An improved kinetic model and optimized configurations for pellet softening

Abstract

In the last decade, several process modifications took place in the Loenderveen (LDV)- Weesperkarspel (WPK) Water Treatment Plant (WTP) of Waternet. There are four process modifications initiated over time: first, a shift from garnet sand to calcite pellets as seeding materials for pellet softening process. Second, the elimination of acid dosing in the pretreatment plant at Loenderveen (LVN). Third, a set point adjustment in total hardness level to 1.4 mmol/L from 1.5 mmol/L. Finally, a switch from the acid (HCl) dosing to CO2 dosing for conditioning of softened water for further treatment process. The existing pellet softening models with linear calcium carbonate crystallization kinetic (Rietveld, 2005, van Schagen et al., 2008a) describing calcium and pH profile over the height of the bed and the supersaturated calcium concentration after bypass mixed water, are not capable to cope with these process modifications. Therefore, an improved prediction model for softening process and a set of optimal operational configurations is needed. Recent researches (Chiou, 2018, Seepma, 2018) showed the first improvement path by using a prediction model based on bi-linear kinetics and hydraulics (Hout, 2016, Kramer, 2016). Hence, in this research a model is developed on the basis of new knowledge on kinetics on 4 regions depending on saturation ratio and hydraulics in the reactor and proposed optimal operational configurations. The proposed new prediction model is based on experimental data from Continuous Stirred Batch Reactor (CSTR) and Plug Flow Reactor (PFR). The chemical model is described in 4 regions depending on the level of supersaturation. The kinetic rate constants for calcium carbonate crystallization on seeding material for these 4 regions are taken from CSTR and PFR experiments. The model is calibrated and validated based on previous experimental data from WPK and full-scale treatment plant (Schooten, 1985, Seepma, 2018, Schetters, 2013). Finally, the calibrated and validated results from studied model were compared with model outcomes proposed by Rietveld., 2005. Calcium Carbonate Crystallization Potential (CCCP) is the amount of supersaturated calcium in the effluent and determines the efficiency of the entire pellet softening process. A scenario analysis is performed for summer and winter based on bypass, linear velocity and fluidized bed height, aiming for the lowest CCCP, high reliability, minimum cost and sustainability. CCCP determines the amount of chemicals used and principal cost of pellet softening process. The high reliability of the process comes from the full-scale plant operations over 30 years. The cost minimizations takes into account the chemical cost of NaOH and CO2 (dosing chemical). Ultimately, an optimal operational configuration will lead to a sustainable operational approach for pellet softening by using as little chemicals as possible. The outcomes from the scenario analysis provided an operational window of 15-25% bypass and linear flow velocity of 69-85 m/h for pellet reactors depending on temperature (0-24°C). This choice of optimal configuration comes with a cost reduction between 3-4% both in winter and summer. Previous optimum configurations suggested a bypass of 50% with linear velocity of 60-70 m/h (Rietveld, 2005) and a cost reduction of 10%. This reduction of cost is less pronounced because of process modifications, improved prediction model with 4 regional kinetics and the set of operational windows. Therefore, the process modifications induced a different set of operational criteria for optimum outcome in the pellet softening process at WPK.