An equilibrium model for simulating the deposition of calcium carbonate in cooling towers

Solids deposition can decrease cooling towers performance since deposits affect the flow through the fill and reduce the air–water contact area. This work aims to develop a mathematical model of calcium carbonate deposition on the fill of a cooling tower. The tower is considered a sequence of equilibrium stages in which the vapor, liquid, and solid phases coexist. The tower model includes mass and energy balances and a thermodynamic model based on the Gibbs free energy minimization method, which determines the composition of the three phases. The model is used to predict the mass of calcium carbonate deposited on the fill as a function of time and stage number in two examples. In the first example, the deposition rate obtained through simulation is compared to the deposition rate experimentally measured in a pilot-scale cooling tower. Despite uncertainties about its operating conditions, the developed model predicted the deposition tendency and the results showed that the species concentration in the cooling water may reduce by up to 96%. In the second example, the deposition rate is calculated for different make-up water compositions at four cycles of concentration. According to the simulations, deposition occurs only when the cooling water is very hard. Simulations also revealed that deposition takes place only in the first stage in the second example. Also, in all the cases where deposition was predicted, the inlet water stream reached the same composition after some time. These results could be applied to optimize the blowdown and make-up flow rates and to help minimize costs with tower cleaning and water treatment.