DESIGN OF SPRAY DRIER Process Equipment and Design – II Reference-Mass Transfer Operations by Alapati Suryanarayana Suryanaray ana (page 571-578) 0801073 4-RAHUL MA 08010734-RAHUL MAY YANK •08010736-SANGADE MAHESH SITARAM •08010737-SANGISHETTY TARUN •
PROBLE PRO BLEM M ST STA ATEM TEMENT ENT
Design of Spray Drier with the data given below:
Centrifuged Urea to be dried from 1.5 wt % to 0.3 wt %
Feed rate of solid
= 1200 kg/hr
Inlet heated air of (T DB)
= 1500 C
Outlet air Temperature emperat ure
= 120 0 C
Inlet temperature of solid
= 50 0 C
Outlet temperature of solid
= 650 C
Specific heat of solid
= 0.32 cal/gm/ 0C
& (TWB) = 500 C
SPRAY DRYER
Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas. This is the preferred method of drying of many thermally-sensitive materials such as foods and pharmaceuticals.. pharmaceuticals
It is highly suited for the continuous production of dry solids in either powder, powder, granulate or agglomerate form from liquid feedstocks as solutions, emulsions and pumpable suspensions. Therefore, spray drying is an ideal process where the end-product must comply to precise quality standards regarding particle size distribution, residual moisture content, bulk density den sity,, and
AT A TOMIZER
Atomization is used to obtain particles of high surface/weight ratio. It is acquired by passing the liquid through a nozzle. To disperse the liquid or slurry into a controlled drop size spray. Depending on the process needs drop sizes from 10 to 500 micron can be achieved with the appropriate choices. Types of atomizer
Single fluid Pressure Nozzle Atomizer Atomizer Two-Fluid Nozzle Atomizer Atomizer Centrifugal (Disk) Atomizer
Pressure Nozzle Atomizer
Pressure nozzle or single fluid nozzle use pressure energy for atomization, with feed pressures in the range of 200 to 600 psi. The pressure energy is converted into kinetic energy and the feed issues from the nozzle has a high- speed film which readily breaks. The pressure nozzles are suitable for feed with relatively low viscosity and free of lumps.
Two-Fluid Nozzle Atomizer
The available energy for atomization in two fluid atomizers is independent of liquid flow and pressure. The necessary (kinetic) energy for atomization is supplied by compressed air in the range 20 -100 psig. The atomization is created due to high frictional shearing forces between the liquid surface and the air having a high velocity even at sonic velocities and sometimes rotated to obtain maximum atomization. In pneumatic nozzle the feed can be mixed with compressed air either internally or externally. The external mixing types are used for abrasive feeds. Two Fluid Nozzle atomizer are convenient for low capacity application and popularly used for high quality ceramic powders: Alumina, Bentonite, Carbides, Clays, Ferrites, Steatites, Titanates and Zirconias.
Centrifugal (Disk) Atomizer
A spray is created by passing the fluid across or through a rotating wheel or disk. The energy required for atomization is supplied by the atomizer motor.
SPRAY CHAMBER
The atomized feed is introduced into a large chamber where droplets are dispersed into the stream of hot air. Drying takes place by contacting the droplets with hot air in a spray chamber. Evaporation of moisture from the droplets and formation of dry particles proceed under controlled temperature and airflow conditions. Powder is discharged continuously from the drying chamber. Evaporation takes place from the surface of the drops and solid material form an impervious shell at the surface which collapses inward due to internal pressure.
Mode of contact
The hot drying gas can be passed as a cocurrent or counter-current flow to the atomiser direction. The co-current flow enables the particles to have a lower residence time within the system and the particle separator (typically a cyclone device) operates op erates more efficiently. efficiently. The counter-current flow method enables a greater residence time of the particles in the chamber and usually is paired with a fluidised bed system.
1. feed storage 3. drying chamber 5. Cyclone 7. separator
2. pump 4. air heater 6. gas scrubber
ADVANT ADV ANTAGES AGES
Very short drying time, which permits drying of highly heat sensitive products.
Product particle size & density are in controllable limits
Relatively low operating cost.
Air is the heated drying media; however, however, if the liquid is a flammable solvent such as ethanol or the product is oxygen-sensitive then nitrogen is used.
Spray drying applications
Food: milk powder, coffee, coffee, tea, eggs, cereal, spices, flavorings Pharmaceutical: antibiotics, medical ingredients, additives Industrial: paint pigments, ceramic materials, catalyst supports
DESIGN ASPECTS
Atomizer : its type & design .
Flow rate of the heated inlet air.
Settling velocity. velocity.
Solid & Gas operating velocity.
Chamber Diameter.
Residence time.
Design of Conical bottom
HERE WE ARE CONSIDERING A CO-CURRENT CO-CURRENT SPRAY DRYER SINCE IT IS THE MOST COMMON ONE
AT A TOMIZER
Centrifugal-disk atomization is particularly advantageous for atomizing suspensions and pastes that erode and plug nozzles Type of Disk atomizer ( Vane, Kesner , Pin ) Ref. patent US20040139625
Diameter of the Disk Atomizer :: 5 – 35 cm .
Rotational speed :: 4,000 - 65,000 rpm
Peripheral speed :: 4,500 – 10,000 m/min
Mean particle size :: 70 – 100 microns
Atomizer design Atomiser selected is centrifugal disk atomiser Mean particle size assumed = 100 microns Feed rate (F) = (1+X )L = 1218 Kg/hr= 44.75 1 s lb/min Herring and Marshal chart(Ref. British Chem. Engg., Dec 1967,V 19 67,Vol ol 12,No. 12,P 1892) peripheral speed vs. mean particle diameter diamet er,, with feed rate as parameter On interpolation , we get peripheral speed = 570.5 ft/s
v
=10433.3 m/min
DISK SELECTION
Disk selected=B-1 Diameter of disk = 0.59 ft Vane height=0.406ft Vane length=1ft No of vanes=60 Rotational speed N= 570.5*60/(periphery of disc) =18467.36 rpm We take N=18500 rpm Ref. Alder and Marshall , CEP 1951 Dec ,p 606)
AT A TOMIZER ( contd. contd. )
Power consumption :
P = 1.02*10-8 * F * ( N * d ) 2 where F = Feed rate of solution in lb/min=44.75 lb/min N = Rotational speed in rpm=18500 rpm d= Radius of disk atomizer in ft=0.59 ft P=power in hp
P=14.744 hp
SPRAY CHAMBER Gs H1 T1
Gs H2 T2
Ls Hl1 t1
Ls Hl2 t2
Spray Chamber Gs = Heated Gas Flow Rate in kg/hr Ls = Solid Flow Rate in kg/hr X1= Moisture content in the entering solid ( kg moisture / kg solid ) X2= Moisture content in the exit solid ( kg moisture / kg solid ) Y1= Humidity of entering air ( kg moisture / kg air ) Y2= Humidity of outlet air ( kg moisture / kg air ) T1= Temperature emperat ure of inlet air
T2= Temperature of outlet air
t1= Temperature emperatu re of inlet solid solid
t2= Temperature of exit
EQUATIO EQUA TIONS( NS( SPR SPRA AY CHAMBER )
Mass Balance eq n : Ls X1 + Gs Y1 = Ls X2 + Gs Y2 ---------------(1)
Enthalpy Balance eq n : Ls HL1 + Gs H1 = Ls HL2 + Gs H2 HL1 = Enthalpy of entering solid in kJ/kg of dry solid
HL2 = Enthalpy of exit solid in kJ/kg of dry solid H1 = Enthalpy of entering air in kJ/kg of dry air H2 = Enthalpy of exit air in kJ/kg of dry air
-----------(2)
EQUATIO EQUA TIONS( NS( SPR SPRA AY CHAMBER )
HL1=( Cps + X1*Cpw)*t1
HL2=( Cps + X2*Cpw)*t2
H1=( Cpa + Y1*Cpv)*T1 + Y1*λ
H2=( Cpa + Y2*Cpv)*T2 + Y2*λ Cps = Specific heat of solid = 1.3395 kJ/kg/ 0C Cpw = Specific heat of water = 4.2 kJ/kg/ 0C Cpv = Specific heat of water vapor = 1.89 kJ/kg/ 0C Cpa = Specific heat of air at avg. temperature = 1.0312 kJ/kg/ 0C λ = Latent heat of vaporization= 2510.7 kJ/kg
CALC CA LCUL ULA ATE TED D DA DAT TA ( co cont ntd. d. )
X1 = 0.015
X2 = 0.003
Y1 = 0.01317(By psychometric chart at given DB & WB)humidity of inlet air
HL1 = 70.125 kJ/kg of dry solid
HL2 = 87.8865 kJ/kg of dry solid
H1 = 191.5097 kJ/kg of dry air
Simultaneously solving Mass balance and Enthalpy balance equations (1) and (2) we get ::
Y2 = 0.0206
Evaporation rate of water=(0.015-0.003)*120 water=(0.015-0.003)*1200=14.4 0=14.4 Kg/hr
Since chamber effciency is assumed to be 70% , therfore net evaporation rate of water is: 14.4/0.7 =20.57 kg water/hr
•
Moisture removed per kg of dry air=(0.02060.01317)=0.00743 kg water per per kg of dry air air
•
Gs= 20.57/0.00743 = 2768.5 Kg/hr
1
Y
Humid Humid Volum Volumee(V ) = 8315 * ( ) *( Ma Mw
T g 273.15 P
)
Ma = Molecular wt of air =29 g/mol Mw = Molecular wt of water 18 g/mol Tg = Temperature of gas in 0C P = Pressure in N/m2 Y = Humidity of air in kg of moisture/ kg of dry air
Humid Volume : Inlet ( Vin )
= 1.225 m3/kg dry air
Outlet ( Vout ) = 1.1493 m3/kg dry air Vavg =(Vin + Vout )/2= 1.1859 m3/kg dry air
Droplet Diameter assumed to be 100 microns.
Operating Velocity Calculations
The pressure within the tower is maintained slightly negative which eliminates the possibility of any dust getting into the atmosphere and making the area unfit for work. The operating velocity in the case of nondusting spray dryer is taken as two times the settling velocity of the drop.
SETTLING VELOCITY In Stoke’s law region i.e. for Reynold’s number less than 2
vs
Dp 2 ( p
f ) g
18
Dp = Drop Diameter in m
ρp = Density of droplet in kg/m kg/m 3 ρf = Density of air at avg temperature in kg/m 3 µ = Viscosity Viscosity of air at avg temperature temperature vs = Settling velocity g = Acceleration due to gravity
CALC CA LCUL ULA ATE TED D DA DAT TA ( co cont ntd. d. )
Density of urea
= 1323 kg/m3
Density of water = 1000 kg/m3
Density of solid particles = (1.5*1000 + 98.5*1323)/100 ρp
= 1318.2 kg/m3
Average temp=(150+120)/2=135 temp=(150+120)/2=1350C
Density of air at avg temperature ρf = 0.865 kg/m3
Viscosity of air at avg temperature temperat ure µ = 23.329*10 23.329*1 0-6 N/m
Settling velocity v s = 0.3078 m/s
CALC CA LCUL ULA ATE TED D DA DAT TA ( co cont ntd. d. ) Dpvs p
Reynolds Number Re
Thereby , STOKE’s Law is applicable and now :
=1.411 =1.411
<2
Operating Velocity ( v a ) = 2 * Settling velocity va = 2 * vs = 0.6155 m/s
COLUMN DIAMETER(SPRAY CHAMBER) Column Area Ac = (Gs * Vavg)/ va Gs= 2738.8 Kg/hr Vavg = 1.1859 m3/kg dry air va = 0.6155 m/s Ac = 1.46578 m 2
Column Diameter Dc
4 * Ac
Dc = 1.3661 m For safety purposes , assuming 15% safety Dc = 1.15* Dc = 1.57104 m
RESIDENCE TIME
td = 50√X1
(Ref. Brown et al Unit Operations, p 564) td = residence time in sec. X1=mass ratio of water to solids in feed=0.015
td = 6.12372 sec
p
w pX1D p 2 12 K f T t
θp = time required to evaporate moisture from droplet
Density of feed . ρp= Density λw = Latent heat of vaporization at normal boiling pt. = 2256.9 kJ/kg Kf = Thermal conductivity of air at avg temperature = 0.033925 Watt/m/K ΔTt = Temperature Temperature driving d riving force for ce
= Drying gas film temp. – Temp. of dry solid surface surfa ce
CALC CA LCUL ULA ATE TED D DA DAT TA ( co cont ntd. d. )
ΔTt = Tavg – (t1+ t2) / 2 = 77.5 θp
= 0.0142 sec
As θp is less than td , hence evaporation of the drops can take place and design is acceptable.
CHAMBER DIMENSIONS It is a cylindrical chamber with w ith a conical bottom.
Total Volume of chamber ( Vt) m3
= Gs * Vavg * td = 5.5239
Minimum height of cylindrical ( hmin)= vs * θp = 0.00437 m portion
Recommended height of(hcyl) = 0.6 * Dc = 0.9426 m cylindrical cylindrical portion
V co n e
V t
2 C cyl
D h
4
Volume of the conical portion of the chamber Vcone = 3.69668 m3 hcone
Dc
2
hcone = Height of the conical portion of the chamber hcone
tan (
3 4 V cone
= 5.7209 m
2
)
Radiu Ra diusof sofch cham amber ber C o n i c a l h ei g h t
Cone angle α= 15.6306 degree
Dc 2h cone
MECHANICAL DESIGN
THICKNESS OF CHAMBER
tmin = tmin= (Dc+100)/1000 = 0.1628 inch
Dc is in inches & t min is in inches
As the next next available thickness thickness is 0.25 inch inch mild steel.
Since head and wall are under same stress , so we can assume the same thickness for both . Ad = 8.3386 * 10-4 m2
DUST COLLECTOR
Outlet area of the dust collector
Ad
Ls
p * vs
Diameter of dust collector
Dd = 32.58 mm = 1.3032 inch ( as this is small , 5 inch nominal diameter is used ) Dd
Length of dust collector Ld = D c /8= 0.19698 0.19698 m
4 * Ad
HOT AIR INLET
Assume operating air velocity v air = 10 m/s
Minimum cross sectional area = (Gs * Vavg)/ vair 2 0.07607 m =
Contamination of outlet air with fine particles requires a higher cross sectional area for outlet duct than that of inlet duct.
CALC CA LCUL ULA ATE TED D DA DAT TA ( co cont ntd. d. )
A rectangular duct having having length to breadth ratio 1.2 :1 is used.
1.2*(breadth)^2=area of inlet pipe= 0.07607 m2
Breadth of inlet duct = 0.2517m
Length of inlet duct =1.2*0.2517= 0.3021 m
The outlet duct duct should should have a higher higher cross-sectional cross-sectional area than inlet duct.
Length of outlet duct = 0.35 m
Breadth of outlet duct = 0.264 m
SUMMARY OF DESIGN DATA
Power consumed=14.744 hp Chamber efficiency=70%(assumed) Total Volume of the chamber= 5.5239 m3 Diameter=1.57104 m Height: Cylindrical portion(0.6D)= 0.9426 m Conical portion= 5.7209 m Total height=6.6635 m Bottom cone angle= 15.6306 degree Mechanical Design: Shell plate thickness= 0.25 inch Hot air inlet duct= 0.3021 m x 0.2517m Hot air outlet duct= 0.35 m x 0.264 m
