Title: ChE 473 Process Drying
1ChE 473 Process Drying
2Dryer 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
3Dryers 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
4A common household example
5What 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
6Solid 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
7Macro Drying Process
- This program will not study these nano and micro
relationships we will develop our controls based
on the macro mechanisms
8What 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.
9Equilibrium 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
10How is the moisture reported?
- Moisture content can be expressed as
- wet / (wet dry)
- wet / dry
11The Drying Process can be described in several
ways
- Batch or Continuous how the material is
processed. - A single charge Batch
- Continuous input and output.
12The 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.
13The 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.
14In 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
15Adiabatic 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
16What 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
17Psychometric chart - displays phase conditions
of water vapour in air
18The Psychometric chart computer program
- Akton Associates Post Office Box 2076
Edmond, Oklahoma 73034 - 405.513.8537
- http//www.aktonassoc.com/
19Properties 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.
20Properties 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.
21Relative 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.
22Relative Humidity
- The water vapor pressure can be calculated by an
exponential equation - p in psia and T in DegF
23Drying is in one of two zones or periods
- Constant rate and Falling rate zones
24Constant 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
25Falling 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
26Batch 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
27Batch 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
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29Control 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
30Drying 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
31Why should we control the drying rate?
- Some products sensitive to excessive heat -
examples - lumber, drying too fast causes the wood to crack
- Pharmaceuticals
32Drying 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.
33Drying 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.
34Drying 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
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