Towards Predictive Modeling of Crystallization Fouling: Taking into Account Fouling Layer Structures
Jie Xiao, Feng Zhang, Jian Han, Xiao Dong Chen
Suzhou Key Laboratory of Green Chemical Engineering
School of Chemical and Environmental Engineering
College of Chemistry, Chemical Engineering and Materials Science
Soochow University, Suzhou, Jiangsu Province 215123, China
The fouling layers formed on heat exchanger surfaces exhibit complicated structures, which essentially determine flow hydrodynamics, fouling kinetics and hence the heat transfer performance. In-depth understanding of deposit layer formation and its structural effects on the fouling process could lead to effective fouling mitigation or even prevention strategies. The exploration tasks, however, are not trivial due to a lack of knowledge of the crystallization process and the complicated deposit structure.
In this talk, I will summarize our past and on-going efforts on numerical simulation of different stages of a complex crystallization fouling process, focusing on deposit layer growth and its impact on the fouling process. The developed method incorporates a pseudo-dynamic scheme where the dynamic fouling process is approximated as a set of sequential steady-state processes taken place in a continuously varying geometric domain. This unique approach allows the characterization of mass, momentum and heat conservations of a continuous flow of liquid over a growing fouling layer. Dynamic evolution of the fouling layer surface and its intricate interactions with hydrodynamics and fouling kinetics can then be rigorously taken into account. The introduced model was validated using the experimental data for a calcium sulphate fouling system. The effects of the solution chemistry and operating conditions on fouling resistance evolution were quantified through a comprehensive parametric study. Furthermore, we are developing methods that can be used to quantitatively evaluate the effects of fouling layers' interior structures on fouling dynamics. We have systematically investigated four representative schemes for fouling layer characterization: i.e., a homogeneous porous medium that is impermeable to water (HoIm), a heterogeneous porous medium that is impermeable to water (HeIm), a homogeneous porous medium that is permeable to water (HoPe), and a heterogeneous porous medium that is permeable to water (HePe). Under the same operational conditions, four models offer significantly different prediction results on the fluid velocity, temperature distribution and fouling resistance. It can be concluded that numerical model development should take the fouling layer structure into account, and the scheme of HePe that best resembles a real fouling layer structure should be a promising option.
Keywords: Crystallization fouling; fouling inhibition; fouling layer structure; CFD model; pseudo-dynamic scheme