Natural gas partial oxidation

Beaudette, T.M., Bochow, C. Jr., Slivensky, D., Natural Gas Partial Oxidation for Chemical Processing in Longview, Texas, Gasification Technologies 2001, San Francisco, CA, October... 

The partial oxidation of natural gas has been considered as an alternative to steam reforming for many years. While natural gas partial oxidation was practiced commercially for ammonia manufacture over 30 years ago (ref. 2), it s use for methanol synthesis remains to be demonstrated commercially. Autothermic reforming (ref. 3), and various other improvements, such as low energy distillation (ref. 4), demonstrate that methanol production can be improved in the short term. 

Use by the refining sector of processes initially developed by petrochemicals such as steam-reforming of natural gas, partial oxidation of residues, Fischer-Tropsch synthesis etc. 

For natural gas, partial oxidation is usually not an economical option as the investment costs for the required cryogenic air separation are high. Only in cases where syngas with a high CO content is needed is partial oxidation of natural gas - mostly then in combination with a steam reformer - an option. Thus partial oxidation is mostly applied where steam-reformable feeds are not available and where local conditions provide relatively cheap heavy feedstocks such as coal or heavy oil. 

Figure 15-11. Integrated membrane plus PSA system in a natural gas partial oxidation (POX) plant (Doshi etal.,...

Hydrogen production from fossil fuels is based on steam reforming of natural gas, thermal cracking of natmal gas, partial oxidation of heavier than naphtha... 

The Tandem process developed in Russia also makes use of two reactors, a feature that allows it to more efficiently combine the processes of steam reforming and partial oxidation [320]. The Syntroleum has developed a process of ATR of natural gas with oxidation by air as a cheaper method of producing syngas at medium-capacity plants with a production capacity in GTL-products beginning from 2000 bbl/day. 

The original method for the manufacture of ethyne, the action of water on calcium carbide, is still of very great importance, but newer methods include the pyrolysis of the lower paraffins in the presence of steam, the partial oxidation of natural gas (methane) and the cracking of hydrocarbons in an electric arc. 

Partial Oxidation. It is often desirable to augment the supply of naturally occurring or by-product gaseous fuels or to produce gaseous fuels of well-defined composition and combustion characteristics (5). This is particularly tme in areas where the refinery fuel (natural gas) is in poor supply and/or where the manufacture of fuel gases, originally from coal and more recently from petroleum, has become well estabHshed. 

As in the case of coal, synthetic natural gas can be produced from heavy oil by partially oxidizing the oil to a mixture of carbon monoxide and hydrogen... 

Synthesis Gas Chemicals. Hydrocarbons are used to generate synthesis gas, a mixture of carbon monoxide and hydrogen, for conversion to other chemicals. The primary chemical made from synthesis gas is methanol, though acetic acid and acetic anhydride are also made by this route. Carbon monoxide (qv) is produced by partial oxidation of hydrocarbons or by the catalytic steam reforming of natural gas. About 96% of synthesis gas is made by steam reforming, followed by the water gas shift reaction to give the desired H2 /CO ratio. 

There has been considerable research into the production of substitute natural gas (SNG) from fractions of cmde oil, coal, or biomass (see Euels SYNTHETIC, Euels frombiomass Euels fromwaste). The process involves partial oxidation of the feedstock to produce a synthesis gas containing carbon... 

The Texaco process was first utilized for the production of ammonia synthesis gas from natural gas and oxygen. It was later (1957) appHed to the partial oxidation of heavy fuel oils. This appHcation has had the widest use because it has made possible the production of ammonia and methanol synthesis gases, as well as pure hydrogen, at locations where the lighter hydrocarbons have been unavailable or expensive such as in Maine, Puerto Rico, Brazil, Norway, and Japan. 

Because the synthesis reactions are exothermic with a net decrease in molar volume, equiUbrium conversions of the carbon oxides to methanol by reactions 1 and 2 are favored by high pressure and low temperature, as shown for the indicated reformed natural gas composition in Figure 1. The mechanism of methanol synthesis on the copper—zinc—alumina catalyst was elucidated as recentiy as 1990 (7). For a pure H2—CO mixture, carbon monoxide is adsorbed on the copper surface where it is hydrogenated to methanol. When CO2 is added to the reacting mixture, the copper surface becomes partially covered by adsorbed oxygen by the reaction C02 CO + O (ads). This results in a change in mechanism where CO reacts with the adsorbed oxygen to form CO2, which becomes the primary source of carbon for methanol. 

In addition to these principal commercial uses of molybdenum catalysts, there is great research interest in molybdenum oxides, often supported on siHca, ie, MoO —Si02, as partial oxidation catalysts for such processes as methane-to-methanol or methane-to-formaldehyde (80). Both O2 and N2O have been used as oxidants, and photochemical activation of the MoO catalyst has been reported (81). The research is driven by the increased use of natural gas as a feedstock for Hquid fuels and chemicals (82). Various heteropolymolybdates (83), MoO.-containing ultrastable Y-zeoHtes (84), and certain mixed metal molybdates, eg, MnMoO Ee2(MoO)2, photoactivated CuMoO, and ZnMoO, have also been studied as partial oxidation catalysts for methane conversion to methanol or formaldehyde (80) and for the oxidation of C-4-hydrocarbons to maleic anhydride (85). Heteropolymolybdates have also been shown to effect ethylene (qv) conversion to acetaldehyde (qv) in a possible replacement for the Wacker process. 

Partial oxidation of natural gas or a fuel oil using oxygen may be used to form acetylene, ethylene (qv) and propylene (qv). The ethylene in turn may be partially oxidi2ed to form ethylene oxide (qv) via advantages (/) and (5). A few of the other chemicals produced using oxygen because of advantages (/) and (5) are vinyl acetate, vinyl chloride, perchloroethylene, acetaldehyde (qv), formaldehyde (qv), phthaHc anhydride, phenol (qv), alcohols, nitric acid (qv), and acryhc acid. 

Partial oxidation of coal to form either synthetic fuel, syngas, or synthetic natural gas represents a potential use of oxygen (see Fuels, synthetic). 

Of the raw material hydrogen sources—natural gas, coal, and petroleum fractions—natural gas is the most often employed in ammonia plants in the 1990s and steam reforming is by far the most often used process. Partial oxidation processes are utilized where steam-reformable feeds are not available or in special situations where local conditions exist to provide favorable economics. Table 5 fists the contribution of the various feedstocks to world ammonia... 

Heavy Hydrocarbon-Based Partial Oxidation Processes. Two major partial oxidation processes are commercially available, the SheU process (38) and the Texaco process (39). Operating conditions in the gas generator vary from 1200°C to 1370°C and from 3100 kPa to 8270 kPa (450—1200 psig). Generally, heavy oils are the hydrocarbon feeds however, the process can also accommodate feeds from natural gas to residual oils. 

Capital costs which foUow the same trend as energy consumption, can be about 1.5 to 2.0 times for partial oxidation and coal gasification, respectively, that for natural gas reforming (41). A naphtha reforming plant would cost about 15—20% more than one based on natural gas because of the requirement for hydrotreatiag faciUties and a larger front-end needed for carbon dioxide removal. 

Conventional Transportation Fuels. Synthesis gas produced from coal gasification or from natural gas by partial oxidation or steam reforming can be converted into a variety of transportation fuels, such as gasoline, aviation turbine fuel (see Aviation and other gas turbine fuels), and diesel fuel. A widely known process used for this appHcation is the Eischer-Tropsch process which converts synthesis gas into largely aHphatic hydrocarbons over an iron or cobalt catalyst. The process was operated successfully in Germany during World War II and is being used commercially at the Sasol plants in South Africa. 

Alternatively it is obtained from eraeking low moleeular-weight aliphatie hydroearbons, or by the partial oxidation of natural gas. 

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