Catalytic stages

In ensuing catalytic stages, usually termed hydrogasrfication and methanation (not to be confused with the noncatalytic, direct hydrogasification processes... 

The Claus process is the most widely used to convert hydrogen sulfide to sulfur. The process, developed by C. F. Claus in 1883, was significantly modified in the late 1930s by I. G. Farbenindustrie AG, but did not become widely used until the 1950s. Figure 5 illustrates the basic process scheme. A Claus sulfur recovery unit consists of a combustion furnace, waste heat boiler, sulfur condenser, and a series of catalytic stages each of which employs reheat, catalyst bed, and sulfur condenser. Typically, two or three catalytic stages are employed. 

A derivative of the Claus process is the Recycle Selectox process, developed by Parsons and Unocal and Hcensed through UOP. Once-Thm Selectox is suitable for very lean acid gas streams (1—5 mol % hydrogen sulfide), which cannot be effectively processed in a Claus unit. As shown in Figure 9, the process is similar to a standard Claus plant, except that the thermal combustor and waste heat boiler have been replaced with a catalytic reactor. The Selectox catalyst promotes the selective oxidation of hydrogen sulfide to sulfur dioxide, ie, hydrocarbons in the feed are not oxidized. These plants typically employ two Claus catalytic stages downstream of the Selectox reactor, to achieve an overall sulfur recovery of 90—95%. 

Function 3 will be first globally described through two main catalytic stages [10-12],... 

Claus Process Parsons, Shell, Amoco Normally 15-20% HjS Production Co., through or more, with product Its licensees such as sulfur-burning, can Ford, Bacon Davis, handle H2S contents Ortloff, Olsen, and down to 5% many others 92 97% removal with 3 catalytic stages. COS CS2 not converted without provisions in design... 

Claus reaction 2H2S+S02- -3S+2H20 one thermal stage, multiple catalytic stages. Several commercial catalysts are available... 

After each catalytic conversion, the condensation/reheat/ conversion sequence is repeated. The number of cycles is dictated by the tradeoff between desired recovery and economics. Usually, a fourth catalytic stage is not justified. 

Tail gas cleanup is required because a well-designed Claus plant with three catalytic stages and fresh catalyst will recover only 95-97% of its feed sulfur (8), which is not generally sufficient to meet current emission standards. In addition, feed impurities and catalyst aging will reduce overall recovery in some plants to about 92% just before catalyst changeout. Therefore, tail-gas cleanup is required. Tail-gas treating processes are generally classified as follows ... 

Recent studies have addressed the combustion of natural gas with pure oxygen, highly diluted with exhaust gases (CO2 and H2O) in order to mitigate both NO and CO2 emissions from power plants. The so-called oxy-fuel combustion includes an air separation unit, which delivers O2 to the catalytic stage, where it is mixed with natural gas and the exhaust recycle stream. An example is the Advanced Zero Emission GT, first jointly studied by Norsk Hydro and ABB Alstom Power [28-31]. 

The modern industrialized world would be inconceivable without catalysts. Catalysis is a multidisciplinary area of chemistry, particularly industrial chemistry where around 85% of all products pass through at least one catalytic stage. Anyone who is involved with chemical reactions will eventually have something to do with catalysts. For example, the contact process for the production of sulfuric acid was introduced as early as 1880. After World War II, some catalysts for crude oil processing appeared on the US and European markets and, from an environmental standpoint, they became crucial from 1970 onwards because of their contribution to the protection of the environment and thus to a generally higher standard of living. 

When the primary stage leaves some asphaltenes in the product, the whole product, including the catalyst and solids, is sent to the secondary catalytic stage, using honeycomb-type catalysts of poreless supports to allow passage of all solids. Thus, flash distillation and catalyst recovery can be easily accomplished. 

Implementation of the enzyme mechanism is the means of increasing selectivity of chemical catalysis, because in this case substrate is converted to product in one catalytic stage, bypassing the formation of substrate intermediates (mediators may be formed). 

In the catalytic converter, after cooling, an additional 25% of the original gas mixture is converted to elemental sulphur. The second and possibly third catalytic stages are operating at a somewhat lower temperature 200-250°C. After the last sulphur condenser, the exhaust gases, containing 3 to 6 mol-% H2S, are incinerated and the resulting SO2 is released to the atmosphere. 

In this study, whole shale oil is catalytically hydrodenitrified to reduce the nitrogen to levels as low as 1 ppm in a single catalytic stage. However, for economic reasons, it appears preferable to denitrify to about 0.05 wt % nitrogen. The resulting synthetic crude resembles a petroleum distillate that can be fractionated and further processed as necessary in conventional petroleum-refining facilities. 

Figure 19.21 shows a schematic of the H-Coal process, which employs a single catalytic stage to produce a synthetic crude oil.37,38 Coal is crushed, dried, and mixed with recycle oil and hydrogen before being preheated to approximately 850°F (454°C). The preheater effluent is fed to the bottom of an ebullated bed reactor. During operation, fresh catalyst (a cobalt-molybdenum extrudate) is fed to the top of the reactor, while spent catalyst is removed from the bottom to maintain constant reactivity and inventory. The upward flow of the coal slurry and hydrogen causes... 

In agreement with the concept of using several catalytic stages, Kwak et al. [100] recently designed a method for converting waste plastics into gasoline, kerosene and... 

Enough heat is retained in the boiler combustion gas stream for the second catalytic stage of the process to proceed using this (Fig. 9.7). The sulfur dioxide/steam output is blended with the remaining two-thirds of the... 

Methane reactants within noncatalytic channels remain unburned and they must be combusted in a fully catalytic stage or in a homogenous flame. A fully catalytic monolith would cause the catalyst to reach adiabatic temperatures and to deactivate. Thus, the only practical option is to complete the combustion in a homogenous combustion process. 

Steam is produced and condensed only once for an entire tower, and the ratio of steam to hydrogen may be varied at will. Steam consumption is therefore only a small fraction of that of Fig. 13.17, but more catalytic stages are required for a given change in deuterium content. 

Overall recovery could be increased slightly by adding a third catalytic stage. However, it was not deemed necessary to use a third stage in the ASARCO-Phelps Dodge pilot plant because it represents technology well established in the Claus process. 

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