production of nitric acid - TSFX

production of nitric acid unit four – 2014 – chemistry

THE PRODUCTION OF NITRIC ACID (HNO3) USES OF NITRIC ACID In terms of production, nitric acid is the third most widely produced acid across the world. It has a wide range of uses in agriculture, industry and medicine where it is used as a fertiliser and in the manufacture of fireworks, explosives, medicines, dyes, food preservatives, pesticides and detergents.

PROPERTIES OF NITRIC ACID Nitric acid: •

Is colourless in its pure form but may become orange or reddish in colour if contaminated by nitrogen oxides.

•

Is highly corrosive.

•

Is a poisonous liquid (freezing point -42˚C, boiling point 83˚C).

•

Reacts with water or steam to produce heat and toxic, corrosive and flammable vapours.

•

Can cause severe burns.

•

Miscible in water at all concentrations.

•

Has an acid dissociation constant (pKa) of −1.4. In aqueous solution, it almost completely (93% at 0.1 mol/L).

•

Will decompose at higher temperatures to form nitrogen oxides.

Nitric acid is both a strong monoprotic acid and a strong oxidant, particularly when hot and concentrated.

© The School For Excellence 2014

Unit 4 Chemistry – The Production of Nitric Acid

Page 1

OXIDISING PROPERTIES The products of the reaction between nitric acid and metals depends upon the reactivity of the metal and the concentration of the acid. As a general rule, oxidising reactions occur primarily with the concentrated acid, favouring the formation of nitrogen dioxide (NO2). Reaction between a reactive metal and dilute acid (<1M): 8

( )

+ 30

(

)

→8 (

)

(

)

+ 3

(

)

+ 9

()

Nitrogen has been reduced from +5 all the way to -3. Reaction between a less reactive metal and more concentrated acid: •

Acid concentration 3 to 6M: 3

( )

+ 8

(

)

→3

(

)

(

)

+ 4

()

+ 2

( )

Nitrogen has been reduced from +5 to +2. •

Acid concentration 12M: ( )

+ 4

(

)

→

(

)

(

)

+ 2

()

+ 2

( )

Nitrogen has been reduced from +5 to +4. Since nitric acid is an oxidising agent, hydrogen (H2) is rarely formed. Only magnesium (Mg), manganese (Mn) and calcium (Ca) react with cold, dilute nitric acid to give hydrogen: Mg(s) + 2 HNO3(aq) → Mg(NO3)2(aq) + H2(g) Reaction with non-metallic elements (with the exceptions of nitrogen, oxygen, noble gases, silicon and halogens) usually oxidises them to their highest oxidation states. The formation of nitrogen dioxide occurs for concentrated acid and nitric oxide for dilute acid. C(s) + 4 HNO3(aq) → CO2(g) + 4 NO2(g) + 2 H2O(l) 3 C(s) + 4 HNO3(aq) → 3 CO2(aq) + 4 NO(q) + 2 H2O(l)



Page 2

ACIDIC PROPERTIES Nitric acid is a strong acid. In moderately dilute solution (~ 0.1 M) it is dissociated to an extent of about 93%, in accordance with the reaction,

Being an acid, it reacts with alkalies to from nitrates

It decomposes carbonates and bicarbonates as:



Page 3

THE PRODUCTION OF NITRIC ACID Nitric acid is made from ammonia in a three-step process known as the Ostwald process. Step 1: Oxidation of NH 3 to NO . Step 2: Oxidation of NO to NO2 . Step 3: Absorption and reaction of NO2 with water. NO2

STEP 1: CATALYTIC OXIDATION OF AMMONIA Air is preheated and mixed with ammonia (which is not preheated as it would decompose) and then passed through a converter where the following reaction occurs:

4 NH 3( g ) + 5O2( g ) → 4 NO( g ) + 6 H 2O( g )

ΔH = −907 kJmol −1

In this reaction, ammonia undergoes catalytic oxidation to form nitrogen monoxide (nitric oxide (NO) and water. This is the start of the oxidation process. The nitrogen in ammonia starts with an oxidation number of -3 (its lowest possible oxidation state) and is converted to +2 in nitrogen monoxide. The ratio of air/ammonia must be carefully monitored and is maintain at between 9 and 12%. If the concentration of ammonia rises much beyond this, the mixture becomes explosive. The catalyst used in this process is 90% platinum alloyed with 10% rhodium for increased strength. The catalyst consists of several woven or knitted gauzes formed from the alloy. The gauze mats are preheated so that the gases are directly heated as they pass over the catalyst. The catalyst may become poisoned by air pollution and contamination from the ammonia which reduces its efficiency. The cost of these catalysts are extremely high and need to be frequently replaced due to the wear and tear they experience under such severe conditions. A cheaper alternative is yet to be developed.



Page 4

Nitrous oxide, nitrogen and water are also simultaneously formed in this step, as shown below.

ΔH = −1267 kJmol −1

4 NH 3( g ) + 3O2( g ) → 2 N 2( g ) + 6 H 2O( g ) 2 NH 3( g ) + 2O2( g ) → N 2 O( g ) + 3 H 2 O( g )

Conditions are carefully controlled in the converter in order to ensure that nitrogen monoxide (NO) is the main product, rather than nitrogen gas (N2) or nitrogen (I) oxide ( N2O ). The yield of nitric oxide depends on the pressure and temperature as shown below. Pressure (atm)

Temperature (˚C)

NO yield (%)

Below 1.7

810-850

97

1.7-6.5

850-900

96

Above 6.5

900-940

95

Typical conditions for the production of NO are therefore: •

High temperatures ( 820 − 930o C )

•

High pressures (11 atm) Temperature Considerations

Even though higher yields would be obtained at lower temperatures (the forward reaction is exothermic), the process is carried out at high temperatures ( 820 − 930o C ). This is because the rate at which the reaction proceeds at low temperatures is too slow to be commercially viable. To compensate for the resultant loss in product yield, the gas mixture is passed over a catalyst a number of times to produce a moderate yield of NO . Pressure Considerations At the high temperatures employed, the NO formed decomposes to form nitrogen and oxygen.

2 NO( g )  N 2( g ) + O2( g ) To avoid this, the gas mixture is passed across the catalyst very rapidly (contact time is approximately 0.003 sec). To achieve this high flow rate, the reaction is performed at high pressures even though lower pressures would result in a higher product yield. The consequential loss in yield of NO is compensated for by the increased reaction rates and the quality of product obtained. Note: Even with all the compromises that are required in this step, the yield of NO is in the order of 95%.



Page 5

STEP 2: OXIDATION OF NITROGEN MONOXIDE The waste heat from the gases leaving the converter is recycled and used in other sections of the plant. The temperature of the nitrogen monoxide mixture is reduced to around 200-250˚C in this process. The gases are then passed through a cooling chamber and their temperature reduced to approximately 50˚C. Any condensed water is transferred to the absorption tower. As the gases are cooled, the nitrogen monoxide oxidises to nitrogen dioxide (the nitrogen in NO is oxidised from +2 to +4 in the nitrogen dioxide). The oxygen consumed in this step may be added from an external source or is provided by excess oxygen in the gaseous mixture exiting the converter. The reaction is:

2 NO( g ) + O2( g )  2 NO2( g )

ΔH = −114 kJmol −1

Temperature Considerations As this reaction is exothermic, high yields of product can be achieved by using lower temperatures. This reaction is unusual in that its rate increases with decreasing temperature meaning that NO CONFLICT arises between the conditions required to optimise rates and yields. Pressure Considerations Yields can be further maximised by using high pressures. The system will respond to high pressures by favouring the reaction that will produce the fewer mole of gas – which in this case is the formation of products.



Page 6

STEP 3: ABSORPTION OF NITROGEN DIOXIDE Water is mixed with the nitrogen dioxide gas in absorption towers to form dilute solutions of nitric acid according to the following overall reaction:

3 NO2( g ) + H 2 O( l ) → 2 HNO3( aq ) + NO( g ) This is a redox reaction in which produces nitrogen in its highest oxidation state (+5 in nitric acid). The towers contain large number of inert plates packed with inert granular materials designed to increase the contact between the gases and water. This reaction is exothermic and continuous cooling is needed. The conversion is favoured by low temperatures and significant reaction occurs until the gases leave the towers. Nitrogen dioxide gas is pumped at 5 to 10 atm across the inert packing material, through which water is trickled from above. Reaction between the water and the gas produces nitric acid, which then dissolves in the remaining water. Small quantities of NO are also produced, which reacts with oxygen from the air in the tower to produce NO2 which then reacts as before. A solution of nitric acid may be produced that is about 45–60% HNO3 . This can easily be increased to 68% (equivalent to 16 M ) by distilling off some of the water.



Page 7

WASTES AND MANAGEMENT •

The Ostwald process is very energy efficient and produces little waste.

•

The oxidation of ammonia is highly exothermic, generating sufficient heat energy to meet the energy needs of the rest of the plant.

•

The main gaseous emissions from the Ostwald process include NO and NO2 . Both gases contribute to photochemical smog, and therefore, careful attention must be paid to minimising how much of these gases are emitted into the atmosphere. One approach involves the heating of these gases using a fuel such as natural gas, naphtha or hydrogen, over a catalyst, so that the NOx is reduced to N 2 . CH4(g) + 4NO2(g)  CO2(g) + 2H2O(l) + 4NO(g) then, CH4(g) + 4NO g)  CO2(g) + 2H2O(l) + 4N2(g) Also: CH4(g) + 4N2O g)  CO2(g) + 2H2O(l) + 2N2(g) H2(g) + NO2(g)  2H2O(l) + NO(g) then, 2H2(g) + 2NO(g)  2H2O(l) + N2(g) Also: H2(g) + N2O g)  CO2(g) + H2O(l) + 2N2(g) In addition, the absorption tower may be modified by increasing its size or operating pressure so as to maximise conversion of NOx to nitric acid.

•

The gas mixture entering the converter is filtered to remove catalytic poisons, which increases the efficiency of the catalysts and hence decreasing the pressure (and energy) required to force the gas through the catalyst bed.

•

Heat exchangers are employed to remove heat released by the reaction in the converter and then using it to heat incoming gases or generate electricity.

•

At the high temperatures and pressures used in the converter, the catalyst slowly vaporises and is lost. Gases leaving the converter are passed through a filter to recover the metals and minimise the impact of these vapours on the environment.

•

Specific catalysts are added to the converter to decompose any N2O formed. Note: Nitrogen(I) oxide is a significant greenhouse gas.



Page 8

HEALTH AND SAFETY •

Concentrated nitric acid is corrosive and causes severe burns to the skin and eyes. Its fumes evolve nitrogen dioxide gas which at low concentrations may cause lung oedema (fluid in the lungs) and fatal with excessive exposure.

•

As a strong oxidant, nitric acid reacts readily with a range of organic materials and metals to produce flammable and/or explosive products.

•

NOx gases are significant greenhouse gases and some can react with water to form acid rain.

Safety Measures Employed: •

There is careful monitoring in nitric acid plants for leaks and spills and all employees are trained to handle such if they do occur.

•

Equipment must be carefully maintained to avoid corrosion.

•

Acid spills are contained using materials such as earth, clay or sand, and then neutralised with a base such as slaked lime ( Ca( OH )2 ) or sodium carbonate.

•

Full protective equipment and breathing apparatus is readily accessible across the plant.

•

The ratio of ammonia to air in the gas entering the converter is continuously measured and controlled to ensure it does not reach explosive conditions.

•

Un-reacted gases are recycled where possible.

•

Various methods are employed to limit NOx emissions, maximise conversion efficiency, and minimise loss of energy.



Page 9

MIXED QUESTIONS QUESTION 1 A number of different oxidation states of nitrogen are involved in the industrial production of nitric acid from ammonia. State the various nitrogen containing compounds involved in the Ostwald Process and the corresponding oxidation states of nitrogen. Solution

QUESTION 2 Describe the theoretical conditions that should be used to maximise the rate of the reaction of ammonia and oxygen to produce nitrogen monoxide. Are these the conditions actually used? If not, why not? Solution



Page 10

QUESTION 3 Which of the following is not a property of nitric acid? A B C D

It is monoprotic. It is a strong acid. It is a good oxidant. It forms nitride salts.

QUESTION 4 Write equations for the following reactions of nitric acid: (a)

With water.

(b)

With ammonia to make ammonium nitrate.

(c)

With potassium hydroxide to make potassium nitrate.

(d)

With zinc metal to form zinc ions and ammonia.



Page 11

QUESTION 5 During the Ostwald process, nitrogen monoxide is made from ammonia at about 900°C and then cooled to 30°C before being reacted with air to make nitrogen(IV) oxide. Both these reactions are exothermic. Why are the temperatures used for these reactions so different? Solution

QUESTION 6 As the gas passes through the catalyst bed in the converter during nitric acid manufacture, its temperature increases. The gas must be cooled before it is mixed with air. (a) (b) (c)

Why does the temperature of the gas rise? Why is it necessary to cool the gas? What side benefit is obtained from the need to cool gases?

Solution



Page 12

QUESTION 7 The flow chart below shows the processes leasing to the production of nitric acid on an industrial scale.

(i)

What is Gas A? What is Gas B?

(ii)

Write an equation for the process occurring in Reactor 1.

(iii) What would be the effect of increasing the temperature in Reactor 1 on the rate of production of gas B?



Page 13

(iv) What would be the effect of increasing the temperature in Reactor 1 on the equilibrium yield of gas B?

(v)

What is the function of the catalyst in Reactor 2?

(vi) At low temperatures, NO2 is in equilibrium with another oxide of nitrogen. Write an equation for this equilibrium.

(vii) What is reagent D, which is added into Reactor 4 with NO2 ? Write an equation for the reaction occurring in Reactor 4.

(viii) Reagent E is recycled back into the cooling tower as shown in the diagram. What is reagent F?



Page 14

QUESTION 8 Nitrogen oxide, NO , is a small but important component of the atmosphere and is produced commercially on a large scale during the manufacture of nitric acid. (i)

Name two processes, one natural and one involving the activities of man, that contribute significant significantly to the NO present in the atmosphere.

(ii)

Write a balanced equation for the reaction by which NO is formed during the production of nitric acid by the Ostwald process.

(iii)

NO produced during the Ostwald process is oxidised to NO2 , according to the equation 2 NO( g ) + O2( g ) → 2 NO2( g )

ΔH = −181 kJmol −1 . In a particular factory, energy

is released at a rate of 570 kJ per minute during this stage. What volume of NO2 at STP is being released each minute?



Page 15

QUESTION 9 (a)

Calculate the atom economy of ethylene oxide, created in the following reaction:

(b)

Would this method of production of ethylene oxide be considered as a “Green” process? Give a reason for your answer.

(c)

Recently, a method of synthesising ethylene oxide from ethene and oxygen using a silver catalyst was developed. What’s the atom economy of this alternative reaction?



Page 16

SOLUTIONS

FOR ERRORS AND UPDATES, PLEASE VISIT WWW.TSFX.COM.AU/MC-UPDATES QUESTION 1 The Ostwald process involves the oxidation of nitrogen, through a series of steps, from its lowest oxidation state to its highest. Starting with ammonia ( NH 3 - oxidation number of N

= −3 ), nitrogen monoxide ( NO - oxidation number of N = +2 ) is formed, followed by nitrogen dioxide ( NO2 - oxidation number of N = +4 ) and finally nitric acid ( HNO3 - oxidation number of N = +5 ). QUESTION 2

Refer to Notes.

QUESTION 3

Answer is D

QUESTION 4 (a)

HNO3( aq ) + H 2O( l ) → H 3O(+aq ) + NO3(− aq )

(b)

HNO3( aq ) + NH 3( g ) → NH 4 NO3( aq )

(c)

HNO3( aq ) + KOH ( aq ) → H 2O( l ) + KNO3( aq )

(d)

4 Zn( s ) + HNO3( aq ) + 8 H (+aq ) → 4 Zn(2aq+ ) + NH 3( g ) + 3 H 2O( l ) or

4 Zn( s ) + HNO3( aq ) + 9 H (+aq ) → 4 Zn(2aq+ ) + NH 4(+ g ) + 3 H 2O( l ) QUESTION 5 Stage 1 of the Ostwald Process: The nitrogen monoxide produced in the converter is cooled to about 30o C to maximise the production of NO2 in the reaction below.

2 NO( g ) + O2( g )  2 NO2( g )

ΔH = −114 kJmol −1

As this reaction is exothermic, high yields of product can be achieved by using lower temperatures. And as reaction rates for this system increase with decreasing temperature, no conflicts are created between rates and yields. In Stage 2 of the Ostwald Process, the reaction once again is exothermic, meaning that higher product yields will be obtained by using lower temperatures. In this case however, the process is carried out at high temperatures ( 820 − 930o C ) as the rate at which the reaction proceeds at low temperatures is too slow to be commercially viable. To compensate for the resultant loss in product yield, the gas mixture is passed over a catalyst a number of times to produce a moderate yield of NO .



Page 1

QUESTION 6 (a)

As the reaction that occurs in the converter is exothermic.

(b)

To maximise rates and product yields.

(c)

Reaction rates are increased as in this particular reaction, rates for this system increase with decreasing temperature.

QUESTION 7

QUESTION 8



Page 2

QUESTION 9 (a)

C2H4O = 44g/mol

% Atom Economy =

CaCl2 = 111g/mol

H2O = 18g/mol

(2 × 44) ×100 = 37.4% (2 × 44 + 111 + 2(18))

(b)

An atom economy of 37.4% is particularly poor, and this is a very wasteful process. This would not be considered a green process, as one the key principles of green chemistry is that it is better to develop reactions with fewer waste products than to have to clean up the waste (eg. achieve high atom economy).

(c)

All atoms in the reactants used in products, therefore 100% atom economy



Page 3

production of nitric acid - TSFX

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