Vortex mixers: New strategies for gas and gas solid mixing

Many industrially important chemical reactions involve mixing of gases alone or in the presence of solid catalysts. Apart from the catalyst activity, the performance of the reactor directly depends on the hydrodynamics of the reactor and efficient mixing is crucial for the safety and yield of the operation. While single phase industrial reactors suffer from poor mixing where rapid mixing of gases are necassary, fluidized bed reactors suffer from non-homogeneous flow regimes such as channelling and slugging which results in formation of hot spots and low yield. Vortex mixers, consisting of a cylindrical chamber connected by two or more tangentially opposed jets, create swirling vortex patterns near the jets, characterized by flow which rotates along an axis, like a tornado can deliver rapid and efficient mixing both in single and multiphase flows. In the scope of this thesis, gas phase flow hydrodynamics, and gas-solid flow hydrodynamics in vortex mixers were studied in two parts. In the first part, flow hydrodynamics of gases in the proposed vortex mixer was explored by utilizing flow visualization and particle image velocimetry (PIV) techniques for 𝑅𝑒 numbers from 𝑅𝑒 = 20 to 280. The detection of the critical 𝑅𝑒 number of flow where both jets fed the same gas, was performed to analyse the flow regime transitions of two gas streams coming from two tangential jets. Results demonstrate that methane flow started to swirl around 𝑅𝑒 = 70, and the instabilities in the flow started around 𝑅𝑒 = 230, below which, gas-flow hydrodynamics was diffusion controlled. Beyond 𝑅𝑒 = 230, unsteady engulfment flow regime occurs and convection dominates the diffusion. To examine the influence of physical parameters on gas-flow hydrodynamics, three gases, namely; hydrogen, nitrogen, and argon were selected due to the viscosity and density differences they offer. The initial critical point of flow, where gases rotate completely in the chamber, is affected by the inertial force in terms of dynamic pressure. While hydrogen had the greatest kinematic viscosity and completed a full rotation at 𝑅𝑒 = 40, nitrogen and argon completed a full turn at 𝑅𝑒 = 70. Following these, continuous rotating flow was observed due to the equilibrium between centrifugal and centripetal forces. Similarly, the formation of engulfment regime was detected earlier for hydrogen, at 𝑅𝑒 = 150, than for nitrogen at 𝑅𝑒 = 200 and argon at 𝑅𝑒 = 220. Gas properties were found to have an influence not only on critical points but also on the shape of the swirling flow pattern. The gas-flow hydrodynamics of different gases, hydrogen, and nitrogen, with argon at various 𝑅𝑒 numbers was also investigated by feeding each jet with a different gas. While the first critical point was found to be at the same 𝑅𝑒 number (𝑅𝑒 = 70) for both Ar/H2 and Ar/N2, the second critical point was discovered to be at 𝑅𝑒 = 120 for Ar/H2 and 𝑅𝑒 = 190 for Ar/N2. Because of the increasing shear stress between fluid particles, the convection dominates diffusion as 𝑅𝑒 number increases. Because xxiii hydrogen has a faster gas velocity and consequently a larger shear stress than nitrogen at the same 𝑅𝑒 number, the critical 𝑅𝑒 number of the Ar/H2 flow was discovered to be lower than the critical 𝑅𝑒 number of the Ar/N2 flow. In the second and final section of the thesis, for a better performance,a new conceptual approach for a fluidized bed were introduced and its hydrodynamics were studied. In classical fluidized beds, solids are the continous phase and the gas is the distributed phase. In this concept, to improve the performance of the fluidized bed by having a flow regime providing homogeneous mixture of solids and gases, where the gas phase is the continuous phase and the solids are the distributed phase is aimed. For this reason, gas-solid hydrodynamics of particles with varying densities and sizes in vortex mixers with varying jet and chamber diameter ratios were examined. The study was performed by visual experiments using a fast camera, and by instantenous pressure values recorded using differential pressure transducers along the jets and the chamber. The minimum fluidization 𝑅𝑒 numbers determined from the flow visulaization study were found to be the same with the results obtained from pressure analysis study. The optimum operation flow regime where solids are distributed homogeneously (between Remf and Ret) can be determined online using the differential pressure transducers for the proposed vortex mixer. The implementation of pressure analysis in these rectors supported the fact that fluidization regimes and hydrodynamics may be monitored and controlled based on pressure signals in industrial systems where fluidization is not visible. Furthermore, it was discovered that particle size, density, and porosity all have complicated effects on gas-solid hydrodynamics.