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DRYING

Learn about various drying methods and the concept of equilibrium moisture content for effective removal of water and organic liquids. Explore batch and continuous drying processes, as well as direct contact, vacuum, and freeze drying techniques.

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DRYING

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  1. DRYING • removal of relatively small amount of water or organic liquids • final processing step before packaging • as a preservative technique esp. food • freeze-dried for biological & pharmaceutical materials 2 methods of drying: 1. batch - material put into dryer & drying proceeds for a given period of time 2. continuous - material continously added to dryer & continously removed 3 catagories of drying: 1. Direct contact with heated air at atmospheric pressure 2. Vacuum drying - heated indirectly either by contact with a metal wall or by radiation 3. freeze drying - water is sublimed from the frozen material

  2. EQUILIBRIUM MOISTURE CONTENT, X* • lowest moisture content obtainable at equilibrium • on dry basis (kg of water/ kg of moisture-free solid) • depends on structure of solid, temperature & moisture content of gas • varies greatly with type of material for any given % relative humidity • decreases with increase in temperature • assumed constant for moderate temperature ranges

  3. EQUILIBRIUM MOISTURE CONTENT, X* • bound water - the minimum moisture a material can carry - intersection of 100% humidity line in equillibrium water content vs relative humidity • Unbound water = excess water held primarily in the voids of the solid • free moisture content, X - moisture above the equilibrium moisture content • - can be removed by drying Moisture amount that can be release/free during drying X = Xt - X* where Xt = total moisture content X* = equilibrium-moisture content – cannot be predicted

  4. Drying Curve

  5. Drying Curve

  6. total moisture content , Xt = RATE OF DRYING CURVES • batch drying • experimental determination data : WS = weight of dry solid W = total weight of wet solid vs time t To obtain as free moisture content X vs time t: free moisture content, X = Xt - X* Measure sample every minutes

  7. RATE OF DRYING CURVES • batch drying • experimental determination To obtain as rate of drying, R : Get slopes of tangents at different values of t : where A = exposed surface area for drying

  8. RATE OF DRYING CURVES X = (0.35 + 0.325)/2 = 0.338 R = -21.5 (-0.07) = 1.493

  9. RATE OF DRYING CURVES Point AB : Warming up (unsteady) period where the solid surface conditions come into equilibrium with the drying air. Point A’ : hot solid Point B-C: constant-rate drying period in which surface of the solid remains saturated with liquid because the movement of water vapour to the surface equals the evaporation rate.  Thus the drying rate depends on the rate of heat transfer to the drying surface and temperature remains constant. Surface temperature  TW Point C : critical free moisture content, XC , where the drying rate starts falling and surface temperature rises. Insufficient water on surface

  10. RATE OF DRYING CURVES Point C-D : first falling-rate drying period which surface is drying out. Rate of water to surface is less that rate of evaporation from surface Point D : surface completely dry Point D-E : second falling-rate period in which evaporation is from inside of solid. Point E : equilibrium moisture content, X*, where no further drying occur

  11. CONSTANT RATE OF DRYING PERIOD To determine the time required for drying from X1 to X2: Experimental drying curve Predicted mass-and-heat coefficients Experimental drying curve: Under similar conditions to actual process Drying curve X vs. t Rate-of-drying curve R vs. X where RC = constant rate of drying WS = kg of dry solid used A = exposed surface area for drying

  12. FALLING-RATE OF DRYING PERIOD To determine the time required for drying from X1 to X2: 1. Graphical integration Most accurate

  13. FALLING-RATE OF DRYING PERIOD To determine the time required for drying from X1 to X2: 2. Special cases a) Rate is linear function of X

  14. FALLING-RATE OF DRYING PERIOD To determine the time required for drying from X1 to X2: 2. Special cases b) Rate is a linear function thru’ origin (a straight line from C to E at the origin) or and

  15. EXAMPLE 9.7-1

  16. EXAMPLE 9.7-2 Repeat Example 9.7-1, but as an approximation assume a straight line for the rate R versus X through the origin from point Xc to X = 0 for the falling-rate period.

  17. Drying Curve

  18. Humidity, H - kg of water vapour in 1 kg of dry air • Saturation humidity, HS • Percentage humidity, HP • Percentage relative humidity, HR ( HR HP) HUMIDITY & HUMIDITY CHART where pA = partial pressure of water vapour in air pAS = saturated partial pressure of water vapour in air H = humidity of air HS = humidity of saturated air • Dew point - temp. at which a mixture of air-water would be saturated

  19. HUMIDITY CHART

  20. HUMIDITY & HUMIDITY CHART • Humid heat, cS - amount of heat required to raise the temp. of 1 kg dry air plus water vapour present by 1K cS (kJ/kg dry air.K) = 1.005 + 1.88H • Humid volume, H - total volume of 1 kg dry air plus water vapour present at 1 atm & given gas temperature H (m3/kg dry air) = (2.83 x 10-3 + 4.56 x10-3H)T T is in Kelvin • H –Humidity found from chart • Total enthalpy of 1 kg of air plus its water vapour, Hy Hy (kJ/kg dry air) = (1.005 + 1.88H)(ToC- 0) + 2501.4H

  21. wet cloth/wick Air flow DRY & WET BULB TEMPERATURE Wet-bulb temperature : decreases in temperature below the dry-bulb temperature until the rate of heat transfer from the warmer air to the wick is just equal to the rate of heat transfer needed to provide for the evaporation of water from the wick into the air stream. Dry bulb temperature: the ordinary temperature you measure with a thermometer

  22. Humidity HUMIDITY CHART Wet bulb temp. =20oC, dry bulb temp. = 30oC,humidity = ? 0.0115

  23. Air: T2, H2 Air: T1, H1 Humidity HUMIDITY CHART Importance of pyschrometric analysis for drying Assumption: evaporation surface is a liquid film H2 Constant wet-bulb temperature process H1 T2 T1

  24. CONSTANT RATE OF DRYING PERIOD To determine the time required for drying from X1 to X2: Predicted mass-and-heat coefficients: Mass transfer of water vapour Heat transfer furnishes the latent heat of evaporation Steady-state : rate of mass transfer = rate of heat transfer Assumptions: Only convective heat transfer to solid surface from hot gas to surface Mass transfer is from surface to hot gas

  25. CONSTANT RATE OF DRYING PERIOD Rate of drying, RC: where A = exposed drying area (m2) T, TW = temp. of gas & surface of solid, respectively (oC) W = latent heat at TW (J/kg) MA,MB = molecular weight of water & air, respectively h = heat-transfer coefficient (W/m2.K)

  26. CONSTANT RATE OF DRYING PERIOD Air flowing parallel to the drying surface (T = 45-150oC, G = 2450 -29300 kg/h.m2,  = 0.61-7.6 m/s) h = 0.0204G0.8 Air flowing perpendicular to the drying surface (G = 3900 -19500 kg/h.m2,  = 0.9-4.6 m/s) h = 1.17G0.37 where G = mass velocity = To determine the time required for drying from X1 to X2:

  27. Example 3 • A granular insoluble solid material wet with water is being dried in the constant-rate period in a pan 0.61m x 0.61m and the depth of material is 25.4 mm. The sides and bottom are insulated. Air flows parallel to the top drying surface at a velocity of 3.05 m/s and has a dry bulb temperature of 60oC and a wet bulb temperature of 29.4oC. The pan contains 11.34 kg of dry solid having a free moisture content of 0.35 kg H2O/kg dry solid, and the material is to be dried in the constant-rate period to 0.22 kg H2O/kg dry solid. a) Predict the drying rate period and the time in hours needed. b) Predict the time needed if the depth of material is increased to 44.5 mm

  28. Example 4 • An insoluble wet granular material is dried in a pan 0.457 x 0.457 m (1.5 x 1.5 ft) and 25.4 mm deep. The material is 25.4 mm deep in the pan, and the sides and bottom can be considered to be insulated. Heat transfer is by convection from an air stream flowing parallel to the surface at a velocity of 6.1 m/s (20 ft/s). The air is at 65.6oC (150oF) and has a humidity of 0.010 kg H2O/kg dry air. Estimate the rate of drying for the constant-rate period using SI and English units.

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  32. Karnival Kerjaya 2014 pada 6-7 Mei 2014 Bersempena dengan program ini, UTM dengan Talent Corp Malaysia Berhad akan menganjurkan Internship Market Day pada 6 Mei 2014 di DSI, UTM. Tujuan program ini diadakan untuk membantu pelajar UTM yang masih belum mendapat tempat untuk menjalani latihan industri.

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