Steam reforming light hydrocarbons

This chapter is concerned with the work reported in the literature on the steam reforming df hydrocarbons which has been done since 1974 when the earlier review by Ross was written. For continuity some reference has had to be made to research covered in that review and some work before 1974 not described there is included here. Hydrocarbon steam reforming is still a process of major importance for the manufacture of hydrogen, synthesis gases, and town gas and, in the last five years, for the production of substitute natural gas. The study of reactions between hydrocarbon and steam on catalytic surfaces has continued to be an area of interest, throwing light on the mechanism of hydrocarbon decomposition and on the properties, of metal surfaces. 

At present, natural gas and light hydrocarbons are the predominant feedstocks for the production of synthesis gas. Synthesis gas can also be produced from heavy hydrocarbons or coal, but in that case noncatalytic partial oxidation processes will be used (see the section Hydrocarbon Steam Reforming). 

Hydrocarbon steam reforming catalysts are classified into natural gas steam reforming catalysts and light-oil steam reforming catalysts according to the feedstock, and primary- and secondary- steam reforming catalysts according to the processes. 

Secunda discharges no process water effluents. AU. water streams produced are cleaned and reused in the plant. The methane and light hydrocarbons in the product are reformed with steam to generate synthesis gas for recycle (14). Even at this large scale, the cost of producing fuels and chemicals by the Fischer-Tropsch process is dominated by the cost of synthesis gas production. Sasol has estimated that gas production accounts for 58% of total production costs (39). 

Steam Reforming. In steam reforming, light hydrocarbon feeds ranging from natural gas to straight mn naphthas are converted to synthesis gas (H2, CO, CO2) by reaction with steam (qv) over a catalyst in a primary reformer furnace. This process is usually operated at 800—870°C and 2.17—2.86... 

Tubular Fixed-Bed Reactors. Bundles of downflow reactor tubes filled with catalyst and surrounded by heat-transfer media are tubular fixed-bed reactors. Such reactors are used most notably in steam reforming and phthaUc anhydride manufacture. Steam reforming is the reaction of light hydrocarbons, preferably natural gas or naphthas, with steam over a nickel-supported catalyst to form synthesis gas, which is primarily and CO with some CO2 and CH. Additional conversion to the primary products can be obtained by iron oxide-catalyzed water gas shift reactions, but these are carried out ia large-diameter, fixed-bed reactors rather than ia small-diameter tubes (65). The physical arrangement of a multitubular steam reformer ia a box-shaped furnace has been described (1). 

Steam Reforming Processes. In the steam reforming process, light hydrocarbon feedstocks (qv), such as natural gas, Hquefied petroleum gas, and naphtha, or in some cases heavier distillate oils are purified of sulfur compounds (see Sulfurremoval and recovery). These then react with steam in the presence of a nickel-containing catalyst to produce a mixture of hydrogen, methane, and carbon oxides. Essentially total decomposition of compounds containing more than one carbon atom per molecule is obtained (see Ammonia Hydrogen Petroleum). 

Most industrial hydrogen is manufactured by the following hydrocarbon-based oxidative processes steam reforming of light hydrocarbons (e.g., NG and naphtha), POx of heavy oil fractions, and ATR. Each of these technological approaches has numerous modifications depending on the type of feedstock, reactor design, heat input options, by-product treatment,... 

Hydrocarbon feedstocks for steam reformers include natural gas, refinery gas, propane, LPG and butane. Naphtha feedstocks with boiling points up to about 430°F can also be used. The ideal fuels for steam reformers are light hydrocarbons such as natural gas and refinery gas, although distillate fuels are also used. Residual fuels are not used since they contain metals that can damage reformer tubes. 

In addition to natural gas, steam reformers can be used on light hydrocarbons such as butane and propane and on naphtha with a special catalyst. Steam reforming reactions are highly endothermic and need a significant heat source. Often the residual fuel exiting the fuel cell is burned to supply this requirement. Fuels are typically reformed at temperatures of 760 to 980°C (1,400 to 1,800°F). 

Process hydrogen is produced by steam reforming light hydrocarbon gases. An alternative method for hydrogen production is partial oxidation of the heavy vacuum bottoms stream or of coal. 

Steam reforming refers to the endothermic, catalytic conversion of light hydrocarbons (methane to gasoline) in the presence of steam [see Eq. (5.1)]. The reforming reaction takes place across a nickel catalyst that is packed in tubes in an externally-fired, tubular furnace (the Primary Reformer). The lined chamber reactor is called the secondary reformer , and this is where hot process air is added to introduce nitrogen into the process. Typical reaction conditions in the Primary Reformer are 700°C to 830°C and 15 to 40 bar46. 

There are basically three processes in usage today for the production of hydrogen or ammonia synthesis gas Steam Reforming for the conversion of light hydrocarbons from natural gas to straight run naphthas Partial Oxidation for heavy hydrocarbons and coal and Electrolysis of Water. 

Coal Feed. As in the case of steam reforming versus partial oxidation, a plant based on use of coal as feedstock is expected to cost more than one based on partial oxidation because of additional equipment requirements, particularly with regard to coal handling operations, gasification and raw gas treatment. Despite these cost differences, coal based plants can be justified if the cost of coal is sufficiently low, that is, relative to light or heavy hydrocarbons. The approximate investment for a coal-based plant appears to be about 2.0 times that for a natural gas plant.The investment is about 33% more for a fuel oil... 

Steam reforming using natural gas and other light hydrocarbon feed stocks will continue to be the most attractive method of manufacturing willfully-produced refinery hydrogen. Feed stocks required for reforming in U. S. refineries will increase from about 150,000 B/D crude oil equivalent in 1980 to 330,000 B/D in the year 2000. 

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