Reaction equilibrium ammonia production

The production of ammonia is of historical interest because it represents the first important application of thermodynamics to an industrial process. Considering the synthesis reaction of ammonia from its elements, the calculated reaction heat (AH) and free energy change (AG) at room temperature are approximately -46 and -16.5 KJ/mol, respectively. Although the calculated equilibrium constant = 3.6 X 108 at room temperature is substantially high, no reaction occurs under these conditions, and the rate is practically zero. The ammonia synthesis reaction could be represented as follows ... 

When produced from natural gas the synthesis gas will be impure, containing up to 5 per cent inerts, mainly methane and argon. The reaction equilibrium and rate are favoured by high pressure. The conversion is low, about 15 per cent and so, after removal of the ammonia produced, the gas is recycled to the converter inlet. A typical process would consist of a converter (reactor) operating at 350 bar a refrigerated system to condense out the ammonia product from the recycle loop and compressors to compress the feed and recycle gas. A purge is taken from the recycle loop to keep the inert concentration in the recycle gas at an acceptable level. 

Other observations of the reaction of hydrazine and nitrogen tetroxide substantiate the production of non-equilibrium combustion products. Non-equilibrium product concentrations were found in combustion gases extracted from a small rocket combustion chamber through a molecular beam sampling device with direct mass spec-trometric analysis (31) (39). Under oxidizer rich conditions excessive amounts of nitric oxide were found under fuel rich conditions excessive amounts of ammonia were found. A correlation between the experimentally observed characteristic velocity and nitric oxide concentration exists (40). Related kinetic effects are postulated to account for the two stage flame observed in the burning of hydrazine droplets in nitrogen dioxide atmospheres (41) (42). 

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. 

L-aspartic acid ammonia lyase, or aspartase (E.C. 4.3.1.1) is used on a commercial scale by Kyowa Hakko, Mitsubishi, Tanabe and DSM to produce L-aspartic acid, which is used as a building block for the sweetener Aspartame, as a general acidulant and as a chiral building block for synthesis of active ingrediants[1]. The reaction is performed with enzyme preparations from E. coli, Brevibacterium jlavum or other coryneform bacteria either as permeabilized whole cells or as isolated, immobilized enzymes. The process is carried out under an excess of ammonia to drive the reaction equilibrium from fumaric acid (1) in the direction of L-aspartic acid (l-2) (see Scheme 12.6-1) and results in a product of excellent quality (over 99.9% e.e.) at a yield of practically 100%. The process is carried out on a multi-thousand ton scale by the diverse producers of L-aspartic acid. Site directed mutagenesis of aspartase from E. coli by introduction of a Cys430Trp mutation has resulted in significant activation and stabilization of the enzyme P1. 

Note that since K 1, we expect this reaction to favor formation of product, leaving an equilibrium mixture that is predominantly ammonia. While this is a valid thermodynamic conclusion, it is incomplete because, in fact, at ambient conditions this ammonia-synthesis reaction proceeds slowly. To be industrially viable, the reaction must be carried out at elevated temperatures, where the equilibrium constant is actually smaller than it is at 25°C compensation is achieved by increasing the reaction pressure and using a catalyst. The controlling factor is a meager reaction rate, but thermodynamics cannot address rates in analyzing any reaction-equilibrium situation, thermodynamics can only bound what will be observed at the completion of a... 

The loop pressure has an important influence on the performance of the ammonia synthesis loop because of its influence on the reaction equilibrium, reaction kinetics, and gas/liquid equilibrium in the product separation. Actual selection of loop pressure is in many cases a compromise between selecting a high pressure to favour the ammonia synthesis reaction, and on the other hand selecting a reasonable pressure to minimise the compression power of the synthesis gas compressor, which compresses the synthesis gas to the desired loop pressure. The loop pressure also has a significant impact on the ammonia refrigeration system, since a high loop pressure favours condensation of the ammonia product in the loop water cooler and saves compression power on the refrigeration compressor. On the other hand, a low loop pressure saves compression power on the synthesis gas compressor, but increases the... 

Ammonia and nitric acid have been selected as process examples in this book and are treated in detail in Sections 6.1 and 6.4, respectively. Urea is produced industrially by reaction of ammonia and CO2 via the intermediate product ammonium carbamate ([H2N-COO][NH4]). While the formation of the carbamate intermediate is exothermic and quantitative under the applied reaction conditions (200 °C, 250bar), urea forms from the intermediate by liberation of vater in a slightly endothermic equilibrium reaction. Existing process technologies differ in their ways of carbamate decomposition as well as ammonia and CO2 recycling. State-of-the-art urea plants produce up to 1.700 tons of urea per day and are often linked to ammonia plants as CO2 is a by-product of NH3 production from natural gas. 

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... 

As we learned in Section 14.3, the equilibrium constant for a reaction that is the sum of two other reactions is the product of the equilibrium constants for the two other reactions. Adding ammonia changes the equilibrium constant for the dissolution of AgCl(i) by a factor of 3.0 X 10 jl.ll X 10 = 1.7 X 10 (17 million), which makes the... 

Energy is released in the exothermic reaction, in which the position of equilibrium favours ammonia production. 

Example 12.10 Modern large ammonia plants mostly carry out the ammonia production reaction at about 400°C and 150 atm pressure. Estimate the equilibrium conversion at these conditions, assuming that all reactants and products behave as ideal gases. 

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