4.2 Continuous Dryer. 4.2.1 Rotary Dryer. 4.2.2 Drum Dryer. 4.2.3 Flash Dryer. 4.2.4 Fluidised Bed Dryer. 4.2.5 Screen Conveyor Dryers.
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NPTEL Chemical Engineering Chemical Engineering Design – II Joint initiative of IITs and IISc Funde d by MHRD Page 1 of 39 Module # 4 D ESIGN OF DRYERS: INTRODUCTION, TYPES OF DRIERS, DESIGN CONSIDERATION OF DRIERS 1. INTRODUCTION 2. PHYSICAL MECHANISM OF DRYING 3. CLASSIFICATION OF DRYERS 4. DRYING EQUIPMENT 4.1 Batch Type Dryers 4.1.1 Tray Dryer 4.1.2 Pan Dryer 4.1.3 Agitated Vacuum Dryer 4.2 Continuous Dryer 4.2.1 Rotary Dryer 4.2.2 Drum Dryer 4.2.3 Flash Dryer 4.2.4 Fluidised Bed Dryer 4.2.5 Screen Conveyor Dryers 5. NOVEL DRYING TECHNOLOGIES 5.2 Microwave Drying 5.3 Supercritical Fluid Extraction and its application to Drying 6. S ELECTION OF D RYING E QUIPMENT 7. D ESIGN OF DRYER
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NPTEL Chemical Engineering Chemical Engineering Design – II Joint initiative of IITs and IISc Funde d by MHRD Page 2 of 39 Lecture 1 : INTRO DUCTION AND TYPES OF DRIERS 1. I NTRODUCTION The term drying refers generally to the removal of moisture from a substance. It is one of the oldest , most commonly used and most energy consuming unit operation in the process industries. Drying is often necessar y in various industrial operations particularly in chemical process industries to remove moisture from a wet solid, a solution or a gas to make it dry and choice of drying medium is depends on the chemical nature of the materials. Three basic methods of dr ying are used tod a y 1) sun drying, a traditional method in which materials dry naturally in the sun, 2) hot air drying in which matrials are exposed to a blast of hot air and 3) freeze drying, in which frozen materials are placed in a vacuum chamber to dra w out the water . The fundamental nautre of all drying porcess is the removal of volatile substances (mainly moisture) from mixture to yield a solid product. In general drying is accompli she d by thermal techniques and thus involves the application of heat, most commonly by conve c tion from current of air. Throughout the convective drying of solid materials, two processes occur simultaneously namely, transfer of energy from the local environemnt in the dryer and t ransfer of moisture from within the solid. Ther efore this unit operation may be considered as simultaneous heat and mass transfer operation. Drying processes and equipment may be categorised according to several criteria, incuding the nature of material and the method of heat supply and the method of o peration . For example In the sugar industry wash ed and centrifug ed sugar crystals are dried to get finisehd product for packing . Drying is an important operation in food processing. Milk is dried in a spray chamber to produce milk powder . A ll the above exa mples indicates that w et material loses moisture in direct contact with hot air/ gas. The hot air/ gas supplies the energy required for drying an d also carries away the moisture released by the solid. For heat sensitive materials much of the resistance to dr ying resides within the material. U nduly high heat and mass transfer rates applied at the surface only result in overheating or over drying of the surface layer resulting in quality problems without major increase in the drying kinetics. The rate of migrat ion of the moisture from within the solid to the evaporation front often controls the overall drying rate. Therefore, drying may be defined as an operation in which the liquid, generally water, present in a wet solid is removed by vaporization to get a rel atively liquid free solid product. Drying of a solid does not demand or ensure complete removal of the moi s ture. Sometimes it is desirable to retain a little mositure in the solid after drying. Dryer and drying process selection for a specific operation is a complex problem, and many factors have to be taken into account. Though, the overall selection and design of a drying system for a perticular material is dictated by the desire to achieve a favourable combination of a product quality and process
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NPTEL Chemical Engineering Chemical Engineering Design – II Joint initiative of IITs and IISc Funde d by MHRD Page 3 of 39 econom ics. In general, with respect to the rate and total drying time, dryer performance is dependent on the factors such as air characteristics, product cheracteristics, eqipment characteristics. But despite the many commercially available drying techniques at present most dehydrated products (i.e. fruits and vegetables) are still produced by the method of hot air drying. Because this is regarded as the simplest and most economical . There are other water/liquid removal processes such as filtration, settling, ce ntrifugation, supercritical extraction of water from gels etc. In all these operations liquid is r emoved by mechanical means but a considerable amount of liquid is still retained in the solid. This residual liquid can be removed by drying. One such example is the p roduction of condensed milk involves evaporation, but the production of milk powder involves drying. The phase change and production of a soild phase as end product are essential features of the drying process. Drying is an essential operation in c hemical, agricultural, biotechnology, food, polymer, pharmaceutical, pulp and paper, mineral processing, and wood processing industries. 2. P HYSICAL MECHANISM OF DRYING Drying does not mean only removal of the moisture but during the process, physical stru cture as well as the apperance has to be preserved. Drying is basically governed by the principles of transport of heat and mass. When a moist solid is heated to an appropriate tem perature, moisture vaporizes at or near the solid surface and the heat requi red for evaporating moisture from the drying product is supplied by the external drying medium, usually air or a hot gas . Drying is a diffusional process in which the transfer of moisture to the surrounding medium takes place by the evaporation of surface moisture, as soon as some of the surface moisture vaporizes, more moisture is tran s ported from interior of the solid to its surface. This transport of m oisture within a solid takes place by a variety of mechanisms depending upon the nature and type of the solid and its state of aggregation. Different types of solids may have to be handled for drying crystalline, granular, beads, powders, sheets, slabs, filter – cakes etc. The mechanism of moisture transport in different solids may be broadly classified into ( i) transport by liquid or vapour diffusion (ii) capillary section , and (iii) pressure induced transport. The mechanism that dominates depends on the nature of the solid, its pore structure and the rate of drying. D ifferent mechanisms may come into play and dominate at different stages of drying of the same material. The following term are commonly used in desig ning of drying system s . Moisture content of a substance which exerts as equilibrium vapour pressure less than of the pure liquid at the same tempera ture is refer ed to as bound moisture. . Moisture content of the solid which exters an equillibrium vapour pressure equal to that of pure liquid at the given temperature is the u nbound moisture .
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NPTEL Chemical Engineering Chemical Engineering Design – II Joint initiative of IITs and IISc Funde d by MHRD Page 4 of 39 The moisture content of solid in excess of the equilibrium mois ture content is refered as free moisture. During drying, only free moisture can be evporated. The free moisture content of a solid depends upon the vapour concentration in the gas. The moisture contents of solid when it is in equilibrium with given partia l pressure of vapour in gas phase is called as equilibrium moisture content . Similalry, the moisture content at which the constant rate drying peroid ends and the falling rate drying period starts is called critical moisture content . During the constant ra te drying period , the moisture evporated per unit time per unit area of drying surface remains constant and in falling rate drying period the amount of moisture evporated per unit time per unit area of drying surface continuously decreases. 3. C LASSIFICA TION OF DRYERS Drying equipment is classified in different ways , according to following design and operating features. It can be classified based on mode of operation such as batch or continuous , In case of batch dryer the material is loaded in the drying equipment and drying proceeds for a given period of time, whereas, in case of continuous mode the material is continuously added to the dryer and dried material continuously removed. In some cases vacuum may be used to reduce the drying temperature. Some d ryers can handle almost any kind of material, whereas others are severely limited in the style of feed they can accept. Drying processes can also be categorized according to the physical state of the feed such as wet solid, liquid, and slurry. Type of heat ing system i.e. conduction, convection, radiation is another way of categorizing the drying process. Heat may be supplied by direct contact with h ot air at atmospheric pressure, and the water vaporized is removed by the air flowing. Heat may also be suppl ied indirectly through the wall of the dryer from a hot gas flowing outside the wall or by radiation . Dryers expos ing the solids to a hot surface with which the solid is in contact are called adiabatic or direct dryers, while when heat is transferred from an external medium it is known as non – adiabatic or indirect dryers. Dryers heated by dielectric, radiant or microwave energy are also non adiabatic. Some units combine adiabatic and non adiabatic drying; they are known as direct – indirect dryers.
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NPTEL Chemical Engineering Chemical Engineering Design – II Joint initiative of IITs and IISc Funde d by MHRD Page 5 of 39 To redu ce heat losses most of the commercial dryers are insulated and hot air is recirculate d to save energy. Now m any designs have energy – saving devices, which recover heat from the exhaust air or automatically control the air humidity. Computer control of dryer s in sophisticated driers also results in important savings in energy. 4. D RYING EQUIPMENT 4 . 1 Batch Type Dryers 4.1.1 Tray Dryer Schematic of a typical batch dryer is shown in f igure 2.1 . Tray dryers usually operate in batch mode , us e racks to h old produ ct and circulate air over the m aterial. It consists of a rectangular chamber of sheet metal containing trucks that support racks . Each rack carries a number of trays that are loaded with the material to be dried. Hot air flows through the tunnel over the r acks. Sometimes fans are used to on the tunnel wall to blow hot air across the trays . Even b affles are used to distribut e the air uniformly over the stack of trays. Some moist air is continuously vented through ex hau st duct; makeup fresh air enters through the inlet . T he racks with the dried product are taken to a tray – dumping station. Figure 2. 1 : Tray dryer Inlet air Exhaust Hot side Temperatures Cold side Te mperatures
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NPTEL Chemical Engineering Chemical Engineering Design – II Joint initiative of IITs and IISc Funde d by MHRD Page 6 of 39 These types of dryers are useful when the production rate is small. They are used to dry wide range of materials , but have high labor requirement f or loading and unloading the materials, and are expensive to operate. They find most frequent application for drying valuable products . Drying operation in case of such dryers is slow and requires several hours to complete drying of one batch . With indir ect heating often t he dryers may be operated under vaccum. The trays may rest on hollow plates supplied with steam or hot water or may themselves contain spaces for a heating fluid . V apour from the solid may be removed by an ejector or vacuum pump . Freeze – drying involves the sublimation of water from ice under high vacuum at temperatures well below 0 o C. This is done in special vacuum dryers for drying heat – sensitive products. 4.1.2 Pan Dryer The atmospheric pan drier has a jacketed round pan in which a st irrer or mill revolves slowly, driven from below. The slow moving stirrer expose s fresh surfaces and thereby raises the rate of evaporation and, hence, of drying. T he pan drier is a batch machine and is limited to small batches. Pan driers may be used firs t to evaporate a solution to its crystallizing concentration and then can function as a crystallizer by sending cold water instead of steam into the jacket. The effect of the stirrer during crystallization prevent s the growth of large crystals and promote s formation of small, uniform crystals. The mother liquor is then drained off and the crystals dried in the same apparatus. 4 .1.3 Agitated Vacuum Dryer The agitated vacuum dryer is one of the most versatile in the range and is similar in principle to a p an dryer . The d r yer essentially consists of a jacketed cylindrical vessel arranged for hot water, steam or a suitable thermal fluid flow through the jacket for heating. Doors are provided on the shell, at the top for loading the feed material and at the bo ttom for discharging. The dryers are available in variety of sizes. The entire drying chamber is well machined to insure small clearance with the agitator blade . T hus ensur e s proper shuffling of the material and avoid s localized over heating. Due to the ag itation of the product in the a gitated vacuum d ryer the drying time is substantially reduced. A choice of the agitator design which can be arranged with or without heating depends on the material characteristics and process requirements. While designing th e shell one has to consider the ext ernal pressure and the shaft
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NPTEL Chemical Engineering Chemical Engineering Design – II Joint initiative of IITs and IISc Funde d by MHRD Page 8 of 39 the third quarter of the circle, producing a nearly uniform fall of the material throughout the cross section of the cylinder. The heated air streams through a rain of particles. This is the most c ommon form of revolving rotary cylinder. It has high capacity, is simple in operation, and is continuous . Table 2. 1 : Rotary dryers practical ranges of dimension and operating parameters Shell i.d. : D = 1 to 10 ft Length, L = 4 D to 15 D Radial flight height: D/12 to D/8; shell rpm: 4 to 5 Pripheral shell speed: 50 100 ft/min The flight count per circle: 2.4D to 3 D Inclination of the shell to the horizontal: up to 8cm/m Avg. solid retention time: 5 min to 2h Mass flow rate of the drying gas: 30 0 to 5000 lb/h.ft 2 Drying capacity: 0.4 to 2.5 lb moisture/(h) (ft 3 dryer volume) Number of heat transfer units in the dryer (NT): 1.5 to 2 Solid hold up m(i.e. fraction of the shell volume occupied by the solid at any time): 5 – 15% Courtesy: Principle of Mass Transfer and Separation Processes, B . K . D utta, 2007.
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NPTEL Chemical Engineering Chemical Engineering Design – II Joint initiative of IITs and IISc Funde d by MHRD Page 9 of 39 Lecture 2 : INTRODUCTION AND TYPES OF DRIERS (CONT.) 4.2.2 Drum Dryer I n drum dryers (Fig 2.3a, b) a liquid containing dissolved solids or slurry carrying suspended solids forms a thin layer on the outside surface of a large rotating drum. For a single drum unit thickness of the film can be controlled by an adjustable scraping blade . I n case of a doubl e drum unit thickness can be controlled by the gap between the drums (figure 2.3a) . A gas , normally air may be blown over the surface for rapid removal of moisture. The rotation of the drum adjusted so that all of the liquid is fully vaporized and a dried deposit can be scrapped off with the help of flexible or adjustable knife . This type of dryer mainly handles the materials that are too thick for a spray dryer and too thin for a rotary dryer. The solid collects on an apron in front of the knife and rolls to a container or to a screw conveyor. The operation of the drum drier is continuous. The drum is rotated continuously by a gear driven by a pinion that receives its motion through a belt, a chain, or a reduction gear from. The speed of the drum may be reg ulated by a variable – speed drive to ad o pt the speed to any slight variation in the feed quality . The speed of the drum regulated depend ing upon the nature of material s ( i.e wet or dry) , if the product material is wet/dry quite a distance before the knife i s reached, the speed should be decreased / increased. The design of the components is similar to that of drum filter. The knife may be held just against the surface . I t may be brought closer by turning the adjusting wheels. The knife supports may be turned t hrough part of a circle so that the angle of the blade of the knife relative to the drum surface may be selected for the greatest shearing effect. In recent years, double drum dryers have replaced single drum dryer in several applications (figure 2.3b) , du e to their more efficient operation, wide range of products and high production rates.
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NPTEL Chemical Engineering Chemical Engineering Design – II Joint initiative of IITs and IISc Funde d by MHRD Page 11 of 39 4.2.3 Flash Dryer The flash driers (figure 2.4) , also called pneumatic dryers, are similar in their oper ating principle to spray dryer . The material s that are to be dried ( i.e. solid or semisolid) are dispersed in finely divided form in a n upward flowing stream of heated air. The se types of dryer are mainly used for drying of heat sensitive or easily oxidiza ble materials. The wet materials that are to dried can be passed into a high – temperature air stream that carries it to a hammer mill or high – speed agitator where the exposed surface is increased. The drying rate is very high for these dryers (hence the ter m flash dryers ), but the solid temperature does not rise much because of the short residence time. A flash dryer is not suitable for particles which are large in size or heavy particles. The special advantage of this type of dryer is that no separate arran gement is required for transporting the dried product. The fine particles leave the mill through a small duct to maintain the carrying velocities (drying gas) and reach a cyclone separator. A solid particle takes few seconds to pass from the point of entry into the air stream to the collector. The inlet gas temperature is high and varies from 650 o C to 315 o C , for example, in 2 seconds, or from 650 o C to 175 o C in 4 seconds. T he thermal efficiency this type of dryer is generally low. A material having an initia l moisture content of 80 % may be reduced to 5 or 6 % in the dried product. Figure 2. 4 : Flash dryer
58 KB – 39 Pages