Physico-Analytical Modeling of an Industrial Low-Temperature Natural Gas Separation Line with Three-Section Cold Recovery and Propane Cooling

This study develops and tests a first-principles dynamic model of an industrial low-temperature separation (LTS) train with three-section cold recovery and propane refrigeration (LTS-4 line, Mubarek Gas Processing Plant). Mass, energy and hydraulic balances of the coolers, the recuperators 20T-01 and 20T-02A–C (including tube-wall heat capacity), the separators and the propane evaporator are coupled with Peng–Robinson phase-equilibrium relations into an index-1 differential-algebraic system with 13 + 12N states. A 58-state thermal core was calibrated on 5 h and validated on 7 h of 2 min supervisory control and data acquisition (SCADA) records that include a 19% throughput step. The 20C-01 gas temperature was predicted with a validation root-mean-square error of 0.49 K (R2 = 0.93), against 1.89 K for a persistence predictor. The linearized core is asymptotically stable; the wall heat capacity, 14.5 times the gas holdup, raises the slowest time constant from 2.4 to 15.2 min. The return-gas loop amplifies the evaporator response to propane-temperature changes by 33%, and one cell per section underestimates the cascade effectiveness (0.64 versus 0.81). Cold-section temperatures were not reproduced (bias up to −5.4 K), indicating an unmeasured disturbance and the need for inter-section measurements.