Weeping and flooding phenomena set the vapor velocity in industrial distillation columns. Presented here are procedures for calculating flooding and weeping velocities in a four-pass sieve-tray column
In industrial distillation, the term “weeping” is primarily used when the liquid in sieve-tray columns starts leaking through the holes or perforations as a result of insufficient counter-current vapor flow. Weeping is undesirable because the liquid should flow across the tray and through the downcomer. This sets the lower limit of vapor velocity for sieve-tray column operation (see box, Definitions). Conversely, the term “flooding” is used in sieve-tray columns where the excessive liquid build-up in the column leads to flooding condition. This is because the liquid cannot move down due to high vapor velocity. This sets the upper limit of vapor velocity for sieve-tray column operation. This article presents the procedure for calculating both flooding and weeping velocities, using both SI (International System of Units) and English Imperial System units.
DEFINITIONS
Four-pass sieve-tray column. In a four-pass sieve-tray distillation column, every alternate tray is similar. A typical four-pass sieve tray will have two side downcomers and one center downcomer, while the next tray will have two off-center downcomers. The next tray will again have two side downcomers and a center downcomer. This cycle continues through the height of the column.
Column cross-sectional area, AT. The area of the column calculated based on the diameter of the tower.
Total hole area, AH. Total area of all the active holes/perforations on the sieve tray
Net area, AN. Column cross-sectional area minus the total downcomer area of the sieve tray
Active/bubbling area, AB. Column cross-sectional area minus the total downcomer area and the downcomer seal area (of the upper tray).
Fractional hole area, Af. The ratio between the total hole area and the active/bubbling area (AH/AB).
Downcomer area, ADC. Area occupied by the downcomers. Area of the side downcomers formed by the column wall and the downcomer panel can be calculated using Table 4 in the Koch-Glitsch Tray Design Manual Bulletin 4900 [1]
Outlet weir height, hw. Outlet weir height maintains a liquid level on the tray and should be high enough to provide sufficient contact between liquid and vapors without excessive pressure drop.
Four-pass sieve-tray columns
The most common types of gas-liquid mass-transfer operations are distillation and gas absorption, and the most common types of contactors used for distillation or gas absorption are packing or tray columns. The different types of packed and tray columns are shown in Figure 1.
FIGURE 1. Different types of trayed and packed columns are categorized here
As the liquid load increases, the number of passes can be increased from single-pass to as high as four- or six-pass trays. Three-pass trays are generally avoided due to the lack of symmetry of the trays. This article discusses only four-pass sieve-tray columns.
All four-pass sieve trays have three types of downcomers (Figure 2). Side downcomers are formed by a column wall and a downcomer panel. The center downcomer is located at the tower centerline and is formed between two downcomer panels offset from the centerline. Two side downcomers and a center downcomer are located on alternating trays, either on all odd-numbered or on all even-numbered trays. The third type of downcomers are off-center downcomers (OCDCs). The OCDC is located between the side and center downcomers on alternating trays — between the trays containing the side and center downcomers.
FIGURE 2. The diagram shows the arrangement of a typical four-pass sieve tray column, with path flow on each tray
Operating range
Satisfactory operation for this type of column is only achieved over a limited range of vapor and liquid flowrates. A typical performance diagram for a sieve plate is shown in Figure 3 [2].
FIGURE 3. On this graph, a typical performance diagram for a sieve plate is shown, with the area of operation between flooding and weeping
The upper limit to vapor flow is set by the condition of flooding, while the lower limit of the vapor flow is set by the condition of weeping.
Entrainment and flooding
Two methods outlined below can be used for calculating flooding velocity.
Fair’s method. The method by J.R. Fair [3] is described as follows: Calculate flow parameter FLV, Equation (1).
(1)
For multi-pass trays, L/V ratio should be divided by the number of passes, which in this case is four [4].
Estimate Souder’s & Brown [5] coefficient CSB (also known as C-factor), using the graphs in Figures 4 and 5. Alternatively, Souder’s & Brown coefficient (CSB) can be calculated empirically using Equation (2) for SI units [6].
FIGURE 4. The C-factor can be estimated from this graph from Ref. 10
FIGURE 5. The C-factor can be estimated from this graph from Ref. 4
(2)
or Equation (3) for English units [7]:
(3)
Flood-point vapor velocity can be estimated using Equation (4).
(4)
Kister and Haas method. Kister and Haas [8] developed the correlation to calculate Souder’s & Brown coefficient (CSB) for sieve-tray flood point given in Equations (5) (English units)[10] and (6) (SI units) [4].
(5)
where:
S = tray spacing, in.
dH = hole diameter, in.
hct = clear liquid height at froth-to-spray regime, in.
(6)
where:
S = tray spacing, mm
dH = hole diameter, mm
hct = clear liquid height at froth-to-spray regime, mm
Clear liquid height at the froth-to-spray regime transition, corrected for the effect of weir height on spray regime entrainment can be calculated using Equation (7) [8].
(7)
where:
hct = clear liquid height at froth-to-spray regime, mm
hct, H2O = clear liquid height at froth-to-spray regime for air-water system, mm
hW = outlet weir height, mm
n = 0.00091 (dH/Af)
For an air-water system, the clear liquid height can be calculated by using the modified Jeronimo and Sawistowski Correlation (Equation (8)):
(8)
Similarly, for English Units, the equation is shown in Equation (9) [10].
(9)
where:
hw = outlet weir height, in.
n = 0.0231 (dH/Af)
and:
Flood-point vapor velocity can then be calculated as follows:
(10)
Weeping
Liquid height over the weir how is calculated using the corrected Francis formula, as shown in Equation (11) (English units) and Equation (12) (SI units):
how = 0.48 × Fw(QL2/3) (11)
where:
how = liquid height over the weir, in.
Fw = correction factor
how = 664 × Fw(QL2/3) (12)
where:
how = liquid height over the weir, mm
Fw = correction factor
FIGURE 6. This diagram shows the liquid height over the weir, from Ref. 2
The correction factor Fw in Equations (11) and (12) can be estimated using the graph in Figure 7 [10].
FIGURE 7. The correction factor from Equation (11) can be estimated from this graph from Ref. 10
Weeping velocity can be calculated using the Eduljee Correlation [9, 11], in either SI units (Equation (13)) or English units (Equation (14)).
(13)
where:
uweeping = vapor velocity at weep point, m/s
dH = hole diameter, mm
(14)
where:
uweeping = vapor velocity at weep point, ft/s
dH = hole diameter, in.
The coefficient K2 in the above correlations can be estimated using the graphs in Figures 8 and 9 [2, 9].
FIGURE 8. The coefficient K2 from Equation (13) can be estimated using this graph from Ref. 2
FIGURE 9. This graph from Ref. 9 can help estimate the coefficient K2 from Equation (14)
Edited by Scott Jenkins
References
1. Koch-Glitsch, Ballast Tray Design Manual, Bulletin 4900, Sixth ed., Koch-Glitsch, 2013.
2. Sinnott, R.K., Coulson, J.M., and Richardson, J.F., “Coulson & Richardson’s Chemical Engineering,” Vol. 6, 4th ed., Elsevier Butterworth-Heinemann, Oxford, U.K., 2005.
3. Fair, J.R., How to Predict Sieve Tray Entrainment and Flooding, Petro/Chem. Engineer, 33(10), pp. 211–218, September 1961.
4. Perry, R.H., Green, D.W., and Maloney, J.O., “Perry’s Chemical Engineers’ Handbook,” 8th ed., McGraw-Hill, New York, NY, 2008.
5. Souders, Mott, Brown, G. Granger, Design of Fractionating Columns; 1- Entrainment and Capacity, Industrial Eng. Chem., 26(1), p. 98, 1934.
6. Lygeros, A.I., Magoulas, K.G., Column Flooding and Entrainment, Hydrocarbon Processing, 65(12), p. 43, 1986.
7. Ward T.J., A New Correlation for Sieve Trays, Chem. Eng., 96 (6), June 1989. p. 177–178.
8. Kister, H. Z., Haas, J. R., Entrainment from Sieve Trays in the Froth Regime, Industrial & Engineering Chemistry Research, 27(12), p. 2,331, 1988.
9. Eduljee, H.E., Design of Sieve-Type Distillation Plates, British Chemical Engineering, 3(1), p. 14-17, January 1958.
10. Kister, H.Z., “Distillation Design,” McGraw-Hill, New York, NY, 1992.
11. Eduljee, H.E., Sieve Plates – Minimum Vapor velocity, The Chemical Engineer, p.123, March 1972.
Further reading
Chase, J.D., Sieve Tray Design Part-I, Chemical Engineering, July 1967, p. 105–116.
Chase, J.D., Sieve Tray Design Part-II,” Chemical Engineering, August 28, 1967, p. 139–146.
Colwell, C.J., Clear Liquid and Froth Density on Sieve Trays, Industrial Engineering Chemistry Process Design and Development, 20(2), p. 298, 1981.
Piling M., “Ensure Proper Design and Operation of Multi-Pass Trays,” Chemical Engineering Progress, (4), p. 22, June 2005.
Piling M., Peng, A.C., “Tray Design for High Load Applications,” Sulzer Chemtech, July 2014.
Lockett, M.J, Banik, S., Weeping from Sieve Trays, Industrial Engineering Chemistry Process Design and Development, 25(2), p. 561, 1986.
Hsieh, C.L., McNulty, K.J., Predict Weeping of Sieve and Valve Trays, Chemical Engineering Progress, p. 71, July 1993.
Lockett, M.J., “Distillation Tray Fundamentals,” Cambridge University Press, Cambridge, 1986.
Kister, H.Z., “Distillation Operation,” McGraw-Hill, New York, N.Y., 1990.
Coker, A. K., Ludwig, E. E., “Ludwig’s Applied Process Design for Chemical and Petrochemical Plants,” Vol. 2, 4th ed., Elsevier Gulf Professional Publishing, Sebastopol, Calif., 2015.
Dutta, B.K., “Principles of mass transfer and Separation Processes,” PHI Learning Private Limited, New Delhi, 2023.
Author
Mohammad Abubaker Minhas is a senior process engineer at Aramco (Dhahran, Saudi Arabia (Ph: +966 55 195 6465 and +1 713-703-2134; Email: [email protected]). Minhas has more than 25 years of experience in process development, design and engineering, with expertise in produced-water and wastewater treatment. He holds a masters degree in chemical engineering from the University of Calgary in Alberta, Canada.