DIPLOMA ENGINEERING N. G. PATEL POLYTECHNIC 637 CHEMICAL ENGINEERING 05 MECHANICAL OPERATION (CEO-I) 320013

DIPLOMA ENGINEERINGN. G. PATEL POLYTECHNIC 637CHEMICAL ENGINEERING 05

MECHANICAL OPERATION (CEO-I) 320013

COMPILED BY:Mr. M. B. DHANGARLECTURER IN CHEMICAL ENGINEERING DEPARTMENTN. G. PATEL POLYTECHNIC, ISROLIChapter IX: MixingPurpose of mixing solids and pastes:The purpose of mixing is to produce a well mixed product that has necessary properties like, visual uniformity, high strength, uniform burning rate, or other desired characteristics. A good mixer is one that produces a well mixed product at lowest overall cost.Mixing EquipmentsFor rubbers, pastes and heavy plastic masses the agitator must visit all the parts of the mixer, or the material must all be brought to the agitator.The action in a mixer can be described as a combination of low-speed shear, smearing, wiping, folding, stretching, and compressing.These mixers are used for viscosity above 2000P.Kneaders and mixer-extruders are used for thick pastes and plastic masses.Impact wheels are used for dry powders.Other mixers can be used for blending pastes, liquids, plastic solids, and powder.Factors considered in selection of equipment:Rate of mixingFor all the rate processes, the rate is directly proportional to the driving force.The mixing index, IS is a measure of how far mixing has proceeded toward equilibrium.For short time mixing, the rate of change of IS is directly proportional to (1 - IS),

Where, k is a constant.The equilibrium value of IS is 1.0 and driving force for mixing at any time can be considered to be (1 - IS).

Where,

With sample containing number of particles = nRibbon blender:Ribbon mixer (blender)

It consists of a horizontal trough containing a central shaft and a helical ribbon agitator.Two counteracting ribbons are mounted on the same shaft, one moving the solid slowly in one direction, and the other moving it quickly in the other.The ribbon can be batch or continuous.The mixing results from turbulence induced by counteracting agitators, not from motion of solids through the trough.Ribbon blenders are effective for thin pastes and for powders that do not flow readily. The power required is moderate.Kneaders:Kneaders

Kneaders Double armedThe universal mixing and kneading machine consists of two counter rotating blades in a rectangular trough curved at the bottom to form two longitudinal half cylinders and a saddle section.The blades are driven by gearing at either or both ends.The mixing action is a combination of bulk movement, smearing, stretching, folding, dividing, and recombining as the material is pulled and squeezed against blades, saddle, and sidewalls. The blades are pitched to achieve end-to-end circulation.The blades may be tangential or overlapping. Tangential blades are run at different speeds, with the advantages of faster mixing from constant change of relative position, greater wiped heat-transfer area per unit volume, and less riding of material above the blades. Overlapping blades can be designed to avoid buildup of sticky material on the blades.Pug mill:Pug millsMixing is done by blades or knives set in a helical pattern on a horizontal shaft turning in an open trough or closed cylinder.Solids continuously enter at one end of mixing chamber and discharge from the other.In the chamber, the material is cut, mixed, and moved forward to be acted upon by each succeeding blades.A double shaft open trough mill is used for rapid or more thorough mixing.The chamber of enclosed mills can be cylindrical or polygonal.Pug mills are used to blend homogenize clays, break up agglomerates in plastic solids, and mix liquids with solids to form thick, heavy slurries.They are built with jackets for heating or cooling.Banbury mixer:Banbury mixers

Banbury mixersIt is used mainly in the plastics and rubber industries. The top of the charge is confined by an air-operated ram cover mounted so that it can be forced down on the charge. The clearance between the rotors and the walls is extremely small, and it is here that the mixing action takes place. The shaft turns at 30 to 40 rpm.The operation of the rotors of a Banbury at different speeds enables one rotor to drag the stock against the rear of the other and thus help clean ingredients from this area.The friction produced in the mixer is more and hence cooling may be required for heat sensitive materialsMuller mixer:Muller mixer

Mulling is smearing or rubbing action similar to that in a mortar and pestle.One design is, the pan is stationary and a central vertical shaft is driven, causing the muller wheels to roll in a circular path over a layer of solids on the pan floor.Second design is, the pan is rotated and wheels are held stationary.Third design is, the wheels are offset from center, and both pan and wheels are rotated.The action results from the slip of the wheels on the solids.Plows guide the solids under the muller wheels, or to the discharge at the end of operation.They are effective in uniform coating of solids, and mixing of heavy solids and pastes.Calculation of mixing index:Effectiveness of mixing and mixing indexLet the overall weight fraction of tracer in the mix = The number of samples taken = NMass fraction of tracer in each sample = xiAverage value of tracer concentration =If mixing is not complete, xi will differ fromThe standard deviation S, is

S diminishes to zero s mixing proceeds, so a low value means good mixing.The ratio that increases with mixing is o (standard deviation at zero mixing) to S (standard deviation after mixing) called mixing index, IP.

REFERENCES:Unit Operations of Chemical Engineering Fifth Ed by Warren L. McCabe, Julian C. Smith, Peter Harriottpage no 235-278Unit Operations I Tenth Ed by K. A. Gavhane page no 111-127All images are taken from Google image.