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MECHANICAL DESIGN
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6.0 Mechanical Design
As per the British Standards, a distillation column is classified as pressurised vessels because it consists
of a ‘shell' which experiences internal pressure. In this regards, the consideration of the mechanical
design of the column is very important. The mechanical design, in this case, entails the mechanical
specifications like the thickness of the shell, skirt design, and weight of the plates. These specifications
are captured in the next section together with the mechanical drawings.
6.1 Construction material
Several factors should be put into consideration when choosing the suitable material for the
construction of the distillation column. Some of these factors are; corrosion resistance, operating
conditions, the simplicity of fabrication, and cost. Corrosion resistance is the most vital factor because
it is necessary for the equipment's operation for a long period, contributes to a low operational cost
during the plant’s life, and less maintenance and replacement. The summary of the material selected
for the construction of the components of a column is provided in table 14 below (Atlas Steels, 2000).
Table 1 – Selection of material for the processing equipment
Equipment
Construction
Why it was selected
material
Shell
Carbon Steel coated
Minimal corrosion occurs in the shell due to the presence
with Stainless Steel
of a small amount of chlorine ions. Carbon steel is a cost-
316 lining
effective material, but it is not corrosion resistant. Coating
with stainless improves corrosion resistance, especially at
high temperatures.
Trays
Stainless Steel 316
Increased resistance to corrosion that can be caused by
chloride ion solutions presents due to the presence of
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Process
some hydrogen chloride. High strength particularly at
Piping
high temperatures as compared to 304. Stainless steel 316
can be molded easily, hence, form specific trays. It has a
yield and minimum tensile strength of 205 and 515 MPa
respectively.
Utility
Stainless Steel 304
Piping
Highly resistant to corrosion. It's surface finishes are
smooth to minimize roughness and fouling in pipes. SS304 is also affordable as compared to stainless steel 304.
6.2 Column insulation
Columns should be insulated to prevent the loss of heat to the surrounding environment and to promote
personal safety. This is because metals can cause burns at high temperatures and more heat in the
surrounding vicinity. The table below indicates three probable options deliberated as far as the
insulation material is concerned (EngineeringToolBox (2), N.D.):
Table 2 - Insulation materials for the distillation column
Insulation material
Thermal conductivity (Wm1K-1) Density (kgm3)
Maximum
Temperature
(°C)
Mineral wool (Stone)
0.043
130
850
Calcium silicate (Ca2O4Si)
0.058
190-260
800
Magnesia (MgO)
0.058
180-220
315
Mineral wool is a preferred insulation material because it has low density, lower thermal conductivity,
and can be operated at high temperature. This is very important in case the temperature in the column
rises significantly. A lower thermal conductivity prevents loss of heat to the surrounding environment
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and enhances better insulation. The selection of 75 mm as an insulation thickness is meant to greatly
minimize reduce the loss of heat to the surrounding environment.
6.3 Manholes
A column will consist of two manholes, one clo...