Question

2.Calculate the specific enthalpy change in kJ / kg-C of the following processes:

a) cyclohexane gas in a process from T0 = 300C to Ft= 900 C

b) hydrocyanic acid gas from T0 = 900C to Ft = 300 C.

Which of the two processes requires more energy? Which one would you use to heat a stream of 100mol / h water that must enter a heater at 25ºC and leave at 80ºC? Would it reach you?

Answer 1

The first process is heating of Cyclohexane

To = 300°C

Tf = 900°C

Cp is calculated from handbook at average temperature of (300+900) /2 = 600 °C

From handbook

Cp(cyclohexane) (g) (600°C) = 291.9201 J/mol°C

Q = nCp(900-300)

Q = ∆H = 1(291.9201) (900-300) =175152.06 J/mol = 175.15206 KJ/mol

M. W of cyohexane = 84.16 g/mol

Mass in 1 moles = 84.16 g

Q = 175.15206/84.16 = 2.08117 KJ/Kg

For hydrocyanic acid

To = 900°C

Tf = 300°C

From handbook cp of hydrocyanic acid at average temperature of (900+300) /2 = 600°C

= 48.4296 J/mol°C

Q = nCp(300-900)

Q = 1(48.4296) (300-900)

Q = -29057. 76 J/mol

Q = -29. 05776 KJ/mol

M. W of HCN = 27 g/mol

Q =∆H= -29. 05776/(27) = -1. 07621 KJ/kg

Out of two process

First process of heating Cyclohexane requires more energy than second process

C)

100 mol/h should be heated from 25°C to 80°C

In second process heat is released and hence this heat is used to heat the water.

So second process is used to heat water

We need more hydrocyanic acid to carry out heating of water

Cp of water at average temperature of

(25+80) /2 = 52.5°C

Cp of water = 75.4 J/mol°C

Q = 100(75.4) (80-25) = 414700 J = 414.7 KJ

Mass of HCN needed = (414.7) /(1.07621)

= 385.333 Kg/h

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