Hi. I'm trying to simulate an exis

[Jack Nicholson]

Hi.
I'm trying to simulate an existed cold box in C2 recovery plant (as attached picture).
To simulate this heat exchanger, you should give outlet temperature except for one, or give another data to simulator such as UA, LMTD, etc.
But here is the problem:
As you can see there are lots of recycle streams, and I can not fix outlet stream temperatures (Because in this way we can't evaluate changing in plant). So I want to find any other constraint on this cold box (such as duty ratio, dT,...). Is there anyone help me???

 

[georgeverghese]

a) Should be based on UA value for each cell in E1211 / E1212.

b) Strictly speaking, the U value for each cell depends on mass flow, so you can set up a calculator to recompute the new value of U for each trial / recycle iteration value for mass flow.

Suggest doing (a) first without including the U value correction. Once this model works okay, then set up (b)the calculator for U value correction.

This may be a difficult model to converge, so loosen all your tolerance limits for recycle convergence (mass, component balance, enthalpy etc) to get the initial approximate solution. Then you can use these values as better trial inputs to the next simulation and gradually tighten recycle convergence limits. Read the simulator ops manual on how to manipulate recycle convergence limits. Also loosen the solution tolerances for ADJUST (Hysis) or Controller ( Pro II) operations initially. Also choose a less complicated EOS for initial solutions if this helps.

You need time and a great deal of determination to get these models simulated and working well.

 

[Jack Nicholson]

Dear georgeverghese, thank you very much.
Quoted:
a) Should be based on UA value for each cell in E1211 / E1212.
the U value for each cell depends on mass flow.
Unquoted.
But in a real plant data, when only temperature changed, the UA data also changed drastically!!!

 

[georgeverghese]

For a given mass flow in a cell, the U value (strictly speaking the h value) will only vary little if there is no phase change as the fluid moves from inlet to exit. Would guess the h value could change in this application only if there is a phase change related to a mass flow change and / or feed temp change or overall heat balance change.

Sometimes these licensors dont tell you everything that is set up in the cold box exchangers. For example, in condensing service in a certain cell, there may be intermediate separators with vapor and liquid split sent further into some other cells. And they may not have given you the internal assembly detail drawings of these inside the cold box.

See if you can figure out how these U or h values changes with mass flow/ feed temp, else you may have to ask the licensor. If you can, post us the performance details of one of these exchangers for all cells / heating or cooling curves for a few modes and any diagrams for internal arrangement and we may try to get some clues from these.