Synthesis gas preparation

Synthesis Gas Preparation Processes. Synthesis gas for ammonia production consists of hydrogen and nitrogen in about a three to one mole ratio, residual methane, argon introduced with the process air, and traces of carbon oxides. There are several processes available for synthesis gas generation and each is characterized by the specific feedstock used. A typical synthesis gas composition by volume is hydrogen, 73.65% nitrogen, 24.55% methane, <1 ppm-0.8% argon, 100 ppm—0.34% carbon oxides, 2—10 ppm and water vapor, 0.1 ppm. 

Synthesis gas preparation consists of three steps ( /) feedstock conversion, (2) carbon monoxide conversion, and (2) gas purification. Table 4 gives the main processes for each of the feedstocks (qv) used. In each case, except for water electrolysis, concommitant to the reactions shown, the water-gas shift reaction occurs. 

The carbon dioxide removed in synthesis gas preparation can be reacted with ammonia, to lonn urea CO(NH2)2- This is an excellent fertilizer, highly concentrated in nitrogen (46.6%) and also useful as an additive in animal feed to provide the nitrogen for formation of meat protein. Urea is also an important source of resins and plastics by reacting it with formaldehyde from methanol. 

The composition of the synthesis gas, particularly the concentrations of hydrogen, carbon monoxide, and carbon dioxide, affects the atmosphere throughout the reactor directly, and also indirectly by its effect on the composition of the recycle gas. Synthesis gas, prepared by partial combustion of methane or some less hydrogen-rich carbonaceous material, lacks sufficient hydrogen for the conversion of all the carbon monoxide to hydrocarbons, and in this sense the synthesis gas is deficient in hydrogen. Stoichiometrically methane has sufficient hydrogen to convert all its carbon to olefins by the two-step process ... 

Catalyst Poisons. Synthesis gas prepared by the partial combustion of sweet natural gas can be charged to the reactors directly without purification. However, synthesis gas containing more than 0.1 grain of sulfur per 100 cubic feet must be purified before use over fluidized iron catalysts. Other catalyst poisons are known, such as chlorine (14), but they are not likely to be encountered in the natural gas to gasoline process. 

An operating ammonia plant using the aforementioned improvements is shown schematically in Fig. 1. This plant8 has a capacity of 1000 short tons/day (900 metric tons/day) and uses natural gas as feedstock. The plant can be divided into the following integrated-process sections (a) synthesis-gas preparation (b) synthesis-gas purification and (c> compression and ammonia synthesis. A typical (Kellogg designed) ammonia plant is shown in Fig. 2. 

Synthesis Gas Preparation. The desulfurized natural gas mixed with steam is fed to the primary reformer, where it is reacted with steam in nickel-catalysl-lilled lubes to produce a major percentage of the hydrogen required. The principal reactions taking place are9... 

Ammonia synthesis is normally carried out at a pressure higher than that for synthesis gas preparation. Therefore the purified synthesis gas that is fed to the ammonia synthesis loop must be compressed to a higher pressure. Synthesis loop pressures employed industrially range from 8 to 45 MPa (80 to 450 bar). However, the great majority of ammonia plants have synthesis loops that operate in the range of 15 to 25 MPa (150 to 250 bar)74. 

Ammonia synthesis is normally carried out at a pressure that is higher than that for synthesis gas preparation. Therefore the purified synthesis gas to the ammonia synthesis loop must be compressed to a higher pressure.74... 

It has been already mentioned briefly, that compared to the synthesis section itself, where of course some progress has been made in converter design and optimization of heat recovery, the more fundamental changes over the years have occurred in synthesis gas preparation and gas compression. It is therefore appropriate to discuss the various methods for the synthesis gas generation, carbon monoxide shift conversion, and gas purification in some detail. Figure 29 shows schematically the options for the process steps for ammonia production. 

CO2 emissions from the amonia production are generated during the synthesis gas preparation. In past more than 50% of generated CO2 had been used to prepare the urea. The urea was then used in production of industrial fertilisers and production of urea formaldehyde polycondensates. Research of its application in the No reduction from the flue gases and... 

Fischer-Tropsch synthesis is the heterogeneously-catalysed formation of hydrocarbons from CO and water. It can be seen as the inverse of the synthesis gas preparation. It is the heart of the respawned gas-to-liquids processes developed by big petrochemical firms in the 9O s. 

The synthesis gas preparation phase of methanol synthesis is presently the most in need of major innovations and developments. As natural gas and liquid hydrocarbons become more costly and less abundant, the less easily processed sources of carbon must be used. First to be considered will be the... 

Steam-hydrocarbon reforming has become the most frequently used method for synthesis gas preparation, and usually natural gas, treated to remove all com-... 

As previously mentioned, two types of ammonia synthesis loops exist. For the inert- containing loop, the inert level in the synthesis loop (most often measured at converter inlet) depends on the inert level in the make-up gas, the production of ammonia per unit make-up gas (the loop efficiency), and the purge rate. The inert level in the make-up gas is solely determined by the conditions in the synthesis gas preparation unit. The ammonia production is determined by conditions around the converter, the gas flow (which may be expressed by the recycle ratio), the inlet temperature and pressure, catalyst volume and activity, and converter configuration. 

The activation of the catalysts occurs by careful reduction of the Cu component (CuO), which is reduced to metallic Cu, whereas Zn and A1 remain as oxide(s), independent of whether the reduction is performed in diluted H2 or in synthesis gas. Industrially, the reducing gas (pure H2 or make-up gas from the synthesis gas preparation) is diluted with inert gas (maximum 2-4% reducing gas) in order to limit the adiabatic temperature rise, which may cause Cu particle sintering. The catalyst is heated to 450 K at low pressure and high space velocity, then H2 is introduced. Finally, the temperature is raised to 490-510 K. As the reduced catalyst is pyrophoric, the reduction procedure is usually performed in the reactor [2]. 

The higher hydrocarbons usually have a higher value for petrochemical production or for sale as liquefied petroleum gas (LPG) than methane. Therefore, in most cases, these products are sold separately, and the term natural gas usually refers to the fraction that contains mostly methane with only smaD percentages, of ethane and higher hydrocarbons. For use as ammonia feedstock, methane is preferable to the higher hydrocarbons because all carbon in the feedstock is converted to carbon dioxide or monoxide, which must be removed from-the ammonia synthesis gas. Therefore, the lower the carbon hydrogen ratio in the feedstock, the smaller and less expensive the purification units in the synthesis gas preparation section will be. 

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. 

The synthesis gas composition depends on the raw material and on the process used in the synthesis gas preparation. Three examples... 

Rostrup-Nielsen, J.R., and P.E. H0jlund Nielsen "Catalyst Deactivation in Synthesis Gas Preparation and important Syntheses", in "Deactivation and Poisoning of Catalysts (Budar, J. and H. Wise, eds.) Marcel Dekker, in press. 

The final stage of synthesis gas preparation is to remove the remaining carbon oxides by methanation. This occurs over a supported nickel oxide catalyst at a temperature of 250-350 °C. The nickel oxide must be reduced to the metallic state before use. 

Another important factor is plant capacity. Until 1960 the capacity of individual installations had increased to a maximum of about 400 MTPD. The units were often multi-train units meaning that several parallel trains were installed in synthesis gas preparation and the synthesis loop - not necessarily the... 

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