liquid mixers

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SEMINAR ON MIXING IN LIQUID DOSAG FORM (AS PER SCHEDULE M) Prepared by: KALPEN N. PATEL (B. PHARM, M.PHARM SEM-I) Guided By : Mrs. CHETNA MODI (M. PHARM) DEPARTMENT OF TECHNOLOGY SHREE KRISHNA INSTITUTE OF PHARMACY, SHANKHALPUR

CONTENTS::

CONTENTS:

INTRODUCTION:

INTRODUCTION Liquid- liquid mixing is considered as a simple operation compared to that of solid-liquid mixing. It involves the formation of a homogenous system. Similar to solid-solid mixing, liquid mixing also involves the application of shear.

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Classification:

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MECHANISM OF LIQUID: Bulk transport The movement of a large portion of a material from one location to another location in a given system Turbulent mixing Mixing due to turbulent flow, which results in random fluctuation of the fluid velocity at any given point within the system Laminar mixing Mixing of two dissimilar liquids through laminar flow Molecular diffusion Mixing at molecular level in which molecular diffuse due to thermal motion. Molecular diffusion is explained by the flick’s law, which depends on the concentration gradient at different regions

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Flow pattern during mixing: Liquids are usually mixed by impellers, which produce shear forces for inducing necessary flow pattern in the container Mixing take place by three components, Radial component acts in a direction vertical to the impeller shaft. Longitudinal component also known as axial or vertical component. It acts as in a direction parallel to the impeller shaft. Tangential component acts in a direction tangent to the circle of rotation around the impeller shaft

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Vortex formation A strong circulatory flow pattern sometimes manifests in to formation of a vortex near the impeller shaft. Vortex can be formed when; The shaft is mounted vertically in the centre of the tank. Blades in the turbines are arranged perpendicular to the central shaft. At high impeller speeds. In unbaffled tank

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Disadvantage: Vortex formation reduces the mixing intensity by reducing the velocity of the impeller relative to the surrounding fluid. When vortex reaches the impeller, air from surface of the liquid is drawn This is an undesirable situation, because air bubbles are difficult to remove The entrapped air causes oxidation of the substance in certain cases The air bobbles in the fluid can create uneven loading of the impeller blades Prevention of vortex formation Baffled containers should be used. Tanks other than cylindrical shape are used to prevent it.

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MIXING VESSELS Apparatus consists of a container (tank) & mixing device and used for a batch process. The general construction of a mixing vessel is shown in figure Mixing device is called Impeller , which is mounted with the help of a shaft. The shaft is driven by a motor. Tank is made up of stainless steel. The size of the tank depends on the nature of the agitation method. It also consists of an outlet, coils, jackets, temperature measuring device etc…

MIXING VESSEL:

MIXING VESSEL Mixing devices are used to supply energy to the system for rapid mixing. Mixing devices are technically called as impellers. Impellers are classified on the basis of the shape & pitch of the blades PROPELLERS: Propeller normally contains number of blades Three bladed design is the most common for liquids . Marine type propeller is similar to the blades of a table fan or a ceiling fan . Figure of three bladed is like marine type propeller The size of the propeller is small. However, large tanks having maximum size of 0.5 meters propeller is used. Small propellers turn at full motor speed up to 8000 rpm.

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Propeller produce the longitudinal movement of the liquid & produce little shear. Flow pattern is axial. Due to high speed of propeller undesirable, vortexing & aeration (air entrapment) take place. This problem can be avoided by the using number of different arrangements of the propellers like, Offset Angled Side entering Push pull Baffled

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Application Are most widely used for liquids of low viscosity. Multi vitamins, elixirs, disinfectants, solutions are manufactured using propellers. Disadvantages Not suitable for those liquids which contain viscosity greater than 5 Pascal*second, e.g. glycerine, castor oil etc. As there is little shear, propeller mixers are not suitable in emulsification. TURBINES: The diameter of the turbine ranges from 30 – 50 percent of the diameter of the vessel. It rotates at a lower speed than propeller. (50-200 rpm)

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Different forms of turbines are used like, Flat blade Disc-flat blade Curved blade Pitched vane Pitched blade Tilted blade Pump impeller Application Due to high shear forces, turbines are effective for high viscous solutions Suitable for liquids of large volume & high viscosity. Advantages It gives greater shearing forces than propellers, so the pumping rate is less. Therefore, turbines are suitable for emulsification. Do not damage dispersed particles at economic speeds. Disadvantages Due to absence if vertical flow, less suitable for suspension. Are expensive to fabricate, Are restricted to a narrow range of speeds.

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PADDLES: Paddle mixers use as an agitator Paddles with two blades or four blades are common. Sometimes, the blades are pitched. In some paddles, the blades are dished or hemispherical in shape & have a large surface area. Because of this shape, paddles pass close to the tank walls & effectively mix viscous liquids, avoiding dead spots & deposited solids. At very low speed it gives mild agitation in an unbaffled tank, where as for higher speeds baffles are necessary In paddles, flow pattern is radial & tangential.

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Application Are used in the manufacture of antacid suspensions (aluminium hydroxide gel & magnesium hydroxide), agar & pectin related purgatives, anti diarrhoeal mixtures such as bismuth-kaolin Advantage Vortex formation is not possible with paddle impellers because of low speed mixing. Disadvantage Mixing of the suspension is poor, therefore, baffled tanks are required

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EQUIPMENTS

LIST OF EQUIPMETNS AS PER SCHEDULE M:

LIST OF EQUIPMETNS AS PER SCHEDULE M SHAKER MIXER OSCILLATORY ROTARY AIR JET MIXER JET MIXER FLOW MIXER OR PIPE MIXER

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SHAKER MIXER In some cases, the container itself moves to mix the liquids. This type of mixer is the example of this type. Oscillatory shaker mixer This is applicable on a small scale. Rotary This is applicable on a large scale. Shaker mixers are rarely used in pharmacy

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AIR JET MIXER Principle: - When compressed air jets are passed from the bottom of a vessel, air bubbles are formed in the liquid phase. The buoyancy of the bubbles lifts the liquids, which are confined to the central portion due to the presence of draft tubes. The liquids flow down from the periphery of the vessel & enter from the bottom due to suction effect. Thus mixing is achieved. Construction & working Assembly is shown in figure. Consists of a vertical tank with centrally located air jet & draft tube. Draft tubes are placed surrounding air jet as shown in figure

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Application Liquids of low viscosity, non-foaming, non-reactive with gas are mixed by using air jet mixer JET MIXER Principle: - It works on the principle of shearing in action. Construction & working The construction of a jet mixer is shown in figure.

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FLOW MIXER OR PIPE MIXER Construction & working The construction of a baffled pipe mixer & a chamber mixer are shown in figures. Liquids to be mixed are passed through the pipe. Mixing takes place mainly through bulk transport in the direction of flow.

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Placing certain devices, such as vanes, baffles, screws, grids & a combination of them, enhances the mixing efficiency. Little additional power supplied to pump the liquid itself accomplishes mixing. For an effective mixing, controlling the feed rate is essential. Suitable metering devices are employed. If input rate is difficult to control then continuous mixing equipment of tank type is preferable, because hold-up of the liquids & back flow or recirculation is possible. Application When large volumes of liquids are to be mixed, flow mixers are used.

REFERENCES:

REFERENCES C.V.S. Subrahmanyam, Pharmaceutical engineering principles and practices, Vallabh Prakashan, 1 st edition, Pg: 160-162, 225-247. G.k. Jani, pharmaceutical engineering-II, B.S. Shah Prakashan, 4 th edition, Pg: 434-452.

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