A flue gas washer is a plant for the absorption of noxious components of industrial gas output. The type of flue gas washers that is considered here is a spray tower. In the tower gas is raising upward and water drops with suspended limestone fall downward. Thereby sulfur dioxide is washed out of the gas. In industrial practice, the plant is operated under long term constant gas and water input. In this article, a mathematical model for equilibrium states of the plant is set up from basic principles. Its discretized version consists of a large system of algebraic equations for the relevant chemical concentrations in the two phases. An efficient iterative algorithm for solving this system is proposed and shown to converge in a wide range of examples. A satisfactory validation of the model is given by comparison with experimental data from existing literature. It is demonstrated that the model along with the algorithm is applicable for the design of spray towers. (c) 2006 Elsevier Ltd. All rights reserved.