SPRAY DRYING

BASIC CONCEPT – SPRAY DRYING

Spray drying - formation of droplets from the bulk liquid – moisture removal

liquid droplets - sprayed –drying chamber the low-humidity hot gas or drying medium is

mixed with the dispersed droplets individual droplets –atomized - by atomizers Spray drying –referred as a suspended

droplet/particle processing technique.

rotary wheel/disc atomizers pressure nozzle or pneumatic-type atomizers. The atomizer is generally located at the top

centre of the drying chamber for most spray drying operations.

The moisture, in the form of vapor, quickly evaporates from the suspended droplets

due to simultaneous and fast heat and mass transfer processes.

Drying of the droplets continues inside the drying chamber - desired particle characteristics are achieved.

The final dried product is produced using a single stage drying process

Separation of the dried particles from the drying gas - cyclones and/or bag-filter houses.

SINGLE STAGE SPRAY DRYER

To produce a hot drying medium, the ambient air ( Tambient) is heated to the desired temperature (Tinlet ).

In modern spray driers, the hot air stream + cooling air stream - to keep the atomizer temperature at a low value

the temperature of the hot air stream is usually kept slightly higher than the temperature required at the atomizer zone.

During this heating, the absolute humidity of the air remains constant while its vapor pressure (RH) is reduced to a very low value

the water activity of the dried product is normally reduced to less than 0.2;

therefore the RH of the air is maintained below 20% RH to reach the desired level of water activity

The outlet air temperature (Toutlet ), which is controlled by the liquid flow, - keep the water activity at the desired level.

At the end of the drying, the drying gas & dried product can approach an equilibrium state.

The rate of evaporation - temperature & vapor pressure differences - surface of the droplets & the drying gas

other important factors: diffusivity of water in air relative velocity of droplet with respect to drying

gas kinematic viscosity conductivity & heat capacity of air

conversion of the liquid droplet to the dried particle weight loss of 50% (due to loss of water) volume loss of 25% (due to shrinkage).

COMPONENTS OF A SPRAY DRYING SYSTEM

drying gas supply and heating system atomization system drying chamber powder separators Additional/Optional

the number of stagesdrying mode - fluidized-bed drier or belt drier

DRYING GAS SUPPLY AND HEATING SYSTEM

ambient air Superheated steam Air – drawn – filtered - heated from 150◦C

to 270◦C Hot air with higher humidity - may result in

a dried product with a slightly higher moisture content.

air dehumidification unit – may be installed

The air can be heated using a direct-contact or an indirect-contact system

Electrical, steam, oil-fired or gas heaters combination of heating methods such as

steam and electrical heating The air pressure inside - slightly lower

than the atmospheric pressure A lower pressure helps to avoid leakage

of product/air from the drier

ATOMIZATION SYSTEM large surface area between the moist droplets and

the drying medium heat and mass transfer processes 1cubic metre of liquid forms approximately 2 × 1012

uniform droplets of 100 μm diameter powder properties and powder collection efficiency –

depends on type of atomizer The atomization process influences

droplet’s size, size distribution, trajectory and velocity, the overall product quality, the drying chamber design energy requirement to form the spray of droplets.

The device - liquid atomization – atomizer Atomizers can be classified based on

the type of the energy used for atomization, the number of orifices the shape of orifice the mode of operation (continuous or intermittent) the geometry of atomizers

Widely used four types of atomizers rotary wheel/disc (centrifugal energy)pressure nozzles (pressure energy)Twofluid nozzles (pressure and gas energy) sonic nozzles (sonic energy)

Rotary atomizers & pressure nozzles – large scale

Pneumatic nozzles – medium scale Sonic atomizers – small scale - use high-

frequency sound energy created by a sonic resonance cup placed in front of the nozzle

Sonic atomizers - difficult to atomize using traditional atomizers and specialty products

Pressure-swirl, sonic and two fluid nozzles - hollow cone-type spray pattern or a fully developed cone

Rotary atomizers produce a wide cone, which is sometimes referred as a ‘spray cloud’

DRYING CHAMBER cylindrical drying chamber with a cone of 40◦–60◦

at the bottom The drier chamber design primarily depends on

the type of the atomizer the trajectory of droplets the properties (such as heat sensitivity, solids

content, etc.) of the material the capacity of the drier single- or two-stage dryingthe cost and the type of air flow (co- or counter-

current) with respect to the feed

To facilitate powder removal and minimize the wall deposition, the drier chambers are usually equipped with an air or mechanical sweeping system

spray driers are mostly operated with a co-current mode - the drying gas and the atomized droplets move in the same direction

When rotary-type atomizers -a rotational airflow is commonly used

provides more uniform temperatures in the drying chamber compared to that of the non-rotational airflow.

POWDER SEPARATORS the air stream from the drying chamber usually

contains about 10–50% of the total powder Powder recovery

economy purposespollution problems

Gravity separators (e.g. cyclones) only or by a combination of gravity and filter separators

SINGLE STAGE SPRAY DRYER

TWO STAGE SPRAY DRYER

cyclone separators

bag filters

wet scrubbers

DRYING OF DROPLETS Drying starts with evaporation of ‘free’ moisture on the droplet surface droplet surface is fully covered with water -the

drying rate = rate for pure water evaporation a droplet that has dissolved or a suspended solid

is being dried - vapor pressure smaller the mass transfer rate gradually becomes lower

drying of the wet ‘porous’ droplets liquid diffusion caused by the liquid density

gradient Vapor diffusion caused by the vapor density

gradient capillary flow caused by the capillary force moisture transfer caused by the internal pressure

gradient moisture transfer caused by evaporation and

condensation in pores

the majority of the free water is removed in a short period of time, leading to a very short (most times negligible) constant-rate drying period

relatively longer falling rate period during drying of small droplets.

spray drying, the maximum water evaporation takes place in a fraction of a second and within a short distance from the atomizer.

MASS AND HEAT BALANCES

in order to evaluate the efficiency of each process independently

Overall mass balance and heat balance

DRIER EFFICIENCY

Tb,i and Tb,o are inlet and outlet drying gas temperatures

Tb,a is the ambient gas temperature adiabatic saturation temperature (Tsat ) corresponding to the inlet air

temperatures