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[astro]

I think you need to separate your thinking about this problem.

First you need to achieve feasible separation in terms of mass transfer, then, second account for equipment specifics associated with entrainment/flooding/weeping/choking (if you need to get that far).

Go back to basics and have a look in your standard student textbook - Perry. Ed 7 devotes chapter 14 to Gas Absorption and Gas-Liquid Systems. Look at diagram 14-2 where it depicts the feasible operating regime of a generic system G/L contactor. If your simulation places your operating point outside the feasible region, the computer won't lie and will tell you just that but use an error message such as "zero liquid flow", or similar. Your situation may well be that the feasible region is very narrow / small and therefore very sensitive to change.

There are following sections devoted to determining the number of theoretical trays or packing transfer units / height required.

Once you've got the theoretical separation that you're after, then start to look at detail issues such as the type of column internals and the specific choices that are associated. Then your problem becomes one of hydraulics and separation efficiency. Go ahead now to figure 14-23 (trays) & 14-46 (packing) and you'll see the impact of the hydraulic balance that is separate from the VLE issues considered previously.

You're tackling a mixed bag of components. When faced with difficulties like this, a good option is to use data from a model that solves or use actual plant data and achieve convergence first. Simulations will tend to solve quicker and more reliably the closer you are to the solution, which unfortunately is of little help if you're starting from scratch with a clean sheet of paper and limited references to consult.