NGL fractionation: single-shell Petlyuk dividing-wall column

A natural-gas-liquids cut (ethane / propane / n-butane) split into three on-spec products by ONE thermally-coupled dividing-wall column instead of two columns in series. This uses the native Petlyuk unit op: a prefractionator whose reflux and boilup are supplied by the main column (the single condenser + single reboiler), solved by an outer Wegstein loop that converges the bidirectional vapor/liquid coupling to self-consistency — the coupling legacy sequential-modular solvers approximate with two connected columns whose recycle fails to converge. Honesty note: the vapor-split and liquid-split ratios are specified inputs (not solved from tray hydraulics), and this is an outer-loop coupling of two rigorous MESH sub-solves, not a single monolithic Petlyuk MESH.