Flooding in Crude Distillation column

[thiru4vino]

Dear Participants!

We have a problem in our atmospheric column. During rainy seasons, crude change over from Lighter crude to heavier crude, we are observing top temperature is higher than our heavy naphtha draw temperature. There are 4 trays between them.

We have a heavy naphtha pump around that may come down in qty during change over to heavier crudes. The return temperature of Pump around from the pre-heat circuit varies from 92 - 75 Deg.C depends on the flow.

This pump around returns to the column at the top. should this be a problem?

As a consequence to this temeperature difference, our heavy naphtha level comes down, and we suspect water condensing at the heavy naphtha tray area.

And we are forced to give approximately 1 ton/hr of stripping steam to the stripper from which heavy naphtha is yielded at the rate of 15 - 25 tons /hour..

our naphtha pump around pumps also face frequent seal failures. We are contemplating that steam is getting condensed at the HN tray area from where both HN to stripper and Naphtha pump around were taken off. This will come to the pump and causing seal failure...

I request the participants help me to solve the problem by discussing their experience..

thanx in advance,

A.thirumaran

 

[25362]

Question: do the rainy season, and the change over to a heavier crude, affect the column's top pressure ? What are the "normal" top temperature and pressure ?

You have already noticed that (liquid) water presence in the column should be reduced to a minimum. Have you checked the HN pumparound stream for water content ?

Besides enhancing corrosion, water settling on the top trays may emulsify with the internal HN reflux when stirred up by the upflowing vapors.

Emulsions take longer to separate from gas as clear liquids, bringing about premature downcomer backup and flooding.

Although I'm many years apart from my connection with toppers, I'd say you try to increase the heat load on a lower pumparound to reduce vapor traffic upwards. This may ease the function of the top trays alleviating the flooding causes. Anyway, I'm sure you are trying all possible ways to supress flooding.

Good luck.

 

[thiru4vino]

Dear 25362!

Thanks for ur reply.

The normal temp is around 120 Deg C and pressure 1.0 Kg/cm2(g).

Both HN pump around and HN yield are taken from the same tray. Can we take it for granted that water presence in HN Pumparound is confirmed as we see water in our HN samples. But we take HN sample after stripper, where stripping steam is sent in.

If we reduce vapor load upwards, should not the temperature at top come down and condensation of water increase???

expecting ur's and others response...

thanks again

chillboy

 

[25362]

You should do whatever possible to keep the tower top temperature at least 5^oC above water's dew point.

You are right: the temperature at the top is a result of a thermal balance related to the heat losses due to rain, the circulating reflux and the upcoming vapors.

But water condensation is the result of reaching the dew point of water.

If the mol ratio of water vapors at the top is, say, 0.85 and the total pressure is 2 bar, the partial pressure would be 1.7 bar which corresponds to a dew point of 115^oC, which is lower than the top temp. of 120 deg C under normal conditions; no water condensing in the tower.

In short: you should reduce also the amount of steam reaching the top at the same time when the hydrocarbon vapors are being temporarily reduced, i/o to maintain a lower water partial pressure corresponding to the T,P conditions in the rainy season with heavier crude oils.

As an ulterior measure install a water trap-out pan on the HN removal tray leading to an external drum where decanted water can be removed.

 

[25362]

As an addendum, if it is at all possible, you could reduce a bit the tower top pressure, i/o to bring the water partial pressure down, as explained above.

 

[thiru4vino]

Dear 25362!

I also observed that when this kinda flooding occurs, the heavy naphtha pump around return temperature also comes down below 80 Deg C and once it returns at the top, it reduces the temperature further to aggravate the problem.

This is like circumreference, we cannot reduce heavy naphtha pump around, as it will lead to even lesser return temperature and thus increase the flooding. We cannot increase also since because of water condensing at this point, heavy naphtha tray condensation comes down and tray will be of water and the pump's amperage and flow starts hunting once we try increase the flow.

During the above said situations, what I normally do is
increase the top temperature
Cut off HN yield and Stripping steam to HN stripper
Run the HN pump around at normal rate provided the amperage is within limits.

It takes atleast 1 - 2 hours to stabilise and smoothen depends on the conditions of crude change over and or rains..

thanks for ur response,

if u have further inputs, u r really welcome...

thanks to u..

Cheers

atm