INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

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

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

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

(OP)
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.

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

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 ?

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

(OP)
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.

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

(OP)
@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.

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

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.

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

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

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

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.

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

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

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

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close