consumption was approximately 22.5 trillion ft. ... 

As a constituent of natural gas, ethane is normally burned with methane as a fuel gas. Ethane s relation with petrochemicals is mainly through its cracking to ethylene. Ethylene is the largest end use of ethane in the U.S. while it is only 5% in Western Europe. Chapter 3 discusses steam cracking of ethane. 

The reverse reaction, steam cracking of methane, involves the same elementary steps as the methanation reaction. The kinetics for that reaction have been developed for a single direct mechanism by Snagovskii and Ostrovskii (39). 

Almost any naphthenic or parallflnic hydrocarbon heavier then methane can he steam-cracked to yield elhylene. The preferred feedstock in the United Slates has been ethane and/or propane recovered from natural gas. or from the volatile fractions of petroleum. However, because of longterm uncertainties pertaining to natural gas, many producers have been turning to heavier petroleum fractions, such as gas oils, as feedstock. The consumption of ethylene throughout the free world is estimated to be about 40 x 10 pounds per year,... 

Carbon dioxide is produced in petrochemical process streams by reactions with oxygenates (mainly oxygen or water). In steam cracking, hydrocarbons (e.g. methane) and carbon react with steam, forming initially carbon monoxide which is then converted into carbon dioxide by the water-gas-shift reaction ... 

Reaction of Steam on Hydrocarbons. The catalytic interaction of steam and hydrocarbons has been used commercially on a large scale. The thermal cracking of hydrocarbons is an important part of petroleum refining and produces a large amount of hydrogen. The reaction of steam on methane from natural gas at about 1100 °C is... 

Metal granules also have been found in cokes formed or deposited on iron, cobalt, and nickel foils in experiments using methane, propane, propylene, and butadiene (7-10). Platelet-type coke, whose properties match those of graphite also was produced in some cases. Lahaye et al. (11) investigated the steam cracking of cyclohexane, toluene, and n-hexane over quartz, electrode graphite, and refractory steel. They report that heavy hydrocarbon species form in the gas phase, condense into liquid droplets which then strike the solid surface, and finally react on the solid surfaces to produce carbonaceous products. The liquid droplets wet and spread out on certain surfaces better than on others. 

Fig. 4, TPCR spectra of steam with cracked methane for A-Ol and B,...

Use Source of petrochemicals by conversion to hydrogen and carbon monoxide by steam cracking or partial oxidation. Important products are methanol, acetylene, hydrogen cyanide, and ammonia. Chlorination gives carbon tetrachloride, chloroform, methylene chloride, and methyl chloride. In the form of natural gas, methane is used as a fuel, as a source of carbon black, and as the starting material for manufacture of synthetic proteins. 

H-Oil unit are processed for sulfur recovery and then sent for separation through the gas recovery facilities associated with the steam cracker. Remaining unconverted residue from the H-Oil operation is used as a fuel oil component for plant fuel. Ethylene is manufactured by steam cracking of ethane, propane, naphtha, and distillate, and products from these operations are separated in conventional gas recovery facilities. Hydrogen for H-Oil is partially supplied by by-product recovery from steam cracker and H-Oil off-gases supplemented by steam reforming of methane. The heavy oils produced in steam cracking of naphtha and distillate are blended with the H-Oil residue to yield plant fuel. 

An alternative to the steam reforming of methane is single-step thermocatalytic decomposition or thermal cracking . Methane and other hydrocarbons decompose at temperatures between 700 and 980 °C and, if no air is present, form hydrogen and carbon ... 

Gas-phase, steam cracking reactions dominate the chemistry of biomass gasification. At temperatures above 650°C, these reactions proceed very rapidly and generate a hydrocarbon rich syngas containing commercially interesting amounts of ethylene, propylene, and methane. Increased pressure appears to inhibit the gasification process. 

In the USA, and to some extent in Great Britain and Norway, ethane is the dominant feedstock for steam cracking. It is recovered from wet natural gas and gives high yields of ethylene, hydrogen and methane. From naphtha, the preferred feedstock in Europe and Japan, additional principal products are propylene, C4 hydrocarbons and pyrolysis naphtha as well as highly aromatic pyrolysis tar. 

Typically steam cracking of naphtha produces ethene (32%), propene (13%) and butadiene (4.5%). By-products consist of aromatics (13.5%), methane, hydrogen, gasoline and fuel oil (total 37%). The exact proportions are a function of temperature. A process by which the olefins are separated is described on p. 89. 

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