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.

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

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%

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 Inc.

PO Box 2076

Edmond, OK 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

Batch Drying

As the air passes over the moist solid, the air temperature will fall by dT

Assuming R = kw, a line that passes through to origin, the above equations can be solved for the outlet moisture:

vwHdFGCdt

wo

wi

v TT

TT

akAH

GCw ln

wo

wic

v

TT

TTKw

CG

akAHln

Batch DryingThe final outlet To temperature to achieve a

desired final moisture is w* is:

Eliminating the wet bulb temperature form the above yields:

Where Toc is the outlet moisture at the critical moisture.

)(**

wiKw

wo TTeTT

cc Kw

Kw

iKw

Kw

oco ee

Tee

TT11

111

**

*

)( wiKw

woc TTeTT o

Batch Dryer, extra credit problem

Use algebraic substitution, show how to arrive at equation (7) from (6) and (6a) in your handout.

10 points credit

Constant rate – Falling rate

The fraction term can be defined as K*

cc Kw

Kw

iKw

Kw

oco ee

Tee

TT11

111

**

*

*** 1 KTKTT ioco

Batch Dryer

This method calculates the outlet temperature required to obtain specified moisture in a batch dryer. It uses inferential moisture calculation based on temperature difference.

A better approach can be taken if the dryer constant, K, is recalculated each time.

Mass Transfer Equations Rate of Evaporation

m evaporation Rate

hv heat transfer coefficient

T inlet gas temperature

Ti interface temperature or Wet Bulb Temperature

A Area

i heat of vaporization BTU/lb

i

iv ATThm

)(

Heat Transfer coefficient can be estimated as

G mass velocity lb/ft^2 hr, note different G!

De airflow channel diameter ft

4.0

6.0026.0

ey D

Gh

Mass Velocity

For packed beds, calculation requires knowledge of void fractions… Void Fractions

Dp/Dt

e for spheres

e for cylinders

0.00 0.34 0.340.10 0.38 0.350.20 0.42 0.390.30 0.46 0.450.40 0.50 0.530.50 0.55 0.60

Packing Factor

Leva, M., Grunmer, M.Chem Eng Progress 43:713 (1947)

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 )(

Drying Rate Control

But the outlet temperature lags the inlet by some amountThis lag is due to the thermal time constant of the solid

Drying Rate Control

First order lag must be applied to the inlet temperature before the difference is calculated.

Continuous Dryer outlet moisture calculation…

The adiabatic drying process has two zones, falling rate and a constant rate. When the product becomes sufficiently dry that there are dry areas on the product surface. Further drying results in a falling rate of moisture removal.

Inferential measurement of product moisture is accomplished by the measurement of temperatures of the gas entering and exiting the dryer and performing a calculation using these temperatures. This technique uses mass and energy balances of the dryer to calculate the product moisture and is valid during the falling rate zone only.

H eater

W et Feed

D ry P roductO ut

A ir Fan

T in

T out

N atura l G as

w

Air O utto dust co llector

Rotary D ryer Counter Current F low

Product MoistureC alculation

The relationship between outlet moisture

and the temperatures are:

wp = k*ln( (Ti - Tw) / (To - Tw) )where:

wp = Outlet Moisture

Ti = Inlet Dry Bulb Temperature

Tw = Wet Bulb Temperature

To = Outlet Dry Bulb Temperature

k = Dryer Constant

Combustion air wet bulb temperature

For water and air systems, the wet bulb temperature is the same at the inlet as it is in the outlet.

Natural gas combustion wet bulb temperature degF is related by an empirical relationship that is:

Tw = 164 - ( 16900/Ti )

Combustion air wet bulb temperature

Combustion air wet bulb temperature

Colder air cannot contain as much absolute moisture as warmer airColder air requires more gas to heat, and therefore adds more moisture to the combustion productRelationship good for temperatures above 300 DegF

Inferential moisture control

PID control of this equation:

wp = k*ln( (Ti - Tw) / (To - Tw) )difficult due to inverse response

The problem in controlling this equation is that the dynamics of the equation result in reverse action, i.e. if the moisture set point is lowered, the instantaneous action would be to increase the inlet temperature, which causes the calculated moisture to increase before the outlet temperature is increased to such an extent as to lower the moisture to its new stable set point. This is because of the dead time and time constant between the inlet and outlet temperatures. Using conventional PID control on this relationship results in unstable control.

Inferential moisture controlIf the inlet temperature signal was transformed through a dead time and time constant control function blocks in the control algorithm, then applied to the inferential calculation, the resultant response would be closer to the actual moisture. This is because the present time observation of outlet temperature is the result of a past inlet temperature.

lagtimede lay

lag

f

in le t_ Tw Tc

dryer_tc

ti_w e ll

tw

con tro l_td

lagtou t_p rocess1 time

de lay

ou tlet_ Tw Tc

tou t_p ro cess2

drye r_ td

to ut_ we ll

lagti_c trl1

con tro l_tc

ftw _ctrl

ti_c trl

d rye r s im u lation

tise t

to u t

In fe ren tia l Dryer Contro ls

x(1)

x(2) x (3 )

x (4 )

w_ calc k * logti_ well tw

tout_ well tw

w_ctrl k *log

ti_ ctrl tw_ ctrl

tout_ well tw_ ctrl

un com pe nsa ted fa llin g ra te co m pe n sa ted fallin g rate m ois ture