Liquid holdup in distillation column

[July]

I am reviewing the column liquid holdup requirements for different services. What is the criteria governing liquid holdup in an atmospheric distillation column upstream of a furnace? What are acceptable minimum residence times?

What residence times are used in a vacuum column?

I look forward to some guidance here.

 

[djack77494]

I have never heard of a concern for residence time inside a distillation column. Except once, but that involved reactive distillation where the concern was to have enough residence time for the reaction to occur. I do not think there would ever be a concern about the vapor phase residence time. There may be some control issues with the liquid phase that would suggest maintaining a level on the trays sufficient to achieve a certain residence time. Is that what you're talking about?
Doug

 

[StoneCold]

July
I have seen 5 minutes used for the feed to a column. BUT it really depends on a lot of things. How big is the feed rate to the column? How steady is the feed rate? What is the shutdown criteria for the furnace? If the feed rate is large and I don't want to spend the money for a 20,000 gallon tank then I might shorten the time and try to deal with variations in the feed rate to the column. If I was concerned about the feed to the furnace down stream I would focus on the size of the sump in the bottom of the column and try to put my hold up there. Especially if most of my product is going out the overheads. Does that make sense? I think that you are going to have to analyze the flowrates, control schemes and safety issues and come up with your own answer. Holdup, while it seems trivial can make or brake the unit you are designing, don't just go with a rule of thumb on this unless you don't mind revamping the unit.

Good luck

StoneCold

 

[Montemayor]

July:

I sense that you "feel" or have been advised that there is an importance to attribute to "residence time" in a distillation column. While the time a quantity of liquid resides within a column may be of interest for some reason or other, the important thing to bear in mind is that for the distillation to succeed in effecting a separation it must have:

1. a constant and steady feed;
2. a positive liquid seal on each tray (assuming you are using trays - whether bubble cap, sieve, or valve) and on every downcomer; otherwise, you don't have a separation stage where you maintain a phase equilibria;
3. a liquid reservoir (or "sump") where you maintain a liquid level for postive control operations and also to serve as a source for feeding a reboiler requirement;
4. a certain amount of liquid reflux is also involved in the external piping and attached overheads condenser and reflux drum.

If you have a packed column instead of trays, you will have a certain amount of liquid hold-up ("wetting) amount within the packed bed. This usually is less than the amount required to maintain trays - although you could also have a substantial amount dedicated to intermediate, re-distribution trays.

Note that you have been very specific in your question and you haven't furnished specific data related to your question. Nevertheless, the important point is that the liquid holdup and required to carry out the operation is determined as per the type of operation and equipment employed. That is the most important thing to consider because otherwise the distillation operation won't work. Therefore, the "residence time" - as Doug infers - will vary for a variety of reasons. If you are looking for surge capacity due to downstream process needs, the usual economic answer is to install an appropriate surge tank on the bottoms product or the overheads product - whichever it is that you are concerned with.

In my experience, there is no such thing as "acceptable minimum residence times" for distillation columns - unless, as Doug points out, we are dealing with reactive distillation (which I assume is not the case since you don't mention it).

I hope these comments help you out.

 

[25362]

Henry Z. Kister in his Distillation -Operation- (McGraw-Hill), Chapter 4, discusses the subject of residence times in the bottom section and other column outlets in detail.

 

[July]

I regret I seem to have not made my question clear. The preflash column bottoms is fed to a furnace and from there to the atmospheric distillation column. The atmospheric column bottoms is fed to a Vacuum unit furnace. My query was about the minimum residence time requirements for the column bottoms liquid, since this will have an impact on the column size.

In the case of a vacuum column, I understand that there is a need to minimise the residence time for the column bottoms liquid from cracking considerations and that this is a function of the bottom temperature.

I thank you all for your response. 25362, I shall look up Kister; thanks for the suggestion.

 

[25362]

Some thoughts:

1. General

Volumes between the HLL and LLL marks are typically -no rules here- designed for residence times in the range of 5 to 15 minutes. These would enable protection of downstream equipment from upsets, and even provide some time for the operators to take corrective action, although hold-time calculations aren't based on the assumption that personnel will promptly respond to level alarms.

Feedforward systems are effectively used to achieve heaters' desired exit temperatures under close control even against sudden load changes, as well as to protect the tubes from failure.

2. Vacuum towers

Precisely because of thermal cracking in the bottom section of vacuum towers the liquid residence time is shortened to only a few minutes. Another step usually used to prevent thermal cracking in the bottom boot and to supress vaporization at the suction of the bottom pump is to provide a circulating quenching stream. Note that the more paraffinic the feed, the lower the cracking temperature.

A vortex breaker, and a suitable suction strainer (to catch coke particles) are provided to protect bottoms' draw-off pumps from losing suction. Besides, to improve the NPSHA of the bottoms-pumps, the overflow of wash vacuum gas oil, sometimes called trim gas oil (TGO), into the bottom sump should be minimized.

 

[johngerhart]

look at the weir height at design and estimate liquid holdup accordingly