High Temperature Conversion Catalysts

The first HTS catalysts were reported to operate for about two years before replacement was required. As production techniques were developed, however, catalyst lives improved so that by 1940, lives of more than 14 years were regularly achieved. There were few poisons which affected the catalyst performance although sulfirr, which was the most common impurity in early plants, did sulfide the magnetite. This reaction was, nevertheless, reversible. If hydrogen sulfide levels exceeded about 300 ppm, sulfided catalysts could not be regenerated 

In modem, single stream ammonia plants there is little scope in the design to make significant changes to the operating conditions in any of the individual catalyst reactors. Operating conditions for the carbon monoxide conversion reaction are shown in Table 9.14. The only practical variable is operating temperature which can be slowly increased as catalyst loses activity. 

The composition of the catalyst can affect its performance in many ways. For example, changes in the formulation to lower the amoimt of hydrogen sitl-fide evolved during reduction led to a significant physical weakening of the catalyst, which became much more prone to damage caused by the condensation of water or contamination by potash. Any water that condensed ditring plant start-up could also wash out any soluble chromium and lead to loss of stability. Most problems led to an increase in pressure drop or maldistribution of gas. Cat 

TABLE 9.14. Caibon Monoxide Conversion Catalysts Operation. 

Carbon monoxide also reacted with magnetite forming carbides which catalyzed the production of hydrocarbons by Fischer-Tropsch reactions. It has since been shown that both reactions can be suppressed by the addition of copper to the existing iron-chromium catalyst. This has allowed operation of HTS catalysts in existing plants down to a steam ratio as low as 0.4 compared with about 0.6 in the early single stream plants. 

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Iron-chromium oxide catalysts, reduced with hydrogen-containing in the conversion plants, permit reactions temperatures of 350 to 380°C (high temperature conversion), the carbon monoxide content in the reaction gas is thereby reduced to ca. 3 to 4% by volume. Since, these catalysts are sensitive to impurities, cobalt- and molybdenum-(sulfide)-containing catalysts are used for gas mixtures with high sulfur contents. With copper oxide/zinc oxide catalysts the reaction proceeds at 200 to 250°C (low temperature conversion) and carbon monoxide contents of below 0.3% by volume are attained. This catalyst, in contrast to the iron oxide/chromium oxide high temperature conversion catalyst, is, however, very sensitive to sulfur compounds, which must be present in concentrations of less than 0.1 ppm. 

Compared with the common high-temperature conversion of natural gas and further carbon oxide conversion on a catalyst [131], the current process promotes process simplification the reaction is implemented at relatively low temperature (860-900 °C instead of 1400-1600 °C for existing non-catalytic processes of methane conversion) and an additional unit for catalytic conversion of carbon oxide is excluded (in NH3 production). 

High-temperature conversion employs catalysts based on iron oxides (80 to 95 per cent weight) and chromium (5 to 10 per cent wei t) which can withstand the presence of small amounts of sulfur products without an excessive loss of activity. They operate between 300 and 450 C, and as high as with volume hpuily space velodties of 300 to 3000 h and lead to residual CO contents of 1 to 2 per cent volume. 

Hydroxylation of phenol was carried out over hydrotalcitcs calcined at different temperatures whose results arc summarized in Table 10. CuNiAI3-5 calcined at 150, 400 and 600"C showed a similar conversion ( 12%) while the sample calcined at 800" C exhibited a higher conversion (18%). In the ease of CuNiAl2-l, a continuous increase in the conversion with an increase in the calcination temperature was observed. The higher activity of the high temperature calcined catalysts (800 C) could be due to the inherent activity of spinel phase, which crystallizes around this temperature, in mediating the reaction. Further, no significant... 

Depending on the temperatures at which the carbon monoxide is shifted, another distinction is made between high-temperature shift conversion (300-500 °C) and low-temperature shift conversion (180-280°C). Low- temperature shift conversion is, however, normally used only if the residual CO content in the converted gas has to be very low. As this is not the case fcx methanol production, and as there is no reason to put up with the high vulnerability to sulfur of the copper catalysts used for low-temperature conversion nor their considerable cost, the following description will be limited to high-temperature conversion. 

No conversion data are presented for gas inlet temperatures over 550 - 600 C, since this was the calcination temperature for all monolith catalysts. The catalysts presented here are therefore not particularly suitable as high-temperature combustion catalysts for hydrocarbons. However, they may have interesting properties for removal of volatile organic compounds at lower temperatures, which is now under investigation. 

The eight kinds of catalysts may be roughly classified as protective catalysts and economic catalysts . Co-Mo hydrogenation catalyst and zinc oxide desulfurizer are the protective catalysts for the primary steam reforming catalysts. The high-temperature shift catalyst protects the low-temperature shift catalyst, and the methanation catalyst are the protective catalyst for ammonia synthesis catalyst. The catalysts for primary- and secondary-steam reforming, low-temperature shift and ammonia synthesis are responsible for the conversions of raw materials and the yield of products, and have direct effect on economic benefits of the whole plant, and are thus called as economic catalysts. The amount of catalysts used depends on the process and raw material. Table 1.2 represents the amount of the eight kinds of catalysts used in the different processes. The total volume of the catalysts is about 330 m in every plant, while there are only two kinds of catalysts with the volume of about 100-140 m when heavy oil or coal is used as raw material. Both shift... 

The gas is then passed through the bed of catalyst in the first (high temperature) conversion stage. About 80 to 95% of the carbon monoxide is converted to carbon dioxide, and a quantity of hydrogen is produced, which is equivalent to that of the carbon monoxide reacted. A typical temperature rise across this reactor is 1(X)°F. The first conversion stage is primarily intended for the production of hydrogen and is, therefore, not considered to be a gas purification step. 

As Table 6.9 shows, conversions of cellulose decrease with the increase of the calcination temperature that corresponds in fact to a less defected catalyst structure. On the high temperature calcined catalysts (i.e., 500°C), under the harsh reaction conditions, the only species able to initiate hydrolyzing of cellulose to smaller oligomers are acidified water molecules (see Figure 6.26 and Table 6.9, entry 6). 

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). 

Mobil s High Temperature Isomerization (MHTI) process, which was introduced in 1981, uses Pt on an acidic ZSM-5 zeoHte catalyst to isomerize the xylenes and hydrodealkylate EB to benzene and ethane (126). This process is particularly suited for unextracted feeds containing Cg aHphatics, because this catalyst is capable of cracking them to light paraffins. Reaction occurs in the vapor phase to produce a PX concentration slightly higher than equiHbrium, ie, 102—104% of equiHbrium. EB conversion is about 40—65%, with xylene losses of about 2%. Reaction conditions ate temperature of 427—460°C, pressure of 1480—1825 kPa, WHSV of 10—12, and a H2/hydtocatbon molar ratio of 1.5—2 1. Compared to the MVPI process, the MHTI process has lower xylene losses and lower formation of heavy aromatics. 

The 1990 Clean Air Act mandates for blended oxygenates ia gasoline created a potentially large new use for DIPE as a fuel oxygenate. Isopropyl alcohol can react with propylene over acidic ion-exchange (qv) catalysts at low temperatures, which favor high equiUbrium conversions per pass to produce DIPE (34). 

At conditions of high temperature and low pressure, for sufficient catalyst activity and acceptable reaction rates, equiUbrium conversions maybe as low as 5%, necessitating recycle of large amounts of unreacted propylene (101). 

If the production of vinyl chloride could be reduced to a single step, such as dkect chlorine substitution for hydrogen in ethylene or oxychlorination/cracking of ethylene to vinyl chloride, a major improvement over the traditional balanced process would be realized. The Hterature is filled with a variety of catalysts and processes for single-step manufacture of vinyl chloride (136—138). None has been commercialized because of the high temperatures, corrosive environments, and insufficient reaction selectivities so far encountered. Substitution of lower cost ethane or methane for ethylene in the manufacture of vinyl chloride has also been investigated. The Lummus-Transcat process (139), for instance, proposes a molten oxychlorination catalyst at 450—500°C to react ethane with chlorine to make vinyl chloride dkecfly. However, ethane conversion and selectivity to vinyl chloride are too low (30% and less than 40%, respectively) to make this process competitive. Numerous other catalysts and processes have been patented as weU, but none has been commercialized owing to problems with temperature, corrosion, and/or product selectivity (140—144). Because of the potential payback, however, this is a very active area of research. 

Binders. To create needed physical strength in catalysts, materials called binders are added (51) they bond the catalyst. A common binder material is a clay mineral such as kaolinite. The clay is added to the mixture of microparticles as they are formed into the desired particle shape, for example, by extmsion. Then the support is heated to remove water and possibly burnout material and then subjected to a high temperature, possibly 1500°C, to cause vitrification of the clay this is a conversion of the clay into a glasslike form that spreads over the microparticles of the support and binds them together. 

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