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Distillation Of Naphtha

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#1 sudheer


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Posted 23 June 2007 - 12:24 AM

For cutting naphtha into three streams with boiling point range C5-90°C, 90°C - 110°C and 110°C - 150°C we are having two columns in series. First column splits naphtha into C5-90°C and 90°C -150°C and second column is supposed to split first column bottom to 90°C - 110°C and 110° - 150°C. The first column bottom product D86 IBP is 80°C and D86-IBP of second column top material is found to be 71°C. Can anyone explain how this is possible. Please note that D86 - 5% of first column top and D86- 95% of first column bottom are exactly same at 90°C. Thanks
Sudheer Pai

#2 Zauberberg


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Posted 23 June 2007 - 01:05 AM

Hello Sudheer,

Actually, I think there is nothing wrong with your plant data. When fractionating multicomponent systems (i.e. naphtha), each component shows different behaviour in a hydrocarbon mixture - compared to single-component distillation. It is also useful to remember that IBP of 1st splitter bottoms product and 2nd splitter overhead product cannot be the same, due to different volumes/fractions of hydrocarbons in these streams (=different compositions). You can make a picture of this difference by imagining what could be the IBP of full-range diesel cut (200-350C), and what is the IBP for D-1 cut (200-250C). The temperatures at which first droplets from both samples evaporate and condense certainly cannot be the same.

To emphasize what I have just said, look at the CDU gas plant, consisting of naphtha stabilizer and naphtha splitter:

- Naphtha stabilizer bottoms (naphtha splitter feed) D86: 50/175 (IBP/FBP)
- Downstream naphtha splitter overhead product (light naphtha) D86: 25/60 (IBP/FBP)
- Naphtha splitter bottoms product (heavy naphtha) D86: 80/177 (IBP/FBP)

Now, can you see the difference in both light naphtha IBP (compared to stabilized naphtha IBP - 50 vs 25) and heavy naphtha end-point (compared to stabilized naphtha end-point, 175 vs 177)? When columns are running efficiently, these differences come only from different fractional compositions of given streams.


#3 sudheer


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Posted 23 June 2007 - 06:11 AM

Thank you very much for the reply. To give more information on this let me give you today's results
1st column top - IBP - 30°, 5% - 39°, 50% - 52°, 95% - 90°, FBP-106° ( Flow - 60cu.m./hr)
1st column bottom - IBP - 80°, 5% - 95°, 50% - 115°C, 95%-146° and FBP - 153° ( Flow - 150 cu.m./hr)
For 2nd column
Overhead - IBP - 71°, 5% - 78°, 50% - 87°, 95%- 113° and FBP - 120° ( Flow - 30 cu.m/hr)
Bottom - IBP - 100°, 5% - 111°, 50% - 124°C , 95% -148° and FBP - 153° ( Flow - 120 cu.m./hr)
( All in deg.C)
Whether we can do something more to improve the performance of column so that cuts can be nearer.

#4 Zauberberg


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Posted 24 June 2007 - 07:56 AM

Hi again Sudheer,

Before you make an operational improvement of existing fractionators, you should check the following data:

1) Design VS actual feed rate
2) Design VS actual feed composition
3) Calculate actual % of flooding and actual % of downcomer backup across the column(s)
4) Compare actual reboiler and condenser duty VS maximum achievable duties
5) Check system pumps capacities

When you check these data, you can try to push more reboiler heat into the system, or decrease operating pressure if there is additional (spare) condensing capacity. If trays are operating below the flood point, there should be an improvement in fractionation efficiency. The idea is to put trays in higher liquid/vapor flows regime - of course, if there is sufficient margin regarding column hydraulics and heat input/heat removal capacities.


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