Shroff S.R. Rotary Institute of Chemical Technology MECHANICAL OPERATION TOPIC:-IMPELLER AND ITS EFFECTS PREPARED BY: BHUPENDRASINH SOLANKI AKSHAYRAJSINH VANSIYA MUBARAK SHAIKH DHARMESH VASAVA ANKIT VASI

Outline of Presentation Introduction Classification Impeller design Relation between reynolds and power number for different impeller Power Requirements for Mixing

Introduction An impeller is a rotating component of a centrifugal pump, usually made of steel,iron,bronze,brass, aluminium or plastic, which transfer energy from the motor that drives the pump to the fluid being pumped by accelerating the fluid otwards from the center of rotation.

Classification Impeller Axial flow impeller Radial flow impeller

Classification Impeller Propeller Paddles Turbines

Stainless stell axial flow impeller Flow pattern in axial flow impeller

Ruston turbine radial flow impeller Flow pattern in radial flow impeller

Impeller design

Why it is needed large liquid-circulation loops developed in stirred vessels make mixing performance poor. For mixing to be effective, fluid circulated by the impeller must sweep the entire vessel in a reasonable time. In addition, the velocity of fluid leaving the impeller must be sufficient to carry material into the most remote parts of the tank. Turbulence must also be developed in the fluid; mixing is certain to be poor unless flow in the tank is turbulent. All these factors are important in mixing, which can be described as a combination of three physical processes: distribution, dispersion and diffusion.

Agitation is the key to many heat and mass transfer operations that rely on mixing. Conventional, mechanically agitated, stirred tank reactors may be used for either batch or continuous processes, though the design and operating constraints are different in the two cases. Low viscosity fluids can usually be mixed effectively in baffled tanks with relatively small high speed impellers generating turbulent flows, while high viscosity (typically above about 10 Pa s) and non-Newtonian materials require larger, slow moving agitators that work in the laminar or transitional flow regimes.

It is convenient to classify impellers as radial or axial pumping depending on the flow they generate in baffled tanks

a) a radial flow, "Rushton", turbine which produces considerable turbulence near the impeller, b) a "pitched blade" impeller with flat, angled blades that generates a diverging but generally axial flow, c) a hydrofoil impeller with carefully profiled blades that develop a strong, more truly, axial flow of low turbulence. Impellers suitable for viscous fluids are: d) a helical ribbon with a blade that travels close to the wall of the tank to force good overall circulation and e) an anchor that produces strong swirl with poor vertical exchange, even when baffled with stationary breaker bars or "beaver tail" baffles.

Energy transfer The power input P to an impeller of diameter D driven at a rotational speed N in a fluid of density ρ and viscosity η can be expressed in terms of a dimensionless Power number, P/(N3D5ρ). This is a form of drag coefficient and is a function of the mixing Reynolds number (ND2ρ/η). For a given pattern of impeller the Po vs. Re function is always of the same form.

Fig. shows typical graphs of the relationship for four impellers .

Axial flow impellers Pitched Blade Turbines have lower turbulent power numbers than Rushton turbines, about 1 in the fully turbulent regime, in part at least because of the lower intensity of the single vortex from each blade end,fig b. Providing the tank is baffled the axial flow will develop. Hydrofoils In recent years improved, more streamlined, axial flow impellers, usually with three or four blades, have been developed. These generate good axial flow with very little turbulence and are widely used when effective bulk motion of the liquid is required. The tip vortices are weaker than those of a pitched blade impeller and the energy is dissipated very uniformly throughout the vessel volume, Figure c.

Helical Ribbons and other proximity agitators The near impossibility of generating turbulence in viscous and non-Newtonian materials means that effective mixing depends on ensuring that all the fluid is moving. This can only be achieved with impellers that are large and which sweep out the whole vessel volume. Several patterns have been developed, amongst them helical ribbons and anchors (fig d and e).

Batch mixing time One measure of mixing performance is the batch mixing time. This has to convey some assessment of the asymptotic approach to homogenization, usually achieved in terms of the addition of a tracer and the subsequent approach to homogenization. The 95% mixing time is often used for this criterion. The moment of tracer addition, is taken as zero, td is the dead time before the addition is first detected, tc the circulation time and tm the 95% mixing time, defined by the last measurement that lies outside the 5% band of the total concentration change. Suitable tracers are pH changes, dyes and salts; even hot liquid can be used. The major difficulties include avoiding spurious affects due to density differences and establishing a reproducible protocol that can be related reliably to the desired process application.

Power Requirements for Mixing UngassedNewtonian Fluids Mixing power for non-aerated fluids depends on the stirrer speed, the impeller diameter and geometry, and properties of the fluid such as density and viscosity. The relationship between these variables is usually expressed in terms of dimensionless numbers such as the impeller Reynolds number (Re)iand the power number Np. P = NpρNi3Di5

Laminar region: The laminar regime corresponds to (Re)i< 10 for many impellers; for stirrers with very small wall-clearance such as the anchor and helical-ribbon mixer, laminar flow persists until (Re)i= 100 or greater. In the laminar regime: Np∝1/(Re)i or P= k1μNi2Di3 ™Turbulent regime: Power number is independent of Reynolds number in turbulent flow. Therefore: P= Np’ρNi3Di5

Constant in above equation Impeller type k1 , (Re)i = 1 Np’, (Re)i = 105 Rushton turbine 70 5 Paddle 35 2 Marine propeller 40 0.35 Anchor 42 Helical ribbon 1000 Np’for turbines is significantly higher than for most other impellers, indicating that turbines transmit more power to the fluid than other designs.

™Transition regime: Between laminar and turbulent flow lies the transition regime. Both density and viscosity affect power requirements in this regime. There is usually a gradual transition from laminar to fully-developed turbulent flow in stirred tanks; the flow pattern and Reynolds-number range for transition depend on system geometry.

Ungassed Non-Newtonian Fluids Impeller Reynolds number based on the apparent viscosity μa:

Gassed fluids All of the changes in hydrodynamic behavior duo to gassing are not completely understood. Power consumption is strong controlled by gas-cavities formation; because this process is discontinuous and appears somewhat randomly, reduction in power consumption is typically non-uniform. The random nature of gas dispersion in agitated tanks means that it is difficult to obtain an accurate prediction of power requirements. However, an expression for the ratio of gassed to ungassedpower as a function of operating conditions has been obtained.

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