Distillation Column 5

[sibongile]

hie
thank you in advance for your help

we have a distillation column that seperates benzene as the top product and a mixture of toluene,xylene and naphthalene residues as bottoms product. the column has 60 valve trays and uses vertical thermosyphon reboiler.

we strongly suspect there is flooding in the column because of the high DP which we get. it is difficult to control the top temperature and hence to get the right spec for the top product.

could this be flooding? and can the problem be solved without opening up the column.

thank you in advance

sibongile.

 

[katmar]

Yebo Sibongile,

Yes, it sounds very much like flooding. The first thing to do is to check the current operating conditions (flows, concentrations, temperatures, pressures) against the original design data.

If it is simple flooding you should find that the pressure drop suddenly increases as you increase the throughput from some initial low value.

If it is due to a mechanical problem (collapsed tray or blocked downcomer) the pressure gradient will be gentle, except in the region of the damaged tray. Of course, you will only be able to see this if you have several points where you can measure the pressure.

There are companies that can scan a column in operation using radioactive sources to see where the flooding is happening. A local tray supplier should be able to refer you to someone who does this, if they do not do it themselves.

The book by Kister (I forget its exact title) has very good tips on trouble shooting columns.

Good luck
Katmar

 

[tkdhwjd]

Simply plot reflux rate vs tower dP to see if the dP exponentially takes off as you start losing control of the overhead cut temperature. If so, you can safely bet that the flooding is causing the upset. You can verify it with Gamma scan. It not only tells you a presence of flooding, but it also locates problemetic trays (if the liquid stacking is due to localized fouling). You can also plot reflux rate and dP over past several years to determine the fouling progress/rate.

 

[alsayed]

If it's indeed flooding then reducing the feed rate will eliminate this. Just reduce the unit feed rate in steps and observe the column operation and Dp the flooding should stop and column operation will improve. Column scans can be expensive.

It sounds you have a downcomer flooding (Backup). This happens when your DP across the tray which calculates to the liquid level in the downcomer is as high or very close to your tray spacing, this due to high reflux rate (liquid loads). To rectify this look at you reflux rate historically, column dp, and the O/H cooling system performance. Also if your reboil ratio is high.

If you have a test-run data its better to find the cause of the flooding using simulation (Hysys, PRO II), high vapor/liquid traffic zones are likely the flooding place (if your trays are all the same design and column diameter don’t vary). Check if you can alter your column operation conditions using the simulation to reduce the column liquid and vapor loading and eliminate or reduce the flooding.

If your tray vendor still exist then just ask him for a tray evaluation (be careful when dealing with tray vendors they will tray to convince you to replace all your trays if your operating above the design limit)

 

[Yordif]

The two most popular scanning companies are Synetix Services(TracerCo) and Tru-Tec. Both have a lot of experince in tower scans and flooding problems.