H2/N2 ratio

Reforming is completed in a secondary reformer, where air is added both to elevate the temperature by partial combustion of the gas stream and to produce the 3 1 H2 N2 ratio downstream of the shift converter as is required for ammonia synthesis. The water gas shift converter then produces more H2 from carbon monoxide and water. A low temperature shift process using a zinc—chromium—copper oxide catalyst has replaced the earlier iron oxide-catalyzed high temperature system. The majority of the CO2 is then removed. 

Figure 9.34. (a) Effect of inlet H2/N2 ratio on the rate of NH3 synthesis over promoted Fe/CaZr0,9ln0,03 under open-circuit (O) and for UWR=-1.0V ( ) (b) Corresponding p (r/r0) ( ) and A (=ArH/(-I/F)) ( ) values.43 Reprinted with permission from the American Chemical Society. 

N2-H2 mixture (with small amounts of Ar and CH4) [33]. The amount of air added to the secondary reformer is adjusted to give the desirable H2/N2 ratio (which is close to 3 for the NH3 synthesis). The secondary reformer is similar to the autothermal reformer described in the previous section. The pressure at the outlet of the secondary reformer is in the range 2.5-3.5 MPa. The outlet temperatures from the primary and secondary reformers are 750-850°C and 950-1050°C, respectively. 

Typical gas compositions from the partial combustion of various fuels are given in Table 5.21. The nitrogen that is needed to produce the desired H2/N2 ratio for ammonia production is usually introduced later in the processing sequence. 

Description Natural gas or another hydrocarbon feedstock is compressed (if required), desulfurized, mixed with steam and then converted into synthesis gas. The reforming section comprises a prereformer (optional, but gives particular benefits when the feedstock is higher hydrocarbons or naphtha), a fired tubular reformer and a secondary reformer, where process air is added. The amount of air is adjusted to obtain an H2/N2 ratio of 3.0 as required by the ammonia synthesis reaction. The tubular steam reformer is Topsoe s proprietary side-wall-fired design. After the reforming section, the synthesis gas undergoes high- and low-temperature shift conversion, carbon dioxide removal and methanation. 

As a rule, the kinetic analysis shows that the maximum reaction rate is obtained if an initial H2/ N2 ratio of 2.5/1 prevails in the reactor, whereas the stoichiometry is 3/1, (Fig. 1.18). The VUSV normally ranges from 10,(XX) to 50,000 h ... 

In contrast to the above-mentioned variables, the dependence of the converter performance on the H2/N2 ratio shows a true maximum (Fig. 81). The optimum conversion at high space velocity [SV = m3 (STP) gas h l- nrf3 catalyst] lies close to an H2/N2 ratio of 2 and approaches 3 at low space velocities. The reason is that equilibrium plays a greater role at low space velocities and has a maximum at a ratio of 3, except for small corrections [33] with regard to the behavior of real gases. Usually, the ratio is adjusted to 3, because in most plants, conversions near equilibrium are attained. 

The TPO profiles for samples used under different operational conditions are presented in Fig. 2. Fig. 2-A shows that when the H2 N2 ratio is changed the nature of coke is similar The amount of coke increases strongly when hydrogen is not fed. The effect of reaction temperature is shown in Fig. 2-B a more condensed coke is produced when reaction temperature is 250 °C, being similar for the rest of the temperatures. 

Surface site heterogeneity of Ru/Si02 for ammonia synthesis, as well as the effect of K promotion, has been studied using SSITKA.K is a well known activity promoter for ammonia synthesis on Ru and Fe catalysts. The Ru/Si02 catalyst was studied at 673 K, 204 kPa, H2/N2 ratio of 3 and GHSV between 5000-23000. SSITKA experiments were carried out by switching between N2 and N2. 

The nitrogen serves as solvent for the removal of CO, simultaneously the H2/N2 ratio of almost 3, required for the ammonia synthesis, is adjusted (see Fig. 5.12). 

Inhibition effect of H2 also exists on the fused iron catalyst, but the effect is less important than that on ruthenium catalysts. There is no consistent point of view on the mechanism of the H2 inhibition effect so far. The author thought that this may relate to the surface basicity of the catalysts.When the H2/N2 = 3, ruthenium catalyst exhibits the higher activity than the iron catalyst at high temperatures, while the activity is reversed at low temperatures. In addition, the situation is the opposite with the decease of the H2/N2 ratio. Thus, ruthenium catalyst is suitable for use at low H2/N2 ratios. 

It can be seen from these figures that the change of activity of A301 iron catalyst is very little with H2/N2 ratio, while it is obvious on Ru catalyst. At 400°C, the activity of Ru catalyst increases with the decreasing of H2/N2 ratio when it is higher than 1.5 and decrease when H2/N2 ratio is lower than 1.5 (Fig. 6.66). Here, the activity is above 22% on ruthenium catalyst and the equilibrium ammonia concentration is about 25% at H2/N2 = 3. At the same time, equilibrium ammonia concentration also decreases with the decrease of H2/N2 ratio. Therefore, the effect of change of H2/N2 ratio is not too obvious. For iron catalysts, the activity does not change with the decrease of H2/N2 ratio until H2/N2 ratio is lower than 0.67 at 400°C. 

At 350°C (Fig. 6.68), the activity of ruthenium catalyst increases from 9.33% to 15% when the H2/N2 ratio is 3 and 0.5, indicating that the relative activity increases by 60%. Fishel et has also investigated the effect of H2/N2 ratio on the turnover frequency of ammonia (TOF). Similar results are shown in Fig. 6.69. The TOF increases in linearity with decrease of H2 molar fraction and increase of N2 molar fraction at 350° C. 

It can be seen that the hydrogen inhibition is closely related to the temperature for ruthenium catalysts. The lower the temperatures, the more serious the inhibition of hydrogen, the larger the effect of H2/N2 ratio is. This is related with catalysts efficiency. The lower the temperatmes, the lower the catalysts efficiency and the larger the effect of H2/N2 ratio on the activity of high active catalysts is. 

For high active ruthenium catalyst, the effect of change H2/N2 ratio on the activity is very little as the outlet ammonia concentration is very close to equilibrium at high temperatures and pressures. However, at low temperatures and low pressures, the catalytic activity increases with decrease of H2/N2 ratio due to reducing hydrogen inhibition. Low H2/N2 ratio not only decreases the operation cost and saves energy but also increases product yield. 

The results show that decreasing the H2/N2 ratio or H2 molar fraction in syngas is a direct and effective way to weaken hydrogen inhibition. Therefore the synthesis ammonia process must be reformed to suffice the demand of low H2/N2 ratio. But it... 

It can be seen from the above calculation that irrespective of the reaction rate, the synthetic ratio (conversion) of ammonia, space-time yield or the TOF of ammonia has a relationship with the outlet concentration of ammonia. Therefore, commonly it can directly use the outlet concentration of ammonia to characterize the activity of ammonia catalysts. It is worth noting that it should be necessary to indicate the reaction conditions, including pressure, temperature, space velocity, H2/N2 ratio, content of inert gas, particle size and volume of catalyst, and inlet concentration of ammonia etc. 

The above equations show that the production of ammonia depends on the inlet gas flow rate, the concentration of ammonia at the outlet and the net values of ammonia at the inlet and outlet of the converter. The production increases with increasing Vj and and from (8.1). The concentration (y>) of ammonia at the outlet is related to the pressme (p), temperature (T), space velocity (S v), H2/N2 ratio, content of inert gases ( i), type (M), volume (Vk), particle size (dp) and reduction R) of catalyst, gas pm-ity (AT), and the concentration of ammonia at the reactor inlet (y>o)- These factors can be expressed by the function below. 

The parameters of gas compositions at the inlet of converter include the H2/N2 ratio, the content of inert gas and of ammonia in the circulating gas. 

Figures 8.11-8.13 shows the effect of H2/N2 ratio on the activity of ZA-5 catalysts. It is seen from the results that H2/N2 ratio has a certain effect on the catalytic activity. For given space, velocity and pressure the optimum H2/N2 ratio changes with the reaction temperature.

Therefore, the best H2/N2 ratios are related to the temperature, pressure, space velocity, inert gases and the activity (outlet concentration of ammonia) of catalysts. 

For a given reactor configuration, under certain conditions, there is bound to be the best H2/N2 ratio that leads to the fastest reaction rate and the highest concentration and production of ammonia. It is clear that the best H2/N2 ratio relates to the degree of reaction and the degree of reaction relates to how close the outlet concentration of ammonia is to the equilibrium ammonia concentration (approach degree of equilibrium). Here we can define iF as a catalyst efficiency which characterizes the degree the catalyst enables the outlet concentration of ammonia... 

According to the theoretical results mentioned above, at the initial reaction stage, (fNHs = 0, so K = 0, the best ratio H2/N2 is 1.5 with the reaction reaching the maximum rate. When the reaction reaches equilibrium situation (for example, space velocity near zero), K = = VnHs) the best H2/N2 ratio is 3.0. When... 

Fig. 8.14 Relationship between the optimum H2/N2 ratio and catalyst efficiency Dotted line calculation value of equation (8.9) Solid line the experimental data...

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