Conversion of Synthesis Gas to Ammonia

The industrial scale reaction of synthesis gas to ammonia in pressure reactors takes place in a cyclic process in which the ammonia formed is removed from the reaction gas and the unreacted synthesis gas returned to the reactor. In addition to the ammonia formed, inert gases and the liberated reaction heat have to be continuously removed from the cyclic process. The excess heat of the product gas is used to heat the feed synthesis gas to the reaction temperature in a heat exchanger integrated into the reactor. Additional waste heat can be utilized for steam generation. The pressure loss in the synthesis gas due to its passage through the synthesis loop is compensated for and the fraction of synthesis gas converted replaced by fresh compressed synthesis gas ( fresh gas ). 

The most important part of an ammonia synthesis plant is the pressure reactor, which is filled with catalyst and in which ammonia formation takes place at temperatures between 400 and 500°C. A maximum temperature of 530°C must not be exceeded, otherwise catalyst damage will ensue. 

Compression in the larger plants is currently carried out with turbocompressors. Plants with capacities 600 t7d still use the previously widely used piston pumps. With turbocompressors, which are almost always driven by steam turbines, plants with a capacity of 1500 t/d can operate in the preferred and economically optimal pressure range of 250 to 350 bar, a single turbocompressor providing the fresh gas compression and the circulation. 

Different reactor types are used in the individual ammonia-synthesis plants. They have in common, that the catalyst mass is to be found in a separate container inside the reactor chamber. Between the catalyst holder and the reactor wall there is a gap through which the cold synthesis gas can be fed in, such that the reactor wall is not heated to the same temperature as the catalyst holder. 

The cold synthesis gas fed into the cycle is, heated with the help of the hot reaction gases inside or outside the 

---

The central part of the synthesis system is the converter, in which the conversion of synthesis gas to ammonia takes place. Converter performance is determined by the reaction rate, which depends on the operating variables. The effect of these parameters is discussed briefly in the following (see also Section 4.5.7). 

Conversion per pass (fractional conversion of synthesis gas to ammonia by passage through the reactor system). 

At the stage of development of the ammonia synthesis reaction as described in the previous sections, the main problem was the low level of conversion of synthesis gas to ammonia. This conld be increased by operation at yet higher pressure, but this would not be cost effective. The other option would be to operate at lower temperature, under which conditions, the equilibrium concentration of ammonia would be appreciably higher. Unfortunately, the catalysts available at the time were not sufficiently active to operate at the lower temperature required to make a meaningful difference to the concentration of ammonia in the product... 

Subsequently, patents covering the conversion of synthesis gas to complex mixtures of organic oxygen compoimds, including methanol, were issued to BASF during 1913. This followed work by Mittasch and Schneider. Full-scale production of methanol was not attempted, however, imtil 1923. By that time high-pressure equipment had been in operation for several years in the new ammonia process. The methanol process was developed by Piers and the plant, built at Leima, used mixed zinc oxide-chromic oxide catalyst. The use of metallic iron for the internal parts of the reactor was avoided to prevent the formation of the volatile iron penlacarbonyl. The would have decomposed on the surface of the catalyst, to deposit finely divided iron metal, which in turn would have promoted the exothermic formation of methane. 

The sequence of the remainii steps of synthesis gas preparation is CO shift conversion, removal of H2S and CO2 by Rectisol wash (cold methanol), and liquid nitrogen wash. As in other partial oxidation processes, the H2S is converted to elemental sulfur. Ube Industries, Japan, commissioned a 1,500-tpd ammonia plant in 1984 using the Texaco coal gasification process. An energy consumption of 10.9 Gcal/tonne of ammonia is stated this is lower than the normal quoted f ure of 11.6 Gcal/ tonne of NH3 for coal-based processes 131]. Another 1,000 tpd coal-based ammonia plant is scheduled for startup at Wehei, China, in 1996. 

Compression of synthesis gas. The synthesis gas composed by hydrogen and nitrogen is compressed to required pressures, usually 10-30 MPa, by piston-type or centrifugal compressors. During ammonia synthesis, single-pass conversion is only 10%-20%, and therefore, most of the synthesis gas must be recycled, compressed and returned to the synthesis loop again. 

In the production of synthesis gas for subsequent conversion to ammonia or methanol, for example, it is usually necessary to remove carbon dioxide formed either by partial combustion of hydrocarbons or by the water gas shift reactions. 

The oxides of carbon are poisons to the iron-based ammonia synthesis catalyst and mnst be removed from the synthesis gas before use. An important reaction that affected the economics of the manufacture of synthesis gas was the conversion of carbon monoxide into equal volumes of hydrogen and carbon dioxide by reaction with steam. This procedure, now cotrunonly referred to as the water-gas shift reaction required a catalyst based on iron and chromium oxides. 

In 1991, the relatively old and small synthetic fuel production faciHties at Sasol One began a transformation to a higher value chemical production facihty (38). This move came as a result of declining economics for synthetic fuel production from synthesis gas at this location. The new faciHties installed in this conversion will expand production of high value Arge waxes and paraffins to 123,000 t/yr in 1993. Also, a new faciHty for production of 240,00 t/yr of ammonia will be added. The complex will continue to produce ethylene and process feedstock from other Sasol plants to produce alcohols and higher phenols. 

Ammonia production by partial oxidation of hydrocarbon feeds depends to some degree on the gasification step. The clean raw synthesis gas from a Shell partial oxidation system is first treated for sulfur removal, then passed through shift conversion. A Hquid nitrogen scmbbiag step follows. 

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

Ammonia Synthesis and Recovery. The purified synthesis gas consists of hydrogen and nitrogen in about 3 1 molar ratio, having residual inerts (CH Ar, sometimes He). The fresh make-up gas is mixed with the loop recycle and compressed to synthesis pressures. AH modern synthesis loops recycle the unreacted gases because of equiUbrium limitations to attain high overall conversions. The loop configurations differ in terms of the pressure used and the point at which ammonia is recovered. 

---