Page 83 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

66
There are two ways of calculating the specific enthalpy of the vapour at its boiling
point. CHEMICAL ENGINEERING
(1) Latent heat of vaporisation at the base temperature C sensible heat to heat the
vapour to the boiling point.
(2) Latent heat of vaporisation at the boiling point C sensible heat to raise liquid to
the boiling point.
Values of the latent heat of acetone and water as functions of temperature are given in
Volume 1, so the second method will be used.
Ž
Latent heat acetone at 56.5 C (330 K) D 620 kJ/kg
Ž
Water at 56.5 C (330 K) D 2500 kJ/kg
Taking latent heats as additive:
H V D 1111[ 0.01 ð 2500 C 0.99 ð 620 C 56.5 25 2.2]
D 786,699 kJ/h

The enthalpy of the top product and reflux are zero, as they are both at the base
temperature. Both are liquid, and the reflux will be at the same temperature as the product.
Hence Q C D H V D 786,699 kJ/h 218.5kW

Q B is determined from a balance over complete system

Input Output
Q B C H F D Q C C H D C H W
H F D 1000 ð 4.00 35 25 D 40,000 kJ/h
H W D 899 ð 4.2 100 25 D 283,185 kJ/h
Ž
(boiling point of bottom product taken as 100 C).
hence Q B D Q C C H W C H D H F
D 786,699 C 283,185 C 0 40,000
D 1,029,884 kJ/h 286.1kW

Q B is supplied by condensing steam.

Latent heat of steam (Volume 1) D 2174 kJ/kg at 274 kN/m 2
1,029,884
Steam required D D 473.7 kg/h
2174
Ž
Q C is removed by cooling water with a temperature rise of 30 C
Q C D water flow ð 30 ð 4.2
786,699
Water flow D D 6244 kg/h
4.2 ð 30