Simultaneous heat and mass transfer studies in drying ammonium chloride in a batch-fluidized bed dryer

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1 Indian Journal of Chemical Technology Vol. 13, September 006, pp Simultaneous heat and mass transfer studies in drying ammonium chloride in a batch-fluidized bed dryer R Kumaresan a & T Viruthagiri b a Department of Chemical Engineering, Mohamed Sathak Engineering College, Kilakarai , India b Department of Technology, Annamalai University, Annamalai Nagar , India kumariisc@yahoo.co.in; drtvgiri@rediffmail.com Received 3 January 006; revised received May 006; accepted 6 June 006 Simultaneous heat and mass transfer occurs during drying. Fluidized bed drier, unlike other types of dryers is more sensitive to the variation of moisture content of the material being dried. Ammonium chloride is used for this study. Heat and mass transfer studies were carried out using the experimental set up which consists of a well-insulated glass fluidized column with calming section and a fluidized section of height 435 mm and diameter 55 mm. Experiments were carried out with the variables- (i) size of ammonium chloride particles from 495 to 91 microns, (ii) inlet air velocity from to m/s, (iii) temperature of inlet air to the fluidized bed dryer from 60 to 75 C, (iv) initial moisture content of ammonium chloride to the fluidized bed dryer from 0.04 to 0.06 kg of water /kg of bone dry ammonium chloride, (v) bed hold of to kg. Humidity of the inlet air to the fluidized bed is determined by measuring the dry and wet bulb temperatures and using the psychrometric chart. Microsoft Excel spreadsheets are used for the calculations. A correlation between Nusselt number and Reynolds number was developed using the least square method. A simplified unsteady state mass transfer equation was developed to determine the diffusion coefficient. Keywords: Fluidized bed dryer, Ammonium chloride, Simultaneous heat and mass transfer IPC Code: C09K5/00, B01J8/00 Drying is a unit operation carried out in the industries for the purpose of removing the moisture present in the wet solids and to obtain the dry solids that can be handled easily. Fluidized bed dryer is very useful for drying heat sensitive and subliming materials like ammonium chloride. The fluidized bed drier offers the following advantages (i) uniform temperature distribution and a perfect control of drying temperature, (ii) better contact of fluid with the entire surface of the material being dried, (iii) high heat and mass transfer rates, (iv) no moving parts and less energy consumption and (v) less dust emission during the operation. Fluidization technique is getting popular in fluid bed drying. Number of experimental and theoretical correlations are available in the literature for drying materials like sand, alumina, grains and glass 1-4. But heat and mass transfer studies for drying subliming and heat sensitive material like ammonium chloride are not done in advanced level. The present work has been carried out to collect data that will be useful to improve the design of fluidized bed dryer handling very fine powdered and corrosive material such as ammonium chloride. Experimental Procedure The fluidized bed dryer column is made up of a glass pipe with 55 mm internal diameter and 435 mm length. Fluidized bed is well insulated, so that the experimentation is carried out under adiabatic conditions. A calming section is provided at the bottom of the fluidized bed dryer column below the distributor. A stainless steel 00 wire mesh is used as distributor. A centrifugal blower is used for the supply of air. The discharge line of the centrifugal blower is 50 mm diameter PVC pipe and has an orifice meter of 5 mm orifice diameter. Water filled manometer is connected across orifice to measure the pressure drop and hence superficial velocity is determined. An air preheater made up of mild steel is connected with 50 mm diameter mild steel piping on both the ends. An electric heater is fitted in the air preheater to supply heat. A current regulator is connected in the electric circuit of the electric heater, so that heat input to the air preheater is controlled. Air preheater is very well insulated with asbestos ropes and cement. Water filled manometer is fitted below the distributor and the fluidized bed dryer column. This manometer is used to measure the pressure drop across the fluidized bed

2 KUMARESAN & VIRUTHAGIRI: HEAT AND MASS TRANSFER STUDIES IN BATCH-FLUIDIZED BED DRYER 441 dryer. Airflow rates are regulated using the valves provided below the fluidized bed dryer. Two numbers of the thermowell tapping are provided at the discharge of the air preheater to measure the hot air inlet temperature and wet bulb temperature of the inlet hot air into the fluidized bed dryer column. Two more thermowell tapping are provided in the fluidized bed dryer column to measure the outlet air temperature from the column and the surface temperature of the ammonium chloride in the bed respectively. A multipoint digital thermometer is connected with different probes to indicate the temperatures measured at the various points in the experimental set-up. A disengaging section is provided at the top of the fluidized bed dryer column to reduce outlet air velocity and to prevent the carryover of ammonium chloride. Figure 1 gives the experimental set-up for simultaneous heat and mass transfer studies in drying ammonium chloride in a batch-fluidized bed dryer. The minimum fluidization velocity, which is one of the most important parameters in carrying out drying studies, is determined initially by conducting fluidization studies using the undried particles. Wet ammonium chloride is prepared. For example, 5 g of distilled water is thoroughly mixed with 100 g of bone-dry ammonium chloride to prepare 5% initial moisture content of feed on dry basis. Similarly the 4 and 6% initial moisture contents are prepared for different bed hold-up. Wet ammonium chloride is fed to the fluidized bed dryer. Air is heated and passed through the fluidized bed dryer until the steady state is achieved. The flow rate of air is adjusted using a valve provided in the discharge line of the blower and the inlet air temperature is adjusted by controlling heat input using the dimmerstat. The inlet air velocity, inlet air temperature, outlet air temperature, surface air temperature, wet bulb temperature of the inlet air and pressure drop across the bed are measured and the drying time is noted. Dried material is taken out from the fluidized bed dryer and the final moisture content is found out by weight loss method. The inlet air humidity is determined from the psychrometric chart and the saturated humidity is obtained from the table corresponding to the surface temperature of solid. The experiment is repeated for various other bed holdup, superficial air velocity, moisture content in the feed, particle size and inlet air temperature. Fig. 1 Schematic diagram of the experimental set-up Results and Discussion Modeling A simplified model for drying solids at the constant rate of drying period in batch-fluidized bed was developed. The drying rate in the fluidized bed is strongly influenced by the characteristics of the material and the conditions of fluidization. Nonporous materials will dry essentially at constant rate, while those with the porous structure will have both constant and falling rates. The drying rate in the falling-rate period is generally modeled using the mechanism of moisture movement by diffusion according to Fick s law equation. The current study focuses on developing a simplified model for predicting the drying rate in the constant-rate period. It is assumed that heat and mass transfer occurs in the dense phase. It is also assumed that all the particles are homogeneous in character, spherical in shape and uniform in size during drying. All the particles within the bed are at the same temperature and hence the moisture content is the same at any time. The drying medium leaving the fluidized bed is in thermal equilibrium with the particles. The resistance to drying is only from the gas phase surrounding the solids and also it is negligible. Again it may be considered that no energy is gained or lost in the dryer so that the process is adiabatic. Plug flow of air is the same from the bottom to the top and complete mixing of air and solid takes place. The exchange of heat between the particle and air is complete. Heat balance Heat lost by hot air = (Sensible heat gained by solid) + (Latent heat of vaporization)

3 44 INDIAN J. CHEM. TECHNOL., SEPTEMBER 006 [(Mass flow rate of hot air)(specific heat of air)(temperature difference)(time for drying)] = [(Mass of dry solid)(specific heat of solid)(temperature rise) + (Mass of water evaporated)(latent heat of vaporization)] (A t U o ρ g )(C pg )(T gi T s )dt=wc ps (T s T ins )+WλdX (1) on integration with boundary condition. t = o at X=X 1 (initial moisture content of the solid) t = t at X = X ( moisture content at any time) t x (A t U o ρ g )(C pg )(T gi T s ) dt=wc ps (T s T ins )+Wλ dx () 0 x 1 taking into consideration, the void fraction and bed height, the above equation can be written in the form of, t x (A t ρ g U o )(Cp g )(T gi T s ) dt= A t ρ s L m (1 ε m )λ dx (3) 0 x 1 where ξ mf =1 {(W/ρ s )/(A t L m )} (4) Initial and final moisture contents are known from the experiment and the rate of drying can be determined. Since, the solids are small and contain free moisture the solids will dry at constant drying rate. Hence, the equilibrium is attained very quickly due to efficient heat transfer. Critical moisture content is close to equilibrium moisture content and we can treat the entire drying process as occurring at constant drying rate. {W [C ps (T s -T ins )+ λ(x 1 -X )]} t= (5) {(A t U o ρ g )(C pg ) (T gi -T s )} W(X 1 -X ) constant rate of drying N c = (6) A s t t is the time at which X=X c A s heat transfer surface area of the n number of particles = n(π d p ) (7) volume of the solids n= volume of one particle n= (W/ρ s )/(π/6d p 3 ) (8) Heat transfer co-efficient (hp) Heat transfer to the solids = h p A s ( T gi T s ) (9) under constant drying rate W(X 1 -X ) N c = (10) A s t N c λ = h p (T gi T s ) (11) W(X 1 -X ) λ h p = (1) A s t ( T gi T s ) Nusselt number (Nu p ) h p d p Nu p = (13) K Reynolds number (Re p ) d p U o ρ g Re p = (14) μ g Mass balance Rate of moisture evaporated = rate of moisture picked up in the hot air N c = k y (Y s - Y) (15) W(X 1 X ) = k y (Y s Y) (16) A s t Mass transfer coefficient (k y ) W(X 1 X ) k y = (17) A s t (Y s -Y) unsteady state mass transfer taking place in a spherical particle can be described with the following differential equation,

4 KUMARESAN & VIRUTHAGIRI: HEAT AND MASS TRANSFER STUDIES IN BATCH-FLUIDIZED BED DRYER 443 X = D[ X/dr +/r X/ r] (18) t where X is the moisture content at any position, r. The boundary conditions are: t=0 r = r X=X 1 X t=t r = 0 = 0 r t=t r =R X=X* after solving the above partial differential equation, one gets the concentration profile as (X X * )/(X 1 X * )=6/π Σ 1/n exp (nπ )(Dt/R ) (19) n=1 where R is the radius of spherical particle and X* is the equilibrium moisture content of particle and is approximately equal to zero as the equilibriums is achieved very quickly, during drying in the fluidized bed. Hence, above equation is reduced to, (X )/(X 1 ) = 6/π Σ 1/n exp (nπ )(Dt/R ) (0) n=1 when n=1 (X )/(X 1 ) =6/π (exp (π )(Dt/R )) (X )/(X 1 ) = exp (9.86Dt/R ) X /X 1 = exp (9.86Dt/R ) ln(1.6449x /X 1 )= 9.86Dt/R D= R /t ln(1.6449x /X 1 ) (1) where t is time in seconds for the drying in the fluidized bed dryer, R is the radius of the particle in meter, D is the diffusion coefficient in m /s Model calculations Particle size = m U 0 = Inlet air velocity = m/s T g i = Inlet air temperature = 61 C W = Weight of the dry ammonium chloride = kg X 1 = Initial moisture content in the feed = 0.05 kg of water/kg of dry ammonium chloride L m = Initial bed height = m ξ mf = 1 {(W/ρ s )/(A t L m )} ρ s = density of ammonium chloride solid = 15 kg/m 3 d t = diameter of fluidized bed dryer = 55 mm = 1 π 55 * * = ξ m U mf - minimum fluidization velocity m/s U mf = ( d Q ) p S 180 Q s - sphericity=0.6 ρs ρ f μ f 3 mf ξ g 1 ξ mf ( ) ( ) * (0.538) 3 U mf = (9.81) ( ) = 0.15 m/s T s = surface temperature = 43.5 C T gi = inlet air temperature = 61 C Q in rate of heat input =(A t U 0 ρ g ) C pg (T gi T s ) Q in = π * = Kw (1.136)(1.083)(1.005)( ) total heat input = heat utilized for water evaporation (A t U 0 ρ) C pg (T gi- T s ) t= W (X 1 -X ) λ s Q in t = W (X 1 -X ) λ s W (X 1 X ) λ s t c = Q in t c time for drying. (0.1) ( ) (39) t c = =8 s

5 444 INDIAN J. CHEM. TECHNOL., SEPTEMBER 006 A s surface area of heat transfer A s = n π d p n = number of particle volume of particle n = volume of one particle A s = (6)(0.1)/(15)( ) = m N c rate of drying Y-inlet air humidity from psychrometric chart at the wet bulb temperature 8.5 C Y = kg of water/kg of dry air k y mass transfer co-efficient N c k y = Y s Y = ( ) W (X 1 -X ) N c = A s t c (0.1) ( ) = (0.7964) (8) = kg/m s h p heat transfer co-efficient N c λ s h p = (T gi T s ) ( )(39)(1000) = ( ) = w/ m C Y s saturated humidity the surface temperature 43.5 C 0.6 (vapour pressure) Y s = (Total pressure-vapour pressure) (0.6)(8.867) = ( ) = kg of water / kg of dry air Pressure is in kpa = kg/m s Δy R ep Reynolds number based on particle size R ep = d p U 0 ρ g /μ ( ) (1.136)(1.083) = = 30.8 Nu p Nusselt number based on particle size h p d p Nu p = K (3.686) ( ) = = Table 1 gives the typical values of mass transfer coefficient and Table gives the typical values of heat transfer coefficient. D-diffusion co-efficient m /s D= R /t c ln ( X /X 1 ) R-radius of the particle m, t c - drying time s D= /8( ) ln( /0.05) = m / s

6 KUMARESAN & VIRUTHAGIRI: HEAT AND MASS TRANSFER STUDIES IN BATCH-FLUIDIZED BED DRYER 445 Table 1 Mass transfer co-efficient Superficial velocity U o = m/s Initial moisture content = 0.06 kg of water/kg of dry ammonium chloride. Hold up in the bed (g) particle size (microns) Mass transfer coefficient (kg water evaporated/m sδy10-4 ) Table Heat transfer co-efficient Superficial velocity U o = m/s Initial moisture content X 1 = 0.05 kg of water/kg of dry ammonium chloride. Hold up in the bed (g) particle size (microns) Heat transfer coefficient (w/m C) Correlation Nusselt number was correlated with the particle Reynolds number using the least squares method. Heat transfer co-efficient was calculated using the relationship W(X 1 -X ) λ h p = A s t(t gi T s ) drying time in the constant drying rate period was calculated using the experimental data Wλ (X 1 -X ) t c = (A t U o ρ g )(C pg )(T gi T s ) Nusselt number was calculated using relationship h p d p Nu p = K Reynolds number was calculated using the relationship d p U o ρ g Re p = μ g heat transfer data is expected to follow a functional form of Nu p =a(re p ) b and the best value for the constants a and b is obtained by the method of least squares. Nu p = a (Re p ) b () taking log on both sides log Nu p =loga +b logre p (3) this is a linear relation of the form Y=A+BX (4) where Y = lognu p X = logre p A = loga B = b By method of least squares, the values of A and B are given as 1 A = ( Y i B X i ) (5) m where m - number of readings X i Y i m X i Y i B = (6) ( X i ) m X i

7 446 INDIAN J. CHEM. TECHNOL., SEPTEMBER 006 a total of 05 experiments were carried out and using the values, A and B were determined. ( ) (05) ( 301.9) B = (344.8) (05) (58.) = A = 1/05 { 18.3 (1.997)344.8} A= 4.48 a=log -1 ( 4.48)= b=b=1.997 Nu p = (Re p ) The above correlation was obtained when 30 < Re p < 70 and < Nu p < Drying of ammonium chloride in the fluidized bed for the particles size from 495 to 915 microns with initial and final moisture content at X 1 =0.06 and X =0.001 kg of moisture/kg of dry ammonium chloride, the following correlation was obtained Nu p = (Re p ) (7) Conclusion The conclusions drawn from the above studies are: (i) Drying period of material in a batch fluidized bed dryer a. Decreases with decreasing of particle size. b. Increases with increasing of hold up of material in the bed. c. Increases with increasing of initial moisture content of the feed material d. Decreases with increasing of air flow rate. (ii) Rate of drying of material in the fluidized bed dryer a. Decreases with decreasing of particle size. b. Increases with increasing of air flow rate. c. Decreases with increasing of hold up of material in the bed. d. Decreases with increasing of initial moisture (iii) Heat transfer co-efficient of the material in the fluidized bed dryer a. Decreases with decreasing of particle size. b. Decreases with increasing of hold up of material. c. Increases with increasing of air flow rate. d. Decreases with increasing of initial moisture (iv) Mass transfer co-efficient of the material in the fluidized bed dryer a. Increases with increasing of air flow rate. b. Decreases with increasing of initial moisture c. Decreases with decreasing of particle size. (v) Pressure drop across the material in the fluidized bed dryer a. Increases with increasing of flow rate. b. Increases with increasing of hold up of material. c. Increases with decreasing of particle size. d. Increases with increasing of moisture (vi) Initial bed height of the material in the fluidized bed dryer a. Increases with increasing of hold up of material. b. Decreases with decreasing of particle size. c. Decreases with increasing of moisture Nomenclature d t = diameter of the column (m) d p = aver diameter of the particle (m) W = weight of the dry solid (kg) L m = height of solid in the column (m) A s = surface area of the solids (m ) n = number of particles N c = constant rate of evaporation (kg/m s) λ s = latent heat of vaporization at T s (kj/k g ) T s = surface temperature ( o C) T gi = inlet air-dry bulb temperature ( o C) T inw = inlet air wet bulb temperature ( o C) T out = outlet air-dry bulb temperature ( o C) h p = heat transfer co-efficient (w/m o C) k y = mass transfer co-efficient (kg/m s(δy) U mf = minimum fluidization velocity (m/s) U o = superficial air velocity (m/s) ρ s = density of solid ( kg/m 3 ) ρ g = density of air (kg/m 3 )

8 KUMARESAN & VIRUTHAGIRI: HEAT AND MASS TRANSFER STUDIES IN BATCH-FLUIDIZED BED DRYER 447 μ = absolute viscosity of air (kg/m s) ξ mf = void fraction at the initial condition X 1 = initial moisture content kg of water/kg of dry solid X = final moisture content kg of water/kg of dry solid Y = humidity of air at inlet kg of water vapour/kg of dry air Y S = humidity of air at surface temperature kg of water vapour/kg of dry air t = time taken (s) D = diffusivity (m /s) K = thermal conductivity of air (w/m o C) References 1 Alvarez P, Drying Technol, 14(3&4) (1996) 701. Norio, J Chem Eng Jpn, 13() (1980) Abid M, Int J Chem Eng, 30(4) (1990) Peishi Chen, Int J Heat Mass Transf, 3() (1984) Kumaresan R & Viruthagiri T, Chem Eng World, 37(3) (00) Kumaresan R & Viruthagiri T, Proc Plant Eng, 1(3) (003) 7. 7 Ibrahim S H, Drying AIChe Modular Instruction, (1983)

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