Closed-loop LPG splitter — dual zero-offset PI control

Two independent, genuinely converged integral (PI) control loops in one train, wired on the canvas (transmitter -> controller -> manipulated unit, not just reported): a feed preheater's duty holds the column feed at its temperature setpoint, and the column's distillate-to-feed split ratio holds the bottoms temperature at its setpoint — a classic temperature-inferred composition control scheme (this solver has no composition transmitter, so temperature is the composition proxy, exactly as most real columns are actually controlled). Both loops reach a genuine zero-offset steady-state operating point via the flowsheet's outer fixed-point iteration (result.control_iterations > 0), each report's measured equal to its setpoint. This is honestly decentralized SISO PI control, not simultaneous MIMO DMC (no predictive horizon, no MV/CV interaction matrix, no constraint handling) — that capability lives in the separate live APC/MPC runtime (see the Control Room), not a static flowsheet example.