Ammonia synthesis temperature effects

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. 

Figure 6.3 The effect of temperature on the ammonia synthesis reaction.

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],... 

About the same time that Haber was making his measurements, Walther Nernst also studied the ammonia synthesis reaction at high temperatures and obtained results that differed significantly from those obtained by Haber.2 Nernst s measurements were made at high pressures (approximately 60 atm). His results are also shown in Figure 15.3, and they do not appear to differ in a major way from those of Haber, until the effect of pressure is taken into account, as we will now see. From equation (15.12), we find that Kx is related to K by... 

This reaction is just similar to the synthesis of ammonia described above. So, the effect of pressure, temperature and concentration will be the same as mentioned in the synthesis of ammonia. Low temperature, high pressure and increased concentrations of S02 and 02 will favour the formation of sulphur trioxide. 

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. 

An important conclusion that can be drawn from the above equation is that the sign of the slope of the plot of ln(fC) versus 1/T depends on the sign of AH° for the reaction. Note that an exothermic reaction (AH° < 0) will show a positive slope (AH° is negative so -AH7R is positive) for the In K) versus 1/T plot. In this case ln(fC) will increase as 1/T increases (T decreases). Thus K increases as T is decreased or, conversely, K decreases as T is increased. This is exactly the temperature dependence of K predicted for an exothermic reaction by Le Chatelier s principle (see Section 6.8). This effect can be shown quantitatively by examining how the value of K for the ammonia synthesis reaction... 

For pore diffusion resistances in reactions having moderate heat evolution, the following phenomena characteristically hold true in industrial ammonia synthesis [212] in the temperature range in which transport limitation is operative, the apparent energy of activation falls to about half its value at low temperatures the apparent activation energy and reaction order, as well as the ammonia production per unit volume of catalyst, decrease with increasing catalyst particle size [211], [213]-[215]. For example at the gas inlet to a TVA converter, the effective rate of formation of ammonia on 5.7-mm particles is only about a quarter of the rate measured on very much smaller grains (Fig. 13) [157]. 

I. A. Smirnov et al. set up a rate equation for ammonia synthesis [371], [372] that takes the effect of water vapor into consideration over a wide range of temperature and pressure ... 

Chlorine compounds. The permanent poisoning effect of chlorine compounds is two orders of magnitude worse than that of oxygen compounds. Concentrations of about 0.1 ppm are viewed as the uppermost allowable limit in order not to affect adversely the life of ammonia catalysts [384]. The deactivation effect is based at least in part on the formation of alkali chlorides that are volatile at the upper synthesis temperatures. 

Because of the interest in ruthenium as a potential catalyst for ammonia synthesis from N2 and H2, the hydrogenation of Nads on Ru surfaces has been investigated (86). Both NHads and NH2,ads were found, in addition to NHs ads- Dietrich ei nl (86) reported that the thermal stability of NHads is the highest of the three N Ha, ads species (x = 1. 2. and 3) on Ru(OOOl) and on Ru(1121) at temperatures up to 400-450 K. On Ru (1010), however, the thermal stability of NH2,ads is higher than that of NHad., and much higher than those on other Ru surfaces. The reason for the enhanced thermal stability of NH2,ads on Ru(lOlO) is not clear. It was also found that coadsorbed N has a positive effect on the thermal stability of NHads on Ru(OOOl), whereas NHads is stable at temperatures up to 400 K, and NHads in the N/NH coadsorbate is stable at temperatures up to 460 K. 

PO synthesis is carried out in a methanol solvent, propene and 40% HP adjusted with ammonia to pH 4.5 pressure is 25 atm [20a]. The feed stream contains 21.5 wt% propene, 57 wt% methanol and 9.4 wt% HP. In some patents, a different feed composition is reported, containing 43 wt% propene, 43 wt% methanol and 8.4 wt% HP (from the feeding of a 60% solution of HP in water, adjusted to pH 4.5 with 1100 ppm ammonia) [20c]. The effect of temperature is shown in Table 6.3, for anupflow feed. By-products are l-methoxy-2-propanol, 2-methoxy-l-propanol (propene glycol monomethyl ethers) and propene glycol (1,2-propandiol). How-... 

Isothermal effectiveness factors for practical reactions cover a wide range, from as low as 0.01 to unity. With normal pellet sizes (I to y in.) r] is 0.7 to 1.0 for intrinsically slow reactions, such as the ammonia synthesis, and of the order of t/ = 0.1 for fast reactions, such as some hydrogenations of unsaturated hydrocarbons. Satterfield and Sherwood have summarized much of the experimental data for effectiveness factors for various reactions, temperatures, and pellet sizes. For reactor design it is important to be able to answer these questions ... 

Thermal treatment of the support at much lower temperatures has also resulted in positive effects on the catalytic performance of Ru in ammonia synthesis. Zhong and Aika [56,57] treated three commercial activated carbons, with different ash contents, at temperatures ranging from 1073 to 1188 K under hydrogen. 

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