7 replies to this topic

[pawan]

I want calculation demo for the crystallization separation.
Feed 10% MEG + 90% Water Say 1000 Kg/hr.
Now I chill this mixture to say -20°C where equilibrium concentration of MEG is say 40%.
So how to calculate amount of crystals & liquid separated.
I am attaching the diagram here.

Attached Files

•  Crystal.pdf   91.75KB   164 downloads

[sgkim]

I want calculation demo for the crystallization separation.
Feed 10% MEG + 90% Water Say 1000 Kg/hr.
Now I chill this mixture to say -20°C where equilibrium concentration of MEG is say 40%.
So how to calculate amount of crystals & liquid separated.
I am attaching the diagram here.

pawan, glad to meet you again.

It seems you are going to find a BETTER method than that is employed presently. Probably you have reviewed already multi-effect evaporation, pervaporation by membranes, extraction by solvents, etc.

Crystallization or decantation which might be in your mind, however, shall not be used so far as EG solution is concerned, which is my opinion. You understand ethylene glycol is mixed infinitely in water (i.e. it has NO solubility in water).

Yes, 40% MEG freezes around -20℃ as can be read from the file attached. But it does not mean it is the saturation point of the solution: note again- EG has no solubility in water. The frozen solid has just the same composition as the residual solution under the freezing point - no crystal pure EG can be produced. Then how can you expect EG be separated from aqueous solution by crystallization ?

Stefano

[pawan]

[/quote]
Yes, 40% MEG freezes around -20℃ as can be read from the file attached. But it does not mean it is the saturation point of the solution: note again- EG has no solubility in water. The frozen solid has just the same composition as the residual solution under the freezing point - no crystal pure EG can be produced. Then how can you expect EG be separated from aqueous solution by crystallization ?

Stefano
[/quote]

Hello Sgkim,
Yes I do not want Multi Effect Evaporation as that is very energy consuming process & therefore, looking for alternates.

I have not reviewed Pervaporation. If U know some supplier who is providing this Tech for MEG Water system, Pls let me know.

I am not really very clear on the term Infinitely OR No solubility. What is the meaning of the crystallization curve in that case. Any binary system is essentially having some boundary conditions where the two components separate out.

If U say as above that EG has no solubility in water - what do you read from the attached curve.

Let me put it again.
Well if I start with 10% MEG & Chill the solution to say -22°C or so then there should be 2 phases - Solid phase will be water freezed out of solution & remaining solution will be ~40% MEG at that temperature. Is it not correct?

Why I am looking for it is that the size of my requirement is ~100 M3/hr of 12% MEG. Which will result in 4 Million USD/yr saving.

[sgkim]

I have not reviewed Pervaporation. If U know some supplier who is providing this Tech for MEG Water system, Pls let me know.

I am not really very clear on the term Infinitely OR No solubility. What is the meaning of the crystallization curve in that case. Any binary system is essentially having some boundary conditions where the two components separate out.

If U say as above that EG has no solubility in water - what do you read from the attached curve.

Let me put it again.
Well if I start with 10% MEG & Chill the solution to say -22°C or so then there should be 2 phases - Solid phase will be water freezed out of solution & remaining solution will be ~40% MEG at that temperature. Is it not correct?

Why I am looking for it is that the size of my requirement is ~100 M3/hr of 12% MEG. Which will result in 4 Million USD/yr saving.

(1) Multi-effect evaporation
In multi-effect evaporation, the heat of vaporization of water is to be used for the evaporation of feed MEG solution, thus the steam consumption can be minimized. Energy consumption depends on the number of evaporators, but it shall be compromized with the additional equipment cost.

(2) Pervaporation
As the word implies, pervaporation uses selective membranes thru which water permeates and vaporizes. Presently I doubt on the economics of the pervaporative dehydration of MEG solution because of the cost of membrane. As the cost of energy soar up, however, it will surely become prevalent in the near future.

If you are not famliar with this, search on the web or refer to the following material for the pervaporation: http://www.ingentaco...6d7qg0ru2.alice

(3) Infinite Solubility vs. No Solubility
I am sure you know, better than me, the solubility of MEG in water. Solubility is generally defined as the maximum quantity of a solute dissolved in 100 gr of a solvent at a certain temperature. And the solubility usually increases with themperature. You know any quantity of MEG dissolves in 100 gram of water(i.e. infinite solubility of MEG in water). Moreover any quantity of water dissolves in 100g of MEG(i.e. inflinite solubility of water in MEG). So MEG or water has NO DEFINITE solubility in each other. That's why I expressed as "NO SOLUBILITY" or "INFINITE SOLUBILITY" - both means the same, I'd say.

(4) Freezing Curve vs. Solubility

If U say as above that EG has no solubility in water - what do you read from the attached curve.

What you had attched on your first posting is "Freezingpoint Curve" and by no means "Solubility Curve" that you may call it. You and I did not read the "Saturation Point" but "Freezing Point" of the solution.

(5) Solid of MEG Solution at Freezing Point - MEG-pure Crystal or Not ?

Let me put it again.
Well if I start with 10% MEG & Chill the solution to say -22°C or so then there should be 2 phases - Solid phase will be water freezed out of solution & remaining solution will be ~40% MEG at that temperature. Is it not correct?

If you chill the 10% MEG solution down to say -22°C, part of the solution will start to form "SOLID" in the solution. But the "SOLID" formed in the solution is not definitely MEG-pure, but MEG-Water mixed solid.
I remember the solid was just like the ICE-CAKE BAR. The SOLID, thererfore is NOT "MEG-pure Crystal".

So your understanding " Pure (or more concentrated) MEG Crystal would be formed leaving the more dilute MEG solution as solution " is NOT correct.

Note i) Freezing means neither Crystallization nor Saturation.
ii) Crystallization is not in the dictionary of MEG-Water solution.

(6)

~100 M3/hr of 12% MEG. Which will result in 4 Million USD/yr saving.

The process being not yet selected, how come to know the figure of annual saving 4 MM USD ?
To fix the figure, what will be the maximum investment cost? Interesting..

Stefano

[pawan]

1. We are already Using Multieffect with total 3 effect system at present. I am looking for alternates for this.
2. As suggested, I will check for pervaporation.

3. Yes Its a freezing point curve Not Solubility curve.

4. U r right, If I chill the solution to -22°C there will not be any "Crystallization" but solidification of either one component or a Mixture. And this is the point of difference where I am not clear.

5. I said, Only water will solidify, as the curve is representing, leaving ~40% MEG solution.

6. But here U say that both MEG & water together will solidify. But this happens when the mixture is at Eutectic point, which is represented in the graph & literature also at much lower temperature & higher MEG concn.

7. Saving estimate is based on steam consumption figures at present & power consumption in chilling, Which is of course tentative if it is possible.

[sgkim]

3. Yes Its a freezing point curve Not Solubility curve.

4. U r right, If I chill the solution to -22°C there will not be any "Crystallization" but solidification of either one component or a Mixture. And this is the point of difference where I am not clear.

5. I said, Only water will solidify, as the curve is representing, leaving ~40% MEG solution.

6. But here U say that both MEG & water together will solidify. But this happens when the mixture is at Eutectic point, which is represented in the graph & literature also at much lower temperature & higher MEG concn.

pawan:

I must have been misundersood. If the solution forms ice crystals only at the freezing point then we get the folloing material balance assuming the feed and effluent concentrations are 10 w% and 40w% respectively:

Feed Dilute Solution 10w% EG: F kg/h
Effluent 40w%EG-water/Crystalline Ice Slurry
Ice, without solution adsorbed = I kg/h and 40w% EG Solution = S kg/h

Overall Mass Balance: F = I + S.........(1)
EG Balance: 0.1F = 0.4S....................(2)
Water Balance: 0.9F = I + 0.6 S ........(3)

From the equations above I and S can be calculated and,
the temperature of 40w% EG and Ice Crystals slurry would be -24 ℃
(The way of separation of Ice from the solution would be a main issue.)

If EG only freezes at the high concentrations, the same fashion could be applied:

[pawan]

So do u agree that water can be freezed from this solution (No Solid of MEG+water mixture)?
& Separation can occur as per curve.

[sgkim]

So do u agree that water can be freezed from this solution (No Solid of MEG+water mixture)?
& Separation can occur as per curve.

Yes, pawan. But the separation of ice crystals would be a main problem, as I pointed out previously.

I noticed the fine ice particles and EG solution were formed a mixed solid like an "ice-cake" on cold surfaces, which did not seem to be separable by using centrifuge. I had not tried, but how about break the cake to slurry and separate with a centrifuge ?

If EG only freezes from EG-Water solution at higher concentrations, it would be preferrable than Ice does at low concentration; washing can be done with pure EG with small portion of remelting for the latter case.

Do you have any experience of freezing either ICE or EG of EG-Water solution?

Stefano