3 replies to this topic

[kaisar]

If I mix two streams with different pressures in HYSYS simulation program, it will set the output mixed stream pressure to the lowest inlet pressure. I don't know why. For what scientific reasons HYSYS makes such setting. Anyone, please give me explanation.

[joerd]

Did you think about your question? How can you mix two streams of different pressures (in real life), because at the mixing point there will be just one pressure. So, if two streams come together at some point, the pressure in each individual line will be equalized, and only be different due to dynamic pressure losses.
That said, however, HYSYS is a simulation tool, with mathematical formulas describing a model of the real plant. To make it easier for you, the user, they will allow you to attach two model streams to a mixer operation, without adding valves etc. to let down the pressure in each stream, and make them all equal. You can still do that, if you want, but you don't have to. I like this behavior, it is much nicer than getting some sort of error message that says: "can't calculate mixer because attached streams have different pressures."
So, by convention, the Hyprotech people decided (in accordance with most other simulation programs) that the outlet pressure of a mixer should be the lowest of all the inlet pressures. In that way, they avoid having streams of a low pressure going into a mixer and coming out of the mixer at a higher pressure, which is physically impossible (unless you're talking about an ejector, but that is not the point here). The streams with higher pressure are just assumed to be let down to the lowest pressure, and then mixed. So this is more a question of user friendliness, in my opinion, than pure science.

[Profe]

Hi Kaisar

In addition to the magnificent explain of Joerd, I invite you to think with some of logic, not for simulation


In order to put another point of view in support to the form to come the pressures in a mixer, we suppose one manifold to which enter two streams A and B at different pressures Pa and Pb, with Pa < Pb and leaves one stream C formed by the mixture of the two, then C = (A + B) with a pressure Pc.
If we remembered the basics of flow through a conduit or pipe: "The movement of the fluid is from the site of greater pressure, to the one of smaller pressure. Not opposite it"
1° If Pc > Pa, A will not flow towards exit C. because we required Pc <= Pa
What happens with B?
2° If Pc > Pb, B will not flow towards C. Then, Pc <= Pb.
But B flows towards A because Pb > Pa and we do not want that B circulates being against to A. We want that both A and B circulate through C. Applying the transitive property of this logic, so that exists flow by C only, if Pc <= Pa < Pb.

You follow me?


I hope will be useful.

[kaisar]

For Profe and Joerd, Thank you very much for your explanation. Now, I clearly understand the problem. I follow you, Profe. I didn't think of that before. Once more,thank you for both of you.