LIQUID MIXING.pptx

Over view on liquid mixing mechanisms and equipments.

1. LIQUID MIXING

2. CONTENTS 180301053 2

3. MIXING Mixing is to be carried out in order to produce a uniform mixture. There must be bulk or convective flow so that there are no dead (stagnant) zones. Secondly, there must be a zone of intensive or high-shear mixing in which the inhomogeneities are broken down. Both these processes are energy-consuming and ultimately the mechanical energy is dissipated as heat.  Depending upon the fluid properties, primarily viscosity, the flow in mixing vessels may be laminar or turbulent, with a substantial transition zone in between the two.  Frequently both flow types will occur simultaneously in different parts of the vessel.  Laminar and turbulent flow arise from different mechanisms, and it is convenient to consider them separately. 180301053 3

4. Liquid mixing mechanisms 180301053 4

5. LAMINAR MIXING Usually associated with high viscosity liquids which may be either Newtonian or non-Newtonian. The inertial forces therefore tend to die out quickly, and the impeller of the mixer must cover a significant proportion of the cross-section of the vessel to impart sufficient bulk motion. Because the velocity gradients close to the rotating impeller are high, the fluid elements in that region deform and stretch.  Repeatedly elongate and become thinner each time the fluid elements pass through the high shear zone.  Extensional or elongational flow usually occurs simultaneously. This is a result of the convergence of the streamlines and consequential increased velocity in the direction of flow. As the volume remains constant, there must, be a thinning or flattening of the fluid elements. Both of these mechanisms (shear and elongation), give rise to stresses in the liquid which then effect a reduction in droplet size and an increase in interfacial area, by which means the desired degree of homogeneity is obtained. 180301053 5

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8. TURBULANT MIXING For low viscosity liquids (less than 10 mN s/m2 ), the bulk flow pattern in mixing vessels with rotating impellers is turbulent.  The inertia imparted to the liquid by the rotating impeller is sufficient to cause the liquid to circulate throughout the vessel and return to the impeller. Turbulent eddy diffusion takes place throughout the vessel but is a maximum in the vicinity of the impeller. Eddy diffusion is inherently much faster than molecular diffusion and, consequently, turbulent mixing occurs much more rapidly than laminar mixing. 180301053 8

9. MIXING EQUIPMENTS 180301053 9

10. MIXING EQUIPMENTS 180301053 10

11. MECHANICAL AGITATION Most commonly used method of mixing liquids. Essentially there are three elements in such devices.  Vessels Baffles Impellers 180301053 11

12. Vessels Vertically mounted cylindrical tanks up to 10 m in diameter. The base of the tanks may be flat, dished, or conical, or specially contoured, depending upon factors such as ease of emptying, or the need to suspend solids. May be mounted horizontally, for the batch mixing of viscous pastes and doughs using ribbon impellers and Zblade mixers.  In such units, the working volume of pastes and doughs is often relatively small, and the mixing blades are massive in construction. 180301053 12

13. Baffles Thin strips about one-tenth of the tank diameter in width. Typically four equi-spaced baffles may be used. Prevent gross vortexing, which is detrimental to mixing particularly in low viscosity systems. generally not required for high viscosity liquids. 180301053 13

14. Selected for a particular duty largely on the basis of liquid viscosity. Propellers ,  turbines ,  paddles ,  anchors ,  helical ribbons  and  screws  are usually mounted on a central vertical shaft in a cylindrical tank. It is necessary to move from a  propeller  to a  turbine  and then, in order, to a  paddle , to an  anchor  and then to a  helical ribbon  and finally to a  screw  as the viscosity. Propellers ,  turbines  and  paddles  are generally used with relatively low viscosity systems and operate at high rotational speeds. For large vessels, more than one impeller are mounted on the same shaft. (a) Three-bladed propeller (b) Six-bladed disc turbine (Rushton turbine) (c) Simple paddle (d) Anchor impeller (e) Helical ribbon Impellers 180301053 14

15. Propellers are frequently of the three-bladed marine type  and may be arranged as angled side-entry units. The anchor and ribbon are arranged with a close clearance at the vessel wall. Helical ribbons or interrupted ribbons are often used in horizontally mounted cylindrical vessels.  Kneaders, Z- and sigma-blade, and Banbury mixers  are used for the mixing of high-viscosity liquids, pastes, rubbers, doughs. (g) Z-blade mixer (h) Banbury mixer 180301053 15

16. Agitator selection guide 180301053 16

17. Agitator arrangements and flow patterns 180301053 17

18. EXTRUDERS Mixing duties in the plastics industry are often carried out in either single or twin screw extruders.  The feed to such units usually contains the base polymer in either granular or powder form, together with additives such as stabilisers, pigments and plasticisers. During processing in the extruder the polymer is melted and the additives mixed. Extrudate is delivered at high pressure and at a controlled rate from the extruder for shaping by means of either a die or a mould. 180301053 18

19. Single-screw extruder:           Shearing which occurs in the helical channel between the barrel and the screw is is not intense, and therefore this device does not give good mixing.  Twin-screw extruder:           May be co or counter-rotatory. There are regions where the rotors are in close proximity thereby generating extremely high shear stresses. Twin-screw units can yield a product of better mixture quality than a single-screw machine. Single-screw extruder   Co-rotating twin-screw extruder 180301053 19

20. STATIC MIXERS Static devices which promote turbulent mixing in pipelines provide an inexpensive way of continuously mixing fluids. Sometimes called "in-line" or "motionless" mixers. pumped through a pipe containing a series of specially shaped stationary blades. Can be used with liquids of a wide range of viscosities in either the laminar or turbulent regimes. The flow patterns within the mixer are complex. 180301053 20

21. Twisted-blade type static mixer elements  180301053 21

22. Sulzer type SMX static mixer Mixing element consists of a lattice of intermeshing and interconnecting bars contained in a pipe 80 mm diameter. It is recommended for viscous materials in laminar flow. The mixer shown is used in food processing, for example mixing fresh cheese with whipped cream.  180301053 22

23. A simple mixing tee, followed by a length of pipe is suitable for mixing low viscosity fluids (50 mN s/m2) providing the flow is turbulent, and the densities and flow-rates of the fluids are similar. 180301053 23

24. With injection mixers in which the one fluid is introduced into the flowing stream of the other through a concentric pipe or an annular array of jets, mixing will take place by entrainment and turbulent diffusion. Such devices should be used where one flow is much lower than the other. Annular Injection 180301053 24

25. THANK YOU REFERENCE   Coulson and Richardson's   Chemical Engineering Design Volume 1 and 2 180301053 25