Page 191 - Chemical engineering design

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CHEMICAL ENGINEERING
Ž
So, enthalpy 45 per cent acid at 40 C D 186 C 0.67 ð 4.186 40 D 73.8kJ/kg
Ž
and enthalpy 60 per cent acid at 40 C D 202 C 0.64 ð 4.186 40 D 94.8kJ/kg
73.8 ð 1860.7 C 94.8 ð 2840.0
Enthalpy of mixed acid D
1860.7 C 2840.0
D 86.5kJ/kg
From enthalpy-concentration diagram, enthalpy of mixed acid
Ž
Ž
(54 per cent) at 0 C D 202 kJ/kg; specific heat D 0.65 kcal/kg C
Ž
so, “sensible” heat in mixed acid above datum of 0 C
D 86.5 202 D 115.5kJ/kg
115.5
Ž
and, mixed acid temperature D D 43 C
0.65 ð 4.186
Energy recovery

In an actual nitric acid plant the energy in the tail gases would normally be recovered
by expansion through a turbine coupled to the air compressor. The tail gases would be
preheated before expansion, by heat exchange with the process gas leaving the WHB.

4.4. COMPUTER-AIDED FLOW-SHEETING
The computer programs available for flow-sheeting in process design can be classified
into two basic types:
1. Full simulation programs, which require powerful computing facilities.
2. Simple material balance programs requiring only a relatively small core size.
The full simulation programs are capable of carrying out rigorous simultaneous heat and
material balances, and preliminary equipment design: producing accurate and detailed
flow-sheets. In the early stages of a project the use of a full simulation package is often
not justified and a simple material balance program is more suitable. These are an aid
to manual calculations and enable preliminary flow-sheets to be quickly, and cheaply,
produced.

4.5. FULL STEADY-STATE SIMULATION PROGRAMS
Complex flow-sheeting programs, that simulate the operation and a complete process,
or individual units, have been developed by several commercial software organisations.
The names of the principal packages available, and the contact address, are listed in
Table 4.1. Many of the commercial programs have been made available by the proprietors
to university and college departments for use in teaching, at nominal cost.