Ammonia partial oxidation process

Of the raw material hydrogen sources—natural gas, coal, and petroleum fractions—natural gas is the most often employed in ammonia plants in the 1990s and steam reforming is by far the most often used process. Partial oxidation processes are utilized where steam-reformable feeds are not available or in special situations where local conditions exist to provide favorable economics. Table 5 fists the contribution of the various feedstocks to world ammonia... 

In the past, coal or heavy hydrocarbon feedstock ammonia plants were not economically competitive with plants where the feedstocks were light hydrocarbons (natural gas to naphtha). Because of changing economics, however, plants that can handle heavy hydrocarbon feedstock are now attracting increasing attention- In addition, the continuous development and improvement of partial oxidation processes at higher pressure have allowed reductions in equipment size and cost. Therefore, the alternate feedback ammonia plants based on a partial oxidation process may become economically competitive in the near future. 

Fig. 1.4-2. Flow chart of a single-train plant for the synthesis of ammonia using the partial oxidation process.

Heavy Oil - Liquid hydrocarbons heavier than naphtha are used for ammonia feedstock by partial oxidation. processes, which will be discussed in a later section of this chapter. 

Hydrocarbons heavier than naphtha can be used as -ieedsteeks-for-ammonia-produclion by partial oxidation processes. Natural gas and naphtha also can be used, but since the plant cost for the partial oxidation process is considerably higher than that for steam reformir, the lighter feedstocks are seldom used. However, the partial oxidation process does offer the advantage of wider choice of feedstock with greater tolerance for impurities. 

The feedstock requirement typically is about 0.76 tonnes of heavy oil per tonne of ammonia [36]. In addition, about 0.14 tonnes of oil or the equivalent in other fuel is required for generation of steam and electricity, or 530 kWh of electricity may be imported. In contrast with the steam-reforming process where most of the fuel is used in the reformir furnace, which requires a premium grade of fuel, the auxiliary fuel for a partial oxidation process is used in a separate unit, which can use coal, for example. 

The gasification pressure in partial oxidation processes has been gradually increased to a range of 60-90 atm, which helps save energy for compression. Texaco s commercial gasifiers for ammonia plants operate at 8,5-9 MPa [371. 

Entrained coal gasifiers are typified by the Koppers-Totzek (KT) and Texaco processes. Most of the present coal-based ammonia plants (except in China) use the KT process. The process is essentially a partial oxidation process as are most coal gasification processes. At least two KT ammonia plants have been built to use either coal or heavy oil although only one (in Finland) is known to have used both feedstocks [48]. However, any entrained gasification units presumably could be readily adapted to heavy oil or lighter hydrocarbons including natural gas. 

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. 

Alternative processes. The most common alternative is to carry out the Haber process but provide the heat for the process internally rather than externally. The extra oxygen used to burn the fuel feedstock is provided in a pure state to ensure that a suitable balance between nitrogen (from the air) and hydrogen is maintained. This is known as the partial oxidation process. Although this process may appear to be more thermally efficient than the steam reforming process it does in fact use about 10% more fuel for an equivalent amount of ammonia. 

The Texaco process was first utilized for the production of ammonia synthesis gas from natural gas and oxygen. It was later (1957) appHed to the partial oxidation of heavy fuel oils. This appHcation has had the widest use because it has made possible the production of ammonia and methanol synthesis gases, as well as pure hydrogen, at locations where the lighter hydrocarbons have been unavailable or expensive such as in Maine, Puerto Rico, Brazil, Norway, and Japan. 

ATR(l) [Autothermal reforming] A process for making CO-enriched syngas. It combines partial oxidation with adiabatic steam-reforming. Developed in the late 1950s for ammonia and methanol synthesis. Further developed in the 1990s by Haldor Topsoe. 

Why synthesis gas And where does the nitrogen come from Synthesis gas, of course, provides the hydrogen air provides the nitrogen. And if the. synthesis gas process is partial oxidation, then there was probably an air separation plant associated with it. That separates the oxygen from the nitrogen for making the synthesis gas, and leaves the nitrogen for feed to the ammonia plant. 

The oxidation and the partial oxidation method, the CO2 from an ammonia plant in a reaction with methane, and water all produce different ratios of CO and H2. In addition, CO2 can be removed by solvent extraction. So, the trick is to use two or three of these processes to get the C0 H2 ratio to about 1 2. 

Hydrazine may he produced by several methods. The most common commercial process is the Raschig process, involving partial oxidation of ammonia or urea with hypochlorite. Other oxidizing agents, such as chlorine or hydrogen peroxide may he used instead of hypochlorite. The reaction steps are as follows. 

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