Effectiveness, catalyst ammonia synthesis

Anon( Ref 67) and Reidel (Ref 79) describe the Phillips Chemical Co plant near Houston, Texas. It is the NEC (Nitrogen Engineering Corp) medium pressure process utilizing natural gas as starting material. Resen(Ref 70) describes the manuf of ammonia at the Lion Oil Co plant in Louisiana. Frankenburg (Ref 51) studied the relationship between the nature and the effectiveness of ammonia synthesis catalysts. Other studies of synthesis catalysts were made by Odelh6g(Ref 44), Enomoto Horiuchi(Refs 53 54), Nielsen(Ref 60), Emmett(Ref 74) and Faith... 

Bare S R, Strongin D R and Somoqai G A 1986 Ammonia synthesis over iron single crystal catalysts—the effects of alumina and potassium J. Phys. Chem. 90 4726... 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

These pioneers understood the interplay between chemical equiUbrium and reaction kinetics indeed, Haber s research, motivated by the development of a commercial process, helped to spur the development of the principles of physical chemistry that account for the effects of temperature and pressure on chemical equiUbrium and kinetics. The ammonia synthesis reaction is strongly equiUbrium limited. The equiUbrium conversion to ammonia is favored by high pressure and low temperature. Haber therefore recognized that the key to a successful process for making ammonia from hydrogen and nitrogen was a catalyst with a high activity to allow operation at low temperatures where the equiUbrium is relatively favorable. 

The effect of alkali additives on N2 chemisorption has important implications for ammonia synthesis on iron, where alkali promoters (in the form of K or K20) are used in order to increase the activity of the iron catalyst. 

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

N2 and CO, respectively [31,32], Empirical knowledge about the promoting effect of many elements has been available since the development of the iron ammonia synthesis catalyst, for which some 8000 different catalyst formulations were tested. Recent research in surface science and theoretical chemistry has led to a fairly complete understanding of how a promoter works [33,34],... 

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... 

In contrast, the use of carbonyl-derived ruthenium catalysts on different supports has been explored in ammonia synthesis [120-122], The use of K2[Ru4(CO)i3] as ruthenium precursor on MgO or carbon yields especially effective catalysts for low-temperature ammonia synthesis [120, 122],... 

The effectiveness is a measure of the utilization of the internal surface of the catalyst. It depends on the dimensions of the catalyst particle and its pores, on the diffusivity, specific rate, and heat of reaction. With a given kind of catalyst, the only control is particle size to which the effectiveness is proportional a compromise must be made between effectiveness and pressure drop. In simple cases t] can be related mathematically to its parameters, but in such important practical cases as ammonia synthesis its dependence on parameters is complex and strictly empirical. Section 17.5 deals with this topic. 

In industry large pellets of a catalyst were employed (e.g., 6-8 mm in size), and the rate of the process was essentially affected by the slowness of the diffusion of ammonia in the pores of the catalyst the efficiency factor at this size of pellets is about 0.5. The effect of diffusion retardation of the ammonia synthesis was studied both at high pressures (99), when the free path of molecules is much smaller than the radius of catalyst pores so that the bulk diffusion is operative, and at pressures near to 1 atm (116), where there is a transition from the bulk to the Knudsen diffusion. 

Equation (341) solves the task of quantitatively describing the effect of water vapor, and also oxygen gas (the last being rapidly converted to water vapor at the conditions of the reaction), on the activity of commercial ammonia synthesis catalysts. This result is of practical importance for ascertaining the necessary degree of purity of the inlet gas mixture with respect to poisons containing oxygen (122). 

Iron-supported-on-MgO catalysts behave in some ways differently from the above catalyst systems. That is, while the catalytic activity of these metallic-iron particles for the atmospheric-pressure ammonia synthesis depends markedly on particle size in the range 1.5-10 nm (206), the Mossbauer parameters (isomer shift, quadrupole splitting, and magnetic hyperfine splitting) are independent of iron particle size in this range (97). This thus rules out an electronic effect in the interpretation of the effect of particle... 

It was shown by these authors that the amount of nitrogen present during pretreatment of a catalyst affects the ultimate activity for ammonia synthesis (206). Specifically, it was found that treating H2-reduced small particles with ammonia at 670 K, followed by re-reduction of the catalyst with a H2 N2 gas mixture, gave rise to an increase in the catalytic activity compared to the activity measured after H2 reduction alone. However, when the catalyst in this high-activity state was further treated with H2 alone at 670 K, the catalytic activity was found to decrease to that value observed before the above ammonia treatment. Subsequent ammonia treatment returned the catalyst to its high-activity state. No such effects were observed for metallic-iron particles greater than 10 nm in size. 

The effect of feed composition cycling on the time-average rate and temperature profile was explored in the region of integral conversion in a laboratory fixed bed ammonia synthesis reactor. Experiments were carried out at 400°C and 2.38 MPa over 40/50 US mesh catalyst particles. The effect of various cycling parameters, such as cycle-period, cycle-split, and the mean composition, on the improvement in time-average rate over the steady state were investigated. 

First attempts to check this hypothesis [23] revealed a superior catalytic activity of iron in amorphous iron-zirconium alloys in ammonia synthesis compared to the same iron surface exposed in crystalline conventional catalysts. A detailed analysis of the effect subsequently revealed that the alloy, under catalytic conditions, was not amorphous but crystallized into platelets of metastable epsilon-iron supported on Zr-oxide [24, 25]. 

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