AGEN/CHEN-474 UNIT OPERATIONS IN FOOD PROCESSING
Oin
a SD, SD, foo foods ds are are tra trans nsfo form rmed ed fro from m pu pump mpab able le liliqu quid id into powder Othe liquid is pumped through a nozzle, where it is atomized Othe droplets are dried by hot air as they fall to the bottom of the chamber
feed air heater
atomizer air out
air out
air broom
product out
Oadvantageous
for heat sensitive products because the particles are never subjected to a temperature higher than the wet-bulb temperature of the drying air Oparticles residence time is short (3 - 30 s)
Ois
divided into 3 distinct processes O- atomization O-drying through the contact between the droplets and heated air O-collection of the product by separating it from the drying air
Othe
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 size Otypes: hydraulic nozzles and rotary nozzles
Oused
to create droplets by forcing the liquid trough a small orifice (0.4-4mm) Omaximum flow rate of 1L/h Opressure range from 300 to 4000 psig Oseldom used when feed is highly concentrated (clogging) Odroplets have narrow range of diameter and the dried product consists of hollow spheres Ooperating costs lower than rotary nozzles
O liquid
is fed to the center of the spinning wheel under centrifugal force O droplets are guided and shaped by vanes in the wheel O droplets are projected horizontally away at 100 to 200 m/s with angular velocities of 10,000 to 30,000 rpm O used for slurries and paste (no clogging) O produces homogeneous spray O mean particle diameter can be controlled by varying rotational speed O widely used in the food industry because it can handle a wide range of liquid viscosities and physical property
O vary
with nozzle type and feed material O estimation can be with following equations, but actual test is required O for rotating wheel atomizers: 0.6
Da
Γ µ αρ L =12.2 ×10 r ρ Nr Γ Γ 0.2
4
l
2
l
Da=average particle size (µm) α = surface tension of liquid (lb/min 2) ρl = liquid density (lb/ft 3) 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
0.1
O for
pressure atomizers O it requires only the pressure drop across the nozzle
Da =
500 1/ 3
∆ P
Da=average particle size (µm) ∆P = pressure drop across the nozzle (psi)
Odepends
on the type of atomizer, the airflow pattern, the production rate, when drying heat sensitive product, the temperature profile of the air in the chamber Othe shape of the drying chamber is a fnc of the droplets trajectory angle as they leave the atomizer Othe 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
Ovaries
with spray dryer design Omost common are air heaters and fans Omost 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) Ocentrifugal fans because they produce high air flow rates
O3
airflow patterns: concurrent, countercurrent, mixed flow
G
concurrent F
countercurrent G
F
G S S G G P
G
P
mixed (combined) used for heat-sensitive product is the concurrent because G product temperature is less than inlet air temperature Oif high-density products G F countercurrent Oif size of dryer is limited - mixed S G flow P Omost
q = ha (T −θ ) As 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)
ν =
& h M CH fg & (θ − T wb )C PF G(T − T wb )C PA + m
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)
Oif
the product separates from the air at the bottom of the conical chamber, it is removed through a auger Oit is common to product to remain entrained in the air stream, so cyclones are used to recover the product
Oflavor
encapsulation: food flavorings are combined with gums and carbohydrates before drying to prevent loss of volatile
Ovolatile
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) Othermal degradation is a problem for droplets that remain in the hot portion of the dryer for too long
2 L fg o
ρ h d
2 p c fg
ρ d h ( M cr − M e) t = + 8k a(T −T wb) 12k a∆T ave 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)
Oin
a PD the food, powder or particles, is continuously dried in a vertical duct while being conveyed by the heated air Oone or more cyclones are used to separate the dried material from the exhaust air Osmall particle sizes (less than 2 mm) and concurrent operation allow the use of relatively air temperatures without overheating the product
wet product fan
exhaust air
feeder cyclone
burner fan
dry product
Otwo
limit velocities are important in fluidization and pneumatic transport of solids, the fluidization velocity, vf , and entrainment velocity, ve v f = ve =
gD
2
p
ε ( ρ w − ρ )
180 µ (1 − ε ) gD
2
p
( ρ s
18 µ
− ρ )
