5 replies to this topic

[pilot]

i, am here looking to design an azeotropic distillation column , but dont know how to calculate the azeotropic temperature at which solution will boil. i am looking for any method or formula related to calculate azeotropic temperature.

actuaaly i am working on iso propanol & methanol, ethyl acetate & hexane. any how to seperate them?

[siretb]

Azeotropes: composition and boiling temperature should be read from a table, this is safest. The CRC handbook of chemistry provides such table.
The topic of azeotropic distillation is far too broad to be covered in a short reply.
Hilmen has published several papers and a thesis on the topic; search the net you should find it.

Search the litteratue and the net to see what has been chosen as third component to enhance the distillation. I downloaded the tesis about a year ago. Sorry but I cannot e-mail it immediately
If you want ,then, to model, you'll has to grab a suitable model (NRTL, UNIQUAC, UNIFAC) ***with proper parameters***
But back to the initial question, look into a book for the boiling temperature of azeotropes

[pilot]

thanks ,
for your suggestions , but it will be more help full if you give the refrance name on net or book. To decide wther particular composition will form azeotrpe or not ,i think that we can use formula

P=x1*Pv1+x2*Pv2; if P=1 atm then it will form azeotrope else not.
where P is total pressure at lower volatiles boiling point. & Pv is vapour pressure

now please tell whether i ma right or not or correct method?

thanking you

[siretb]

NO. You do not check for an azeoptrope just by applying an oversimplified formula.
You look into a book to check if azeotropes are reported.
If you do not have a book, but have a suitable model available (suitable is the important word), and UNIFAC alone will not be good enough, for instance), you generate for a given pressure the x-y curve and check whether it meets (crooses or tangents) the diagonal, for a binary mixture.

[pilot]

Can you give me idea how i can get the values of x, y (equilibrium data) or can you give any site or books name/ any formula like the relating relative volatility with x, y / vapour presure. or can you show me by giving any example.

like i had seen perry there is a formula given to show that is it any possibility of azeotrope or not or at what temp. but i m not getting that can u explain me it is in distillation chapter.

thanks for keep on guiding me & co-operating with me.

[siretb]

Pilot,

I think that you should review a book on thermodynamics

A very good source, albeit expensive on VLE data is the serie of books issued by DECHEMA / Gmehling. Search the Internet for the above keywords. It contains azeotropic data, VLE VLLE data and more.

To get back to your first post, we'll consider the n-hexane ethyl-acetate system.
According to CRC handbook of chemistry we have an azeotrope, boiling at 65.1°C with 60.1% wt hexane. This is supposed to be reliable data.
Now,; assuming we donnt have this information, have no access to experimental VLE data, but have to rely on a computer and a model.
We select NRTL (or UNIQUAC) and, fortunately have the interaction parameters available. For the sake of illustration (only) we use here the ASPEN PLUS implementation of NRTL, with parameters retrieved from their database.

We generate the x-y curve at 1 atm. We see that we get a minimum in boiling temperature, and that the vapour composition equals the liquid composition at some boiling. Hence we have a minimum boiling azeotrope.
T(azeo)=65.2 x=0.66 (good enough, though not in perfect agreement with table)
T(azeo) < T (hexane)=69° < T (acetate)=77°

At this stage you must realize that, since we have an azeotrope, we cannot have ideal behaviour. So the formula you indicated is not applicable. Let's check
Pv1 (hexane)=0.89 atm at 65°C
Pv2 (acetate)=0.66 atm at 65°
P=0.66*0.89 + (1-0.66)*0.66 = 0.81 atm NOT 1 atm (your simplified formula)
This is because we have non ideal behaviour and the activity coefficients are not equal to 1. If we correct for the activities we have
P= 0.66*(gamma=1.13)*0.89 + (1-0.66)*(gamma=1.5)*0.66=1.0 atm OK

The K values are tabulated in the attached document. Note that, at the azeotrope we have K(hexane)=K(acetate)=1.00 vapor == liquid
You will easily see that the K values are not equal to one elsewhere. The relative volatility will be the ratio of K(hexane)/K(acetate)

This means that it will not be possible to completely separate the two component by plain distillation. We have to use azeotropic distillation, extractive distillation, pervaporation or others.

To sum up
When one has azeotrope the ideal model cannot be used
The safest way to get azeotropic data is to use tables or books; second is to use computers
VLE X-Y curves are best read from experimental data (books), or generated by suitable models ; interaction parameters are almost always needed.

The enclosed file is just listed here as example, and should not be used as such for design purposes.

Attached Files

•  TEMP.txt   19.46KB   105 downloads