5 replies to this topic

Hi,
I have some questions. What is the main point should i consider when choosing the type of distillation column?

I need to design a pilot scale plant and i need to separate fatty alcohol from all those ester and hydrocarbon. My distillation column is operate under vacuum condition.

According to what i had read, packed column is more preferably for vacuum distillation column and for small scale. So,which one should i choose?

If i want to design packed column, where can i find the related information?

[Art Montemayor]

1) There are several main points to consider when selecting the type of internals in a distillation column:

a) If the fluid is dirty or contains contaminants, a tray (not bubble) type is preferred;
b) If a viscous fluid is distilled, a tray type is usually preferred;
c) If definite and specific side streams are required, then a tray type is selected;
d) If a low pressure drop is desirable, packing is selected;
e) If personnel entry is to be minimized (because of the inherent dangers in confined spaces), packing is desirable;
f) If corrosion of internal is a concern, then packing is desirable;
h) If total capital and maintenance costs are a concern, then packing is usually selected.
i) If total, overall height of tower is a concern, then packing (structured) may be a solution.
j) If the diameter of a column is less than what is needed to weld or install trays, the practical answer is to select a packing.

2. You should select the internals that make sense to you – using the guidelines above and your engineering training as to common sense. Apply the common sense as it relates to your Scope of Work. Study well all the constraints, restrictions, and hazards you know you have to avoid. Apply all the means to achieve the goals and targets you are assigned to reach. This is your decision – not anyone else’s – since only you know all the Scope of Work.

3. To design a packed column, you should apply all the teachings and instructions received in your Unit Operations course which you surely have taken by now (or are in the process of taking). I have to assume you are a Chemical Engineering Student and, as such, this is a silly question and no further discussion is needed on this item.

[fallah]

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1) There are several main points to consider when selecting the type of internals in a distillation column:

a) If the fluid is dirty or contains contaminants, a tray (not bubble) type is preferred;
If a viscous fluid is distilled, a tray type is usually preferred;
c) If definite and specific side streams are required, then a tray type is selected;
d) If a low pressure drop is desirable, packing is selected;
e) If personnel entry is to be minimized (because of the inherent dangers in confined spaces), packing is desirable;
f) If corrosion of internal is a concern, then packing is desirable;
h) If total capital and maintenance costs are a concern, then packing is usually selected.
i) If total, overall height of tower is a concern, then packing (structured) may be a solution.
j) If the diameter of a column is less than what is needed to weld or install trays, the practical answer is to select a packing.

Dear Art
Let me to add one additional point:
k) If due to sensitivity of fluid to heat , it is needed to have lower hold-up in the tower.
Regards

[Zauberberg]

Packed columns are favorable for vacuum applications, since they have much lower pressure drop compared to trayed towers. This means that you can go for cheaper and less robust vacuum system in order to obtain the same pressure profile across the tower, or you can go for higher product purity with the same tower dimensions due to increased relative volatilities at reduced pressures. However, packed towers have some unique drawbacks, such are: sensitivity to liquid and vapor distribution, pressure surges etc.

I'd suggest you to visit Karl Kolmetz' website and look for technical articles. I am sure you will find many interesting references there.

Best of luck,

http://www.kolmetz.c...l-articles.html

[Art Montemayor]

Fallah:

It is a good point that you bring up. My listing is by no means complete. I hope that you and others continue to add to the listed reasoning and guidelines - especially from personal experience.

All students:
Please note that I have only given guidelines - and these based on my experience. Other contributors to this Forum will hopefully always add their knowledge and experience.

Never - I repeat NEVER - take any one person's guidelines or recommendations (including my own) without taking the time to analyze the experience level and the knowldege behind the recommendation. To these factors, always add the value of rational, logical, common sense. This is the part where a separation of mediore and superlative engineers is made. Good future engineers will always take a recommendation, disect it, and analyze it in detail to see if it makes for good sense. The secrets behind good engineering decisions are almost always based on hard, practical, good sense - and almost never based on an un-referenced equation or arcane information that is not backed up by facts.

What Fallah brings up makes for good sense. For example, if you know (or have been advised) that one of the fluids you intend to separate by distillation will polimerize, form sludge, or gums up then common sense forces you to eliminate any packing at all. You must apply a basic tray design and allow for intermittant maintenance and cleanup through use of good, safe human entry and access to the internals. This, ultimately, means a bigger and taller tower (& structural ladders, platforms and supports - including lighting) than what HySys (or any other calculation) predicts. These are factors that only experience and good engineering judgment can resolve - but certainly, even an un-experienced engineer should know better than to allow a packed bed to convert itself into a solid mass of polymer!

Too many times an engineer forgets that total dependency on equations and mathematical solutions is a dangerous way to resolve a problem unless one takes the time to fully comprehend and apply the equations with good engineering judgment and sense. The human brain is a wonderful tool and this is the kind of resolution it can bring about much safer and more efficiently than the biggest computer ever imagined. And the best part of the human brain is that it is always ready, available, and free to use - instantly. And what is more, EVERYONE of us has one and is free to use it!

[djack77494]

When you are employing vacuum distillation, it is usually because your material is heat sensitive and will degrade at high temperature. I like packing in such applications because :

1) Lower pressure drop = higher vacuum & lower temperature, and
2) Minimal liquid retention = less residence time for undesirable reactions to occur.

Both packing and trays have their own advantages and disadvantages, some of which have been enumerated here.
Doug