Ammonia synthesis temperature dependence

The internal utilization ratio of ammonia synthesis catalysts depends on particle size and radius of micro-pore of catalyst, reaction rate constant, operation temperature and pressure as well as the difference between the concentration of reaction components in gas bulk and equilibrium concentration etc., among which the most... 

Urea is produced from liquid NH and gaseous CO2 at high, pressure and temperature both reactants are obtained from an ammonia-synthesis plant. The latter is a by-product stream, vented from the CO2 removal section of the ammonia-synthesis plant. The two feed components are deUvered to the high pressure urea reactor, usually at a mol ratio >2.5 1. Depending on the feed mol ratio, more or less carbamate is converted to urea and water per pass through the reactor. 

Because the ammonia synthesis reaction is an equiUbrium, the quantity of ammonia depends on temperature, pressure, and the H2 to-N2 ratio. At 500°C and 20.3 MPa (200 atm), the equiUbrium mixture contains 17.6% ammonia. The ammonia formed is removed from the exit gases by condensation at about —20° C, and the gases are recirculated with fresh synthesis gas into the reactor. The ammonia must be removed continually as its presence decreases both the equiUbrium yield and the reaction rate by reducing the partial pressure of the N2—H2 mixture. 

The reactor can operate with either a liquid-phase reaction or a gas-phase reaction. In both types, temperature is very important. With a gas-phase reaction, the operating pressure is also a critical design variable because the kinetic reaction rates in most gas-phase reactions depend on partial pressures of reactants and products. For example, in ammonia synthesis (N2 + 3H2 O 2NH3), the gas-phase reactor is operated at high pressure because of LeChatelier s principle, namely that reactions with a net decrease in moles should be mn at high pressure. The same principle leads to the conclusion that the steam-methane reforming reaction to form synthesis gas (CH4 + H20 O CO + 3 H2) should be conducted at low pressure. 

The optimum ammonia synthesis reaction rate depends on several factors including pressure, temperature, H2-to-N2 molar ratio, concentration of impurities and catalyst activity. Therefore the H2-to-N2 molar ratio is adjusted to suit the requirements in the ammonia synthesis. This adjustment occurs before the compression step. 

The thermodynamic equilibrium is most favourable at high pressure and low temperature. The methanol synthesis process was developed at the same time as NH3 synthesis. In the development of a commercial process for NH3 synthesis it was observed that, depending on the catalyst and reaction conditions, oxygenated products were formed as well. Compared with ammonia synthesis, catalyst development for methanol synthesis was more difficult because selectivity is crucial besides activity. In the CO hydrogenation other products can be formed, such as higher alcohols and hydrocarbons that are thermodynamically favoured. Figure 2.19 illustrates this. 

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... 

Equations for describing ammonia synthesis under industrial operating conditions must represent the influence of the temperature, the pressure, the gas composition, and the equilibrium composition. Moreover, they must also take into consideration the dependence of the ammonia formation rate on the concentration of catalyst poisons and the influence of mass-transfer resistances, which are significant in industrial ammonia synthesis. 

Contradictory data on the kinetics of ammonia synthesis, especially in the earlier literature, in some circumstances may reflect a lack of attention to the influence of impurities in the gas. If oxygen compounds are present in the synthesis gas, reversible poisoning of the adsorbing areas, in accordance with an equilibrium depending on the temperature and the water vapor-hydrogen partial pressure ratio, must be taken into account when developing rate equations (see also Section 3.6.1.5). 

The potential for ruthenium to displace iron in new plants (several projects are in progress [398] of which two 1850 mtpd plants in Trinidad now have been successfully commissioned [1488]) will depend on whether the benefits of its use are sufficient to compensate the higher costs. In common with the iron catalyst it will also be poisoned by oxygen compounds. Even with some further potential improvements it seems unlikely to reach an activity level which is sufficiently high at low temperature to allow operation of the ammonia synthesis loop at the pressure level of the synthesis gas generation. 

In the kinetic modelling of catalytic reactions, one typically takes into account the presence of many different surface species and many reaction steps. Their relative importance will depend on reaction conditions (conversion, temperature, pressure, etc.) and as a result, it is generally desirable to introduce complete kinetic fundamental descriptions using, for example, the microkinetic treatment [1]. In many cases, such models can be based on detailed molecular information about the elementary steps obtained from, for example, surface science or in situ studies. Such kinetic models may be used as an important tool in catalyst and process development. In recent years, this field has attracted much attention and, for example, we have in our laboratories found the microkinetic treatment very useful for modelling such reactions as ammonia synthesis [2-4], water gas shift and methanol synthesis [5,6,7,8], methane decomposition [9], CO methanation [10,11], and SCR deNO [12,13]. 

As previously mentioned, two types of ammonia synthesis loops exist. For the inert- containing loop, the inert level in the synthesis loop (most often measured at converter inlet) depends on the inert level in the make-up gas, the production of ammonia per unit make-up gas (the loop efficiency), and the purge rate. The inert level in the make-up gas is solely determined by the conditions in the synthesis gas preparation unit. The ammonia production is determined by conditions around the converter, the gas flow (which may be expressed by the recycle ratio), the inlet temperature and pressure, catalyst volume and activity, and converter configuration. 

However, Murakami et al. (2005) suggested that the ammonia synthesis rate may not depend on only electrolysis potential. Other factors such as catalytic activity of electrode material, partial pressure of gaseous reactants, and temperature are cmcial parameters for the kinetics of ammonia synthesis (Mamellos, Zisekas, Stoukides, 2000 Skodra Stoukides, 2009). 

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