4 replies to this topic

[matt.o]

I am designing an expansion to our plant that includes a new distillation column and reboiler.  I need some assistance with the reboiler design.  We need approximately 750,000 BTU/h with a high turndown (4:1).  45# Steam is the best and only choice for our heating fluid.

 

I'm using Aspen Plus and Exchanger Design and Rating (EDR) to design the reboiler.  

 

I selected a kettle because a thermosiphon would require an additional packed section in the column, which would raise the overall height of the column and the overhead condenser.  There would also be additional height requirements for the thermosiphon at the base of the column.  So, we've decided it best to go with a kettle to minimize the overall height of the structure. 

 

I've referenced this website, other websites, Kern's Process Heat Transfer, and Kister's Distillation Operation. I am familiar with the requirements: max heat flux <= 12,000 BTU/h-ft2 and outside film coef <= 300 BTU/h-ft2-F.  

 

We believe an internal kettle (in the column) would minimize the overall height and capital cost.  In particular, I am interested in the bathtub design (described by Kister) to minimize holdup volume (liquid phosgene and HCl).  

 

For the internal design, I've got an EDR file that reports the heat flux = 9000 BTU/h-ft2 and the outside film coef = 600 BTU/h-ft2-F.  I can't add tube length or increase the diameter any further and still fit it in the column.  

 

1st Question:  Is there anyway I can design for a high outside film coef beyond Kern's criteria?  Or can I combine the fouling + film coef to reduce to 300 BTU/h-ft2-F?  For instance, has industry every intentionally fouled the outside of the tubes?  If so, would this meet Kern's recommendation?

 

 

If we couldn't go with the internal (need more tube area to DTLM and ho), then I have an external kettle design that meets both of Kern's criteria (300 BTU/h-ft2-F and 2500 BTU/h-ft2).  I suspect this design will be more costly due and will be less accepted due to the higher phosgene holdup.

 

2nd Question:  The column would operate continuously, so I need a way to prevent heavies (MCB) accumulation in the kettle.  Is it reasonable to cut a small slit or opening in the bottom of the outlet weir to allow heavies to always drain out?  Or would it be more reasonable to give operations a drain/blowdown line on the kettle to occasionally purge if heavies accumulate?  

 

3rd Question:  As for level control, I want to maintain a calm level zone either in the sump of the column (internal design) or a weir at the far end of a kettle (external).  Would I also need a level indication (for steam override) for the kettle itself to ensure the tubes don't dry out?

 

I appreciate any help you've already provided through this website (especially Art Montemayor), and any additional help you can provide.

 

- Matt O.

 

[Bobby Strain]

You really should seek the services of an expert. Probably Chris can provide valuable assistance. If you are manufacturing phosgene, then your employer can well afford proper consultants.

 

Bobby

[Art Montemayor]

Matt O.:

 

Before responding to your 3 questions, I would like to make the following comments:

• I presume you are embarking on a preliminary design to prepare an estimated capital cost for the expansion project you describe.  In preparing to go forth, you seek experienced Forum advice on the scope of your design.
• I believe our Forum can add some value to your preliminary design and scope since I would expect some very experienced engineers such as Chris Haslego, Srfish, Katmar, and others will jump into this and lend their ideas and suggestions.
• 750,000 Btu/h is not a large reboiler capacity and I would guess that your distillation tower is approximately 1.0 meter in diameter.  Correct me if I am wrong because the size of the tower and the reboiler leads to the feasibility of using a “stab-in” reboiler within the tower sump.  A 1.0 meter diameter tower also fits very well on top of a 1+ meter diameter kettle reboiler – which helps the economics of using a kettle because it reduces tower support costs and piping.  Please tell us your tower dimensions and height requirements as well as any bottoms heat exchanger requirement.
• If you require a 4:1 turndown capability in your distillation reboiler, then the kettle type (or similar type) is the type to use.  A thermosyphon simply can’t operate with that wide a range.

To answer your questions the best way I know how:

1. A kettle (or pool boiling) reboiler has a drawback in that the related heat transfer coefficient is a relatively low one.  This is inherited with the natural convection that comes with the kettle pool boiling.  I would guess that you could get by with a design heat flux of 8,000 Btu/hr-ft².
2. If you are handling “heavies” or sludges in your bottom product (I don’t know what MCB means), then you can always install a liberal drain on the kettle’s belly, on the upstream side of the internal weir.  Note that I’m attaching a workbook with sketches and we can use this as the thread goes along to explain the different ways you can hook up the kettle or use a “stab-in” reboiler bundle.
3. As far as level control on the kettle reboiler, you would have a level control on the downstream side of the internal weir.  This, too, I can show on the submitted sketches.

 

I hope this helps to start this thread.  I'll wait now for our Forum experts to join in.

 

 Reboiler Design & Selection.xlsx   31.79KB   1091 downloads

[matt.o]

Thanks for the reply, Art.  Your assumptions about the tower diameter and project development are correct.  Also, I have not considered a column sitting on top of a kettle.  I suspect those economics will be favorable to an external kettle.  

 

Here's a description of the tower dimensions:

• With a kettle reboiler (near perfect equilibrium stage):
  • Tower ID = 42"
  • Tower Height = ~ 55 ft T/T;  2 packed beds 20 ft tall each
  • Reboiler elevation ~ 5 ft above grade (We plan to use a steam trap pump (example) to remove condensate in a vacuum from the reboiler;  This lowers our height requirement for the whole system).  
  •  Condenser elevation ~ 75 ft above grade (required for reflux flow to gravity drain back to column)
• With a thermosyphon:
  • Tower ID = 42" + smaller ID section (18"?)
  • Tower Height = ~ 70 ft T/T; 3 packed bed (2 packed beds 20 ft tall + 1 packed bed 5 - 10 ft tall in small ID section)
  • Thermosyphon elevation ~ 5 ft above grade (same reason as kettle above)
  • Condenser elevation ~ 95 ft above grade

Our team has all but ruled out the thermosyphon design because it requires too much height.  I don't like it because it doesn't have much turndown and requires additional packed bed.  

 

The light key is HCl and the heavy key is phosgene.  The purpose of the column is to purify anhydrous HCl at the top of the column and recover phosgene at the bottom.  Monochlorobenzene (MCB) is heavier than phosgene and is present in very low concentrations (100's of ppm) in feed.  

 

Let me know if you need more specific information.

[srfish]

In some cases the Kern limit of 12,000 BTU/hr-ft2 critical heat flux can be exceeded. Now most "Process Heat Transfer" publications use a Mostinski type correlation that was developed by Palen. As a consultant to engineering contractors I have seen heat exchanger manufacturers quote heat fluxes as high as 25,000 BTU/hr-ft2 for light hydrocarbon mixtures.