CFD Analysis of a Kenics Static Mixer with a Low Pressure Drop under Laminar Flow Conditions

Abstract

Kenics static mixers enhance mixing under laminar flow conditions. The ideal static mixer achieves efficient mixing at a low pressure drop. A modified design of the Kenics static mixer is analyzed, featuring gaps between the mixing elements, which can achieve the same level of mixing as the conventional design but with fewer mixing elements and a substantially lower pressure drop. The mixing effects at the entrances and exits of the mixing elements are enhanced by the introduction of gaps between the elements. The performances of mixers based on the right−left and right−right configurations with different gap lengths are characterized in terms of pressure drop, coefficient of variance of concentration, residence time distribution, and extensional efficiency with computational fluid dynamics simulations. Furthermore, the coefficient of variance of concentration is measured experimentally with several three-dimensional (3D) printed devices. The gaps reduce the mixing length when the design is based on the right−right configurations, and the gap-to-diameter ratio is 0.5 or 1.0 compared to the corresponding conventional design. Furthermore, Taylor dispersion is suppressed with the introduction of gaps, which enables a narrower residence time distribution. The presence of gaps between mixing elements introduces an additional degree of freedom, which can be utilized to strike a compromise between the required mixing length and pressure drop.

Mixing Efficiency and Residence Time Distributions of a Side-Injection Tubular Reactor Equipped with Static Mixers

“…Instead, empty reactors in laminar regime have to rely on molecular diffusion to even out radial gradients. 8,9 Therefore, a plug-flow (PF) behavior can only be achieved in an empty tube if the reactor is extremely long 9,10 or if the tube is bent or coiled to reduce axial dispersion. 11,12 Alternatively, the process can be carried out in a series of CSTRs, but with a very large investment cost, as theoretically a PFR performance is reached with an infinite series of CSTRs.…”

Influence of Buoyancy Effects on the Mixing Process and RTD in a Side-Injection Reactor Equipped with Static Mixers

“…This indicates that 10 million is the lowest number of cells above which the results are independent of the mesh. As a further confirmation, since pressure drops are particularly susceptible to perturbations, the pressure losses inside the reactor were compared to the ones obtained using an empirical correlation reported in eq where K L is a dimensionless factor and it varies in a range of 820–1280, depending on technical details such as the number of bars; for a standard Sulzer SMX with 8 bars, the K L factor can be assumed roughly equal to 1070. , The obtained results are reported in Figure , showing a good agreement between CFD and empirical correlation predictions.…”

“…This indicates that 10 million is the lowest number of cells above which the results are independent of the mesh. As a further confirmation, since pressure drops are particularly susceptible to perturbations, 16 the pressure losses inside the reactor were compared to the ones obtained using an empirical correlation 26 reported in eq 5…”

“…In these cases, the mixing process among the streams of reactant is based only on diffusion, 14,15 and often, the resulting mixing times are too large, causing undesirable situations in terms of process planning, productivity levels, and product quality control. 13 Considering an empty tube, a plug flow behavior can be obtained only if the tube is extremely long with respect to its diameter 15,16 and it possibly follows a coiled geometry. 17,18 Alternatively, active mixers such as ultrasonic micromixers and pulse-flow mixers can be employed, but their efficiency is hindered by their high cost, difficulty of maintenance, and high failure rate.…”

“…In these cases, the mixing process among the streams of reactant is based only on diffusion, , and often, the resulting mixing times are too large, causing undesirable situations in terms of process planning, productivity levels, and product quality control . Considering an empty tube, a plug flow behavior can be obtained only if the tube is extremely long with respect to its diameter , and it possibly follows a coiled geometry. , …”