5 replies to this topic

[ankur2061]

Dear All,

 

I have a Crude Column where I have a Kero draw-off and Diesel Draw-off through chimney trays. The draw-off feeds the Kero and Diesel stripper via level control valve. No pumparound is provided.

 

There are conflicting opinions on the residence time for the draw-off chimney trays. I have also looked up Kister's book on distillation operations and the subject has been treated somewhat ambiguously without any firm guidelines on the residence time.

 

Some of the opinion I have been getting for the residence time is as follows:

 

1. Since there is no pump-around residence time is not critical.

 

2. A minimum residence time of 1 minute is required irrespective of whether there is a pump-around or draw-off by level control to the stripper under pressure difference

 

3. A minimum residence time of 2-3 minutes is required whether it is a pump-around or a level control draw-off under pressure difference.

 

Assuming that the column is already in existence and the existing configuration provides a residence time of less than 1 minute do I need to modify the column by removing the trays above and increasing the chimney riser height?

 

Is the residence time criteria an absolute must? Will my draw-offs see flow fluctuations due to lower residence time? The operating philosophy is to run the atmospheric column at a fixed throughput (feed rate) and no turn-down or turn-up is envisaged.

 

Under steady state operating conditions and constant feed, are problems anticipated in draw-off due to low residence times?

 

Would appreciate answers from experts in design and operation of crude distillation units.

 

Regards,

Ankur.

 

 

 

 

Edited by ankur2061, 18 March 2015 - 04:26 AM.

[PingPong]

Residence time is not the right word, what sometimes matters is hold-up time or surge time, that is: the time between the high (maximum) and low (minimum) operating levels on the chimney tray. But if the chimney tray is a partial drawoff tray, with an overflow weir that sends excess liquid to the tray below, the liquid level on the chimney is constant, no matter how high the overflow weir is.

 

Whether it all matters in your case can not be determined without a sketch of the layout of the chimney tray and what is above and below it. And what the controls on the main column and on the sidestripper look like.

Edited by PingPong, 18 March 2015 - 04:57 AM.

[Zauberberg]

The minimum liquid level (and thus the volume and thus the residence time) on a partial draw-off tray is the level required to overcome draw-off nozzle cavitation at the maximum draw-off rate of the product. Lieberman recommends this minimum height to be equal or higher to the nozzle diameter + 2 inches. Nozzle cavitation occurs when the static head of liquid on the draw-off tray is insufficient to overcome nozzle entrance head losses plus the head loss in the horizontal section of the draw-off line feeding the stripper. Once when nozzle cavitation occurs, the draw-off limit from the tray is reached and cannot be increased.

 

Residence time is a dynamic parameter which changes if, for example, you increase the cut point of the adjacent product above, and hence is normally not used as a typical design parameter. By increasing Naphtha cut point beyond certain value you can completely dry out the Kerosene draw off tray if you maintain the same draw-off rate of Kerosene. An equivalent example would be if one starts reducing the feed to a 2-phase separator while constant liquid outflow is maintained from the separator. Then the concept of residence time does not have much of a meaning.

 

Adding X inches of liquid above the top of the draw-off nozzle (X is calculated head loss for the maximum draw-off rate, plus extra margin) should be more than sufficient to cover any operating scenario. Why do I say above the nozzle? If you locate the nozzle somewhere in the middle or upper height of the tray sump, then all the liquid below the nozzle does not contribute in developing the required head to push the liquid towards the stripper. In this case you may have 2 hours residence time, but due to nozzle cavitation the product draw-off rate will be limited.

[Bobby Strain]

I always leave things as they are for a revamp. Some, however, keep applying fixes until it no longer is workable.

Bobby

[Zauberberg]

This is very good point. If the draw-off trays used to function trouble-free, there are no reasons to perform any modifications. Unless their operation is impacted with some of the revamp modifications in the column system (e.g. if you are installing Kero pumparound loop).

[Padmakar Katre]

Dear All,

 

I have a Crude Column where I have a Kero draw-off and Diesel Draw-off through chimney trays. The draw-off feeds the Kero and Diesel stripper via level control valve. No pumparound is provided.

 

There are conflicting opinions on the residence time for the draw-off chimney trays. I have also looked up Kister's book on distillation operations and the subject has been treated somewhat ambiguously without any firm guidelines on the residence time.

 

Some of the opinion I have been getting for the residence time is as follows:

 

1. Since there is no pump-around residence time is not critical.

 

2. A minimum residence time of 1 minute is required irrespective of whether there is a pump-around or draw-off by level control to the stripper under pressure difference

 

3. A minimum residence time of 2-3 minutes is required whether it is a pump-around or a level control draw-off under pressure difference.

 

Assuming that the column is already in existence and the existing configuration provides a residence time of less than 1 minute do I need to modify the column by removing the trays above and increasing the chimney riser height?

 

Is the residence time criteria an absolute must? Will my draw-offs see flow fluctuations due to lower residence time? The operating philosophy is to run the atmospheric column at a fixed throughput (feed rate) and no turn-down or turn-up is envisaged.

 

Under steady state operating conditions and constant feed, are problems anticipated in draw-off due to low residence times?

 

Would appreciate answers from experts in design and operation of crude distillation units.

 

Regards,

Ankur.

 

Hi,

If this is an existing system and operating fine, I don't see any reason to apply the residence time criteria and have some modifications. As long as there is auto level control of the chimney tray and even on side strippers I don't foresee any issue in operation. As suggested by Zauberberg you can check Liquid Froude number for the draw-off nozzle to ensure it meets the gravity flow line criteria. If the column is being revamped for higher capacity, draw-off nozzle losses to be critically assessed.

 

Residence time criteria is applied when the draw-off chimney is catering to once thru thermosyphon reboiler and there we need hold up volume to accommodate the duty step changes whereas in your case I presume you side strippers too would be on level control hence I think no issues if you retain the draw-off trays as is.