DESIGN OF SPRAY DRIER

Process Equipment and Design – II

Reference-Mass Transfer Operations by Alapati Suryanarayana (page 571-578)

• 08010734-RAHUL MAYANK•08010736-SANGADE MAHESH SITARAM

•08010737-SANGISHETTY TARUN

PROBLEM STATEMENT

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 (TDB) = 1500 C & (TWB) = 500 C

Outlet air Temperature = 1200 C Inlet temperature of solid = 500 C Outlet temperature of solid = 650 C Specific heat of solid = 0.32 cal/gm/0C

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.

It is highly suited for the continuous production of dry solids in either 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, and particle shape.

ATOMIZER

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 Two-Fluid Nozzle 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 co-current 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 more 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 2. pump3. drying chamber 4. air heater5. Cyclone 6. gas scrubber7. separator

ADVANTAGES

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, 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, 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. Solid & Gas operating velocity. Chamber Diameter. Residence time. Design of Conical bottom HERE WE ARE CONSIDERING A CO-CURRENT

SPRAY DRYER SINCE IT IS THE MOST COMMON ONE

ATOMIZER

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+X1)Ls = 1218 Kg/hr= 44.75

lb/min Herring and Marshal chart(Ref. British Chem.

Engg., Dec 1967,Vol 12,No. 12,P 1892) peripheral speed vs. mean particle diameter, 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)

ATOMIZER ( 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 H2 T2

Gs H1 T1 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 of inlet air T2= Temperature of outlet air

t1= Temperature of inlet solid t2= Temperature of exit solid

EQUATIONS( SPRAY CHAMBER )

Mass Balance eqn :

Ls X1 + Gs Y1 = Ls X2 + Gs Y2 ---------------(1)

Enthalpy Balance eqn :

Ls HL1 + Gs H1 = Ls HL2 + Gs H2 -----------(2)

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

EQUATIONS( SPRAY 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/0CCpw = Specific heat of water = 4.2 kJ/kg/0CCpv = Specific heat of water vapor = 1.89 kJ/kg/0CCpa = Specific heat of air at avg. temperature = 1.0312 kJ/kg/0Cλ = Latent heat of vaporization= 2510.7 kJ/kg

CALCULATED DATA ( contd. )

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)*1200=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=(0.0206-0.01317)=0.00743 kg water per kg of dry air

• Gs= 20.57/0.00743 = 2768.5 Kg/hr

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.

1 273.15 ( ) = 8315 * ( )*( )

g

a w

Y THumid Volume V

M M P

Ma = Molecular wt of air =29 g/molMw = Molecular wt of water 18 g/molTg = Temperature of gas in 0C

P = Pressure in N/m2

Y = Humidity of air in kg of moisture/ kg of dry air

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 non-dusting spray dryer is taken as two times the settling velocity of the drop.

SETTLING VELOCITY

2pD ( )

18

p fs

gv

Dp = Drop Diameter in m

ρp = Density of droplet in kg/m3

ρf = Density of air at avg temperature in kg/m3

µ = Viscosity of air at avg temperature

vs = Settling velocity

g = Acceleration due to gravity

In Stoke’s law region i.e. for Reynold’s number less than 2

CALCULATED DATA ( contd. ) 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=1350C Density of air at avg temperature ρf = 0.865

kg/m3 Viscosity of air at avg temperature µ =

23.329*10-6 N/m

Settling velocity vs = 0.3078 m/s

CALCULATED DATA ( contd. ) Reynolds Number =1.411

< 2

Thereby , STOKE’s Law is applicable and now :

Operating Velocity ( va ) = 2 * Settling velocity

va = 2 * vs = 0.6155 m/s

eRp s pD v

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 m2

Column Diameter

Dc = 1.3661 m

For safety purposes , assuming 15% safetyDc = 1.15* Dc = 1.57104 m

4* cc

AD

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

θp = time required to evaporate moisture from droplet

ρp=Density of feed.

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 driving force = Drying gas film temp. – Temp. of dry solid surface

21X

12

ppp

f t

w D

K T

CALCULATED DATA ( contd. )

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 a conical bottom. Total Volume of chamber (Vt) = Gs * Vavg * td =

5.5239 m3

Minimum height of cylindrical (hmin)= vs * θp = 0.00437 m

portion

Recommended height of(hcyl) = 0.6 * Dc = 0.9426 m

cylindrical portion

Volume of the conical portion of the chamber

Vcone = 3.69668 m3

hcone = Height of the conical portion of the chamber

hcone = 5.7209 m

Cone angle α= 15.6306 degree

2

4

cylCcone t

D hV V

2

3 4 conecone

c

Vh

D

tan( )2 2

c

cone

Radiusofchamber D

Conicalheight h

MECHANICAL DESIGN

THICKNESS OF CHAMBER tmin = tmin= (Dc+100)/1000 = 0.1628 inch

Dc is in inches & tmin is in inches

As the next available thickness is 0.25 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

Diameter of dust collector

Dd = 32.58 mm = 1.3032 inch ( as this is small , 5 inch nominal diameter is used )

Length of dust collector

Ld= Dc/8=0.19698 m

*

sd

p s

LA

v

4* dd

AD

HOT AIR INLET

Assume operating air velocity vair = 10 m/s

Minimum cross sectional area = (Gs * Vavg)/ vair

= 0.07607 m2

Contamination of outlet air with fine particles requires a higher cross sectional area for outlet duct than that of inlet duct.

CALCULATED DATA ( contd. ) A rectangular duct 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 should have a higher 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