Dynamic models of NaOH–H2O falling-film absorbers/desorbers are needed for process analysis and future controller design, but time-resolved experimental data for the target system are not yet available. A dynamic segment-resolved model was developed for a horizontal-tube falling-film heat exchanger. Each of the N = 10 segments is described by four coupled ordinary differential equations governing inner fluid temperature, wall temperature, external solution mass flow rate, and NaOH mass fraction. Absorption and desorption are represented within a unified framework using mode-dependent heat-transfer assumptions and thermophysical property data for NaOH–H2O. For absorption, the simulated steady-state concentration change is Δw = 0.105, close to the reference value of Δw = 0.10; the small residual deviation is attributed to the surfactantfree operating condition of the model. For desorption, the simulated NaOH concentration of approximately 0.45 lies within the reference range of 0.44 to 0.49. Energy-balance errors below 0.05% confirm the numerical consistency of the implementation. The model yields steady-state results consistent with reported reference values and provides a physically sound dynamic formulation suitable as a preliminary plant model for future model-based control studies. Its primary current limitation is the absence of time-resolved experimental data for transient validation and parameter identification. A sensitivity analysis confirms that the steady-state results are analytically independent of the PT1 time constant τ , and quantifies the strong dependence on αe and on the number of discretization segments N.