13 replies to this topic

[jiahao123]

Hi everyone.

We have a design that has sized our distillation column at 15m wide. Is this feasible or not? If not, how can we correct this problem?

We require an estimated 90m3/s distillate production rate.

[Art Montemayor]

jiahao123:

 

As a chemical engineering student, what you are asking our Forum members is simply ridiculous.

 

It isn't ridiculous because of the size of the distillation column you have "designed".   It is ridiculous because of the lack of factual, correct, and concise details of what it is that you have done - and your expectation that experts can respond with details.  Without any basic data other than what you supply, no one can give you any helpful advice on what you have designed.  Supply your work and all the basic data surrounding the assignment you have been given and how you have attempted to resolve it and our members will, as in other past threads, probably pitch in and try to give you helpful advice on how you have either done the correct design or how you can best do it.

 

Cooperate, and you will graduate. 

[rdcrags]

I am not a mechanical engineer, but I remember being told that the maximum diameter is actually limited by the column wall strength's ability to support the beams that are needed to support trays of such large diameters. I recollect that your example is right at that maximum. Note: It is not cost effective to increase the wall thickness beyond that needed for the design pressure in order to support the internals.

[jiahao123]

 

Dear Admin. Thanks for your advice.

 

The task that has been assigned was to produce methanol from carbon dioxide, The production route restriction is that we are not allowed to have the bulk of the methanol coming from Carbon Monoxide.  So we have used the carbon dioxide hydrogenation route.

 

The simulation software is HYSYS 8.8 and the we have used a total condenser in the column. The production rate of methanol is 3x10⁶ Tons per annum assuming 340 days production, 24/7.  Hence, the requirement of approximately 90 m³/s methanol production rate.  Similar to the posting by another member in this forum, we have used the fluid package of NRTL  with modified BIPs in this column.

 

Using the formulations from the textbook, we have arrived at the conclusion that the column diameter is 15 m which sounds unreasonably large, but could be correct given that we have an unusually large production rate of methanol.

 

Hence, I was wondering if there is a possibility that such a wide column could exist given that, from Heuristics the widest column is generally <4 m (Turton et al, 3^rd Ed).

 

-JH

Edited by Art Montemayor, 17 February 2017 - 12:19 PM.

[pavanayi]

Jiahao,

Check your numbers. Something does not seem to add up. Your annual production target and your 90 m³/s do not seem to agree. Check conversions and units.

As Art mentioned, provide your work and data. As a starting point, sketch a PFD of your separation train and provide the material balance you have calculated. 

[Art Montemayor]

jiahao123:

 

I repeat what I stated before: your request is ridiculous.

 

I don’t state this to insult you or to be negative.  I do it to help you understand that you are failing in your attempt to do chemical engineering studies and related work.  It is my hope that you correct this flawed manner of trying to attack and resolve routine engineering problems and challenges.  You are failing in your pure, simple basic manner of attacking engineering problems.

 

Please refer to the attached spreadsheet and find the very elementary and basic (grade school mathematics) calculation made for your review and acceptance.  Please note the answer that Excel arrives at by using my input.  If my input (which is YOUR input) is correct, then the answer is correct.  Do you agree with my answer?  Do you see now why I recommended you do this first and submit your work for our members?  Do you recognize now that all you have done to date is a waste of time and effort due to the incorrect calculation of your basic production rate?  Engineers depend on the use of logic, common sense, science, and mathematics.  Common sense usually dominates over all other tools because we know, by experience, that humans make mistakes, wrong assumptions, or wrong calculations.  That is why we always review and check all calculations before blindly putting faith on the results.  When someone tells me he/she will produce 90 m3/sec, I immediately become suspicious and challenge.  This is what happens in the real life of an engineer and what I would want for you to learn as a student.

 

Do not be embarrassed by the error, but rather accept the fact that what our experienced members tell you is meant for your benefit and key learnings as a student.  I can assure you that there is no one on our Forums who has not made or generated an error or mistake in practicing engineering.  The important factor is that we always use peer review and checks to make sure our mistakes are not overlooked by us.  Ergo, submit your work and you will reap great benefits from others.  Pay heed to persons like Pavanayi.

 

 Methanol Production rate Calculation.xlsx   10.72KB   46 downloads

[Bobby Strain]

Wow! That's a big error, ain't it? And all that work wasted. But the lesson to learn is to always start at the beginning, and verify each step before proceeding to the next step. If something seems wrong, it probably is. I wonder where the error originated. Would be best to determine that so it doesn't repeat in future work.

 

Bobby

[latexman]

Art,

 

I punched it out on my TI-85 before opening your spreadsheet.  I had the same answer to 2 sig figs.  OP is off by a factor of 700.

[pavanayi]

I guess the most probable reason for the error is not ensuring units of measure in Hysys match the units of measure of data input. It is a very easy and common mistake to happen. It has happened to most people, and its not easy to trace back. The importance of developing a consistent material balance (in excel or similar) cannot be over-emphasized before starting your simulation programs.

 

To OP: the above posts by the esteemed members show you the importance of checking your numbers twice. It is good that you are questioning yourself and you did not go ahead and submit the work to your professor with a column diameter of 15m. Now you can re-do the calculations and find out a more reasonable solution.

[jiahao123]

Hi Admin,

 

I think there is a misunderstanding, the fault is entirely on my part.  I had made the error of distillate production rate = vapour rate inside the column. 

My sincerest apologies for wasting everybody's time.

It should be 90 m³/s of vapor rate inside the column.

 

I apologize also for not uploading the files, this is an assignment that we have and there are several other students with the same assignment and I really hope that the efforts of my team members is not stolen by another team who just downloads the file.

[Art Montemayor]

jiahao123:

 

There is absolutely nothing to apologize for.  If you have realized where and how you erred in dealing with basic data, then the efforts and time contributed by our Forum members has not been in vain because you have profited by the experience.  That, basically is the goal that our experienced members seek: to help students help themselves by adopting the learnings that have been handed down to them.

 

I will close my writings on this thread with the following summary meant for you and all other students reading this:

• Always bear in mind that every engineering problem can be quickly analyzed by first analyzing the basic, correct facts as has been pointed out by our members.
• As Pavanayi states, always check your numbers first.  Make sure they represent what you mean to base yourself on.  That means relying on the basic scope of what your project has - in this case, the production of liquid methanol at the outlet of the distillation train.
• Embrace the advice Bobby Strain gives: always start at the beginning, and verify each step before proceeding to the next step.  This is smart, organized, and basic common sense.
• Do the simple, basic requirements to check your data, like Latexman did: do a simple hand calculation and verify the validity of your input before going on.
• As Pavanayi reiterates, the importance of sketching a PFD and developing a consistent material balance (in excel or similar) cannot be over-emphasized before starting your simulation programs.

These are key learnings from experienced engineers who have successfully undergone and labored through the same training that you presently are undergoing.  All students who aspire for a successful career would be wise to follow these recommendations.

[colt16]

jiahao123:

 

How did you obtain this 90m3/s of vapor rate inside the tower?

 

Does this mean perhaps your column is too short?

How did you decide the number of stages to be used in your simulation? 

 

If you use the minimum trays, then you get the highest energy consumption, i.e. highest reboiling duty, highest vapor rate inside the tower. This will give the shortest tower (i.e. least stages) and will be the result of the Fenske calculation.

 

If you wanted the smallest possible diameter, then you will get the lowest energy consumption, i.e. minimum reflux, lowest vapor rate. This is the highest tower (i.e. most stages) and will be the result of the Underwood calculation.

 

Practical designs selects a point in between the Fenske and Underwood extremes. 

[jiahao123]

Hi.

We used shortcut column to determine Rmin and Nmin

Then used 1.4Rmin as actual reflux.

Is there a better method to determine the no of trays?

[colt16]

I have a column that is roughly 90,000m3/h (~25m3/s) of vapor traffic inside the column using a 6.7m diameter tower. 

 

Could you run the case for Nmin and see what is the vapor traffic? If your results are correct this will give a much, much higher vapor traffic than 90m3/s.

 

Edit:

Just out of curiosity, is there a chance that 90m3/s is actually the reboiler circulation rate and not the vapor traffic? (trying to cover all angles)

Edited by colt16, 20 February 2017 - 10:33 PM.