Size Reduction

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Scope: This course is designed to impart a fundamental knowledge on the art and science of various unit operations used in pharmaceutical industry . Objectives : To know various unit operations used in Pharmaceutical industries. To understand the material handling techniques. To perform various processes involved in pharmaceutical manufacturing process. To carry out various test to prevent environmental pollution. To appreciate and comprehend significance of plant lay out design for optimum use of resources. To appreciate the various preventive methods used for corrosion control in Pharmaceutical industries. 2

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UNIT-I Flow of fluids Size Reduction Size Separation UNIT-II Heat Transfer Evaporation Distillation UNIT- III Drying Mixing UNIT-IV Filtration Centrifugation UNIT- V Materials of pharmaceutical plant construction, Corrosion and its prevention 3

Chapter 1- Size Reduction:

Chapter 1- Size Reduction ( Comminution / Diminution/ Pulverisation) Objectives, Mechanisms & Laws governing size reduction Factors affecting size reduction Principles, construction, working, uses, merits and demerits of Hammer mill, ball mill, fluid energy mill, Edge runner mill & end runner mill. 4

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Definition- Size reduction or comminution is the process of reducing substances to smaller particles OR Process of reducing large solid unit masses into small unit masses i.e. Coarse or fine particle When the particle size of solids is reduced by mechanical means it is known as milling. The size reduction operation can be divided into two major categories depending on whether the material is a solid or a liquid. If the material is solid, the process is called grinding and cutting, if it is liquid, emulsification or atomization. 5

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Advantages/ objectives:

Advantages/ objectives Content Uniformity- Mixing becomes effective if particle size is uniform and small. With smaller particle size , number of particles per unit dose increases. Larger the number of particle, better is the mixing. Therefore better content uniformity can be achieved Uniform flow- Smaller and controlled particle size promotes the flow of powder, this helps in tablet compression and capsule filling. 7

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Effective extraction of drugs- Smaller particle allows rapid penetration of menstrum or solvent into the tissue or cells, extraction becomes effective. Effective drying- Rate of drying increases as size reduction increases the surface area. Improved physical stability- Smaller particle size decreases the sedimentation rate, thus increases the stability of suspension and emulsions. Improved dissolution Rate- Size reduction increases the surface area, facilitate intimate contact of solid particle. 8

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Improved rate of absorption- Smaller is the particle size, faster is the absorption. Disadvantages- Drug Degradation- It is possible due to heat produced during milling. Increased Surface area also facilitate decomposition. Very small particle size posses very strong cohesive forces which results in aggregation of particles. Increased SA promotes the adsorption 9

Mechanism of size reduction:

Mechanism of size reduction 10

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Hammer mill:

Hammer mill 26

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Principle- Operates on the principle of impact between rapidly moving hammers mounted on a rotor and the powder material. Construction- It is either horizontal or vertical shaft type mill. Hammers are made up of hardened steel or stainless steel with impact surface made of an extremely abrasive resistant material such as haystellite and carbaloy SS hammers are sufficient for pharmaceuticals. 27

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Hammers may be bar shape or stirrup shape. Bar shape is used for tablet granulation. Hammer blade can be flat or sharp edges or both on each side. Hammer may be either rigid or swing type. The entire unit is enclosed in a chamber containing grid or removable screen through which material passes. Screens are prepared using metal sheet of varying thickness with perforated holes or slots 28

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Working- Speed of hammer 8000-15000 rpm Feed material is placed into the hopper, which flows vertically down and then horizontally, hammers are in continuous motion Rotating hammer beat the material to yield smaller particle. These particles pass through screen. Particle exit tangentially to yield smaller sized particles. Screens are interchangeable for varying size 29

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Fineness of the powder is regulated by altering Rotor speed Feed rate Clearance between hammer and grinding plates Number and types of hammer Aperture size of screen. Uses- Fine to moderate powder (10-400mm) Nonabrasive to moderately abrasive , brittle material can be used to crush Dry materials, wet filter press cakes, ointments, slurries 30

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Advantages- Easy to setup, install, dismantle and clean up Minimum scale up problem. Requires small place Different screens can be used to handle different types of feed Speed and screen can be changed rapidly. Operated in closed environment, dust can be reduced 31

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Disadvantages- Screens may get clogged Heat build-up is more, therefore product degradation is possible. Wearing of mill is more with abrasive material Not suitable for sticky, fibrous and hard material 32

Ball Mill/Pebble Mill:

Ball Mill/Pebble Mill 33

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Principle- Operates on the principle of impact between rapidly moving balls and powder material, both enclosed in a hollow cylinder. At low speed balls rollover each other and attrition (rubbing action) will be predominant. In ball mill attrition or impact or both are responsible for the size reduction. Construction- It consist of a hollow metallic cylinder mounted on a metallic frame. It rotates on its longitudinal axis. Length of cylinder is slightly greater than its diameter. 34

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Cylinder is lined with chrome, in pharma industry it is lined with rubber or porcelin . Cylinder contains balls which occupies 30-50% place of the mill Weight of the balls is constant Ball size depends upon size of the feed and diameter of the mill Balls are made with steel, iron or stoneware. Balls acts as grinding medium 35

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Working- Grinding material is filled into cylinder upto 60% capacity (material to void ratio). Fix numbers of balls are used. Mill is allowed to rotate at longitudinal axis Speed of balls is very important. At low speed balls rollover each other and attrition will be predominant action. This mode of attrition is used for wet grinding. 36

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At correct speed (critical speed), the centrifugal force just occurs, as a result balls are picked up by the mill wall and carried to the top, where they break the contact with wall and roll down to bottom. In this manner impact stress is induced and the size reduction becomes more effective. At higher speed balls will thrown out to the wall by CF. Hence grinding will not occur. This may cause compression but not sufficient for size reduction 37

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Uses: Fine grinding with particle size 5-100mm or less Ball mill at low speed is used for dyes, pigments, insecticide etc. Advantages: Produce very fine powder. It is used for batch operation. It can be made continuous operation by including a chamber next to cylinder. Suitable for both wet and dry grinding. 38

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This mill is closed system hence toxic substances can be ground. Also, sterility can be maintained Milling can be done in inert atmosphere Installation, operation and labour cost is low Various ball shape can be used 39 Milling of sticky material

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Disadvantages: Noisy machine Wearing from the balls and from the casing Ball milling is a slow process Soft tacky and fibrous material cannot be milled by ball mill. The rate at which energy is applied is limited, because it depends on acceleration of balls. 40

Fluid Energy Mill / Jet Mill / Micronizer / Ultrafine Grinder:

Fluid Energy Mill / Jet Mill / Micronizer / Ultrafine Grinder 41

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Principle: Operates on the principle of impact and attrition. In this mill feedstock is suspended within a high velocity air stream. Milling takes place because of high velocity collisions between the suspended particles. Construction: It consist of an Elliptical shape pipe of height about 2Meter and diameter ranging from 20-200mm. The mill surface may be made of SS or tough ceramics. 2-6 grinding nozzles are placed tangentially or opposed to the initial flow path of powder. 42

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Compressed air of 600 kilopascal to 1 megapascle is used. Inert gases can be used to minimize or eliminate the oxidation of susceptible material Venturi feeder is provided in the path of airflow. An outlet with a cyclone separator or bag filter is fitted to allow the escape of air. Working: Powder is introduced through the inlet of venturi . The air entering through the grinding nozzle transport the powder in elliptical or circular track of the mill. 43

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In the turbulent stream of air, the suspended particles colloid with each other and break. Thus impact and attrition force operates in size reduction Small particles entrapped by air are carried out to outlet and removed by cyclone separator or bag filter. The coarser particles undergo recirculation in the camber because of their own weight. These re-circulated particles colloid again with new incoming feed. 44

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The powder remains in the mill, untill its size is reduced sufficiently. Later it leaves via the eieve . Hence this mill produces fine particles with narrow size distribution Uses: For antibiotics and vitamins When better absorption is desirable , this mill is preferred. Ultrafine grinding can be achieved. Moderately hard material can be processed. 45

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Advantages: It has no moving parts, hence no heat generation. Suitable for heat labile material. ( sulphonamaides , antibiotics, vitamins) Rapid and efficient method for fine powder (30mm or less) No wear of the mill , hence no contamination Disadvantages: Not suitable for soft, tacky and fibrous material Equipment is expensive 46

Edge Runner Mill:

Edge Runner Mill 47

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Principle: Operates on the principle of compression (crushing) due to heavy weight of stones. Shearing force is also involved during the movements of stones. Construction: It consist of two heavy rollers (weigh several tone). Rollers move on a bed, which is made of stones or granite. Each roller has central shaft and revolve on its own axis. Rollers are mounted on a horizontal shaft 48

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Working: The material to be ground is placed on the bed. It is kept in the path of stone wheel using scraper. The stones revolve around their own axes and at the same time travel around the shallow bed stone. The outer part has to travel more than inner so that size reduction is achieved by shearing as well as crushing. The material is ground for sufficient period, powder is collected and passed through sieve. 49

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Uses: Grinding of tough material Used for plant based products Advantages: Does not require attention during operation Disadvantages Occupy more space, Time consuming, High energy consumption Not suitable for sticky material Contamination is possible with roller material 50

End Runner Mill:

End Runner Mill 51

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Principle: Size reduction is achieved by crushing (compression) using heavy weight steel pestle. Shearing stress is also involved during the movement of mortar and pestle Construction: It is a mechanical mortar and pestle, consist of steel mortar fixed to a flanged plate Underneath the flanged plate, a bevelled cog fitting is attached to horizontal shaft with pulley 52

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The pestle is dumb-bell shaped so that balancing and efficient grinding by its weight can be achieved. Bottom of the pestle is flat rather than round. Pestle carries an arm, which is hinged, this helps in emptying and cleaning. The narrow central portion of the pestle is longer than the band of the arm around it. 53

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Working: The material to be ground is placed in mortar. Scrapers put the material in the path of pestle. Mortar revolves at high speed. Pestle is placed in the mortar. Rotating mortar causes the pestle to revolve. Size reduction is achieved by shearing and crushing Material is collected and passed through sieve. 54

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Advantages: Suitable for fine grinding Disadvantages: Not suitable for drugs which are in unbroken or slightly broken condition 55

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Summary 56 Sr. No Mill Working principle 1 Hammer mill, impact 2 ball mill impact and attrition 3 fluid energy mill impact and attrition 4 Edge runner mill compression (crushing) 5 End runner mill compression (crushing)

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