Catalyst iron oxide high temperature shift

This reaction is first conducted on a chromium-promoted iron oxide catalyst in the high temperature shift (HTS) reactor at about 370°C at the inlet. This catalyst is usually in the form of 6 x 6-mm or 9.5 x 9.5-mm tablets, SV about 4000 h . Converted gases are cooled outside of the HTS by producing steam or heating boiler feed water and are sent to the low temperature shift (LTS) converter at about 200—215°C to complete the water gas shift reaction. The LTS catalyst is a copper—zinc oxide catalyst supported on alumina. CO content of the effluent gas is usually 0.1—0.25% on a dry gas basis and has a 14°C approach to equihbrium, ie, an equihbrium temperature 14°C higher than actual, and SV about 4000 h . Operating at as low a temperature as possible is advantageous because of the more favorable equihbrium constants. The product gas from this section contains about 77% H2, 18% CO2, 0.30% CO, and 4.7% CH. ... 

A high temperature water-gas shift reactor 400°C) typically uses an iron oxide/chromia catalyst, while a low temperature shift reactor ( 200°C) uses a copper-based catalyst. Both low and high temperature shift reactors have superficial contact times (bas on the feed gases at STP) greater than 1 second (72). 

As shown above, Eq. (5.2) is exothermic, and high temperatures are unfavorable for complete conversion. In the High Temperature Shift (HTS) conversion, the synthesis gas is passed through a bed of iron oxide/chromium oxide catalyst at around 400°C and a pressure of 25 to 28 bar. The CO content of the gas is reduced to about 3% (on a dry gas basis), and this is limited by the shift equilibrium at the actual operating temperature. The catalyst is not... 

Prior to 1960, the same type of catalyst was used in both steps. This catalyst is mainly iron and chromium oxides, about 55% Fe, and 6% Cr. It is active only at relatively high temperatures (350 -430°C) and is known as a high-temperature shift (HTS) catalyst. About 1960 a new copper-based catalyst came into use, which is active at lower temperatures (200 -260 C) and is known as low-temperature shift (LTS) catalyst. Most LTS catalysts contain zinc and alumina in addition to copper and are poisoned by sulfur and chlorine compounds. 

The first reactor (high-temperature shift) is loaded with high-temperature catalyst, generally chromium-promoted iron oxide, which operates at 623—673 K (Ledjeff-Hey, Roes, Wolters, 2000). The second reactor (low-temperature shift) is loaded with low-temperature catalyst of copper-promoted zinc oxide, which operates at 473 K (Ledjeff-Hey et al., 2000). 

Bulk catalysts comprise mainly active substances, but some binder is often added to aid the forming/shaping operation. This is the case for iron oxide for the water-gas shift (WGS) reaction, iron molybdate for the oxidation of methanol to formaldehyde, and vanadyl pyrophosphate for butane oxidation to maleic anhydride. However, in some cases, bulk catalysts are used as prepared, without the need for addition of the binder. Typically, this involves catalysts prepared by high temperature fusion (eg, the iron-based ammonia synthesis catalyst). The need for the addition of binder, or the requirement for pelleting, solely depends on the strength required for the catalyst under the reaction conditions and the reactor type that is used in. This requires consideration of attrition resistance, and oxide... 

A breadboard gasoline fuel processor was assembled by Moon et d. [67]. Fixed bed reactors served for reforming by steam supported partial oxidation (see Section 7.1.1), followed by high and low temperature water-gas shift Commercial iron oxide/ chromium oxide catalyst was applied for high temperature shift at a 4200 h gas hourly space velocity and 450 °C reaction temperature, while the copper/zinc oxide low temperature water-gas shift catalyst was operated at 250 °C and 5600 h gas hourly space velocity. 

Fes04 is an active component of high-temperature shift catalysts. Iron oxides prepared by different methods have different compositions and crystal phase. Some results show that the catalysts derived from the precursors of 7 — Fe203 and Fe (0H)3 — are the most active ones. 

The usual modern practice is to use a two-stage process. In the first stage, high-temperature shift conversion, an iron oxide/chromium oxide catalyst operates in the range 350-500 C. The carbon monoxide concentration can be reduced to about 3% (dry basis) in a single adiabatic bed. The second stage, low-temperature shift conversion, uses a bed of copper oxide/zinc oxide/alumina operating at 200-250 °C to reduce the carbon monoxide content to between 0.2 and 0.4% (dry basis). 

When the steam to carbon ratio in the reforming section is reduced, the conditions in the shift section must be carefully evaluated. If the steam to dry gas ratio becomes too low, severe problems may arise due to conversion of the iron oxide in the high temperature shift catalyst to iron carbide, which will promote formation of undesirable by-products (hydrocarbons and oxygenates) (see Sect. 6.3.3.1). 

Since then the basic formulation of high temperature shift catalyst (HTS) has been iron oxide, stabilized with chromium oxide, although the methods of stabilizing the catalysts and producing it in large quantities have been refined as operating conditions have changed... 

Reforming is completed in a secondary reformer, where air is added both to elevate the temperature by partial combustion of the gas stream and to produce the 3 1 H2 N2 ratio downstream of the shift converter as is required for ammonia synthesis. The water gas shift converter then produces more H2 from carbon monoxide and water. A low temperature shift process using a zinc—chromium—copper oxide catalyst has replaced the earlier iron oxide-catalyzed high temperature system. The majority of the CO2 is then removed. 

Iron oxide is an important component in catalysts used in a number of industrially important processes. Table I shows some notable examples which include iron molybdate catalysts in selective oxidation of methanol to formaldehyde, ferrite catalysts in selective oxidative dehyrogenation of butene to butadiene and of ethylbenzene to styrene, iron antimony oxide in ammoxidation of propene to acrylonitrile, and iron chromium oxide in the high temperature water-gas shift reaction. In some other reactions, iron oxide is added as a promoter to improve the performance of the catalyst. 

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