(497a) CFD Simulation and Experimental Validation for An Industrial Crystallizer of Potassium Chloride

Abstract: Some pioneering studies have been done very well to understand the thermodynamics, kinetics and processes of carnallite dissolution and KCl crystallization, but little work has been done to study the influence of hydrodynamics in an industrial crystallizer of potassium chloride. In this paper, the computational fluid dynamic (CFD) solver is used to simulate the fluid flow field in such units and optimize both the impeller design and the operating parameters, in order to reduce the energy consumption for KCl production from carnallite. According to the CFD optimization design, a new impeller (axial-flow hydrofoil impeller) was retrofitted into an industrial crystallizer of potassium chloride at Qinghai Lake Factory of China in 2008, and its performance was evaluated against data collected using the original pitched blades impeller. The DTB(draft tube and baffle) crystallizer available to 1,000,000 tons/year KCl production is optimized by the CFD simulation. The structure and main dimensions of the industrial crystallizer are crystallizer high H=14.8m with a columniform upper part (diameter D=12.2m) and a coniform under-part, draft tube with H=7.5m, D=3m, baffle with H=10.4m, D=2.7m, and impeller with D=2.5. In CFD simulation, the fluid flow fields are solved from the Navier-Stokes equations, where the multi-reference frame (MRF) is used and the RNG k-å turbulence model is chosen for strong swirling flows in FLUENT6.3. The KCl crystal size distribution is described with Rosin-Rammler model. The impeller design and the operating parameters are optimized by CFD simulation, which include the impeller shape, rotational speed and the position, the feedstock flow rate, fluid viscosity, temperature et al. Figure 1 and 2 show the typical fluid flow field and the typical crystal particles concentration field in DTB crystallizer with the axial-flow hydrofoil impeller, respectively. CFD Simulation results show that: the fluid velocity in the cycle zone is larger while it is smaller in the sedimentation zone when using the axial-flow hydrofoil impeller to mix the feedstock, which favors the growth of KCl crystals. Moreover, the fluid velocity distributions in both the cycle zone and the sedimentation zones are significantly better than that using both the pitched blade impeller and the Rushton turbine, the power consumption is also reduced significantly during crystallization. Then, the influence of the rotational speed of the impeller was studied. CV coefficient of variation become smaller after the first big, so an optimum rotational speed can be obtained as 65-70rpm with an optimum position 5.7m from the crystallizer bottom. The feedstock flow rate and temperature have little influences on both velocity distribution and energy consumption. An industrial application example of DTB crystallizer with the productivity of 1,000,000 tons/year KCl is demonstrated at Qinghai lake factory in china, in which the pitched blade impeller is substituted by the axial-flow hydrofoil impellers, and the operating parameters are optimized further by CFD simulation. With the improved impeller and the optimum operating conditions, the product quality is increased, that is the percentage of product with the crystal size more than 2mm is raised from 50% to 90%; the amount of the natural gas consumption is decreased from 22m3 to 15m3 per ton of product. Moreover, the power consumption used by the new impeller also is reduced by 15% in contrast with the original pitched blade impeller.