Table 2.

General characteristics of some conventional dryers and suggested areas for further improvement

Dryer type	General characteristics	General characteristics
Tray	Materials are placed on trays and directly make contact with drying medium (typically hot air)	Uniformity of air flow distribution
	Heat transfer mode is typically convective.	Uniformity of final product quality and moisture content
	Conductive is possible by heating tray	Hybrid mode by combining with the microwave heat input
Rotary	A cylindrical drying chamber rotates while material tumble in the chamber	Precise prediction of particle motion, particle residence time distribution and uniformity of final moisture content
	Drying medium (typically hot air) is charged into the chamber contacts the material in cross flow	Effect of polydispersity and cohesiveness of solids on drying kinetics
	Flights are used to lift the material	Design of flights, internal heat exchangers, delumpers
	Internal heat exchangers installed to allow conductive heat transfer	Effect of solid holdup and hot air injection on drying kinetics
		Model-based control
Flash	Flash dryer is used to remove surface moisture. Material is charged into a fast moving drying medium stream, drying occurs while the drying medium conveys the material pneumatically	Modeling of particle motion including effects of agglomeration attrition and geometry of dryer
	Cyclone is normally used to separate the drying medium and the material	Use of pulse combustion exhaust, superheated steam, internal heat exchangers, variable cross-section ducts, hot air injection along length of dryer duct
Spray	Atomizer mounted on top of a drying chamber sprays liquid/suspension and forms droplets	Effects atomizer design on droplet trajectories, product properties, agglomeration, size reduction
	Drying medium (typically hot air) is supplied into the chamber concurrently or counter currently	Effect of chamber geometry
	Hot air exits the chamber at the chamber outlet and carries dried powder	Injection of supplementary air
	Separation of hot air and powder takes place in Cyclone	Use of superheated steam
		Uniformity of product quality and final moisture content
Fluidized Bed	Similar to fixed bed dryer but operating hot air velocity is higher to ensure the particles are suspended in the sir stream	Effect of particle moisture content/polydispersity on fluidization hydrodynamics, agglomeration, heat and mass transfer
	Large contacting surface areas between the drying medium and the material if compare with fixed bed dryer	Effect of agitation, vibration, pulsation, acoustic, radiation on drying kinetics and characteristics
	Conventional fluidized bed is not suitable for drying fine powders (due to channeling and slugging) and coarse particles (due to formation of big bubbles)	Design of internal heat exchangers
	However, modified FBD such as vibrating FBD, agitating FBD, etc. can be used to dry difficult-to-fluidized particles	Classification of particle type based on fluidization quality at varying particle moisture content and stickiness
	If the materials are polydispersed, the hot air stream may carries over some fine particles	Mathematical modeling of fluidization hydrodynamics, heat and mass transfer by taking into account agitation, vibration, pulsation, internal heat exchanger, varying particle moisture content, etc.
	A cyclone is used to separate the fine particles from the gas stream	Over 30 variants possible
Vacuum	Need to maintain high vacuum; expensive	Combined mode of heat transfer, e.g., MW vacuum drying
	Drying chamber is operated at reduced pressure or vacuum	Hybrid drying, e.g., vacuum superheated steam drying, etc.
	Boiling point of water/solvent is reduced thus reducing the operating temperature	Use of internal heating media
	However, absence of drying medium in the vacuum drying chamber disables convective heat transfer but enhances mass transfer at low temperatures	Enhancement in drying kinetics by incorporating radiant heat input, internal heating media, etc.
Freeze	Vacuum freeze drying is expensive in terms of capital costs and operating costs due to very low vacuum required at very low temperature	Use of magnetic/electric/acoustic fields to control nucleation and crystal size of ice during freezing; permits better quality product
	Drying times are long; most operated batchwise
	Suitable only for very high value products like pharmaceutical products
Batch dryer	Not all dryers can operate in batch mode	Effects of intermittent/cyclic/variable heat inputs and variable operating profiles on drying kinetics and characteristics as well as product quality
	Good for low capacity needs	Use of heat pump including chemical heat pump
	Tray, rotary, drum, fixed bed, fluidized bed vacuum dryers etc. can be operated batchwise	Reduction in labor costs
		Model-based control
		Intermittent drying

Mujumdar and Law (2010)