How are hydraulic calculations are performed for a column
in normal, flooding and weeping conditions?
Products: Aspen HYSYS
Last Updated: 20-Jan-2017
Versions: 2006, V7.0, V7.1, V7.2, V7.3, V8.0, V8.2, V8.3, V8.4, V8.6, V8.8, V9.0
Article ID: 000030780
Primary Subject: Dynamics
Problem Statement
Problem Statement
In Aspen HYSYS Dynamics, how are hydraulic calculations are performed for a column in normal, flooding and
weeping conditions?
Solution
1. Aspen HYSYS uses the Francis weir equation to calculate the flow rate leaving a tray. If the tray floods, then at
some point that equation no longer applies. Aspen HYSYS currently handles this by limiting the value used for the
liquid height over the weir in the flow equation.
The Francis Weir equation is the starting point for calculating the liquid flowrate leaving a tray:
LN = C*ρ*lw*h1.5
Where:
LN = liquid flowrate leaving tray N
C = units conversion constant
ρ = density of liquid on tray
lw = weir length
h = height of liquid above weir
The vapour flowrate leaving a tray is determined by the resistance equation:
Fvap = k * sqrt (density *ΔPfriction)
where:
Fvap = vapour flowrate leaving tray N
k = conductance, which is a constant representing the reciprocal of resistance to flow
ΔPfriction = dry hole pressure drop
A. For columns, the conductance (k) is proportional to the square of the column diameter (k α (Tray diameter)2). The
pressure drop across a stage is determined by summing the static head and the frictional losses.
B. Currently, Downcomer Volume, Active Area and Number of Flow Paths are not used in the column hydraulic
calculations. Instead, the volume of liquid and the total area are used to compute the height of liquid. Using the total
weir length, the height of liquid over the weir is calculated and that determines the liquid flow via the Francis weir
equation.
2. Flooding:
Aspen HYSYS has the ability to model flooding inside the column.
When calculating flooding in dynamics, it is assumed that every tray can be treated like a vessel (i.e. flooding is a
symptom of liquid build up in every tray).
For the flooding condition, the bulk liquid volume approaches the tray volume. This can be observed on the Holdup
page in the Dynamics tab (via either the Column Runner or the Tray Section property view).
The key variables to look at are primarily: Liquid Height on Tray or Height over weir; they can be found on the
property view of the Tray Section of the column, then Performance / Hydraulics. Note that while Aspen HYSYS
does model tray flooding to some degree, it is rather rudimentary (i.e. downcomer backup, entrainment etc. are not
rigorously modeled). When the column does flood, the user will see the higher pressure drops, liquid levels etc.
associated with this condition, however, in terms of the exact point where this happens (or its exact response), the
results will be highly approximate. The packed column model does offer better support for flooding, and there are
long term plans to improve the tray flooding handling.
The flowrate is determined by
Flow = k * sqrt(density*friction pressure losses).
The relevant parameters are taken from the steady-state solution.
3. Weeping:
Aspen HYSYS has the ability to model weeping inside the column. If ΔPfriction is very small, the stage exhibits
weeping. It is therefore possible to have a liquid flow to the stage below even if the liquid height over the weir is
zero.
Weeping can start to occur on a tray when the dry hole pressure loss drops below 0.015 kPa. It allows liquid to drain
to the stage below even if the liquid height is below the weir height.
When this occurs, a small crest height is calculated so that liquid will flow down to the next stage even if the liquid
level is below the weir height:
h (Crest height) = weepFactor * 2.0 * weepDp * (1.0 - vapToAboveKAvg);
Where:
weepFactor is an adjustable parameter enabing the user to adjust the weeping effect
weepDp is 0.015 kPa
vapToAboveKAvg is the damped ratio gasΔp/weepΔp, truncated to lie between 0.05 and 1.0.
Under normal circumstances this ratio is unity, resulting in no weeping.
Note that Aspen HYSYS Dynamics assumes the liquid is present as clear liquid.
Keywords
column dynamics, column, dynamics, hydraulics, Francis, weir, Francis weir equation
How does Aspen HYSYS predict weeping in Column
Internals Analysis tool?
Products: Aspen HYSYS
Last Updated: 17-May-2017
Versions: V9.0
Article ID: 000044581
Primary Subject:
Problem Statement
How does Aspen HYSYS predict weeping in Column Internals Analysis tool?
Solution
One of the factor affecting column distillation operation is Weeping caused by low vapour flow. The
pressure exerted by the vapour is insufficient to hold up the liquid on the tray. Therefore, liquid starts to
leak through perforations. If you want to view a video presentation of weeping, please click here.
Aspen HYSYS has the capability to predict this phenomenon using the Column Hydraulic Internals tool in
V9.0. The Hydraulic Plots will show in red the problematic areas. If you review the plots by tray, you will
be able to see more details with the error message indicating that weeping was detected.
The program takes as reference the book "Distillation Design" by Henry Kister ISBN 0-07-034909-6 to
predict weeping. Specifically look for the weeping methods of Hsieh & McNulty, and Lockett.
The basic correlations involved as used by Wallis (Wallis, G.B., "One-Dimensional Two-Phase Flow",
p.339, McGraw Hill, New York (1969). are:
In addition, the equation for the calculation of the characteristic length Z:
Please find the attached paper with the above reference for further details.
Keywords
Weeping, Column, Internals, Error, Hydraulic Plot, Trays.
