NUMERICAL SIMULATION OF WALL-BOUNDED FLOWS THROUGH SCREEN-TYPE STATIC MIXERS

Abstract

Mixing is a critical operation in most chemical processes. It is found in a multitude of operations ranging from simple blending to complex multiphase flow contacting. While mechanically agitated tanks, bubble and packed columns were traditionally employed, many process industries are now shifting to in-line static mixing as an alternative mixing method because of their good performance at low operating costs and enhanced safety conditions. A multitude of designs is available on the market, one variant of them has proven efficient in processing multiphase operations. This relies on the use of woven meshes as screen-type static mixers (STSM). This thesis numerically investigates the flow behavior and mixing performance of STSMs using single-phase flows under laminar and turbulent regimes. In addition, it proposes a new mixer design based on modifications to the flow through STSM in order to enhance their performances. For this purpose, a three-dimensional wall bounded flow model was developed and studied using computational fluid dynamics (CFD). The effect of varying the mixer geometry, number of mixer elements, inter-mixer spacing, and operating conditions on the hydrodynamics and mixing performance were detailed. The validation of the numerical results was performed by means of pressure drop measurements where a maximum relative error of 13.3% was recorded. Under laminar flow conditions where the Reynolds number based on the empty pipe diameter (Repipe) ranged between 30 and 1850, screens were found capable of flattening the parabolic velocity profile until extended downstream distances. Moreover, the nature of the flow was found to be three-dimensional and cannot be simplified. Using a Lagrangian particle tracking technique, mixing was quantified, and screens were found inefficient at promoting radial mixing, however, this is counterbalanced by their high potential in delivering good dispersive mixing. Under turbulent conditions where the Reynolds number varied between 9,000 and 56,000, the flow through screens revealed that the mean flow energy dissipation cannot be overlooked as it constitutes a major component of the total energy dissipation rate. In addition to the analysis of the velocity field, the study highlighted the high dispersive mixing potential of STSM and its low distributive mixing capability. Moreover, residence time distribution analysis showed that near plug flow conditions could be attained using these mixers. Based on the outcome of these studies and to overcome the distributive mixing limitations of screen mixers a new design was proposed. It relies on the use of woven meshes in conjunction with specially designed downstream inserts. These inserts aimed at enhancing distributive mixing while maintaining the high dispersive action. The flow through this new mixer was then analyzed under turbulent conditions (5,000 < (Repipe) < 30,000) using a Eulerian approach. The results showed that the added cost of operation due to the presence of additional inserts is counterbalanced by further enhancing both dispersive and distributive mixing, where the new mixer provided about 95% homogenized flow, a feature which was almost absent in a STSM.