This invention relates generally as indicated to the synthesis of ammonia and more particularly to apparatus in which such synthesis may be advantageously carried out.
The synthesis of ammonia from natural gas occurs by a process in which natural gas, steam and air are combined in a series of well-known chemical reactions, culminating in a synthesis step. The synthesis reaction is conducted at high pressures and relatively moderate to high temperatures to obtain efficient conversion, since the rate of reaction is enhanced by higher temperatures. The equilibrium reaction, however, is enhanced by high pressure and low temperature and is significantly retarded by temperatures on the order of approximately 900.degree. to 950.degree. Fahrenheit. Because the synthesis reaction is exothermic, this is a significant consideration, as is the requirement for a minimum temperature of about 650.degree. to 700.degree. Fahrenheit to initiate the reaction, thus necessitating careful and accurate temperature control.
Moreover, since the synthesis reaction occurs at high pressure, the synthesis apparatus must be constructed in such a way and of such materials to stand prolonged use, at pressures, for example, of 2000 pounds per square inch or greater. To satisfy such requirements, a synthesis converter has previously been used which included a thick walled bottom forging adjacent the portion of the vessel containing the catalyst bed. The forging was fabricated from a material comprising an alloy of chromium and molybdenum. Over the course of time, however, it was found that the forging developed cracks which were hydrogen induced, resulting from the high temperature to which it was exposed and the many cycles that the forging was subjected to as a result of shut downs in operation.
Additionally, because the forging is positioned adjacent the catalyst bed, the exothermic synthesis reaction exposes the chromium alloy to temperatures on the order of 950.degree. to 1000.degree. Fahrenheit. It has been found that such temperatures cause the reaction between ammonia and the chromium-molybdenum alloy to proceed rapidly and form an objectional chrome-nitride layer along the interior surface of the forging.
Because the forging is subjected to high temperatures, it has been the practice to fabricate the forging from a 5% chromium-1/2% molybdenum alloy. However, 5% chromium alloys are not acceptable under the applicable codes for pressure vessel components in West Germany and consequently, the synthesis converter could not be used in that country.
A modified form of synthesis conversion apparatus which has also been utilized eliminated the thick walled bottom forging and bottom catalyst dump and replaced the same with an outlet elbow, which enabled the wall thickness to be reduced considerably since it was unnecessary to have integral reinforcement or a side outlet nozzle. Such apparatus, however, utilized an outlet fabricated of a material containing the aforedescribed 5-Cr-1/2-Mo alloy to withstand the temperatures to which the outlet was subjected. Hence, such apparatus suffers from the same limitations with respect to nitride and code as described above. Furthermore, such apparatus requires piping which operates at objectionable temperatures and at high pressures.