31 replies to this topic

[peruanolimense]

Hi, mi name is Carlos Reynoso, I'm a student from Peru.

 

I have a question about distillation using McCabe-Thiele method... The distillation column is like this:

 

 

As we can see in the image, we have a normal column, except that there is a feed (B ) on to the partial condenser. That makes things a bit difficult...

The question is about the net flows:

•  
• Undoubtedly, there is a net flow in the Stripping section (where the boiler is, the lower part of the column). The net flow in that section would be:
•  

 

This is correct 100% sure, but the problem is with other net flows. I do not know if there are more net flows. In a normal, usual column, we have the net flow in the stripping section (the one we have in the upper image), and the net flow in the rectifying section. However, as we have a feed (B ) in the partial condenser, I do not know if we have a net flow in the rectifying section. Does anyone know if we have a net flow in the rectyfying section? (as we have feed in the partial condenser).

 

Some people say there is only the net flow in the stripping section, and no net flow in other sections of the column. Other people think that we have the net flow in both the stripping section and the rectifying section, and finally, other people think we have the net flow in the stripping section and between the feed to the column (A) and the feed to the partial condenser (B ).

 

As for now, I only need to know the question to the following answer:

Do we have more net flows in the distillation column of the above image? (100% sure there is a net flow in the stripping section)

 

Thanks, please sorry for my low english-level, and again, thankyou very much!

Have a nice day.

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Edited by peruanolimense, 31 December 2021 - 06:09 AM.

[lnbsak]

I could not see any image.

[breizh]

Hi,

I believe you should find pointers in the document attached . Check page 239 link underneath

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

You need to perform mass balances ( global and particular) 

 

Good luck 

Breizh 

[peruanolimense]

I could not see any image.

For those who caould'nt see the image, I have uploaded them as attached files, you should not have any problem now!

 

 

Hi,

I believe you should find pointers in the document attached . Check page 239 link underneath

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

You need to perform mass balances ( global and particular) 

 

Good luck 

Breizh 

Once I get home, I will take a look at it in more detail, but what I've seen in a quick overview this morning, was that there was a feed in the boiler, not in the partial condenser. I know a boiler "works" like a partial condenser (""""works"""""), but the problem is there's no feed to the column in that example, so the problem is not solved. However, I will take a better look at it this evening, thanks!

[breizh]

Hi ,

Consider this document as a supportive document ,for you to perform your task .

Good luck

Breizh 

[peruanolimense]

Hi ,

Consider this document as a supportive document ,for you to perform your task .

Good luck

Breizh 

Hi, I've been reading the document you recommended me, and it has been of great help, however, I still do not know how to procede when we have feed in the partial condenser and in the column.... Thanks

[Bobby Strain]

Ask your professor.

 

Bobby

[latexman]

The derivation of the McCabe-Theile graphical method is widely available in texts, handbooks, YouTube, etc. Glom onto a detailed version, modify it to include your condenser feed, and you have it!

[peruanolimense]

Ask your professor.

 

Bobby

 

Hi Bobby! 

Unfortunately, he is not keen on answering questions... He does not answer questions... However thanks for your answer!

 

The derivation of the McCabe-Theile graphical method is widely available in texts, handbooks, YouTube, etc. Glom onto a detailed version, modify it to include your condenser feed, and you have it!

I will take a look, however, I've investigated Seader, Treybal, King and McCabe books and I have seen nothing... Thanks by the way!

[Bobby Strain]

This is one I found when I searched McCabbe-Thiele with condenser feed.

    youtube.com/watch?v=86lwEGtm8qQ

 

Bobby

Edited by Bobby Strain, 29 December 2021 - 03:29 PM.

[latexman]

Good find, Bobby. Right after that came this one:

https://m.youtube.co...Kpo&pp=QAFIAQ==

It’s titled Binary Distillation with Multiple Feeds.

There are so many more resources now than when I was a student.

[latexman]

Carlos,

It is McCabe, not McCabbe. Warren L. McCabe. He was Professor Emeritus at my college, NCSU, when I was student, 1975-1979. Besides being one of the founding fathers of ChE, he was a good guy to know.

[peruanolimense]

This is one I found when I searched McCabbe-Thiele with condenser feed.

    youtube.com/watch?v=86lwEGtm8qQ

 

Bobby

Thanks Bobby!

 

Good find, Bobby. Right after that came this one:

https://m.youtube.co...Kpo&pp=QAFIAQ==

It’s titled Binary Distillation with Multiple Feeds.

There are so many more resources now than when I was a student.

 

 

Carlos,

It is McCabe, not McCabbe. Warren L. McCabe. He was Professor Emeritus at my college, NCSU, when I was student, 1975-1979. Besides being one of the founding fathers of ChE, he was a good guy to know.

Thanks for the video Latexman, I will research a bit more. Thanks to both of you

Yeah, I know it is McCabe, not Mcccabe, however, I misspelled it and don't know how to edit it. Thanks!

[breizh]

Hi,

You may find this resource useful :
http://www.separatio...on/MainSet1.htm

 

Breizh

[latexman]

“ don't know how to edit it. “

Everyone can edit their own posts.

[peruanolimense]

“ don't know how to edit it. “

Everyone can edit their own posts.

Yeah, I've edited the post, what I don't know how to edit is the title. However, i think my English is not that bad for someone whose native tongue is Spanish!

Thanks Latexman!      

[latexman]

I fixed it as best I could.  With no space, no capital C is allowed, so . . . 

 

¡Tu inglés es mucho mejor que mi español!

[peruanolimense]

I've invented a possible exam problem, could I upload the resolution I've made in this post so that between all of us we could se the possible errors? 

 

 

I fixed it as best I could.  With no space, no capital C is allowed, so . . . 

 

¡Tu inglés es mucho mejor que mi español!

Not so bad! Most people don't know that before the sentence it goes ¿ and ¡ 

Some people even think Peru, Spain and other countries are in Mexico 

[latexman]

If you post your work, you will get replies.  Many heads are better than one!

[peruanolimense]

This is the statement of the problema I've invented (I studied McCabe-Thiele in spanish, so maybe there's somethings that are not well written, sorry for that):

 

 

 

 

To the upper distillation column, it is fed onto the partial condenser 100 kmol/h (courrent B ) of a mixture that has 70% (in moles) of the compound we are interested in. To the propper column, it is also fed 100 kmol/h (courrent A) of the same mixture, with 40% (in moles) of the same compound. We obtain a Residue with 5% in moles of that compound, and a Distilate of 80% in moles. 

Consider that V_R/R = 1.5 · V_R/R)_minimum 

Consider too that q_B = q_A = q_LD = q_D = 1 and q_R = q_VR = 0

Consider as compound 1 the compound of interest

1. Molar flows of distillate and residue
2. Feed equations
3. Net flows of the column
4. Molar flows of both phases in the column sectors
5. Operative equations
6. Minimum number of plates
7. Number of plates when used the specified value of V_R/R
8. Most efficient position for the feed A

The diagram we will use is:

Okey so I proceede to explain the answers I've obtained:

1. The molar flow of D and R

By making a mass balance we obtain two equations with two unknown variables (D and R)

A + B = D + R  --> 100 + 100 = D + R

A·x_A + B·x_B = D·y_D + R·x_R  --> 100·0.4 + 100·0.7 = D·0.8 + R·0.05

 

From here we obtain D and R

D = 133.33 kmol/h

R = 66.67 kmol/h

 

      2. Feed equations

 

In general, the equation used to determine the equation of the feed is the following:

y = q/(q-1) · x - x_A/(q-1)

As q = 1 for both A and B, the feed equations are:

A: y = 0.4

B: y = 0.7

 

     3. Net flows of the column

Because we have a feed in the partial condenser, there is no net flow in the rectifying sector (I think) 

So:

Stripping sector:

L_m - V_m = Δ_R = R = 66.67 kmol/h

z_ΔR = x_R = 0.05

 

Medium sector: 

Mass balance: L_n + A = V_n + R

V_n - L_n = Δ_T = A - R = 33.33 kmol/h

z_ΔT = (A·x_A - R·x_R)/Δ_T = 1.10

 

     4. Molar flows of both phases in the column

We have to determine firstly V_R/R)_min

We know that: L_m/V_m)_max = (y_ΔR - y_A,eq)/(x_ΔR - x_A,eq)

y_ΔR = x_ΔR = 0.05

y_A,eq is the point where the feed line cuts the equilibrium curve in the y x diagram, in this case: y_A,eq = 0.755

x_A,eq is the point where the feed line cuts the equilibrium curve in the y x diagram, in this case: x_A,eq = 0.4

 

We obtain: L_m/V_m)_max = 2.014

 

By a mass balance (i'm not going to make it because it would make the resolution too long) we know that:

L_m/V_m)_max = (1+V_R/R]_min)/V_R/R)_min    so we obtain:

V_R/R)_min = 0.986

As we know the relation between V_R/R) = V_R/R)_min · 1.15      we obtain:

V_R/R) = 1.47887

Now we can answer the question;

 

Stripping section:

L_m = V_m + R

We know that V_R = V_m and that V_R/R = V_m/R = 1.47887

so we can obtain V_m and consequently L_m:

L_m = 165.26 kmol/h

V_m = 98.59 kmol/h

 

We can calculate now the medium section:

L_n + V_m + A = L_m + V_n

Balance to the vapour: V_m + (1-q_A)·A = V_n   --> V_n = 98.59 kmol/h

Balance to the liquid: L_n + A·q_A = L_m   --> L_n = 65.26

 

     5. Operation lines:

Stripping operation line: y_m = L_m/V_m· x_m - R·x_R/V_m

y_m = 1.6762·x_m - 0.0338

 

Medium section operation line: y_n = L_n/V_n·x_n + A·x_A/V_n - R·x_R/V_n

y_n = 0.6619·x_n + 0.3719

 

    6. Minimum number of plates

3.6 - partial boiler - partial condenser  --> 1.6      (see final image)

 

   7. Number of plates

4.8 - partial boiler - partial condenser      --> 2.8  (see final image)

 

   8. Most efficient position of A

third plate counting from the bottom of the column

 

 

 

 

    What do you think about it???

[Bobby Strain]

I think you didn't "invent" the problem.

 

Bobby

[peruanolimense]

I think you didn't "invent" the problem.

 

Bobby

 

I did, in fact, I made an Excel so I could see which values of composition of Residue, Distillate and of V_R/R) I could use so that the operative lines could be reasonable (aswell as the number of plates). 

Try to find two compunds with that y x diagram, I bet you won't find them.

 

Here's the Excel I mentioned:

 

 

I did not ask you to believe me, I just wanted someone to take a look at my workings. It is completely fine for me if you think I didn't invent the problem myself, but at least (though I did invented the problem...) I try to solve it by myself. If hypothecally the problem was not invented by my (which indeed, it is invented by me), what would be the problem? 

 

Although you don't believe me, thank you for your answer Bobby, appreciate it.

 

Thanks

[Bobby Strain]

And someone will surely look at your work and offer help. And, you should not take a comment from me too seriously.

 

Good luck.

Bobby

[latexman]

in·vent /inˈvent/ verb
create or design (something that has not existed before); be the originator of.

cre·ate /krēˈāt/ verb
bring (something) into existence.

I suspect a language misunderstanding, because like Bobby, I doubt you are the first person in the whole world to ever work on this specific distillation arrangement. And your xy diagram looks similar to ethanol/water to me.

Anyway, I looked at your work, and nothing obviously wrong came to my mind. I followed your logic and equations, but did not check your math. It looks good too me.

[peruanolimense]

in·vent /inˈvent/ verb
create or design (something that has not existed before); be the originator of.

cre·ate /krēˈāt/ verb
bring (something) into existence.

I suspect a language misunderstanding, because like Bobby, I doubt you are the first person in the whole world to ever work on this specific distillation arrangement. And your xy diagram looks similar to ethanol/water to me.

Anyway, I looked at your work, and nothing obviously wrong came to my mind. I followed your logic and equations, but did not check your math. It looks good too me.

 

Yeah, I did not invent the problem, but I do believe I invented the statement. The y x diagram is based in ethanol/water, but it is made artificially by me. You can't calculate a relative volatility because it would change constantly, furthermore, look at the diagram passed the azeotrope, the curve is completely "broken" (my fault  )

 

However, lets go to the interesting part. 

Firstly, thanks both of you for answering my doubts. There's no need to revise the mathematical operations, they may be wrong (I doubt it because I revised it a lot), but that is not what we are here for. 

I do think that it is correct, because I'm sure that when a feed enters the partial boiler, there is no net flow in the boiler, and because a partial condenser and a partial boiler are the "same" thing (except they do exactly opposite things), I think it is correct to say there's no net flow in the condenser part.

Do you think that drawing the plates till the y_D is correct (the pink line in my picture)? Maybe that is the only point where I doubt a bit.

 

Thanks to yall