separationsteknik - Åbo akademi | startsidausers.abo.fi/rzevenho/sep1617-oh9-1pp.pdf ·...
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Separationsteknik/ Separation technology 424105
9. Mixning, omrörning och blandning/ Mixing, stirring and blending
Ron ZevenhovenÅbo Akademi University
Thermal and Flow Engineering Laboratory/ Värme- och strömningsteknik
tel. 3223 ; [email protected]
9.1 Overview, Flow types
oktober 2017
Mixing, stirring & blending /1
The purpose of equipment for mixing, stirring or blending is to remove or reduce non-uniformities and gradients in composition, temperature, particle size, ...
The result is – (Better) dispersion of other phases in a bulk phase,
homogenising miscible (fluid) mixtures; for example L/L, L/S, S/S, G/L, G/L/S
– Improved heat transfer to walls or heat exchangesurfaces
– Improved mass transfer, especially for immiscibleliquids, G/L and L/S dispersions, and to support chemical reaction
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oktober 2017
Mixing, stirring & blending /2 A mixture can be considered homogeneous if at each point
the concentration is equal to an average value Inhomogeneities are characterised by 1) scale = size of
unmixed fluid elements and 2) intensity = concentration differences between unmixed regions. With chemistry, this leads to side-reactions (→ by-products).
Miscible liquids mixing can be achieved in turbulent pipe flow after ~ 50 diameters (plus valves etc.)
Most mixtures will show tendency for segregation and an energy input is needed is to get the correct balance between mixing and segregation.
Liquid mixing is usually done in a vessel with a rotating impeller.
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oktober 2017
Impellers for mixed vessels Impellers have the task
to create turbulent flow in axial, radial and tangential directions.
Most important types, giving different types of flow patterns:– a. Flat-blade paddle – b. Pitch-blade paddle– c. Flat-blade turbine– d. Marine-type propeller– e. Spiral mixer– f. Anchor mixer
Picture: Z875/16Åbo Akademi - Värme- och strömningsteknik Biskopsgatan 8, 20500 Åbo
Flow: tangential, radial
Tangential flow may rotate the entire fluid mass without much mixing
Is created with blade- and turbine-type impellers, especially with smooth vessel walls
The centrifugal effect may create a vortex at the surface
Baffles at the wall put a limit to tangential flow
Meeting the vessel wall makes the fluid flow return towards the impeller, creating radial flow
Created by blade- and turbine-type impellers
Picture: Z876Åbo Akademi - Värme- och strömningsteknik Biskopsgatan 8, 20500 Åbo
Flow: axial, baffled
Axial flow arises with propeller-type and pitched impellers
A guidance tube can enhance the axial flow circulation
A cruciform baffle at the vessel bottom can further improve mixing and dispersion
Baffles at walls and bottom considerably increase power input requirements
As a rule, baffle width × number of baffles≈ 0.4 × vessel diameter (for example 4 baffles á 0.1 diameters)Pictures: Z87, CRBH83
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Off-setting; Liquid level Using the impeller in a off-set
position can improve mixing by minimising vortex formation
Liquid filling level with respect to the position of the impeller affects mixing as well
Too low level Correct level
Too high levelPictures: Z87
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9.2 Power requirements, Design
see e.g. BMH99 for mixing (energy) needed to keep dispersed particles in dispersion
oktober 2017
Power requirements /1 Power requirements depend on fluid
properties, process parameters, sizes and size-ratio’s
Dimensional analysis gives a relation between power P (W) and Reynolds number Re, Froude number Fr, power number Poand size ratio’s D/T, D/Z etc., where
with rotation rate n (1/s), density ρ, dynamic viscosity η, impeller diameter D and gravity g
Dnρ
PPo;gDnFr;
ηρnDRe
Picture: Z87
Note: product n·D (m/s) is thetip-speed of the impeller
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Power requirements /2 For geometrically similar stirred vessels, Po = f(Re,Fr) If vortexing is not occuring, Po = f(Re) For Re < 10 (laminar), Po ~ 1/Re → P = Po· η· n2· D3
For Re > 104 (turbulent) Po ~ constant → P = Po· ρ· n3· D5
Lower without bafflesbecause some some gas is sucked in which reduces fluid density
Power consumption curve for a turbine-type impeller
Picture: BMH99
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Power requirements /3
Power number for various impeller types
Po
Re
transition
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Mixture properties For non-homogeneous mixtures, average values for
density (ρ) and viscosity (η) may be used (gives goodresults for turbulent mixing).
With dispersed phase ”d” dispersed at volume faction ε(m3/m3) in a continous phase ”c”:mixture density
mixture viscosity
cd
dcaverage
cdaverage
5.111
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A typical design
An example for a well- performingL/L mixing vessel:
6-blade turbine impeller, circular vessel with 4 baffles, Z/T = 1, D/T = 1/3, B/T = 1/12, C/Z = 1/2 This set-up with 4 baffles with
B ≥ 0.1·T is referred to as ”fully baffled” Typically, for a vessel with volume V
(m3) and flow throughput Φv (m3/s), good (turbulent) mixing is obtained in time tmix = 4· V/Φv = 4· tresidence, average
Picture: Z87
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Heat transfer
For heat exchange (primarily for temperature control) a mantle volume can be put around the whole vessel (left), or spirals can be put inside the vessel (centre, right). If necessary, condensers or evaporators are used.
For the various types of impeller, the heat transfer to/from wall or spirals is given by heat transfer coefficients from Nusselt number, such as Nu = f(Re, Pr, D/T, T/D, T/C, w/D, ηwall/η,....) Picture: Z87
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Sources
BMH99 Beek, W.J., Muttzall, K.M.K., van Heuven, J.W. ”Transport phenomena” Wiley, 2nd edition (1999)
CRBH83 J.M. Coulson, J.F. Richardson , J.R. Blackhurst, J.H. Harker”Chemical engineering”, vol. 2, 3rd ed. Pergamon Press (1983) Ch. 13.2
SH06 J.D. Seader, E.J Henley ”Separation process principles” John Wiley, 2nd edition (2006) 8.1 **
Z97 F. Zuiderweg ”Physical separation methods” (in Dutch: FysischeScheidingsmethoden) TU Delft 1987 (vol. 2)
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* * See ÅA course library