DRYER DESIGN

AGEN/CHEN-474UNIT OPERATIONS IN FOOD PROCESSING

SPRAY DRYER-SD

in a SD, foods are transformed from pumpable liquid into powderthe liquid is pumped through a nozzle, where it is atomizedthe droplets are dried by hot air as they fall to the bottom of the chamber

SPRAY DRYER

air heaterfeed atomizer

air out

air out

product out

airbroom

SPRAY DRYING

advantageous for heat sensitive products because the particles are never subjected to a temperature higher than the wet-bulb temperature of the drying airparticles residence time is short (3 - 30 s)

SPRAY DRYING OPERATION

is divided into 3 distinct processes- atomization-drying through the contact between the droplets and heated air-collection of the product by separating it from the drying air

ATOMIZATION

the type of atomizer is important because it determines: the energy required to form the spray, size and distribution of the droplets, available heat transfer area, drying rate, droplet speed and trajectory, and final product sizetypes: hydraulic nozzles and rotary nozzles

Pressure Atomizer

used to create droplets by forcing the liquid trough a small orifice (0.4-4mm)maximum flow rate of 1L/hpressure range from 300 to 4000 psigseldom used when feed is highly concentrated (clogging)droplets have narrow range of diameter and the dried product consists of hollow spheresoperating costs lower than rotary nozzles

Rotary Atomizer

liquid is fed to the center of the spinning wheel under centrifugal forcedroplets are guided and shaped by vanes in the wheeldroplets are projected horizontally away at 100 to 200 m/s with angular velocities of 10,000 to 30,000 rpmused for slurries and paste (no clogging)produces homogeneous spraymean particle diameter can be controlled by varying rotational speedwidely used in the food industry because it can handle a wide range of liquid viscosities and physical property

1- DROPLET-SIZE CALCULATIONvary with nozzle type and feed materialestimation can be with following equations, but actual test is requiredfor rotating wheel atomizers:

1.0

2

2.06.0

24102.12

Γ

Γ

Γ×=

LNr

rD l

la

αρµρ

Da=average particle size (µm)α = surface tension of liquid (lb/min )ρl = liquid density (lb/ft )r = disk radius (ft)Γ = spray mass velocity per foot of disk periphery (lb/ft.min)N = disk speed (rpm)L = disk periphery (ft)

2

3

1- DROPLET-SIZE CALCULATION

for pressure atomizersit requires only the pressure drop across the nozzle

3/1500P

Da ∆=

Da=average particle size (µm)∆P = pressure drop across the nozzle (psi)

2- DRYER CHAMBER DESIGN

depends on the type of atomizer, the airflow pattern, the production rate, when drying heat sensitive product, the temperature profile of the air in the chamberthe shape of the drying chamber is a fnc of the droplets trajectory angle as they leave the atomizerthe chamber must be sized so that the largest droplets is dry before it contacts a wall

rotating wheels 1-600

pressure nozzles 10-800

pneumatic nozzles 6-300

milk 30-250

coffee 80-400

Range of Droplets and Particle Sizes obtained in Spray Dryers (µm)

3 - AUXILIARY EQUIPMENT

varies with spray dryer designmost common are air heaters and fansmost common heater in FI is steam heater (saturated steam at 150 to 200C is used to heat the air up to 10C bellow the steam temperature)centrifugal fans because they produce high air flow rates

4- AIRFLOW PATTERNS3 airflow patterns: concurrent, countercurrent, mixed flow

P

F

P

G

G

S G

F G

G

G S

concurrent countercurrent

4- AIRFLOW PATTERNS

most used for heat-sensitive product is the concurrent because product temperature is less than inlet air temperatureif high-density products -countercurrentif size of dryer is limited - mixed flow P

G

G

FS G

mixed (combined)

5- CALCULATION OF HEAT INPUT

( ) sa AThq θ−=ha = dry air film coefficient (J/m^2K)θ = temperature of solid (K)T = temperature of thedry air (K)As = surface area of the solid (m^2)

6- CALCULATION OF THERMAL EFFICIENCY

( ) ( ) PFwbPAwb

fgCH

CTmCTTGhM

−+−=

θν

&

&

MCH=chamber evaporation capacity (kg water/s)hfg = latent heat of vaporization (J/kg)G = airflow rate (kg/s)m = feed flow rate (kg/s)CPA = heat capacity of air (J/kgK)CPF = heat capacity of the feed (J/kgK)T = air temperature (C)θ = feed temperature (C)

.

7- PRODUCT COLLECTION

if the product separates from the air at the bottom of the conical chamber, it is removed through a auger it is common to product to remain entrained in the air stream, so cyclones are used to recover the product

8- USE OF SPRAY DRYERS

flavor encapsulation: food flavorings are combined with gums and carbohydrates before drying to prevent loss of volatile

volatile retention is a problem with SD - loss of volatile is minimized by increasing the particle diameter (as), decreasing feed temperature (lower the liquid-phase diffusion coefficient), and decreasing the air temperature (minimizing particle expansion)thermal degradation is a problem for droplets that remain in the hot portion of the dryer for too long

9- FOOD QUALITY FACTORS

10- Drying Time

avea

ecrfgcp

wba

ofgL

TkMMhd

TTkdh

t∆

−+

−=

12)(

)(8

22 ρρ

do = initial diameter of droplet (m)dc = droplet diameter at critical moisture Mcr (m)ka = thermal conductivity of air (W/mK)DTave = average temp. difference between air

and product (C)

FLASH OR PNEUMATIC DRYERS

in a PD the food, powder or particles, is continuously dried in a vertical duct while being conveyed by the heated airone or more cyclones are used to separate the dried material from the exhaust air small particle sizes (less than 2 mm) and concurrent operation allow the use of relatively air temperatures without overheating the product

SCHEMATIC OF A PNEUMATIC DRYER

burner

fan dry product

exhaust airfan

cyclone

feeder

wet product

VELOCITY CALCULATIONS

two limit velocities are important in fluidization and pneumatic transport of solids, the fluidization velocity,vf, and entrainment velocity, ve

( )( )

( )µ

ρρεµρρε

18

11802

2

−=

−−

=

spe

wpf

gDv

gDv