Mechanical Design of Distillation Column
MECHANICAL DESIGN OF DISTILLATION COLUMN Given data: Working pressure = 84 kPa = 0.084 N/mm2 Inside Diameter (Di) = 649
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- Sumant Pilgaonkar
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MECHANICAL DESIGN OF DISTILLATION COLUMN Given data: Working pressure = 84 kPa = 0.084 N/mm2 Inside Diameter (Di) = 649 mm Material of construction: Stainless Steel Allowable Stress (f) = 1350 MPa Top chamber and bottom chamber height = 0.5 m Density of material = 8030 kg/m3 Number of trays = 9 Plate spacing = 24 inches = 0.6 m Hole pitch (rectangular) Diameter = 15 mm Tray thickness = 5 mm Design Pressure (P) = 0.084 x 1.1 = 0.0924 N/mm2 1. Shell design ( )
(
)
(
) (
( ) Hence Do = 361 mm 2. Head design A hemispherical head is selected: ( )
)
( ) Hence
= 6 mm
3. Shell thickness at different heights The thickness of the shell calculated as above is satisfactory up to a certain distance from the top of the shell. In order to determine the distance x, different types of stresses acting on the shell are taken into consideration. a) Axial stress due to pressure: (
)
(
)
b) Stresses due to dead loads: i.
Compressive stress due to weight of shell up to a distance x (
) (
)
( )
( )
ii.
( )
Compressive stress due to weight of insulation at height x ( ) (
) ( )
(
)
, ( ) (
) ( ) (
iii.
)
Compressive stress due to weight of the liquid in the column up to a height x (
) (
)
(
)
( )
iv.
)
Stress due to weight of attachments ( (
)
) (
) (
)
( (
v.
(
)
(
)
Total compressive dead weight stress upto height x: (
)
(
)
c) Stress due to wind load(fw) (
)
(Assuming Pw = 1300 N/mm2) Determination of value of x: We have, (
)
(
)
(
)
)
Where,
(
)
(
)
Hence the thickness of 6 mm will be same for the entire height. Design of supports: Columns of diameters greater than 600 mm are usually mounted on the skirt support. Hence, we require skirt support. A. Skirt Design The cylindrical shell of the skirt is designed for the combination of the stresses due to vessel dead weight, wind load and seismic load. The skirt thickness is uniform and is designed to withstand the maximum values of tensile or compressive stresses. (
(
)
(
W (
)
H k p w H D o 2 2 π 2 t sk (
)
)
)
3.82 tSK (
(
)
(
)
(
3.51 tSK
)
)
We use a thickness of 2 mm + CA.
B. Nozzle design: The nozzles provided are as follows: Nozzle
Number
Size in mm
Liquid inlet at top
1
200
Liquid outlet at bottom
1
200
Vapor Outlet at top
1
200
Manhole
5
500
The design procedure for all nozzles is the same For nozzle on torispherical head
Consider the 200 mm diameter nozzle (
P di 2fJ - P
)
0.0949 ´ 200 2 ´1350 ´ 0.85 - 0.0949
Actual thickness taken = 6 mm
Area available for compensation i)
( Where,
(
ii) (
)
Compensation need not be provided.
)
)
(
)
C. Tray design: A column of this type used in the industry, is made up of a shell, fabricated from sheets by welding. It has no flanged joints. This requires a special design of trays, which can be easily dismantled and removed. Such trays are sectional trays. These trays are made if stainless steel, with a thickness of 2 mm and with holes of 5 mm diameter. Downtakes and weirs are bolted at the appropriate positions. The downcomers on the even number of trays are located at the ends. The shape of the downcomer is segmental with a size of 60 mm x 350 mm. The downcomers on the odd number trays are at the centre. The shape of the downcomer is rectangular with a size 60 mm x 520 mm.