Ammonia synthesis commercial application

We first considered applications of this approach within process engineering. Steady-state flowsheeting or simulation tools are the workhorse for most process design studies the application of simultaneous optimization strategies has allowed optimization of these designs to be performed within an order of magnitude of the effort required for the simulation problem. An application of this strategy to an ammonia synthesis process was presented. Currently, flowsheet optimization is widely available commercially and has also been installed on the FLOWTRAN simulator for academic use. 

Ammonia synthesis is one of the most important processes of chemical industry tens of millions of tons of this product are synthesized annually in various countries of the world. On a commercial scale the reaction is operated on promoted iron catalysts at temperatures near to 500°C and high pressures, mostly at 300 atm. At present K20, A1203, and CaO in amounts of several parts by weight per 100 parts of catalyst are usually employed as promoters. The application of high pressure is caused by the reversibility of the reaction molar fraction of ammonia corresponding to the equilibrium... 

At integrating (305) for the conditions of a flow system (93, 98), it proved to be convenient to introduce a constant k proportional to k. The value of k was also calculated from data obtained in circulation flow systems (4, 96, 99-103). If the volume of ammonia reduced to 0°C and 1 atm, formed in unit volume of catalyst bed per hour, is accepted as a measure of reaction rate, then k = (4/3)3 1 m)k (101). The constancy of k at different times of contact of the gas mixture with the catalyst and different N2/H2 ratios in the gas mixture can serve as a criterion of applicability of (305). Such constancy was obtained for an iron catalyst of a commercial type promoted with A1203 and K20 at m = 0.5 (93) from our own measurements at atmospheric pressure in a flow system and literature data on ammonia synthesis at elevated pressures up to 100 atm. A more thorough test of applicability of (305) to the reaction on a commercial catalyst at high pressures was done by means of circulation flow method (99), it confirmed (305) with m = 0.5 for pressures up to 300 atm. Similar results were obtained in a large number of investigations by different authors in the USSR and abroad. These authors, however, have obtained for some promoted iron catalysts m values differing from 0.5. Thus, Nielsen et al. (104) have found that m 0.7. 

In 1905 Haber reported a successful experiment in which he succeeded in producing NH3 catalytically. However, under the conditions he used (1293 K) he only found minor amounts of NH3. He extrapolated his value to lower temperatures (at 1 bar) and concluded that a temperature of 520 K was the maximum temperature for a commercial process. This was the first application of chemical thermodynamics to catalysis, and precise thermodynamic data were not then known. At that time Haber regarded the development of a commercial process for ammonia synthesis as hopeless and he stopped his work. Meanwhile, Nernst had also investigated the ammonia synthesis reaction and concluded that the thermodynamic data Haber used were not correct. He arrived at different values and this led Haber to continue his work at higher pressures. Haber tried many catalysts and found that a particular sample of osmium was the most active one. This osmium was a very fine amorphous powder. He approached BASF and they decided to start a large program in which Bosch also became involved. 

Around 1900 Fritz Haber began to investigate the ammonia equilibrium [11] at atmospheric pressure and found minimal ammonia concentrations at around 1000 °C (0.012 %). Apart from Haber, Ostwald and Nernst were also closely involved in the ammonia synthesis problem, but a series of mistakes and misunderstandings occurred during the research. For example, Ostwald withdrew a patent application for an iron ammonia synthesis catalyst because of an erroneous experiment, while Nernst concluded that commercial ammonia synthesis was not feasible in view of the low conversion he found when he first measured the equilibrium at 50 - 70 bar [12] - [14],... 

Ammonia lyases in their natural role are involved in the metabolism of amino acids and also play a role in, for instance, the degradation of amino sugars, but only a limited amount of these enzymes have been characterized biochemically. Application of a broad range of different ammonia and lyases in organic chemical synthesis on an industrial scale has thus far not occurred, which is due to both their limited commercial availability and their lack of stability under process conditions. Exceptions are the commercially applied aspartase, which is an ammonia lyase that is utilized for the synthesis of L-aspartic acid from fumaric acid, and phenylalanine lyase. The latter is an example of a commercial application of an ammonia lyase in a process for the production of L-phenylalanine and more importantly L-phenylalanine derivatives. 

The adiabatic fixed-bed reactor with periodic flow reversal has three commercial applications, oxidation of SO2 for sulfuric acid production, oxidation of volatile organic compounds (VOCs) for purification of industrial exhaust gases, and NO, reduction by ammonia in industrial exhaust gases. Other possible future applications are steam reforming and partial oxidation of methane for syngas production, synthesis of methanol and ammonia, and catalytic dehydrogenations (Matros and Bunimovich, 1996). 

Having introduced the two main processes for the production of ammonia synthesis gas, the commercial application of the synthesis reaction can now be considered. 

Since the first commercial application of the Haber process for ammonia production in 1913, a wide variety of synthesis loop designs have been developed. A history of the early developments is given in Chapter 1, and has also been reviewed elsewhere. However, by the 1950s and early 1960s, a broad consensus about the optimum design conditions for an ammonia synthesis loop had been reached. A typical design from this period will be described. This will be used to demonstrate how the elements of the synthesis loop are applied to produce a practical design, and will also serve as a base case for the discussion of modern developments. A flowsheet for this type of synthesis loop is shown in Fig. 7.4. 

The Koppers-Totzek (K-T) gasifier produces a medium-Btu gas (in the general range of 300 Btu/scf) and has been commercially employed in many different syngas applications, with particular emphasis in the area of ammonia synthesis. The process is carried out at just over atmospheric pressure but at very hi temperatures of over 1870°C. The data in Table 3 [16] give the expected K-T gasifier product composition for an Illinois coal (62% C, 19.1% ash, 4.4% H2, and 5% S plus 02 and H20) that has been ified with a steam-coal ratio (wt/wt) of 0.27 and an oxygen/coal ratio (wt/wt) of 0.7. K-T units vary in size between those that convert about 300 t coal per day and those that convert over 750 t coal per day. 

The first widespread commercial application of membrane separations is to use hollow-fiber membrane separators for hydrogen recovery from processes, ammonia plants and petrochemical purge streams, and H2/CO ratio adjustment in synthesis gas (Gardner et al., 1977 Bollinger et al., 1984 Koros and Mahajan, 2000). The reported ideal hydrogen selectivities with respect to different gases, such as CO2, CO, N2, and CH4, appear to be reasonable, ranging from 170 for H2/CH4 to 6.75 for H2/CO2. The operation of these permeators, however, is restricted to low temperatures because of the polymeric material used for the synthesis of hollow fibers. 

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