February 12, 2009

Thumb Rules for Tray Towers

Similar to my previous post, here I am putting few good thumb rules for Tray towers also.

Again be careful while using these thumb rules.

  1. For ideal mixtures, relative volatility can be taken as the ratio of pure component vapor pressures.

  2. 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.

  3. Perform the easiest separation first (least trays and lowest reflux) while sequancing columns

  4. If relative volatility, nor feed composition vary widely, take products off one at a time as the overhead.

  5. If the relative volatility of components do vary significantly, remove products in order of decreasing volatility.

  6. If the concentrations of the feed vary significantly but the relative volatility does not, remove products in order of decreasing concentration.

  7. The most economic reflux ratio usually is between 1.2 Rmin and 1.5 Rmin.

  8. The most economic number of trays is usually about twice the minimum number of trays.

  9. Typically, 10% more trays than are calculated are specified for a tower.

  10. Tray spacing should be from 18 to 24 inches, with accessibility in mind.

  11. 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.

  12. A typical pressure drop per tray is 0.1 psi (0.007 bar)

  13. 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%.

  14. 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.

  15. The most common weir heights are 2 and 3 inch and the weir length is typically 75% of the tray diameter.

  16. Reflux pumps should be at least 10% over designed.

  17. The optimum Kremser absorption factor is usually in the range of 1.25 to 2.00.

  18. Reflux drums are almost always horizontally mounted and designed for a 5-min holdup at half of the drum capacity.

  19. 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.

  20. Limit tower heights to 175-ft (53 m) due to wind load and foundation considerations.

  21. The length / diameter ratio of a tower should be no more than 30 and preferably below 20.

  22. A rough estimate of reboiler duty as a function of tower diameter is given by:

  23. 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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