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Procedure for distillation, absorption & stripping column sizing using HYSYS or UniSim


HYSYS/UniSim includes a utility to calculate diameter, height, pressure drop, etc. for gas-liquid columns.  (Note that the default 1.5 m diameter has no relationship to reality, and should never be used.)  Following is the procedure for tray columns, after you have obtained a converged column design.  For packed columns, the column height is the number of theoretical trays (100% efficiency) multiplied by the Height of an Equivalent Tray (HETP) or Stage (HETS).  This utility incorporates a calculation of HETP, but not tray efficiency.


Heuristics (rules-of-thumb) are cited in several places below.  Some sources for these are the Front Matter Rules of Thumb in Walas;  heuristics; Chapter 11 in Turton et al., Chapter 14 in Seider et al.


Do not use the tray sizing utility for liquid-liquid extraction (LLE).  Use the LLE heuristics.  See also Perry’s section 15.   The delta P for a LLE column is approximately the weight of the two phases in the column per cross-sectional area.


1.      Open your case study.  If the green calculation light is not on, turn it on.  

2.      Click on Tools, Utilities, and add the tray sizing utility.

3.      Click on Select TS (tray section), click on a converged column (preferably a distillation column), Main TS object, OK.

4.      Click on Add Section.

5.      Click on Design Specs.  Hit F1.  This should give you the help page for Setting Tray Sizing specifications.  Read this page carefully along with the information windows on the underlined green items.  Click on each type of internals in item 7 and select that most suitable for your column.  For item 8, you want to select the design mode to let this utility select optimal values.  For sieve, valve & bubble-cap trays, note that the recommended number of flow paths and the tray spacing depend on the tower diameter.  The tower diameter, in turn, depends on the number of flow paths, so you will probably have to iterate to achieve the recommended values.

6.      Return to the Tray Sizing Specs page and enter any required values.  This should cause the utility to calculate and converge.  If weeping occurs in a sieve-tray column, switch to valve trays.

7.      Click on Performance Results to see the calculated results. See also the Performance Plots.  If necessary to reach recommended values (from help in 5 above), return to the Tray Sizing Specs page and make the required changes. 

8.      When you are satisfied with the results, from the Performance Results copy the column diameter, height, percent of flooding, and total pressure drop (bottom to top, Section Delta P).  If you have used 100% column efficiency, to get the actual pressure difference you’ll have to divide by a realistic column efficiency and add a safety factor (see the heuristics listed above and Table 14.6 in the 7th edition of Perry's Handbook).  Return to the Column Design Connections page, and adjust either the top and/or bottom pressure to give the total pressure drop you’ve just determined.  Return to the utility Performance Results.  Note that the pressure drops and diameter(s) will have changed slightly.

1.      Go to File, Print, and print out the first page (Tray Results) of the performance results.

2.      Click on Performance Plot, and print out plots of the Flow and Delta P.

3.      Compare the above results with those calculated using appropriate heuristics (e.g., % flooding, diameter, height, height/diameter, pressure drop / tray, etc.).  If you see any strong discrepancies, correct them in the Tray Sizing Utility Design Specs.   If the height/diameter exceeds the value in heuristics, you will probably have to break the column into two parts.  If you do this, make certain to account for the change in hydrostatic head for the liquid flowing between the two parts.  (Draw a diagram to clarify your thinking.)

4.      Make certain you have recorded the number of actual (not theoretical with 100%) trays, the top and bottom pressures, tower diameter and tower height, all corrected for tray efficiency and safety factor.  You will need these to estimate the cost of installing and operating the column.

5.      Repeat using the tray efficiency entered into the column calculations.  Double-click on the column, enter the actual number of trays (without the safety factor) on the Design Connections page, and the efficiencies on Parameters Efficiencies.  Make any changes required to get convergence.

6.      Packed columns:  Do NOT use tray efficiencies for packed columns.  The height is equal to the number of trays times the HETP (Height Equivalent to a Theoretical Tray); this substitutes for the tray efficiency in tray columns.  Add a safety factor to the height, say 10%.


Distillation, stripping and absorption

Clarkson's Department of Chemical and Biomolecular Engineering


W.R. Wilcox, Clarkson University, July 13, 2009.


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