Assignment on dryers
- 1. GCE4241 ASSIGNMENT ON DRYERS Submitted By- S.A.M. SABBIR AHASAN Roll: 136025 Department of Glass & Ceramic Engineering, RUET Submitted To- Prof. Dr. Md. SHAMIMUR RAHMAN Dean, MechanicalFaculty, RUET Head, Department of Glass & Ceramic Engineering, RUET
- 2. Summary High quality ceramic products are finding application in multiple domains such as pottery, archeology, construction, design & decoration, foundries, electronics & electrical and etc. as the demand of ceramic products are increasing in the respective sectors, the pressure on producers to deliver the right amount of the ceramic or compound products has increased significantly. The ceramic manufacturing (#ceramicmanufacturing)) process involves majorly 4 steps: Step- 1) Selecting the raw material Step -2) Preparation of Raw material (i.e. clay of production) step-3) providing the desired shape and size to ceramic product (i.e. Molding the ceramics) step-4) Drying (to maintain the mold shape). The ceramics materials are sensitive to the surrounding atmosphere hence drying is very important for precisely designing the mold, determining the high-efficient heating rate and maintaining the desired level of drying temperature and duration (#importanceofdrying). With the time of science of ceramic material, production process and demand are changing and accordingly the required type and rate of drying. The quality ceramic production process requires drying during the preparation process such as grinding, mixing, granulation as first phase of drying, second phase of drying appears after the molding to remove the water/moisture level present to achieve the desired strength and maintain the mold form. The quality of ceramic is highly affected by the 1)rate of rise of temperature in dryer 2)irregular deformation and shrinking 3)irregular or none-uniform rate of drying 4)Creating of black spot or none dryable spot due to complex shape(#qualityceramicproduct). The drying processes are very important for the accurate design of molds and the achievement of highly quality majorly with the ceramic having complex shapes or large sizes in which the uniform density distribution is very difficult to achieve.The following topic will be discussed here: Dryers Classification of dryers Their operations and maintenance Sources of heat for drying drying schedule (Rate of drying) Defects at green stage- causes & remedies Heat and mass transfer in dryers Drying mechanism of non-porous and porous solids Shrinkage and case hardening, Through circulating and suspended bed drying Drying equipment- for solids pastes slurries and solution.
- 3. Contents The title page Summary Contents Introduction Dryers Classification of dryers Their operations and maintenance Sources of heat for drying drying schedule(Rate of drying) Defects at green stage- causes & remedies Heat and mass transfer in dryers Drying mechanism of non-porous solids Drying mechanism of porous solids Shrinkage and case hardening through circulating and suspended bed drying Drying equipment- for solids pastes slurries and solution Conclusion References
- 4. Introduction A dryer can be described in a number of ways, but the most fundamental concept is that a dryer is a machine to accomplish energy transfer. In the usual case of a dryer processing ceramic products, the energy transfer is primarily by convection processes where sensible enthalpy (heat) is transferred to the product creating latent enthalpy (heat) in the dryer atmosphere (as well as temperature increase ¡n the product in stage two of drying). In the case of microwave or radio frequency drying, energy in the form of radiation is transferred to the product, creating latent heat in the dryer atmosphere. Regardless of the source of energy, water vapor is added to the dryer atmosphere. if drying is to be accomplished, thereby increasing the latent heat content of the atmosphere. In considering dryers, the “system” of space includes the dryer and its environment (ambient conditions). In this view of the dryer and its environment, the process is not adiabatic, because heat can he transferred to the environment surrounding the dryer. Adiabatic exchanges between air segments within the dryer are not considered in this treatment. Regardless of the method of applying energy to the product being dried, there must be air movement within a dryer to move the mass of evaporated water away from the product. This means that any dryer must have the following essential components: • Hot supply: An entry point for air flowing into the dryer. In the case of a convection dryer, the hot supply obviously allows air at elevated temperature to enter the dryer, and that hot air is the energy source in drying. In the case of a dryer employing radiant energy, the supply air may or may not employ heated air. The convention is to call the source of air used in the dryer the “hot supply” regard less of the type of dryer. • Dryer exhaust: The exit duct for air from the dryer to be taken or discharged to ambient. This duct is called the dryer stack or smoke stack. In real or operating dryers, air leakage occurs, and this form of air exiting the dryer will be included in “dryer loss.” Dryer Dryer is a device which is used to remove moisture content from a moist body.It is a most common and important equipment in industrial and domestic sector.For example ovens,industrial dryers etc. Classification of dryers Drying equipments may be classified in several ways. The two most useful classifications are based on— 1. The methods of transferring heat to the wet solids 2. The methods of handling of the wet materials/solids handling/supplying feed into the dryer.
- 5. Methods of transferring heat: This method classifies driers as— A. Direct dryers: a. Batch type: i. Tray and compartment dryers ii. Fluidized bed dryer. b. Continuous type: i. Pneumatic conveying dryers ii. Rotary dryers iii. Spray dryers iv. Through circulation dryers v. Tunnel dryers B. Indirect dryers: a. Batch type: i. Agitated pan dryers ii. Freeze dryers iii. Vacuum -Rotary dryer - Tray dryer b. Continuous type: i. Cylinder dryers ii. Drum dryers iii. Steam tube rotary dryer C. Infra-red / Radiant /Dielectric heat dryer Operations of dryer Various types of dryers are listed today All of them have separate operation and maintenance principle.But the common principle of dryer is- Products or moist wares introduced into the dryer Starting and controlling of dryer Release of product after cooling Operations of dryer may be Manual (Sun or natural dryer) Man or machinized (Batch dryer) Fully Automated (Continuous dryer)
- 6. Maintenance of dryer Checking efficiency after a particular time period Checking burners or heating elements Checking refractory lining Checking of temperature controllers Cleaning Should not open the door at high temperature Wipe down of outside of dryer Sources of heat for drying Sources of heat may be the followings: Suralun (natural) Hot air (natural) Combustion of gaseous fuel (natural gas,H2 etc) Combustion of liquid fuel (diesel,petrol,kerosene etc) Resistive heat source (electric heating oil) Microwave heat source (infra-red heating) Drying schedule(Rate of drying) An illustration of typical drying schedule is shown in the following figure 0 1 2 3 4 5 6 warm up period constant rate period critical moisture content falling rate period equilibrium moisture content X-axis= time Y-axis= Concentrationof water
- 7. Warm up period First step of drying It is initial adjustment period Drying occurs at equilibrium temperature Constant rate period Moisture evaporates at constant rate Most of the moisture removed in this stage Critical moisture content It is the critical moisture content at which the constant rate period converts into the falling rate period Falling rate period Drying rate gradually decreases Decrement is liner or more complex function of time Depends on the moisture movement Equilibrium moisture content The lowest moisture content to which a material may be dryed with a given condition of air temperature and humidity. Defects at green stage- causes & remedies Problems with clay bodies can occur at any stage of pottery production. These clay body defects can usually, be avoided, however. For all of clay's versatility and incredible flexibility as a medium, it does need to be understood. Clay does make certain requirements of those who work with it, or problems will arise.
- 8. 01 of 07 Cracking and Warping While Drying Drying is actually potentially quite hard on clay objects. Common causes of cracking all center on the process of water being drawn out of the clay body. To avoid cracking during drying o keep the clay walls and floors as even as possible o avoid S-cracks by compressing pot floors during throwing o do not dry greenware quickly. Allow a minimum of a week on the shelf. Check the ware to make certain it is bone dry before loading into a kiln. o the thicker the ware, the longer the drying process should be o dry objects evenly and from all sides. This is especially important with tiles and plates. Part of the drying process includes your ware shrinking. For more on this, see Why Clay Bodies Shrink. 02 of 07 Cracking from Thermal Shock When ware is heated or cooled too quickly, cracks can form as the stresses of expansion or contraction overcome the strength of the bonds within the clay body. To avoid thermal shock o Do not heat or cool the kiln too quickly o Do not allow cool air into the kiln too early, such as opening the kiln lid while the ware is still too hot. o If you hear pinging from inside the kiln, the kiln is cooling too quickly. The pinging is the noise of cracking occurring. Close the lid or door, and shut any dampers. o Do not put fired ware (even ovenware) into an oven or microwave if the ware is damp, and o Do not put ware into an oven or microwave directly from the refrigerator or freezer.
- 9. 03 of 07 Dunting Dunting is the term used for cracks in the clay body that develop specifically due to rapid cooling at the end of the firing schedule. Dunting is most likely to occur in clay bodies that contain 25% or more silica. 04 of 07 Blackening, Bloating, and Carbon Coring These closely related problems happen during the ware's first firing. They are caused when the naturally-occurring carbon in the clay is not able to burn off properly. To avoid all of these defects, o raise the temperature slowlyduringthe bisquefiring,and o if you are firingina fuel-burningkiln,donotallow the kilntogo intoreduction.Provide plentyof oxygenforthe flame. 05 of 07 Blebbing Blebs are air pockets that are caught in the clay body when the ware was being shaped. During firing, the air swells up like a balloon under the surface of the clay. To avoid blebbing o wedge yourclaythoroughlybefore usingit,oruse clay that hasbeenprocessedinade- airingpugmill,and o if blebbingisanongoingproblemwithaparticularclaybody,tryaddinggrog (or change clay bodies). Although blebs are a clay body fault, they also affect the glaze. Blebbing can cause glazes to pit, pinhole, and blister. 06 of 07 Deflocculation When you are shaping damp clay, very rarely you may find that the clay may suddenly become so apparently wet as to be unworkable. In extreme cases, a handful of apparently normal clay, when shaken, will liquefy.
- 10. This is deflocculation; the clay particles have become ionized and are literally repelling each other. What do you do with this rare problem? o If you are usinga claybodycontainingnephelinesyenite,use yourclaywithinthree monthsof mixingitwithwater o If you are havingtrouble witha claybodyhighin the feldspars,talc,orfrit,try adding 0.4% (by dryweight) Epsomsaltsasyou mix the clay. o If you are workingwithathrowingclaybodyhighinkaolin,tryadding0.5 - 1% (bydry weight) epsomEpsomasyoumix the clay. 07 of 07 Clay - Glaze Fit Problems occur when glazes and clays don't fit well with each other. Clay bodies with less than 10% silica may be hard to fit with glazes. If that is the case, consider raising the silica content. Heat and mass transfer in dryers Heat is transferred from heat source to green body by the following basic methods Conduction: It is a heat transfer process in solid.Heat is conducted by molecular vibration,heat energy transferred from higher temperature zone to lower temperature zone.Amount of heat conduction, Q = KA dT/dX Where, Q = amount of heat K= Thermal conductivity dT = Temperature gradient dX= thickness Convection: In this process heat is transferred through a liquid or fluid medium in dryer.The heat is convected to the green body through air medium Q = hA (Ta-Tb) Where, A=area h= Heat transfer co-efficient
- 11. Ta & Tb = Temperature Radiation: In this process heat is transferred from one region to another by electromagnetic radiation or wave without any medium Q ∞ T4 Or, Q = AƏT4 Where, A=area T=Absolute temperature Ə=5.6703 x 10-8 W/m-2/k-1 Mass transfer during drying Water removing process(Drying mechanism) Volatile mass removing: Volatile compounds remove due to heat without liquification they sublimed so change of mass occurs in the green body. Drying mechanism of non porous solids The term "drying" is a relative one, and simply means that there is a further reduction in the moisture content from some initial level provided by mechanical dewatering to some acceptable lower level. For example, a moisture content of 10-20% by volume would normally allow particles to flow freely, yet suppress dust formation. The necessity for drying may be to make a product suitable for sale (e.g. paint pigments), or for subsequent processing (e.g. in pyrometallurgical operations). When a solid dries, two fundamental and simultaneous processes occur: (1) heat is transferred to evaporate liquid; (2) mass is transferred as a liquid or vapor within the solid and as a vapor from the surface. These factors governing the rates of these processes determine the drying rate. Commercial drying operations may utilize heat transfer by convection, conduction, radiation, or a combination of these. Industrial dryers differ fundamentally by the methods of heat transfer employed. However, irrespective of the mode of heat transfer, heat must flow to the outer surface and then into the interior of the solid. When a solid is dried experimentally, data are usually obtained relating moisture content to time. Consider the drying of a non-porous, insoluble material such as sand in a tray. The surface of the sand is exposed to a drying medium such as hot dry air passing over the surface. Figure 1. shows a typical drying curve.
- 12. Figure 1. Typical Drying Curve. When a solid is dried experimentally, data are usually obtained relating moisture content to time. Consider the drying of a non-porous, insoluble material such as sand in a tray. The surface of the sand is exposed to a drying medium such as hot dry air passing over the surface. Figure 1. shows a typical drying curve. Immediately after contact between the wet solid and the drying medium, the solid temperature adjusts until it reaches a steady state. The solid temperature and the rate of drying may increase or decrease to reach the steady state condition (AB). At steady state, the temperature of the wet solid surface is the wet bulb temperature of the drying medium. Temperatures within the drying solid also tend to equal the wet bulb temperature of the gas, but the lag in movement of mass and heat result in some deviation. Once the stock temperatures reach the wet bulb temperature of the gas, they are quite stable and the drying rate also remains constant. This is the constant rate drying period (BC) which ends when the solid reaches the critical moisture content. Beyond this point the surface temperature rises and the drying rate falls off rapidly (CD). Not always distinguishable, there may be another change in drying rate(DXE1).If this occurs, it is referred to as the second falling rate period. The falling rate periods can take a far longer time than the constant rate period even though the moisture removal may be less. The drying rate approaches zero at some equilibrium moisture content (XE1) which is the lowest moisture content obtainable with the solid under the drying conditions used.
- 13. Drying mechanism of porous solids A porous medium: any medium in which are present a matrix and a void space In a porous medium the water (or any other fluid) flows through a very complex network of pores and capillaries.The latter form the void space of the medium. Darcy’s law of moisture movement in porous solid Darcy found a proportionality relation between discharge water per unit time (Q) and the increment of the hydraulic head which can be expressed as the following: Heat transfer,mass balance,boundary action also shows a variation in porous solid drying than normal non porous solid. Shrinkage and case hardening through circulating and suspended bed drying Drying shrinkage: Drying shrinkage may be defined as the contraction of solid body due to removing of moisture content Determination of drying shrinkage: To determine the drying shrinkage a bar is made A straight line is scratched on the green body L1 and the length is determined Then the body is dried.After cooling L2 is measured By using the following equation percentage of shrinkage can be measured, % of shrinkage = 𝐿1−𝐿2 𝐿1 𝑥 100%
- 14. Proper drying shrinkage improves strength, high amount of shrinkage causes crack,defect,wasping etc. Shrinkage mechanism: Drying occurs in two steps Constant rate Falling rate At constant rate maximum water removed so shrinkage occurs.At falling rate period limited amount of moisture is removed.In this stage does not occur any shrinkage or shows a very little shrinkage. Fig: Shrinkage of clay body & Drying rate moisture Case Hardening Case-hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal (called the "case") at the surface. For iron or steel with low carbon content, which has poor to no hardenability of its own, the case-hardening process involves infusing additional carbon or nitrogen into the surface layer. Case-hardening is usually done after the part has been formed into its final shape, but can also be done to increase the hardening element content of bars to be
- 15. used in a pattern welding or similar process. The term face hardening is also used to describe this technique, when discussing modern armour. Fluid bed dryer Fluid Bed Dryer is designed to introduce the hot air stream at the /base of the product container which is filled with the material. Induced draught is created by means of blower and fresh air is sucked into the unit. This hot air stream expands the material at certain velocity and creating turbulence in the product. The phenomenon is known as fluidisation and offer conditions which are almost for drying. Fluidisation produces full agitation of solid particles by hot air, heat transfer is extremely high and uniform. The product is dried fast without appreciable loss of heat. Filter bags prevent particle escaping from the dryer. SUITABILITY The conventional drying method of drying product in tray dryer is rather inadequate and troublesome. They occupy large floor pace, have very high drying time, are labour intensive and offer uneven drying. On contrast fluid Bed Dryer occupy comparatively lesser floor space, are very easy to operate and can dry material in least time as compared to tray dryer. In Fluid Bed Dryer, temperature distribution throughout the product is uniform and the heat transfer rate is very high. Due to the reduced drying time high production rates are achieved. As the product is in close contact with the drying air at low temperature, and also for short duration, the physical and chemical properties of the products are generally not effected and therefore the dryer can effectively be used for heat sensitive products. Due to the continuous movements of product during drying, lump formation, case hardening etc. are minimised. The Fluid Bed Dryer are most suitable for drying, granular, crystalline, coarse or similar material in pharmaceutical, fine chemical, fine chemicals, dyes, food, allied products. The Fluid Bed Dryer is not suitable for drying liquids or pasty materials. Drying equipment- for solids pastes slurries and solution For drying slurries or solution the following drying equipment’s are used Spray dryer Spin flash dryer Mesh belt dryer Pressure spray dryer Spray dryer A spray dryer mixes a heated gas with an atomized (sprayed) liquid stream within a vessel (drying chamber) to accomplish evaporation and produce a free flowing dry powder with a controlled average particle size.
- 16. Advantages • Short drying time, • Single step drying process Spin Flash Dryer Dryer while mechanical dewatering of feed slurry is significantly less expensive than thermal drying, this process results in a paste of filer cake that cannot be spray dried and can be difficult to handle in other types of dryers. The Spin Flash dryer, as one option available for continuous powder production from pastes and filter cakes with out the need of grinding. The Spin Flash dryer was developed and introduced in 1970 and is widely used to produce a uniform powder on a continuous basis from highly viscous fluids, filter press wet cake, thick/cohesive paste, sludge, and semi hard lumps-lumpy materials etc. This equipment is ideally suited for such type of applications. Features: Continuous operation and single step drying with disintegration of products. Uniformed product quality and particle size High thermal as well as drying efficiency Compared with conventional dryers, there is lower operating cost from spin flash drying system Effective dust collector ensures emission of clean exhaust process air. No pollution and environmentally friendly
- 17. Fig: Spin flash dryer Mesh belt dryer A layer 25 to 150 mm(1 to 6 in) thick of material to be dried is slowly carried on a traveling metal screen. • The chamber consists of a series of separate sections, each with its own fan and air heater
- 18. At the inlet end of the dryer, the air usually passes upward the screen and the solids; near the discharge end, where the material is dry and may be dusty, air is passed downward through the screen. • The air temperature and humidity may differ in the various sections, to give optimum conditions for drying at each point Advantage: It is quick method because of its large surface area, material is dried in few seconds. Pressure spray dryer It is similar as spray dryer but high pressure nozzle is used.For high pressure paste or semisolid or slurry easily atomized and drys quickly.
- 19. Conclusion Drying is an important and often necessary unit operation for many processes. It can determine the quality of the final product, establish its shelf life, or expose defectiveness and lead to defective products. It is also extremely energy intensive and time consuming, so a lot of attention should be paid to optimizing the drying step so as to make the total process competitive. Drying involves the removal of water or other solvents from the product, primarily from the surface, then from the cracks and pores. If done too quickly, the solvent leaves a weak structure behind and this could cause imperfections or fractures, as witnessed in the ceramic industry. If performed at very high of temperatures, it could degrade the product, a typical issue in the food and pharmaceutical industry. Also, drying can be a risky step as the removal of the solvent (when drying coatings and paints for instance) can create a volatile atmosphere. A dryer can be described in a number of ways, but the most fundamental concept is that a dryer is a machine to accomplish energy transfer. In the usual case of a dryer processing ceramic products, the energy transfer is primarily by convection processes where sensible enthalpy (heat) is transferred to the product creating latent enthalpy (heat) in the dryer atmosphere (as well as temperature increase ¡n the product in stage two of drying). In the case of microwave or radio frequency drying, energy in the form of radiation is transferred to the product, creating latent heat in the dryer atmosphere. Regardless of the source of energy, water vapor is added to the dryer atmosphere. if drying is to be accomplished, thereby increasing the latent heat content of the atmosphere. In considering dryers, the “system” of space includes the dryer and its environment (ambient conditions). In this view of the dryer and its environment, the process is not adiabatic, because heat can he transferred to the environment surrounding the dryer. Adiabatic exchanges between air segments within the dryer are not considered in this treatment. Regardless of the method of applying energy to the product being dried, there must be air movement within a dryer to move the mass of evaporated water away from the product.