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[ciwstevie]

Hi,

I'm new to this forum and not that experienced with process simulation yet. I am trying to model a dryer in Aspen Plus.
So far everything works fine, but I am trying to automate things a little and therefore would like to have a Design specification that does the following:
The air stream leaving the dryer should be at saturation.
Its temperature is not know from the start and the incoming air flow should be varied to exactly achieve saturation of the outcoming air.
The option I am using for the moment is to specify the vapour fraction to be just below 1 but this is not very satisfying. It is hard to make the solution converge as I am having 2 recycle streams (the air is recycled around the dryer and there is heat recovered from the air to preheat the cold incoming air).
So I have to play manually with the starting value for air flow and the tolerances to make things converge. Often when having an error, the error message for the design specification is that the optimum might be quite flat (what is logical as I am trying to move as close to a vapour fraction of 1 and that value can be achieved with any air flow higher than the one I am looking for).

I hope this is not too confusing and someone has an idea.
Short version: Am looking for a way to specify a humid air flow to be at saturation to be able to use this criterion in a design specification.

Any hints welcome.

Cheers,

Ciwstevie

[ciwstevie]

Well, solved the problem myself.....

Instead of setting the vapour fraction to just below 1 as design criterion to ensure saturation, I added a heater to that flow that was specified to have the stream at its dew point in the outlet. Then I used the heat load of that heater to be equal to be zero as design criterion in order to make sure the stream is at saturation already before the heater.

Now convergence is no problem any more as the function for the heat is continuous (contrary to the relative humidity that i used before) around the setpoint.