Separationsteknik / Separation technology

Transcription

1 Separationsteknik / Separation technology Mixning, omrörning och blandning / Mixing, stirring and blending Ron Zevenhoven Åbo Akademi University Thermal and Flow Engineering Laboratory / Värme- och strömningsteknik tel ; ron.zevenhoven@abo.fi

2 9.1 Overview, Flow types

3 Mixing, stirring & blending /1 The purpose of equipment for mixing, stirring or blending is to remove or reduce nonuniformities 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 exchange surfaces Improved mass transfer, especially for immiscible liquids, G/L and L/S dispersions, and to support chemical reaction oktober /16

4 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. oktober /16

5 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: Z87 oktober /16

6 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 turbinetype impellers Picture: Z87 6

7 Flow: axial, baffled Axial flow arises with propellertype 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 Pictures: Z87, CRBH83 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) 7

8 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 Pictures: Z87 Too high level 8

9 9.2 Power requirements, Design see e.g. BMH99 for mixing (energy) needed to keep dispersed particles in dispersion

10 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 Po and size ratio s D/T, D/Z etc., where Re nd η ρ ;Fr n D g ;Po P ρn D with rotation rate n (1/s), density ρ, dynamic viscosity η, impeller diameter D and gravity g Note: product n D (m/s) is the tip-speed of the impeller Picture: Z87 oktober /16

11 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 n 2 D 3 For Re > 10 4 (turbulent) Po ~ constant P = Po ρ n 3 D 5 Power consumption curve for a turbine-type impeller Lower without baffles because some some gas is sucked in which reduces fluid density Picture: BMH99 oktober /16

12 Power requirements /3 Po transition Power number for various impeller types Re 12/16

13 Mixture properties For non-homogeneous mixtures, average values for density (ρ) and viscosity (η) may be used (gives good results for turbulent mixing). With dispersed phase d dispersed at volume faction ε (m 3 /m 3 ) in a continous phase c : mixture density average mixture viscosity average d c d d c 1 c Picture: 13

14 A typical design An example for a well- performing L/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 (m 3 ) and flow throughput Φ v (m 3 /s), good (turbulent) mixing is obtained in time t mix = 4 V/Φ v = 4 t residence, average Picture: Z87 14

15 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, η wall /η,...) Picture: Z87 15/16

16 Sources * * See ÅA course library 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 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: Fysische Scheidingsmethoden) TU Delft 1987 (vol. 2) 16/16