Jump to content



Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

Vertical Tank Selection

New Blog Entry

Low Flow in Pipes- posted in Ankur's blog

Which Column Is Best To Use?

separation distillation column design stripper temperature pressure

This topic has been archived. This means that you cannot reply to this topic.
10 replies to this topic
Share this topic:
| More

#1 C.Eng.Student

C.Eng.Student

    Junior Member

  • Members
  • 13 posts

Posted 06 March 2017 - 10:02 AM

Feed into column:

FEED Molar Mass kmol/hr Mole Fraction kg/hr Mass Fraction EO 44.05 472.18 0.86235047 20799.529 0.901889666 H2O 18.02 5.35 0.009770797 96.407 0.00418031 CO2 44.01 0.25 0.000456579 11.0025 0.00047708 CH2O 30.03 60.73 0.110912245 1823.7219 0.079078518 AA 44.05 4.91 0.008967218 216.2855 0.009378369 C2H4 28.05 2.43 0.004437951 68.1615 0.002955555 O2 32 1.24 0.002264633 39.68 0.001720567 CH4 16.04 0.46 0.000840106 7.3784 0.000319935 TOTAL - 547.55 1 23062.1658 1

 

Composition out of the column:

DISTILLATE Molar Mass kmol/hr kg/hr Mole Fraction Mass Fraction EO 44.05 4.7218 207.99529 0.068161608 0.09710942 H2O 18.02 0 0 0 0 CO2 44.01 0.25 11.0025 0.003608878 0.005136878 CH2O 30.03 60.1227 1805.484681 0.867902058 0.842949715 AA 44.05 0.0491 2.162855 0.000708784 0.0010098 C2H4 28.05 2.43 68.1615 0.035078298 0.031823431 O2 32 1.24 39.68 0.017900037 0.018525909 CH4 16.04 0.46 7.3784 0.006640336 0.003444848 TOTAL   69.2736 2141.865226 1

1

 

Bottom (To go into the final distillation column where EO and water will be separated):

Bottom Molar Mass kmol/hr kg/hr Mole Fraction Mass Fraction EO 44.05 467.458 20591.53371 0.977380862 0.984284793 H2O 18.02 5.35 96.407 0.011186 0.004608299 CO2 44.01 - - - - CH2O 30.03 0.6073 18.237219 0.001269768 0.000871747 AA 44.05 4.8609 214.122645 0.01016337 0.010235161 C2H4 28.05 - - - - O2 32 - - - - CH4 16.04 - - - - TOTAL 256.25 478.276 20920.30057 1 1

 

this is a table that I have to show the temperatures of the inlet and outlet streams, is it feasible to have a distillate temperature in the negatives? If so, how can I make it feasible in the physical sense? (i.e. what coolant would I use, material etc)

temperature = boiling point of component * mole fraction of component in section FEED     DISTILLATE   BOTTOM   EO 9.22715   EO 0.729329   EO 10.45798 H2O 0.97708   H2O 0   H2O 1.1186 CO2 -0.03584   CO2 -0.2833   CO2   CH2O -2.10733   CH2O -16.4901   CH2O -0.02413 AA 0.181138   AA 0.014317   AA 0.2053 C2H4 -0.46022   C2H4 -3.63762   C2H4   O2 -0.41443   O2 -3.27571   O2   CH4 -0.13568   CH4 -1.07241   CH4   temp 7.231873   temp -24.0155   temp

11.75775

 

 

Phase:

 

Feed - Liquid

Distillate - Gas

Bottom - Liquid

 

Pressure:

Feed - 3 atm

Distillate - 1 atm

Bottom - 1.03 atm

 

I started off designing this as a simple distillation column but after doing some reading I have seen a number of columns being mentioned, I am wondering what column is best for this separation?

 

Is it a standard distillation column or something else? 

 

(If you see any other mistakes, please can you point them out, thanks!)

 

Thanks for all your help.



#2 C.Eng.Student

C.Eng.Student

    Junior Member

  • Members
  • 13 posts

Posted 06 March 2017 - 10:04 AM

Apologies, this did not turn out the way it showed whilst I was writing this out, I have attached the information on the excel sheet, please refer to that whilst reading the post, thanks.

 

Attached File  Individual Project Calcs - Different Temp Method.xlsx   99.25KB   18 downloads


Edited by C.Eng.Student, 06 March 2017 - 10:18 AM.


#3 Saml

Saml

    Gold Member

  • Members
  • 301 posts

Posted 07 March 2017 - 10:04 AM

There is no way someone can help here.

You don't say if this is an assignment for your mass transfer class.

Or is part of a project.  

And what are the columns you are referring to in " I have seen a number of columns being mentioned"

And the Excel spreadsheet looks like a good tool for the one that made it, but not very clear to convey what you want, or to be understood for someone that see it for the first time.



#4 C.Eng.Student

C.Eng.Student

    Junior Member

  • Members
  • 13 posts

Posted 07 March 2017 - 11:02 AM

There is no way someone can help here.

You don't say if this is an assignment for your mass transfer class.

Or is part of a project.  

And what are the columns you are referring to in " I have seen a number of columns being mentioned"

And the Excel spreadsheet looks like a good tool for the one that made it, but not very clear to convey what you want, or to be understood for someone that see it for the first time.

This is a design project that I am a part of, the process I have been given is the production of Ethylene Oxide from the direct oxidation of Ethylene. I have been tasked with designing the light ends column.

 

When I said I had seen a number of columns mentioned I meant, strippers, flash drums, gas fractioners and others

 

I made the excel spreadsheet whilst trying to carry out some calculations. Yes apologies, it is a bit difficult to follow. 

 

Please do say if there's any other information I may have missed.



#5 Pilesar

Pilesar

    Gold Member

  • Members
  • 1,347 posts

Posted 07 March 2017 - 11:55 AM

Have you completed your plant design overview with a process flow diagram and material balance? Is it based on a real process? If you do not get the 'big picture' correct, the detailed design will tend toward unworkable fantasy. Not knowing how you are going to separate the components gives me concern for the success of your project as you should not begin detailed design until the overall PFD is settled. Research should be able to find a PFD to use as a go-by. Has any of your reading suggested alternatives such as separating the acid gases from the EO by use of an alkyline stream? Why did you choose a column at atmospheric pressure? I am sympathetic since I designed an EO plant for my senior project also and made some huge mistakes. Diligence on the front end will pay off. The tendency is just to get started with the detailed design, but it is wise to nail the overall scheme down in order to reduce re-work.



#6 C.Eng.Student

C.Eng.Student

    Junior Member

  • Members
  • 13 posts

Posted 07 March 2017 - 12:26 PM

Have you completed your plant design overview with a process flow diagram and material balance? Is it based on a real process? If you do not get the 'big picture' correct, the detailed design will tend toward unworkable fantasy. Not knowing how you are going to separate the components gives me concern for the success of your project as you should not begin detailed design until the overall PFD is settled. Research should be able to find a PFD to use as a go-by. Has any of your reading suggested alternatives such as separating the acid gases from the EO by use of an alkyline stream? Why did you choose a column at atmospheric pressure? I am sympathetic since I designed an EO plant for my senior project also and made some huge mistakes. Diligence on the front end will pay off. The tendency is just to get started with the detailed design, but it is wise to nail the overall scheme down in order to reduce re-work.

Hi Sir, 

 

yes I have, I am attaching it here. Attached File  Appendix 1 - PFD.pdf   638.87KB   19 downloads. I have also conducted the material balance however as I am only designing this part, the excel sheet only shows the material balance around the Light Ends Column.

 

Yes to an extent, we have looked at the way Shell produce this chemical and have based it off that.

 

In literature, we have read 'components are separated from the EO/Water solution in the light ends column before entering the distillation column'. However I have read in a number of places that a stripper is used to separate the components.

 

I have not read anything about separating the acid gases from the EO by use of an alkyline stream.

 

The reason why I went for atm pressure is because the boiling points of my components are very low, if I increase the pressure the EO will turn to vapour. Whilst calculating my temperatures, I have found them to be very low and this is where my problem lies, my temperatures and pressures. I have read that the column temperature is well above 0 degrees Celsius, but I do not understand how, the increase in the pressure means the temperature will rise and so the EO is likely to leave the top of the column, which is something I do not want.

 

Do you have any advice or pointers to give that would help me solve this initial problem? Any help at this point is greatly appreciated.



#7 Pilesar

Pilesar

    Gold Member

  • Members
  • 1,347 posts

Posted 07 March 2017 - 01:15 PM

As the pressure increases, the boiling point of all of your components also increases. I think you may have some confusion on this. It may help to discuss this with your professor. If you want to operate at a higher temperature, then increase the column pressure. The PFD for C-005 does not show a condenser, so I assume this is a stripper only.

  Your overall material balance shows a lot more water in your feed than your Excel spreadsheet. At least one of these documents is incorrect. If you base your design off of your project partner doing the upstream unit instead of using the official material balance, will you be prepared for extensive re-work when your project partner finds his mistakes? This is why the official material balance is so important -- it allows the individual equipment designers to work independently. 



#8 C.Eng.Student

C.Eng.Student

    Junior Member

  • Members
  • 13 posts

Posted 07 March 2017 - 02:16 PM

As the pressure increases, the boiling point of all of your components also increases. I think you may have some confusion on this. It may help to discuss this with your professor. If you want to operate at a higher temperature, then increase the column pressure. The PFD for C-005 does not show a condenser, so I assume this is a stripper only.

  Your overall material balance shows a lot more water in your feed than your Excel spreadsheet. At least one of these documents is incorrect. If you base your design off of your project partner doing the upstream unit instead of using the official material balance, will you be prepared for extensive re-work when your project partner finds his mistakes? This is why the official material balance is so important -- it allows the individual equipment designers to work independently. 

Hi Sir, 

 

thanks so much for your responses.

 

I understand the boiling point increases too but when doing research I learnt the boiling point for EO didn't increase enough for it to be feasible. Is there a method I can use to calculate the increase in BP?

 

The material balance on the pfd was a preliminary one, after conducting further research I learnt that there were some mistakes within the material balance. These were corrected and that's what is seen in the excel sheet attached. 

 

Hence, my problem stills lies in working out the correct (or feasible) temperature and pressure in the column.

 

What would you advise I do?

 

Thanks so much for all the help you've given and will give.



#9 Pilesar

Pilesar

    Gold Member

  • Members
  • 1,347 posts

Posted 07 March 2017 - 03:15 PM

Your spreadsheet shows that you have done many calcs. Do you have access to a steady state process simulator (e.g. Pro/II, AspenPlus, Hysys)? It would help to get a converged model of your process and then adjust parameters so you can understand the system better. Choose a liquid activity coefficient method such as NRTL. Choose twice the minimum number of stages and no condenser and see what product streams result. Add your liquid feed to the top stage and specify the bottoms flow to be the total mass of your EO + Water. Vary the reboiler duty to get the desired flow rate. It is unlikely that your products will be pure enough on the first attempt. Change pressures, number of stages, product cuts, feed temperature, etc. The more you play with it, the closer you will come to understand how to make it work. If you are not fluent in simulator, perhaps another on your project team can help. If nothing you do can achieve product specs, then you may have an infeasible problem caused by the material balance and the problem constraints. You might have to add a condenser or add steam to the column or make some other changes to your PFD. If your design class professor does not help you, consider going to the professor of your distillation class for advice.

  There are real-world problems to operating a column containing water at temperatures below its freezing point. That may result in points subtracted from your grade if you don't resolve this.



#10 C.Eng.Student

C.Eng.Student

    Junior Member

  • Members
  • 13 posts

Posted 07 March 2017 - 03:23 PM

Your spreadsheet shows that you have done many calcs. Do you have access to a steady state process simulator (e.g. Pro/II, AspenPlus, Hysys)? It would help to get a converged model of your process and then adjust parameters so you can understand the system better. Choose a liquid activity coefficient method such as NRTL. Choose twice the minimum number of stages and no condenser and see what product streams result. Add your liquid feed to the top stage and specify the bottoms flow to be the total mass of your EO + Water. Vary the reboiler duty to get the desired flow rate. It is unlikely that your products will be pure enough on the first attempt. Change pressures, number of stages, product cuts, feed temperature, etc. The more you play with it, the closer you will come to understand how to make it work. If you are not fluent in simulator, perhaps another on your project team can help. If nothing you do can achieve product specs, then you may have an infeasible problem caused by the material balance and the problem constraints. You might have to add a condenser or add steam to the column or make some other changes to your PFD. If your design class professor does not help you, consider going to the professor of your distillation class for advice.

  There are real-world problems to operating a column containing water at temperatures below its freezing point. That may result in points subtracted from your grade if you don't resolve this.

 

Yes I have access to AspenPlus, I'll go to University and try get the process up tomorrow morning.

 

Unfortunately my design project supervisor is not knowledgable in distillation/stripping so he cannot help that much.

 

And to make things worse, my distillation professor is in China for two weeks! So things are a little tricky right now.

 

But thanks for all your time and help, it's very much appreciated. 

 

If you have any other thoughts please do post them here, thanks! :)



#11 Pilesar

Pilesar

    Gold Member

  • Members
  • 1,347 posts

Posted 07 March 2017 - 11:06 PM

I looked back at my EO/EG class project from over 30 years ago. The project was direct oxidation of ethylene to make EO. I have a compressor on the light ends column feed. I seriously recommend you NOT use my project as an example to follow. I made incredible mistakes but experience has been a good teacher. Attached is the PFD for my project provided by the professor as a starting point. Attached also the first page of my equipment list showing the light ends column size. Also the three calc pages for my light ends column. Perhaps these will be of some value to you.
  I cannot offer much further help -- even to explain my calcs as I don't really understand exactly what I wrote. I have designed many columns since then and am still active designing columns, but I certainly cannot vouch for these calcs and I suspect my professor was merciful to me. I could not even spell 'sieve tray' properly. I worked the hardest I ever have in that design class and survived. But real world engineering is so much more fun.
 

Attached Files






Similar Topics