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280 .272 (4)
Lecture

Fluid Mixing Lecture Notes.docx
Fluid Mixing Lecture Notes.docx

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School
Massey University
Department
280
Course
280 .272
Professor
Patrick Janssen
Semester
Spring

Description
280.272 Notes Fluid Mixing Types of Mixers Type Picture Energy Produced By Mixing vessels with an impellor (or an agitator) Impellor Motor In-line static mixer (combines folding and slicing) Pump In-line rotational mixer (high shear rate) Agitator Motor Jet mixer (convert pressure to kinetic energy – inducing momentum) Pump Gas lifting (compressed gas at the bottom of tank pulls liquid up to Compressed Gas drag forces) 1 280.272 Notes Vessel Impellor Agitator Types  Axial Flow (A,B,C)  Radial (D,E,F)  Close Clearance (G,H)  High Shear (I) A Marine Propellor Blade is angled to ensure speed is High Re because uniform through width of liquid inertia is high and B Hi-Eff Hydrofoil Cheaper option due to fabrication viscosity is low C Pitched Blade Turbine Turbine: not very sophisticated D Rushton Turbine Used in optimised places Rely on high inertia E Flat-banded Turbine to mix the liquid F Parabolic Bladed A modern design of D throughout the vessel G Anchor 90 to 95% of liquid is swept Low Re because H Helical Improvement on G – the angle inertia is low and generates top to bottom flow viscosity is high I Toothed Rotor and High shear inside the agitator Stator Process Classification A process can be classified due to the phases being mixed or the underlying dynamic processes (there are usually more than one in each application) Elements of Mixer Vessel Design When designing a mixing vessel, find the best suited design for the process, know the power of the motor and height needed for the impellor. Need to Know:  Fluid Mechanics of Impellors (pumping vs shear rate and circulation)  Scale up and similarity  Impellor power characteristics (a correlation between P, N and D) P = Power N = speed (rps – revolutions per second) 2 D = diameter of agitator 280.272 Notes Vessel Impellors as Pumps Vessel Impellors can be thought of as an enclosed pump P = Power ρ = density  derived from power work rate H = total head or shear Q = primary flow rate The primary flow rate is discharged through the impellor – which is approximately 1/3 of the sytem flow rate Fluid Pumping vs. Shear Rate High pumping rate (Q) results in a low shear (H) and visa versa. Processes can be flow controlled or shear controlled but often it is a combination of both. Ultimate Goal: Match the pumping vs. shear balance of the impellor to the process requirements Examples: The propellor has constant pitch which results in less sheer and axial velocity. The pumping rate is high so the shear is low. This is a FLOW-CONTROLLED impellor. The rushton turbine has high shear zones arounf the blade which results in low pumping rate and high sheer. This is a SHEAR- CONTROLLED impellor. 3 280.272 Notes Effect of D/T on Pumping Shear From Similarity Laws: T = diameter of the vessel ( ) ( ) H = shear form of head Large diameter but slow speed favours pumping over shear. However a small, high speed agitator are cheaper. Vessel and Impellor Dimensions  Baffel width = D  Height of Impellor = C  Height of Liquid above Impellors = Cv  Heigh of the liquid = Z  Width of Impellor = W “Standard” values have been proven for convience:  There are usually 3 to 6 Baffles in a system and B/T = 1/12  D/T is usually 0.2 to 0.6  Blade Width W/D = 1/5  Impellor Height C/D = 1 to 2 (consider coverage during emptying)  Impellor Coverage Cv/D = 2 to 4 The tank aspect ratio = Z/T. Multiple impellors when tank aspect ratio is greater than 2 to 1. Similarity Geometric Similarity: All linear dimensions incr2ased by the same factor (f). The surface areas are scaled by f and the volume are scaled by f . Dynamic Similarity: Rate-limited physical processes. Appropriate use of dimensionless numbers or ratios may help.
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