Size Separation PDF

**SIZE SEPARATION** Size separation is a unite operation that involves the separation of a mixture of various sizes of particles into two or more portions by means of screening surfaces. Size separation is known as *sieving*, sifting, *classifying* or *screening*. This technique is based on physic...

**SIZE SEPARATION** Size separation is a unite operation that involves the separation of a mixture of various sizes of particles into two or more portions by means of screening surfaces. Size separation is known as *sieving*, sifting, *classifying* or *screening*. This technique is based on physical differences between the particles such as size, shape and density. Size reduction of a solid material never gives particles of same size, but gives particles of varying sizes, i.e., distribute in different sizes. These materials must be subjected to a separation technique to obtain narrow size range. **Applications** Size separation of solids has important applications 1. As a method to determine particle size and size distribution, which are useful in the production of tablets and capsules. 2. To test the efficiency of a size reduction equipment or process. 3. To optimize the process conditions such as methods of agitation, time of screening, feed rate etc. 4. To recover valuable products or byproducts. 5. To prevent environmental pollution. **Official standards for powders** According to pharmacopoeia the powders can be specified into five grades depending on the size of the mesh of the sieve through which the powder can pass. i. ii. **Moderate coarse powder (22/60)**: iii. **Moderate fine powder (44/85):** iv. **Very fine powder (120):** When a drug is reduced in size, particles with different sizes are produced. It easy to get fine powder rather than coarse powder because material can be subjected to continuous grinding until fine powder is obtained. Various grades of powders are used in different pharmaceutical processes e.g., extraction of active constituents and in the formulation of various types of dosage forms. For the formulation of preparations meant for external application to the skin e.g., ointment, pastes, creams, lotions, liniment, dusting powders etc. and for ophthalmic preparations very fine powder should be used to avoid irritation duo to the presence of gritty particles. For oral preparations like suspensions, powders, tablets, capsules fine powder should be used because the rate of absorption of a drug depends on the particle size, smaller the particle size quicker will be the rate of absorption. If sustained action of a drug is to be produced then large size particles will be used. **SIEVE** Sieves are the simplest and sieving in the most frequently used methods for size separation. **Types of Sieves:** The primary consideration for sieve is given to the size shape of aperture opening. Square meshes are arranged as per the specification. Sieves commonly used in pharmaceutical processing include: - Woven wire sieves - Bolting cloth sieves - Closely spaced bars (screens) - Punched plates **Woven wire sieves:** Woven wire sieves are general- purpose sieves and widely used in pharmacy practice. The types of woven wire are - Plain weave - Twill weave **Bolting cloth sieves:** Silk, nylon and cotton are generally woven from twisted multi-strand fibres, Nylon cloths are generally designated by their micrometer opening and available in different grades. These are used for the separation of fine powders. Hummer screen uses this type of screen. **Modes of motion in size separation** Screening is methods of separation particles according to size alone. The basic technique involved is passing the particles through a series of sieves of uniform size. In this, the particles drop through the openings due to gravity. Coarse particles can drop easily through large openings, but it is difficult to screen the fine powders. This process can be hastened by inducing some type (mode) of motion (movement) to the particles. Size separation is basically assisted by three methods. 1. Agitation 2. Brush the sieve 3. Centrifugal force These modes of shaking help to shake the material so that sieving will be quick and entire sieving area can be utilized. 1. Sieves may be agitated in a number of different ways: a. b. Agitation methods are not continuous methods. However, these can be made continuous by inclination of the sieves. Separate outlets are made for undersize and oversize particles. **Advantages** 1. Agitation methods are inexpensive 2. Simple method **Disadvantages** 1. Agitation methods have lower limit of the particle size. 2. If the powder is not dried apertures become clogged with particles leading to improper sieving. 3. During agitation attrition (particles colliding with each other) occurs causing size reduction. **2- Brushing Methods** In this case, a brush is used to move the particles on the surface of the sieve and to keep the meshes clear. The brush is rotated in the middle in the case of a circular sieve, but the spiral brush is rotated on the longitudinal axis in the case of the horizontal cylindrical sieve. One example is the brush sifter (Figure 2). This is used for size separation of greasy or sticky powders such as waxes and soaps. Brush sifter **Fig. 2:** brush sifter 2. **Centrifugal Methods** In this method, a high-speed rotor is fixed inside a vertical cylindrical sieve, so that on rotation the particles are thrown outwards by centrifugal force. The currents of air can be generated by means of a jet of air into the equipment, which helps in separating the particles. A few examples of equipment that work on this principle are [cyclone separators](https://en.wikipedia.org/wiki/Cyclonic_separation) and air separators. **Advantages of Centrifugal Method** - Centrifugal methods are extremely useful in cases where conventional sieving tends to block the sieves. - Extremely useful for fine powder, because sieves have the limitation of the mesh size. **Sieve Analysis -- Testing of Powder** The milled material is subjected to size separation in order to obtain the powder of desired size or size distribution. Size distribution analysis is important in different areas as mention below Quality control tool for the analysis of raw materials. Testing the efficiency of size reduction equipment or process. Selecting the sieve system for commercial equipment. Errors can arise if the sieve are overloaded or if insufficient time is allowed for the particles to pass through. **Sieve Shaker Machine:** **Principle:** The powdered drug is separated according to its particle size using a number of sieves in a nest. These are subject to different types of agitation, so that size separation is rapid. **Working:** Sieves are arranged in a nest with the course at the top. A sample (50 g) of the powder is placed on the top sieve (fig. 3). This sieves set is fixed to the mechanical shaker apparatus and shaken for a certain period of time (20 minutes). The powder retained on each sieve is weighed. 1\. A sieve-nest usually comprises 6 to 8 sieves with an aperture progression based on 1/2 or 21/2 change in diameter between adjacent sieves. 2\. Initial weight (W~o~) of powder sample was taken on the first sieve (i.e., topmost sieve). The sieve-set was closed and shaking was started. After shaking for a stipulated time, the sieve-set was taken out. All the sieves were disassembled. 3\. The powder retained on each sieve was collected on a paper (bearing the mesh number) and weighed.  Fig. 3: laboratory sieve shaker machine **Equipment for Size Separation** **CYCLONE SEPARATOR** **Principle** In cyclone separator centrifugal force is used to separate solid from fluids. The separation process depends on particle size and particle density. It is also possible to allow fine particles to be carried with the fluid. **Construction** It consists of a short vertical, cylindrical vessel with a conical base. The upper part of the vessel is fitted with a tangential inlet. The solid outlet is arranged at the base. Fluid outlet is provided at the center of the top portion, which extends inwardly into the separator. Such an arrangement prevents the air short-circuiting directly from the inlet to the outlet of the fluid. **Working** The solids to be separated are suspended in a stream of fluid (usually air or water). Such feed is introduced tangentially at a very high velocity, so that rotary movement takes place within the vessel. The centrifugal force throws the particles to the wall of the vessel. As the speed of the fluid (air) diminishes, the particles fall to the base and collected at the solid outlet. The fluid (air) can escape from the central outlet at the top. **Uses** 1\. Cyclone separators are used to separate solid particles from gases. 2\. It is also used for size separation of solids in liquids. 3\. It is used to separate the heavy and coarse fraction from fine dust.  What is cyclone separator? How it works? What kind of industry use it? - Quora Fig. 4: cyclone separator  **BAG FILTER** **Principle** In a bag filter, size separation of fines (or dust) from the milled powder is achieved in two steps. *In the first step*, the milled powder is passed through a bag (made from cloth) by applying suction on the opposite side of the feed entry. This facilitates the separation. *In the next step,* pressure is applied in order to shake the bags so that powder adhering to the bag falls off, which is collected from the conical base. **Construction** It consists of a number of bags made of cotton or wool fabric. These are suspended in a metal container. A hopper is arranged at the bottom of the filter to receive the feed. At the top of the metal container, a provision is made for vacuum fan and exhaust through discharge manifold. At the top of the vessel a bell-crank lever arrangement is made to change the action from filtering to shaking. **Working** \(a) Filtering period: During this period the vacuum fan produce a pressure lower than the atmospheric pressure within the vessel. Gas to be filtered enters the hopper, passes through the bags, and out of the top of the apparatus. The particles are retained within the bags. \(b) Shaking period: During this period the bell-crank lever first close the discharge manifold and air enters through the top so the vacuum is broken. At the same time, it gives a violent jerking action to the bags so that they are freed from the dust. The fine particles are collected at the conical base. **Uses** 1\. Bag filters are used along with other size separation equipment, e.g., a cyclone separator. 2\. Bag filters are used to remove the fines from cyclone discharge. 3- Bag filter is connected to the discharge end of the fluidized energy mill 4\. Household vacuum cleaner is a simple version of bag filter. Bag Filters, For Food Industry, Rs 150000 Smd Engineering Equipments & Systems \| ID: 11087440448  Fig.5: Bag Filter