6 replies to this topic

[Rous]

Hi! I am a chemical engineering student and I am working on my final degree project. I have several questions, and I hope someone could help me out.

My project is about a styrene distillation train which is composed of four vacumm distillation columns.

the feed composition is:

Mass Frac
STYRENE 0,64
ETHYLBENZENE 0,32
BEZENE 0,01
TOLUENE 0,02
WATER 0,005
TBC 1,5e-05
CUMENE 0,004985
Total Flow kg/hr 7200

Temperature K 303,15
Pressure atm 0,445
Liquid Frac 1

the objective of this proyect is to obtain a 99,95% purity Styrene.

Top condenser pressure are:
1 column: 0,27 bar
2 column: 0,092 bar
3 column: 0,0089 bar
4 column: 0,011 bar

I have chosen a total condenser for the four distillation columns, and now my problem is that I don´t know where I should place the steam vapor ejector. I have read all the posts regards with this topic but I still don´t understand where it should be placed: on the vapor stream that exists the column´s top and feeds the top condenser?

the type of reboilers I have choosen is: falling film evaporator, how can I design this type of heaters? what are the standard values for the parameter: U?

Thanks in advanced

[Art Montemayor]

Rous:

I am replying in spite of feeling very bad about you not doing your required and necessary research for the material that you seek and need. The information you seek in found in great detail in the archives of our forum and you clearly have not done a search.

Once again, I repeat the basic information that all students and young grads should seriously study and learn regarding the production of partial vacuums and their application. It's sad for me to see that the concept is not understood at all by students and this clearly indicates one of two things (or both): Chem Engrg professors are failing in teaching it or students are not paying attention and/or not doing the work demanded of them. Either way, it is a terrible tragedy because it shows that the basic principles of vapor pressure, pressure drop, phase changes, heat transfer, and NPSH are not being applied or they are not understood. These are among the most important of the fundamental principles that make up the Unit Operations concept.

I have, for the above reasons, prepared and freely distributed a workbook ("Producing and Maintaining a Vacuum") through this Forum.

Please read and study the attached workbook containing my comments and prepared not only for you - but for all students seeking knowledge to become better and capable engineers.
 Styrene_Distillation.zip   234KB   174 downloads

[Rous]

Thank you very much for the explanation, and I am really sorry Art Montemayor, it was not my intention making you feel bad. I recognized that I didn´t have the concept very clear about how the steam ejectors works, but with the sketch I think I can start designing it now!

I didn´t mean to write an evaporator, sorry again, I meant a falling-film type reboiler to minimize styrene polymerization, as I had found in bibliography (Hydrocarbon Processing). In order to design this type of heat exchanger I need an aproximate value of the parameter U, and I don´t seen to find much information on this type of reboiler in bibliography, just simple explanations and drawings. Is there any book or web page where I can find this information?

Thanks again!

[Art Montemayor]

Rous:

Don’t feel bad. Just feel fortunate that you followed your basic instincts and came to this Forum for help. For your information, you can go to the following websites and obtain a lot of help for your project:

http://www.niroinc.c...lation_tech.pdf

http://www.wlv.com/p...abook/ch5_2.pdf

http://www.tomsadest...ds/brochure.pdf

www.kolmetz.com/pdf/odcdrev65.pdf

The last URL is one that I highly recommend to you and all other ChE students to download and study very carefully. It is an excellent treatise on distillation by Sulzer Chemtech. I have a copy of the Adobe .pdf file and if you have trouble downloading it, I can send you a copy of the paper. It contains a lot of information that you should include in your project report write up and can save you a lot of time in researching this type of information.

I don't know where you are at, since you have not filled in your Personal Profile when you became a member. Therefore, I don't know much about your situation and conditions. However, you can also try to find out if such firms as:

Tomsa Destil, S.L.
Avenida de Aragón, 336 Edif. 6
28022 Madrid, España
Telefono: +34 91 329 49 38
http://www.tomsadestil.com/

will help you out by recommending an overall “U” for the very specialized falling film reboiler. This type of reboiler is usually proprietary design that is not shared with just anybody. You may find that you can use the HTRI program to design a falling film evaporator and apply this to the reboiler application – but I would caution you to be very careful in doing this without some expertise help. The process design is just one problem; the mechanical design is yet another big problem, and the instrumentation/controls are a challenge. This is to be expected when you enter a field where the process equipment is primarily of special or proprietary design.

I have never done this type of reboiler; otherwise I would be more than happy to give you estimates on the film coefficients or the overall “U” to be expected on the application. In your application, you are correct and smart in taking caution with the possibilities of polymerization problems – especially in the reboiler area. You are being wise, in my opinion, on looking to avoid this type of possible pit fall by applying falling film technology which has certain advantages. Two items that stand out are:

1. The technology allows for very low delta T on a body without losing stability. As a result, the technology is suited for seven or eight effect operation and also for applications with fixed steam pressure and system vacuum such as a pre-evaporator system;
2. The second item is that the liquor is distributed across the tube bundle via a distribution tray. This insures wetting of all the tubes and minimizes dry spots so that the falling film technology is more resistant to scaling.

The trade off in applying innovative or proprietary technology is that you are starved of the basic design information that can allow you to estimate the sizes and costs of such equipment. You may just have to apply a “best-guess” over-all “U” to the reboiler design and go with that for your report – advising your readers that this is the best good judgment engineering that you can do in this type of situation.

Liquid enters the top of the evaporator and a liquid film is formed by gravity, which then flows down the heat transfer surface. During evaporation of the falling liquid, vapor fills the center of the tubes and as the momentum of the vapor accelerates, the film becomes thinner. Also, the liquid accelerates in velocity as it descends inside the tubes because of gravity and the drag of the vapor. Since the vapor is working with gravity, a falling film reboiler produces thinner films than a rising film thermosyphon reboiler for any given set of conditions. This gives rise to shorter residence times and a further improvement over the thermosyphon types in heat transfer. With these devices, liquid is usually separated from the vapor in the bottom portion of the reboiler’s bonnet or channel.

The falling-film reboiler is particularly useful in applications involving heat sensitive chemical solutions. This is due to a low “driving force” or temperature difference between the heat-transfer medium and the liquid (Delta T's less than 15 ^oF compared to 25^oF or more for a rising film).

Be also aware that this type of reboiler requires additional and unique equipment and piping. For one thing, I believe the installation requires that the reboiler be fed with a metered/controlled liquid quantity and that the reboiler be situated ABOVE the vapor inlet to the distillation column – since the vapors generated are exiting the BOTTOM of the reboiler, not the top. This is a radical change of conventional piping and requires additional structural steel and presents accessibility problems. It is a formidable challenge - even to an experienced engineer.

I hope that other Forum members/readers who have had any experience with this type of reboiler can lend a helpful hand here and offer some comments.

I strongly recommend that you do extensive research and study on the subject of falling film reboilers in search engines like Google. There is a lot of information there if you do the work required to get it.

I hope this helps you out and gets you started on a successful learning mission. Good Luck!

[rosneft]

Hi I was wondering if you could possibly repost "Styrene_Distillation.zip" as it is no longer available.

Regards

[process.eng88]

I'm also working on designing a falling film reboiler for my distillation column for my design project.

I hope Art Montemayoror some other forum member can kindly upload the pdf document of the following link: (which isn't working anymore):

http://www.tomsadest...ds/brochure.pdf

[process.eng88]

please kindly repost "Styrene_Distillation.zip" as it is no longer available.