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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April 02, 2007

Packed Towers - Few Thumb Rules

Packed Towers
  1. Packed towers almost always have lower pressure drop than comparable tray towers.
  2. Packing is often retrofitted into existing tray towers, to increase capacity or separation. Thus same size of packed towers can handle more than tray towers.
  3. For gas flow rates of 500 ft3/min (14 m3/min) use 1 in (2.5 cm) packing, for gas flows of 2000 ft3/min (57 m3/min) or more, use 2 in (5 cm) packing.
  4. Ratio of tower diameter to packing diameter should usually be less than 15.
  5. Due to the possibility of deformation, plastic packing should be limited to an unsupported depth of 10-15 ft (3-4 m) while metallic packing can withstand 20-25 ft (6-7.5 m).
  6. Liquid distributor should be placed every 5-10 tower diameters along the length for pall rings and every 20 ft (6.5 m) for other types of random packing.
  7. Packed columns should operate near 70% flooding.
  8. Height Equivalent to theoretical stages (HETS) for vapor liquid contacting is 1.3-1.8 ft for 1in pall rings and 2.5-3 ft for 2.0 in pall rings
  9. Design pressure drop should be as follows

    System Pressure drop (in water/ft)
  • Absorbers & regenerators
    Non foaming systems 0.25 – 0.40
    Moderate foaming systems 0.15 - 0.25

  • Fume scrubbers
    Water absorbent 0.40 - 0.60
    Chemical absorbent 0.25 - 0.40
  • Atmospheric or pressure distillation 0.40 – 0.80
  • Vacuum distillation 0.15 – 0.40
  • Maximum for any system 1.0

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