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-   -   Two miscible liquids in a CSTR - (http://www.cfd-online.com/Forums/fluent/126483-two-miscible-liquids-cstr.html)

AryaStark91 November 18, 2013 23:27

Two miscible liquids in a CSTR -
 
I need to simulate the mixing of Toluene and Sulfuric acid. Based on my understanding, these two liquids are miscible.

Both the liquids are added to the CSTR initially and later mixed using an impeller attached to the CSTR. My query is if this problem can be considered as a single phase problem.

If we consider it to be a multiphase model, what could I specify as the Primary and the Secondary phase? - Considering that there is no distinct region which contains toluene exclusively. Both the liquids are present in the tank.

Using the software - Fluent 6.2.16

Can someone please take the time out to guide me regarding this?

Thank you in advance for your time. :)

smhosseini November 19, 2013 06:33

Hi Arya,
It's better to consider the liquid with lower density as primary phase, you can use mixture model and initialize your domain with volume fraction of each phase. But your problem is single phase and If you consider multiphase model, you'll solve excess equations and you'll spend more cost and time.

AryaStark91 November 19, 2013 11:28

Quote:

Originally Posted by smhosseini (Post 462511)
Hi Arya,
It's better to consider the liquid with lower density as primary phase, you can use mixture model and initialize your domain with volume fraction of each phase. But your problem is single phase and If you consider multiphase model, you'll solve excess equations and you'll spend more cost and time.

Hello smhosseini,

I realize my queries are very basic, but I am in need of some assistance. Could you provide me with further assistance regarding Single Phase modelling?

My aim is to find out the mixing pattern for a particular rotational speed of the impeller. Further, I want to change the rotational speed and observe the change in mixing pattern. Ultimately, I want to find out the optimum rotational speed that gives the best mixing.

From the little that I gauged by reading the documentation files, I gather that I must be using either the Moving Reference Frame/Sliding Mesh models for this.

I have chosen to ignore thermal effects for this problem.

Must I enable the species transport model, and create a mixture containing sulphuric acid and toluene - if I wish to consider them as a single phase?

Any input and guidance on this problem would be welcome.

Thank you in advance for your time! :)

smhosseini November 19, 2013 12:22

You have mass transfer because of mixing between two species. Since you should consider two species and mass transfer in your domain.

AryaStark91 November 19, 2013 14:30

My equipment consists of a Continuous Stirred Tank Reactor, with a 3-bladed impeller attached to provide rotational motion.

There is no outlet. Initial conditions : toluene and sulphuric acid are both present in the tank at t=0 before the impeller is given rotational motion.

Alternatively, Sulphuric acid can also be added at the inlet, if the simulation model requires it. Suggestions regarding this are welcome.

Amount of sulphuric acid taken is 5% of the amount of toluene.

If I consider single phase flow and also enable mass diffusion in the species transfer option, how do I define the regions where sulphuric acid is present and the regions where toluene is present? (considering they are both initially present in the CSTR)

Thanks in advance for your time.

smhosseini November 20, 2013 02:40

I think you cannot have two discrete area for each component in real condition and in this type of project, the secondary component is usually injected to domain at time zero. I think you should inject Sulphuric acid initially. You can have an inlet with specific volumetric rate for special time and then, when the amount of Sulphuric acid was entered to the domain, you should stop iteration and delete the inlet. Or you can patch the amount of Sulphuric acid at a volume near the inlet of tank at time zero.

AryaStark91 November 20, 2013 02:56

Quote:

Originally Posted by smhosseini (Post 462641)
I think you cannot have two discrete area for each component in real condition and in this type of project, the secondary component is usually injected to domain at time zero. I think you should inject Sulphuric acid initially. You can have an inlet with specific volumetric rate for special time and then, when the amount of Sulphuric acid was entered to the domain, you should stop iteration and delete the inlet. Or you can patch the amount of Sulphuric acid at a volume near the inlet of tank at time zero.

Mohsen,

That seems like the ideal way to go about this problem. I shall surely try it out.

Also, which model would I be using in order to inject sulphuric acid at the inlet at a specific rate? From what I gather, the discrete phase model asks for details like the diameter of the droplets (sulphuric acid), the numerical simulation method to solve equation for the trajectory of particle motion (trapezoidal, runge-kutta) - data which I do not have.

Can you shed some light on this?

smhosseini November 20, 2013 03:19

You don't need any other models, you should only turn mass transfer on and choose the Sulphuric acid as the only component of inlet flow.

AryaStark91 November 20, 2013 06:34

Quote:

Originally Posted by smhosseini (Post 462648)
You don't need any other models, you should only turn mass transfer on and choose the Sulphuric acid as the only component of inlet flow.

Mohsen,

I have been looking, but I did not find an exclusive "Mass transfer" option for single phase flows. Can you tell me more about this option and how I can select it?

Thanks in advance!

smhosseini November 20, 2013 07:10

Define -> Models -> species -> transport & reaction
For more help please read Fluent user guide -> Modeling Species Transport and Finite-Rate Chemistry

AryaStark91 December 3, 2013 13:12

Hey Mohsen,

In one of your replies, you had mentioned this procedure - "I think you should inject Sulphuric acid initially. You can have an inlet with specific volumetric rate for special time and then, when the amount of Sulphuric acid was entered to the domain, you should stop iteration and delete the inlet."

Can you tell me how to perform this operation in FLUENT? As in, how do I create a problem where the number of iterations stop at the time when the amount of Sulphuric acid is entered into the domain?

Thanks in advance!

smhosseini December 3, 2013 14:19

Hi Arya,
You can define the velocity inlet. You know the inlet area. Since you know the volumetric flow rate. Now, you should determine a solution time that cause the amount of Sulphuric acid.

In the other way, you can patch the amount of Sulphuric acid on the anywhere of your domain at time zero, for example near the inlet place. In this case, you don't have a defined inlet in your simulation.

Best regards.


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