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The Effect of Pressure on Naphtha Stabilizer

The Effect of Pressure on Naphtha Stabilizer

The Effect of Pressure on Naphtha Stabilizer


Since our naphtha stabilizer's condenser is lacking of condensing capacity at higher reflux rates (in order to ensure proper fractionation efficiency), what should be the best way to correct this unpleasant situation? Working on higher pressures allows us to achieve higher condenser duties, but we are limited to 6.5 barG, which is not enough for decent separation. Current working pressure is 5.5 barG.
We have 2 condensers in series; cooling water flow is in series also (outlet from the 2nd condenser goes to the inlet of 1st condenser??). The amount of C5 in LPG goes up to 40%, and amount of C4 in the bottoms is too high (4-10%), which creates problems in downstream naphtha splitter.
Replacing bare tubes with low fin tubes looks like good idea, along with parallel CW flow through condensers. Does anybody have similar experiences and problems in such services?

RE: The Effect of Pressure on Naphtha Stabilizer

Evidently improving the condensers' performance would be a step in the right direction.

Besides, do you have sufficient trays and are these in working conditions ? What type of trays are used ?

One usually uses the Fenske equation (to get the minimum number of theo. trays at total reflux), the Underwood equation (for the minimum reflux for infinite number of trays), as well estimating the trays' efficiency using the Gilliland correlation, for a pre-selected mol fraction of C4's and C5's as keys in the distillate and in the bottom product at the prevailing tower conditions.

Is the feed quality constant ? Do you use on-line analysers ?

Good luck.

RE: The Effect of Pressure on Naphtha Stabilizer


The feed comes from the main CDU column overhead accumulator at 115C, after preheating with stabilizer's bottom product. There are 45 valve trays in naphtha stabilizer (combined single and dual-pass trays, no damages registered during turnaround). Design bottom temperature is 145C, and top temperature 52C. The best result we ever accomplished was at 170C bottom temperature and 48C top temperature, but we were making so much gas, so this kind of operation couldn't be continued for a very long time. Separation efficiency was very good - 3.5% C5 in the overhead product, and 0.5% C4 in the bottoms product. I guess we're just lacking of condensing capacity, and that's all.
I was interested if raising the pressure could help (let's say to 15bar, if equipment could hold the pressure). Then we should be working at higher temperature levels, to achieve higher condenser duties.
But what happens with decreased relative volatility effect? Does it reduce separation efficiency so much, and "kills" benefits of higher condensing capacity at higher pressures?
According to the vapour pressure charts, it has very little influence on C4/C5 and C5/C6 splitting.

What modifications should we make on condensers? Is it worthy, or we need to install another one, upstream or downstream of the existing?

RE: The Effect of Pressure on Naphtha Stabilizer

It might be worthwhile to run an estimate of the tower's fractionating performance on the actual feed composition and desired (top/bottom) results following D.S.J. Jones' Elements of Petroleum Processing -Wiley- Chapter 8: "Straight-Run Light Ends Units" to evaluate what can be expected from a rise in pressure and a change in top/bottom temperatures.

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