Continuous Distillation Packed Column: Liquid Level Buffer Sizing at Column Bottom

[plantprowler]

When designing a continuous distillation column is there a design procedure / rule of thumb to select how much of a liquid level one must keep at the column bottom?

This is a vacuum column (10 mmHg abs), in the petrochemicals industry with a forced circulation reboiler. The column has structured packing and the diameter is 900 mm.

Would it be reasonable to select the level to size for (say) 15 minutes of bottom draw off? The required holdup for 15 mins is coming quite small, approx. 300 L which I can get in less than 1 m of liquid level for this column dia.

 

[georgeverghese]

What have you got for the reboiler - hot oil operated kettle or a fired heater ? We used 30minutes for the holdup time in a deC4 column sump for a fired heater forced recirc reboiler - rationale being to enable sufficent liquid in the sump to dissipate the residual heat in the fired heater upon shutdown, with the recirc pumps still running. What would happen in your unit on shutdown ?

 

[plantprowler]

Thanks. Column uses steam heating for reboiler. Under the operating conditions of high vacuum the bottoms temperature should not exceed 120 C.

Shutdown in our unit probably means shut the steam inlet to reboiler & blow away any residual Steam pressure. Then drain all the column material out.

 

[plantprowler]

@george

Just curious: When you write "Column sump" was it just a column bottom end section (cylindrical) of same dia as rest of column but with no plates / packings? Or was it something different.

 

[georgeverghese]

Looks like you are depending on operating procedures to prevent the buildup of high temp vapors in the column from the reboiler as it absorbs the residual heat from the large metal mass in the tube bundle in the kettle reboiler ?

Not clear what blowing away residual steam is to be built into the reboiler design.

We had a reboiler feed sump and reboiler return sump sections in the column bottom sump, separated by a vertical plate that extended to partway down to the low point in the dished end. This sump was of a larger dia than the rest of the column to enable this 30minutes holdup time.

 

[plantprowler]

What is the advantage of separating the reboiler feed & return sections? Is that for the purpose of getting an extra separation stage?

 

[georgeverghese]

The reboiler feed is actually the exit stream from the last stage above, while the reboiler return sump is a mix of the return stream from the reboiler plus some minimal underflow from the reboiler feed.

 

[plantprowler]

Correct. But I'm just trying to visualize what would be the downside if you just eliminated the vertical plate.

 

[georgeverghese]

In a once through reboiler, where the product compartment of the kettle is operated on level control, and the column sump is purely for holding reboiler feed, that would be possible. But usually, there is no pump to transfer liquid across from the column sump to the kettle - level in the column sump is approx the same as that in the kettle heating compartment.