Feedstock Purification

Hydrocarbons that can be fed to ammonia plants include natural gas, associated gas, liquid petroleum gas, and naphthas boiling up to 220 = C. Higher hydrocarbons are not used in primary steam reforming because it would lead to coke formation on the catalysts. Hydrocarbons are usually contaminated with variable quantities of different sulfur compotmds and often contain chlorides. These catalyst poisons must be removed before the other catalysts in the plant can operate in a satisfactory manner. 

Impurities in natural gas are usually simple sulfur compoimds such as hydrogen sulfide and mercaptans and are easily removed. Where the gas supply is also used for domestic purposes a stenching agent such as a thioether or thiophene may have been added. These less-reactive compoimds have to be subjected to a hydrogenolysis treatment before they can be removed. Naphthas contain several hundred parts per million of more complex organic sulfur compounds and may be pre-treated in a hydrofiner before use in ammonia or hydrogen production. Variation in the sulfur content of feed does not affect the performance 

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Feedstock Purification Manufacture of Synthesis Gases Hydrogen, Ammonia, Methanol, product bulletin. United Catalysts, Inc., Louisville, Ky. 

Steam-Reforming Natural Gas. Natural gas is the single most common raw material for the manufacture of ammonia. A typical flow sheet for a high capacity single-train ammonia plant is iadicated ia Figure 12. The important process steps are feedstock purification, primary and secondary reforming, shift conversion, carbon dioxide removal, synthesis gas purification, ammonia synthesis, and recovery. 

Feed Ga.s Purifica.tion. Because nickel-based reforming catalysts are quite sensitive to sulfur, halogen, and heavy metal poisons which may be found ia natural gas, a feedstock purification system is normally required. Sulfur compounds, ia both organic and inorganic forms, are the most common... 

Figure 5.9. Potential configurations of reformer feedstock purification equipment. (Reproduced by permission of Johnson Matthey Catalysts. Copyright Johnson Matthey PLC )...

Feedstock Purification. In feedstock purification, mainly desulfurization, adsorption on active carbon was replaced by catalytic hydrogenation over cobalt-molybdenum or nickel-molybdenum catalyst, followed by absorption of the H2S on ZnO pellets with formation of ZnS. By itself this measure has no direct influence on the energy consumption but is a prerequisite for other energy saving measures, especially in reforming and shift conversion. 

The viability of feedstock purification should be evaluated. Basic raw materials such as crude oil and minerals inevitably contain impurities. In most cases, it is best to remove impurities as early as possible in the processing chain. 

This section describes batchwise fixed-bed adsorbers in which the adsorbent is replaced with fresh material, or removed and regenerated after it is exhausted, then reinstalled. Commercial examples include columns used for chemical feedstock purification, decolorizing solutions, and wastewater treatment. The goal is generally to employ material balance and rate equations to predict adsorber performance, possibly to analyze experimental data (e.g., breakthrough curves and temperature histories), to diagnose problems, or to assess properties or conditions. Unfortunately, various conditions often result in nearly identical behavior, so diagnosing causes may be difficult. 

Feedstock purification. This process removes poisons, including sulfur and chloride, to increase the life of the downstream steam-reforming process and that of the other catalysts. 

For coal and heavy oil based syngas production, feedstock purification is, to date, not possible. Hence during partial oxidation the organic sulfur compounds are converted into H2S as we have a reducing atmosphere, at least after the oxygen has been consumed. Thus, H2S and other unwanted impurities like ash, tar, and solid carbon (soot) have to be separated from the raw syngas before it can be used further. 

In feedstock purification - mainly desulfurization - the development was from adsorption of sulfur-compounds on active carbon to catalytic hydrogenation of sulfur-compounds followed by adsorption of hydrogen sulfide on hot zinc oxide. This concept has been of major importance for the development of processes based on reforming at severe conditions including reforming of naphtha. See also Sect. 6.3.1. 

Feedstock purification as such has no influence on the energy consumption. But efficient purification is a prerequisite for other energy saving measures, especially in reforming and shift conversion. 

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