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