Refinery Static Mixer CFD: Multiphase Mixing and Pressure Drop Before Supply

A process equipment maker had an order to supply a static mixer to a refinery. The mixer brings a hot vapour stream from a regeneration section together with a recycled caustic liquid, and the refinery's tender asked for a CFD report on mixer performance before the unit could be supplied.

Shirsh Technosolutions carried out the static mixer CFD in ANSYS Fluent as a three-dimensional multiphase study. The first element layout would not give a stable flow solution at the specified line pressure, so we flagged it early, proposed other mixing element shapes and re-ran the model on the modified geometry. The revised design gave a converged flow field and a pressure drop prediction, and the refinery's engineering team reviewed the report and cleared the mixer for supply.

Model and mesh. The mixer was modelled in ANSYS SpaceClaim with the outer pipe, the side injection nozzle and the full series of mixing elements. The fluid volume was meshed with local refinement around the element edges and plates, where the velocity gradients are steepest, and near-wall resolution was checked before the runs.

Multiphase CFD in ANSYS Fluent.

• Three-dimensional, steady-state, turbulent flow with the mixture multiphase model for the vapour and liquid phases.

• Realizable k-ε turbulence with enhanced wall treatment, with mass, momentum and energy solved together.

• Separate mass flow inlets for the main vapour stream and the injected caustic, a pressure outlet at the specified operating pressure and no-slip walls.

• Fluid properties for both phases taken from the owner's process datasheet.

Design iteration. With the first element layout, the solution at the specified line pressure would not settle: the flow field diverged and the turbulence quantities could not be resolved. We reported this to the supplier and the refinery straight away, explained that changing the element configuration was the practical way forward, and suggested alternative mixing element shapes. The modified design uses pairs of crossed baffle plates alternating with half-disc segments, each set turned relative to the one before, so the flow is split and rotated as it moves along the pipe.

Results read for the owner. For the modified mixer we reported pressure and velocity contours, velocity vectors, profiles at several sections along the pipe, liquid volume fraction on cross-sections, streamlines of each stream and a volume rendering of the flow. The pressure drop was taken from the difference in total pressure between inlet and outlet, and the mass balance between inlets and outlet was checked to confirm convergence.

Common questions

What does a CFD study of a static mixer tell you? It shows how the injected stream spreads across the pipe, where the flow speeds up or recirculates around the elements, and how much pressure the mixer costs the line. Element shape, number and spacing can then be compared before the mixer is fabricated.

Why model a gas and liquid static mixer as multiphase? The two streams have very different densities, so a liquid injected into a vapour line behaves nothing like two gases blending. A multiphase model tracks each phase and shows whether the liquid really reaches the whole cross-section.