Download - ChE 473 Process Drying
ChE 473 Process Drying
Dryer Control
In order to control any process, we need a good understanding of the process itselfWhat is the drying process?Dryer classifications and typesProcess analysis Macro vs. Nano, Micro
Dryers – A common yet costly unit operation
Dryers used in chemical processing, food processing and pharmaBatch or continuousEnergy intensiveFrequently over dried at added costs, dusting, product lossDrying 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
Hot dry air Hum id air
W et Material
D ry Material
Solid drying process is very complexwith micro and nano mechanisms
Liquid movement due to capillary forcesDiffusion due to concentration gradientsLiquid vapor flow due to pressure differencesVapor diffusion due to vapor pressure differences,
concentration differencesOsmotic pressure created by colloidal bodies has
soluble and insoluble fractions Vapor Effusion – A relationship of vapor flow to pore
diameterThermodiffusionVaporization-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 MoistureThe 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 – BatchContinuous 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 non-adiabatic.
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 circulationBlown through a bed of solids, through-circulation; solids stationary; wood, corn etcDropped slowly through a slow moving gas stream, rotary dryerBlown through a bed of solids that fluidize the particles; solids moving; frequently called fluidized bed dryerSolids 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 inHg650 ft
F 45 50 55 60 65 70 75 80 85 90 95 100 105 110
0.005lbm/lbm
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
1
Btu/lbm 75
70
65
60
55
50
45
40
35
30
25
20
15
10
ft^3/lbm 13.2 13.8 14.4
Tw Wet Bulb Temperature Lines
Relative Humidity Lines
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 axisThe dew point temperature - temperature where condensation begins to form as the water is condensed from the wet air; not shown on the chartThe 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:
is the water vapor pressure at the dew point temperature
is the water vapor pressure at the dry bulb temperature.
ow
w
p
pRH *100
wp
owp
Relative Humidity
The water vapor pressure can be calculated by an exponential equation:
p in psia and T in DegF
0.385
3.7071exp*10*04466.2 6
tp
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 DryingIf 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
vwoi HFTTGC )(
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 Temperaturea Mass transfer coefficientR Rate coefficientdA Surface Area
)( ww TTaRdAdF
a RdA
dF wG
T i
T o
T w
T
H v
)( ww TTaRdAdF vwHdFGCdT
a = M ass transfer coeffic ientR = R ate coeffic ientC = A ir S pecific H eat
E vapora tion M ode l; A ir tem pera ture decreases as the m o istu re is rem oved from the so lid
C onstant w et bu lbtem perature
W ater heat o fvaporization
hr
lbsR
Ffta
2
1
Control of the drying process
Drying is considered a self regulating processA 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.
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
vwoi HFTTGC )(
Why should we control the drying rate?
Some products sensitive to excessive heat - examples:lumber, drying too fast causes the wood to crackPharmaceuticals
Drying Rate Control
But the outlet temperature lags the inlet by some amountThis lag is due to the thermal time constant of the solidWe need to compensate for this time difference for proper controlIn 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