Phase Equilibrium by Karthiga Nagarajan

Equilibrium Criteria

karthigan1 — 2020-10-19 04:06:40 UTC

Equality of chemical potential (or fugacity) of species across all the phases at constant T and P

Gibbs Phase Rule

karthigan1 — 2020-10-19 04:07:00 UTC

Independent variables required to describe the state of the system
F = N - pi + 2

Phase Diagram

karthigan1 — 2020-10-19 04:07:10 UTC

Identify single and multi-phase regions at the given conditions
Describe phase transformation
Evaluate composition of single and multi-phases in equilibrium

Phase Diagram

karthigan1 — 2020-10-19 04:10:24 UTC

Vapourization curve (P-T curve) along which vapour and liquid phases exist in equilibrium.
Critical point: Highest P and T beyond which vapour and liquid phases becomes identical.

Phase Diagram

karthigan1 — 2020-10-19 04:12:01 UTC

Envelope bounded by saturated liquid (bubble) and saturated vapour (dew) curve.
Bubble point: First bubble of vapour forms from liquid mixture (or) last bubble of vapour condenses into liquid phase.
Dew point: First droplet of liquid forms from vapour mixture (or) last droplet of liquid vapourizes from liquid phase.
Critical point: Composition dependent

Raoult's Law

karthigan1 — 2020-10-19 04:15:20 UTC

Low pressure; chemically similar mixtures.

Modified Raoult's Law

karthigan1 — 2020-10-19 04:15:39 UTC

Low pressure; chemically dissimilar

Henry's Law

karthigan1 — 2020-10-19 04:15:49 UTC

For species with limited solubility in liquid phase
VLE system T>Tc of the species

Equilibrium Composition Plot

karthigan1 — 2020-10-19 06:01:41 UTC

Plots the composition of vapour and liquid phases in equilibrium.
Used to comprehend the ease of separation
For good separation the Ki should be different from 1

Bubble problem

karthigan1 — 2020-10-19 06:02:24 UTC

Evaluate composition of bubble
and
Bubble pressure/temperature

Dew Problem

karthigan1 — 2020-10-19 06:06:12 UTC

Evaluate composition of liquid droplet
and
Dew pressure/temperature

Flash Problem

karthigan1 — 2020-10-19 06:06:51 UTC

Evaluate composition and amount of vapour and liquid phases in equilibrium

Azeotrope

karthigan1 — 2020-10-19 06:10:28 UTC

Constant boiling mixtures
Composition of vapour and liquid phases will be equal.
High boiling (or) minimum pressure azeotrope
Low boiling (or) maximum pressure azeotrope

High boiling azeotrope

karthigan1 — 2020-10-19 06:17:36 UTC

Also known as minimum pressure azeotrope.
Boils at temperatures higher than its pure species
Formed by mixtures that deviate negatively from ideal solution

Low boiling azeotrope

karthigan1 — 2020-10-19 06:25:52 UTC

Also known as maximum pressure azeotrope.
Boils at temperatures lower than its pure species
Formed by mixtures that deviate positively from ideal solution

Bubble Point

karthigan1 — 2021-11-10 03:02:40 UTC

Why the composition of liquid is same as overall composition at bubble point?
The system comprises a liquid phase in equilibrium with a minute bubble. Mole number of species in the bubble is very small compared to the overall mixture. It is therefore the composition of liquid phase is same as overall composition.

Dew Point

karthigan1 — 2021-11-10 03:04:23 UTC

Why the composition of vapour is same as overall composition at dew point?
The system comprises a vapour mixture in equilibrium with a liquid droplet. Mole number of the liquid phase is very small compared to the overall system. The composition of vapour phase is therefore the same as overall composition.

Henry's Law Constant

khooweizhi99 — 2022-11-23 07:08:02 UTC

Hi Prof! Just to check for questions that use Henry's Law would the Henry's constant be provided?

Yes. Property data will be provided

Prof, where can i find a 3d model of the azeotropic diagram which you showed in lecture that can be rotated, i want to see all sides of it, up,down,left,right and i dont trust my imagination

2023-10-24 14:08:35 UTC

3D Diagram

2023-10-24 23:13:05 UTC

Hi prof I was wondering if the points circled in orange represent anything? Do we just need to know that the the RKAC1 line is the vaporisation curve for species 1 and UBHC2 is the vaporisation curve for species 2? So the points themselves don't represent anything?

pressure-composition diagram

2023-10-24 23:27:39 UTC

Hi prof, for the left axis, did you mean the composition is x1=0 and y1=1 (instead of x2=1)?

2023-10-24 23:50:13 UTC

Hi prof, just to clarify: the pressure-composition diagram use the more volatile species (1) and shows the composition of species 1's liquid and vapor phases (x1 and y1 respectively). So taking the bubble point B for example:

x1=0.6, x2=0.4

y1=0.74, y2=0.26

Is my current understanding correct?

2023-10-25 01:33:27 UTC

For the topmost graph, why are there 2 critical points for the mixture? Which one do we use as the critical point for that mixture?

2023-10-25 02:19:48 UTC

Hi prof, for the 3 different compositions (80%,60% and 20%), is it referring to species 1 or 2? Do we just designate the composition of each loop as the more volatile species?

2023-10-26 03:47:09 UTC

Hi Prof, may I clarify that in the demonstration on phase transformation on both P-xy and T-xy diagram, overall composition of species 1: Z1, means (moles of species 1 in liquid phase + moles of species 1 in vapor phase)/(moles of species 1 in liquid phase + moles of species 1 in vapor phase + moles of species 2 in liquid phase + moles of species 2 in vapor phase)?

Prof, could you expl how to determine which is min/max boiling azeotrope?

snehaathreya03 — 2023-10-26 07:35:54 UTC

calculating fugacity by poynting eqn

2023-10-26 10:34:37 UTC

If we use poynting's equation and lewis randall rule, fugacity of liquid A at azeotropic point will be 0.55*1600=880?

Wk10 Q5

2023-10-27 07:39:15 UTC

Hi prof, why can we use modified raoult’s law for this question? I thought that this can only be used at VLE @ low P, but we don’t know the P here?

Phase Equilibrium-Problem Solving Slide 15

2023-11-12 13:29:18 UTC

Hi prof, just checking if there are errors in the values of y1 and y2? For example, using the equation under point 3), @T=365K, y1 = (0.4*137.834)/100 = 0.551 but the value shown on the slide is 0.4135

What type of question would require K factor calculation?

2023-11-20 18:04:13 UTC

for a given temperature, can i use the k-factor difference to show that it will flash?

2023-11-21 08:39:06 UTC

since K factor shows the tendency to be in certain state, if i find Ki/Kj to be not 1, does that prove the mixture will flash?

Dew temperature calculations with Modified Raoult's Law

2023-11-22 05:08:26 UTC

In the Thursday lecture, we went through Question 5 on using Modified Raoult's Law as the VLE model to calculate bubble and dew pressures. Actually, bubble temperature calculation using Modified Raoult's Law is also rather straightforward if model parameters for the chosen activity coefficient model are given (e.g. constants A12 and A21 for 2-constant Margules). However, dew temperature calculations seem much more difficult using this VLE model because we need to guess temp as well as the composition of the liquid right? Which I don't think an iterative method will allow us to solve and software will become necessary right?

Tutorial 11

2023-11-22 11:37:58 UTC

Dear prof,

Are you going to share tutorial 11 problems discussion notes with us?

Practice Problem

2023-11-22 13:25:01 UTC

For this question, I found the answer to be 0.578 bar. My equation is:

Ln[P/P_1^sat]/Ln[P/P_2^sat] = x_1^2/x_2^2

Ln[x/0.7]/Ln[x/1] = (0.371)^2/(0.629)^2

x = 0.578

Only if I change ln(gamma_1) = Ax_1^2 to ln(gamma_1) = Ax_2^2, then I get the correct answer. Is this difference related to my solution or Activity coefficient equation given has error?

Review slides + Video from last year

2023-11-23 05:21:45 UTC

Hi Prof Karthiga, I remember that you mentioned that you may be uploading review slides + a video from last year to help us in our Final Exam preparations. Will you still be doing that?

2023-11-23 14:27:29 UTC

For this question, I calculated γ1 and γ2 using P/Psat. After that, I submitted them into lnγ1 = Ax2^2 and lnγ2 = Ax1^2 and got 2 different values of A. May I know what is wrong?

Taking this module inspires me to dropout and apply to McDonalds

2025-10-30 07:49:18 UTC

Hi Prof, in the exam, could you give a hint on the question on what Eqn to use please? E.g. Hint: Use Henry's Law

2025-10-30 08:02:49 UTC

Students still require the knowledge of how to use the equation to solve the problem, so it doesn't make the exam too easy also.

Assuming Ideal Solution

2025-10-30 08:06:13 UTC

You mentioned that in the Henry's Law question that it can be assumed to be ideal solution due to water's dominance. What would be the red line to say it is or isn't an ideal solution. Like x = 10^-4? 10^-5?

Need reality check

2025-10-30 08:07:47 UTC

I'm already lost since week 4. should i just drop this mod and take the F grade at this point

karthigan1 — 2025-11-07 09:07:37 UTC

Abt Henry's Law

2025-11-17 09:03:15 UTC

Hi prof, could u pls explain the reason behind the following statement--

(VLE)when the system temperature is higher than the critical temperature of the species, it then has a limited solubility in liquid phase

thx!

Iterative method

2025-11-19 05:09:13 UTC

Hi Prof, if I remember correctly, during the lecture, you mentioned that if an iterative method is being tested, we just need to show the workflow and complete one iteration.

But my question is: if we only do one iteration and we don't get the correct $T_{\text{bubble}}$, we actually cannot get the correct composition of the bubble (vapor phase)? So what shall we do?