mechanical unit operation cha 2EBUY PPt.pptx

transportation and storage of solids in bulks

1. Chapter Two 2. Transportation and storage of solids in bulks 2.1 Storage of solids When huge amount of particulate solids of coarse material like gravel, coal and sand are to be stored, the most economical method is to store in large piles. There are two types of storage Outdoor storage Confined storage Outdoor storage: Coarse solid are stored outside in large piles, when hundred or thousand of ton of material are involved. Out storage can be affected by environmental problem such as dusting or leaching of soluble material from the piles. Dusting is control by covering the piles Examples: Coal storage and Gravel storage

2. Cont….. Confined storage: Solids that are valuable and can be affected by the environment in out door piles are stored in Bin’s, Hopper and Silo’s. These are the most versatile containers or equipments used to store particulate solids. All these containers are loaded from the top by some kind of elevator and discharge is ordinarily from the bottom. In storage containers the pressures felt at the bottom is not equal to the weight of the particular mass. 1. Silo is a storage structure for storing bulk materials like storage of grain, coal, cement, food products etc. Silo is tall and relatively small in diameter which is used for bin storage. 2 . bin is typically much shorter than a silo, and is typically used for holding dry matter such as concrete or grain. Bins may be round or square. A bin is not so tall and usually fairly wide.

3. Cont…. 3. Hopper is small vessel with a sloping bottom for a temporary storage before feeding solids to a process. It is filled at the top and Discharge is from the base. Fig: mass flow in hoppers, bin and silo respectively

4. 2.2 Transportation of solids Conveyors and elevators are used to transport solids. An industrial plant typically contains more than one type of conveyor or elevator. There are two main conveying (transporting) mechanisms of particulate solids. These are: Mechanical conveying Pneumatic conveying a) Mechanical conveying Conveying is defined as the transportation of particulate solids from place to place. For ease of a process, mechanical conveyors are used for both short and long distance to transport at a rate of 6000ton/hr of material up to 20km.

5. Cont……… There are three types of mechanical conveying. Belt conveyor Bucket elevator Screw conveyor 1. Belt conveyor Belt conveyors are the most popular type of mechanical conveyor. Is the most important way of transportation of particulate solids among the mechanical conveying equipments. Mechanical conveyors use moving parts to transport material. It is used for conveying of waste from chemical plant. i.e conveying of fuel and coal

6. Cont……… Belt conveyors can be used for almost any material. They come in many sizes and can be inclined. Typically they are connected in series to form a long belt conveying system. The main component of a belt conveyor is a continuous belt. It can be made of polyester coated with rubber, plain or coated canvas, woven wire, or a steel ribbon. Belt conveyors are usually flat but in bulk solids handling they are typically troughed to prevent material from falling. Belt conveyors are driven at one end by a motor, not shown, that rotates the drum to move the belt. At the other end of the conveyor there is an idle drum. Belt conveyors are typically used for transporting, proportioning feeding, discharging, and metering bulk solid materials.

7. Cont…… Advantages Disadvantages Cannot transport anything at temperatures greater than 120 0C. Small scale experiments needed to determine operating parameters. Can be developed to convey almost any type of solid material. Easy to adapt to plant layout.

8. Cont…… 2 . Bucket elevator In which particles carried up ward in buckets attached to a continuously moving vertical belt which passes over a derived roller at the top end and is mainly used for lifting solids vertically upward. Bucket elevators are usually used when there is not enough space for a horizontal conveyor. Bucket elevators consist of buckets mounted on a single or double chain.

9. Cont…… The buckets are loaded at the bottom of the apparatus and dumped through a chute at a higher level. From the chute, the materials are usually loaded on a conveyor or stored in a silo.

10. Cont…… Advantages Can operate at speeds of about 1.5 m/s. Can handle large capacities of material in a short time. Disadvantages Buckets undergo wear and tear. No universally accepted design.

11. 3. Screw conveyor Screw conveyors are the simplest and cheapest for transfer of materials at rates of up to 40 ton/hr over distances of up to 65 ft. Screw conveyors are flexible; depending on their diameter, they can be curved to some extent. Screw conveyors consist of a special heat-treated and tempered carbon or stainless steel spiral that rotates with an ultra-high-molecular-weight polyethylene (UHMWPE) food-grade tube. The transport of material in a screw conveyor occurs by the turning of a helix screw in a trough casing. The particles should be smaller than the screw pitch. The screws can have constant, variable, or tapered pitch.

12. Cont…… Screws with round cross sections are flexible and strong, and screws with flat cross-sections are more suited for high velocity applications. The trough casing can be round, rectangular, or U-shaped.

13. Cont……. Advantages Closed tube prevents product contamination Inexpensive to purchase and operate Flexible conveyors can be routed around obstacles Product handled gently Product is usually very accurate and highly repeatable batches are possible Conveyor can be easily emptied at the end of a batch operation Disadvantages Particles must be smaller than the pitch Abrasive materials will most likely cause wear Small scale experiments needed to determine operating parameters Need to run full of products

14. b) Pneumatic conveying Are non-Mechanical Conveyors Is used to transport huge amount of particulate solid through a gas stream (air). Instead of using moving parts to convey material, gas flow is used to transport materials horizontally, vertically, or on an incline. The two types of pneumatic conveyors are dense phase and dilute phase. In dense phase pneumatic conveying the particle concentration is very high. Dilute phase pneumatic conveying has a small solid concentration.

15. Cont……… In dense phase pneumatic conveying, the material is first placed in a hopper. The hopper feeds the material into a dense phase transmitter, which mixes the feed with a gas, usually air. The material travels by plug flow to its destination. In dilute phase conveying, the material is fed into an airlock feeder. The solid material is suspended into air at a ratio of about 3 to 5 kilograms of feed to 1 kilogram of air. The material is then conveyed as a suspension along the pipe line. Throughout the entire process air penetrates through the feed stream. If chemical reactions with air are a concern, other gases can be used.

16. Cont….. Advantages No dust contamination Flow direction can be varied Low maintenance costs Can handle multiple products with one system Virtually no limitation in capacity, product type, distance, or routing Disadvantages Particles must be dry Possibility of product breakage Wear and tear on the pipes High cost compared to other systems High amounts of filtration required

17. Hydraulic Transport   A method of displacing solid materials by a stream of water. Hydraulic transport is used in moving earth or rock by hydro-mechanized means, in building earth structures (such as dams or dikes), in removing slag and ashes from large boiler rooms, in transporting mineral products, in removing waste products in ore concentration processes, and in transporting various materials (such as wood chips, paper pulp, and raw materials used in the manufacture of sugar and alcohol).

18. Cont…….. Hydraulic transport systems are either pressurized or non-pressurized. In non-pressurized systems the hydraulic mixture is moved on inclined troughs or chutes and pipelines that are only partially filled. In this case the pressure on the free surface of the hydraulic mixture is equal to the atmospheric pressure. In pressurized systems the hydraulic mixture in the pipelines is under an excess pressure. This excess pressure is provided by pumps (boring pumps, coal suction pumps, and the like). The pressure resulting from the difference in altitudes between the beginning and the end of a pipeline is sometimes sufficient for hydraulic transport. For instance, when transporting rock into a mine shaft in order to fill an excavated space.

19. Transportation of solids in cement factory

20. Chapter-three size Reduction operations What is the meaning of size reduction? Size refers to physical Dimension of an object. Reduction refers to decrement or the process of decreasing the size. Size reduction is the operation carried out for reducing the size of bigger particles into smaller one of desired size and shape with the help of external forces.

21. Objectives Of Size Reduction In the materials processing industry, size reduction is usually carried out in order to: Increase the surface area because, in most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase. Break a material into very small particles in order to separate the valuable amongst the two constituents. Achieve intimate mixing. To dispose solid wastes easily . To improve the handling characteristics. To mix solid particle more intimately.

22. Cont….. Advantages Content uniformity Uniform flow Effective drying Increases surface area or viscosity Uniform mixing and drying Improve rate of absorption, Smaller the particles greater is absorption. Improve dissolution rate. Disadvantages Drug degradation Contamination

23. Cont……. Factors Affecting Choice of equipments for size reduction Mode of operation: batch/continuous Capacity Size of feed and product Material properties Integration with other unit operation : drying, classification, mixing, transportation, storage

24. Cont…….. Mechanism of size reduction Impact —particle concussion by a single rigid force (hammer). Compression—particle disintegration by two rigid forces (nutcracker). Shear —produced when the particle is compressed between the edges of two hard surfaces moving tangentially. Attrition —arising from particles scraping against one another or against a rigid surface.

25. Cont….. Factors affecting size reduction Hardness : - It is a surface property of the material. It is frequently confused with a property named strength. Thus , it is possible for a material to be very hard, but if it is brittle also then size reduction may present no special problems. Material structure:- Some substances are homogeneous in character. Mineral substances may have lines of weakness along which the materials splits to form flake-like particles. Abrasiveness:- Abrasiveness is a property of hard materials (particularly those of mineral origin). It may limit the type of machinery that can be used. During the grinding of some very abrasive substances the final powder may be contaminated with more than 0.1 percent of metal worn from the grinding mill.

26. Cont….. Softening temperature: During size reduction process sometimes heat is generated which may cause some substances to soften, and the temperature at which this occurs can be important. Waxy substances, such as stearic acid, or drugs containing oils or fats are examples that may be affected. Some methods can be used to overcome this like cooling the mill, either by a water jacket or by passing a stream of air through the equipment. Moisture content: It is found that materials do not flow well if they contain between about 5 and 50 per cent of moisture. Under these conditions the material tends to cake together in the form of balls. In general, grinding can be carried out satisfactorily outside these limits.  Crushing strength: The power required for crushing is almost directly proportional to the crushing strength of the material.