Ritik kumar mixing.pptx
- 1. Submitted To: Prof. Shiv Kumar Katiyar Head of the Department Submitted By: Ritik Kumar M.Sc. 1stSemester Roll no. 221155081018
- 2. Mixing operation: Mixing is basically a process in which components are treated in such manner so that particles of each component are available to the adjacent particles of the other component that are required to be mixed. Factors affecting mixing: 1. Particle size: Smaller particle results in homogeneous mixture than the larger one. 2. Particle shape: Particle should be spherical for uniform mixing. If there is irregular shape of the particle then they become interconnected in such a way so that their separation is relatively difficult after mixing than if the particle shape is regular. 3. Particle charge: If the particle has some electrostatic charge that cause attraction forces between particles, then there are more chances of segregation or separation.
- 3. 4. Nature if the particle: Particle hardness, elasticity, porosity, texture, angularity and particle vibrations are also the factors that affect mixing phenomena greatly. 5. Relative Density: If the components are of different density, the denser material will sink through the lighter one, the effect of which will depend on the relative positions of the material in the mixer. a. If the denser particles form the lower layer in a mixture at the start of a mixing operation, the degree of mixing will increase gradually until equilibrium is attained , not necessarily complete mixing. b. If the denser component is above, the degree of mixing increases to a maximum , then dropping to equilibrium as the denser component falls through the lighter one , so that segregation has started.
- 4. 6. Viscosity: Mixing is also affected by viscosity. More viscous particles causes improper mixing as their higher viscosity affects their speed (slow) to flow that is produced by forces to get mixing . so basically increasing viscosity reduces mixing extent. 7. Surface tension of liquids: Surface tension of liquid is also an important factor that effects mixing. High surface tension reduces extend of mixing. In fact cohesiveness is the tendency of material to adhere to itself causes difficulty in mixing as agglomerates are formed. 8. Moisture: Moisture present in particles heap also affects mixing phenomena. For example mixing of dry clay is more rapid and efficient than wet mixing of clay. In fact proper mixing requires specific moisture content in bulk.
- 5. 9. Temperature: Temperature also affects mixing as viscosity is changing with temperature changing. 10. Flow characteristics: Flow properties are directly related to particle size . as due to increase/decrease in particle size gravitational forces according to size increases/decreases. 11. Liquid quality: In solid-liquid mixing liquid (water) quality is also an important factor . Liquid quality includes pH value, salt level, organic matter; foreign matter etc. affects greatly the efficiency of mixing. 12. Speed/rpm of impeller: Speed of impeller affects the homogeneity of the mixture. As with less rpm mixture is more homogenous than with greater rpm.
- 6. 13. Mixer volume: Mixer volume also affects mixing phenomena. Mixer volume should be such that over filling should not be done as it decreases efficiency of mixing and mostly material can’t be mixed thoroughly. 14. Type of agitator: The shape, size, location and type of agitator present also affects the extent of mixing achieved and the time required for mixing of specific components . As the type of agitator required for mixing depends upon the nature of substances that need to be mixed. However, generally speaking, a. Impeller type mixers are used for solid-liquid mixing. b. Paddle type mixers are used for solid-solid mixing operations. c. The blades are further modified for kneading and dispersing purpose cohesive solids are needed to be mixed.
- 7. 15. Type of mixer: Type of mixer greatly effects on mixing phenomena . as in mixing there is specific flow patterns due to which mixing taking place. Suitable flow pattern for mixing can be obtained as a result of balanced components in mixer . a. If the impeller shaft is vertical, excessive radial movement, especially if solids are present , will take materials to the vessel periphery, where they drop to the bottom and may revolve as a mass below the impeller. b. If the tangential component is leading, a vortex is created and may deepen until it extents the impeller, when aeration occurs. c. If the longitudinal component is inadequate, liquids and solids may present in films without mixing.
- 8. 16. Mixing time: Mixing time is also very important for proper mixing. There is always an optimum mixing time for specific conditions in which mixing is taken place. As degree of mixing reaches to its limiting equilibrium value asymptotically. 17. Mechanism of mixing: The mixer must apply suitable shear forces to bring about local mixing and a convective movement to ensure that the bulk of the material passes through this area . so this mechanism also affects the mixing process.
- 9. 1. Three-bladed marine propeller: Many varieties are existing: a. with cut out or perforated blades for grinding and breaking up lumps. b. with saw tooth edges for cutting and scratching action, c. And with other than three blades . The stabilizing ring sometimes is in included. to minimize shaft flutter and vibration particularly at low liquid level. Where? They are used at relatively high speeds (up to 1800rpm) with low viscosity fluids, up to about 4000cP
- 10. 2. Flat vertical blades turbine: The simple geometry of this design has stimulated extensivetesting so that expectation of their action is on a more rationalbasis than that of any other kind of impeller. Where? It is suitable for the vast majority of mixing duties up to 100,000CP or so at high pumping capacity.
- 11. 3. Horizontal plate blades turbine: Where? The horizontal plate to which the impeller blades of this turbineare attached has a stabilizing effect. 4. Shrouded turbines: It is consisting of a rotor and a stator. Where? Ensures a high degree of radial flow and shearing action, and arewell adapted to emulsification and dispersion
- 12. 5. Cage beaters: Mostly they are mounted on the same shaft along with a standard propeller. More violent action may be achieved with spined blades . Where? It imparts a cutting and beating action. 6. Anchor paddles: Anchor paddles fit the contour of the container. Where? It prevents sticking of pasty materials, and promotes good heattransfer with the wall.
- 13. Types of mixtures for dry and paste materials 1. Tumbler Mixers: Free-flowing non-segregating powders may be readily mixed in batch by use of tumbler mixers. Tumbler mixers operate by tumbling the mass of solids inside a revolving vessel. Blenders are available in various geometries, affecting material movement, mixing efficiency and ease of cleaning between batches. These vessels take various forms, such as those illustrated , and may be fitted with baffles or stays to improve their performance. A tumbling batch blender can be of four types, which are described as follows: a. Horizontal cylinder : This cylindrical mixer has a tubular vessel mounted on trunnions. Internal baffles or lifter bars are mounted along the inner walls of the vessel. The inlet is typically located at the top center of the vessel and the outlet at the bottom center. The blender tumbles and the internal baffles gently lift and aerate the material preventing it from sliding along the blender bottom; they also de- lump the material. See figure a
- 14. b. Double cone blender : The double cone blender consists of two cone-shaped sections, typically with 45o slopes. The cone sections are welded at their ends to a center band. The blender is mounted between two trunnions that permit the unit to tumble end over end. An opening in one of the ends of the cones serves as inlet and outlet, or the inlet can be in one cone end with the outlet in the other. Cleaning access is through the outlet. The blender tumbles, and the material in the vessel spreads out. The transition area at the band between the cones prevents the material from sliding along the inner wall and instead causes the material to fold over itself. This provides gentle mixing with only very slight shear. See figure b Horizontal and Vertical Trough Mixers
- 16. C. V-cone blender and Y-cone blender : The V-cone blender is similar to a double cone unit, but consists of two large diameter pipe sections cut at a 45◦-angle and welded together to form a V. In the same way, the Y-cone blender has a third section that extends the volume of the blender in a bisectional direction with respect to the other pipe sections. Inlets are typically located at both ends of the V (or of the Y); the outlet is at the V point (or at the bottom of the Y). The unit is also mounted on trunnions to allow it to tumble and can be equipped with a spray line for liquid addition and an agitator for de- lumping. The units tumble end over end as in the double cone blender. The free-falling action combined with increased frictional contact between the material and the long vessel sides result in less gentle mixing than in a double cone blender.See figure c or d
- 17. 1.Vertical Screw Mixers : In vertical screw mixers, a rotating vertical screw is located in a cylindrical or cone shaped vessel. The screw may be mounted centrally in the vessel or may rotate or orbit around the central axis of the vessel near the wall. Materials are lifted from the bottom to the top of the hopper and are then exchanged with materials on the way up. Such mixers are schematically shown in . A vertical screw blender may be desired for larger batches handled in a small space, while the orbiting screw mixer (Fig. 17.4b) is used for difficult mixes. The latter arrangement is more effective and stagnant layers near the wall are eliminated. Vertical screw mixers are quick, efficient, and particularly useful for mixing small quantities of additives into large masses of material. Specialized atmospheres as well as normal temperatures and pressures are accessible for multipurpose operations. see figure a or b on next slide
- 19. 2.Fluidized Bed Mixers: Food powders can also be mixed by aeration using a fluidized bed. The resulting turbulence of passing air through a bed of particulate material causes material to blend. Materials are moved upward by air jets, causing differential movement. Stationary vessels using gas-flow agitation are used primarily for batch mode mixing. Materials to be mixed have to be relatively fine and fairly narrow in their size distribution, as well as not too cohesive. Powders to be mixed can be charged to more than 70% of the vessel volume. Mixing times required in fluidized beds are significantly lower than those required in conventional powder mixers. The mixing is largely convective with the circulation patterns set up by the bubble motion within the bed. An important feature of the fluidized bed mixer is that several processing steps (mixing reaction, coating, drying, etc. may be carried out in the same vessel. Additional equipment can include blowers, dust collectors, and pressure regulators, which will enlarge the system as a whole. A particular type of the fluidized mixer is the fluidized paddle mixer . The mixer has twin troughs, each with a center mounted rotating shaft. Flat paddles are welded to spokes on each shaft. The paddles lift the material from the bottom and throw it into a zero gravity, fluidized mixing zone, settling a random displacement pattern for the material.figure seen on next slide…