ChE 433 DPCL Dryer Control Lecture

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Transcript ChE 433 DPCL Dryer Control Lecture

ChE 473 Process Drying
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
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
A common household example …
Clothes dryer appliance
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
Wet Material
Hot dry air
Humid air
Dry Material
Solid drying process is very complex
with 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
Macro Drying Process
This program will not study these nano
and micro relationships; we will develop
our controls based on the macro
mechanisms
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.
Equilibrium Moisture
The solid’s 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%
How is the moisture reported?
Moisture content can be expressed as:
wet / (wet + dry)
wet / dry
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.
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.
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 nonadiabatic.
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
Adiabatic dryers, solids are exposed to the
heated gasses in various methods:
Blown across the surface cross circulation
Blown through a bed of solids, throughcirculation; 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
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
Psychometric chart - displays phase conditions
of water vapour in air
29. 225
650 ft
1
inHg
Btu /lbm
75
0.0 5
70
65
60
55
Tw Wet Bulb Temperature Lines
0.0 35
0.0 3
40
0.0 25
35
0.0 2
30
25
0.0 15
20
0.0 1
15
10
ft^ 3/lb m 13 .2
F 4 5 50
55
0.0 4
50
45
Relative Humidity Lines
0.0 45
0.0 05
lbm /lbm
60
65
70
13. 8
75
80
85
90
95
14. 4
100 105 110
The Psychometric chart computer
program
Akton Associates
Post Office Box 2076
Edmond, Oklahoma 73034
405.513.8537
http://www.aktonassoc.com/
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.
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.
Relative Humidity
The relative humidity is calculated as a ratio
of partial pressures:
p
RH  100*
p
w
o
w
p w is the water vapor pressure at the dew point
p wo
temperature
is the water vapor pressure at the dry bulb
temperature.
Relative Humidity
The water vapor pressure can be calculated by an
exponential equation:
  7071.3 
p  2.04466*10 * exp

 t  385.0 
6
p in psia and T in DegF
Drying is in one of two zones
or periods…
Constant rate and Falling rate zones
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
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
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:
G
C
(
T
T
)
F
H
i
o
w
v
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
Batch Drying
The outlet temperature value will be between
the inlet and the wet bulb temperature. The
rate of evaporation dFw is equal to:
d
F
a
R
d
A
(
T

T
)
w
w
Ti Inlet Dry Bulb Temperature
Tw Wet Bulb Temperature
a Mass transfer coefficient
R Rate coefficient
dA Surface Area
Ti
To
Tw
Constant wet bulb
temperature
dFw
G
dA
Hv
Water heat of
vaporization
T
a R
a = Mass transfer coefficient
R = Rate coefficient
C = Air Specific Heat
dFw  aRdA(T  Tw )
GCdT  dFw H v
Evaporation Model; Air temperature decreases as the moisture is removed from the solid
R
lbs
hr
a
1
ft 2  F
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
Drying Rate Control
To control the drying rate, you control
the temperature differences.
G
C
(
T
T
)
F
H
i
o
w
v
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
Why should we control the
drying rate?
Some products sensitive to excessive
heat - examples:
lumber, drying too fast causes the wood
to crack
Pharmaceuticals
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.
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.
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