Phenomenological modeling of black liquor evaporators, found in the pulp and paper industry, presents a very challenging task. The physicochemical phenomena that occur do not lend themselves readily to known mechanisms, and in many instances, data to support these hypotheses are difficult to obtain. In this work, a distributed-parameter model (a system of partial differential equations) based on first-principles knowledge about the fluid dynamics and heat-transfer processes is developed for a falling film lamella type evaporator. Primarily, the model describes falling film evaporation on one lamella. The model is solved using orthogonal collocation on finite elements in the presence of scaling and disturbances in the mass feed rate, feed dry solids content, and wall temperature.