How do you calculate overflash in a crude distillation column?
Answers
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07/11/2017
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Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
There are two types of overflash, one is measured and other is true overflash. overflash is the liquid leaving bottom tray in wash section divide by total feed rate. Measured overflash is the liquid from bottom tray + entrainment from flash feed while true overflash is excluding the entrained liquid. Generally in atmospheric column liquid from wash section is not measured and sent directly to stripping section internally with down pipe / externally by gravity or pump.
In your plant if it is not measured then you can collect the plant data, make a plant match simulation in any simulator and then you can see the liquid leaving from wash section bottom trays i.e. your overflash.
if you do not want to make a whole column simulation then you can just make a feed stream then put a flash vessel and define the temperature and pressure in flash vessel similar to the flash zone in column. Flash the stream then add a column with 1.5 to 2 theoretical stages and feed the liquid from flash vessel to this column, define stripping steam rate similar in your crude column to this two stage column. add vapor stream from overhead of this two stage column to the vapor stream from flash vessel. Flash vessel vapour rate + vapor rate from two stage column (this is nothing but stripping section of column) - all product rates from the column will give your more or less close overflash rate. Note that here we neglect the liquid coming from wash section to stripping section and not 100% accurate calculation.
If you want to check exact overflash then make a simulatin model with plant data and then you can see the liquid leaving from bottom tray of wash section.
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06/11/2017
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
When the overflash is not metered, it is possible to calculate using the available flow and temperature readings. Draw a heat and material balance around the lower section of the column, being sure to break the feed line, bottom product and overflash stream. Using the available data, write the mass and heat balance equations. Input the available observed data. Calculate the overflash flow rate by difference. I have seen units run from 2% to 22%.
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