7.1 Filtration
Filtration is defined as the unit operation in which the insoluble solid component of a solid liquid suspension is separated from the liquid component by passing the latter through a porous membrane or septum which retains the solid particles in the upstream surface. The solid liquid suspension is called feed slurry, the liquid component that passes through the membrane is called the filtrate and the membrane is called the filtering medium. The separated solids are known as the filter cake.
Filter media
Filter media may be rigid or flexible. Rigid filter media may be loose such as sand or gravel, diatomaceous earths or charcoal. There are also rigid fixed media like porous carbon, porcelain, fused alumina, perforated metal prate,…
Flexible media may consist of woven fabrics of cotton, silk, wool and jute. Many synthetic materials are finding application, including nylon, polypropylene, polythene, polyvinylchloride…
Different filtration equipments are known. These include:
Pressure filters where a pressure high than atmospheric pressure is maintained upstream of the medium to induce the flow of the filtrate. There may be vertical plate pressure filter and horizontal plate filters.
Vacuum filters where low pressure is maintained downstream of the medium. There are:
continuous rotary drum vacuum filter, rotary vacuum disc filter.
7.2 Ultrafiltration
Ultrafiltration is widely employed in the increasingly important biotechnological industry for separation of fermentation products, particularly enzymes. Its largest commercial use is in the dairy industry for recovery of proteins from cheese whey and for pre-concentration of milk for cheese making. This process used membranes with a very small pore size but a bit larger than for reverse osmosis.
7.3 Reverse osmosis
It is well known that when a plant or an animal membrane separates two solutions of different concentrations, pure water passes through the membrane. The movement of water occurs from a solution with high concentration of water to a solution with low concentration of water. This process is called osmosis and it is driven by osmotic pressure. If a pressure higher than the osmotic pressure is applied to the side where the
76 concentration of water is higher, the process of osmosis is reversed. This process is called reverse osmosis.
7.4 Centrifugation
Centrifugation is a unit operation involving the separation of materials by the application of centrifugal force.
7.4.1 Separation of immiscible liquids:
The centrifugal force Fc acting on an object of mass m, rotating in a circular path of radius R at an angular velocity of ω is:
r = »Þ
,If two liquids with different densities are placed in a cylinder rotating about a central axis, the more dense liquid will tend to move towards the wall thus separating from the other liquid.
Figure 7.1: Liquid separation in a centrifuge
7.4.2 Separation of insoluble solids from liquids:
This process is usually called centrifugal clarification where small quantities of insoluble solids are removed from a liquid by centrifugation.
7.4.3 Centrifugal filtration:
This tem describes another method of separating insoluble solids from liquids involving centrifugal force. However, in this case the mechanism of separation is filtration and the flow of the filtrate is induced by centrifugal force. In this case, the centrifugal bowl is leaned with filtration medium.
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7.5 Sieving:
This is a mode of mechanical separation where particles are separated base on their size.
The separating medium is called a sieve. The material is shaken or agitated above a mesh or cloth screen; particles of smaller size than the mesh openings can pass through under the force of gravity.
The volume of material handled by the sieve depends on:
• nature and the shape of the particles,
• frequency and the amplitude of the shaking,
• methods used to prevent sticking or bridging of particles in the apertures of the sieve and
• tension and physical nature of the sieve material.
Industrial sieves include rotary screens which are horizontal cylinders either perforated or covered with a screen, into which the material is fed. Other sieves are vibrating screen generally vibrated by eccentric weights.
7.6 Sedimentation
• Sedimentation uses gravitational forces to separate particulate material from fluid streams.
• The particles are usually solid, but they can be small liquid droplets, and the fluid can be either a liquid or a gas.
• Sedimentation is very often used in the food industry for separating dirt and debris from incoming raw material, crystals from their mother liquor and dust or product particles from air streams. In sedimentation, particles are falling from rest under the force of gravity.
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Chapter VIII Mixing
Mixing is the dispersing of components, one throughout the other. It occurs in innumerable instances in the food industry and is probably the most commonly encountered of all process operations.
Mixing can be, particles mixing where small particles are mixed to make an homogeneous mixture, it can also be particle mixing in liquids it can also be about mixing of different liquids.
8.1 Mixing equipments:
Liquid Mixers:
The propeller mixer is probably the most common and the most used in liquid mixing.
Powder and Particle Mixers
The essential feature in these mixers is to displace parts of the mixture with respect to other parts.
Figure 8.1: Mixers (a) ribbon blender, (b) double-cone mixer
Dough and Paste Mixers
Dough and pastes are mixed in machines that have, of necessity, to be heavy and powerful. Because of the large power requirements, it is particularly desirable that these machines mix with reasonable efficiency, as the power is dissipated in the form of heat, which may cause substantial heating of the product.
79 Figure 8.2: Example of a Kneader
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