6 replies to this topic

[Art Montemayor]

WorkingBugs:

I am not knowledgeable in the Acetone-butanol-ethanol (ABE) fermentation process, but I do know a little about packed distillation towers. I don’t believe you should be trying to use 1” Pall rings in a 12” ID distillation column. If I had to use my guessing cap, I would say you should apply a ½” or 5/8” size of Pall ring. I haven’t the calculations to back me up, but my suspicion is that the 1” size ring will not give you enough contact space and your HETP will be very high and that is not even taking into consideration that you will suffer a lot of by-passing and channeling. Have you checked your decision with the experts – the fabricator and supplier of your Pall rings? I certainly recommend you get in contact with your proposed packing supplier and make sure of “your” selection. Note that I highlight the word YOUR. That means that you are taking ALL THE RESPONSIBILITY for the resulting performance of the distillation performance. I would not recommend you do that unless you are collecting the commensurate amount of monies to justify that risk.

You don’t have the experience in having done this type of design and implementation work before, so I’ll make some unsolicited recommendations for you to ponder:

• I assume that you are working at the production plant level and that you are to design and have a purification train built for separating the feed Acetone, Butanol, and Ethanol. I have to do this since you have given us zilch input or scope of work information. If so, you should specify what you want to separate to a packing manufacturer who can work with you and help with the packing design.
• To do the above, you have to pre-select a packing supplier and that is what you should do first.
• Have the packing supplier supply all the packing, supports, distributors, and re-destributors required by the packing supplier's own design. Try to have the packing supplier assume the responsibility for the performance expected and be prepared to supply him/her with all the required basic data and backup information. Do not take that liability yourself - unless you are independently wealthy.
• Make sure you allow for the recommended bed spacings and heights. You will require multiple beds withing the same tower, so make sure you allow for packing removal at the bottom of each bed and packing introduction at the top of each bed.
• Make sure you allow for sufficient tower sump capacity. For a 12” tower I would allow for at least 3 feet of sump height. I would allow a minimum of 12” between beds for distribution and access. I would allow 24” above the top bed for disengagement space. I would estimate that each bed would be about 56” tall, making it 3 packed beds in the tower.
• Based on the above, I estimate the over-all tower to be (3+14+2+2) = 21 feet tall – and that doesn’t include the tower support skirt that is needed, assuming you used a thermosyphon reboiler. I estimatel the skirt would add another 6 feet, making the total tower height 27 ft – twice the height of your packed beds. A tower this thin with a 27:1 L/D ratio is going to require substantial structural support - don't forget that you will need platforms and access ladders to inspect, dump, and load packing. You will also need lighting.

Since you starve us with basic data and scope of work, we can’t furnish any more details than what I have given above. The quality of the response can’t be any better than the quality of the input data.

I hope this helps.

[smalawi]

Hi

Agree with Art's line of thought, just my 2 c

- try to keep the column dia around 500-600 mm and not less, you will have wall effects that affects the column performance.

- working from the above dia, suggest you check what packing will give you reasonable flood at max-min flow. this includes turndown and startup conditions

- most important, choose the right material for your system and a packing type/size that will not easily foul (pall rings have good fouling resistance in general)

- your manway size depends on your column internals size, inspection needs and column dia. you cant have a manway in a 300 mm, its going to be too small "a hand hole"

would suggest you have a read at H Kister books on distillation design & operation, you will find all you need there including design guidelines. also as Art suggested, try to use your internals vender and best to talk to few

cheers,

SM

[siretb]

Hello all

I agree, for the most of it with what has already been said. I disagree that 500-600mm is a diameter under which a design should not be made.

My comments:

Pall rings are no longer state of art packing, and you should be able to find more efficient packing (better price/efficiency/capacity compromise)
1" nominal size in a 12" ID column is bit outside of the recommendations, but is no way completely unnacceptable. You just have to take into account additional wall effects. Yet I'd prefer 5/8" nominal size. I have seen many industrial columns in the 250-500mm diameter range.
I would seriously consider a structured packed column.

I have not found any information indicating how you reached the 12" ID and the 1" pall / 14' deep bed. I have designed and run colmuns not larger than 300mm ID (10') without problems. Of course insulation is critical.
When you dump the packings into the column, special care is needed.

No water in the feed?

[Art Montemayor]

WorkingBugs:

Congratulations. You are now doing the smartest thing a young chemical engineer can do in your present situation: define and evaluate what you have as a scope of work and start to identify your work path and resources. Your lack of experience immediately should lead you to consult with those that you can identify as having the experience that you lack and using them as your starting resource. And in order to maximize the effectiveness of that kind of resource you have to make sure that you pass on all the basic data and scope to your resources in order to make sure you receive their maximum, usable, and correct input. That is where good communications come into play.

I have been in similar situations when I was a young graduate and so, will take this opportunity to emphasize some valuable inputs that you have already received and that, in my opinion, you should heed:

• Confirm and make sure your estimates for the Packed bed height is correct. As I stated, you may be able to use a packing supplier to work with you on this application. By making sure that this first step is correct, you can then verify the estimated overall height required for the distillation column and accessories. This is important because this is the largest part of the total investment.
• What Siretb seriously recommends as a structured packed column may be the smartest solution to effecting a good separation with the shortest overall height. Again, this can be verified with the right packing manufacturer and your knowledge of the cleanliness of your feed strea
• Bear in mind that structured packing may not work out with a small diameter column (12” or even 24” ID) without having to have body flanges on the column – which adds costs and introduces flange leaks and maintenance. Again, this is another point to work out with a packing supplier.
• Finalize your decision on what kind of reboiler you intend to use. This may also impact on the overall height. This also applies to your overheads condenser and reflux requirements.
• As both Siretb and Smalawi advise you, be careful and fully aware that “wall effects”, fouling, channeling, and good distribution become very important subjects when you deal with small diameter packed columns. The perfect perpendicular position of the column and its internal liquid distributors becomes very important – especially in relatively tall, slender columns. This is why I caution you on making sure you allow for any required structural steel that may be needed to reinforce the column integrity and alignment – as well as personnel access.

Both Seader and Peters and Timmerhaus are good sources for taking the basics into consideration. I used these texts when I was in your position 50 years ago. However, bear in mind that Drs Peters, et. al. Never – to my knowledge – designed, built, and operated a chemical processing unit. They were excellent academicians and teachers but I doubt that they ever had to do what I did and what you are going through. You have an admirable challenge – and I envy you. Do the best and smartest things you can do, using all available resources and original thinking. Never let your pride hold you back and always seek help.

What we need from you in order to help you is ALL the basic data and scope of work you can feed the Forum. Without that, we can’t offer suggestions, comments, advice, or recommendations. I hope we can help you further and out Forum has always served as way station to those that seek help.

Hope to continue to hear from you.