6 replies to this topic

[Fr3dd]

Hi again,

Once again i'm asking for your help. As part of the SWS system I'm currently designing, there is a Stripper column with a reboiler. This reboiler shall be a Vertical Thermosyphon (TEMA Type BEM) and shall handle the product of the last tray of the stripper.

Hot fluid (column product) in the tubes, and heating media (MP Steam) in the shell. I have the following questions:

1. Is there any guideline to set the tube arrangement?, I've read that a 90° arrangement is recommended if mechanical cleaning will be performed, but a 45° or other arrangement increases the heat transfer, reducing the area. Considering that the system condenses MP Steam in shellside, I think that mechanical cleaning may not be necessary. What Do you Think?.

2. I've checked some variables related to tube vibration, are there any other variables (like flow velocity shellside or tubeside, etc) that I should care about in this design?.

If there is another variable or situation I should care about, please tell me; I'll be glad to read all your recommendations.

Thanks in advance ,


Fr3dd

[Art Montemayor]

Fr3dd:

What is an “SWS system”?

What engineering courses and other studies have you taken that prepare you for this design assignment? Have you ever designed a distillation reboiler before? This is the Refining, Hydrocarbons, Oil, and Gas Forum, so I assume that you are not a student but a professional engineer.

What are your reasons for selecting a BEM TEMA type of heat exchanger? Are you familiar with the hydraulic and process conditions required of a thermosyphon reboiler?

What do you mean by a “45 and 90° tube arrangement? Are you referring to the tube PITCH in a tubesheet? Where have you read that this is recommended?

Have you done any research on thermosyphon reboiler design? If so, what have you found?

[chemsac2]

Fr3dd,

Answers to your question:

1) Tube pitch arrangement is set mainly by ease of cleaning. If your shellside material is fouling, you would go for inline configurations like 90 or 45 deg pattern. However, for better shellside heat transfer coefficient and to accomodate more number of tubes in smaller shell ID, 30 deg pattern is preferred.

As for your aplication, I would go for 30 deg pattern. Since steam is on shellside, fouling is going to be very less. In any case, using BEM type exchanger with 90 deg pattern does not make sense at all. You go for 90 deg Shell for fouling fluids, but BEM type (fixed tubesheet exchanger) is not amenable to shellside cleaning.

For condensing services, condensing coefficient is very high and hence turbulence created by tube arrangement is not much rate controlling. It may help reduce condensate film in horizontal condensers, but I am not so sure about it for vertical downflow condensation of steam as in reboilers.

Thus, for vertical thermosiphon reboilers using steam on shellside, I would go for BEM with 30 deg pattern.

2) Exchanger design and that too a thermosiphon reboiler requires study of lot many parameters. I would recommend you not to venture into it unguided. Few critical issues that you need to check are:
- Ensure type of boiling is not in film boiling (certainly not in unsteady film boiling)
- Ensure you have sufficient % vaporization at reboiler outlet (generally kept at around 25-30%). May have to change static head to meet this requirement
- Flow regime in tubes should preferably be annular
- Design heat flux to critical heat flux ratio should be below 0.6
- There are requirements for inlet and exit piping as well

Regards,

Sachin

[Fr3dd]

Hello Mr. Montemayor,

I'll answer your questions one by one.

- By using the acronym "SWS" I mean "Sour Water Stripping", sorry for the confusion this caused.

- I'm a Chemical Engineer, I don't have an extensive experience but I've worked in the design of equipment before (under supervision, of course). However, this is the first time I'm dealing with the design of this specific kind of equipment.

- I've been consulting books, articles and international standards, I've found some guidelines about the operation of this kind of equipment regarding to its position in relation to the column elevation, the consideration to take into account for estimating the required head, etc. Some people here in the forum gave me recommendations about the column bottom arrangement for feeding this kind of equipment.
The TEMA type was specified during the conceptual engineering phase, and I haven't found any objection of this type of exchanger for this application. I think that an one-pass shell works for this application since the heating media (MP Steam) is expected to condensate in order to provide enough heat for process fluid evaporation.


- Well, by saying "tube arrangement" I was referring to the Tube Pattern (Triangular 30° or 60°, square 45° or 90°), I didn't use the term "Tube Pitch" since I understand pitch as the center-to-center spacing of the tubes. However "tube arrangement" wasn't the exact term to be used.
It is recommended by The "Standards of the Tubular Exchangers Manufacturers Association", 8th edition (maybe an old version) section 5 that triangular or rotated triangular arrangement should not be used if the shell side is to be cleaned mechanically; in the other hand, square tube pattern is recommended when mechanical cleaning of the tubes shall be performed. That's exactly what I'm asking, using steam as heating media (shellside) is it common to use mechanical cleaning?; this may be defined later, but I'd like to take that prevision during the design phase.

-I have made some research about vertical thermosiphons indeed, I've found information about recommended tube length and outside diameter, flow areas of inlet and outlet lines, inlet velocities, etc. I was asking about important guidelines just to make sure I'm considering all the main variables; all the recommendations of experienced engineer as you are invaluable.

I hope you can help me to find an answer, as usual using this forum. Many Thanks,

P.D. There was a mistake in my previous post, Where says "Hot fluid (column product)..." should say "Cold Fluid (column product).."

[djack77494]

Congratulations to you for the researching you've done into what looks to be unfamiliar territory. As you surmised, a single shell pass is what you want. You're condensing steam on the shell side, so the service is very clean. Therefore you can go with an economical triangular tube pitch and not worry about shellside mechanical cleaning. With condensation occuring on the shell side and boiling occuring on the tube side, you should have a high heat transfer coefficient - this is an efficient combination for transfering heat. Your tubeside like the shellside is single pass. Liquid enters at the bottom and is vaporizing as it travels up the tube. Look for about 25 to 30% of it to vaporize, as mentioned by Sachin.

[Fr3dd]

Ok, perfect. Then a 30° Tube pattern works fine!. Thank you djack77494 and chemsac2 for your recommendations.

[fatimah]

Hi there

i suggest you to try 60 degree tube layout as sometimes it can give you lower surface area than 30 degree. Good Luck!