Lecture 22.pptx
- 1. Separation Processes-I (ChE-206) Lecture No. 22 Drying
- 2. Drying • Drying is the removal of moisture (either water or other volatile compounds) from solids, solutions, slurries, and pastes to give solid products. • In the feed to a dryer, moisture may be: • embedded in a wet solid • a liquid on a solid surface • or a solution in which a solid is dissolved • The term drying also describes: • A gas mixture in which a condensable vapor is removed from a non-condensable gas by cooling • The removal of moisture from a liquid or gas by sorption • In this topic of drying, we will deal with drying operations that produce solid products.
- 3. Some Applications • Drying is widely used to remove moisture from: • (1) crystalline particles of inorganic salts and organic compounds to produce a free-flowing product • (2) biological materials, including foods, to prevent spoilage and decay from microorganisms that cannot live without water • (3) pharmaceuticals • (4) detergents • (5) lumber, paper, and fiber products • (6) dyestuffs • (7) solid catalysts • (8) milk • (9) films and coatings • (10) products where high water content entails excessive transportation and distribution costs. • Not all drying processes have been successful; the beer industry, for decades, has been trying to market dehydrated beer with no success whatsoever.
- 4. Drying Mechanism • Because drying involves vaporization of moisture, heat must be transferred to the material being dried. • The common modes of heat transfer are: • (1) convection from a hot gas in contact with the material • (2) conduction from a hot, solid surface in contact with the material • (3) radiation from a hot gas or surface • (4) heat generation within the material by dielectric, radio frequency, or microwave heating. • These different modes can sometimes be used symbiotically, depending on whether the moisture to be removed is on the surface or inside the solid.
- 5. Industrial Example • The continuous production of 69,530 lb/day of MgSO47H2O crystalline solids containing 0.015 lb H2O/lb dry solid is an example of an industrial drying operation. • The feed to the dryer in Figure 18.1 consists of a filter cake from a rotary drum vacuum filter.
- 6. Categories of Wet Solids •Granular or crystalline solids that hold moisture in open pores between particles. •Inorganic materials (crushed rocks, sand, catalysts, titanium dioxide, zinc sulfate and sodium phosphate) •Can be dried very rapidly to lower moisture contents Wet solids First- Category •Fibrous, amorphous and gel-like materials that dissolve or trap moisture in fibers or very fine pores. •Organic solids (tree plant, vegetable, and animal materials such as wood, leather, soap, eggs, glues, cereals, starch, cotton and wool) •Drying to lower moisture content is possible only when using a gas of low humidity Wet Solids Second-Category
- 7. Equilibrium-moisture content of solids • A hypothetical equilibrium isotherm • Moisture content, X, is expressed as mass of moisture per 100 mass units of bone-dry solid. • This is the most common way to express moisture content and is equivalent to wt% moisture on a dry-solid basis. • This is analogous to expressions for humidity and is most convenient in drying calculations where the mass of bone-dry solid and dry gas remain constant while moisture is transferred from solid to gas. Less common is wt% moisture on a wet-solid basis, W. • The two moisture contents are related by the expression:
- 8. Equilibrium Isotherm • Equilibrium-moisture content, X , is plotted for a second-category solid for a given temperature and pressure, against relative humidity, HR.
- 9. Some Important terms • In some cases, humidity, H, is used with a limit of the saturation humidity, Hs. At HR = 100%, equilibrium-moisture content is called bound moisture, XB. • If the wet solid has a total moisture content, XT > XB, the excess, XT-XB, is unbound moisture. • At a relative humidity < 100%, the excess of XT over the equilibrium moisture content, i.e., XT – X* , is the free-moisture content.
- 10. • Free Moisture: The difference between the total water content of solid and the equilibrium water content. • Bound Moisture: Part of the moisture present in a wet solid to such an extent to prevent it from developing its full vapor pressure and from being easily removed. Such moisture is described as “bound moisture” and is more difficult to remove than unbound water. • Unbound Moisture: If a material contains more water than indicated by intersection with the 100% RH-line, the water in excess is called unbound moisture. It is easily lost by evaporation until the equilibrium moisture content of the solid is reached. • Total Moisture Content: Total amount of moisture in the solid: bound plus unbound water or free moisture plus equilibrium moisture content.
- 11. Removal of moisture • In the presence of a saturated gas, only unbound moisture can be removed during drying. For a partially saturated gas, only free moisture can be removed. • But if HR = 0, all solids, given enough time, may be dried to a bone-dry state. • Bone-dry Solid: A material or the product that contains no liquid contents.
- 12. Example of equilibrium-moisture isotherms • Experimental equilibrium-moisture isotherms at 25C and 1 atm for second-category materials • At low values of HR <10%, moisture is bound to the solid on its surfaces as an adsorbed monomolecular layer. • At intermediate values of HR = 20-60% multimolecular layers may build up on the monolayer. • At large values of of HR > 60% moisture is held in micropores.
- 13. Effect of Temperature • Temperature has a significant effect on equilibriummoisture content. • For cotton at 96–302F • At an HR of 20%, equilibrium moisture content decreases from 0.037 to 0.012 lb H2O/lb dry cotton.
- 14. Drying Periods • The decrease in average moisture content, X, as a function of time, t, for drying either category of solids in a direct-heat dryer was observed experimentally by Sherwood. • The final equilibrium-moisture content is X*. • Although both plots exhibit four drying periods, the periods are more distinct in the drying-rate curve. • A-B: From A to B, the wet solid is being preheated to an exposed-surface temperature equal to the wet-bulb gas temperature, while moisture is evaporated at an increasing rate. • B-C: Constant rate period • C: Critical moisture content • C-D: first falling rate drying period • D-E: second falling rate drying period
- 16. • From A to B, the wet solid is being preheated to an exposed-surface temperature equal to the wet-bulb gas temperature, while moisture is evaporated at an increasing rate. • At the end of the preheat period, if the wet solid is of the granular, first category, a cross section has the appearance of Figure 18.29a, where the exposed surface is still covered by a film of moisture. • A wet solid of the second category is covered on the exposed surface by free moisture. The drying rate now becomes constant during the period from B to C, which prevails as long as free moisture covers the exposed surface. • This surface moisture may be part of the original moisture that covered the surface, or it may be moisture brought to the surface by capillary action in the case of wet solids of the first category or by liquid diffusion in the case of wet solids of the second category. • In either case, the rate of drying is controlled by external mass and heat transfer between the exposed surface of the wet solid and the bulk gas. • Migration of moisture from the interior of the wet solid to the exposed surface is not a rate-affecting factor. • This period, the constant-rate drying period, terminates at point C, the critical moisture content.
- 17. • At C, the moisture just barely covers the exposed surface; and then until point D is reached the surface tends to a dry state because the rate of liquid travel by diffusion or capillary action to the exposed surface is not sufficiently fast. • In this period, the exposed-surface temperature remains at the wet-bulb temperature if heat conduction is adequate, but the wetted exposed area for mass transfer decreases. Consequently, the rate of drying decreases linearly with decreasing average moisture content. This is the first falling- rate drying period. • During the period from C to D, the liquid in the pores of wet solids of the first category begins to recede from the exposed surface. In the final period from D to E, evaporation occurs from liquid surfaces in the pores, where the wet-bulb temperature prevails. • However, the temperature of the exposed surface of the solid rises to approach the dry-bulb temperature of the gas. • During this period, the second falling-rate drying period, the rate of drying may be controlled by vapor diffusion for wet solids of the first category and by liquid diffusion for wet solids of the second category. The rate falls exponentially with decreasing moisture content.
- 19. Drying Equipment Drying Equipment Mode of Operation Batch (when production rate is less than 500 lb/h of dried solid) Continuous (when production rate is more than 2000 lb/h) Mode of Heat Transfer Direct-heat Dryers (convective or adiabatic) contact material with hot gas Indirect-heat Dryers (non- adiabatic) provide heat to material by conduction or radiation from a hot surface Degree to which material is Agitated Feed is stationary Agitation may be necessary, if the material is sticky. For example fluidized bed dryer
- 20. Batch Dryers • Equipment for drying batches includes: • (1) tray (also called cabinet, compartment, or shelf) dryers • (2) agitated dryers. • Together, these two types cover many of the modes of heat transfer and agitation discussed above.
- 21. Tray Dryers • The oldest and simplest batch dryer • Useful when low production rates of multiple products are involved and when drying times vary from hours to days. • The material to be dried is loaded to a depth of typically 0.5–4 inches in removable trays that may measure 30 x 30 x 3 inches and are stacked by a forklift on shelves about 3 inches apart in a cabinet. • If the wet solids are granular or shaped into briquettes, noodles, or pellets with appreciable voids, the tray bottom can be perforated so that heating gas can be passed down through the material (through-circulation) as shown in Figure 18.2b. • Otherwise, the tray bottom is solid and the hot gas is passed at velocities of 3–30 ft/s over the open tray surface (cross-circulation), as in Figure 18.2a.
- 23. Agitated Dryers • Indirect heat with agitation and, perhaps, under vacuum, is desirable for batch drying when any of the following conditions exist: • (1) material oxidizes or becomes explosive or dusty during drying • (2) moisture is valuable, toxic, flammable, or explosive • (3) material tends to agglomerate or set up if not agitated • (4) maximum product temperature is less than about 30°C. • Heat-transfer rates are controlled mostly by contact resistance at the inner wall of the jacketed vessel and by conduction into the material being dried. • A wide variety of heating fluids can be used, including hot liquids, steam, Dowtherm, hot air, combustion gases, and molten salt.
- 24. If material tends to agglomerate or set up if not agitated • Feed is a liquid, slurry, or paste • This dryer consists of a shallow (2–3-ft high), jacketed, flat- bottomed vessel, equipped with a paddle agitator that rotates at 2–20 rpm and scrapes the inner wall to help prevent cake buildup. • Capacity of up to 1,000 gallons • Heat-transfer surface from 15 to 300 ft2 • The material to be dried occupies about 2/3 of the vessel volume. • The degree of agitation can be varied during the drying cycle. • With a thin liquid feed, agitation may vary from very low initially to very high if a sticky paste forms, followed by moderate agitation when the granular solid product begins to form. • Several hours are required for drying • Vacuum may also be applied
- 25. Double-cone (also called tumbler) vacuum dryer • When any or all of the above four conditions apply, but only mild agitation is required. • V-shaped tumblers are also available • The conical shape facilitates discharge of dried product, but, except for the tumbling, no means is provided to prevent cake buildup on the inner walls. • Heat-transfer surface areas of 1 to 56 m2 . • Additional heat-transfer surface can be provided by internal tubes or plates. • Up to 70% of the volume can be occupied by feed.
- 26. Ribbon or paddle-agitated, horizontal-cylinder dryer • When any or all of the above four conditions are relevant • The cylinder is jacketed and stationary. • The ribbons or paddles provide agitation and scrape the inner walls to prevent solids buildup. • Dimensions range up to diameters of 6 ft and lengths up to 40 ft. • The agitator can be rotated from 4 to 140 rpm, resulting in overall heat-transfer coefficients of 5 to 35 Btu/h-ft2 - F. • Typically from 20 to 70% of the cylinder volume is filled with feed. • Drying times vary from 4 to 16 hours.
- 27. Continuous Dryers • A wide variety of industrial drying equipment for continuous operation is available. • The following descriptions cover most types, organized by the nature of the wet feed: • (1) granular, crystalline, and fibrous solids, cakes, extrusions, and pastes • (2) liquids and slurries • (3) sheets and films • Types include: • Tunnel dryers • Belt or band dryers • Turbo-tray tower dryers
- 28. Tunnel Dryers • The simplest, most widely applicable, and perhaps oldest continuous dryers • Suitable for any material that can be placed into trays and is not subject to dust formation. • The trays are stacked onto wheeled trucks, which are conveyed progressively in series through a tunnel where the material in the trays is contacted by cross-circulation of hot gases. • Hot gases can flow countercurrently, cocurrently, or in more complex flow configurations to the movement of the trucks. • As a truck of dried material is removed from the discharge end of the tunnel, a truck of wet material enters at the feed end. • The overall drying operation is not truly continuous because wet material must be loaded into the trays and dried material removed from the trays outside the tunnel, often with dump truck devices. • A typical tunnel might be 100 ft long and house 15 trucks.
- 30. Belt or Band Dryers • Carry the solids as a layer on a belt conveyor, with hot gases passing over the material. • The endless belt is constructed of hinged, slotted-metal plates, or, preferably a thin metal band, which is ideal for slurries, pastes, and sticky materials. • More common are screen or perforated-belt or band conveyor dryers.
- 31. Turbo-Tray Tower Dryers • When floor space is limited but headroom is available, the turbo-tray or rotating-shelf dryer is a good choice. • For rapid drying of free-flowing, non-dusting granular solids. • Annular shelves, mounted one above the other, are slowly rotated at up to 1 rpm by a central shaft. • Wet feed enters through the roof onto the top shelf as it rotates under the feed opening. • At the end of one revolution, a stationary wiper causes the material to fall through a radial slot onto the shelf below, where it is spread into a pile of uniform thickness by a stationary leveler. This action is repeated on each shelf until the dried material is discharged from the bottom of the unit. • Fans that provide cross-circulation of hot gases at velocities of 2 to 8 ft/s across the shelves. • The bottom shelves can be used as a solids-cooling zone. Because solids are showered through the hot gases and redistributed from shelf to shelf, drying time is less than for cross-circulation, stationary-tray dryers. • Typical turbo-tray dryers are from 2 to 20 m in height and 2 to 11 m in diameter, with shelf areas to 1,675 m2 .
- 32. Direct-Heat Rotary Dryers • For free-flowing granular, crystalline, and flaked solids of relatively small size, when breakage of solids can be tolerated, is the direct heat rotary dryer. • It consists of a rotating, cylindrical shell that is slightly inclined from the horizontal with a slope of less than 8 cm/m. Wet solids enter through a chute at the high end and dry solids discharge from the low end. • Hot gases (heated air, flue gas, or superheated steam) flow counter- currently to the solids, but co-current flow can be employed for temperature-sensitive solids. • Bulk solids occupy 8– 18% of the cylinder volume, with residence times from 5 minutes to 2 h.
- 34. Indirect-Heat, Steam-Tube Rotary Dryers • When materials are: • (1) free flowing and granular, crystalline, or flaked • (2) wet with water or organic solvents • (3) subject to undesirable breakage, dust formation, or contamination by air or flue gases • It consists of a rotating cylinder that houses two concentric rows of longitudinal finned or unfinned tubes that carry condensing steam and rotate with the cylinder. • Wet solids are fed into one end of the cylinder through a chute or by a screw conveyor. • A gentle solids-lifting action is provided by the tubes. • Dried product discharges from the other end
- 36. Fluidized Bed Dryers • Free-flowing, moist particles can be dried continuously with a residence time of a few minutes by contact with hot gases in a fluidized-bed dryer. • This dryer consists of a cylindrical or rectangular fluidizing chamber to which wet particles are fed from a bin through a star valve or by a screw conveyor, and fluidized by hot gases blown through a heater and into a plenum chamber below the bed, from where the particles pass into the fluidizing chamber through a distributor plat. • The hot gases pass up through the bed, transferring heat for evaporation of the moisture, and pass out the top of the fluidizing chamber and through demisters and cyclones for dust removal. • The solids are circulated by the action of the hot gases in the bed and by baffles, and sometimes mixers, but eventually pass out of the chamber through an overflow duct, which also serves to establish the height of the fluidized bed.
- 37. Advantages • Fluidized-bed dryers have become very popular in recent years because they: • (1) have no moving parts • (2) provide rapid heat and mass transfer between gas and particles • (3) provide intensive mixing of the particles, leading to uniform conditions throughout the bed • (4) provide ease of control • (5) can be designed for hazardous solids and a wide range of temperatures (up to 1200C), pressures (up to 100 psig), residence times, and atmospheres • (6) can operate on electricity, natural gas, fuel oil, thermal fluids, steam, hot air, or hot water • (7) can process very fine and/or low-density particles • (8) provide very efficient emissions control
- 38. Spray Dryers • When solutions, slurries, or pumpable pastes—containing more than 50 wt% moisture, at rates greater than 1,000 lb/h— are to be dried. • The drying chamber has a conical shaped bottom section with a top diameter that may be nearly equal to the chamber height. • Feed is pumped to the top center of the chamber, where it is dispersed into droplets or particles from 2 to 2,000 mm by any of three types of atomizers: (1) single-fluid pressure nozzles (2) pneumatic nozzles (3) centrifugal disks or spray wheels • Hot gas enters the chamber, causing moisture in the atomized feed to rapidly evaporate. • Gas flows co-currently to the solids, and dried solids and gas are either partially separated in the chamber, followed by removal of dust from the gas by a cyclone separator, or, as shown in Figure 18.13a, are sent together to a cyclone separator, bag filter, or other gas–solid separator. • The hot gas can be moved by a fan.
- 40. Book • Seader, J. D.; Henley, E. J.; Roper, D. K., Separation Process Principles: Chemical and Biochemical Operations. 3rd Ed.; John Wiley & Sons, Inc.: 2011. • Chapter 18: Drying of Solids