ChE 473 Process Drying

1
ChE 473 Process Drying
2
Dryer Control

• In order to control any process, we need a good
  understanding of the process itself
• What is the drying process?
• Dryer classifications and types
• Process analysis Macro vs. Nano, Micro

3
Dryers A common yet costly unit operation

• Dryers used in chemical processing, food
  processing and pharma
• Batch or continuous
• Energy intensive
• Frequently over dried at added costs, dusting,
  product loss
• Drying accounts for 12 manuf. costs

4
A common household example

• Clothes dryer appliance

5
What is the Drying Process

• Removal of small amount of liquid, usually water
  Large amounts of water normally removed by
  press or centrifuges. Thermal methods employed.
  Heat and Mass transfer

6
Solid drying process is very complexwith micro
and nano mechanisms

• Liquid movement due to capillary forces
• Diffusion due to concentration gradients
• Liquid vapor flow due to pressure differences
• Vapor diffusion due to vapor pressure
  differences, concentration differences
• Osmotic pressure created by colloidal bodies has
  soluble and insoluble fractions
• Vapor Effusion A relationship of vapor flow to
  pore diameter
• Thermodiffusion
• Vaporization-condensation mechanism

7
Macro Drying Process

• This program will not study these nano and micro
  relationships we will develop our controls based
  on the macro mechanisms

8
What is the Drying Process

• Drying - water liquid vaporization not as
  efficient as centrifuge, 1050 BTU/lb of water
  removed. 
• Final moisture varies dried table salt contains
  0.5 water, dried coal 4.
• Solids can have many different forms, flakes,
  granules, crystals, powders, etc. The liquid can
  be on the surface, within the surface in cellular
  structures, such as wood. Consider the method of
  handling, dusting, rough or gentle treatment.

9
Equilibrium Moisture

• The solids moisture content is a function of
  the humidity of the drying air. The moisture
  cannot be lower than the equilibrium moisture
  content corresponding the humidity of the
  incoming air.
• 50 RH air equilibrium moisture
• Wool 12.5 Newspaper 5.5

10
How is the moisture reported?

• Moisture content can be expressed as
• wet / (wet dry)
• wet / dry

11
The Drying Process can be described in several
ways

• Batch or Continuous how the material is
  processed.
• A single charge Batch
• Continuous input and output.

12
The Drying equipment can be described as dryer
types

• Dryer Types the classification as to the method
  solids travel through the heated zone, the heat
  source and transfer method.

13
The Drying Process can be classified as

• Classifications
• Adiabatic Dryers are the type where the solids
  are dried by direct contact with gases, usually
  forced air. With these dryers, moisture is on the
  surface of the solid.
•  
• Non-Adiabatic Dryers When a dryer does not use
  heated air or other gasses to provide the energy
  required the drying process is considered a
  non-adiabatic.

14
In the case of Adiabatic Dryers

• The process can be considered to be two related
  processes
• Solids Drying
• Air Humidification
• We will view dryer control from the air
  humidification process

15
Adiabatic dryers, solids are exposed to the
heated gasses in various methods

• Blown across the surface cross circulation
• Blown through a bed of solids, through-circulation
  solids stationary wood, corn etc
• Dropped slowly through a slow moving gas stream,
  rotary dryer
• Blown through a bed of solids that fluidize the
  particles solids moving frequently called
  fluidized bed dryer
• Solids enter a high velocity hot gas stream and
  conveyed pneumatically to a collector Flash Dryer

16
What can the Psychometric Properties tell us
about the drying process?

• In many ( or most ) cases, the nano and macro
  drying mechanisms are not know.
• However, we do know air properties
• Lets make use of the air properties to control
  our dryer

17
Psychometric chart - displays phase conditions
of water vapour in air
18
The Psychometric chart computer program

• Akton Associates    Post Office Box 2076   
  Edmond, Oklahoma 73034
• 405.513.8537
• http//www.aktonassoc.com/

19
Properties shown on psychometric chart

• The air temperature - dry bulb temperature of the
  stable air water vapour mixture on the x axis
• The dew point temperature - temperature where
  condensation begins to form as the water is
  condensed from the wet air not shown on the
  chart
• The wet bulb temperature is the temperature at
  which adiabatic heat is transferred during the
  drying of solid or humidification of air. For a
  dryer, moisture in the solid is transferred to
  the air. The air will gain moisture while the
  solid looses moisture, therefore or
  humidification of the air occurs. This process
  will occur at a constant wet bulb temperature.
  The dry bulb air temperature will decrease during
  this process and be lower exiting the dryer or
  chamber. This temperature is shown as a series of
  curved lines sloping downward.

20
Properties shown on psychometric chart

• Relative humidity is the ratio of the water
  vapour pressure at the dew point to the water
  vapour pressure at the dry bulb temperature. This
  ratio is usually expressed as a percent. This
  ratio is multiplied by 100 to obtain the
  percentage reading. These lines are the curved
  lines sloping upward.
• Vertical line on the right shows the absolute
  moisture pounds of moisture per pound of dry air.

21
Relative Humidity

• The relative humidity is calculated as a ratio of
  partial pressures
• is the water vapor pressure at the dew point
  temperature
• is the water vapor pressure at the dry bulb
  temperature.

22
Relative Humidity

• The water vapor pressure can be calculated by an
  exponential equation
• p in psia and T in DegF

23
Drying is in one of two zones or periods

• Constant rate and Falling rate zones

24
Constant Rate Zone a.k.a. first period of drying

• Layer of saturated air on solid surface
• This rate is determined by the capacity and
  properties of the inlet gas or vapor
• Solid temperature is equal to the wet bulb
  temperature during this period
• Free water drying

25
Falling Rate Zone a.k.a. second period of drying

• inflection point at the critical moisture
• begins when the surface or free water is removed
• solid temperature increases form wet bulb temp to
  that approaching the inlet air, gas, temperature

26
Batch Drying

• If air is passed over a moist solid, air
  temperature will be reduced as the water is
  evaporated. Calculated through an enthalpy
  balance
• Ti Inlet Dry Bulb Temperature
• To Outlet Dry Bulb Temperature
• G Air Mass Flow
• C Air Heat Capacity
• Fw Mass rate of water evaporation
• Hv Heat of vaporization

27
Batch Drying

• The outlet temperature value will be between the
  inlet and the wet bulb temperature. The rate of
  evaporation dFw is equal to
• Ti Inlet Dry Bulb Temperature
• Tw Wet Bulb Temperature
• a Mass transfer coefficient
• R Rate coefficient
• dA Surface Area

28
(No Transcript)
29
Control of the drying process

• Drying is considered a self regulating process
• A change is heat input will, after time, result
  in a change in product moisture, assuming all
  other conditions are constant

30
Drying Rate Control

• To control the drying rate, you control the
  temperature differences.
• Ti Inlet Dry Bulb Temperature
• To Outlet Dry Bulb Temperature
• G Air Mass Flow
• C Air Heat Capacity
• Fw Mass rate of water evaporation
• Hv Heat of vaporization

31
Why should we control the drying rate?

• Some products sensitive to excessive heat -
  examples
• lumber, drying too fast causes the wood to crack
• Pharmaceuticals

32
Drying Rate Control

• But the outlet temperature lags the inlet by some
  amount
• This lag is due to the thermal time constant of
  the solid
• We need to compensate for this time difference
  for proper control
• In our experiment we will measure this lag time
  as well as calculate it, knowing the properties
  of the material being dried.

33
Drying Rate Control

• We want the temperature difference to be the
  difference between the inlet and the outlet
  temperatures, but the inlet temperature must be
  lagged before the difference is taken.
• We must reference the inlet temperature at a
  previous time that caused the current outlet
  temperature.

34
Drying Rate Control

• First order lag must be applied to the inlet
  temperature before the difference is calculated.
• The reset setting in the temperature difference
  controller is set to the same time as the first
  order inlet temperature lag

35
(No Transcript)