Commercial ammonia converters

Equation (341) solves the task of quantitatively describing the effect of water vapor, and also oxygen gas (the last being rapidly converted to water vapor at the conditions of the reaction), on the activity of commercial ammonia synthesis catalysts. This result is of practical importance for ascertaining the necessary degree of purity of the inlet gas mixture with respect to poisons containing oxygen (122). 

In most processes the reaction takes place on an iron catalyst. The reaction pressure is normally in the range of 150 to 250 bar, and temperatures are in the range of 350°C to 550°C. At the usual commercial converter operating conditions, the conversion achieved per pass is only 20% to 30%53. In most commercial ammonia plants, the Haber recycle loop process is still used to give substantially complete conversion of the synthesis gas. In the Haber process the ammonia is separated from the recycle gas by cooling and condensation. Next the unconverted synthesis gas is supplemented with fresh makeup gas, and returned as feed to the ammonia synthesis converter74. 

The previous sections mainly considered the individual process steps involved in the production of ammonia and the progress made in recent years. The way in which these process components are combined with respect to mass and energy flow has a major influence on efficiency and reliability. Apart from the feedstock, many of the differences between various commercial ammonia processes lie in the way in which the process elements are integrated. Formerly the term ammonia technology referred mostly to ammonia synthesis technology (catalyst, converters, and synthesis loop), whereas today it is interpreted as the complete series of industrial operations leading from the primary feedstock to the final product ammonia. 

Fig. 11.3 Comparison of the steady-state ammonia converted during the ammonia oxidation on commercial and FeZ-18 catalysts. Effect of water on ammonia oxidation reaction is studied on the commercial catalyst. Feed 500 ppm NH3, 5 % O2, and 0 or 2 % water. Total flow rate 1,000 seem. Balance gas Ar. (Adapted from Metkar et al. [42] and used with permission.)...

Although ammonia converters and synthesis loops vary considerably in design, there are a number of general operating principles which can be applied to most commercial plants ... 

For many years into the future there will be a continuing requirement for the large-scale production of ammonia, principally for fertilizer use. The main objectives for commercially sized plants are high efficiency, low capital cost, and high reliability. The trends in plant design to achieve these objectives can be considered on a number of different levels, ranging from broad issues such as choice of feedstock to details of the ammonia converter design. 

Ammonia Production Processes 235 6.4.3 Commercial Ammonia Synthesis Converters... 

When this reaction was first discovered, a considerably higher (ca 1300°C) temperature was required than that used in the 1990s. Thus, until Haber discovered the appropriate catalyst, this process was not commercially attractive. As of this writing (ca 1995), the process suffers from the requirement for significant quantities of nonrenewable fossil fuels. Although ammonia itself is commonly used as a fertilizer in the United States, elsewhere the ammonia is often converted into soHd or Hquid fertilizers, such as urea (qv), ammonium nitrate or sulfate, and various solutions (see Ammonium COMPOUNDS). 

An additional mole of ammonium sulfate per mole of final lactam is generated duting the manufacture of hydroxylamine sulfate [10039-54-0] via the Raschig process, which converts ammonia, air, water, carbon dioxide, and sulfur dioxide to the hydroxylamine salt. Thus, a minimum of two moles of ammonium sulfate is produced per mole of lactam, but commercial processes can approach twice that amount. The DSM/Stamicarbon HPO process, which uses hydroxylamine phosphate [19098-16-9] ia a recycled phosphate buffer, can reduce the amount to less than two moles per mole of lactam. Ammonium sulfate is sold as a fertilizer. However, because H2SO4 is released and acidifies the soil as the salt decomposes, it is alow grade fertilizer, and contributes only marginally to the economics of the process (145,146) (see Caprolactam). 

The ammonolysis of phenol (61—65) is a commercial process in Japan. Aristech Chemical Corporation (formerly USS Chemical Division of USX Corporation) currently operates a plant at Ha verb ill, Ohio to convert phenol to aniline. The plant s design is based on Halcon s process (66). In this process, phenol is vapori2ed, mixed with fresh and recycled ammonia, and fed to a reactor that contains a proprietary Lewis acid catalyst. The gas leaving the reactor is fed to a distillation column to recover ammonia overhead for recycle. Aniline, water, phenol, and a small quantity of by-product dipbenylamines are recovered from the bottom of the column and sent to the drying column, where water is removed. 

The oxime is converted to caprolactam by Beckmann rearrangement neutralization with ammonia gives ca 1.8 kg ammonium sulfate per kilogram of caprolactam. Purification is by vacuum distillation. A no-sulfate, extraction process has been described, but incineration of the ammonium bisulfate recovers only sulfur values and it is not practiced commercially (14). 

The catalyst temperature is about 1100°C. Precious metal catalysts (90% Pt/10% Rh in gauze form) are normally used in the commercial processes. The converters are similar to the ammonia oxidation converters used in the production of nitric acid (qv) although the latter operate at somewhat lower temperatures. The feed gases to the converter are thoroughly premixed. The optimum operating mixture of feed gas is above the upper flammabiUty limit and caution must be exercised to keep the mixture from entering the explosive range. 

A convenient laboratory synthesis of high purity CA is hydrolysis of cyanuric chloride (7). On a commercial scale, CA is produced by pyrolysis of urea [57-13-6]. When urea is heated at - 250 ° C for about an hour, it is converted to crude CA with evolution of ammonia. 

Most commercial liquid ammonia contains up to several ppm of colloidal iron compounds, possibly the iron oxide catalyst commonly used in manufacturing ammonia. Reduction converts these compounds to colloidal iron which strongly catalyzes the reaction between alcohols and sodium and potassium. The reaction of lithium with alcohols is also catalyzed by iron but to a markedly lesser degree. The data in Table 1-4 illustrate the magnitude of these catalytic effects. The data of Table 1-5 emphasize how less than 1 ppm... 

A suspension of sodium amide2 (0.1 mole) in liquid ammonia is prepared in a 500-ml. three-necked, round-bottomed flask fitted with a West condenser, a ball and socket glass mechanical stirrer (Note 1), and a dropping funnel. In the preparation of this reagent a small piece of clean sodium metal is added to 350 ml. of commercial anhydrous liquid ammonia. After the appearance of a blue color, a few crystals of hydrated ferric nitrate are added, whereupon the blue color is discharged. The remainder of the 2.3 g. (0.1 mole) of sodium (Note 2) is then rapidly added as small pieces. After all the sodium has been converted to sodium amide (Note 3), a solution of 16.4 g. (0.1 mole) of ethyl phenyl-acetate (Note 4) in 35 ml. of anhydrous ethyl ether is added dropwise over a 2-minute period, and the mixture is stirred for 20 minutes. To the dark green suspension is added over an 8-minute period a solution of 18.5 g. (0.1 mole) of (2-bromo-... 

Feed gases to most, if not all, methanation systems for substitute natural gas (SNG) production are theoretically capable of forming carbon. This potential also exists for feed gases to all first-stage shift converters operating in ammonia plants and in hydrogen production plants. However, it has been demonstrated commercially over a period of many years that carbon formation at inlet temperatures in shift converters is a relatively slow reaction and that, once shifted, the gas loses its potential for carbon formation. Carbon formation has not been a common problem at the inlet to shift converters. It has been no problem at all in our bench-scale work, and it is not expected to be a problem in our pilot plant operations. 

Emission control from heavy duty diesel engines in vehicles and stationary sources involves the use of ammonium to selectively reduce N O, from the exhaust gas. This NO removal system is called selective catalytic reduction by ammonium (NH3-SGR) and it is additionally used for the catalytic oxidation of GO and HGs.The ammonia primarily reacts in the SGR catalytic converter with NO2 to form nitrogen and water. Excess ammonia is converted to nitrogen and water on reaction with residual oxygen. As ammonia is a toxic substance, the actual reducing agent used in motor vehicle applications is urea. Urea is manufactured commercially and is both ground water compatible and chemically stable under ambient conditions [46]. 

Commercially produced amines contain Impurities from synthesis, thus rigid specifications are necessary to avoid unwanted Impurities In final products. Modern-day analytical capability permits detection of minute quantities of Impurities In almost any compound. Detection In parts per million Is routine, parts per billion Is commonplace, and parts per trillion Is attainable. The significance of Impurities In products demands careful and realistic Interpretation. Nltrosatlng species, as well as natural amines, are ubiquitous In the environment. For example, Bassow (1976) cites that about 50 ppb of nitrous oxide and nitrogen dioxide are present In the atmosphere of the cities. Microorganisms In soil and natural water convert ammonia to nitrite. With the potential for nitrosamine formation almost ever-present In the envlronmeit, other approaches to prevention should Include the use of appropriate scavengers as additives In raw materials and finished products. 

---