Content of inert gas

The content of inert gas (Methane, Argon, Helium etc.) in the feed gas depends on the methods of gasification and purification for the feed gas. The copper-ammonia solution washing process is employed to purify the synthesis gas, which is produced by coal as the raw materials, the content of inert gas in the synthesis gas is generally from 0.5% to 1%, more than 1% with the methanation purification process and only a few ml-m with the methanol and liquid nitrogen wash purification process. 

Because the hydrogen and nitrogen gas continuously react and hence consumed in the circulation loop, the content of inert gas continues to accumulate. High concentration of inert gas does not favor reaction equilibrium and kinetics. 

Although the existence of inert gas has without poison on the iron catalyst, from the perspective of chemical equilibrium, increasing the content of inert gas tp ) is equivalent to the reducing the effective pressure The relationship for operating pressure (po), effective pressure (pe) and content of inert gas (pi) is as follows  

po and the corresponding equilibrium concentration of ammonia (po) to reduce to the pe and the corresponding equilibrium concentration of ammonia (pe) reduces the driving force in reaction. 

The effect of the content of inert gas on the activity of ZA-5 catalysts under different conditions is shown in Table 8.13, Figs. 8.15 and 8.16. 

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The exothermic reaction between nitrogen and hydrogen occurs in the presence of suitable catalysts and results in volume reduction, the highest ammonia concentrations being obtained at the highest possible pressure and the lowest possible temperature. The upper limit for the applied pressure is determined by the cost of compression of the gas mixture and the cost of the high pressure plant. The reaction temperature is determined by the type and activity of the catalyst. The removal of ammonia from the reaction gas should be as complete as possible to favor the fresh formation of ammonia. Other important parameters are the contents of inert gas and oxygen compounds in the unreacted synthesis gas. 

It is well known that the conversion of hydrogen and nitrogen per pass is only 20%-30% for the present catalytic ammonia synthesis technology (Table 1.4). Most synthesis gases need to be returned to the reaction system, which increases power consumption. In order to increase conversion per pass, it must increase the outlet ammonia concentration of reactor. Accordingly, it can be seen from Table 1.4 that it is necessary to increase reaction pressure for small and medimn scale aimnonia plants and Topspe process, or to reduce the content of inert gas in sjmthesis gas for Topspe and Braun processes, or to reduce ammonia concentration in the inlet of converter for small and medium scale ammonia plants and Kellogg process. But all of these operations will add the power consumption or unit gas consumption. 

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. 

Equation (8.7) indicates that the optimum reaction temperature T is related to equilibrium temperature Te and performance of catalyst i.e., activation energy of El, E2 for forward and reverse reactions. The optimum temperature is indirectly affected by both the pressure and contents of inert gas in the reactants because the equilibrium temperature is affected by both. 

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. 

It can be seen from Table 8.13 that increase in the inert gas significantly reduces the catalytic activity. For example, under the conditions of 15 MPa, 440° C and 10,000h , the ammonia concentration at the outlet of A301 catalyst drops from 21.6% to 16.29% when the content of inert gas increases from 0 to 15%, and the respective catalytic activity is reduced by 26%. Increasing the content of inert gas by 1% leads to a reduction of the ammonia concentration at the outlet (net ammonia) by 0.2 to 0.35 percent on average. This behavior is similar to that shown in our earlier results ... 

Compared to the traditional flow, converter in the by-pass type flow can use the high content of inert gas and CH4 as feed gas. 

In order to increase the output of ammonia or reduce consumption of fuel as raw materials, the N2 and H2 in vent gas are further synthesized to ammonia using an auxiliary converter. It is not economic at the same temperature and pressure as main synthesis system on Fe catalyst because the content of inert gas in vent gas is very high, and if it continues rising, then the pressure can increase the energy consumption. 

The difference between the two process flows is that the reactant gas entering the auxiliary converters of 23 and 25 with Ru catalyst is the vent gas from the main converter 22 with Fe catalyst, but not the outlet gas from the main converter or the circular gas. Actually, in Fig. 9.10, the reaction takes place on Fe catalyst only once, and again for quadratic on Ru catalyst in Fig. 9.11, the main reaction takes place on Fe catalyst, and supplemental synthesis is carried out on Ru catalyst. In comparison with the traditional process, the ratio of H2 to N2 in the converter 22 decreases to 2.9 1, the flow of the vent gas increases to 6.3%, and the content of inert gas reaches 5.6%. In this method, the amount of the vent gas can be reduced because the content of the inert gas in the vent gas is higher, the compression cost can be decreased, and the yield can be increased. Besides, the load of hydrocarbon reformation in the primary is alleviated because the hydrogen of vent gas is used. 

Cooling purification process, i.e. the purification process of washing by methanol and liquid nitrogen at low temperature ensures that the content of inert gas (CH4 + Ar - - He) in the recycling gas is less than 2%. 

The process mentioned is about the cooling purification process. For most of modern ammonia plants, it is more popular to use the thermal purification method. In the thermal purification process, the final purification is methanation, and the content of inert gas (CIH-I-Ar) is usually higher. The increase of the inert gas content will reduce the concentration and the net value of the outlet ammonia. The research results show that, while the inert gas content increases by 1%, the net value will reduce by about 0.3%-0.4%. In the condition No. 2 of Table 9.5, if the inert gas content increases to 6%, the net value will decrease from 10.69% to 9.49%. Then the amount of inlet gas of converter and the catalyst volume are 64.45 x 10 m h and 107.4 m , respectively. If the inlet concentration of ammonia of the converter is reduced to 2% (condensing temperature is about —30°C), the net value of ammonia will reach 11.15%, and the amount of inlet gas of converter and the catalyst volume are 54.85 x 10 m h and 91.4 m , respectively. It will be seen from this that, for the thermal purification process, it is necessary to limit the inert gas content below 6%-8% and to decrease the condensing temperature of ammonia to about —30°C. 

The catal3dic activity will gradually decrease with the prolongation of catalyst service time. At this time, it can be known from Eq. (9.4) that in order to keep the liquid ammonia output, it should increase the reaction pressure, reduce the content of inert gas and inlet ammonia of the converter, to retain A

out) in the equation unchanged or the flow of recycle gas (Vjn) must be increased which is the easiest way. The flow of recycle gas (Vjn) is related to catalytic activity (outlet concentration of ammonia). Here the recycle ratio is related to catalytic activity (net value of ammonia). The so-called recycle ratio is the ratio of recycle gas to fresh syngas (Fig. 9.29). 

It will be seen from this that the recycle ratio of gas in synthesis system is dependent on the outlet concentration, net value of aimnonia and the content of inert gas of the synthesis loop. Recycle gas flux (recycle ratio) increases with the decrease in net value and the increase in the inert gas content. Because value... 

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