The column top temperature is equal to the dew point of the overhead vapor. This corresponds to the 100% point on the EFV curve of the top product at its partial pressure calculated on the top tray.
A trial and error procedure is used to determine the temperature:
1. The temperature of reflux drum is fixed, keeping in view the maximum temperature of the cooling medium (water or air).
2. Estimate a tower overhead temperature, assuming steam does not condense at that temperature.
3. Run a heat balance around top of tower to determine the heat to be removed by pumpback reflux. Calculate the quantity of pumpback reflux.
4. Calculate the partial pressure of the distillate and reflux in the overhead vapor. Adjust the 100% point temperature on the distillate atmospheric flash vaporization curve to the partial pressure.
5. Repeat these steps until the calculated temperature is equal to the one estimated.
6. Calculate the partial pressure of steam in the overhead vapor. If the vapor pressure of steam at the overhead temperature is greater than the partial pressure of steam, then the assumption that steam does not condense is correct. If not, it is necessary to assume a quantity of steam condensing and repeat all steps until the partial pressure of steam in the overhead vapor is equal to the vapor pressure of water at overhead temperature. Also, in this case, it is necessary to provide sidestream water draw-off facilities.
7. To calculate overhead gas and distillate quantities, make a component analysis of total tower overhead stream consisting of overhead gas, overhead distillate, pumpback reflux, and steam. Next make a flash calculation on total overhead vapor at the distillate drum pressure and temperature.
8. The overhead condenser duty is determined by making an enthalpy balance around the top of the tower.
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