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.
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.
