Liquid Mixing Ashis Kumar Podder

Liquid Mixing Liq-Liq mixing involves the application of shear to form a homogeneous system. Nature of liq. mainly determines the ease of mixing liquid. They can be classified into: Miscible liq.: They are miscible in all proportions. [E.g. Mixture of ethanol and water, after shave lotion, elixir] Partially miscible liq.: They are miscible in one another at one particular proportion. [E.g. Mixture of p-cresol and water] Immiscible liq.: They are not miscible. [E.g. Mixture of vegetable oils and water]

Mixing Vessels The mixing apparatus consists of a container (tank) and a mixing device. The general construction of the mixing tank: Impeller, a mixing device, mounted with the help of a shaft, which is driven by a motor. The tank is made up of stainless steel. The top of the tank may be open or closed. The size of the tank depends on the nature of the agitation method. The tank bottom is round (not flat) to eliminate sharp corners into which the fluid can accumulate. It also carries an outlet, coils, jacket, temperature measuring device etc. wherever necessary. Lid Shaft

Mechanism of liq. mixing There are 4 main mechanisms by which liquids are mixed. They are: Bulk transport Molecular diffusion Turbulent transport Laminar transport Usually, more than one processes are in operation in practical mixing of solutions. Bulk transport: movement of relatively large amount of liquid from one position to another. This mechanism tends to produce a large degree of mixing, but leaves the liquid within the moving bulk unmixed. For the bulk transport to be effective, other devices may be used, such as paddle, revolving blades, baffles etc. Molecular diffusion: primary mechanism for mixing at the molecular level is diffusion resulting from the thermal motion of molecules. This type of mixing occurs whenever there is a concentration gradient (According to Fick’s law).

Mechanism of liq. Mixing (cont.) Turbulent mixing: occurs due to haphazard movement of molecules in various directions when forced to move in a turbulent manner. Turbulent mixing results in random fluctuation of the fluid velocity at any given point within the system Within a turbulent fluid, there are small groups of molecules moving together as a unit, referred to as eddies. These eddies reduce in size and are replaced by new eddies. The constant changes in speed & direction of movement means that induced turbulence is a highly effective mechanism of mixing. Laminar flow : Laminar flow is frequently encountered when highly viscous fluids are being processed. The stirring is relatively gentle. In this mechanism, layers fold back upon themselves. Thus the number of layers increases. So, the mixing involves reduction of fluid layer thickness by producing folding effect. The applied shear stresses between the interface of the 2 dissimilar liquids to be mixed.

Flow pattern during mixing Liquids are mixed usually by impellers, which produce shear forces for inducing the necessary flow pattern in the mixing container. Mixing occurs due to the resultant effect of 3 components acting on liquid: Tangential / Circular component Radial component Axial / Longitudinal component The type of flow depends on Type of impeller Characteristic of fluid Size proportion of tank, baffle & impellers

Tangential component: Direction: acts in the direction tangent to the circle of rotation around the impeller shaft. Effect: if shaft is placed vertically & centrally, tangential flow follows a circular path around the shaft & creates a vortex in the liquid. 2. Radial component: Direction: acts in the direction perpendicular to the impeller shaft. Effect: excessive radial flow takes the material to the container wall, then the material falls to the bottom and rotates as a mass beneath the impeller. 3. Axial component: Direction: acts in the direction parallel to the impeller shaft. Effect: inadequate longitudinal component causes the liquid and solid to rotate in layers without mixing. Adequate longitudinal pattern is best used to generate strong vertical currents particularly while suspending solids are present.

Impeller & classification Impellers are mixing devices that provide a definite flow pattern in liquid during mixing, moving at various speeds. Classes: Broadly 2 classes depending on the angle the blade makes with the plane of impeller rotation: Axial flow impellers (Types: propeller, pitch blade turbines) Radial flow impellers (Types: turbines, paddles, anchor agitators) Radial flow impellers: Impellers where they make 90˚ angle with the shaft & blades are parallel to the axis of drive shaft. In this type of impeller, the direction of flow of liquid is perpendicular with the axis of rotation. More possibility of vortexing. Axial flow impellers: Impellers where blade makes an angle less than 90˚ with the plane of rotation. When the axial flow impellers rotate, the flow of the liquid is parallel to the axis of the rotation. So it is called axial flow. There is less possibility of vortexing.

Different mounting positions of propeller (fig) Baffles Baffles are flat metal strips that placed vertically to the internal surface of the tank. Application of baffle: -To avoid vortex in large tanks -Facilitates mixing of the liquid even from remote regions of the mixer. Different mounting positions of propeller (fig) Propellers are mixing equipments that produce axial flow in a mixture. Propellers are usually mounted in 4 different positions:- 1. Centered 2. Vertical off centre 3. Angular off centre 4. Side entering

Centered: # shaft blade is positioned in the centre of the tank # two different flow patterns are produced on either sides. # it is the best position as the current will be distributed downwards and thus facilitate homogenization. # it can produce vortex Vertical off-centre: # shaft is placed vertically in the tank near one end of the tank (off-centre position) # a single flow pattern is produced # discourages formation of vortex Angular off-centre: # shaft with blade is angularly fixed at a certain angle from the centre line # more diverse flow patterns are produced

# propellers are mounted at the bottom sides of the container wall Side entering: # propellers are mounted at the bottom sides of the container wall # used to blend low viscosity fluid # discourages formation of vortex # may have leakage problem or risk of contamination. Differences between “axial” & “radial” flow impellers Differences between “propeller” & “turbine” flow impellers

Vortex What is vortex? Vortex: If a low viscosity liquid is stirred in an un-baffled tank by an axially mounted agitator, tangential flow follows a circular path around the shaft & a swirling flow pattern is developed. This is vortex. (Fig.) How is it formed? In an un-baffled tank, a vortex is produced due to the centrifugal force on the rotating liquid. This creates a swirling motion in the liquid & the surface tends to go upward near the vessel rim & downward near the shaft. So a V-shaped surface is formed which is the vortex.

Vortex……..contd. Reasons: Problems: If the shaft is placed symmetrically in the tank. If the blades of the turbines are arranged perpendicular to the central shaft. At high impeller speeds Unbaffled tank Problems: Vortex decreases mixing intensity by reducing velocity of the impeller relative to the surrounding fluid. When vortex reaches the impeller, air from the surface of the liquid are drawn (air entrapment) and air bubbles are produced. Air bubbles in the fluid can create uneven loading of the impeller blades. Entrapped air causes oxidation of the substances in certain cases. Possibility of foam formation Possibility of throwing out some material out of container.

Prevention of vortex formation Impeller should be in any one of the following positions that can avoid symmetry such as off central, inclined, side entering, etc., and should be deep in the liquid. Baffled containers should be used. In such case impeller can be mounted vertically at the center.

Prevention of vortex formation….Contd. Two or more impellers can be mounted on the same shaft where greater depth is required. This system is known as push and pull mechanism. The bottom impeller is placed about one impeller diameter above the bottom of the tank. It creates zone of high turbulence.

Prevention of vortex formation…………..Contd. Tank other than cylindrical in shape are used to prevent vortex formation. However, such shapes may facilitate the formation of dead spots. Mixing: Theory and Equipment

Mixing Device Propellers Turbines Paddles Based on shape and pitch , the are classified into 3 types: Propellers Turbines Paddles Impeller type Flow component Propellers Axial Turbines Axial or tangential or both Paddles Radial and tangential Paddles with pitch Radial, tangential and axial

Propellers It consists of number of blades, generally 3 bladed design is most common for liquids. Blades may be right or left handed depending upon the slant of their blades. Two are more propellers are used for deep tank. Size of propeller is usually small (Ratio of diameter between propeller and container is 1:20) and may increased up to a max. of 0.5 m for large tanks. Small size propellers can rotate up to 8000 rpm and produce Axial / Longitudinal movement.

Advantages of propellers: Used when high mixing capacity is required Advantages of propellers: Used when high mixing capacity is required. Effective for liquids which have maximum viscosity of 2.0 pascals.sec or slurries up to 10% solids of fine mesh size. Effective gas-liquid dispersion is possible at laboratory scale. Example: Multivitamin elixirs, Disinfectant solutions are prepared using propellers Disadvantages of propellers: Propellers are not normally effective with liquids of viscosity greater than 5 pascal.second, such as glycerin castor oil, etc.

Turbines A turbine consists of a circular disc to which a number of short blades are attached. Blades may be straight or curved. The diameter of the turbine ranges from 30-50% of the diameter of the vessel. Turbines rotate at a lower speed than the propellers (50-200 rpm). Flat blade turbines produce radial and tangential flow but as the speed increases radial flow dominates. Pitched blade turbine produces axial flow. Near the impeller zone of rapid currents, high turbulence and intense shear is observed. Shear produced by turbines can be further enhanced using a diffuser ring (stationary perforated ring which surrounds the turbine). Diffuser ring increase the shear forces and liquid passes through the perforations reducing rotational swirling and vortexing.

Advantages of Turbines: Turbines give greater shearing forces than propellers though the pumping rate is less. Therefore suitable for emulsification. Effective for high viscous solutions with a wide range of viscosities up to 7.0 Pascal. second. In low viscous materials of large volumes turbine create a strong currents which spread throughout the tank destroying stagnant pockets. They can handle slurries with 60% solids. Turbines are suitable for liquids of large volume and high viscosity, if the tank is baffled.

Paddles A paddle consists of a central hub with long flat blades attached to it vertically. 2 blades or 4 blades are common. sometimes the blades are pitched and may be dished or hemispherical in shape and have a large surface area in relation to the tank in which they are used. Paddles rotate at a low speed of 100 rpm. They push the liquid radially and tangentially with almost no axial action unless blades are pitched. In deep tanks several paddles are attached one above the other on the same shaft. At very low speeds it gives mild agitation in unbaffled tank but as for high speeds baffles are necessary.

Advantages of paddles: Uses of paddles: Paddles are used in the manufacture of antacid suspensions, agar and pectin related purgatives, antidiarrheal mixtures such as bismuth-kaolin. Advantages of paddles: Vortex formation is not possible with paddle impellers because of low speed mixing. Disadvantages of paddles: Mixing of the suspension is poor therefore baffled tanks are required.

Paddles Turbines Propellers Example Simple Paddle Curved – Blade Standard 3- Blade Size Diameter of paddle is 50 -80% of the inside diameter of vessel Diameter of turbine is 30-50% of the inside diameter of vessel. Diameter of propeller is much smaller in diameter than paddles or turbines Uses Suitable for: mixing low viscosity liquids. mixing viscous liquids, thin pastes and emulsions. Mixing heavy susps., mixing of thin liquids Dissolution of solids. Rotation speed 20 - 120 r.p.m. (low speed) 120 - 200 r.p.m. (moderate speed) 400 – 1750 r.p.m. (high speed ) Flow type Tangential & radial (no axial) Tangential & axial (no radial)

Return flow with Draft tubes Draft tubes are placed to control the direction and velocity of the flow to the impeller. These are mounted around the propeller. In case of turbines, draft tubes are mounted immediately above the impeller. Uses: Draft tubes are fitted to equipment used in the manufacture of certain emulsions. When solid particles tend to float on the surface of the liquid, they are dispersed using draft tubes. Air jet mixers of continuous mixing of liquids employ draft tubes. Disadvantage: Draft tubes add to the fluid friction in the system. These reduce rate of flow.

Factors influencing mixing of liquids in tanks Material related factors- Properties of liquids: physical properties of materials to be mixed. e.g.: Density, Viscosity and miscibility. Equipment related factors- Shape of impeller: Propeller type, straight, vertical, curved, or pitched. Position of impeller: Central, off-center, side entry, vertical or inclined etc., Shape and size of the container: cylindrical or other geometric forms. Presence or absence of baffles. Cost of equipment and its maintenance. Process related factors- Speed of rotation of the impeller. Time required for mixing. Amount of power that can be expended. Ease of operation. Batch size.

Equipments for Liquid mixing Air jet mixer Jet mixer Flow mixer or line mixer or pipe mixer

Air jet mixer Principle: Construction: When compressed air jets are passes from the bottom of a vessel, air bubbles are formed in the liquid phase. The buoyancy of the bubbles lifts the liquids, which are confines to the central portion due to the presence of drat tubes. The liquids flow down from the periphery of the vessel and enter from the bottom due to suction effect. Construction: Compressed air or suitable gas is allowed to pass at high pressure from the inlet provided at the bottom of the tank. This causes buoyancy of the bubbles which lifts the liquid from bottom to the top of the vessel.

Mixing: Theory and Equipment Jet mixer It consists of a vessel and a mixing device known as Jet. A typical jet-mixing system has 12 jets arranged radially in a cluster in the centre of the mixing tank. The cluster of jets is called an eddy jet mixer. An individual jet has two concentric nozzles with a suction chamber between them. A single eddy jet mixer may have 4 - 24 jets, but 8 - 12 is the usual configuration. Likewise, a single tank may have 1 or more eddy jet mixers, though 1 is most usual. Mixing: Theory and Equipment

Jet mixer The tank contents are pumped through the top of the mixer to the primary nozzles. Then, after passing through the common suction chamber, the 12 streams of fluid are discharged into the tank through the secondary nozzles. As the pressurized fluid flows through the primary nozzle and into the chamber, it creates a suction because of its high velocity-typically 6-10 m/s for low-shear mixing and 10-20 m/s for high shear. This suction draws fluid from the tank into the chamber, where the large velocity difference causes intense mixing. The mixed fluid is then expelled from the jet through the secondary nozzle. This forms a cone that entrains the surrounding liquid, and a plume that spreads horizontally before rising to the surface. The plume imparts most of its energy to the fluid in its path, causing circulation and mixing throughout the tank. Mixing in the tank is less intense than in the suction chamber.

Applications of Jet mixer Jet mixers tend to be used in situations that require turbulence, rapid approach to homogeneity, and high local shear rates. Jet mixing is normally used for liquids and slurries having viscosities below 1 Pascal-sec; a mechanical agitator is generally more efficient for higher-viscosity fluids.  Mainly useful for processes of liquid blending. e.g., neutralization or extraction.

Flow mixer or line mixer or pipe mixer It may consist a baffled pipe or an empty chamber. Liquids to be mixed are passed through the pipe/ chamber. Mixing takes place through bulk transport in the direction of flow. The power supplied to pump the liquid itself accomplishes mixing. For effective mixing controlling the feed rate is essential. Baffled pipe If the input rate is difficult to control and fluctuations in the added proportions of the liquids are unavoidable, continuous mixing equipment of tank type is preferable, because hold-up of the liquids and back flow or recirculation is possible.

Applications of pipe line mixer Used when large volume of liquids are to be mixed. Used for continuous mixing.

Advantages of liquid mixing Liquid mixing promotes heat transfer between liquid and a heating source. This step is essential in the crystallization of drug substances. Uniform heat transfer in the solution yields crystals of same size. Liquid mixing is essential in the manufacture of number of dosageforms. E.g.: Suspensions Emulsions Solutions Aerosols