I cannot close a loop

[miafatsas]

Hi all,

I am trying to close a loop in the attached model using subscripts but it gives me an error. Even if I don't use subscripts gives an error. It cannot accept the feed back loop on the A in. I really don't understand why is this happening and where I am wrong. This is a very important part for my model and I would really appreciate your help in this issue.

Thanks,
A

[Administrator]

You've got a simultaneous equation. "a in" depends on "a out2", which depends on "a in".

Can you describe what you are trying to do any maybe we can suggest the correct thing to do?

[miafatsas]

Thank you for your prompt reply!

The model that I have developed, it is based on mass balance. I have a reactor which in the model i attached is the A. Mass comes in and out the reactor. Now I want to recycle a part of the mass that is going out form the reactor, which in this case is the mass coming out from the A out 2. It is very important to close those loops, as I am looking for an overall cost reduction. A way to manage this is to get what ever comes out from the A out 2 and add a variable with this value. But then I need to change by hand this value and I am afraid that this will cause an error eventually.

Thanks,
A

[Administrator]

So "A in" is actually "mass in"? Is that correct?

What are "B" and "C"?

I find it very difficult to look at a diagram with a/b/c and not know what they actually are. Wouldn't it be easier if you labelled them as "mass in", "Mass out" etc?

[miafatsas]

I am sorry if the model is not clear !

c is volumetric flow rate and B is concentration. I just wanted to make a model really quickly and send it to you.

The flow rates are Mass flow rates. I added the C and B as in the actual model the mass is related with the change in volumetric flow rate and concentration.

The problem as i said is that i want a part of the outflow mass to be recycled back to the inflow mass

Thanks,
A

[Administrator]

The outflow needs to be delayed in some way. Is the mass in the reactor for a certain amount of time?

[tomfid]

The real physics of the situation surely requires that there must be some state variable in the A in <-> A out loop. It could be a buffer stock, pressure in the vessel, or similar.

Occasionally a process is very fast with respect to other features of interest in the model, in which case it's appropriate to use SIMULTANEOUS or FIND ZERO functions.

However, so far, this structure seems incomplete. The red flag is the outflow of A does not depend on the stock of A. That fails an important physical reality check and essentially makes the stock superfluous; there's no concept of residence time of material in A.

I think you need to articulate the physics of what's happening here more clearly.

[miafatsas]

Thank you for your reply.

You are right, now I understood what I was doing wrong and now I fixed the problem!

Thanks,
A