Page 188 - Chemical engineering design

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FLOW-SHEETING
3. Oxidation of NO
The greatest heat load will occur if all the oxidation occurs in the cooler-condenser (i.e.
none in the WHB) which gives the worst condition for the cooler-condenser design.
Mols of NO oxidised D mols in mols out D 41.3 6.75 D 34.55 kmol/h
From reaction 4, heat generated D 34.55 ð 57,120
6
D 1.97 ð 10 kJ/h D 1.97 GJ/h
4. Formation of nitric acid
850.6
HNO 3 formed, from flow sheet, D D 13.50 kmol/h
63
The enthalpy changes in the various reactions involved in the formation of aqueous nitric
acid are set out below (Miles):
2NO 2 g ! N 2 O 4 g H D 57.32 kJ 6a
1
N 2 O 4 g C H 2 O l C O 2 g ! 2HNO 3 g H DC 9.00 kJ 6b
2
HNO 3 g ! HNO 3 l H D 39.48 kJ 7
Combining reactions 6a,6b and 7.
1
Reaction 8. 2NO 2 g C H 2 O l C O 2 ! 2HNO 3 l
2
overall enthalpy change D 57.32 C 9.00 C 2 39.48
D 127.28 kJ
127.28
heat generated per kmol of HNO (l) formed D ð 10 3
3
2
D 63,640 kJ
6
heat generated D 13.50 ð 63,640 D 0.86 ð 10 kJ/h
D 0.86 GJ/h
Note, the formation of N 2 O 4 and the part played by N 2 O 4 in the formation of nitric acid
was not considered when preparing the flow-sheet, as this does not affect the calculation
of the components flow-rates.

5. Heat of dilution of HNO 3
The heat of dilution was calculated from an enthalpy concentration diagram given in
Perry’s Chemical Engineers Handbook, 5th edn, p. 3.205, Figure 3.42.
Ž
Ž
The reference temperature for this diagram is 32 F(0 C). From the diagram:
enthalpy of 100 per cent HNO D 0
3
enthalpy of 45 per cent HNO D 80 Btu/lb solution
3
specific heat 45 per cent HNO D 0.67
3
Ž
So, heat released on dilution, at 32 F D 80 ð 4.186/1.8 D 186 kJ/kg soln.
Ž
Heat to raise solution to calculation datum temperature of 25 C D 0.67 25 0 4.186
D 70.1 kJ/kg.
Ž
So, heat generated on dilution at 25 C D 186 70.1 D 115.9 kJ/kg soln.