116 February 1998CHE 446

Dimensionless Groups Used in Bravo-Fair Correlations for Mass TransferCoefficients of Packed Towers (See handout from Kister)

1. Reynolds No. = Re = V / ( ap) = Inertial Forces / Viscous Forces [ap=area/vol]

2. Schmit No. = Sc = / ( D) = Momentum Transport / Diffusion [D is diffusivity]

3. Sherwood No. = Sh = k / (D ap) = Convective MT / Diffusive MT [k is MT Coeff]

4. Weber No. = We = V2 / (ap ) = Kinetic energy of fluid to surface tension stress

5. Froude No. = Fr = V2 ap / g = Kinetic energy / potential energy

6. Capillary No. = Ca = V / = Viscous forces (liquid-wall interaction) / cohesion of liquid molecules

Random Versus Structured Packing

1. At low liquid rates ( 100-200 psia.

5. Structured packings perform less well with aqueous liquid systems and thosewhich possess high . The latter property indicates that larger droplets form, whichindicates that poorer wetting will occur on the surface of metal structured packings.

6. Liquid holdup: similar values for structured and random packings.

7. Structured packings are very susceptible to corrosion.

8. Structured packings are much less sensitive to surges and plant upsets. A packedtower operating in the loading region can be easily induced into the flooding region ofoperation with plant upsets.

29. Inspection and maintenance of structured packings are much more difficult thanrandom packings.

10. Cost: A tradeoff exists. The cost per unit mass is 3-10 times more expensive forstructured compared to random packings; but, the former is more efficient (lower HETP)due to lower pressure drop. Also, pumping costs are less for structured packings becauseof the lower pressure drop and the shorter columns.

Tray Towers Versus Packed Towers

1. Smaller pressure drops per unit length (lower pumping costs): PACKED (e.g.0.04 psi/stage for packed; 0.15 psi/stage for tray; important issue for vacuum distillatn)

2. Smaller liquid holdup (susceptibility to flooding): PACKED (e.g., importantissue for batch distillation)

3. Higher L/G ratio (slope of operating line) PACKED; or MULTIPASS TRAYS

4. Lower L/G ratio: TRAY

5. Easier cooling of liquid: TRAY

6. Easier side stream withdrawal (plus easier incorporation of other tower internalssuch as inter-reboilers and cooling coils): TRAY

7. Better adaptation to L/G systems that foam: PACKED

8. Better for L/G systems that corrode: PACKED (random)

9. Easiest for cleaning: TRAY

10. Operation for small column diameter: PACKED (maintenance issues)

11. Operation for large column diameter: TRAY (packings have maldistributionproblems when DT / dp is too large)

12. Presence of solids TRAY (solids can clog void space of packed beds)

13. Feed composition variation: TRAY (feed tray position can be varied)

14. Ability to predict column performance/more accommodating of design errorsTRAY

15. Chemical reactions TRAY (Larger holdup, or residence time, of liquid)

316. Lower weight: TRAY

17. Effect of temperature gradient between ambient and internal: less significant forTRAYS (expansion/contraction of shell can crush packings.)

18. Lower capital costs (often): TRAY (Hairston article)