17 replies to this topic

[Art Montemayor]

Ignorance:

OK, I'll help you out. And I'm sure that others will also because you've been smart enough to give us most of all the basic data and you are very direct and specific in your queries. Besides that, I know what you are missing and why. Most of the information is not to be found in academic texts - especially the ones you have named and studied. Most of the basic, practical answers lie in the heads of guys like me who have been around for decades and have the hands-on experience to know it. This has a valid and respected reference for all the reviewing professors that have looked at and read the Final Year Project for many of the students that I've helped to gain their degree. Some of those students are probably now reading and participating in the Industrial Forum.

What I will ask you to do, since you've been smart in giving your basic data correctly, is that you fill in (or give us) all the information in your profile when you registered as a member. I feel that if we are to help you out with such a serious and important project that we should know each other - especially the country and university that you are attending. Now for some of your questions:

1) Forget about the 8 meter height you have read about in whatever book you have. Packing beds are kept normally at a height from 10 to 15 feet. This is for good liquid and vapor distribution, and not for the weight of the bed. It is very important to keep good liquid-vapor contact and it has been found that the correct maximum height is dependent on the liquid-vapor involved. Liquid re-distributor plates are recommended at the top of every bed. Keep the space between each bed (the bottom of one and the top of the other) such that you can install a recommended 24" diam. manway there. This is a common sense engineering requirement. You have to give safe, efficient access to personnel to inspect, maintain, and work inside the tower in the future. And 24" is empirically the size that allows quick and safe entry/exit. Note that all these engineering answers are empirical and found in field experience only - not in books.

2) Liquid and vapor disengagement heights are very important and you are smart to bring them up. These are dependent on the liquid, vapor, and the process involved. Good engineering judgment is critical to fix this figure. Without any specifics, I would allow for at least 1.5 meters.

3) Again, the height of the separation between the top of the bed and the liquid redistributor is up to the type of fluid(s) and the operation. I normally would allow for 0.5 meter for a 2 meter diam. column.

I will continue answering the rest when I return from a home chore I have to do. In the meantime, can you furnish the personal information?

March 17, 4PM;
I'm back now for a short while and can finish this up as follows.

4) The “typical” size of appropriate manways is, in my opinion and that of many other experienced engineers, to be the size that ensures that a human being can ingress and egress a confined space with speed, safety, and efficiency even when he has safety equipment on his person (breathing mask, backpack, safety harness, safety hat, etc., etc.). As you can imagine, a hefty, healthy, 6ft-3”, 250 lb Texan has a big problem making it through an 18” diameter. Experience has proven that 24” is acceptable for this job scope. The quantity and location is totally up to you, the designer. What would you require, if it was you that was going to have to go into your packed tower on a monthly basis? I’ll bet you would like to see more than just one, 18” manway on the tower!

5) This is a very good and astute question. It seems that you have been doing your homework! This is the heart of good, practical engineering. You have captured the origin of a very troublesome and expensive problem before it has raised its ugly head! The horse sense of the problem is that if you are operating at a vacuum, you will have a distinct resistance of the fluids to exit the column against a superior, outside pressure. In fact, this will give you all kinds of NPSH problems in trying to pump the bottoms liquid out. (I’ll assume that you are familiar with what NPSH means and its importance to the ability to be able to pump a liquid through a pump). The obvious horse sense solution that a design engineer does is that he/she elevates the tower sump high enough so that it yields an appropriate NPSH to the bottoms pump. Welcome to the real world! You will require to do some serious fluid flow pressure drop calculations in your bottom piping (as well as in your vacuum overheads vapor line and overheads condenser liquid piping).

6) These are tower nozzles, not “openings”. You need them for obvious reasons:

• to feed your feed;
• to produce your overhead liquid;
• to produce your bottoms product;
• to feed your reboiler;
• to receive your reflux;
• to measure your temperature and pressure at distinct points up and down you tower;
• to take samples;
• to measure liquid levels;
• to…….what the heck! Why am I doing your thinking for you? You should figure this out. It makes for good, common horse sense.

Your thinking is correct. You’d better be prepared to defend everything YOU designed. It’s the least that is expected of you and your product.

Good luck.

[Art Montemayor]

Eng:

If you are a normal Chemical Engineering student at University Putra Malaysia (UPM), then I’m very, very impressed with your University. I suspect that you are above-average; nevertheless, I’ve had continuing good impressions of Malaysian ChEs and you are another good example of that very favorable impression.

I want to compliment you on your excellent logical and mental thinking and the manner that you clearly express yourself and your problem. Any University should be proud to have such students as you.

However, I detect a material weakness in your use (actually mis-use) of the English language. I feel certain that you have been properly and correctly trained in the use of the English language – and probably other languages as well. That is why I also feel certain that you know exactly what it is that you are writing in a grammatical manner. To purposely abuse the rules of grammar in any language is not only an insult to that language, it also corrupts the writer’s ability to communicate correctly and accurately. Many young people – especially immature teenagers – fail to see the error in pursuing such an interest. And in that failure, they lose the ability to make a strong, valid, credible point or to communicate accurately. This handicap in communications can be fatal to a young, bright Chemical Engineer. It can ruin a career simply by leaving the wrong impression in the minds of supervisors or future bosses. This isn’t fair to society, nor to the young Chemical Engineer in question. I highly recommend that you reflect on the following comments regarding erroneous/corrupt words and grammar that you have employed and learn by always employing the correct, legible manner of writing any language.

oh, i --- Always start a sentence with a Capital Letter;

i've --- The personal pronoun “I” is always capitalized;

bcoz --- There is no such word in the English language; do you mean “because”?

ur --- There is no such word in the English language; do you mean “your”?

formula given in perry --- All proper names are Capitalized, such as “Perry”.

I hope you heed my hints. The use of slang and corrupted words or grammar may be fashionable in a teenage “chat room”; however, it is never tolerated in serious engineering discussions or in Board of Directors of important companies like Petronas where many of the graduating engineers may appear in the future.

I thank you for the kind submittal of your personal data – especially that of your University. My sincere congratulations go to your Head of Department, PROFESOR MADYA DR. ROBIAH BT. YUNUS. By the level of your knowledge, the staff at UPM has clearly done a very commendable job in training you for a respectable engineering career.

Good luck to you and all the ChEs at UPM.

[joerd]

Ignorance: Did you download KochGlitsch's IMTP brochure? http://www.kochglits...ures/KGIMTP.pdf
It seems that will answer a lot of the questions you have. Take a look at other material on their website, too. You'll find some good data on distributors as well, for example.

[Art Montemayor]

Eng:

You continue to communicate efficiently and promptly. This will always be your valuable tool in engineering. With this ability and talent cultivated, you will have no problem in your career when you seek advice, help, and assistance. Keep it up and you can only get better.

Yes, I've faced the same type of problem and resolved it. It was a re-distillation column for Nitric Acid in the manufacture of Nylon derivatives. The column was approx. 11 feet in diameter. The overhead vapor line was 48" OD. The Column was approximately 55 feet tall.

I uploaded a vacuum Workbook in this Forum some years ago and a lot of people have used it in the past. I believe it might be of some help for you as a refresher in looking at vacuum applications in chemical plants and how we deal with it. I also have accumulated a lot of literature involving vacuum equipment and its design and operations. Some of the material is rather large: 2-3.5 MB and in .pdf format. I am attaching the vaccum workbook for your (& other students) use as needed.

At the present time I have a very serious and pressing family problem and I find my self with little time to concentrate on this Forum. I will probably be unavailable from time-to-time for about a week or so. In the mean time, I offer the following things for you to think about.

Your overhead vapor nozzle cannot be larger than your column. Common sense tells you that the Souders-Brown relationship is what fixes the diameter of your column and that the superficial velocity inside the column should be less than that for the vapor line emitting from the same column. Therefore, by common sense logic, the vapor line diameter has to be smaller than the column diameter. Look at my Vacuum Workbook and see the recommended max. allowable vapor velocities for the vacuum level in question.

Agreeably, there are going to be special problems when dealing with a vacuum operation. Prepare yourself to learn a lot of useful and very valuable lessons in this project. For example, witness the lesson of the criticality of the height of the column sump in order to be able to pump the bottoms liquid out. I already alerted you to look out for the very critical pressure drop allowed in the overheads line. If this is almost an impossibility, then you can build the overheads condenser on top of the column. And if that doesn't work out, then you can build the condenser INTO the top of the column. This has been done before. I saw this quite a lot at BASF in Ludwigshafen, Germany. Ingenuity is not limited to the Germans. Malaysians are also more than capable of using their ingenuity to overcome a barrier or "insurmountable" problem. This is the greatness of engineering. It doesn't get better than this!

I will return to this thread from time-to-time to check how you and others are interacting and also attacking the problem(s) you discover. You are doing everything in the proper manner. Do not ease up on your attack on this project. Just because you seem to have some very large problems, doesn't mean that they can't be overcome. In the end, you will discover the strength and power of good, logical and common sense engineering combined with the proper and correct theory taught you in university will resolve your problems.

If you need help in fluid flow, then go to the other threads in these forums where I have uploaded other workbooks for students and young engineers. These may also be of help. Don't fail to look at the NPSH application in your bottoms pump. I can assure you that this is going to be a favorite subject for your reviewing professors in asking you how you can explain the application. It is practical, it is theoretical, and it is found in a lot of real-life pumping situations. I have a tutorial on NPSH and how to calculate it if you need it. Just let me know in this thread.

I regret having to leave the Forums for a duration, but it involves a family death and my obligations call me elsewhere for now.

Regards
 Producing_and_Maintaining_a_Vacuum.zip   56.49KB   401 downloads

[Art Montemayor]

Eng:

I'm attaching a Winzip file with some information that may come in handy when designing Vacuum columns.

You will also note the information on NPSH. I highly recommend you visit the McNally Institute web site and read all the information on pumps that is offered there. You will not regret investing this time and effort to obtain the knowledge and know-how found there.

I hope this information helps you and others out when confronting difficult vacuum design and pumping operations.

 Ovhd_Condensers_and_NPSH.ZIP   800.56KB   406 downloads

[Art Montemayor]

Well, Eng now comes the time for you to use all the valuable and important training in reasoning and logic that persons like Professor MADYA have worked so hard to teach you.

I am assuming that you have thoroughly read the Excel Workbook on Producing a Vacuum that I asked you to download. This is very important because that workbook has all the answers to your questions and to your mis-understanding of what occurs when you produce a vacuum. Don’t feel bad about your misunderstandings over vacuum systems. I prepared that workbook years ago specifically to train young graduate engineers working under me who always had trouble in understanding just exactly what happens in a vacuum environment. Let’s confront your questions and comments:

“I found that vacuum is usually vented from the reflux drum”
No, vacuum is never vented. Vacuum is produced by evacuating matter from a system. That matter has a vapor pressure – which is what you measure and call a “vacuum”. What you vent (preferentially) should be any outside air or inerts that leak into the partial vacuum within your system. And you vent these undesirable, high vapor pressure culprits by sucking them out with a vacuum “pump”.

“what if i do not have vapor product??? (I'm using a total condenser, because I want my product to be in liquid phase). Isn't that sound like a conflict?”
No. There is no conflict. Think hard and think logically about what is happening when you create a vacuum in the system. Apply the basic, natural laws and the valuable training you have had in the physical sciences – especially vapor pressure. As I mentioned above, it is the vapor pressure in the system that you are measuring as a vacuum. Therefore, when you totally condense, you have created a very good method for establishing, and controlling a vacuum as well as for expelling any high vapor pressure contaminants. Bear in mind that the vacuum device is always over-sized and the vacuum created is controlled by “bleeding” atmospheric air back into the system as it is required to maintain the vacuum pressure desired – which is the vapor pressure of your condensed overheads product at the cooled-down temperature that you achieve. Also, note that this is the way that you establish the pressure in your distillation column! The fact is, the pressure in the column is set by the vapor pressure you establish in your overheads condenser. I’m sure that you have been told that more than once in your Unit Operations courses and laboratory. Therefore, if you maintain your overhead product at a temperature that corresponds to the vapor pressure you want (which is a partial vacuum), then you have set the stage for the successful operation of the column – except that you have to ensure that any culprit atmospheric air getting into your system through the many minute leak sources is evacuated by your vacuum producing device – a “pump” or a steam ejector. Whatever excess vacuum is produced is compensated for by bleeding back some air from outside the system, in order to maintain a constant, designed partial vacuum. Don’t forget: you can’t produce an ABSOLUTE vacuum; you can only produce a partial vacuum (you can never get rid of the last molecule of air).

“I don't feel right about this. My product will be evaporating and will be vented out by steam, I'm losing my product.”
You are worried over nothing. If you subcool your product, it won’t evaporate as much. All your product can contribute is its vapor pressure. So the more you subcool, the less the evaporation. Don’t forget: Everything in the Universe has a vapor pressure. If you maintain your liquid product under a pressure that is slightly higher than its corresponding vapor pressure at that temperature, then it can’t evaporate. That’s why a little subcooling is very helpful to maintain overheads product. If you review your teachings on vapor pressure and partial pressures, you will suddenly come to the realization of just how simple distillation, reflux, and equilibrium really are and how it all comes together to work under your column design. It’s that simple. A total condenser is the right equipment and operation for what you are doing.

I hope I’ve succeeded in explaining how you really don’t have a problem in front of you but, instead you have a neat and simple working solution to an engineering challenge.

[victor]

Hello.

I'm doing my Chemical Engeenering Final Project that consists of a packed tower design. I was searching for some information in the web when I found this forum. You may can help me.

I need the weight of the internals elements of the column such as liquid distributors, redistributors, supports, also the weight of manways. According to Coulson, there are several handbooks with this kind of information but I couldn`t find anyof them at my University. Maybe there is information in the ASME code, but I couldn't find it either. Do you know if I could find something in the web? Where can I find the ASME code?
There is also software for the mechanical design of this equipment. Do you know if there is free software available?

Sorry... as you can see, my English is not very good. I'm Spanish.

Thank you.

[Art Montemayor]

Victor:

Welcome to our Forums.

Next time, please start a new thread with your query. I realize your request is similar to the subject started on this thread, but you would gather more and better attention to your needs if you develop an independent thread.

Since I obtained both an English and Chemical Engineering degree in college, I couldn’t resist the idea of challenging your statement asserting that your “English is not very good”. I did a language check and found the following:

Engeenering – Engineering (simple spelling error)

You may can help me. - You may be able to help me. (a difficult English construction)

couldn`t - couldn’t (a typical apostrophe mistake due to different key boards --European)

anyof – any of (a spacing error; something that everyone does – at least every normal busy human)

My point (which I can’t resist making) is that if the above is classified as not very good English, then I wish all Chemical Engineering students – especially the American ones – would write their queries as well as you do! I find your communication and ability to organize and express your thoughts impeccable and an example to other non-English speaking students. Now for your Final Year Project:

Do not worry about not being able to come up with equipment weights for the various tower components. Coulson, like most – if not all – engineering authors, lacks the expertise to be able to authoritatively tell you just how this is done in real-life industrial projects. There are two primary methods employed –

1. Use manufacturer’s published catalogs. Koch is one that comes immediately to mind. I have their catalogs for packing supports and re-distributors at home and not here at work, so I can’t tell you some common weights for common diameters. However, you can search the Internet and request this information from them. They usually do not cater to students (since students don’t buy too much of their products), but sometimes they can be supportive if you explain your need with specific information like product description and size.
   
2. The next method is one that is more laborious because you have to develop the estimate. What you do is calculate the thickness of the steel components and simply employ a spreadsheet layout to develop the total weight of the components. Manways are easy. You know the thickness of the blind cover, either from a catalog or by simple calculations and the nozzle, flange, bolts & nuts are easy to add on. Always add a contingent amount of weight to allow for extras and for weld material. You can estimate the packing supports by calculating vertical steel plates welded inside a circular ring and supported by an internal flange and intermediate columns (if required). Re-distribution trays are easier because they require little or no stress calculations.

Your final year project should not require you to come up with accurate or exact weights. The important thing here is that you are aware that all these extra things and equipment are necessary to complete your project and that you also are taking them into account. That is the important thing.

If you have specific sizes and descriptions and can’t come up with any estimate, let us know and we can estimate them for you. What university in Spain are you attending?

¡Buena suerte y que te vaya bien!

[victor]

Hello Art,

Thank you very much for your rapid answer...

I am aware that it is very difficult for the engineering student to come up with all the exact information (and not only about mechanical design, I've suffered a lot to get the data needed to obtain the height and diameter of the column). Ok, I understand you, in many cases I should simply use approximate parameters or estimations and indicate where more exhaustive data can be found, shouldn't I?

I already know something about Koch, because I've chosen some of their products as internals of my packed column. As soon as I've received your answer I've requested their catalog (but as you've told me, they may not answer me, I am used to that since I started the project, )

In case that I need to use the second method you mention,... I have still to look for more information but... I don't know the thickness of the distributors or supports plates either.
On the other hand, in the book "Applied Procces Design" (E.E.Ludwig) I found aproximates weights of some of this elements but, unfortunately, for metal or ceramic material, and I've chosen plastic as material. I will have probably to consider this weights if I don't find another reference. What do you think about that?

I am finishing the Chemical Engineering degree at the University of Alicante. The final project consist of the design of the packed tower (absorption reactor) belonging to the bromine production from brine procces.


Thank you again for your help.
Bye!

PD: I would thank you if you correct again any mistake you find in my English...

[Art Montemayor]

Victor:

Good response; excellent attitude.

Your reasoning is sound and starts to form what you will be using the rest of your professional life as a practicing, professional engineer: good, sound engineering judgment.

Your choice of "Applied Process Design in Chemical & Petrochemical Plants" by Ernie Ludwig is not only sound and good – it is probably one of the best sources you could find. Ernie Ludwig got his information directly from the processing plants that he helped design, build, and maintain when he was working for Dow Chemical at Freeport, Texas (a little bit down the Texas Coast, where I was born and raised). His data and information are based mainly on his empirical plant experience of many years. You couldn’t, in my opinion, have picked a better and more authoritative source for your design.

If you have internal component dimensions and weights for different materials, you can easily convert these to those of steel or other materials as well. For example, use the weight you have for steel and factor it (using the proportional density differences) for plastic. Using the allowable stress values, also factor it for thickness of components. Of course, it’s only an estimate – but then, that’s what the U de Alicante wants you to do: ESTIMATE. Just make sure you furnish a conservative estimate and state it as such in your report to the judges.

Three or four years ago on special request I helped a young Alicante Chemical Engineering student - through this Forum - who was struggling with her Final Year Project Report. We worked together to get her into a situation where she could produce what the U de Alicante was, in my opinion, looking for. We spent many months communicating by Excel spreadsheets and she not only succeeded with high honors in her Project, she also made a dedication to me in the report. She sent me a copy of her beautiful report which I treasure and show everyone I know – to demonstrate what can be accomplished when one decides to firmly attack what seems to be an insurmountable obstacle. She also rewarded me with a large box of delicious (& famous) Alicante Turrones.

I have also assisted 3 other Spanish students to obtain their degree through the Final Year Project and it has been very rewarding. One of them was from U de Salamanca and is now working in England for a very large Pharmaceutical firm. You communicate better than they were able to do at the beginning. They all improved their English after we finished their project. They learned more than just Chemical Engineering design and decision-making. Do not fear the Project judges; challenge them with your reasoning and logical decisions. You will find that they not only will respect and honor your logical reasoning but they will subsequently give you higher grades. This Final Year Project is a test of your work ethic and of your ingenuity ("el ingenio" - la base de ingenieria). Both are essential and critical values for any successful engineer – and this is what the judges panel is looking for. Do not be afraid to make bold decisions - as long as you can logically defend them in writing and in front of the panel. Be assertive and confident - but always make sure your reasoning and your calculations are sound and based on good engineering judgment.

Buena suerte y mano a la obra.

[victor]

Hello again...!

Yes, you are right, my teachers want me to estimate, I know because in the preliminary calculations I made a lot of estimations, thinking that they were to turn down them and surprisingly for me... they didn't. However, I think that for a real project more accurate data would be needed and if you are part of a company you probably can access easily to the information provided by other companies (I don't know, what do you think about that?) ... that's why I tried to find all the exact information I could.


About the weights of internals... yes, by using densities factor I can estimate the weights, but theres is still some problems... In the E.Ludwig isn't the weights of bed limiters, gas injectors... And the weights that Ludwig shows for distributors are for maximum diameter 1,5 m. Ok, again I could estimate if I consider the weight is proportional to diameter, anyway... what about bed limiters and the rest of elements?... I could consider all the internals have a similar weight, I know... if I'm worrying too much about that it is because I've seen in other projects that this information is provided by the manufacturer... but I haven't found anything.

Another thing... do you think is there any problem choosing ceramics packing (Intalox saddles in my case) with plastic internals, and with a Niquel Alloy (Hastelloy) shell? I mean, different materials corrosion resistant in the same equipment...

I expect to solve this as soon as possible because when I have the system loads, I know exactly what to do to complete nearly all the mechanical design... (nearly, of course...

What's your opinion about that estimations?
They seem to be logical, but... I don't like the results... a 160 kg plastic support taking more than 10 ton of ceramic packing!...

Thank you very much.

You can answer to my mail if you want...
destilador69@hotmail.com

[Art Montemayor]

Victor:

You are correct. For a real project more accurate data would be needed and if you are part of the design team you have ready access to the information you require for determining the total description of the equipment and internal components that you proposed to install. In fact, you would probably subcontract out the design and fabrication work to engineering specialist companies because your normal operational group is too small and doesn’t have original design capabilities – just operational capabilities.

You will find that those professors that are knowledgeable amongst your University’s staff will know this and don’t expect you to design the bolts, flanges, nuts, washers, etc., etc. in order to estimate the correct, exact weight of all components. That isn’t realistic nor is it practical. It proves nothing as far as the principal purpose for doing the Final Year Project. They expect you to estimate – but also to be able to explain how you did it and why. I would not spend too much time on this subject because it can be considered wasteful and taking you away from the real objective.

There is no problem in selecting ceramic packing (Intalox saddles) together with plastic internals and Hastelloy shell – as long as you do research and prove to yourself that you are not risking any undue corrosion or other detrimental effects because of the material mix. I see nothing wrong with what you are proposing – but do the research anyway and document it in case the question comes up. You must be ready to defend your decisions. One possible outcome might be the development of a hazardous incident. You certainly do not want the judges' panel to catch you suggesting something that could be dangerous to human life or the investment.

Think logically. If you chose plastic internals (like bed supports), then you must be prepared to confront a “trade off” – you must pay a price. Nothing comes free. One of the immediate trade offs that I know of is that you will have to accept shorter than usual packed bed heights (shorter than 5 meters). In other words, you will not be subjecting the plastic bed supports to 10 tons of weight. It is fairly obvious that you must make concessions and design your bed heights for the amount of safe weight that you can put on the bed supports. If I had to take a guess, I would say that you may wind up with ceramic packing bed heights of approximately 2 to 3 meters high. That means more separated beds and a taller tower. That’s a price you may have to pay for the exotic materials selected. This finding should be part of your report. What you are finding is not unusual. In order to solve one problem, you often create another, milder (or more tolerable) one. This is a fact of life in engineering and why we professionals live by the “refran”: There are no free rides or free lunches.

One alternative might be a result of an obvious question: If you desire to use plastic internals to combat corrosion, then why not also make the bed packing plastic? And if this is a possibility, doesn’t this lighten up the weight of the packing bed? Be prepared for this type of question from the judges’ panel when you present your findings. I would suspect that the plastic packing would not be as efficient as the ceramic and probably make the required tower higher. In other words, I would expect a trade off somewhere.

[victor]

Hello Art,

I've finally solved the problem I told you choosing Hastelloy internals, that can take the weight of the ceramic packing. At first, I did not because I thought that koch could not offer this, but actually they can.


Your remarks about the trade off (I didn't know this term, I call it "economic optimization") have made I think about a lot of things. About the choice of the packing material, you are right, plastic is a good corrosion resistant, much lighter and even as efficient as ceramic.
The reasons why I chose the ceramic Intalox saddles (when I began the design) is that I found more information about them, it's a common packing, quite efficient, and ceramic is often used to avoid corrosion. But now I am aware that plastic could be better option. However, we must consider the cost. The same happens with the selection of kind of packing (why not rasching rings or on the contrary, modern 3rd generation packings?)

What I try to tell you is that to find the best option a trade off is needed in a lot of cases (as you explained to me) but I handed in this part of the project and my teachers don't force me to do that. The important thing is to point all this out. So I won't make any change.
(as some author say... this selection is more an art than a science... or something like that... )


Now I have all the information about the loads and I can already begin with the mechanical design (as usual the data searching takes more time that the calculations...)


Thank you again for your time.
I will probably write again shortly...