Towards better understanding of the flow inside industrial processes: a CFD study

Sammanfattning: Computational Fluid Dynamics (CFD) is a method to numerically pre- dict flow patterns. In this thesis the method is used to predict the flow in a hydrochloric acid regeneration plant located at SSAB EMEA in Borl ?ange and to investigate the flow in a burner region of a rotary kiln at LKAB in Kiruna. The first part of the thesis concerns the topic of waste hydrochlo- ric acid (HCl) from the pickling process at SSAB. The waste acid is regenerated using a technique called spray roasting where waste HCl is sprayed into a hot reactor through four spray nozzles at the top of the reactor. The regeneration (drying) process is driven by four natu- ral gas burners placed symmetrically along the periphery of the reactor and causes the water in the waste acid droplets to evaporate. Through chemical reactions the iron chloride is oxidized to form hematite and chloride gas; the latter to be used for manufacturing of new HCl which is used in the pickling process. The byproduct in form of hematite is sold and used in e.g. the electronic industry and as filling material for road constructions. Good quality hematite powder enables profitable sale of the product, and hence an enhanced economy for the whole regenera- tion process which in turn motivates continued regeneration rather than shipping away the waste acid and continually buying new HCl. Due to the hostile environment inside the reactor, measurements and optical visualizations are difficult to perform. This constraint leads to poor understanding of the dynamics inside the reactor, since the process is a black box where waste acid is injected and hematite and chloride gas are collected; what happens between the two stages is not adequately known. Here CFD is a useful tool for the modeling of the physics inside the reactor. In this thesis, the flow inside the reactor is resolved and modeled, leading to improved understanding of the regeneration process. Also, a purely numerical approach is considered where the impact of the wall-grid resolution on the bulk flow is investigated. The second part of the thesis deals with the aerodynamics in a rotary kiln, which is a large rotating oven used in the mining industry for drying purposes. At one end of the kiln a large diffusion flame is placed in order to support the process with heat so that moisture from the material can be evaporated. The flame is fed with secondary air to achieve complete combustion. Traditionally the flame is controlled and stabilized by cre- ating a short and intense combustion zone by feeding just sufficient air for complete combustion. In the kiln used in this project, the material is sensitive to shock drying and uses oxygen to complete the oxidization stage. Hence more secondary air than what is needed for combustion is fed through the secondary air channels, making the flame longer and harder to control. Earlier research of the process has been focused on the cold flow from the secondary channels and has found a possible problem related to vortex shedding in the burner region. In this work the burner is inserted into a water model from the earlier work and PIV/PLIF mea- surements are performed to see concentration fields of the burner fluid. Numerical work is also performed in order to study the details of the vortex shedding and burner interaction. Careful validation is performed with experimental data from two interesting cases, the square cylinder and free jet, before the two models are combined to see the effect of vor- tex shedding on the mixing of the jet and secondary air. In analyzing the results of the vortex shedding and centerline concentration, dynamic mode decomposition is used to find the predominant structures of the system. In this thesis, the CFD software Ansys CFX has been used, which is commercially available, as well as the open source project OpenFOAM.