Why is the top of a CDU normally wider than its lower body?
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26/03/2018
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AMIT KALYAN CHANDRA, INDIAN OIL CORPORATION LIMITED, chandraak@indianoil.in
1) In a Crude Column, the crude ( around 50%) flashes as it enters the column in the flash zone leading to an increase in vapor volume.
2) A higher cross sectional area at the top is required to accommodate the increased volume of flashed vapor. Higher will be the area less will be the vapor velocity and less will be the chances of entrainment flooding in the top section.
3) The bottom section does not handle high vapor volumes as it deals with reduced crude oil (RCO) that is mostly in liquid phase at the bottom or stripping section of the column.
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25/02/2018
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Ganesh Maturu, Self, maturu.ganesh@gmail.com
In CDU and VDU columns, distillate product is more than 70% and hence vapor-liquid load above feed tray is significantly higher compared to section below feed tray. Required diameter is higher in top section compared to bottom section. In case of strippers, bottom load is higher and hence bottom part is wider compared to top section. These designs are economic designs and saves CAPEX.
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23/02/2018
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Dipankar Phukan, Indian Oil Corporation, dipankarphukan79@yahoo.com
It is because of higher vapor load and lighter density of the products as you move up the column
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23/02/2018
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Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Thermal cracking of Reduced Crude oil (RCO) is possible with heavy Crude processing. Reduced volume of the crude column bottom section can reduce the cracking severity by lowering the residence time of RCO.
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22/02/2018
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
An optimized design for a crude distillation column, with installed cost included in the optimization, will usually have these features:
• Equal diameter from the top tangent line to the bottom of the flash zone.
• A much smaller diameter below the flash zone.
The upper portion is influenced by the liquid and vapor rates through the column. The lower section, which has a relatively very low vapor rate, is sized for a reasonable liquid holdup rate which is needed for smooth control. Even for crudes with high bottoms product rates, the liquid has a much less effect on the required column diameter than does the vapor, which, again, is very low below the flash zone.
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22/02/2018
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Dave Collings, Jacobs, dave.collings@jacobs.com
The CDU distillation tower is not always wider at the top. I would say that most of them are a single diameter. The top of this column is often an area of high reflux (top pumparound reflux). So, liquid traffic is controlling the column diameter. I would say in most columns, vapor traffic determines the diameter, so this is an exception.
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