14 replies to this topic

[ceciliatan]

Dear all,
I had to design a vacuum distillation column for styrene plant. But im not being able to draw the mechanical drawing of the vacuum pressure vessel as i doesn't know exactly where (the position at the Pressure Vessel) to add flanges for steam ejector, feed, liquid and vapor outlet. Hope to get some illustrations.

[Art Montemayor]

What kind of design do you mean? A process design or a mechanical design?

Are you trying to draw a sketch of the styrene vacuum column or a mechanical fabrication drawing? What is the quantity and identity of all the nozzles that you need to locate on the column? All this information is normally detailed in a Vessel Data Sheet. Have you such a Data Sheet and are you going to prepare one? You have to define the shape, size, type, configuration of the vessel complete with all the nozzle types, sizes, and locations.

What do you mean by “illustrations”? You should be furnishing the Data Sheet. If you don’t have one, we can furnish you with one to start on.

[ceciliatan]

Dear Sir,
I would have to draw a mechanical fabrication drawing for a vacuum column. Normally, for a normal distillation column, there is only 3 nozzle which is for feed, top and bottom outlet. Im not sure if it is the same for a vacuum column as the column will be connected to steam ejectors and of course there are more flanges to be fixed at the column. I doesn't have any Vessel Datasheet and no idea how many flanges are there in the column. Really hope to get some illustration on the Mechanical Drawing of vacuum column. Many thanks for your kindness.

***Please find attached was the mechanical drawing of phase separator that i had drawn during my previous semester. I had not being able to draw such diagram for a Vacuum Distillation Column as there was no example available from literature and im not sure if there is an additional flanges.

Attached Files

•  Mechanical AUtoCad drawing.bmp   1.29MB   170 downloads

[pavanayi]

Ceciliatan,
Are you a chemical engineering student? What year are you in?
Have you ever visited a plant or seen a distillation column physically? Let alone a vacuum column?

[Art Montemayor]

Ceciliatan,

I realize you are a student, and that is why I feel I have to explain some basic facts regarding Chemical Engineering to you:

Chemical Engineers don't draw fabrication drawings. We SPECIFY what is to be fabricated - particularly identifying the different and various sized nozzles on pressure vessels needed for our processes. For that purpose we resort to using Data Sheets - in close cooperation with mechanical engineers who also participate in the preparation of the final Data Sheet. This Data Sheet is sent to vessel fabrication shops where mechanical engineers design and estimate the time and cost of such a vessel. They work in close coordination,supervising a Draftsman (or "Designer") who actually does the drawing of the subject vessel as a fabrication drawing. Engineers are involved in the production of the drawing only insofar as to make sure that it is an accurate and respresentative drawing. The engineers approve the finalization of the drawing and its suitability for use as a fabrication drawing.

If you are studying Drafting, then I can understand your dilemma in not knowing anything about the type, number, and quality of nozzles on a vessel. You are not supposed to know that and it is'nt expected of you - now or in the future.

If you are a Chemical Engineering student, however, I expect a lot more. A lot more.

Please tell us your line of study so that we can possibily be of help to you. I hope that you understand the importance of a Data Sheet and the role that it plays in the entire circle of fabricating a pressure vessel.

The drawing you attached is an AutoCad drawing. I don't operate the AutoCad computer program - nor do I need to. As a professional engineer, I have designers who do that for me if the need arises. However, I know exactly what the drawing should and does contain and why. I approve the applicability of such drawings. Many clients insist on a professional engineer's stamp on such drawings to certify their authenticity and correctness. Designers are not empowered to stamp such drawings.

I hope I have explained how the system works.

[ceciliatan]

Dear Sir,

I am chemical engineering students. I do understand that this is not a job of a chemical engineering. But however, our university requires us to do a drawing (as per attached) for our design project. Hence, I don't really know how to start on the drawing as I'm not sure of the flanges position. Hope to get some guidance on that or some rough schematic diagram of a vacuum distillation column together with its flanges.

[ceciliatan]

Yup I am a chemical engineering student and in final year. Yup i seen a distillation column but not a vacuum distillation column. Hence, I not sure if both have the same type of designs.

[pavanayi]

aaahhhh.. I am sinking..... I am drowning... help me!! help me!!...

Not withstanding the ramblings above, it is nice to know that the universities take the pain to ensure that an engineer (A chemical engineer in particular) has a feel of processes/steps that happen between an engineer finishing his/her design and the design being transformed into a physical plant equipment. But it doesn't help if the process engineer himself/herself doesn't know what they are doing. Does it?

Dear friend, I dont think I or any other chemical engineer in this forum might have seen/designed/operated a " Normal distillation column with 3 nozzles, one each for feed, top and bottom products".
I am at a loss where to start . I dont want to ask questions like "Do you know a distillation column has a reflux at the top?" or "Do you know a distillation column normally has a reboiler take off and a vapour return back to the column?" And that is only the process side of a very basic configuration!!!. God forbid if it was an atmospheric distiilation column in a refinery!!!

At this moment, the only advice I can give you is to re-visit the plant, and re-visit the distillation column, carefully noting how many pipes go in/come out of the particular distillation column. While at it, you might also be able to see/obtain a vessel drawing for the column.

All the best!!

Edited by pavanayi, 23 February 2011 - 05:54 AM.

[pavanayi]

In any case, I am also attaching a PFD which I saw on the internet, for the vacuum tower of the styrene plant. See if your doubt is cleared from the PFD, regarding the location of the "flanges" for the vacuum system.

Attached Files

•  str-01.pdf   11.75KB   143 downloads

[Art Montemayor]

All interested Students:

I don't know what university ceciliatan attends or what he/she is being taught (or how). All I know is that I am a degreed, professional Chemical Engineer who has never taught at the University level but who has recruited, interviewed, hired, trained, and fired a lot of chemical engineering graduates in the last 50 years that I have practiced the profession.

And based on that experience and all the other engineering work that I have done, I can only offer my frank and personal opinions on what a Chemical Engineering Student should concentrate his/her efforts on and learn while he/she is in the University. I consider it a lamentable and regretable shame that a Chemical Engineer is being cheated out of the basic learnings and facts about being a Chemical Engineer out in Industry. I seriously feel that a student is being cheated by being forced to take AutoCad drawing prior to - and instead of - learning the important basics and necesities of Engineering - like knowing and learning how to prepare a basic Data Sheet and what is so important to being an engineer. And prior to learning about the importance of Data Sheets, the student should learn and master how a distillation column is designed, how it works, and how it is put together or fabricated in order to meet the requirements of separating various components AS PER THE DATA SHEET.

In every part of this world where I have lived and worked, a Chemical Engineer is fully expected to assume or take a LEAD and responsible role in managing and supervising projects, production, or other major important engineering efforts. We are not expected to be followers. That is why Chem Engineers are forced to take - and excel in - some very difficult and complex university subjects and courses, such as: the sciences, math, computing, economics, etc., etc. Traditionally, engineers take the hardest courses that a university has to offer. Autocad drawing is certainly not one of those.

I regret that some universities have failed to live up to their responsibilities of delivering to the engineering student the type and quality of education that the student deserves - and pays for. I can fully understand that some Chemical Engineering students are laboring and suffering - not because of their inability or their IQ, but rather because of some ill-organized or un-prepared engineering faculty. This is bad news for the engineering profession and for society as a whole.

[ceciliatan]

Dear Sir,

Really sorry for interrupting you. And im really thankful and appreciate on your explanation regarding the real things to be done by a chem eng in industries and I do hope to be a good design engineer someday. Just to inform sir that I had already completed the process design of the column earlier such as no of stages, tray efficiency,etc). But as an additional task, our course requirements requires us to draw a simple fabrication drawing based on the mechanical design of vessel (minimum thickness, support, analysis of stress) . Hence, I'd like to ask if the fabrication to be drawn are similar as a normal distillation column and the flanges for an ejector could be ignored (except for a bigger diameter & shorter vessel length)?

[Art Montemayor]

Ceciliatan:

No reasonable and logical request is an interruption. I will gladly address your concerns.

Please give importance to what I wrote in my previous post in this thread. The importance of a Data Sheet cannot be over-emphasized.

If I were your instructor or your boss and you were assigned to design a vacuum distillation column for a styrene plant, I would expect you to:

• Do a detailed process design;
• Prepare a PFD of the design;
• Prepare a preliminary P&ID of the design;
• Prepare the Data Sheets for the major equipment related to the process design.

I would expect the above because you claim to have already done the “process design” – which means that you have completed a heat and mass balance and can identify the energy requirements and the mass and energy involved in the process.

You also should have a complete process description of the vacuum distillation unit which includes the number of stages, the internals, the diameter of the column as well as its height, reboiler and condenser requirements. Since it is operating under a partial vacuum, you should also have sized the vacuum system required. To do that, you should have a detailed listing of all the flanged connections and nozzles related to the column and the system.

Since you prepared a PFD and P&ID, you should also know how many instruments and what types are required to safely operate the unit – as well as all safety relief valves required.

Since you should have done all the above, then you should already know the information that you are asking about with the exception of the mechanical design – which is not a part of Chemical Engineering Design. The mechanical design (such as the wall thickness of the column, structural, etc.) is the responsibility of a mechanical and structural engineer. The quantity, type, and size of the nozzles are all the responsibility of the Chemical process design engineer – you. You are responsible for specifying (on Data Sheets) the sizes and types of reboiler(s) and condensers(s) to the mechanical engineer.

You are not dealing with a “normal” distillation column if you are designing a vacuum distillation. The mechanical design of a vacuum tower is much more complicated than that of a pressurized tower. Be careful where you thread; you are not a mechanical engineer. That is not supposed to be your expertise.

[pavanayi]

Ceciliatan,
Not withstanding the above reply, if you are still not clear, please refer to the section of the PFD which I had attached.
You can see that the vacuum ejectors are not connected to the distillation column vessel, but to the overhead reflux drum (mostly, it is the case, but in some places, the overhead vapour line directly connects to the venturi.). I hope that removes any lingering doubts.

Edited by pavanayi, 23 February 2011 - 04:15 PM.

[ceciliatan]

Dear pavanayi,
Really thanks for your guidance and concerns in helping me. I really appreciate it.

Ceciliatan,
Not withstanding the above reply, if you are still not clear, please refer to the section of the PFD which I had attached.
You can see that the vacuum ejectors are not connected to the distillation column vessel, but to the overhead reflux drum (mostly, it is the case, but in some places, the overhead vapour line directly connects to the venturi.). I hope that removes any lingering doubts.

[ceciliatan]

Dear Sir,

I would once again say many thanks for the time you had spend in explaining your experience in industries. Thank you.