What makes outside shapes of distillation columns differ from one another? i.e. shape of pre-flash differs from CDU, CDU differs from VDU?
Answers
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06/07/2010
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Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
I concur with the responses by Keith and Ralph.
The key drivers for cross-sectional area are the ACFS,vapors or Am3/sec, vapors and the feasible / comfortable capacity factors of the internals [trays or structured packing or grid packing] and the design margins. Vessel cost and structural considerations favor only a limited number of diameters in any given tower. Sometimes the bottom swage down below the flash zone is internal in crude atmospheric towers.
Best practice designs for crude atmospheric towers often favor high liquid wetting flux pump-arounds using structured packings [as Lesson Learned from complex distillation tower theory / computer modeling]. In these instances, the hot gpm per ft2 or hot Am3/hr-m2 can become an important factor in determining cross-sectional area in conjunction with ACFS, vapors or Am3/sec, vapours.
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05/07/2010
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keith bowers, B and B Consulting, kebowers47@gmail.com
Distillation column cross sectional area necessary to meet design thruput is determined by the volume of vapor and liquid that must be processed on that 'tray'.
A preflash column is sized to allow sufficient disengaging space for the light ends so 'flashed.'
A flashed crude column will typically have a larger mid-section since much of the 'work' of distillation is done in that section of the column. The top end may be either smaller, or larger, depending on operating pressure and the light-ends yield of the design crude. The bottom of the so-called Atmospheric column will be sized to accommodate the work necessary to separate the gas-oil from the reduced crude.
A vacuum distillation column is similarly designed but has a much larger cross section because of the low density of the vapor due to the very low absolute pressure. Often the very bottom section is much smaller because both the vapor content and liquid rate are so much lower that the reduced crude feed and residence time at the typical high temperature must be minimized to avoid thermal degradation that will form deposits in the column and plug it .
Sometime the physical strength requirements are such the very bottom section is too small to support the column and, and a 'skirt' must be added around the bottom section to provide necessary strength
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05/07/2010
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Usually due to vapor rates being much lower in some sections, such as below the flash zone of the atmospheric column and the vacuum column. Save the cost if a smaller diameter is needed in that section. Another example: In a reboiled absorber, the vapor rate in the absorber section, above the feed, is much lower and can have a smaller diameter.
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