15 replies to this topic

[Kalid]

Dear All,

 

Is it possible to design a distillation column without the use of simulation software?

 

I am separating ethanol and water, using 8 stages with a total condenser in mind.

 

How can I predict the duty required for the condenser?

 

 

 

 

 

[Art Montemayor]

How do you logically think we old, retired chemical engineers designed all our distillation columns during the 20th century?

 

By guessing the correct answers?  We did it with our brains, pencil and paper, and a slide rule.

[Bobby Strain]

It is quite time consuming since you seem not to have completed the requisite courses. Time is better spent in other endeavors. So, wait until it is necessary for your classes.

 

Bobby

[breizh]

https://escholarship...96n0xv#page-489

 

Hi ,

May be you should consider this resource ?

 

 

Breizh

[Kalid]

Thanks for your responses! Sorry if I have sounded rude, naive and offended any of you!

 

 I have good knowledge of the distillation process and all the fundamentals. Just don't know where and how to start. I have no desired product quality I want to achieve. The information I know is I want 8 stages. I can choose the flowrates and operating conditions, its a blank canvas. 

[MTumack]

How can you solve a problem that is not defined?

 

This is equivalent to trying to divide by zero.

[Art Montemayor]

James:

 

You are not being rude nor have you offended anyone.  I think we have tried to answer your basic questions but find a difficulty in the manner you express your query.  For example, you contradict yourself by stating that you “have good knowledge of the distillation process and all the fundamentals” but “just don't know where and how to start”.  How can that be????  If you really know your distillation fundamentals, then you surely know where to start with designing the distillation process in the following manner:

• Identify the conditions and composition of the Feed stream;
• Identify the conditions and composition of the product streams - overheads and bottoms;
• Calculate the reflux rate;
• Calculate the number of theoretical stages needed to satisfy the above conditions;
• Calculate the reboiler duty;
• Calculate the overheads condenser duty;
• Calculate the height and diameter of the column;
• Select the type and number of distillation trays (or packing) used.

To specifically answer your question:

Yes, you can hand calculate all the above steps, without using any simulation program.

 

Please clearly - and with details - explain why you want to separate ethanol from an aqueous mixture and you don’t know the Feed or product compositions.  That doesn’t make any sense!  Before going anywhere you have to know where it is that you want to wind up at.

 

This all may be your failure to communicate correctly or to candidly tell us ALL of the facts behind this query.

[Kalid]

Thank you for your kind response! 

 

The reason for my project is to design a column which can be used to teach students about the fundamentals of distillation.(seems ironic) The information I have been given is the mixture to be separated is ethanol/water and there are to be 8 stages. The process is to be batch distillation. The composition of the product is not essential. The feed can be chosen by myself, along with all parameters. But I think the best way would be to work backwards, I know my number of stages , so I can define my overhead and bottoms compositions. 

[Art Montemayor]

Finally!  The communications failure has been overcome.

 

Although you may know a lot about distillation and the Unit Operations calculation methods to describe the operation, you have initially failed to tell us the most important detail of all:  this topic is about BATCH DISTILLATION - not continuous, steady state distillation.  There is a world of difference in how to describe and calculate this specific method.   Good luck.

[MrShorty]

As one who came to his current position around the usual academic routes, I could easily be wrong in this. It seems to me that the McCabe Thiele method/diagram's whole purpose in life to design and analyze binary distillation columns. I see discussions applying McCabe Thiele analysis to both continuous and batch distillation columns. I am not certain if you have yet considered it, nor have the others mentioned it, but it seems to me that it would be a good starting place for the kind of exercise you seem to be describing.

[Kalid]

Are there any recommended texts or design procedures for batch distillation? 

[breizh]

Hope this helps,

Breizh

[Kalid]

Thanks Breizh for the documents!

 

Is there a way in which I can calculate the flow of vapour up the column, based on the heat input from my reboiler?

I have been told my column can not be bigger than 50mm in diameter. 

[Art Montemayor]

James:

 

You need to go back to your basic Distillation theory and study it very carefully.

 

In this topic, you have to think DIFFERENTIALLY.  As I've stated before, batch distillation is NOT a continuous process.  It is a dynamic process.  Therefore, by simple logic, the overheads vapor rate is varying if you want to control its purity.  It looks simple, but it is complex because it is continuously changing.  Your overhead vapor rate will be continuously changing as the process progresses - unless you very carefully (and skillfully) control the variables in this process.  That is difficult to do.  You would have to vary your reboiler heat rate, your reflux, your overhead condenser load, etc.  A lot depends on how you want to operate the batch process.  Will you accept a varying distillate purity?  Will you accept a varying overheads?  

 

You have a much harder problem than you might have imagined, based on what you have been telling us.  I don't think we still have the total picture of this problem identified - and it might be because you don't understand the basic process.  Study the material Breizh has submitted.  The description is there.  And there is bountiful information on the different ways to operate a batch distillation in the internet.  Google for Batch Distillation and you will find a lot of it.

[Art Montemayor]

James:

 

Attached is a document with sample calculations of a Binary Distillation operation that I forgot to attach to my previous post.  I found this on the internet where you will find a lot of material on this subject.

 

Perhaps this can help organize your thoughts on how to attack this type of dynamic problem.

Attached Files

•  Binary_Batch_Distillation.xlsx   184.73KB   89 downloads

[Kalid]

Thanks for the information, much appreciated! I can accept a varying distillate purity this is not a problem. I understand the process is dynamic as the composition of the still will change as it is heated of time. If I keep the reboiler temperature constant over time, is it correct to say the vapour flow will decrease? of course I have the effect of the reflux too. Another concept I am struggling to understand is how I can calculate the relative volatility? Sorry for the constant questions. 

 

The parameters of the design I have:

 

Column diameter: 50mm

Mixture: Ethanol/water

Mole fraction of still: 0.5 ethanol

Volume of still: 10 litres

Stages: 8

Edited by James12, 25 April 2017 - 03:05 AM.