April 03, 2007

Tray Towers - Few Thumb Rules

Tray Towers
  • For ideal mixtures, relative volatility can be taken as the ratio of pure component vapor pressures.
  • Tower operating pressure is most often determined by the cooling medium in condenser or the maximum allowable re-boiler temperature to avoid degradation of the process fluid.
  • For sequencing columns:
  • Perform the easiest separation first (least trays and lowest reflux)
  • If relative volatility, nor feed composition vary widely, take products off one at a time as the overhead.
  • If the relative volatility of components do vary significantly, remove products in order of decreasing volatility.
  • If the concentrations of the feed vary significantly but the relative volatility does not, remove products in order of decreasing concentration.

  • The most economic reflux ratio usually is between 1.2 Rmin and 1.5 Rmin.
  • The most economic number of trays is usually about twice the minimum number of trays.
  • Typically, 10% more trays than are calculated are specified for a tower.
  • Tray spacing should be from 18 to 24 inches, with accessibility in mind.
  • Peak tray efficiencies usually occur at linear vapor velocities of 2 ft/s (0.6 m/s) at moderate pressures, or 6 ft/s (1.8 m/s) under vacuum conditions.
  • A typical pressure drop per tray is 0.1 psi (0.007 bar)
  • Tray efficiencies for aqueous solutions are usually in the range of 60-90% while gas absorption and stripping typically have efficiencies closer to 10-20%.
  • The three most common types of trays are valve, sieve, and bubble cap. Bubble cap trays are typically used when the low-turn down is expected or a lower pressure drop than the valve or sieve trays can be provided.
  • The most common weir heights are 2 and 3 inch and the weir length is typically 75% of the tray diameter.
  • Reflux pumps should be at least 10% overdesigned.
  • The optimum Kremser absorption factor is usually in the range of 1.25 to 2.00
  • Reflux drums are almost always horizontally mounted and designed for a 5-min holdup at half of the drum capacity.
  • For towers that are at least 3 ft (0.9 m) in diameter, 4 ft (1.2 m) should be added to the top for vapor release and 6 ft (1.8 m) should be added to the bottom to account for the liquid level and reboiler return.
  • Limit tower heights to 175-ft (53 m) due to wind load and foundation considerations.
  • The length / diameter ratio of a tower should be no more than 30 and preferably below 20.
  • A rough estimate of reboiler duty as a function of tower diameter is given by:

Q = 0.5 D2 for pressure distillation.
Q = 0.3 D2 for atmospheric distillation.
Q = 0.15 D2 for vacuum distillation.
Where Q is in Million Btu/hr and D is lower diameter in feet



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