Page 175 - Chemical engineering design

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CHEMICAL ENGINEERING
From reaction 1, at 96 per cent yield,
96
NO produced D 100 ð D 96 kmol
100
5
oxygen required D 96 ð D 120 kmol
4
water produced D 96 ð 3 D 144 kmol
2
The remaining 4 per cent ammonia reacts to produce nitrogen; production of 1 mol of N 2
requires 3 mol of O 2 , by either reaction 2 or 1 and 3 combined.
2
4
nitrogen produced D D 2kmol
2
oxygen required D 2 ð 3 D 3kmol
2
All the oxygen involved in these reactions produces water,
water produced D 3 ð 2 D 6kmol
So, total oxygen required and water produced;
water D 144 C 6 D 150 kmol

oxygen (stoichiometric) D 120 C 3 D 123 kmol
Excess air is supplied to the oxidiser to keep the ammonia concentration below the
explosive limit (see Chapter 9), reported to be 12 to 13 per cent (Chilton), and to provide
oxygen for the oxidation of NO to NO 2 .

1
Reaction 4. NO(g) C O 2 ! NO 2 (g) H Ž D 57,120 kJ/kmol
2 298
The inlet concentration of ammonia will be taken as 11 per cent v/v.
100
So, air supplied D ð 100 D 909 kmol
11
Composition of air: 79 per cent N 2 , 21 per cent O 2 ,v/v.
So, oxygen and nitrogen flows to oxidiser:

21
oxygen D 909 ð D 191 kmol
100
79
nitrogen D 909 ð D 718 kmol
100
And the oxygen unreacted (oxygen in the outlet stream) will be given by:

oxygen unreacted D 191 123 D 68 kmol
The nitrogen in the outlet stream will be the sum of the nitrogen from the air and that
produced from ammonia:
nitrogen in outlet D 718 C 2 D 720 kmol