Drying

Drying  CHE133

Classi Cla ssific ficati ation on of Drying  •

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1. Batch Batch drying- mate material rial is inserted inserted into into a drying equipment and stay their for a period of time until desired drying dr ying is achieved.

2. Continuous drying-material is continuously added and removed from the dryer dr yer..

Meth Me thod odss of Dryin Drying  g  •

1. Air drying

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2. Vacuum drying

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3. Freeze drying

 Vacuum Drying  •

Moisture evaporate faster at low pressures.

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Heat is added indirectly by heated walls.

Freeze Drying 

Equipment for Drying  •

1. Tray dryers

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2. Vacuum Shelf Dryers

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3. Continuous tunnel dryer

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4. Spray dryers

 Tray Dryers

 Tray Dryers Materials are spread in trays from 10 mm to 100 mm thickness. Steam heated air is recirculated thru the material by a fan. Generally only about 10 to 20% of the air is fresh(not recirculated).  After drying the trays are replaced with new trays containing new materials.

 Vacuum Shelf Dryers •

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Made of cast iron cabinets  with tightly fitted doors so that it can be vacuum.

Generally hot water is used to heat the walls of the cabinets.

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 Vacuum Shelf Dryers Generally used for expensive products or heat sensitive products.

Continuous tunnel dryer

Continuous tunnel dryer

Length of a Drying Conveyor Calculation •

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One thousand kilogram per hour of a material with 30% moisture content (d.b.) is dried in a belt conveyor drier with a capacity of 50 kg of bone dry solid per meter of drier. The total drying time is 5.3 hours. Estimate the length of a the drying conveyor.  A. 74.2 m

B. 81.5 m C. 106 m

D. 98 m

Spray Dryers

Spray Dryers Dried milk powder is generally formed using spray drying.

Spray dryer •

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Coffee extract containing 70% water is to be spray dried using air at 420 K in a 6-m diameter by 6-m high chamber with centrifugal disc atomizer. The capacity of the spray drier is _____ lb/hr of feed, if the final product will have a moisture content of 1%(w.b.) see table 12-36, Perry, 7 th ed. Note : this reference gives an evaporation rate of 3.8 kg/s.  A. 20500

B. 35000 C. 40500

D. 43200

Drying Equipment •

5. Rotary dryers

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6. Rotary Drum dryers

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7. Grain dryers

Rotary dryers

Rotary Drum dryers

Grain dryers

Drying Curve

Drying Rate Curve

Drying Equation 1 •

2 ) R is rate of drying. (kg water/hr.m Type equation here.

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Q is weight of bone dry solid. (kg)

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 A is drying area,m2

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 W is total moisture content dry basis (db) (kg water/kg dry solid)

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θ

is drying time in hours

Drying Dr ying Equation Equation 2 •

 W = X + X*

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 W is total moisture moisture content content dry basis (d.b.) (kg water/kg dry solid)

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X is moisture content. (kg water/kg dry solid)

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X* is equilibrium moisture content

Drying Equation 3: Constant Rate of Dry Dryin ing  g 

Sample problem •

 The critical moisture content of banana is 2%. It took 4 hours to dry dr y from 20% to 10%. How long will it take to dry from 20% to 5%?

Drying Equation 3: Falling Rate Period

sample problem 1 •

 A wet material having a critical moisture content of 15% (d.b.) and an equilibrium moisture content of 3% (d.b.) took 6 hours to dry from 45% (d.b.) to 10%(d.b.). The time it will take to dry to 15%(d.b.) is

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 A. 4.9 hours

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B. 3.9 hours

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C. 5.5 hours

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D. none of these

Sample Problem 2 •

 The rate of drying at constant rate period is 20 lbs/hr . 400 lbs of dry solid containing 300lbs water enter the drier. If the critical free moisture is 0.5(d.b.) and the equilibrium moisture content is 0.05 (d.b.), the drying time to dry the material to 5% moisture(w.b.) is

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 A. 67.5 hours

B. 47.5 hours

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C. 56.5 hours

D. 76.5 hours

Seat work  •

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 Wet solid are to be dried from 40% to 10% in 5 hours under constant drying conditions. The critical moisture content is 20% and the equilibrium moisture content is 7% . All moisture contents are on dry basis. The time needed to dry from 15% to 5% free moisture under the same drying condition is __.  A. 3.42 hours

B. 1.846 hours

C. 0.542 hours

D. 2.129 hours

Sample Problem 3 •

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Given a solid material containing 120% moisture on a dry basis, the equivalent moisture content on a wet basis in percent moisture is  A. 20

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B. 54.5

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C. 12

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D. none of these

Seat work  •

 The amount of water to be removed from 2000 kg/hr of feed to be dried from 110% (d.b.) to 5% (w.b.) is____ kg/hr.

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 A. 998

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B. 948

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C. 451

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D. 910

Sample Problem 1

Sample problem 4

Seat work 1

Seat work 3

Figure 9.5-1b

Exercise 9.7.2

The bone-dry sample weight was 3.765 kg dry solid. The top exposed surface has an area of 0.186 m2. The equilibrium moisture content is 0.190 kg. The following sample weight was obtained in the drying test. Time, h

0.8

Weight, kg

4.808

1.4

2.2

3.0

4.2

4.699 4.554 4.404 4.241

a. Calculate the free moisture content in kg water/kg dry solid for  each time t.  b. Calculate the slopes (Hint: i.e. change in free moisture content over time. There are 4 slopes). c. Calculate the rates of drying in kg water/h.m2. d. Determine the critical moisture content. e. Estimate the time it will take to dry from 0.10 kg water/kg dry solid to 0.04 kg water/kg dry solid.

Drying/ChE Calc  What will be the volumetric flow rate (in ft3/min) of entering air required to remove 10 lbs of water per hour from rayon, if the air enters at 80°F and 25% RH and leaves at 170°F and 55% RH?  The operating pressure is 14.3 psia.  A. 12

B. 8.6

C. 7.12

D. 13.0

1st order drying equation  The drying rate is proportional to the moisture content. If half of the moisture content of a material is dried in 2 hours, then three fourths  will be dried in 4 hours.  A. true

B. false

Continuous dryer

 Water balance

Enthalpy balance

Calculation of enthalpies

Cs = 0.24 + 0.45H (in BTU/lbm.R) λ=2501KJ/KG

Continuous dryer

Example 9.10-2

Seat work   A continuous countercurrent dryer is being used to dry 453.6 kg dry solid/ h containing 0.04 kg total moisture/kg dry solid to a value of 0.002 kg total moisture/kg dry solid. The granular solid enters at 26.7°C and is to be discharged at 62.8°C. The dry solid has a heat capacity of 1.465 kJ/kg.K, which is assumed constant. Heating air enters at 93.3°C, having a humidity of 0.010 kg water /kg dry air and is to leave at 37.8°C. Calculate the air flow rate if the heat losses from the dryer is estimated as 2931 W.

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Exercise 2

 The bone-dry sample weight was 3.765 kg dry solid. The top exposed surface has an area of 0.186 m2. The equilibrium moisture content is 0.190 kg. The following sample weight was obtained in the drying test.  Time, h  Weight, kg

0.8 4.808

1.4

2.2

3.0

4.2

4.699 4.554 4.404 4.241

a. Calculate the free moisture content in kg water/kg dry solid for each time t. b. Calculate the slopes (Hint: i.e. change in free moisture content over time.  There are 4 slopes). c. Calculate the rates of drying in kg water/h.m2. d. Determine the critical moisture content. e. Estimate the time it will take to dry from 0.20 kg water/kg dry solid to 0.04 kg water/kg dry solid.

sterilization

No is the original number of bacteria. N is the number of bacteria after time t of sterilization.

Decimal reduction time, D

 Arrhenius equation

 Thermal death time, 12D

sterilization

Derivation of Rc equation

Derivation of Rc equation

 The bone-dry sample weight was 3.765 kg dry solid. The top exposed surface has an area of 0.186 m2. The equilibrium moisture content is 0.190 kg. The following sample weight was obtained in the drying test. 20 pts  Time, h

0.8

 Weight, kg

4.808

1.4

2.2

4.699

4.554

3.0

4.2

4.404 4.241

a. Calculate the free moisture content in kg water/kg dry solid for each time t. b. Calculate the slopes (Hint: i.e. change in free moisture content over time. There are 4 slopes). c. Calculate the rates of drying in kg water/h.m2. d. Determine the critical moisture content. e. Estimate the time it will take to dry from 0.20 kg water/kg dry solid to 0.04 kg water/kg dry solid.

Heat of conduction

Heat of convection

Heat of radiation

Rate of drying, R  =

  

 The temperature of the sample surface, Ts is 32.2C and hc is 62.45 W/m2 K.

Fick’s 2nd

law of diffusion

Diffusion control drying 

Diffusion control drying 

Diffusion control drying