Ammonia synthesizing

There is a lot of evidence that chemical equilibrium is dynamic. One example of this evidence is relevant to the ammonia equilibrium. Imagine carrying out two ammonia syntheses with the same starting conditions, but using D2 (deuterium) in place of H2 in one of them (Fig. 9.2). The two reaction mixtures reach equilibrium with almost exactly the same composition, except that D2 and ND3 are present in one of the systems instead of H2 and NH3. Suppose we now combine the two mixtures and... 

The ammonia syntheses of heteronuclear complexes with use of elemental copper, lead halides, and aminoalcohols (3.273) [679-681] are significant, since... 

These variants of ammonia syntheses represent an especial interest due to the possibility of obtaining, on their basis, in some cases, complex compounds with unusual or rare structures [202]. In this respect, it is necessary to mention the synthesis (3.274), as a result of which the anionic complex 825 (84% yield) with nontypical coordination (Sec. 2.2.3.5, structure 113) for the rhodanide group Cu2S — CN — Cu was isolated [674] ... 

Vertical ammonia synthesizer at 300 atm with five cold shots and an internal exchanger (Fig. 19-22d). The nitrogen and hydrogen feeds are reacted over an Al203-promoted spongy iron catalyst. The concentration of ammonia is also shown in the figure. 

FIG. 19-22 Temperature and composition profiles, (a) Oxidation of S02 with intercooling and two cold shots, (b j Phosgene from CO and Cl2, activated carbon in 2-in tubes, water-cooled, (c) Cumene from benzene and propylene, phosphoric acid on quartz with four quench zones, 260°C. (d) Vertical ammonia synthesizer at 300 atm, with five cold shots and an internal exchanger, le) Vertical methanol synthesizer at 300 atm, Cr203-Zn0 catalyst, with six cold shots totaling 10 to 20 percent of the fresh feed. To convert psi to kPa, multiply by 6.895 atm to kPa, multiply by 101.3. 

Determine the outlet temperature and equilibrium composition of ammonia synthesized in an adiabatic reactor from the following reaction ... 

This reaction requires an electric arc to provide the high activation energy and is therefore quite expensive. The availability of cheap ammonia synthesized by the Haber-Bosch process signaled the demise of this process. 

Initially, the reactors in the days of the early methanol and ammonia syntheses operated at approximately 1000 atmospheres or 15,000 psi. This, over the years, has been reduced to 12,000, then to 5,000 and then to 750 but presently the trend is back up toward 2,500 psi (Figure 12). It is likely that, when single line 5,000 or 10,000 ton per day plants are constructed, the optimum pressure will be in the range of 2,500 to 4,000 psi. The economics of pressure and size are all closely related to many factors such as capital investment, reactor size, synthesis gas generating pressure, interconnecting piping size, method of generating synthesis gas, and problems of maintenance. 

The yield is nearly 11%, but the unreacted compounds reenter the cycle leading to an overall yield of 100%. Nearly 100% of the ammonia synthesized worldwide is produced by the Haber-Bosch process. Even though this technology is expensive, no low cost alternatives have been developed so far. 

When we move to very high-pressure exothermic processes (such as methanol and ammonia syntheses), different engineering approaches have resulted in a variety of other reactor types... 

Synthesis of ammonia. The synthesis reaction is dependent on the conditions of equilibrium and the kinetics of the reaction. The latter is dictated by the efficacy of the catalyst, which in turn is chosen because of its cheapness and activity. Iron is the only realistic catalyst, but its activity can be greatly increased by the use of suitable promoters. It is prepared by melting iron oxide, refractory oxides such as potassium and aluminium oxides. A solid sheet forms on cooling, and is broken down into 5-10 mm lumps. The whole is then reduced in the ammonia synthesizer, where the oxide is converted to iron crystallites separated by the refractory oxides and covered in part by KOH as a promoter. The KOH can enhance the reactivity twofold. This catalyst must be used within the temperature range 400°-540 °C. Below this the catalyst becomes uneconomically inactive above, it sinters and loses surface area. An improved iron catalyst of higher activity and longer life is a feature of the AMV process. It is important to note that much of the reason for improved and continued activity is due to the careful removal of poisons such as CO, CO2, and H2S. 

As a classical example, we will look at the industrial synthesis of nitric acid, a compound whose production revolutionized the fertilizer industry, and the world population growth is strongly correlated with the amount of nitric acid (or ammonia) synthesized. 

The low temperature water-gas shift (LTWGS) catalysts commercially used for hydrogen and ammonia syntheses processes are based on Cu-Zn0-Al203. These catalysts work at 185-275°C. Non-pyrophoric and very active LTWGS catalysts, as needed for fuel cell applications, are based on supported noble metals, like Pt/Ce02-Al203, in monolytic structures. Catalysts based on gold, such as... 

The most important details in the technology of ammonia synthesis were kept confidential until the factory in Oppau was opened publicly, thus allowing outsiders to visit. The British Nitrogen Products Committee had estimated that the cost of ammonia synthesized from its elements would be about 50% more expensive than that from calcium cyanamide. This is why calcium cyanamide factories were built during, and even after, the First World War. 

Natural gas is processed mainly for the recovery of liquid hydrocarbons useful in gasoline, pure hydrocarbons as butane, propane, ethane, or mixtures of them, hydrogen sulfide (and sulfur or sulfuric acid), and carbon black but significant amounts of gas are also converted into ammonia, synthesized by the Fischer-Tropsch reaction, or oxidized into chemical products such as formaldehyde. Conventional operations, however, consist of mainly two operations, viz., recovery of liquids (absorption, etc.), and purification of the liquid. 

FIQURE 2.5 Energetics for the ammonia syntheses in the gas phase and at a surface. 

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