 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.

 To produce a hot drying medium, the ambient air ( T ambient ) 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).

 drying gas supply and heating system  atomization system  drying chamber  powder separators  Additional/Optional  the number of stages  drying mode - fluidized-bed drier or belt drier

 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

 large surface area between the moist droplets and the drying medium  heat and mass transfer processes  1cubic metre of liquid forms approximately 2 × 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’

 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 drying  the 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.

 the air stream from the drying chamber usually contains about 10–50% of the total powder  Powder recovery  economy purposes  pollution problems  Gravity separators (e.g. cyclones) only or by a combination of gravity and filter separators

 cyclone separators  bag filters  wet scrubbers

 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.

 in order to evaluate the efficiency of each process independently  Overall mass balance and heat balance

 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