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