Numerical modelling of swirling drying flow

Abstract: Waste hydrochloric acid (HCl) from the pickling process at SSAB EMEA in Borl¨ange, Sweden, 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 natural 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 anew is used in the pickling process. The produced byproduct in form of hematite is sold and used in e.g. the electronic industry and as filling material for road constructions. A good quality of the hematite powder vouch for profit of the sold product, and hence an enhanced economy for the whole regeneration process which in turn motivates a continued regeneration rather than shipping away the waste acid and continuously buying new HCl.Due to the hostile environment inside the reactor, measurements and optical visualizations are difficult to perform. This limitation lead to a poor understanding of the dynamics inside the reactor as the process is a black box where waste acid is injected and hematite and chloride gas is collected; what happens in-between the two stages is not known to any larger extent. Here Computational Fluid Dynamics (CFD) is a useful tool for the modeling of the physics inside the reactor. In this licentiate thesis, the commercially available CFD software Ansys CFX has been used to model the gas flow and temperature inside the regeneration reactor as part of the main goal of modeling the whole regeneration process. As a first approach, the gas flow and temperature behavior are of certain interest as these quantities determine the dynamics of the injected droplets through their journey inside the reactor, and hence the regeneration process. The present work comprise two papers devoted to the actual application of HCl regeneration, and one paper which emphasize the importance of grid independency studies and the impact of grid element size in regions with high gradients. In the two application-focused papers (Paper A and C) the gas flow and temperature profile inside the reactor is resolved together with the particle (droplet) distribution. In Paper C the study is developed to also comprise the influence on theses quantities due to different locations and spray directions of the spray nozzles located at the top of the reactor.