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
b. Dissolving
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
e. Emulsification
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.
j. Crystallization
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.
NOW LET US TRY TO UNDERSTAND BASIC IMPELLER DESIGN.
As like fan (for air handling) or hydel power turbines, much research has been conducted towards agitator impeller design and it is a very wide subject with much more further scope to research and improve. Impeller is designed mainly keeping in mind the purpose or application and sometimes custom built.
The basic designs are as follows.
A. Marine Propeller
Looks like a table fan blade. Suitable for high flow and low shear e.g. blending, dissolving, heat transfer, etc. Unsuitable for solid suspension, dispersion, extraction, gas dispersion, etc. flow efficiency- 50-60%. Getting outdated but manufacturers with their old proven designs are still sticking to that.
B. Pitched Blade Turbine
Looks like a table fan blade. Suitable for high flow and low shear e.g. blending, dissolving, heat transfer, etc. Unsuitable for solid suspension, dispersion, extraction, gas dispersion, etc. flow efficiency- 50-60%. Getting outdated but manufacturers with their old proven designs are still sticking to that.
C. Hydrofoil
Hydrofoil is supposed to be a foil in water and looks like pitched blade turbine with an angled cut on the lower periphery. Its flow efficiency is better than marine propeller, replacing the later in most applications and is most desired for mixing purposes but unsuitable for high shear application.
D. Aerofoil
aerofoil is having a classic stand fan blade type look and its properties are almost like hydrofoil impellers. Of interest for mining people for mineral washing and beneficiation for energy efficiency.
E. Curved Vane
Curved Vane impeller looks like straight Pelton wheel designed to hold moving particles for some time e.g. for air absorption.
F. Stator Rotor
Old outdated design supposed to bring highest shearing action and usually specified for oil-water emulsion.
G. Saw Tooth Cutter
Saw Tooth Cutter is reasonably good for handling medium to high viscous liquids. It is most economic for emulsification. Bad design for blending simple solutions as well as handling semi solids (grease, honey etc.). Suggested for oil-water emulsion, lubricants, paints, etc.
H. Anchor
Anchor (with its namely look) is used for heat transfer from bottom or scraping viscous liquids.
I. Helical Ribbon
Helical Ribbon looking like screwed ladder or double helix model of our gene. With its diameter close to tank diameter, is used for viscosity above 80 poise.
Another very effective design for handling viscous fluid is to use pitched blade turbine placed at bottom of draft tube (equaling 70% of tank dia.) at medium speed.