Hi
if you are using commercial softwares, then they may not be used to do optimization studies like NLP, MINLP or similar optimization tools can do. I think ASPEN has got some sort of optimizer with it but they got limitations with them. Like it can't optimize height on its own.
What you can do is use sequential procedures to find the trade offs. for examples..first you do the simulations with different heights and see the effect on your objective function. Then you choose second variable..say absorbent flowrate and see the effect on objective function. This will give you some sort of idea about the trade-offs in the absorber and desorber. Then you can choose optimal parameters and design the columns. But you can't do things simultaneously. This could give you suboptimal results. You can pose some limitations like min flowrates etc.
If you want to optimize the papameters and variables simulataneously, then you need to model these columns on your own and use some sort of optimizer. This could lad you to optimal solution hopefully. But have to be careful which kind of optimization you are using. For example, if you have to optimize number of trays alongwith other variables then it is integer variable and then problem becomes mixed integer NLP. If variables are only continous then it is NLP. Also you can try stochastic optimizers.
There are some books on process design and synthesis which can help you.
Regards
Prashant
