1-3 Design of Tray and Packed Column
1-3 Design of Tray and Packed Column
1-3 Design of Tray and Packed Column
Mass Transfer
& Separation Processes
Approximate Tray and Packed Columns
Design
Chemical Engineering Department
Faculty of Engineering
Cairo University
Prepared by
Dr. Ahmed Fayez Nassar
A. F. Nassar 1
Tray Columns
Types of Plates
Design of Trays / Operation Checks
Plate Selection
Packed Columns
Packed Column Parts
Packing Types / Materials / Properties
Hydrodynamics of Packed Columns & Operation Checks
Comparison between Packed and Tray Columns
Spray Columns
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Columns
Columns
Plates
Perforated
Valve
Sieve Bubble Cap with no Turbo Grid
(floating)
down-comer
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Cross Flow Plate
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Bubble Cap Tray
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Shower Type Plate
liquid
Perforated with
Turbogrid
no down-comer
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Tray Parts
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Design of Plates
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Design of Plates
Procedure
1. Calculate the maximum and minimum vapor and liquid flow-rates, for the turn down ratio
required.
2. Collect, or estimate, the system physical properties.
3. Select a trial plate spacing.
4. Estimate the column diameter, based on flooding considerations.
5. Decide the liquid flow arrangement.
6. Make a trial plate layout: downcomer area, active area, hole area, hole size, weir height.
7. Check the weeping rate, if unsatisfactory return to step 6.
8. Check the plate pressure drop, if too high return to step 6.
9. Check downcomer back-up, if too high return to step 6 or 3.
10. Decide plate layout details: calming zones, unperforated areas. Check hole pitch, if
unsatisfactory return to step 6 .
11. Recalculate the percentage flooding based on chosen column diameter.
12. Check entrainment, if too high return to step 4.
13. Optimize design: repeat steps 3 to 12 to find smallest diameter and plate spacing
acceptable (lowest cost).
14. Finalize design: draw up the plate specification and sketch the layout.
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Design of Plates
a) Tray Spacing
Usually it is between 30 and 90 cm, most used between 50 and 60 cm.
Tray spacing should be smaller for low flow rates and vice versa.
b) Hole active area
Standard hole active area ratio is greater than 10%.
For other ratios apply the following corrections for K1:
hole: active area multiply K1 by
10% or greater 1
8% 0.9
6% 0.8
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Design of Plates
c) Diameter
Souders-Brown equation
l v
v f K1
v
v f : flooding vapor velocity (m/s) based
on the net column cross - sectional
area (without t he down comer)
K1 : constant from graph (with restrictio ns)
l , v : density of liquid and vapor on the plate
c) Diameter
Qv
An
(0.70 to 0.90)v f
Qv : volumetric flow - rate of vapor (gas)
An : net cross sectional area of column Ac Ad
Ac R 2
Ad : cross sectional area of down - comer ( 12% of AC )
Calculate the diameters at top, bottom, above feed and below feed trays. If the
diameters are close, take the largest value as the column diameter. If the
diameters are different, take 2 different values for the top and bottom sections.
The velocity should normally be between 70 to 90% of that which would cause
flooding. For design, a value of 80 to 85% of the flooding velocity should be used.
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Design of Plates
d) Liquid-flow arrangement
The choice of plate type (reverse, single pass
or multiple pass) will depend on the liquid
flow-rate and column diameter. An initial
selection can be made using this figure.
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Design of Plates
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Design of Plates
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Design of Plates
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Design of Plates
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Design of Plates
g) Weeping
The lower limit of the operating range occurs when
liquid starts to leak through the plate holes. This is
known as the weep point. The vapor 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 vapor flow velocity is still well
above the weep point. The minimum design vapor
velocity is given by:
vh min (m/s )
K 2 0.925.4 d h (mm)
v
K 2 : a constant depends on the depth of clear
liquid on the plate which is equal to hw plus
the depth of the crest of liquid over the weir how
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Design of Plates
g) Weeping
23
L
how (mm ) 750 w
l lw
lw : weir length, m
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Design of Plates
g) Weeping
lw 0.77 DC
for 12% Downcomer Area
Vw v
vh ( m / s )
Ah
For weeping, not to happen:
vh vh min
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Design of Plates
h) Pressure drop
It is the sum of 3 terms:
• Dry pressure drop, due to vapor flow through the holes (an orifice loss), hd
• Head of clear liquid on the plate (hw + how)
• Residual loss, hr, which accounts for the energy to form the vapor bubbles
and the fact that on an operating plate the liquid head will not be clear
liquid but a head of "aerated" liquid froth, and the froth density and height
will be different from that of the clear liquid
2
v
hd 51 h v , in mm
ht hd hw how hr
Co l
if the hydraulic gradient
12.5 1000 is significant, add h 2
hr 12.5 mm
l
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h) Pressure drop
• Co = function of plate thickness,
hole diameter, and the hole to
perforated area ratio
• Ah = hole area, the total area of all
the active holes
• Ap = perforated area (including
blanked areas)
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Design of Plates
i) Down-comer back-up
The down-comer area and plate spacing
must be such that the level of the liquid
and froth in the down-comer 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.
i) Down-comer back-up
2
L
hdc 166 wd
l Am
Lwd : liquid flow rate in down - comer
Lwd Lw , if no weeping and single pass
Am Ad (down - comer area) or
Aap (clearance area under downcomer)
whichever is the smaller
Aap hap lw
hap hw (5 to 10 mm)
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Design of Plates
j) Entrainment
It takes place when gas velocity is too
high that entrains liquid droplets to the
upper plate. Due to this, the upper
plate concentration changes so,
separation efficiency decreases.
The amount of liquid carried by the
uprising vapor should be less than
0.1 (kg entrained/kg liq). Excessive
entrainment may lead to flooding!
vn
%flooding
vf
vn : actual velocity based on An
Lw v
FLV
Vw l
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Design of Plates
Ad hb
tr
Lwd l
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Comparison Between Cross-Flow Trays
Capacity
Nearly the same (sieve then valve then bubble-cap)
(flow rates)
Erection &
Easy Difficult Difficult
Maintenance
Plates Efficiency
Columns
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3- Packed Column
Accessories
• Demister
(mist eliminator)
• Liquid Distributer
(Liquid collector)
• Bed Limiter
• Packing Support
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Demister
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Demister
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Distributor / Redistributors
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Bed Limiter
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Packing Support
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Design of Packed-Column
Procedure
1. Select the type and size of packing.
2. Determine the column height required for the specified separation
(H = HTU NTU or H = HETP NTP).
3. Determine the column diameter (capacity), to handle the liquid and
vapor flow rates.
4. Select and design the column internal features: packing support, liquid
distributor, redistributors..
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Type of Packing
HTU G
The principal requirements of a packing are:
• High surface area per unit volume (a) low HTU K og aS
• Large void fraction (high capacity + low pressure drop)
• Strong (don’t break during loading or operation)
• Small density (low dead weight)
• Corrosion resistance
• Easily wetted by liquid
• Low cost
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Type of Packing
Packing
Types
Natural Fabricated
Stones / Unstructured
Coal / Coke Structured
Gravels (Random)
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Random Packing
Lessing Rings
Berl Saddle
Intalox Saddle
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Structured Packing
• The advantage of structured packing over random packing is their low HTU
(typically less than 0.5 m) and low pressure drop (around 10 mm H2O/m)
• The cost of structured packing/m3 will be significantly higher than that of
random packing, but this is offset by their higher efficiency.
• The applications have mainly been in distillation, but structured packing can
also be used in absorption
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Structured Packing
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Packing Material of Construction
Packing
Material
Ceramic
Metallic Wood Plastic
(unglazed)
Steel
Aluminum Silica Alumina
Copper
Adv: strong, small Adv: good wetting, no Adv: good wetting, Adv: light, no
thickness with high void corrosion no corrosion corrosion
fraction Disadv: fragile, large Disadv: low void Disadv: bad
Disadv: corrosion, high thickness with low void fraction, possibility wetting
cost, bad wetting
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fraction of rottenness 47
Installing Packing
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Size of Packing
• In general, the largest size of packing that is suitable for the size of column
should be used, up to 75 mm.
• Small sizes are more expensive than the larger sizes.
• Use of too large packing size in a small column can cause poor liquid
distribution.
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Height of Packing
38 0.6 – 0.75
50 0.75 – 1.0
Column Diameter
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Column Diameter
0 .1
* 2
13.1 V Fp l
l
w
K4
v l v
*
Vw : gas mass flow rate per
unit column cross
sectional area (kg/m 2s)
FP : packing factor (m -1 )
l : liquid viscosity (Pa s)
L v
FLV w
Vw l
From graph get K4
From equation get Vw*,
then calculate the diameter
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H va2
Pdry f g
dp 2
Pdry : dry pressure drop
d p : equivalent diameter of packing
va : actual velocity through packing
f : friction factor
v Q
va s
S
vs : superficia l velocity
: void fraction
134 d p va g
f 2.34 , Re
Re g
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Hydrodynamics of Packing (flooding)
Pwet c Pdry b
Pwet f ( L, type of packing, d p )
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If very low liquid rates have to be used, below FLV = 0.01, the packing wetting
rate should be checked to make sure it is above the minimum recommended by
the packing manufacturer.
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Channeling often occurs in a packed tower. This phenomenon takes place when
the fluid moving down the column moves towards the region of greatest void
space; this occurs at the region near the wall where the packing is not tightly
packed. Thus, liquid redistributors are used (every 3 – 5 m) to redirect the fluid
flow towards the column center.
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Tray Column Vs. Packed Column
Choice of plates or packing
To do this correctly, you need to compare the price of both columns for the same operation.
But following points help in the choice:
1. Plate columns can handle wider range of liquid and gas flow-rates than packed
columns.
2. Packed columns are not suitable for very low liquid rates.
3. Plate columns can be designed with more assurance than packed columns regarding
good liquid distribution, particularly in large columns.
4. It is easier to make cooling in a plate column; coils can be put on the plates.
5. Withdrawal of side-streams from is easier from plate columns.
6. It is easier to clean plate columns; manways can be installed on the plates. But with
small diameters, it may be cheaper to use packing and replace the packing when it is
fouled.
7. For corrosive liquids, a packed column will usually be cheaper than the equivalent
plate column.
8. The liquid hold-up is much lower in a packed column than a plate column.
9. Packed columns are more suitable for handling foaming systems.
10. The pressure drop per equilibrium stage (HETP) can be lower for packing than plates;
and packing should be considered for vacuum columns.
11. Packing should always be considered for small diameter columns (< 0.6 m), where
plates would be difficult to install, and expensive.
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Columns
Columns
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Characteristics of Spray Columns