Possibility of Electromembrane Softening Treatment of Network Water at a Thermal Power Station

Abstract

The possibility of electromembrane softening treatment of network water at a thermal power station is shown. Technology for the manufacturing of an inert anode free from noble metals or their compounds is proposed. Performed resource tests have demonstrated that such an anode can be in operation in chloride–sulfate solutions for several years almost without wear. A mathematical model describing the electrical current utilization efficiency as a function of process parameters and electrolysis regimes in the electromembrane softening of water has been developed. The results of calculating the current utilization efficiency coefficient by this model are in satisfactory agreement with experimental results. The current utilization coefficient in the formation of NaOH does not depend on the anolyte composition but appreciably decreases with increasing catholyte concentration and slightly grows with increasing current density at a constant catholyte concentration. Experiments have not disclosed any essential dependence between the specific water and CO 3 2- ion transports and the process parameters. Experiments on estimating the depth of the electromembrane softening of initial water for the heat supply networks of Kharkiv have demonstrated that the electromembrane method provides the residual hardness of 0.3–0.4 mg-equiv/dm3, which is almost four times lower than for water subjected to soda–lime treatment. The experiments performed on a test bench setup have confirmed that the application of heterogeneous cation-exchange membranes (both MK-40 (OAO Shchekinoazot) and CMI 9000 (Membrane International)) provides the possibility to attain a high current utilization ratio above 88%. The carbonate index of water after electromembrane softening corresponds to the normative requirements to water for heat supply networks with a heat transfer agent temperature below 150°C (no higher than 0.4 (mg-equiv/dm3)2]. This provides the possibility to use the treated water immediately in heat supply networks without further ion-exchange softening. Some technical and economic parameters of the softening process and the technical parameters of an industrial electromembrane softener are given, and the possibilities of the utilization of wastes and byproducts are considered.