General Process Considerations In The Selection Of Agitators For Simple Applications And Impeller Design
Classification towards Purpose
a. Blending of miscible liquids
Easiest of agitating applications and normally axial turbine or hydrofoil impellers are preferred. Jet mixing by recirculation with an ordinary pump is equally good at lower cost
In dissolving, we want provision of high flow rate and low shear past the solid surface. Unless the solid is polymeric or sticky or viscous in nature, it is an easy operation. Data solicited are solid percentage, physical characteristics with interim changes, temp, solubility and permissible dissolving time.
c. Dispersion (liquid- liquid, solid-liquid)
Dispersion refers to mixing of non-miscible liquids or of solids in liquids, into somewhat homogeneous mass whose stability is measured by its life before reasonable separation occurs. Power input varies greatly depending on purpose and impellers generally used are pitched blade turbine, or saw tooth cutter. This is most critical of mixing problems and unless properly understood, the design is liable to fail.
d. Heat exchange
Used to speed up heat transfer by forced convection. Apparently simple but for critical applications, following data helps.
- Tank dimensions and details of jacket and coils, preferably G.A.drawing.
- Heat transfer co-efficients
- Specific heats and thermal conductivities
- Temp. of batch at start and end of cycle
- Temp. – Viscosity curve of components
- Whether solid suspension is included
Imparting a high power to break the molecular chains and to form the oil-water emulsion with a non-ionic surfactant. Other applications are mainly in paints & lubricants sector. Some emulsion may be steady for years and some break within minutes. Static mixers are preferred than agitators for high instantaneous power resulting better emulsion but washing pump & pipeline is problematic.
f. Solids suspension
It is simple physical (like mixing) operation but power consumption varies greatly on purpose e.g. a> complete motion of solids, b> complete suspension of solids, c> complete uniformity. Power req. is in ratio of 1: 2: 5 for said operations.
g. Chemical reaction
It can be considered as combination of blending, dissolving, heat transfer, extraction, gas dispersion, and solid suspension etc. Usually an easy task from agitator designer’s point but to be sure, pilot plant study is always recommended. A haphazard selection is vulnerable and over design (like peripheral impeller tip speed) has various detrimental effects on the final product.
h. Extraction including washing and leaching
This is normally a continuous counter- current (fluidized bed) process like solid suspension involving water to be well mixed up with other ingredients and the ingredients separates out by gravity separation. Usually of interest for mining people.
i. Gas dispersion, absorption, and stripping
Gas is impregnated from bottom as small bubbles and intimately distributed throughout the liquid usually resulting a chemical reaction. Generally curved vane impeller or multiple turbines are preferred with high speed. Fully baffled tanks should be tall and narrow in construction. Pressurized chamber accelerates the process. A better way is by static mixer employing a liquid pump and a pressurized semi- permeable solid wall to impregnate gas under pressure.
It is opposite of dissolving and is accomplished by cooling a saturated solution or by heating to drive out the solvent. The heat transfer requires a good flow. Satisfactory handling of crystals is of prime importance. Pilot plant data are desired. Generally crystals deposit at the bottom but if process deserves to be uniformly suspended, much study on the crystal structure & sensitivity is to be made for speed selection. Fluid-foil or aerofoil impellers with high flow and low shear are suggested.