Mixers

Mixing is one of the important unit operation widely used from reaction to

finishing stages in chemical industry. A production (chemical) engineer has to

operate the running and modifications with the installed type for the particular

process or recommend one in case of plant erection or process design. The

selection or modification with these equipments is highly subject to the set of

properties of material being handled. Mixers are meant to shift the non-

homogeneity of a batch to a homogenous state. The degree of success in this

operation is never the hundred percent, since we can’t assume that a molecule A

will take the very next position to the molecule B and so on. Our auditing analysis

is therefore subject to spot sampling methods that take concentrations at

randomly selected locations in a sample in to consideration to estimate the

overall mixing efficiency of the entire batch.

Different mixers being employed follow three1 mixing phenomenon: the small-

scale random motion (diffusion), the large-scale random motion (convection) and

the interchange of particles by virtue of slip zones (shear). Mixers are employed

both for solids and liquids phases with varied interests. These include be but not

limited to blending of ingredients, cooling or heating purposes, drying or roasting

of solids, reaction engineering, coatings, agglomeration or even size reduction.

Confining our focus the mixing of solids, powders and particles, our selection is

subject to a set of properties2 which include: particle size distribution, bulk

density, true density, particle shape, surface characteristics, flow characteristics,

friability, state of agglomeration, moisture or liquid content of solids, density,

viscosity and surface tension, and temperature of ingredients. Referring our case

to the nature of solid particles, we have two classifications to cater for: non-

1 Perry’s Chemical Engineering Handbook, Solid-Solid Operations and Equipment, Section 19, pg. 122 Perry’s Chemical Engineering Handbook, Solid-Solid Operations and Equipment, Section 19, pg. 10

cohesive solids and cohesive solids. Further is the discussion over the types of

mixers for this classification.

For Non-Cohesive Solids

Mixers for non-sticky solids have a wide variety available for consideration. This may

range from free-flowing powders’ preferences to heavy pastes’ options. Mixing could be

carried3 out by agitation, tumbling, centrifugal action and impact forces. Following is a

brief about the options for discussed type of solids:

Internal Screw MixerIt consists of a vertical vessel with a screw rotating to achieve the circulation of material

and secondly the elevation of material. This results in intermixing of solid grains as well

as shear action to the ones in contact with the screw or the walls of container. The feed

enters from bottom usually with aim to nullify the gravity factor which could escape the

molecules without desired mixing. Two vessel shapes are normally used for this

category, the cylindrical and the conical. On the basis of screw movement, it could

further be classified4.

3 McCabe Smith, Unit Operations of Chemical Engineering, Mechanical Separations, Vol 7, pg 9794 Perry’s Chemical Engineering Handbook, Solid-Solid Operations and Equipment, Section 19, pg. 15

Type I: Conical shaped vessel, with a screw orbiting the centre of axis of the vertical tank and also rotating around its own axisType II: Conical shaped vessel, with a screw fixed at the centre and rotating around its own axisType III: Cylindrical vessel with a screw in centre and rotating around its own axis

Internal screw mixers are normally used for free-flowing grains and other light solids. In

type II, the conical tank is such tapered that with increase in height the area swept

increases.

Ribbon MixerSuitable for light and finely divided materials to fibrous sticky feed, ribbon mixer is

operated in both batch and continuous orders. It consists of a horizontal vessel with two

blades or ribbon in opposite directions with rotating at different speeds to intermix the

feed batch. The operation size5 could be as large as 34m3 for heavy ribbon mixers. The

size of ribbon widely affects the operational advantages. For example, broad ribbon can

be used for lifting or conveying purposes, while a narrow ribbon can cut the materials.

The type is further classified on construction parameters with differing features6 of

ribbon cross section and pitch, clearances between outer ribbon and shell, and number

of spirals etc.

5 McCabe Smith, Unit Operations of Chemical Engineering, Mechanical Separations, Vol 7, pg 9796 Perry’s Chemical Engineering Handbook, Solid-Solid Operations and Equipment, Section 19, pg. 12

Tumbling MixersTumbling mixers include mixers in which the vessel rotates, mixing the feed to optimum

levels, similar to tumbling mill that aim for size reduction. The closed vessel rotating

about its axis can handle heavy solids and dense slurries through diffusion mixing. The

shapes could include5 and are not limited to V-mixer, double cone, rotating cube. The

equipment main include internal sprays for addition of liquids to facilitate the mixing

process. Baffles could also be installed in an attempt to reduce segregation.

The retention time in a tumbling mixer is fixed since initial blending is rapid followed by

gradual uniform mixing. Excess time may affect and even decrease the quality of mixing

levels.

For Cohesive Solids

Cohesive solids employ maximum consideration in terms of mixer selection and

operating parameters due to influence of rheology and forces of cohesion. The power

consumption for this type of mixers is high due to these factors. Mixing of sticky solids

however is supported7 by shearing, folding, stretching and compressing elements.

7 McCabe Smith, Unit Operations of Chemical Engineering, Mechanical Separations, Vol 7, pg 980

Muller MixersSuitable for heavy solids and pastes, muller mixer applies smearing, rubbing and

somewhat skidding action to achieve the desired mixing levels. Its working action is

similar to that of mortar and pestle. It has several types, three most common include:

Stationary-pan mixer, Rotating-pan mixer and Countercurrent-pan-muller mixer. The

roller wheels move in a circular chamber to ride over the material, producing shearing

action that causes size reduction as well as intermixing of material.

Change-can MixerUsed for blending of viscous liquids & light pastes like food and paints, its most used

types are pony and beater mixer. In pony mixer, the can rotates to produce centrifugal

action to mix the materials, while in beater mixer, the agitator rotates to do the job. The

vertical shaft contains blades and paddles which are slightly twisted. When the mixing is

complete, agitator head is raised, lifting the blades out of can and the material can be

drained by tilting it.

Kneader Mixers

It is used to mix deformable or plastic solids by squashing the mass flat, folding it over

and squashing it once more7. It may tear the mass and shear it between the blades and

walls of mixer. The power requirements for this type of mixers are relatively high, esp. in

case of stiff materials. Examples (in order of power requirements) include: two-arm

kneader, disperser, masticator (intensive mixers), banbury mixer (internal mixers), and

continuous kneaders.

The two-arm kneader with minimum power requirements handles suspensions, pastes

and light plastic materials. A disperser with heavier construction body and more power

consumption is suitable for additives and coloring agents into stiff materials. Masticator

with maximum work capacity can work on scraping rubber and plastic materials. The

body is heavier than disperser and consumes more power.

In internal mixers, the chamber is sealed during the working time; making dispersions of

feed in liquid usually water. Example of such a type is Banbury mixer, a heavy-duty two

arm mixer in which the agitators are in form of interrupted spirals. The turning frequency

is 30 to 40 rpm. Kneaders work both in batch and continuous operation with equipment

parameters differing accordingly.

Paddle Mixers

These mixers are open air chambers with agitators scooping the materials and dropping

them again to those chambers, thus achieving the mixing levels via relocation of

material. The lifting action of blade is adjusted to the desired pace. This cross missing

configuration provides a homogenously mixed product. The use is effective even in

cases where bulk densities of component materials vary greatly. The design and bends

of the paddles are precisely made to ensure shear mixing in addition to convective one.