Chapter 8, Problem 8.78P

A liquid mixture of benzene and toluene containing 50.0 wt% benzene at 100°C and pressure P_ois fed at a rate of 32.5 m³/h into a heated flash tank maintained at a pressure $P_{\tan k}\ll P_0$. When the feed is exposed to the reduced pressure in this unit, a portion of it evaporates. The liquid and vapor product streams are in equilibrium at 75°C and $P_{\tan k}$. The liquid product contains 43.9 mole% benzene. When carrying out the requested calculations, assume volume additivity of liquid benzene and toluene, use Raoult’s law and the Antoine equation where necessary, and neglect the effect of pressure on enthalpy.

1. Calculate the molar flow rate (mol/s) and molar composition (component mole fractions) of the feed stream. Then calculate the minimum value of Po(atm) needed to keep the feed stream in the liquid state until it enters the flash tank.
Calculate $P_{\tan k}$(atm), the mole fraction of benzene in the vapor, and the molar flow rates of the liquid and vapor products.

Calculate the required heat input rate in kilowatts.

An hour after the system is started up, a chromatographic analysis of the vapor product is run and the benzene mole traction is found to be 3% higher than the value calculated in Part (b). The system temperature and pressure are rechecked and found to have the correct values. Give several possible explanations of the discrepancy between the calculated and measured values.

Briefly explain why the product temperature is lower than the feed temperature. What would be required to run the unit isothermally?