Ammonia process steps

Steam-Reforming Natural Gas. Natural gas is the single most common raw material for the manufacture of ammonia. A typical flow sheet for a high capacity single-train ammonia plant is iadicated ia Figure 12. The important process steps are feedstock purification, primary and secondary reforming, shift conversion, carbon dioxide removal, synthesis gas purification, ammonia synthesis, and recovery. 

The TFTs are made on transparent glass substrates, onto which gate electrodes are patterned. Typically, the gate electrode is made of chromium. This substrate is introduced in a PECVD reactor, in which silane and ammonia are used for plasma deposition of SiN as the gate material. After subsequent deposition of the a-Si H active layer and the heavily doped n-type a-Si H for the contacts, the devices are taken out of the reactor. Cr contacts are evaporated on top of the structure. The transistor channel is then defined by etching away the top metal and n-type a-Si H. Special care must be taken in that the etchant used for the n-type a-Si H also etches the intrinsic a-Si H. Finally the top passivation SiN, is deposited in a separate run. This passivation layer is needed to protect the TFT during additional processing steps. 

VOC emissions from the processing steps for separating ammonia, coke oven gas, tar, phenol, light oils, and pyridine from the foul gas... 

Fourth, the increased capacity may allow some changes in equipment and/or processing steps that can produce substantial savings. For example, a major reduction in the price of ammonia occurred in the late 1960s because the increase in the... 

In the aerobic treatment step, which in general follows the anaerobic treatment step and the specific treatment processes for removal of ammonia and hydrogen sulphide, residual amounts of organic pollutants are removed. A proper sequence of process steps and proper process conditions in the aerobic treatment step can also lead to the removal of nitrogen and phosphorus containing pollutants... 

This paper will take note of a number of troublesome components of Claus feed, including olefins, CO2, ammonia, cyanide, and COS. Proper management of these components usually requires thoughtful selection of preparatory process steps as well as the Claus step itself. 

Hot gases from the ammonia destruction step contain nitrogen, a trace of residual NH3, as well as H2, S, SO2 and sulfur vapor. With proper design these gases can be fed into a Claus type process to make sulfur. If the latter process uses Selectox catalyst ... 

When ammonia is made from natural gas using the steam reforming process, several reaction stages are needed and catalysts are key to the economic operation of each stage. The chemistry for the different process steps and the basic materials that make up the catalysts are shown in Figure 6.582. 

A comparison of the process steps in a conventional ammonia process (Figure 6.16) with the MEGAMMONIA process (Figure 6.17) is given in Table 6.6. The MEGAMMONIA process has one less process unit than the conventional process and two less catalytic reactors291. 

In the DSN processes ammonia and air react over a platinum catalyst, and the process gas stream is cooled to the point where weak acid condensate is formed. Tail gases are also reheated by hot process gases and provide energy recovery in a gas expander. DSN processes differ from weak acid plants in that additional or different process steps are needed to achieve superazeotropic strengths of nitric acid100,104. 

Much ingenuity has been exercised by chemists in attempts to effect this change through a series of reactions. In the older Le Blanc soda process sulphuric acid is used in the first step and coal (carbon) in another, and neither of these auxiliary substances is recovered. The soda process which is employed exclusively today is known as the Solvay or ammonia process and uses ammonia as an auxiliary substance. The ammonia is almost 100 per cent recoverable, however, and can be used over and over indefinitely. The successive steps in the process are as follows ... 

The final step is desulfurization. A number of processes have been developed which are suitable for removing H2S from coke-oven gas. These include using solutions of potassium carbonate, monoethanolamine (MEA), or ammonia to absorb the H2S. If ammonia solution is used as the absorbent, desulfurization is frequently combined with the ammonia removal step. Recovered H2S can be converted to elemental sulfur or sulftiric acid. The product remaining after all the above steps is cleaned coke-oven gas, some of which is used to heat the coke ovens and produce more coke with the rest going to the boiler house and/or the blast furnace for direct injection. 

Synthesis gas from the gasification of fossil fuels is contaminated by several gaseous compounds that would affect its further use in different ways. The purification steps depend on the type of syngas process and the feedstock. Table 22.9 illustrates how the synthesis gas stream can vary with different feedstocks. Figure 22.8 illustrates the different process steps that may occur before ammonia synthesis takes place.74... 

Fig. 22.8. Alternative process steps for generation and purification of synthesis gas. (Courtesy of Wiley-VCH. Bakemeier, H., Huberich.T., et al. "Ammonia" in Ullmann s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A 2, VCH Verlagsgesellschaft, Weinheim 1985, pp. 143-242.

After the sulfur is removed, most traditional ammonia processes have employed the purification steps that are shown in Figs. 22.4 and 22.6. However, ammonia plants have been built that use hydrogen purification via PSA, membrane separation with polymeric membranes, and cryogenic separation. PSA achieves the greatest product purities (about 99.999% pure hydrogen), but it is capital-intensive. Polymeric membranes are the least capital-intensive, but they achieve the lowest... 

Synthesis gas is compressed to the synthesis pressure, typically ranging from 140 to 220 kg/cm2g and converted into ammonia in a synthesis loop using radial flow synthesis converters, either the two-bed S-200, the three-bed S-300, or the S-250 concept using an S-200 converter followed by a boiler or steam superheater, and a one-bed S-50 converter. Ammonia product is condensed and separated by refrigeration. This process layout is flexible, and each ammonia plant will be optimized for the local conditions by adjustment of various process parameters. Topsoe supplies all catalysts used in the catalytic process steps for ammonia production. 

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. 

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