Johnson Matthey Technology Review the Ammonia Oxidation Process for Nitric Acid

Thomas Malthus (1766–1834) maintained that the earth'due south population increases more rapidly than its food supplies, and only disease, state of war, poverty and vice foreclose big-scale starvation. He could not foresee the world of the twentieth century, nor the role to be played by platinum gauzes in producing fixed nitrogen fertilisers without which his gloomy predictions might well be realised.

It is at present estimated that the full world output of nitric acid has risen from nearly 18.three million tonnes (100 per cent HNO₃) in 1962–63 to just nether 23.v one thousand thousand tonnes in 1965–66 (1). At this level of production the world's nitric acid plants are believed to operate at an average of 82 per cent of all available product chapters. The estimated production of the major manufacturing countries is shown in Table I (i). It may as well be estimated that this total plant capacity represents a platinum gauze inventory of effectually 200,000 oz troy (about 6250 kg), excluding the weight of spare gauzes that almost plants hold.

Table 1

The World's Output of Nitric Acid (1)

(Millions of tonnes 100 per cent HNO₃)

	1962–63	1965–66
U.s.A.	3.four	4.7
United states of americaS.R.	two.5	3.seven
Westward Germany	2.2	2.55
France	i.73	2.2
Italy	1.05	1.3
Norway	1.0	1.25
Netherlands	0.seven	0.95
United Kingdom	0.6	0.90
Rest of the world	5.12	5.85

The manufacture of nitrogen fertilisers represents by far the largest proportion of the use of nitric acid. These fertilisers, more often than not with high nitrogen contents, provide the active nitrogen in the class of ammonium nitrate or as nitrophosphate formed by the activity of nitric acid on phosphate rock. For these processes "weak" nitric acid (55 to 65 per cent HNO_three) is usually employed. Sure territories, such as Nihon, are exceptions to this generalisation, since nitrate fertilisers for domestic agriculture may not be suitable in that location, and consumption of the acid by the explosive and chemical industries is thus more of import. In the U.S.A., about 65 per cent of the nitric acid produced is consumed in the manufacture of fertilisers, for instance, ammonium nitrate, and an additional 4 to 5 per cent is used for products such equally potassium nitrate and nitrophosphates. The remainder (nearly 30 per cent) is used past the explosives, plastics and chemical industries. Tabular array II shows an estimated breakdown of the earth'southward employ of nitric acid (i).

Table Ii

The Earth's Usage of Nitric Acrid (1)

	Millions of Tonnes 100 per cent HNO3
	1962–63	1965–66
Fertiliser production	14.vi (80%)	xviii.ix (81%)
Explosives, plastics and chemical industries	three.7(twenty%)	4.5(19%)

Fig. 1

Three ammonia converters installed in a modern nitric acrid plant operated by Imperial Chemical Industries Limited. Each converter has 3 rhodium-platinum catalyst gauzes, 114 inches in bore, woven past Johnson Matthey & Co Limited.

In 1962–63 nitric acid represented near 25 per cent of the world's fixed nitrogen production, but this proportion had dropped to an estimated 22.5 per cent past 1965–66. This turn down is probably temporary, and is largely attributed to the sustained expansion of fertilisers obtained directly from ammonia, in detail urea and complex fertilisers based on ammonium phosphate. Low-toll ammonia from very big modern plants employing new processes based on hydrogen obtained from petroleum fractions (output about 1000 tonnes per day), together with increasingly large nitric acid plant facilities (well-nigh 500 tonnes per 24-hour interval), tin can be expected to result substantial reductions in nitric acid production costs. Such reductions may take significant effects on potential applications in which cost considerations have limited or prohibited the use of nitric acid until now.

One such sector is the manufacture of nitrophosphate-type fertilisers.

With a world population that is expected to be nigh 7000 meg by the end of this century, and with two-thirds of them undernourished by even the almost frugal standards, the outlook for nitrogen fertiliser product seems assured, and nitrate fertilisers based on nitric acid are likely to play a very major role in fulfilling full fertiliser demand.

The Nitric Acid Process

The foundation of the modern nitric acid process was laid by Kuhlmann in 1838 when he filed a patent for the catalytic oxidation of ammonia over platinum sponge, although Milner had before oxidised this gas to nitric oxide over manganese dioxide in 1789. Pilot plant calibration experiments were carried out by Ostwald and Brauer in the period 1901 to 1904, and the first plant to produce 300 kg of nitric acid per day was commissioned at Gerthe, nigh Bochum, in 1906. A tenfold increase in production was achieved by 1908. These early plants used crimped platinum strips wound into a coil, merely in 1909 Kaiser filed a patent for the use of platinum in the grade of a gauze woven of wire 0.06 mm diameter with 1050 apertures/cm², and such gauzes are all the same used today. The contribution of Ostwald and his co-workers to the development of the nitric acrid process, and the trials that led to the establishment of this new technology, have been described elsewhere (2, 3).

Until the early 1920s ammonia was obtained from gas works liquors and the impurities information technology contained — mainly sulphur and arsenic — resulted in comparatively short catalyst lives. The really large-scale product of nitric acid had to await two other major technological developments: the availability of cheap, high-purity synthetic ammonia from the Haber process, and stainless steel equally a material of construction for efficient absorption systems working under force per unit area. The contribution the latter development has made to plant design may be judged by the fact that in 1919 a U.S.A. plant to produce 280 tonnes per mean solar day of nitric acid required 24 atmospheric force per unit area acid absorption towers, each 35 ft foursquare and 60 ft high — a total volume of i.63 meg cubic feet, not counting 12 large oxidation towers (four).

The modern nitric acrid procedure proceeds essentially in 3 stages:

• Liquefied ammonia is vapourised and pre-heated before being mixed with preheated air. The mixture at about 300°C, and usually containing 10.5 to 12 volume per cent of ammonia, is passed downward through a pad of platinum blend gauzes in a converter. Nitric oxide is formed equally a result of an extremely rapid, highly exothermic reaction at the platinum surface in accordance with the overall equation

  

• This reaction will be discussed in fuller detail in a later section.

• The nitric oxide that is produced at the platinum gauze is oxidised further by means of secondary air reacting homogeneously in the gas phase in accordance with the overall equation

  

• Germination of dinitrogen tetroxide gain via an intermediate nitrogen dioxide, NO₂. The nitric oxide oxidation reaction has a negative temperature coefficient, hence low temperatures favour high yields. Elevated pressures also enhance the yields of the tetroxide, making possible the product of higher strengths of acid.

• Dinitrogen tetroxide is absorbed in water with the formation of nitric acid in accordance with the simplified equation

  

• The assimilation proceeds at greater rates, and with higher efficiencies, at elevated pressures and low temperatures.

• In a modern plant the heat liberated by the ammonia oxidation reaction is commonly recovered, enabling the plant to be self-supporting for its power requirements once the reaction has reached optimum conditions and capacity working is attained.

Mod Proprietary Processes

In that location is no universally optimum overall pattern of a nitric acrid establish; weather condition of pressure, temperature, flow rate and other factors for each phase in the process may be varied to suit particular local requirements and situations. At least xv proprietary nitric acid processes are available now; the pick presented to operators is wide, and the selection of the "best" process to suit a given requirement involves the evaluation of numerous technical and economic factors (v, half-dozen).

The main blueprint variables for the ammonia oxidation and subsequent stages of the constitute are temperature, pressure and gas period rate. Their main furnishings in these sections are shown in Table III (7). Since the oxidation of nitric oxide and the absorption stages are ever carried out at pressures in the range 4 to nine atmospheres, plants are by and large described by the pressures inside the ammonia oxidation coverter: depression, or atmospheric, medium (3 to v atmospheres) or loftier (v to 9 atmospheres). Ammonia oxidation at low pressures results in higher conversion efficiencies and lower platinum metal losses compared with medium or high pressure level oxidation. Longer operating periods between shut-downs for gauze maintenance are besides possible, just such converters are bulkier, more expensive and require costly hot-gas compressors to raise the gas pressure afterwards oxidation to that required in the subsequent stages.

Table 3

Effect of Major Procedure Variables on Nitric Acid Manufacture (7)

Process Modify	Oxidation of NHthree	Oxidation of NO	Absorption of NO2
Increased temperature	Greater yield	Lower yield	Reduced absorption, lower HNOthree strength
Increased pressure	Oxidation rate slightly reduced. Weight of NH3 oxidised increased per unit time	Increased yield	Improved absorption, greater HNO2 strength
Increased flow rate	Optimum operating temperature increased. Weight of NH3 oxidised increased. NO yield improved	Reduced secondary reactions. More than turbulence and increased yield	Small improvements only, every bit absorption mostly controlled by gas composition and reaction rates

The American loftier pressure plants—all derived from the Du Pont process—operate with a converter pressure effectually viii atmospheres and with a gauze pad temperature in the range 900 to 950°C. They besides have loftier gas flow rates per unit area of gauze, and hence the platinum loss rates are loftier, in practice running to 250 to 400 mg/ton 100 per cent HNO₃. Among the European process licensors, Uhde and Stamicarbon too offering high pressure plants with the gauze pad operating at temperatures most 900°C. These, however, have lower gas flow rates and the platinum losses are correspondingly reduced to 180 to 200 mg/ton. Medium force per unit area converters with gauze temperatures of 845 to 880°C generally experience platinum losses from 85 to 125 mg/ton, while converters operating at atmospheric pressure level with gauze temperatures about 800°C have the lowest metal loss rates, around fifty mg/ton (five, viii). It must be mentioned that procedure licensors' claims for platinum loss rates generally appear to be conservative in the light of actual operating feel.

Table IV summarises the principal features of some commercial nitric acid processes available to operators under licence. It is based on licensors' specifications and is reproduced by courtesy of the Noyes Development Corporation, New York (7).

Table IV

Master Features of Some Commercial Nitric Acid Processes

Proper name	Approximate operating pressures, atmospheres (abs)			Product acid per cent HNOthree	Typical requirements per tonne of 100% HNO3		Distinctive features
	Combustion	Oxidation	Absorption
Bamag (Monopressure)	one,4 or 8	ane,four or 8	1,4 or 8	55 to 70	dependent on used	pressures	Uses perforated plate towers with cooling water coils on the plates
Bamag (Combination)	1,4 or 8	four or 8	4 or 8	55 to 70	NH3	280 kg	Similar to above
					Pt	45 mg
					cooling H2O	170 mthree
					pure H2O	100 kg
Chemico	8 to 9	eight to 9	8 to 9	55 to 65	(steam turbine and plus expander)	510°C	(with steam turbine and 680°C plus expander)
					NH2	287 kg	NH3	287 kg
					Pt	128 mg	Pt	128 mg
					cooling H2O	135 thouthree	cooling H2O	100 chiliad3
					pure H2O	500 kg	pure H2O	540 kg
C & I	8	viii	viii	55 to 67 plus	NH3	292 kg	Uses cascade cooling and a single belfry for oxidation/absorption/bleaching
					Pt	170 mg
					cooling H2O	117 thou2
					pure HtwoO	400 kg
Grand Paroisse	3 to 4	vii to 8	vii to viii	56 to 70	NH3	282 kg	Oxidation towers cooled externally past water. Absorption tower cooled past internal water coils. v unit, single shaft, turbo-compressor used
					Pt	100 mg
					cooling H2O	200 m3
					pure HiiO	710 kg
Hercules	8	8	eight	57 to 60	NH3	285 kg	Uses high gas superheat to obtain maximum power recovery. Specially designed h2o-cooled absorption trays ensure a minimum 99% absorption efficiency
					Pt	174 mg
					cooling H2O	85 to 95 one thousand3
					pure H2O	500 kg
Kuhlmann	ane	4	5	55 to seventy	NH3	280 kg	Uses a especially designed tray to induce oxidation and absorption in the liquid phase
					Pt	45 mg
					cooling H2O	150 miii
Montecatini	iv to five	4 to 5	iv to 5	53 to 62	NH3	286 kg	Uses separate oxidation and absorption towers fitted with refrigerated trays
					Pt	100 mg
					cooling H2O	100 m3
Pechiney-St Gobain	four to 5	4 to 5	4 to 5	commonly up to 60	NH3	286 kg	Uses particularly designed water-cooled trays in oxidation assimilation columns
				Pt	100 mg
SBA (Société Biscuit de fifty'Azote)	1	three to five	3 to 5	58 to 70	(for 55% HNO3)		Packed columns for 58% acid. Additional plate-blazon columns used for 70% acrid
				NHthree	281 kg
				Pt	45 mg
				cooling H2O	150 mthree
				pure H2O	650 kg
SBA	3.3	3 to 5	8	50 to 70	Similar to above		Similar to above. Uses catalytic tail-gas combustion for maximum energy recovery
Stamicarbon	1	5 to half dozen	5 to 6	50 to 65	NH3	285 kg	Particularly designed plate coolers used in conjunction with packed columns
					Pt	45 mg
					cooling H2O	250 miii
					pure H2O	730 kg
Uhde (normal force)	7 to 8	7 to 8	7 to eight	55 to 60	NHiii	285 kg	79% of steam produced used for driving air compressor, eleven% for heating tail-gas, x% every bit production steam. Bubble tray absorbers used
					Pt	45 mg
					cooling H2O	250 thousand3
Uhde (loftier forcefulness)	(a ) 4.5	4 to five	4 to 5	60 max.	NHiii	283 kg	Uses multi-stage, packed oxidation and absorption columns; each stage fitted with own cooler and pump. Figures listed refer to process (a )
	(b ) one	four to v	four to five	68 min.	Pt	threescore mg
					cooling HtwoO	165 miii
					pure H2O	250 kg
Weatherly	ix.5	9.0	9.0	55 to 60 (or college)	NHthree	284 kg	Uses butted heat-exchanger associates and a single oxidation/absorption/bleaching column fitted with bubble trays
					Pt	165 mg
					cooling H2O	142 m
					pure H2O	750 kg

The cost of producing ammonia has fallen recently post-obit the introduction of new processes for manufacturing depression cost hydrogen from petroleum fractions. With low ammonia costs, the advantages of higher conversion efficiencies achieved by atmospheric pressure level converters may no longer outweigh the relatively high majuscule and operating costs of such mixed-pressure plants. There is a trend in Europe, therefore, to follow American do and to adopt abiding-pressure processes in which the converter and the following sections of the plant operate at the same force per unit area (normally in the range iii to 9 atmospheres). The pick of the economically optimum operating pressure depends on a number of factors, for example, ammonia costs, steam credit value and local tax situations (9).

Note: The above table is intended equally a general, comparative guide simply. Information technology is based on licensors' specifications, and bodily plant performances depend on variable and private establish features such every bit daily capacities, water temperatures, types of drive and other associated items selected to suit local conditions.

The Gauze in Operation

The heart of the nitric acid plant is the converter where ammonia is oxidised to nitric oxide. Consisting essentially of a reactor trounce wherein the pre-heated ammonia-air mixture by and large flows vertically downwards through the gauze pad, information technology is usually situated immediately on elevation of the waste oestrus boiler. A baffle or diffuser system located in the top of the converter ensures that the gas mixture reaching the gauzes is homogeneous, and provides an even gas distribution over the entire gauze surface. This is important in order to avoid "hot spots" with bellboy hazard of local gauze damage, and becomes more than difficult to reach with increasing converter diameter.

Stainless steel or an aluminium-magnesium alloy are used as constructional materials for the ammonia pre-heater, the mixed-gas ducts leading to the converter and the converter hood itself. This is to ensure that ammonia at the pre-rut temperature (250 to 300°C) does non come into contact with surfaces capable of catalysing its decomposition into nitrogen and hydrogen before making contact with the platinum gauzes. Iron oxide—used every bit an ammonia synthesis catalyst, and as capable of catalysing the reverse reaction—is particularly liable to cause ammonia losses by pre-decomposition. It should be advisedly excluded from the system by avoiding the use of mild steel components liable to rust germination. The interior surface of the converter hood is heated by radiation from the gauze pad and external water cooling is required, specially for high pressure level converters whose gauzes operate at temperatures up to almost 950°C. Fig. 2 illustrates the marked issue of temperature on the decomposition of ammonia on various metallic surfaces (x). At normal pre-estrus temperatures, ammonia decomposes nearly 10 times faster on mild steel than on stainless steel. It is estimated that from 1.5 to 3 per cent of the ammonia fed to most plants may exist lost past pre-decomposition and thus never reach the platinum gauzes.

Fig. 2

The rates of decomposition of ammonia on various constructional materials increase sharply with rising temperature. This graph illustrates these rates, using a 10 book per cent ammonia-air mixture (Spratt).

The Ammonia-Air Mixture

The ammonia feedstock is vaporised, superheated and filtered to remove any dust or oil particles before pre-heating. Atmospheric air may be scrubbed with water and is then carefully filtered before being pre-heated in a separate oestrus exchanger. Good mixing of the gases is essential, and for this purpose venturi-blazon or sparge-type mixers are usually employed.

In certain proportions, ammonia and air form explosive mixtures. At i atm. force per unit area, the lower explosive limit is thirteen.8 book per cent NH₃, falling to xiii.0 and 12.4 volume per cent at 5 and 8 atm. pressures respectively. The stoichiometric composition for the reaction

is fourteen.2 volume per cent NH₃, although a somewhat more important ratio is the kinetic-stoichiometric limerick, at which the standoff chances for oxygen and ammonia molecules are in the ratio 5/4 : 1. This ratio corresponds to an ammonia concentration of 12.7 volume per cent in air. In practise, ammonia concentrations in the range x.v to 12 volume per cent are used, and in this region it is apparent that at that place is very little excess oxygen supplied to the reaction system. The variation in the NH₃ : O_two ratio every bit the reaction proceeds is illustrated in Fig. iii (11).

Fig. 3

Graph illustrating the changing ammonia: oxygen ratio as the reaction proceeds on a platinum gauze pad. The initial ammonia concentration was 11.0 volume per cent (Oele)

The Reaction at the Gauze

The oxidation of ammonia is a typical case of a very fast heterogeneous reaction with residence times from 10⁻³ to ten⁻⁴ seconds and hourly space velocities exceeding 10⁶. Such weather condition entail considerable linear gas velocities, ofttimes greater than 20 metres/minute. Since the hateful free path of ammonia molecules under the force per unit area and temperature conditions usually encountered in a converter is of the social club of 0.3 micron, compared with a wire bore around 60 microns and fifty-fifty larger aperture diameters, ammonia mass transfer is not influenced by molecular diffusion limitations. The physical transport of the ammonia molecules to the platinum surface is the rate-determining factor during the greater office of the reaction period.

If ammonia and oxygen were allowed to react under equilibrium conditions, nitrogen and h2o would be the only products. Reaction conditions are such, however, that nearly every molecule that hits the platinum gauze surface is converted to nitric oxide in accord with the overall equation

If the gas flow rate is too high, some un-reacted ammonia passes through the gauze pad, and will then react with nitric oxide (12)

If the rate is also low, some nitric oxide volition decompose on the hot platinum surface (thirteen)

Since about 80 to ninety per cent of the reaction is completed on the upper gauze of a 3- to 5-ply pad, or in the top 20 per cent of the layers in a thick pad used in high pressure level converters, the gas leaving this zone will contain both ammonia and nitric oxide. In social club to reduce to a minimum the reaction between these gases, it is necessary to provide very good contact betwixt individual gauzes and to avert any gratuitous spaces between them. This is achieved by using gauzes that are perfectly flat and gratis from folds or buckles that could separate them from each other.

Numerous studies of the kinetics of the principal reaction have been made (14, 15, 16). Amidst the postulated intermediates are hydroxylamine, nitroxyl, NHO, and the radical NH. It is more often than not agreed that reaction gain between ammonia and an activated oxygen boundary layer that covers most of the platinum surface.

The formation of nitric oxide is favoured past low operating pressures and, since the reaction has a positive temperature coefficient, likewise by high temperatures. The optimum temperature, moreover, increases with the rate of gas menstruation.

The final function of Mr Connor'due south commodity, dealing principally with the problem of platinum losses from gauzes during performance and with the production and handling of gauzes, will be published in the Apr event of Platinum Metals Review.

1

Individual communication, Nitrogen, published past the British Sulphur Corporation, London

2

L. B. Hunt, Platinum Metals Rev., 1958, ii, 129

iii

A. Mittasch, Saltpetersäure aus Ammoniak, Verlag Chemie, Weinheim, 1953

4

A. M. Fairlie, Chem. Met. Eng., 1919, 20, 8

v

D. C. Oosterwijk, The Choice of a Nitric Acid Found, Trans. Inst. Chem. Eng., 1966, 44, (ii), CE. 38

vi

Anon, Chem. Proc. Engng., 1966, 47, (one), eleven – 17

7

Nitrogen Fertiliser Chemic Processes, Noyes Evolution Corporation, New York, 1965

8

E. Bahari, Chem. & Proc. Eng., 1965, Jan, 16

nine

A. Horton, European Chemic News, Big Plant Supplement, September 30, 1966, 76

10

D. A. Spratt, Chem. Soc. Special Publication, 10, 1957

xi

A. P. Oele, Chemic Reactor Engineering science, Ed. Thousand. Rietema, Pergamon Press, London, 1957

12

H. Wise and Thousand. F. Frech, J. Chem. Phys., 1954, 22, 1463

13

50. Andrussow, Z. Angew. Chem., 1926, 39, 322

14

Eastward. Molinari et al., J. Catalysis, 1965, 6, (3), 341 – 353

15

J. Zawadski, Faraday Soc. Discussions, 1950, viii, 140

16

R. H. Harrison and L. A. Lobe, Chem. Eng. Prog., 1953, 49, 349

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