9 replies to this topic

[LimeTree.]

Dear all,
I am designing a total of 5 distillation columns to separate pyrolysis oil into petrol/diesel/benzene/heavy-hydrocarbons. 

The reason for 5 is so that the strict properties of petrol can be upheld. 

 

My question is for initial design, how do I select temperatures for the Feed, Before/after condenser and before/after reboiler. 

I need these temperatures to complete an energy balance. 



For the feed I was going to keep it as liquid at its boiling point and therefore just find the temperature of the stream by using the individual components boiling points. Lets call this temperature; X.

Would I be right in assuming that the temperature of the column is the same throughout? And therefore before the condenser the temperature is also X. Therefore if I use a total condenser, then I just remove the sensible heat and latent heat using the condenser until the stream is now at its new temperature which makes it liquid at its boiling point? Say, X2.?

Would you recommend this?

How should I go about deciding on temperatures of the distillate and bottom?

Any help in the right direction would be greatly appreciated.

I'm really sorry about the explanation, i'm not sure if this will make sense. 
Kind regards,
Luke

[Bobby Strain]

Looks like you need to study much more before we can help you in any way.

 

Bobby

[LimeTree.]

Thank you for the reply Bobby.

Unfortunately that doesn't help in the slightest. I am aware that I am not a master at this, hence my question. 

Attached is a very crudely (haha) drawn diagram of a labeled distillation column. 

For a simplified example...

Feed:
Octane, 60 mols
Nonane ,20 mols

Decane, 20 mols 

Individual Boiling points respectively are 125°C,151°Cand 174.1°C.
Therefore, the streams boiling point would be (0.6*125)+(0.2*151)+(0.2*174.1)=140.02°C.
This should be adjusted using perhaps a depriester chart, but as this is initial designs, this should suffice. 
So now I know my feed stream temperature. By making the assumption of well lagged I should be able to say that there is negligible heat loss across the column.

Could I then just do the same thing for the condenser? Ive done the mass balance, so I can calculate the boiling point of the stream? Then assume that it only removes latent heat, so that the temperature of vn = the temperature of Ln and D? 

Then I could also say that Lm is liquid at its boiling point and vm is vapour at its boiling point?





 

 

Attached Files

•  Distillation Column.jpg   19.97KB   9 downloads

[Art Montemayor]

Lime Tree:

 

Unfortunately, what Bobby is stating is the honest truth.  Based on your assertions and algorithm, you are displaying little knowledge on how a distillation column operates and how to design it - OR your instructors have failed in teaching you the same.  This comment is not mean to offend you, but rather to help you understand that you seem to lack a lot of basic understandings on a distillation Unit Operation.  Without knowing the basics, the Forum member are going to be communicating over your head while trying to help you.  This will result in hindrance and confusion rather than help.  We all have to speak a common language at the same level of knowledge.

 

We on the Forum cannot teach you Unit Operations; that is the function of your university.  What you seem to seek is the answer to the problem and not the methodology and procedure to resolve the design of a distillation problem - which is what your instructors are charged with teaching you.  You don't project the basic knowledge of how a column is designed, so it is hard to imagine how you have either been assigned this task or are doing it as general interest.  If it is an assignment, then identify is as such and submit your detailed work to date on this task for the Forum to see where you are making errors or not applying the correct procedure(s).

 

Thank you.

[LimeTree.]

Art Montemayor, thank you for your response.

I could upload the mass and energy balance to date but I don't see how this would help you explain the method behind choosing temperatures for the condenser and reboiler.

Please judge the following statement:

1) I will have a total condenser which will only remove latent heat and therefore; vapour at its boiling point will go in, liquid at its boiling point will come out.

Please can I have a response that helps with my problem? Even if your response goes "over my head" It atleast gives me a starting point and I will endeavor to understand your riddles.



 

[LimeTree.]

I've attached an example,

The reboiler temperature is 100°C which makes sense as the bottom stream comprises mostly of water. 

However, the distillate temperature of 25°C does not make sense to me, as the boiling point of acetone is 56.5°C. So why would a temperature of 25°C be chosen?
Is it simply because you can choose whatever temperature you desire (within reason) and just design the column accordingly? 

Thank you,
Luke

Attached Files

•  example.JPG   88.61KB   12 downloads

[gegio1960]

luke,

 

I totally agree with Bobby and Art answers and my first advice to you is to take in high and serious consideration what these two "very-senior-experts" are trying to communicate to you.

The second advice is to go to your teacher and to ask him the same questions you've written in your post.

The third advice is to qualify yourself and your scope in such a way that we can understand the best paths to help you.

And now, following your request, I'll try to give some answers:

 

Please judge the following statement: I will have a total condenser which will only remove latent heat and therefore; vapour at its boiling point will go in, liquid at its boiling point will come out.

The statement could be right if you've clear the concepts of subcooling, bubble point and dew point (see below).

 

the distillate temperature of 25°C does not make sense to me, as the boiling point of acetone is 56.5°C. So why would a temperature of 25°C be chosen? Is it simply because you can choose whatever temperature you desire (within reason) and just design the column accordingly?

The difference between 56.5°C and 25°C is the subcooling (you remove sensible heat from the liquid in the condenser).

 

The outlet temperature of the product is chosen on the base of its destination and of the cold utility available (other than the dew point of the product itself at the condenser pressure).

 

Good luck!

[gegio1960]

luke,

just to anticipate a little bit the other concepts/pitfalls involved in your original problem... you need to have a strong understanding of the word azeotrope and a very clear picture of the components included in the feed to the system.

In the industrial practice you should have available, as a basis of design, the chromatographic analysis of the feed made in accordance with the ASTM rules.

[PingPong]

LimeTree,

 

What kind of pyrolysis oil is that? From what process unit?

Do you have a detailed composition of that pyrolysis oil?

 

You should realise that not all distillation columns have a reboiler. I would expect that there is a fractionator with a feed from a fired heater, sidestrippers (stripping steam or reboiled) for several of the products and stripping steam in the bottom of the fractionator.

Edited by PingPong, 27 January 2014 - 11:54 AM.

[Pilesar]

Temperatures, compositions, and pressures vary within distillation equipment containing more than one separation stage. You need to understand the distillation concept from its basic functions. How do components separate inside a column? Why not use just a single flash drum? What are the advantages of multiple stages of distillation? If you do not have a good grasp on vapor-liquid equilibrium concepts, then your progress in chemical engineering academics may be at risk. I suggest you get a tutor quickly to get caught up if you are convinced chemical engineering is right for you. The only way to succeed is to really learn this stuff as it is fundamental.-- better to learn it as soon as you can.

 

If I had your assignment, I would take each distillation column one at a time.

1) Select a pressure for the distillation (start slightly above atmospheric pressure if no other info available).

2) Perform distillation calcs to converge on a mass and energy balance.

3) Check the temperatures of the top and bottom products. (These temperatures are not affected by the feed temperature.)

4) Can cheap utilities condense and reboil at the temperatures found? If not, go to step 1 and adjust the pressure.

 

Feed temperature is generally only a secondary consideration to optimize utility and equipment costs. With multiple columns, look to find where heat exchange can be integrated using process streams instead of utility streams. A distillation column can have more than two products using side draws to save cost.