Drying Flashcards
In what ways can products be damaged during drying?
-Contamination or crystal damage (pharmaceuticals)
-Shrinkage (paper)
-Cracking (wood)
-Loss of flavour (fruit)
Why is drying carried out
-To reduce transport costs
-To make a material more suitable for handling e.g. soap powders, fertilisers
-To provide definite properties e.g. maintaining the free-flowing nature of salt
-To remove moisture which may otherwise lead to corrosion e,g, drying of gaseous fuels or benzene
What are the main considerations for dryer type selection?
-Form of feed and product -particulate, sheet, slab, solid/liquid
-Mode of operation - batch/continuous
-Mode of heat transfer - convective/conductive
-Condition of solids - require static/moving bed , fluidised or dispersed
-Contact between gas and solid - parallel/perpendicular/through-circulation
-Gas flow pattern - cross/ co or counter current
What are the two primary steps when selecting a dryer
1) List all the dryers capable of handling the material to be tried
2) Remove costly options on basis of annual, capital and operating costs
Advantages of continuous over batch
-Easy to integrate into the rest of the process
-Lower unit cost of drying
Disadvantages of continuous over batch
-As the rate of throughput of material decreases, the capital cost is main component of total running cost so the relatively cheaper batch plant is better
-Throughputs ~ 50,000kg/day -> continuous
-Throughputs ~ 5,000kg/day -> batch
What is the formula for humidity?
kg water / kg dry air
What assumptions are used with the simplified drying curve?
1) In the constant rate period drying takes place from a saturated surface of the material, of specific area, by diffusion of the water vapour through a stationary hot air film into the hot air stream
2) In the falling rate period there is a uniform rate of decrease
What are the 2 key principles during the constant rate period of a drying curve
-Removal of free moisture from the surface of the solid
-Moisture removal is controlled by external evaporation
What are the 2 key principles during the falling rate period of a drying curve
-Surface is dry, no free moisture
-Moisture removal is controlled by internal diffusion/the internal structure
At dynamic equilibrium how is moisture transferred
The rate of heat transfer to the surface by convection is equal to the rate of mass transfer from the surface
How do industrial dryers operate?
-Adiabatically
-All heat goes into evaporation
-Wet bulb temperature is constant during adiabatic drying
What 2 mechanisms are applicable for moisture removal in the falling rate period
-Diffusion
Internal moisture from solid diffuses to the surface
Uses the simple drying curve
Applicable to non-porous solids e.g wood
-Capillary flow
Water is drawn to the surface by capillary action
Evaporation front moves into solid as material close to the surfaces dries
Applicable to porous solids e.g sand and depends on structure of pores and waists
How does drying work with the diffusion model?
-Typical mechanism for slow drying materials, diffusion coeff is small or material is thick
-Drying process controlled by diffusion rather than evaporation
-Moisture at surface is near an equilibrium value
-Air flow rate has little to no effect
-Air humidity and temperature provides driving force for diffusion
-Temp increases diffusion and hence drying rate
How does drying work with the capillary model?
-As drying proceeds, water content in bed reduces
-Recession of water surface into bed is impeded by narrow pores and waists
-These exert capillary pressure on fluid in larger pores below
-Initially, evaporation front remains static while fluid below is sucked up to replace that evaporated
-As drying continues surface pores lose water and the evaporation front moves into the solid
-Front remain in place until voids below fill with air through larger connecting pores as the surface layer recedes
-Eventually water becomes discontinuous as air breaks up connections so capillary action can no longer happen
