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10/09/2016
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S Banik, Centre for High Technology, sbdr@rediffmail.com
The subject has been discussed extensively. Vaporization of crude is always higher than the overhead and side products. This excess vaporization can be called as Overflash. It is normally kept at 3-6 LV%.Wash section de-superheats vapour and entrained liquid, de-entrain liquid contaminants from vapour , provides some fractionation. Too little overflash may lead to coking of wash section. Ideally process design and hardware design are integrated to have the best results.
Measurement of overflash is difficult but worth attempting as better accuracy of column simulation can be achieved. An overflash measuring device was developed and successfully implemented in an Indian refinery. Using the measured value of overflash it was observed that real life simulation could be achieved.
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30/05/2016
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tanmay patle patle, borl, patletenendra@gmail.com
Measure of flash of rco in distillation column
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19/05/2016
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Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
There are two kinds of overflash one is true overflash and another is measured overflash. True overflash is a actual liquid coming from the bottom tray of wash section and measured overflash is a liquid from bottom tray + the entrainment (entrainment of liquid from feed to flash zone chimney tray, this can be from 0.1% to max 2% for good design but it can be even up to 10% for worst feed inlet and feed nozzle design).
Overflash has a significant impact on your atmospheric gas oil 95% or yield, too low overflash means less liquid to wash section trays and poor fractionation. Some times it may happen that there is not enough liquid on trays and tray may operate in spray regime and you may have flooding, this will lead to loss of gas oil product. Generally in wash section you have poor tray efficiency compared with top section and reason is that there you have low reflux (indirectly overflash).
Also as i mentioned above there will be some entrainment from the feed to wash section, if you do not put enough liquid on wash section tray then this entrainment can be get carried to gas oil draw tray and you may pollute your gas oil product.
Higher overflash means you put lot more energy than required in furnace, it is only loss of energy but it will not deteriorate your product quality. incase if your trays are designed with flooding approaching to max limit and then if you try to increase the overflash by incrasing the COT or stripping steam then there is risk of flooding the trays, because more vaporization will create more vapor liquid traffic to wash section and may lead to flooding.
Generally 3-5% overflash are good design but i have seen in few cases it is up to 15%.
in operating point if you are not loosing the gas oil then you can reduce the overflash quantity for tray column because with trays you have minimal risk of fouling but if it is a packing column then you need certain amount of liquid to avoid fouling and keep the packing wet.
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11/05/2016
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Your guess is correct. The overflash is defined as the liquid flow from the lowest tray above the flash zone into the flash zone. With a proper design, the overflash flow rate can be measured. Otherwise, it can be calculated, but not as accurately as measuring it. There is a specific quantity of heat to be contained in the feed stream to the column. That total amount is equal to that required to vaporize all products above the flash zone plus the sensible heat in the bottoms product. However, if only that specific amount of heat is in the column feed stream, the trays between the flash zone and the lowest side draw would be too dry and would not fractionate as needed. At that condition, the flow of liquid from the lowest tray above the flash zone down into the flash zone would be zero. Therefore, the heat input to the feed stream is deliberately made to be that amount plus a little more, called the overflash. I have seen overflash liquid flow as high as 30% of the column feed. That constitutes a waste of fuel and column vapor and liquid handling capacity. The most common % overflash used in design is 5% of the column feed.
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10/05/2016
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Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Overflash is an internal reflux stream that returns excess material lifted to the Gas Oil section. However the purpose is not about providing liquid flow exactly. It helps in optimizing the Furnace Coil outlet temperature (COT). Too low COT may result in lower recovery of Gas oil and lighter material in the VDU feed. Too high COT will take heavy material up to Gas Oil section (asphaltenes / metals) and lead to quality issues (color / 90 - 95% recovery point etc.,). Low recovery Gas Oil in ADU / CDU overloads the VDU overhead system. Typically furnace outlet temperature can be increased to the level of obtaining 4-6 Vol% (product with drawn) of overflash flow. Overflash lines tend to get choke up with start up high levels in the bottom, higher end point material if the lines are not steam traced / insulated. It is important that this line is kept live all the time. Measuring overflash flow will be a measure of energy optimization.
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09/05/2016
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Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
In atmospheric distillation column, overflash is slip stream from last drawn product (Gas Oil) to flash zone. This is to ensure that the GO draw off tray is not made dry and reduced crude (RCO) vapor doesn't enter into GO zone.
Higher rate of over flash means higher coil outlet temperature, that is more energy consuming. An optimum of 3 to 6% of crude charge is being maintained as overflash depends upon the unit requirement.
Overflash is the indication of drawing gas oil product with out affecting its end point, thus maintain its important property of recovery percentage.
Strictly speaking, consider a crude feed which is homogeneous at all levels of its storage tank and absolutely there is no variation in heater outlet temperature and draw product exactly to its yield potential, then zero overflash also will not affect product quality. But this in ideal situation which never comes in practical life.
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09/05/2016
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Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Overflash in the atmospheric column serves the same purpose as overflash in the vacuum column. You must flash more liquid in the crude than you can recover in the column. The overflash is refluxed onto trays or packing above the flash zone, to knock out entrained crude oil and wash it back down into the bottom of the column.
The advantage of overflash is that it keeps contaminants such as metals and asphaltenes out of light products like diesel or atmospheric gas oil. The disadvantage is that it you lose yield. In units that have a vacuum tower downstream, the yield loss is not always a big deal as the liquids can still be recovered in the vacuum column. In constrained vacuum units excessive overflash in the crude column will result in increased vacuum residue yield.
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