DRYERS I. Introduction Drying is the removal of water, or other volatile liquids, by evaporation. Most solid materials require drying at some stage in their production. The choice of suitable drying equipment cannot be separated from the selection of the upstream equipment feeding the drying stage. The overriding consideration in the selection of drying equipment is the nature and concentration of feed. Drying is an energy-intensive process, and the removal of liquid by thermal drying will be more costly than by mechanical separation techniques. [1] When the feed is solids, it is important to present the material to the dryer in a form that will produce a bed of solids with an open, porous structure. For pastes and slurries, some form of pretreatment equipment will normally be needed, such as extrusion or granulation. Accordingly, the process of drying can be divided into two stages. During the first stage, in which the material surface is wet, the rate of evaporation is constant. This is known as constant rate period. In the next stage the surface being dry, the water must force itself to the surface by diffusion, which is slower than evaporation. This is therefore known as falling rate period. The entire process therefore involves heat transfer as well as material transfer. [2] II. Types of Dryers [1] [2] Figure 1. An Industrial Dryer Source: pharmabasics.com
Transcript
DRYERSI. Introduction
Drying is the removal of water, or other volatile liquids, by evaporation. Most solid materials require drying at some stage in their production. The choice of suitable drying equipment cannot be separated from the selection of the upstream equipment feeding the drying stage.
The overriding consideration in the selection of drying equipment is the nature and concentration of feed. Drying is an energy-intensive process, and the removal of liquid by thermal drying will be more costly than by mechanical separation techniques. [1]
When the feed is solids, it is important to present the material to the dryer in a form that will produce a bed of solids with an open, porous structure. For pastes and slurries, some form of pretreatment equipment will normally be needed, such as extrusion or granulation.
Accordingly, the process of drying can be divided into two stages. During the first stage, in which the material surface is wet, the rate of evaporation is constant. This is known as constant rate period. In the next stage the surface being dry, the water must force itself to the surface by diffusion, which is slower than evaporation. This is therefore known as falling rate period. The entire process therefore involves heat transfer as well as material transfer. [2]
II. Types of Dryers [1] [2]
Because of the very wide range of dryer designs available, classification is a virtually impossible task. The main factors to be considered when selecting a dryer are: (1) feed condition: solid, liquid, paste, powder, crystals (2) feed concentration, the initial liquid content (3) product specification: dryness required, physical form (4) throughput required (5) heat sensitivity of the product (6) nature of the vapour: toxicity, flammability and (7) nature of the solids: flammability (dust explosion hazard), toxicity.
A large number of dryers have been designed for each group of materials, with different arrangements and methods of heat transfer, as also the heating mediums. Dryers can therefore be classified according to the material, solids or liquids, according to the method of heat transfer; direct or indirect, or according to the heating medium; air, steam, hot water, etc. On the basis of the method of heat transfer, dryers can be
Figure 1. An Industrial DryerSource: pharmabasics.com
classified as (a) direct dryers (b) indirect dryers. In each category they can further be divided into batch and continuous types. As a general rule, production rates of 5000 kg per day (0.06 kg/s) are best handled by batch dryers and rates over 50,000 kg per day (0.06 kg/s) in a continuous dryer.
1. Batch Type Dryers
Batch dryers are normally used for small-scale production and where the drying cycle is likely to be long. It is much more versatile and it can often be used for different materials. The humidity may be controlled during the drying operations, and this is especially important in cases where the humidity has to be maintained at different levels for varying periods of time.
1.1. Tray Dryer
The simplest type of batch dryer is the tray dryer, which is essentially a cabinet of large compartment with a number of trays. These trays may either be fabricated from sheets or from screens. In these dryers, steam, gas or electrically heated air is used as a drying medium. The air is passed by means of a fan over a radiator or over finned tubes and then over the trays. A portion of the air is let out at the discharge, the remainder is reheated and recirculated. An amount of fresh air equivalent to the volume discharged is admitted at the fan.
The spacing of trays is such as to maintain low pressure losses about 7.5 cm. Tray areas are 0.3-1 m2 with a depth of material of 10-100 mm, depending on the particle size of the product. Air velocities of 1-10 m/s are used and, in order to conserve heat, 85-85 per cent of the air is recirculated. Even at these high values, the steam consumption may be 2.5-3.0 kg/kg moisture removed. The capacity of tray dryers depends on many factors including the nature of the materials, the loading and external conditions, although for dyestuffs an evaporative capacity of 0.03-0.3 kg/m2 ks (0.1-1 kg/m2 h) has been quoted with air at 300-360 K.
Batch vacuum shelf or tray dryers are generally used for materials which are excessively heat sensitive. They consist of a cast iron rectangular or steel cylindrical chamber, fitted with a vacuum-tight charging door. The door is either of the quick-acting type or is provided with several wing-nuts and swivel bolts.
The trays are made of mild steel, stainless steel, enameled iron or other special materials, and are fabricated from sheets of 3 mm to 6 mm thick. A shelf is fabricated from steel structural sections like angles or tees, on which about 10 to 20 trays may be supported. The shelf must be sufficiently rigid to avoid deflection of the framework due to
Figure 2 Schematic Diagram of a Typical Batch Tray Dryer
Source: www.nptel.ac.in
dead load of trays and the material. It is necessary to ensure that the trays remain flat under the load of the materials.
Table 1.1. Features of Tray DryersOPERATING CONDITIONS
Method of Operation ConductionEvaporation Rate 0.02-2.5 (lb/hr)/sq ft
Used for a wide range of materials Close control can be maintained over the
drying conditions and the product inventory
Suitable for drying valuable products
Have high labor requirements
1.2. Pan Dryer
This consists of a flat bottom shallow cylindrical pan with a steam jacket at the bottom and on the sides. An anchor shaped scraper type of agitator is used to move the wet material over the surface heated by the steam. The material is removed through an outlet at the bottom.
The scraper is rotated at speeds usually between 2 and 20 rpm with the intention of merely preventing the solids from forming a hard cake on the heating surface. When drying slurries and wet pastes, clearance between the scrapers and heated surfaces must be minimal to prevent a skin of dried material building up on these surfaces. The driving system must, therefore, be designed for heavy torque conditions.
The usual size of the pan is up to 3 meters in diameters, with capacities up to 5000 litres. The drive to the stirrer is either from the bottom or from the top of the pan with a bevel reduction gear, which in turn is driven by a V-belt and motor. The pan is designed for an external pressure of about 3 to 6 kg/cm2, depending on the pressure of steam. In addition, if a vacuum pan is to be designed the design pressure will be increased further by 1kg/cm2.
Table 1.2. Features of Pan DryersOPERATING CONDITIONS
Method of Operation Conduction
Figure 3 A Jacketed Pan Dryer(a) pan (b) anchor-agitator (c) cover (d) bevel gear (e)shaft
ADVANTAGES DISADVANTAGES The minimal agitator to wall clearance
keeps the wall free from product crust. The pan bottom can be easily lowered
for fast cleaning and inspection.
1.3. Rotary Vacuum Dryer
This is similar in principle to a pan dryer, where a jacket is used for the heating medium and an agitator is used for moving the wet material over the heated surface. The unit consists of a horizontal jacketed cylindrical shell, closed at the ends by suitable heads. The dryer is built in variety of sizes, ranging from about 75 cm diameter by 4 m long to about 1.6 m diameter by 12 m long. A central shaft is supported in bearings outside the shell. It is sealed by stuffing boxes against leakage through the holes in the heads.
Table 1.3. Features of Rotary Vacuum DryersOPERATING CONDITIONS
Method of Operation ConductionEvaporation Rate 6.1-16.4 (lb/hr)/sq ft
Thermal Efficiency 45 %ADVANTAGES DISADVANTAGES
Versatile in process applications High energy efficiency Low operating costs compared to other
types of dryers Large heat transfer area available on
paddles and shaft give maximum heat transfer rates
Formation of the temperature difference between the internal and surface of material
1.4. Tumbler Dryer
Figure 4 A Rotary Vacuum Dryer with StirrerSource: yamato-sanko.co.jp
This type of dryer is, to some extent, replacing the cylindrical rotary vacuum dryer. It has two opposing jacketed cones on a common jacketed short cylindrical base. When the cones are in a vertical position the dryer can be rapidly discharged. The unit is provided with supporting trunnions running in suitable bearings. The trunnions are hollow. A vacuum connection is made through one of the trunnions. The connection pipe is turned upwards in the cone and is fitted with a dust filter at its end. Inlet and outlet pipes pass through the other trunnion for supply of a suitable heating medium to the jacket. A special design of a rotating valve is used for this purpose. One of the trunnions is driven through a reduction gear and chain drive,
Rotational speeds will range from 12 rpm for small units to 3 to 4 rpm for large commercial installations. The horse powers vary between ½ and 15. The base cylindrical diameters are about 1 m to 3 m.
Table 1.4. Features of Tumbler DryersOPERATING CONDITIONS
Method of Operation ConvectionEvaporation Rate 5.6 (lb/hr)/cu ft
In this type of dryer, the drying gas is passed through the bed of solids at a velocity sufficient to keep the bed in a fluidized state; which promotes high heat transfer and drying rates.
Fluidised bed dryers are suitable for granular and crystalline materials within the particle size range 1 to 3 mm. They are designed for continuous and batch operation.
Dryers with grid areas up to 14 m2
have been built and evaporative capacities vary from 0.02 kg/s m2 grid area for the low-temperature drying of food grains to 0.3 kg/s m2 for the drying of pulverized coal by direct contact with flue gases. Specific air rates are
Figure 5 A Tumbler DryerSource: sspindia.com
Figure 6 A Fluidised Bed DryerSource: nptel.ac.in
usually 0.5-2.0 kg/s m2 grid area and the total energy demand is 2.5-7.5 MJ/kg moisture evaporated. The exit gas is nearly always saturated with vapor for all allowable fluidization velocities.
Table 1.5. Features of Fluidised Bed DryersOPERATING CONDITIONS
Method of Operation ConvectionEvaporation Rate 50-160 (lb/hr)/cu ft
Rapid and uniform heat transfer Short drying times Good control of the drying conditions Low floor area requirements
Power requirements are high compared with other types
2. Continuous Dryers
2.1. Band Dryer
With the trays on trucks, the dryer can be made continuous by passing the trays with the wet material continuously through a drying chamber. Trays may be placed on a conveyor instead of using trucks. In some cases the material may be placed or attached directly to a conveyor belt. The conveyor belt is made of perforated metal plate or woven wire.
A section of the dryer is used for locating the heater which may be usually of steam heated finned tubes. A fan which is placed above the heater pulls the air through the heater and Figure 7 A Band Dryer
Source: tradeget.com
circulates it through the wet material. Recirculation and reheating may be automatically controlled.
Conveyor widths vary between 40 cm and 250 cm. Lengths range up to 50 m, so that the material can be retained in the chamber for sufficient time. The entire chamber is fabricated out of structural steel sections with steel sheets welded to it. Doors are provided at either end with the necessary ports. The chamber is properly insulated.
Table 1.6. Features of Band Dryers
OPERATING CONDITIONSMethod of Operation Conduction
Evaporation Rate -Thermal Efficiency 46-58 %
ADVANTAGES DISADVANTAGES Uniform drying due to movement of
product Effective use of drying air circulation fans Can easily be controlled belt speed and
drying time Very versatile and can handle a wide
range of materials
Bed of wet material can be permeable Importance to distribute carefully since
there is no opportunity to rearrange it
2.2. Rotary Dryer
It consists of a long cylindrical shell mounted horizontally with a slight slope. The shell is either rotated or may be kept stationary. If it is stationary, an agitator is made to revolve within the shell at a slow speed. The wet material is fed at the upper end, and moves gradually towards the lower end due to the rotation of the cylinder or movement of the agitator. Warm air or a stream of hot gas travels countercurrent to the material. The rate of feed, the speed of rotation or agitation, the volume of the heated air or gases and their temperatures, are so regulated that the material is completely dried before it is discharged at the lower end.
The simplest shelves are longitudinal baffles, plain or serrated, of about 5 cm to 10 cm width on the periphery of the cylinder. The retention time of the material in the
Figure 8 The Rotary Dryer and Its PartsSource: Alibaba.com
dryer will be determined by several factors such as the slope of the dryer shell, its speed of rotation and the length, the arrangement of flights, etc. The retention time at a given speed of rotation is inversely proportional to the slope of the shell. The rotational speed in rpm x dryer diameter in m lies between 75 to 105. The slope is 18 mm to 54 mm.
The rotating shell diameter varies between 0.3 and 3 m, while the length of the shell may range between four and ten times the diameter. The shell is generally 6 to 8 mm thick, and is made as one piece. It may be fabricated from mild steel, stainless steel clad or lined.
Table 1.7. Features of Rotary DryerOPERATING CONDITIONS
Method of Operation Conduction (indirect) /Convection (direct)Evaporation Rate 6.1-16.4 (lb/hr)/sq ft
Thermal Efficiency 85 %ADVANTAGES DISADVANTAGES
Covers smaller area Convenient to be transported and
installed Moisture content is greater than the
specified value Effects of operating parameter changes
predictable
Maintenance is inconvenient Kiln/rolling action difficult to quantify Sensitive to load and gas velocity
variations
2.3. Film Drum Dryers
These dryers are operated under atmospheric conditions or under vacuum. The feedstock is supplied continuously to the effective drying surface of the drums and the dried product is removed by scraper knife. The material is fed by various arrangements. Pasty materials are fed by feed rollers from top. Viscous materials are fed at the nip between drums. Slurries are normally fed by submerging the drum partially in a trough.
The drum, which may be ½ m to 2 m diameter and 1 m to 4 m length can be fabricated from a plate or cast to the required shape. The materials used are cast iron, bronze, chromium plated steel or stainless steel. The drum is heated internally by steam and has, therefore, to withstand internal pressure.
Figure 9 Schematic Diagram of a Film Drum DryerSource: nptel.ac.in
Drums are rotated at speeds in range of 3 to 20 rpm. As the drum rotates a film of about 1 to 3 mm thickness is formed, which is scraped by doctor knives. Three types are generally used, stationary single-bladed, oscillating single-bladed and multiple adjustable abutting or overlapping bladed. The knife is silicon-carbon steel with fairly small thickness.
Table 1.8. Features of Film Drum Dryers
OPERATING CONDITIONSMethod of Operation Conduction
ADVANTAGES DISADVANTAGES Takes less time to dry Occupies less space Rapid drying takes place due to rapid
heat and mass transfer Can be enclosed in vacuum chamber to
reduce the drying temperature
High maintenance cost Skilled operators are essential to
thickness control of film Not suitable for less solubility products
III. Selection Criteria Based on Operating Parameters [3]
With such understanding on the types of dryers, one would have a good idea of which type of dryer to use in given applications. The dryer selection criteria based on major operating parameters are highlighted in this section.
1. Scale of Production
Figure 10. Classification of Dryers Based on the Scale of Production
Figure 11. Classification of Dryers Based on the Physical Form of Feed
2. Physical Form of Feed
IV. Design of Rotary Dryers
Design of a rotary dryer only on the basis of fundamental principle is very difficult. Few of correlations that are available for design may not prove to be satisfactory for many systems. The design of a rotary dryer is better done by using pilot plant test data and the full scale operating data of dryer of similar type if available, together with the available design equations. A fairly large number of variables are involved such as solid
to be dried per hour, the inlet and exit moisture contents of the solid, the critical and equilibrium moisture contents, temperature and humidity of the drying gas. The design procedure based on the basic principles and available correlations is discussed below. In this case we assume that the solid has only unbound moisture and as shown in Figure 12 in stage II the solid is at the wet bulb temperature of the gas.
Figure 12. Temperature profile for solid and gas in a counter current rotary dryer
1. Rules of Thumb [4]
Rotary cylindrical dryers operate with superficial air velocities of 5-10ft/.sec, sometimes up to 35 ft/sec when the material is coarse.
Residence times are 5-90 min. Holdup of solid is 7-8%. An 85% free cross section is taken for design purposes. In countercurrent flown, the exit gas is 10-20°C above the solid; in parallel flow,
the temperature of the exit solid is 100°C. Rotation speeds of about 4 rpm are used, but the product of rpm and diameter in
feet is typically between 15 and 25.
2. Design Proper
DESIGN DESCRIPTION
Rotary dryers have the feed materials pass through a rotating cylinder together with a stream of hot gas. Internal lifters or flights elevate the feed and drop it in a curtain from the top to the bottom cascading along the length of the dryer. Material moves from one end of the dryer to the other by the motion of the material falling due to the angle of inclination of the drum.
DESIGN SELECTION
A cocurrent direct-heat rotary dryer will be used to dry the raw material. This will be used because it is suited for relatively free-flowing and granular materials. Also, it is suited for low and medium temperature operations. It is relatively low capital cost and labor cost. Cocurrent dryers are more suitable for material that must be dried to very low moisture contents or where the last traces of moisture are difficult to remove.
PARTS
PARTS FUNCTION(S)Dryer Shell It can be made from a variety of materials, including
carbon steel, or special stainless steel alloys.Material lifters It is used to “pick up” material, carry it over, and shower it
through the stream of gas. It also helps maximize efficiency of heat transfer between the material and the gas.
Chain Drive Assembly It includes chain and sprocket, reducer, and motor. This is the motor behind the actual rotation of the drum. A gear and pinion setup could also be used here in place of the chain and sprocket. A reducer takes down the speed of the motor for higher torque applications.
Wet Feed It is where feedstock is fed into the system, typically by a feed screw or chute.
Air Seal It is where the entering steam meets the drum.Support Roller It supports the weight of the drum which made out of
steel.Riding Ring It adds structural support for the drum, and a place for
pressure to be absorbed. The riding ring rides on the support roller.
Thrust Roller It pushes on the riding ring to stop the drum from drifting, or moving horizontally.
Exhaust Gas It is where spent gases and hot air (and small particulates) exit the system. Typically goes to a scrubber or bag house. Exhaust gas almost always needs to go through some sort of ventilation system before it is expended into the atmosphere, in order to clean the exhaust air and remove anything hazardous from it.
Product Discharge It is where product exits the system.
DESIGN CONSIDERATION
1. Rotary dryers usually operate with 10 to 15 percent of their volume filled with material (Perry and Green, Perry’s Chemical Engineers’ Handbook, 7th ed., sec 12-55).
2. The mass velocity of the gas is in the range of 2000 to 25000 kg/m2-h (400 to 5000 lb/ft2-h) (Harriot, McCabe & Smith, Unit Operations in Chemical Engineering, 5th ed., pp. 795).
3. Rotary dryers are operated most economically when the number of heat transfer units is between 1.5 and 2.5 (Harriot, McCabe & Smith, Unit Operations in Chemical Engineering, 5th ed., pp. 796).
4. Air-mass velocities in rotary dryers usually range from 0.5 to 5.0 kg/m2s. An air rate of 1.4 kg/m2s can usually be safely used (Perry and Green, Perry’s Chemical Engineers’ Handbook, 7th ed., sec 12-55).
5. Dryer diameters range from 1 to 3 m (3 to 10 ft) (Harriot, McCabe & Smith, Unit Operations in Chemical Engineering, 5th ed., pp. 796).
6. The L/D (length-diameter) ratio found most efficient in commercial practice lies between 4 and 10. Slopes of rotary-dryer shells vary from 0 to 8cm/m. (Perry and Green, Perry’s Chemical Engineers’ Handbook, 7th ed., sec 12-54).
7. The flights are usually offset every 0.6 to 2 m to ensure more continuous and uniform curtains of solids in the gas (Perry and Green, Perry’s Chemical Engineers’ Handbook, 7th ed., sec 12-53).
8. The radial flight heights in a direct dryer will range from one-twelfth to one-eighth of the dryer diameter (Perry and Green, Perry’s Chemical Engineers’ Handbook, 7th ed., sec 12-56).
9. A value of 30 product rpm and diameter with a typical range between 25 and 35 (Walas, Chemical Process Equipment, pp.247).
DESIGN REQUIREMENT1. Flow rate of heating air 2. Outlet humidity 3. Dryer geometry
A. Dryer Cross-sectional area
B. Dryer Diameter C. Dryer Length D. Dryer Volume
4. Flight geometryA. Flight arrangementB. Number of flightsC. Height of flights
5. Rotational speed6. Residence Time7. Power Requirement
DESIGN CALCULATION
1. FLOW RATE OF HEATING AIRThe flow rate of heating air is found using Equation 24.2 (Harriot, McCabe & Smith,
Unit Operations in Chemical Engineering, 5th ed., pp. 772),
qT=mg(1+H b)csb(Thb−T ha)where:H b = humidity of gas at inlet
csb = humid heat of gas at inlet humidity
mg = mass rate of dry gas
The humid heat can be found using Figure 23.2 (Harriot, McCabe & Smith, Unit Operations
in Chemical Engineering, 5th ed., pp. 744), csb= 0.25 Btu/lb-°F. Then, substitute all the
known values in Equation 24.2 to compute for the flow rate of heating air,
mg=qT
(1+H b)c sb(T hb−T ha)
2. OUTLET HUMIDITY
In computing for the outlet humidity, we have to determine first the average rate of
mass transfer, mv. It can be calculated using Equation 24.9 (Harriot, McCabe & Smith, Unit
Operations in Chemical Engineering, 5th ed., pp. 774),
mv=ms(X A−X B)where:ms= mass rate flow of bone dry solids
Xa = initial moisture contentXb = final moisture contentmv = average rate of mass transfer
The outlet humidity, Ha, is determined using Equation 24.10 (Harriot, McCabe & Smith, Unit Operations in Chemical Engineering, 5th ed., pp. 774),
H a=H b+mv
mg
3. DRYER GEOMETRYA. DRYER CROSS-SECTIONAL AREASince we have the flow rate of heating air and mass velocity of gas, we can now
compute for the cross-sectional area,
A=m g
G
B. DRYER DIAMETERThe dryer diameter can be found by using the cross-sectional area which is
computed as,
D=( 4 Aπ )
0.5
C. DRYER LENGTHThe dryer length can be computed by using Equation 24.29 (Harriot, McCabe &
Smith, Unit Operations in Chemical Engineering, 5th ed., pp. 796),
qT=0.125πDLG0.67 ∆T
where:D= dryer diameter, ftL = dryer length, ftG = mass velocity of the gas of dryer cross-section, lb/h-ft2
∆ T = average temperature difference, taken as logarithmic mean of wet-bulb depressions at inlet and outlet of dryer
However, we have to determine first the value of ∆ T by using Equation 24.7 (Harriot, McCabe & Smith, Unit Operations in Chemical Engineering, 5th ed., pp. 773),
∆ T=(T hb−T wb)−(T ha−T wa)
ln [T hb−T wb
T ha−T wa ]Therefore,
L=qT
0.125 πDG0.67 ∆T
Note: L/D ratio should be in the range between 4 and 10 according to Perry and Green (1999).
D. DRYER VOLUMEWe can solve for the dryer volume using Equation 24.29 (Harriot, McCabe & Smith,
Unit Operations in Chemical Engineering, 5th ed., pp. 796),
qT=0.5G0.67
DV ΔT
Then, computing for the dryer volume,
V=DqT
0.5G0.67 ΔT
4. FLIGHT GEOMETRYA. FLIGHT ARRANGEMENTFlights attached to the shell lift up the material and shower it as a curtain through
which the gas flows. The shape of the flights depends upon the handling characteristics of the solids.
B. NUMBER OF FLIGHTS
Number of Flights=Dryer LengthDistance
C. HEIGHT OF FLIGHTS
Flight Height= 112
D
5. ROTATIONAL SPEEDThe rotational speed can be computed with the use of the product of rpm and
diameter value.
6. RESIDENCE TIMEThe time of passage in rotary dryers can be estimated by the relationships
developed by Friedman and Marshall (Perry and Green, Perry’s Chemical Engineers’ Handbook, 7th ed., sec 12-56) as given here:
θ= 0.23 L
S N0.9D+0.6 BLG
F
where:B = 5(Dp)-0.5 = a constant depending upon the material being handledDp = wt. average particle size of mannitol crystals, (microns)F = feed rate to dryer (lb dry material/(h×ft2 of dryer cross section) θ = residence time, minS = slope (ft/ft)N = speed (r/min) L = dryer length (ft)G = air-mass velocity (lb/h×ft2) D = dryer diameter (ft)
7. POWER REQUIREMENTBy using Equation 12-60 (Perry and Green, Perry’s Chemical Engineers’ Handbook,
7th ed., sec 12-60), we can compute for the total power required to drive a rotary dryer with lifters,
bhp=N (4.75dw+0.1925DW+0.33W )
100,000
where:bhp = brake horsepower required (1 bhp = 0.75 kW)N = rotational speed, r/minW = total rotating load (equipment plus material), lbw = live load (material), lbD = riding-ring diameter, ft (D = d+2)d = shell diameter, ft
