ChE 473 Process Drying - PowerPoint PPT Presentation

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ChE 473 Process Drying

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... Solids Drying Air Humidification We will view dryer control from the air humidification process Adiabatic dryers, ... – PowerPoint PPT presentation

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Title: 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
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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
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