Equilibrium, concentration of ammonia

Figure 2.1. Equilibrium concentration of ammonia in a mixture of initially 1 3 N2 H2 as a function of temperature for several total pressures. Note the slight deviation due to non-ideality of the gases.

It is important to understand the factors that control the position of a chemical equilibrium. For example, when a chemical is manufactured, the chemists and chemical engineers in charge of production want to choose conditions that favor the desired product as much as possible. In other words, they want the equilibrium to lie far to the right. When Fritz Haber was developing the process for the synthesis of ammonia, he did extensive studies on how the temperature and pressure affect the equilibrium concentration of ammonia. Some of his results are given in Table 6.2. Note that the amount of NH3 at equilibrium increases with an increase in pressure but decreases with an increase in temperature. Thus the amount of NH3 present at equilibrium is favored by conditions of low temperature and high pressure. 

Figure 17-10 shows two theoretical curves for the titration of zinc(Il) with EDTA at pH 9.00. The equilibrium concentration of ammonia was 0.100 M for one titration and 0.0100 M for the other. Note that the presence of ammonia... 

C and 30% RH can you calculate the corresponding equilibrium concentrations of ammonia and nitric acid in the gas phase Why ... 

To make the Haber-Bosch process viable we require (1) a high equilibrium concentration of ammonia, and (2) that the equilibrium concentration of ammonia be produced in a short time (the kinetic factor). In originally studying this reaction, Haber was aware of the following facts ... 

One of the more valuable contributions of the laboratory to the synthetic ammonia industry was the supplementary information furnished relative to details essential to proper plant designs. For example, the equilibrium concentration of ammonia for synthesis reaction... 

It is likely that natural ecosystems (forest, grassland) emit no or only small amounts of ammonia because normally there is a deficit of fixed nitrogen in landscapes. Reported emissions factors over forests span three orders of magnitude and are likely be influenced by re-emission of wet deposited ammonium. Older publications considerably overestimated emission by using simple models considering soil ammonium concentrations obtained from relative decomposition and nitrification rates, where Henry s law gives the equilibrium concentration of ammonia gas in the soil, and a simplified diffusion equation yields the flux to the atmosphere, for example, Dawson (1977) calculated it to be about 47 Tg N yr b... 

It was Fritz Haber (1868-1934) who devised the nitrogen fixation process which was to come into general use. He started work on the reaction between nitrogen and hydrogen in 1904, initially determining the very low equilibrium concentrations of ammonia obtained at atmospheric pressure. With Le Rossignol he then constructed an apparatus that would operate at 150-200 atmospheres pressure, but there was still no catalyst available which would promote rapid reaction at 550 C. Haber eventually used uranium, and after the feasibility of the method had been... 

You are given the initial concentrations of nitrogen and hydrogen as well as the equilibrium constant for their reaction to form ammonia. You are asked to find the equilibrium concentration of ammonia. 

The industrial synthesis of ammonia is based on the reaction above. Which factor(s) will increase the equilibrium concentration of ammonia ... 

A high temperature favors the decomposition of ammonia, the endothermic reaction. But at low temperatures, the forward reaction is too slow to be commercially useful. The temperature used represents a compromise between kinetic and equilibrium requirements. It is high enough that equilibrium is established rapidly but low enough that the equilibrium concentration of ammonia is significant Moderate temperamre (about 500°C) and very high pressure (700-1000 atm) produce a satisfactoryyield of ammonia. 

Haber et al. obtained the equilibrium concentration of ammonia on pure iron catalyst at 1,000°C and 101 kPa (Fig. 1.2). They obtained 0.012% ammonia at 1,020°C and 101 kPa. This result indicated that ammonia could be synthesized even at pressure of 101 kPa. 

Nevertheless, either increasing the outlet ammonia concentration or reducing space velocity is confined by the equilibrium ammonia concentration. Where, the catalyst efficiency ( ex/ e) is defined as the ratio of outlet ammonia concentration ( ex) to equilibrium ammonia concentration (y e) under same conditions, which indicates the degree of the reaction close to the equilibrium. The efficiency of the present catalysts used in industry is about 90% at higher temperatures, as shown in Table 1.5. For example, the efficiency of ZA-5 catalyst is about 95% at about 475°C at the space velocity of 3 x 10 h and pressure of 15 MPa, which is very close to the equilibrium concentration of ammonia. With low space velocities, the operating temperature can be decreased as the outlet ammonia concentration is close to equilibrium concentration. As shown above, the efficiency of ZA-5 is about 98% at above 450°C and at the space velocity of 1 x 10 h Thus, it is impossible to further increase the outlet ammonia concentration under these conditions. As a result, the activity at low temperatures must be increased, since equilibrium ammonia concentration is higher at low temperatures. The outlet ammonia concentration is 16.68% on ZA-5 catalyst at 400°C and with the space velocity of 3 x 10 h and pressure of 15 MPa, the equilibrium concentration is 32.83%, and therefore the catalyst efficiency is only 50.8%. If the catalytic efficiency can be increased to more than 95% at 400°C by increasing the catalyst s activity at this temperature, the synthetic quotient per pass could be about 50%. 

According to equation (8.3), the highest concentration of ammonia and maximum reaction rate are shown in Figs. 8.11-8.13 when Sy, Feat and Wcat are the same. The ratio of the maximum concentration of ammonia in the curves to the corresponding equilibrium concentration of ammonia under the same conditions is the catalyst efficiency K. The H2/N2 ratio with the maximum concentration of ammonia as a function of K is plotted as a solid line in Fig. 8.14. By the least square method (relative coefficient = 0.9957), we obtain... 

Here, 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. 

C), decreasing the reaction rate so that 77 is increased. However, when the reaction temperature is less than 350°C, 77 tends to increase, especially for large size catalysts. This is because when the temperature is less than 350°C, although the equilibrium concentration of ammonia increases, the catalytic activity decreases more significantly, resulting in a decrease of reaction rate, thus 77 rises with decreasing temperature. At the same time, it can be seen from Table 8.19 that with an... 

It was already generally accepted that molecular nitrogen was too inert to react directly with hydrogen at moderate temperatures. Unfortunately, at high temperatures where reaction with hydrogen becomes possible, the decomposition of ammonia also takes place, hence the need for a catalyst to allow synthesis at the reaction temperature. Perman used iron as a catalyst in 1904 and studied the formation of ammonia from its elements, but, unfortunately for him, in the presence of moisture which we now know retards the reaction. Haber immediately realized that the equilibrium concentration of ammonia should not be infiuenced by the presence of moisture. 

The equilibrium concentration of ammonia in a nitrogen and hydrogen mixture is extremely small under low-pressure conditions, and its formation is barely seen. Desorption of this molecule from the catalyst surface is, on the other hand, the last step in the synthesis reaction, and its adsorption becomes of relevance when approaching the equilibrium state. In addition, decomposition of ammonia has to proceed through the same elementary steps as its formation, as a consequence of the principle of microscopic reversibility, so that information on the reaction mechanism is to be expected from the study of possible surface intermediates. 

Increasing the loop pressure gives a higher equilibrium concentration of ammonia and also increases the rate of reaction. Hence, the maximum production capacity is attained at the maximum operating pressure of the equipment. On the other hand, the synthesis gas compression power is also increased. 

As described in Chapter 7, synthesis of ammonia takes place in a circulatory system commonly known as a loop. Only a fraction of the synthesis gas is converted per pass, as limited by the equilibrium concentration of ammonia at the exit conditions of the converter. The relevant part of the loop is shown in Fig. 8.1. 

From these values it is clear that if we could reach equilibrium at 1 atm and room temperature, we would have almost complete conversion to NH3 the equilibrium is very favorable for the reaction. Alas, the reaction rate at this temperature is fsO, and no catalyst is known that will make the reaction go at temperatures below about 300°C. (Fame and fortune await the student who can find one ) Most industrial ammonia synthesis reactors operate in the temperature range 350-520°C, at which the rates of the reaction are satisfactorily rapid (in the presence of a catalyst). At this temperature and 1 atm the calculated equilibrium concentration of ammonia is small enough to make the reaction completely impractical. Industrially, the reaction is conducted at high pressures, as discussed below. 

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