In the manufacture of aniline, liquid nitrobenzene at 20 ŽC is fed to a vaporiser where it is vaporised in a stream of hydrogen. The hydrogen stream is at 30 ŽC,and the vaporiser operates at 20 bar. For feed-rates of 2500 kg/h nitrobenzene and366 kg/h hydrogen, estimate the heat input required. The nitrobenzene vapour isnot superheated.
The Correct Answer and Explanation is :
To estimate the heat input required for vaporizing nitrobenzene in a stream of hydrogen, we follow these steps:
Given Data:
- Mass flow rate of nitrobenzene = 2500 kg/h
- Mass flow rate of hydrogen = 366 kg/h
- Initial temperature of nitrobenzene = 20°C
- Initial temperature of hydrogen = 30°C
- Pressure = 20 bar
- Final state of nitrobenzene = Saturated vapor (not superheated)
Assumptions:
- The process operates at constant pressure (20 bar).
- The enthalpy change of hydrogen is negligible.
- Heat input is mainly used to heat and vaporize nitrobenzene.
Steps to Calculate Heat Input:
The total heat required consists of:
- Sensible heat to heat nitrobenzene to boiling point
( Q_1 = m_{\text{NB}} C_p (\Delta T) )
Where:
- ( C_p ) of liquid nitrobenzene ≈ 1.8 kJ/kg·K
- Boiling point of nitrobenzene at 20 bar ≈ 250°C
- ( \Delta T ) = 250 – 20 = 230 K [
Q_1 = (2500 \text{ kg/h}) \times (1.8 \text{ kJ/kg·K}) \times (230 \text{ K})
] [
Q_1 = 1035000 \text{ kJ/h} = 287.5 \text{ kW}
]
- Latent heat to vaporize nitrobenzene
( Q_2 = m_{\text{NB}} \times \lambda )
Where:
- Latent heat of vaporization of nitrobenzene at 20 bar ≈ 320 kJ/kg [
Q_2 = (2500 \text{ kg/h}) \times (320 \text{ kJ/kg})
] [
Q_2 = 800000 \text{ kJ/h} = 222.2 \text{ kW}
]
- Total heat input required
[
Q_{\text{total}} = Q_1 + Q_2
] [
Q_{\text{total}} = 287.5 + 222.2 = 509.7 \text{ kW}
]
Explanation:
The heat required includes two main contributions: heating nitrobenzene from 20°C to its boiling point at 250°C and then vaporizing it. The specific heat capacity determines how much heat is required to increase the temperature, while the latent heat quantifies the energy needed for phase change. Since the hydrogen stream does not undergo a phase change and has a much lower heat capacity, its contribution is negligible. Therefore, the total estimated heat input required for this process is 509.7 kW.