MECHANICAL OPERATIONS | CONVEYING | SEDIMENTATION | MIXING OF SOLIDS

           

OVERVIEW

        Mechanical Operations involves the study of separation operations using mechanical/physical forces acting on solid particles. The forces acting can be gravitational, centrifugal as well as kinetic. Mechanical operations involve physically changing a material.

            Chemical engineers  are expected to have the knowledge of mechanical operations as the material available to us is not in desirable form . So, we first need to use mechanical operations to reduce them to required size and then they can be passed through a number of unit processes in the plant. 

            Rate of chemical reactions is proportional to the surface area available. Area can be calculated only if the size of the particle is known to us. Hence, Sizing operations become essential. Mechanical Operations involve Size reduction, Screening, Conveying of solids, Sedimentation, Filtration, Mixing of solids and many more. We shall briefly look at each of them.

1. SIZE REDUCTION / COMMINUTION :

Objectives of  Size Reduction :

1. To increase the surface area which further increases the rate of reaction.

2. Reduced solids are easy to convey.

3. Sometimes comminution may be done to separate two constituents present as a dispersive mixture.

4. For materials like pigments, smaller the size, more is the coloring and covering power.

5. More intimate mixing of solids can be achieved if particle size is small.

The material obtained from the mines and fields is very large and in the form of lumps. Before taking it into processing , it has to be reduced in size. Size reduction refers to various processes through which  large lumps are reduced to either smaller particles or powder.Various forces can be employed in size reduction such as :

a. Impact force

b. Attrition force

c. Shear force

d. Compression force.

Depending upon the principle force employed in size reduction,we have different machines which are called either crushers (for coarse crushing) or mills (for intermediate crushing). Some of them are listed below alongwith the major force :

a. Jaw Crusher (Compression)

b. Roll Crusher (Compression)

c. Gyratory Crusher (Compression)

d. Hammer Mill (Impact)

e. Impactor  (Impact)

f. Tumbling Mills (Impact)

g. Ultrafine Grinders (Attrition)

Power Consumption in Comminution :

The energy fed to the crushing equipment is utilized in the following ways :

1. In producing elastic deformation of the particles before fracture occurs.

2. In producing inelastic deformation which results in size reduction.

3. In causing elastic deformation of equipment.

4. In noise, heat and vibration in the plant.

5. In friction between the particles and between the particles and the machine.

6. In friction losses in the plant itself.

A scientist OWENS suggested that only 10% of  total power is usefully employed.

2.  SCREENING/ SIEVE ANALYSIS :

Sieving is a technique of measuring the size of the particles.  It is an old, inexpensive and readily usable technique for separation of particles in size bands. It is not applicable to emulsions, cohesives and agglomerating materials.

 Screening surface can be made up of :

a. Parallel rods : for coarsest screening

b. Punched plates : made up of sheet steel. Punched in various patterns.

c. Woven wires : for finest screening. Wires made up of steel, copper or bronze. Can have two types of openings :

 ~ square opening - gives coarse screening.

 ~ rectangular opening - gives fine and intermediate screening.

3. CONVEYING OF SOLIDS :

Conveying refers to transportation. Solids are to be transported from one place to another once they have been stored. For this purpose we have various types of conveyors available in the market. Selection of conveyors  depends upon a number of factors like capacity requirement, length of travel, cost, etc. Some of them are listed below:

~ Belt conveyors (horizontal long distance transport)

~ Screw conveyors (horizontal conveying)

~ Bucket elevators (spaced and continuous type, for vertical transport)

~ Continuous flow elevators (used in a small region like in the plant itself)

~ Vibrating conveyors (horizontal transport, slightly inclined wrt horizontal)

~ Pneumatic conveying. (air is the conveying medium, particles suspended in air)

Here we refer not only to horizontal transport but the vertical transport as well. Horizontal transport normally uses belt and screw conveyors and vertical transport can be done using bucket elevators (spaced and continuous) and pneumatic conveyors. 

Pneumatic conveying  is used for both - horizontal and vertical transport and is the most versatile way of conveying. It can be used almost anywhere and everywhere. The only requirement is the space to lay the pipes.

4. SEDIMENTATION :

Sedimentation is a method of solid-liquid separation using the concept of settling. A slurry containing solids is split up into two fractions : first is the clear liquid and second being the thickened slurry. The settling can occur under the effect of gravitational acceleration, centrifugal acceleration or electromagnetism. 

Depending upon the concentration of solids in the slurry, we have two types of settling.

~ Free settling - observed when the concentration of solids is less than 1% . In this type of settling the particles fall with terminal settling velocity and their motion is neither affected by other particles nor by the walls of the vessel. 

~ Hindered settling - observed when the concentration of solids in the slurry is more than 1% . In this type of settling the particles fall with a velocity smaller than the terminal settling velocity and their motion is affected by other particles as well as by the walls of the vessel. 

Factors affecting Sedimentation :

1. Diameter of particles.

2. Strength of gravitational field.

3. Relative density of particle wrt fluid.

4. Density of fluid.

5. Viscosity of fluid.

6. Particle concentration.

P.S. Diameter of vessel and Mass of particle have no effect on the rate of sedimentation.

 

The vessels in which Sedimentation is carried out are called thickeners. They are of two types :

~ Batch thickener

~ Continuous thickener

5. FILTRATION :

Filtration  basically refers to the separation of a slurry containing solids into two fractions using a permeable medium. This permeable medium is called the filter medium .One of the fractions is the clear liquid called filtrate and the other fraction is the solids .Filtrate gets passed through the filter medium and the solids are collected over the filter medium.

Filtration can be classified into various categories on the basis of various parameters.

Rate of filtration can be improved by the addition of substances known as "filter aids".

Some commonly used filtration equipments are :

~ Plate and frame filter

~ Rotary vacuum drum filter

~ Candle filter

~ Leaf filter

~ Filter press

~ Belt filter

~ Membrane filter.

6. MIXING OF SOLIDS :

Mixing of solids is a lot different from that of liquids. In case of solids, no flow currents are being generated and the power consumption is also more. Even after mixing, the two fractions can be identified.

There are different types of mixers available as per the types of solids to be mixed.

Mixers for Cohesive solids :

- Kneaders

- Muller Mixer

- Change Can Mixer

- Masticators

- Mixing Rolls

- Pug Mills

- Mixer Extruders. 

Mixers for Free Flowing solids :

- Ribbon Blenders

- Impact Wheels

- Tumbling Mixers

- Internal Screw Mixers.

Thanks for reading. If you found it useful, do let me know in the comments section.