This work studies the control of a single falling-film evaporator and a multiple-effect falling-film evaporator plant. The specific case studied is the concentration of solids found in the liquor that is part of the production of sulfate (kraft) pulp. The concentrated stream is the feed to the recovery boiler, a unit operation that plays a significant role in the economics of the pulp mill. The disturbance compensation of a single-loop strategy based on proportional-integral feedback controllers is compared against a model-based strategy that uses optimal control theory.