Nitrous gases

Figure 4-1. Assembling a nitrous gas compressor at a GHH manufacturing plant.

Process 1 Ammonia eombustion at atmospherie pressure and absorption of the nitrous gas at 3-12 bar... 

Process 2 Ammonia eombustion at 3-12 bar and subsequent nitrous gas absorption at the same pressure level... 

Process 3 Ammonia combustion at an intermediate pressure of 2-5 bar and nitrous gas absorption at 7-15 bar... 

Finally, the dual-pressure eyele of Proeess 3 ean be seen sehematieally in Figure 4-9, and in its physieal eonfiguration in Figure 4-10. Note that the lineup starts with the axial flow air eompressor to the left, the radial flow nitrous gas eompressor is in the middle, and the expander in the right foreground. Figure 4-11 shows the typieal eontrol and instrument diagram for Proeess 3. 

Figure 4-9. Ammonia combustion at air compressor discharge pressure of 2-5 bar, absorption at nitrous gas compressor discharge pressure of 7-15 bar (Process 3, dual-pressure cycle).

Figure 4-10. Compressor train for dual-pressure installation consisting of an axial flow air compressor with adjustable stator blades and a radial flow nitrous gas compressor and expander. Mass flow air = 139,000 Nm /h, nitrous gases = 122,500 Nm /h Pressure air = 0.82/4.75 bar, nitrous gases = 4.38/10.8 bar Power input total = 16,840 kW Power recovery by expander = 10,950 kW.

Nitrous gas compressor with air-sealed labyrinth shaft seals... 

Figure 4-12. Duty range for turbocompressors in nitric acid plants. The diagram refers to atmospheric air and gases with similar properties, such as nitrous gas (A = axial, R = radial flow compressor).

Due to die extremely toxie nature of nitrous gases, losses to die atmosphere must be avoided. Nitrous gas eompressors and expanders are fitted with suitable shaft seals, often equipped with eonneetions for seal air and vents to the staek. The simplieity of the seal and its assoeiated system make it a reliable element and easy to maintain. 

Seleetion of a tail gas expander depends on the temperature and pressure on the inlet eonditions of the nitrous gas. The pressure equals that of the eompressor outlet minus the pressure losses in the eyele. Depending on proeess and plant size, these losses amount to 0.3-2.0 bar. The inlet temperature may vary widely from plant to plant. Figure 4-19 shows an expander in the 10,000 kW power output eategory. 

Figure 4-18. Labyrinth seal for nitrous gas equipped with seal air connection.

Nitrous gases originating from the combustion units in nitric acid plants carry small amounts of unreacted ammonia, NH3. The ammonia may react with the nitrous gas to form microscopic particles of ammonium nitrate that adhere to solid surfaces. Within a short time, there is a growing layer of ammonium nitrate salt covering the internal surface of the nitrous gas compressor (Figure 4-27). This layer can obstruct the flow passages because it tends to increase the power consumption, provoke excessive vibrations, and even present a safety hazard since ammonium nitrate explosions can occur. 

Figure 4-27. Ammonium nitrate salt deposits in the inlet of a nitrous gas axial compressor.

To remove these undesirable but unavoidable salt deposits, a signi-fieant amount (about 1 % of mass flow) of boiler feedwater at 80-90°C is periodieally injeeted. Injeetion nozzles are typieally loeated in the inlet seetion, in the return ehannels, and in the labyrinth seals on the diseharge side of nitrous gas eompressors. This periodie flushing of the eompressor with water does have eertain disadvantages ... 

In the latter case, the plant is not purged and some nitrous gas will remain in the equipment. The entire installation is shut down within 6-8 min after the first signal is initiated. 

As mentioned earlier, a variety of turbomaehines are required to produee nitrie aeid. In the ease of small plants, these maehines represent a substantial investment. A eombination of proven air eompressor modules with newly developed expansion turbines and nitrous gas eompressors on a single gearbox provides a new, low-eost alternative and gives the plant owner/eontraetor useful potential for faeulty optimization (Figures 4-36 through 4-38). 

Dalton s work provided a system for representing chemical reactions, but inevitably, conflicts arose when trying to resolve Dalton s idea on chemical combination with experimental evidence. According to Dalton, one volume of nitrogen gas combined with one volume of oxygen to give one volume of nitrous gas (nitric oxide). Dalton referred to combination of atoms as compound atoms. Using Dalton s symbols, this reaction would be represented as... 

But when one volume of nitrogen reacted with one volume of oxygen, the result was two volumes of nitrous oxide, not one. Joseph Louis Gay-Lussac (1778-1850) had experimentally demonstrated that two volumes of nitrous gas result from combining one volume of nitrogen with one volume of... 

This idea occurred to me in 1805. I soon found that the sizes of the particles of elastic fluids must be different. For a measure of azotic gas and one of oxygen, if chemically united, would make nearly two measures of nitrous gas, and those two could not have more atoms of nitrous gas than the one measure had of azote or oxygen. Hence the suggestion that all gases of different kinds have a difference in the size of their atoms and thus we arrive at the reason for that diffusion of every gas through every other gas, without calling in any other repulsive power than the well-known one of heat. ... 

The catalytic oxidation of ammonia by air over platinum gauze at 900 °C gives nitric oxide (reaction 9.12), which is then oxidized to nitric acid by air and liquid water in a nitrous gas absorber (reactions 9.13 and 9.14) ... 

The nitrous gas absorption step (reactions 9.13 and 9.14) is slow, especially if concentrated HN03 is required, since cooling to 2°C is then necessary. Consequently, large countercurrent towers of stainless steel are needed, with associated high capital cost. The recovery of the heat of reaction of this step is inefficient because of the low temperature of the source gases that must be maintained. It has been suggested that the energy of reaction 9.12 could be more effectively recovered if it is run in a fuel cell (see Exercise 15.8). 

Didzatoluene-3-sulfonic Acid, wh nds, mp -not reported sol in hot w si sol in cold vf insol in ale prepd from 6 arainotoluene-3-sulfonic acid nitrous gas in a saturated... 

Diazotoluene-4-sulfonic Acid, prisms, burns brightly when heated on a Pt foil explodes by impact insol in cold w was prepd by introducting nitrous gas in an aq ale spin of 2-aminotoluene-4-sulfonic acid (Ref, p 569)... 

Bromo-4-diazotoluene-3-sulfonic Acid, CyHg OjBrS brn-red powd, expl on heating or on impact sol in cold w with slow decompn dec rapidly in hot w prepd by action of nitrous gas on a coned aq or aic soln of 5-bromo-4-amlnotoluene-3-sulfonic acid (Ref, p 568)... 

CyH O B S ndls, deflgr violently on heating almost insol in w ale prepd by passing nitrous gas in an aq suspension of 3,5-dibromo-2-aminotoluene-4 sulfonic acid... 

The SH-process involves continuous nitration of hexamethylenetetramine by concentrated nitric acid, with the production of nitrous gas. The RDX is filtered from the residual acid and stabilized by boiling in water under pressure and purified by recrystallization from acetone. 

MedloCK has latterly patented a method, whereby the nitrates which exert such a powerful action, upon lead are removed, together with the organic matter, if the latter he not present in great excess. It consists in suspending coils of iron wire or pieces of sheet-iron in the water, and after a stated period filtering off the deposit which forms. By this means the nitrie acid is decomposed by the iron,.which Is transformed into sos-quioxlde and falls to the bottom, whilst nitrous gas is set free and Oxidizes the carbonaceous matters, which remain still in the fluid so as to yield carbonic aoid and a lower oxide cf nitrogen. 

If one views the gigantlo scale in which this maun-fecture is carried on at the present day, by a Tennant Or a Mdspkatt, he must smile on looking bock at tbe, almost puny manner in which they operated in former times but, then, add to their process the continuous system—the introduction of steam into the chambers by Kestneb, concentration in the plathmm vesselB, and Gay-Lussac s column for the recovery of the nitrous gas—and manufacturers have almost all that could be desired at present. 

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