Mixing Ppt 4m451x

MIXING Definition: Mixing may be defined as an operation in which two or more components, in a separate or roughly mixed condition, are treated so that each particle lies as nearly as possible in with a particle of each of the other ingredients.

Objectives of mixing 1.Simple physical mixture: simply the production of a blend of two or more miscible liquids or two or more uniformly divided solids. •degree of mixing must commonly be of a high order, as many such mixtures are dilutions of a potent substance (e.g. small amount of steroid mixed in large amount of inert diluents) •correct dosage must be ensured

2. Physical change:Mixing may aim at producing a change that is physical the solution of a soluble substance •lower efficiency of mixing will often be acceptable

3. Dispersion: dispersion of two immiscible liquids to form an emulsion or the dispersion of a solid in a liquid to give a suspension or paste. •Usually good mixing is required to ensure stability. 4. Promotion of reaction: Mixing will usually encourage (and control at the same time) a chemical reaction, so ensuring uniform products. •products or processes where accurate adjustment to pH is required and the degree of mixing will depend on the process

TYPES OF MIXTURES • Positive mixtures: are formed from materials such as gases and miscible liquids, where irreversible mixing would take place, by diffusion, without the expenditure of energy provided time is unlimited. such materials do not present any problems in mixing. • Negative mixtures: Suspensions of solids in liquids are examples of negative mixtures require work for their formation, and the components of which will separate unless work is continually expended on them. • Negative mixtures are more difficult to form and a higher degree of mixing efficiency is required.

• Neutral mixtures: Neutral mixtures are static in their behavior the components having no tendency to mix spontaneously, nor do they segregate when mixed. • e.g. Pastes, ointments and mixed powders.

SOLID MIXING MECHANISM Solids mixing proceeds by a combination of one or more of the following mechanisms: 1.Convective mixing: A relatively large mass of material is moved from one part of the powder bed to another  this is called convection. •Depending on the type of mixer employed, convective mixing can occur by an inversion of the powder bed, by means of blades or paddles, or by means of a revolving screw etc. •Macromixing

2. Shear mixing •In this type, forces of attraction are broken down so that each particle moves on its own between regions of different components and parallel to their surface •When shear occurs between regions of different composition and parallel to their interface, it reduces the scale of segregation by thinning the dissimilar layers. •Shear occur in a direction normal to the interface of such layers is also effective since it too reduces the scale of segregation. •using agitator arm or a blast of air.

3. Diffusive mixing •Mixing by “diffusion” is said to occur when random motion of particles within a particle bed causes them to change position relative to one another. •Such as exchange of positions by single particles result in reduction of the intensity of segregation. •Diffusive mixing occurs at the interfaces of dissimilar regions that are undergoing shear and therefore results from shear mixing. •Micromixing

Solid-solid mixing steps In the solid- solid mixing operation four steps are involved: 1.Expansion of the bed of solid 2.Application of three dimensional shear forces to the powder bed 3.Mix long enough to permit true randomization of particle 4. no segregation after mixing

Methods of solid-solid mixing The powders may be mixed by following methods: 1.Spatulation 2.Trituration 3.Tumbling 4.Geometric dilution

Classification of mixers : a) Batch Type mixers 1. 2. 3. 4. 5. 6. 7. 8.

Tumblers V-cone blender or twin shell blender Double cone blender Tumbling blender with agitator mixing blade Ribbon blender Sigma blade mixer Planetary mixer Fluidized mixer

b) Continuous Type mixers 1. 2.

Barrel type continuous mixer Zigzag type

Based on flow properties 1: free flowing solids V cone blender Double cone blender 2.Cohesive solids Sigma blender Planetary blender

• BATCH MIXERS • In tumbling mixers, rotation of the vessel imparts movement to the materials by tilting the powder until the angle of the surface exceeds the angle of repose when the surface layers of the particles go into a slide. • A common type of mixer consists of a container of one of several geometrical forms, which is mounted so that it can be rotated about an axis. The resulting tumbling motion is accentuated by means of baffles or simply by virtue of the shape of the container.

• The efficiency of tumbling mixers is highly depended on the speed of rotation. • If the rotation speed is too slow  does not produce desired tumbling or cascading motion nor does it generate rapid shear rates. • If the rotation speed is too high  produce centrifugal force sufficient to hold the powder to the sides of the mixer and thereby reduce efficiency. • If the rotation speed is optimum depends on the size, shape, r.p.m. Commonly in the range of 30 to 100 rpm.

Agitator mixers • Agitator mixer for powders can take a similar form to paddle mixers for liquids, but their efficiency is low. Planetary motion mixers are effective, but special design are to be preferred. • This type of mixers employs a stationary container to hold the material and brings about mixing by means of moving screws, paddles or blades. • Use: Since the mixers do not depend entirely on gravity as do the tumblers, it is useful in mixing wet solids, sticky pastes etc. • The high shear force effectively break up lumps or aggregates.

MECHANISM-MIXING OF FLUIDS 1. Bulk transport – the movement of a relatively large portion of the material being mixed from one location in the system to another constitutes bulk transport. •accomplished by means of paddles, revolving blades 2. Turbulent Mixing--the phenomenon of turbulent mixing is a direct result of turbulent fluid flow, which is characterized by a random fluctuation of the fluid velocity at any given point with in the system. •fluid velocity at a given instant may be expressed as the vector sum of its components in the x, y, and z directions.

3. Laminar mixing: two dissimilar liquids are mixed through laminar flow, •the shear that is generated stretches the interface between them. •Suitable for liquids which require moderate mixing

4. Molecular diffusion- primary mechanism responsible for mixing at the molecular level is diffusion resulting from the thermal motion of the molecules.

Mixing Apparatus for fluids • A Container and • A Mixing Device or Impeller

Mixing equipments for liquid-liquid mixing The mixing apparatus consist a container (tank) and a mixing device. A mixing device is called impeller, which is mounted with the help of shaft. The shaft is driven by a motor. Three main types of impeller are used namely-

Paddles

Propeller

Turbine

• The propellers are small impellers that produce an longitudinal movement of liquids.

• Characteristics: • For liquids of low viscosity simple flat paddles are used and the emphasis is on radial and tangential movements. • Paddles for more viscous liquids generally have a number of blades, often shaped to fit closely to the surface of the vessel, avoiding ‘dead spot’ and deposited solids. • An alternative design for the more viscous range of liquids is the planetary motion mixer, which has a smaller paddle that rotates on its own axis, but travels also, in circular path round the mixing vessel.

Shaker Mixers • Mixers in this category cause the container material to be agitated, either in an oscillatory or a rotatory movement. • Oscillatory movement: applied to small scale • rotatory movement: applied to large vessels

• The mixing operation has 2 requirements: 1.Localized mixing, sufficient to apply shear to the particles of the fluid. 2.A general movement, sufficient to take all parts of the bulk of the material through the shearing zone and to ensure that a uniform final product is obtained. • For readily miscible liquids  flow alone is sufficient. • For two immiscible liquids to produce an emulsion  shear force is essential. • The movement of the liquid at any point in the vessel will have three velocity components and the complete flow pattern will depend upon variations in these three components in different parts of the vessel.

• Excessive radial movement, especially if solids are present, will take materials to the container wall, when they fall to the bottom and may rotate as a mass beneath the impeller. • If the tangential component is dominant, a vortex forms and may deepen until it reaches the impeller, when aeration occurs. • If the longitudinal component is inadequate, liquids and solids may rotate in layers without mixing, even when rotation is rapid and in the presence of vortexing.