4 replies to this topic

[jprocess]

Dear All,
When evaluating a condensate stabilisation unit, we found that the total feed flow rate in summer case is only 14% of winter case!
So this very low turndown may cause some problems in terms of condensate products specifications like RVP, transient column traffic and poor separation.
Now my questions:
1. What are the typical values for column turndown?
2. How to cope with probable problems in summer case? An option to increase the turndown value can be recycling a part of vapor and liquid products from stabilser overhead and bottom to the inlet of unit. But as we checked with simulation following this option we should recycle around 70% of products to reach a turndown of 30% and also we may not meet the spec value for condensate RVP.

Your valuable comments are appreciated.

[Zauberberg]

The largest turndown I have ever seen is 25-30% of design capacity. You will hardly get anything better than that, even with the special type of column internals (which I doubt it is the case here). Also, you have to bear in mind that reboilers can reach their turndown point much before the column does. Especially recirculating thermosyphons and once-through configurations.

Depending on the type of service, internals, column configuration etc., turndowns can be as low as 85% and as high as 25-30% of maximum throughput. Some tray manufacturers claim it is possible to have even 10% turndown, but I wouldn't bet on that one.

Few months ago I was working on a project similar to yours (actual tower capacity was less than 10% of design), and the problem was similar: inability to meet condensate RVP. Tower pressure was unusually high (28barg), while reboiler duty was only 68-72% of design value. The problem was solved by reducing the tower pressure to 23barg, at the expense of loosing certain amount of condensate in the overhead (gas) product due to high vapor-blowing hydraulic regime inside the tower. Recycling product is not a true option: flowrates required are very high (you are recycling something that will not contribute a lot to the vapor-liquid traffic inside the column). And you'll have problems with recycling vapor since you will need a special inlet device for two-phase feed, or separate inlet must be provided for vapor/gas inlet stream.

I think you have to conduct very comprehensive test-run of the unit, and see how the whole plant works in different regimes. At this moment, it is hard to give you anything more than general conclusions and guidelines.

Best of luck,

[eilpar]

I appreciate Zauberberg's recommendations. There are options depending on how long you would like to operate at low capacity and accept the deviations in the product quality.

For one of our projects the study report suggested to have a smaller column for the low capacity operation as the duration was about 6 months.

In another project the column manufacturer recommended only 40% turn down; but the operation wanted about 35%. As the difference was not too large and the duration was short it was recommeneded to increase the RR to make up for internal traffic.

Both these options are expensive and may have impact on the product quality. Hence a detailed study may be called for.

All the best

PAR

[jprocess]

Dear Dejan,
Thanks a lot for your valuable comments.

Also, you have to bear in mind that reboilers can reach their turndown point much before the column does. Especially recirculating thermosyphons and once-through configurations.

May I know why?

Few months ago I was working on a project similar to yours (actual tower capacity was less than 10% of design), and the problem was similar: inability to meet condensate RVP. Tower pressure was unusually high (28barg), while reboiler duty was only 68-72% of design value. The problem was solved by reducing the tower pressure to 23barg, at the expense of loosing certain amount of condensate in the overhead (gas) product due to high vapor-blowing hydraulic regime inside the tower.

In our case the operating pressure is around 10 barg.

Recycling product is not a true option: flowrates required are very high (you are recycling something that will not contribute a lot to the vapor-liquid traffic inside the column).

Could you please explain me more?

And you'll have problems with recycling vapor since you will need a special inlet device for two-phase feed, or separate inlet must be provided for vapor/gas inlet stream.

There is a three phase separator at the inlet of unit.

Thanks again.

[jprocess]

Dear,
Thanks a lot for your valuable comments.

For one of our projects the study report suggested to have a smaller column for the low capacity operation as the duration was about 6 months.

This option is not feasible for us!

In another project the column manufacturer recommended only 40% turn down; but the operation wanted about 35%. As the difference was not too large and the duration was short it was recommeneded to increase the RR to make up for internal traffic.

But this one can be studied.

Thanks again.