Ammonia synthesis catalyst types

Since theoretical calculation of effectiveness is uncertain and is moreover sensitive to operating conditions, for industrially important cases it is determined by such reaction tests. Common types of curve fits may be used. For ammonia synthesis catalyst, for instance, an equation is provided by Dyson Simon (IEC Fundam 7 605, 1968) in terms of temperature and... 

Bridger, G.W. Snowden, C.B. (1970) Ammonia synthesis catalysts. In Catalyst Handbook. Wolfe Scientific Books, 126-147 Brindley, G.W. Bish, D.L. (1976) Green rust a pyroaurite type structure. Nature 263 353 Bromfield, S.M. Williams, E.G. (1963) An examination of the biological reduction method for estimating active iron in soils. J. Soil Sd. 14 346-359... 

The passivation by oxygen of a commercial ammonia synthesis catalyst was studied with adsorption microcalorimetry by Tsarev and co-workers (240). Two types of adsorbed oxygen at 293 K were found to participate in the formation of a passivating layer. One type was characterized by differential heats of adsorption near 420 kJ mol" that were close to the heat of iron oxidation and which were independent of surface coverage for several mono-layers. The other form was obtained after a large dose, sufficient for coverage of the entire metal surface with a molecular monolayer. Subsequent adsorp-... 

KBR offer various schemes for production of ammonia. The most widely used converter is the KBR horizontal inter cooled ammonia converter using iron based ammonia synthesis catalyst. The ammonia loop is of the inert free type, because the synthesis gas is treated in a molecular sieve unit followed by a cold box (purifier unit). The make-up gas is mixed directly with the recirculating gas in the synthesis gas compressor. The ammonia chillers for condensing the ammonia product are combined in one unit (unitised chiller)... 

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... 

Catalysts have often been referred to, but few details have been given. For heterogeneous reactions of the above type, catalysts are mostly manufactured as the metal oxide on a ceramic support and reduced in situ to their active state. (The iron oxide ammonia synthesis catalyst is a major exception, in that it has no support but is simply the oxide with some promoters.) The shape of typical catalysts varies from lumps to pellets to granules, and in addition to being firm... 

It will be seen from above discussion that the activity of ammonia synthesis catalyst correlates not only with the chemical compositions, but also the crystal types and crystal structure of iron oxides. The relationships between the activity and the Fe +/Fe + ratio can be interpreted perfectly by the molecular ratio / of the iron oxides, which have the different crystal structures in their precursors. At the same time, it also gives the theoretical explanation for those results of the classical catalysts (Fig. 3.27). For example, for the classical volcano-type activity curve, when Fe +/Fe + = 0.5, then / = f (Eqs. 3.16 and 3.17), so the catalyst has the good activity both sides at Fe +/Fe + = 0.5, due to / < 1, so the activity of the catalyst decreases. 

Presently, the Fe-based ammonia synthesis catalysts used in industry are produced by traditional molten method, and are also known as fused iron catalysts. The advantages of molten method include simple processes, easy operation, low cost and excellent performance of product. Even though there are some differences in the chemical composition and catalytic performance of various types of fused iron catalysts, their manufacture process is mainly the same as follow. 12... 

The same type of fused iron catalyst may exhibit different structures and activities after reduction under different conditions (e.g., temperature, pressure, space velocity and gas composition etc.). Reduction condition is the external factor which affects the physical-chemical properties of catalysts. Thus, different reduction conditions are required for catalysts with different t3rpes, particle sizes or different types and content of promoters. The selection of the optimized reduction condition is very important to obtain a high performance for ammonia synthesis catalysts. It is the main reason to study the reductive performance and related kinetics of catalysts. 

At present, the two major types of ammonia synthesis catalyst are the fused iron catalyst and the ruthenium catalyst, but the fused iron catalyst is still the primary catalyst in use. A fused iron catalyst may be classified in several ways according to the operating temperature as medium-temperature and low-temperature according to the state before use as pre-reduction and oxidized state or according to shape as irregular shape and regular shape. The ruthenium catalyst is a low temperature catalyst, but is not widely used because of its high price. 

Table 8.39 Main types of ammonia synthesis catalysts in the world... 

In methanol synthesis, the case for radial flow converters is less obvious. Tops0e has earlier proposed, the use of one-bed radial flow converters in large methanol plants. Later analyses, partly based on a change of catalyst type, have, however, led to the conclusion that axial flow should be preferred even in very large methanol synthesis converters. The reasons for this difference in reactor concept between ammonia synthesis and methanol synthesis are in the differences between the properties of the catalysts. As mentioned above, the ammonia synthesis catalyst is ideally suited for the radial flow principle. This is not true for the methanol synthesis catalyst. The reasons for not using the radial flow principle in methanol synthesis are the following ... 

S. Tauster, Poisoning Experiment with Iron-Type Ammonia Synthesis Catalysts, Doctoral Thesis, Polytechnic Institute of Brooklyn, June 1964 (available university microfilms, 64-10, 728). 

The second chapter Structure and Surface Chemistry of Industrial Ammonia Synthesis Catalysts is written by Dr. Per Stoltze. This chapter deals with the structure and surface chemistry of iron-based ammonia synthesis catalysts of the type used by industry. Certain studies of single crystal surfaces are included to the extent that they serve to add information to the main topic. This chapter includes a presentation of the unreduced catalyst the reduction process and the bulk and surface structure of a reduced catalyst. A thorough discussion is given of the different states of sorption of nitrogen and of chemisorption of hydrogen, carbon oxides, ammonia and oxygen. The last part of the chapter gives a detailed account of the mechanism of ammonia synthesis on iron. 

Figure 4-8 shows a continuous reactor used for bubbling gaseous reactants through a liquid catalyst. This reactor allows for close temperature control. The fixed-bed (packed-bed) reactor is a tubular reactor that is packed with solid catalyst particles. The catalyst of the reactor may be placed in one or more fixed beds (i.e., layers across the reactor) or may be distributed in a series of parallel long tubes. The latter type of fixed-bed reactor is widely used in industry (e.g., ammonia synthesis) and offers several advantages over other forms of fixed beds. 

At integrating (305) for the conditions of a flow system (93, 98), it proved to be convenient to introduce a constant k proportional to k. The value of k was also calculated from data obtained in circulation flow systems (4, 96, 99-103). If the volume of ammonia reduced to 0°C and 1 atm, formed in unit volume of catalyst bed per hour, is accepted as a measure of reaction rate, then k = (4/3)3 1 m)k (101). The constancy of k at different times of contact of the gas mixture with the catalyst and different N2/H2 ratios in the gas mixture can serve as a criterion of applicability of (305). Such constancy was obtained for an iron catalyst of a commercial type promoted with A1203 and K20 at m = 0.5 (93) from our own measurements at atmospheric pressure in a flow system and literature data on ammonia synthesis at elevated pressures up to 100 atm. A more thorough test of applicability of (305) to the reaction on a commercial catalyst at high pressures was done by means of circulation flow method (99), it confirmed (305) with m = 0.5 for pressures up to 300 atm. Similar results were obtained in a large number of investigations by different authors in the USSR and abroad. These authors, however, have obtained for some promoted iron catalysts m values differing from 0.5. Thus, Nielsen et al. (104) have found that m 0.7. 

The following picture has been formed of reduced iron catalysts corresponding to the promoted types used for ammonia synthesis. 

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