FLUID FLOW FOR CHEMICAL ENGINEERING EKC 212 FLUID FLOW FOR CHEMICAL ENGINEERING CHAPTER 9 (PART 1) AGITATION & MIXING OF LIQUIDS Dr Mohd Azmier Ahmad Tel: +60 (4) 5996459 Email: chazmier@eng.usm.my

AGITATION & MIXING OF LIQUIDS “Many processing operations depend for their success on the effective agitation & mixing of fluids” ……McCabe Agitation It is an induced motion of a material in a specified way. Pattern is normally circulatory. Normally taken place inside a container. Mixing Random distribution, into & through one another of two or more initially separate phases

Purposes of agitation of liquids Suspending solid particles. Blending miscible liquids e.g. methyl alcohol & water. Dispersing gas through liquid in the form of small bubbles. Promoting heat transfer between liquids & a coil/jacket.

Liquids are agitated in a tank Bottom of the tank is rounded Agitation vessel Liquids are agitated in a tank Bottom of the tank is rounded Impeller creates a flow pattern. Small scale tank (< 10 litres) are constructed using Pyrex glass. For larger reactors/tank, stainless steel is used. Speed reduction devices are used to control the agitation speed. Mixing Flow : 3 patterns (axial, radial, tangential flow) Three main types of impellers : propellers paddles (iii) turbines Typical agitation process tank

Impeller: Three main types of impellers : (i) Propellers; (ii) paddles & (iii) turbines

(i) Propellers: Create an axial-flow. Flow of currents is pushed in downward direction. High speed for low viscosity liquid. Effective in very large tanks. suitable for mixing of low viscose liquids, particle suspension. In a deep tank, 2 or more propellers may be mounted on the same shaft.

Two or four blades turning on a vertical shaft (ii) Paddles: Two or four blades turning on a vertical shaft Suitable for stirring low & middle viscosity liquids at moderate speeds (~ 20-150 rpm). Paddles push the liquid radially (radial flow). Anchor & turbine are type of paddle agitator. Ratio of paddles diameter to the vessel diameter is typically 50-80%. Width of blade is 1/6 to 1/10 of its length. Anchor

Multi-bladed paddle agitators with short blades (iii)Turbine: Multi-bladed paddle agitators with short blades Diameter ~ 30-50% of vessel diameter. Suitable for wide range of viscosity. For low viscosity, it generates strong currents which continue throughout vessel. Principal currents produced: radial & tangential. 6 blades 5 blades 4 blades 3 blades 12 blades 8 blades Velocity patterns in turbine agitator

Mixing Flow patterns (3 types): (i) Axial flow. Impeller makes an angle of less than 90o with the plane of rotation thus resultant flow pattern towards the base of the tank (i.e. marine impellers). More energy efficient than radial flow mixing. More effective at lifting solids from the base of the tank. used for blending & solids suspension

(ii) Radial flow. Impeller are parallel to the axis of the drive shaft. The currents travel outward to the vessel wall & then either upward or downward. Higher energy is required compared to axial flow impellers. Used for dispersion

(ii) Tangential / swirling flow At high impeller speeds, vortex is formed & may be so deep that it reaches the impeller. The high the speed , the deep the vortex. Result : Power absorbed by liquid is limited. Effective volume reduced and mixing effect worsen. If solid particles present within tank; it tends to throw the particles to the outside by centrifugal force. vortex

Tangential or swirling flow field is ineffective at dispersing the solids (grouped below the impeller). Large surface vortex is visible at the top of the shaft. More like a centrifuge than a mixer. Method of preventing vortex - baffles - impeller in an angular off-center position

Prevent Vortex : Off-centering & baffles

(i) Baffles on the tank walls Preventing vortex (i) Baffles on the tank walls Vertical plates perpendicular to the tank wall. Suitable for large tanks with vertical agitators. Propeller drive the liquid down to the bottom of the tank, where the stream spreads in all directions toward the wall, flows upward along the wall & returns to the propeller suction from the top. Flow pattern for an axial flow propeller

For paddle agitators & flat blade turbines give good radial flow in the plane of the impeller, with the flow dividing at the wall to form two separate circulation patterns. One portion flows downward along the wall & back to the center of the impeller from below. Other portion flows upward toward the surface & back to the impeller from above. Flow pattern for a radial flow turbine/paddle

With baffles, most impellers show their true flow characteristics. Most common baffles are straight flat plates of metal (standard baffles).  Most vessels will have at least 3 baffles.  4 is most common and is often referred to as the "fully baffled" condition. The number of blades ranges from 4 to 16.

Agitation in an unbaffled vessel leads to swirling flow with vortex formation & poor distribution Standard baffling promotes flow that results in good solids distribution

(ii) Impeller in an angular off-center position Mount the impeller away from the center of the vessel & tilted in the direction perpendicular to the direction of flow. Flow pattern with off-center propeller

Improving mixing : Draft tube Controlling the flow velocity & direction of flow to the suction of the impeller. Reducing the short cut especially for particle suspension. Useful when high shear at the impeller itself is desired e.g. emulsion manufacture (solid particles tend to float on the surface of the liquid in the tank). Draft tubes are mounted around the propellers or mounted immediately above the impeller for turbine/paddle.