General Mixing Applications
Seeking the optimal mixing solution always begins by analyzing the required agitation duties. Single-duty agitators are seldom used. Together with the customer, we are able to define the optimal solution based on these principal duties or a combination of the most important ones:
- Blending of fluids
- Suspension of particles
- Dispersion of gas
- Dispersion of particles or droplets
- Mass transfer of reactants
- Heat transfer
Blending of fluids
Mixing two soluble liquids in a macroscopic perspective is referred to as the blending duty. The blending time is mostly dependent on the impeller’s pumping capacity. Relatively light-duty hydrofoil impellers, such as HDL or THETA units, are normally applied for blending due to their good pumping efficiency. When the viscosity of the fluid increases, blending requires heavier duty impellers, such as ZETA, SIGMA and ALPHA units.
Typical process steps in which blending is the main agitation duty:
- Paste mixing
- Slurry and sludge mixing
- Polycondensation (e.g. esterification)
- Fermentation (bulk phase and nutrient blending)
- Esterification, transesterification
- Oleochemicals
- Reaction support
- Neutralization
- pH Adjustment
- Biomass mixing
- Storage tanks for food products, e.g. wine, milk, edible oil
- Storage tanks for biodiesel
- Yogurt fermentation
- Cheese production
Suspension of particles
Agitation is the most common duty of suspension, keeping the vertical flow high enough to prevent particles from settling. Applications include fluid/solid reactors, crystallizers, storage tanks, feed tanks and buffer tanks for slurries. In some cases, particles are lighter than liquid, in which case the suspension duty is to prevent the formation of a floating layer on the surface.
Suspension to a large extent involves optimizing the mixing energy needed to reach the required degree of suspension. Impellers with a low shear force and radial flow component, such HD, HDL and THETA units, are the most efficient choices for this duty.
Suspending slurry tank emptying all the way down to the bottom is a special case and requires a different approach, such as an RA impeller.
Maintaining droplet or bubble suspension is a more complex duty that involves not only preventing floating or settling but also preventing an increase in the droplet or bubble size. Generally, a higher shear impeller, such as our proprietary HySol3 unit, is needed in mixing to break up the merged bubbles or droplets.
Typical process steps in which maintaining suspension is the main agitation duty:
- Reactions with catalysts
- Paste mixers
- Crystallizers (cooling crystallization, evaporation crystallization, recrystallization)
- Polymerization reactors with solid adding (e.g. PTA in EG to form PET).
- Precipitation polymerization
- Solvent polymerization
- Bulk polymerization
- Solid dissolving
- Precipitations
- Absorptions
- Slurry storage
- Feed tanks
- Buffer tanks
- Reversed suspensions (floating layer drag- in)
- Biogas fermenters (hydrolyses, primary and secondary fermenters)
- Bioethanol fermenters, liquefaction, saccharification, beer well, yeast
- Propagators, coagulation tanks, grain breaking, hydrolyses
Dispersion of gas
Typically, when the mass transfer of a gaseous reactant through the gas liquid interphase is the limiting factor of a chemical reactant, dispersion of injected gas is the principal duty of the agitator. The duty of agitation in this case is to break up the injected gas into fine bubbles and mix it evenly with the liquid/slurry.
The agitator’s capacity of gas/liquid surface generation is mostly dependent on the impeller’s shear forces and resistance to flooding. Our proprietary RP6 Concave, GasGen4, GasSol4 and HySol3 Turbine impellers are designed for dispersing gas.
Typical process steps in which dispersion of gas is the main agitation duty:
- Aeration in fermentation processes (membrane gassing, sparger gassing, sparging lance gassing)
- Amino acids, antibiotics, xanthan, vitamins, polysaccharides
- Alkoxylation (e.g. ethoxylation, propoxylation)
- Carbonization, hydration, chlorination, sulfurization
- Stripping
- Evaporation
- Degassing
- Hollow shaft generator gassing
- Pressure and atmospheric leaching
- Gold bioleaching
- Ammonization and digestion reactors
- DAP (di-ammonium phosphate) reactors
- Attack and maturation tanks for phosphoric acid production
- Wastewater treatment
Dispersion of particles or droplets
Dispersing particles to the liquid is typically a principal agitation duty when mixed particles are extra small or hydrophobic, agglomerates need to be broken, or the emulsion of insoluble liquids needs to be generated. The heavy dispersion capacity requires extra high shear forces from special agitators, such as Tooth Disc Turbine, RP6 Concave or Jet Cage units. In special cases, the liquid dispersion droplet size distribution needs to carefully controlled with a very specific impeller configuration, for example using a SOLVEX SXL Pumper.
Typical process steps in which dispersion of particles/droplets is the main agitation duty:
- Emulsion polymerization (e.g. PVC)
- Suspension polymerization (e.g. PVC)
- De-agglomeration
- Agglomeration (e.g. NBR, HNBR, EPR)
- High shear powder dissolving
- Extraction processes
- Dispersion and emulsifying with jet cage (rotor-stator) systems
- Ointments and creams
- Colors and paints
- Mashing
- Pulping tanks
- Solvent extraction dispersion pumps
- Mixer settler units for salt in oil extraction
- Thickener dissolving in low viscous fluids
Mass transfer of reactants
For process units involving fast chemical reactions, microscale mixing of the reactant is more critical than the blending of raw materials, so mass transfer is the principal agitation duty. An effective mass transfer requires relatively intensive turbulent energy, so the impeller should provide a radial mixing component or shear forces.
Traditionally, generic pitched blade turbines have been applied for mass transfer duty. Today, however, special hydrofoils, such as the GasSoL4, HySol3 or a combination with axial flow turbines (HDL, THETA) and radial impellers (RP6) are increasingly applied for this duty.
Typical process steps in which mass transfer is the main agitation duty:
- Chemical reactions
- Synthesis reactions
- Polymerization
- Saccharification
- Fermentation
- Esterification, transesterification
- Refinement
- Synthesis
- Oleochemicals
Heat transfer
Cooling or heating the liquid by coils, vertical tube baffles or wall jackets determines the agitation heat transfer duty. The agitation liquid turbulence and liquid velocity in turn determine the heat transfer coefficient.
Although heat transfer is seldom the principal agitation duty, it is often strongly connected to a chemical or physical reaction, such as evaporation or crystallization. For low-viscosity fluids, good impellers for heat transfer are the HDL, THETA, GasSol4, HySol3. For high-viscous fluids, ZETA, SIGMA and ALPHA units are suitable.
Typical process steps in which heat transfer is the main agitation duty:
- Crystallizers
- Polymerization (e.g. esterification, suspension polymerization)
- Fermentation of Newtonian or non-Newtonian fluids
- Mixing cocoa mass
- Fruit cooking
- Fats and waxes
- Oleochemicals