Week-11, Lecture-54 Distillation Column –
Process Design
Shabina Khanam
Associate Professor
Department of Chemical Engineering
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Distillation Column – Plate hydraulic design
Process Design
Plate efficiency
Design of Multi-component system
Design method for binary system
Distillation and continuous process
Plate – Design Procedure
Weep Point
• The lower limit of the operating range occurs when liquid leakage
through the plate holes becomes excessive. This is known as the
weep point.
• The vapour velocity at the weep point is the minimum value for stable
operation.
• The hole area must be chosen so that at the lowest operating rate the
vapour flow velocity is still well above the weep point.
• The minimum design vapour velocity is given by:
Plate – Design Procedure
Weep Point
uh = minimum vapour
velocity through the
holes (based on the
hole area), m/s,
dh = hole diameter, mm,
K2 = a constant
Plate – Design Procedure
Weir Dimensions
Weir height
• The height of the weir determines the volume of liquid on the plate and
is an important factor in determining the plate efficiency.
• A high weir will increase the plate efficiency but at the expense of a
higher plate pressure drop.
• For columns operating above atmospheric pressure the weir heights
will normally be between 40 mm to 90 mm (1.5 to 3.5 in.); 40 to 50 mm
is recommended.
• For vacuum operation lower weir heights are used to reduce the
pressure drop; 6 to 12 mm.
Plate – Design Procedure
Weir Dimensions
Weir length
• With segmental downcomers the
length of the weir fixes the area of
the downcomer.
• The chord length will normally be
between 0.6 to 0.85 of the column
diameter.
• A good initial value to use is 0.77,
equivalent to a downcomer area
of 12 percent.
Plate – Design Procedure
Hole Size
The hole sizes used vary from 2.5 to 12 mm; 5 mm is the preferred size.
Larger holes are occasionally used for fouling systems.
The holes are drilled or punched.
Punching is cheaper, but the minimum size of hole that can be punched
will depend on the plate thickness, typical plate thicknesses used are: 5
mm for carbon steel, and 3 mm for stainless steel.
When punched plates are used they should be installed with the
direction of punching upward as reversing the plate will increase the
pressure drop.
Plate – Design Procedure
Hole Pitch
• The hole pitch (distance between the hole centres) lp should not be
less than 2.0 hole diameters, and the normal range will be 2.5 to 4.0
diameters.
• Within this range the pitch can be selected to give the number of
active holes required for the total hole area specified.
• Square and triangular patterns are used; triangular is preferred.
• The total hole area as a fraction of the perforated area Ap is given
by the following expression
Plate – Design Procedure
Perforated Area
Recommended values of calming
zones are: below 1.5 m diameter, 75
mm; above, 100 mm.
The width of the support ring for
sectional plates will normally be 50
to 75 mm: the support ring should
not extend into the downcomer area.
Plate – Design Procedure
Plate Pressure Drop
• The pressure drop over the plates is an important design consideration.
• There are two main sources of pressure loss: that due to vapour flow through
the holes and that due to the static head of liquid on the plate.
• The total is taken as the sum of the pressure drop calculated for the flow of
vapour through the dry plate (the dry plate drop hd); the head of clear liquid on
the plate (hw + how) and a term to account for other, minor, sources of
pressure loss, the so-called residual loss hr
• It is convenient to express the pressure drops in terms of millimetres of liquid.
ΔPt = total plate pressure drop, Pa(N/m2),
ht = total plate pressure drop, mm liquid.
Plate – Design Procedure
Dry Plate Drop
The pressure drop through the
dry plate can be estimated
using expressions derived for
flow through orifices, where
the orifice coefficient C0 is a
function of the plate thickness,
hole diameter, and the hole to
perforated area ratio.
Plate – Design Procedure
Residual Head
• Methods have been proposed for estimating the residual head as a
function of liquid surface tension, froth density and froth height.
• This equation is equivalent to taking the residual drop as a fixed value
of 12.5 mm of water
Total plate drop
Plate – Design Procedure
Downcomer Design
The downcomer area and plate spacing
must be such that the level of the liquid
and forth in the downcomer is well
below the top of the outlet weir on the
plate above.
If the level rises above the outlet weir
the column will flood.
The back-up of liquid in the downcomer
is caused by the pressure drop over
the plate and the resistance to flow in
the downcomer itself.
Plate – Design Procedure
Downcomer Design [Back-up]
• In terms of clear liquid the downcomer back-up is given by:
hb = downcomer back-up, measured from plate surface, mm,
hdc = head loss in the downcomer, mm.
• The main resistance to flow will be caused by the constriction at the downcomer
outlet, and the head loss in the downcomer is
Lwd = liquid flow rate in downcomer, kg/s,
Am = either the downcomer area Ad or the clearance area under the downcomer; Aap
whichever is the smaller, m2.
Plate – Design Procedure
Downcomer Residence Time
• Sufficient residence time must be allowed in the downcomer for the
entrained vapour to disengage from the liquid stream; to prevent
heavily "aerated" liquid being carried under the downcomer.
• The downcomer residence time is given by:
tr = residence time, s,
hhc = clear liquid back-up, m.
Thank You!
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