Iranica Journal of Energy & Environment 4 (4): 391-397, 2013ISSN 2079-2115 IJEE an Official Peer Reviewed Journal of Babol Noshirvani University of TechnologyDOI: 10.5829/idosi.ijee.2013.04.04.11

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Corresponding Author: Mostafa Keshavarz Moraveji, Department of Chemical Engineering, Faculty of Engineering,Arak University, Arak 38156-8-8349, Iran. Tel.: +98-9363098063Fax: +98-861-4173450,E-mail: m-moraveji@araku.ac.ir

391

Investigation of Some Effective Parameters on the Fluidized Bed Grain Dryers

Mostafa Keshavarz Moraveji, Reza Davarnejad and MarziehFarjami

Department of Chemical Engineering, Faculty of Engineering,Arak University, Arak 38156-8-8349, Iran

(Received: April 24 2013; Accepted in Revised Form: Nov. 13, 2013)Abstract: Having accurate values of the parameters which mainly govern the industrial processes in hand isthe key aspect to handle the process and optimize it. To deal with this challenge, some mathematical modelshave been developed and modified to be epidemic. This Study aims to present the simulation of gas-solid flowin fluidized bed dryer. Numerical solution of two-dimensional, axis-symmetrical cylindrical model for bothphases is the very base on which this study is conducted. Some of the versatile parameters like inlet gasvelocity and temperature, diameter and density of particles during drying process went under magnification andalso challenges of heat transfer along the bed are investigated and discussed in detail. Solid temperaturesin the center and on the surface which are greatly affected by time as well as results of the study are shown.At the end, model outputs are compared with experimental data which shows reasonable agreement and goodmatch.

Key words: Gas-solid flow Fluidized bed Grain dryer Heat transfer Properties of wheat

INTRODUCTION industrial dryers, drying is a process which requires high

Drying is the key process in majority of industries reducing the cost of process [2-4].which deals with separation and chemical processes as Mathematical modeling of fluidized bed drying iswell as those industries with solid products. Actually, the crucial for optimizing the performance of existing systemsaim of drying process is to eliminate a liquid using a dry and designing new dryers. It is highly recommendedgas. The most troublemaker liquid is often water while the that modeling be done in 2 different models parallel togas is dry air. Also air drying is the most common each other. The first model (CFD) is based on a continuumapplicable commercial technique to do drying of food and assumption of phases and the second one is anchemical products. Main parameters which govern drying engineering model such as two-phase or three-phaseefficiency are reported as drying air temperature, its models. The second model relies on a bubble phaserelative velocity and product initial moisture content [1]. without solid and a dense phase consisting of gas and

Fluidized beds are currently used commercially to solid particles [5, 6].dry granular materials, cereals, polymers, chemicals, Palancz [7] has presented a model to describe thepharmaceuticals, fertilizers, crystalline products and continuous drying process of particles based on theminerals. Drying process of solids usually includes two two-phase model, which is followed and modified by Laiseparate stages; namely, constant and falling rate periods. and Chen [8]. Hajidavalloo and Hamdullahpur [9, 10] alsoMost food materials have short constant rate periods and have proposed a mathematical model of simultaneous heatthey may be dried entirely in the falling rate period. In the and mass transfer in fluidized bed drying of large particles.gas-solid fluidization, the solid phase is assumed to be in Afterward, Urgschweiger and Tsotsas [11] performedthe “fluid like” state. Because of the latent heat of water some experiments in order to model continuous fluidizedevaporation and relatively low energy efficiency of bed drying under steady state and dynamic conditions.

input energy. Input energy plays a significant role in

2( ) (r ) ( ) ( ) arrs s s s s s s s s s s r s g su u v u u mu

t r r z r r∂ 1 ∂ ∂ ∂ ∂

+ + = − + − − −∂ ∂ ∂ ∂ ∂

2( ) (r ) ( ) ( ) g - arrs s s s s s s s s s s z s g s s sv u v v v v mu

t r r z z z∂ 1 ∂ ∂ ∂ ∂

+ + = − + − − −∂ ∂ ∂ ∂ ∂

2 | |150 1.75 for <0.8

( )g g g s g g

zg p s p s

s u v vs d d

−= +

∅ ∅

2.65| |3 for 0.84

g s g g sz Dz g

p s

v v s sC

d−−

= ≥∅

0.68724 (1 0.15 ) 1000

0.44 1000

Dz sz szsz

Dz sz

C Re ReRe

C Re

= + <

= ≥

( )s s g s psz

g

v v dRe

−=

Iranica J. Energy & Environ., 4(4): 391-397, 2013

392

Additionally, Assari et al. [12] have presented a model to viscosity is 2.173×10 at 100°C in blown to the fluidizedsimulate the drying process for two-dimensional bed. Wheat density is 1200 kg/m and its heat capacity iscylindrical column based on mass, energy and momentum assumed to be 1260kJ / kg °C.conservations. This study reveals the simulation resultsof a fluidized bed dryer. This dryer was used to dry the Analysis and Modeling of Fluidized Drying Process:spherical grains along the column. Fundamental theory of formulation of two-phase flow is

MATERIALS AND METHODS references [8-12]. Two sets of conservation equations are

Material: In this study, wheat is introduced as a solid [8, 11, 12], background theorem for the work iswith diameter of 3 mm and inlet temperature of 25°C to the considering two different fluids (wet solid particles andfluidized bed and the drying operation are carried out gas) for each phase. Equations of mass, momentum,under the various conditions. Air, the common dry gas, is thermal energy and continuity for each phase are the mainused here with temperatures of 70 and 100°C and issues which are presented here in detail.

5

3

discussed in detail in general [6-7] and specified

used for gas-solid models. By means of previous attempts

Equation of motion for solid, momentum equation in r-direction:

(1)

Solid momentum equation in z-direction:

(2)

In the above equations, right-hand terms are pressure gradient, drag, normal solid stresses and the sourcemomentum due to the solid evaporation, respectively. In the equations of motion, and are drag coefficients betweenr z

the gas and solid particles (drag coefficients are used from the literature [13]).

(3)

(4)

+ = 1.0 (5)s g

C is the drag coefficient in z-direction [6]:Dz

(6)

and

(7)

2( ) (r ) ( ) ( ) ag g g g g g g g g g g r g s gu u u v u u mut r r z r∂ 1 ∂ ∂ ∂

+ + = − + − +∂ ∂ ∂ ∂

2( ) (r ) ( ) ( ) g - ag g g g g g g g g g g z g s g g gv u v v v v mut r r z z∂ 1 ∂ ∂ ∂

+ + = − + − −∂ ∂ ∂ ∂

( ) (r ) ( ) - as s s s s s s s s s sx u x x v mt r r z∂ 1 ∂ ∂

+ + =∂ ∂ ∂

( ) (r ) ( ) - ag g g g g g g g g g gx u x x v mt r r z∂ 1 ∂ ∂

+ + =∂ ∂ ∂

m

1( ) (r ) ( ) ( ) ( ) - a ( )s ss s s s s s s s s s s s s s wg g

T TI u I v rk k m c Tt r r z r r z z z

∂ ∂∂ 1 ∂ ∂ ∂ ∂+ + = + + +

∂ ∂ ∂ ∂ ∂ ∂ ∂

1( ) (r ) ( ) ( ) ( ) - a ( ) ( )g gg g g g g g g g g g g g g g g wg g v s g

T TI u I I v rk k m c T ah T T

t r r z r r r z z∂ ∂∂ 1 ∂ ∂ ∂ ∂

+ + = + + + −∂ ∂ ∂ ∂ ∂ ∂ ∂

0.44

0.78

1.77 if Re 30

5.70 if Re<30ic

ic

In Re

In Re

−

−

= ≥

=

g g pic

s g

u dRe

s u=

Iranica J. Energy & Environ., 4(4): 391-397, 2013

393

The expression for the friction coefficient in the radial direction is the same as that of the axial direction.Equation of motion for gas in r-direction:

(8)

Gas momentum equation in z-direction:

(9)

On the left, the first and second terms represent rate of accumulation and net rate of outflow across the closedsurface, respectively. The right-hand terms are considered the pressure gradient, drag, gravitational force and the sourcemomentum term due to the evaporation of wet solids, respectively. Friction force due to shear stresses is neglected here.Continuity equations for solid phase:

(10)

Continuity equation for gas phase:

(11)

stands for the moisture evaporation from the particle surface.

Thermal energy equations for the solid phase:

(12)

For gas phase:

(13)

Transfer coefficient for convective heat transfer between The energy due to conduction is laid on two terms onsolid and gas is given by: the right side of equations 12 and 13 and the third one is

h = c u J pr (14) exchange due to convention.v g g g h2/3

boundary conditions because they cannot be evaluated(15) with analytical analysis [12]. Zero is the value which is

particles. The grids have been assumed on r–z axis. The(16) domain length in the radial direction is 7 and 120 cm in the

evaporation exchange, while the last one shows energy

Numerical modeling is used to solve initial and

assumed for gas velocity in all direction on the wallsurface. However, this is not completely true for the solid

axial direction while the average voidage of gas is 0.8.

Iranica J. Energy & Environ., 4(4): 391-397, 2013

394

Fig. 1: Schematic of fluidized bed

Experimental Set up: An experimental apparatus has beenbuilt by Assari et al. [12] and BasiratTabriziet al. [14]foran investigation into grain drying process in fluidized bed.Air is blown to the electrical heaters in room temperaturethen it is heated to be in the preset temperature. In orderto have uniform flow and desired velocity, hot airafterward will be forced to go through a distributor. Then Fig. 3: Experimental vs. theoretical temperatures of solidhot air with the known velocity and temperature enters the in fluidized bed at 343 K air temperature andcylinder upward from the bottom. Cylinder bed is 14 cm in velocity of 4m/sdiameter and 120 cm in height. The grain temperature isthen measured by the thermocouples installed along the According to evidences, as inlet gas temperaturebed. Figure 1 shows the schematic of system increases, drying period for solid spheres goes down andconfiguration. vice versa. Although higher gas temperature at entrance

RESULTS AND DISCUSSION detrimental effect on the integrity of drying materials. By

In this paper simulation of grain drying process in period will begin earlier. Drying rate is very high at thefluidized bed is carried out with wheat and dry air. Wheat initial stage of the drying process, but it decreasesparticles were fed at initial temperature of298 K, 3 mm exponentially when all the surface moisture evaporatesdiameter, density of 1200 kg/m and heat capacity of 1260 and the drying front diffuses inside the material. In other3

kJ/kg°C to the fluidized bed and filled 0.6 m height in the words, at the initial stage, temperature of solid increasescylinder. In this section, the effects of parameters such as until it approaches the initial temperature of air. At theinlet gas velocity and temperature as well as size and second stage (constant temperature), solid temperaturedensity of grains on temperature of particles in the bed are will remain equal to saturation temperature correspondinganalyzed, as it is shown in Figures 2 through 7. A to the initial conditions. Also temperature of particles forcomparison has been done between experimental data and 4 and 5 m/s of gas velocity obtained at 373k. These resultssimulation results for solid temperature at gas temperature and experimental data are shown in Figure 4.of 373 and 343 K. Experimental data is obtained from DiMattia et al. [16] have found that fluidized bedliteratures[12,15]. These results are shown in Figures 2 dryers reach their very high efficiency at low fluidizationand3. The differences between the experimental and the velocity. Figure 5 shows the temperatures of solid in thetheoretical results are due to precision of the measurement center and on surface at a given time. A little differencetools and the heat loss from the apparatus walls, which is between the temperatures can be observed; however,more sensible in the beginning of falling rate period. temperature of particles in the center of bed is higher than

Fig. 2: Experimental vs. theoretical temperature of solid influidized bed at 373 K air temperature and velocityof 4m/s

leads to higher drying efficiency, it actually has

lowering the gas temperature, the constant temperature

290

310

330

350

370

390

0 50 100 150 200 250 300

Tem

per

atu

re (

K)

Time (s)

simulation

experimental

290

310

330

350

370

390

0 50 100 150 200 250 300

Tem

per

atur

e (

K)

Time (s)

simulation

experimental

Iranica J. Energy & Environ., 4(4): 391-397, 2013

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Fig. 4: Experimental vs. theoretical temperatures of solid particle diameter, the longer time for drying it needs.in fluidized bed at 373 K air temperature Therefore, drying process will be commonly done either

Fig. 5: Theoretical temperatures of solid in fluidized bed show the way to choose optimized parameters in order toin the center and on the surface at 373 K air have highest drying efficiency. A comparison wastemperature and velocity of 4m/s conducted between model outputs and experimental data

Fig. 6: Theoretical temperatures of solid in fluidized bed Nomenclaturein different diameters of particles at 373 K air Englishtemperature and velocity of 4m/s

Fig. 7: Theoretical temperatures of solid in fluidized bed I Enthalpy, kJ/kgat different densities of particles at 373 K air J Heat transfer dimensionlesstemperature k Thermal conductivity, kJ/m °C

the bed surface. In this simulation various parameters areinvestigated by changing one variable and keeping othersconstant. The effect of effective parameters such as sizeand density of particles on temperature is separatelysurveyed. At the initial stage, size of wheat particles ischanged from 2.2, 3and 5mm in diameter. The relatedresults are shown in Figure 6.

Figure 7 shows the effect of change in density (900,1200 and 1500 kg/m ) on the temperature of particles.3

Results implies that in the same density, the greater the

with small wheat particles or with low density spheres dueto the increase in temperature of particles.

CONCLUSIONS

In this study, simulation of gas-solid flow in fluidizedbed was carried out in order to verify effective parameterson drying wheat particles on the bed. Essential parameterswere monitored during the process and the related graphswere recorded. Results help to manage the system and

to find out how accurate the model is. Gas inlettemperature has the major effect on drying efficiency andactually the higher the gas inlet temperature, the higherthe process’ efficiencywill be. However, it may bedetrimental to particles integrity and, thus, an upper limitshould be considered for that. The drying temperature ofsolids shows a rising and flat period, which is welldescribed by two proposed fluid models (gas-solid flow).

a Specific surface, 1/mc Specific heat kJ/kg °CCdz Two phase drag coefficientd Particle diameter, mD Molecular diffusion, m /s2

F1, F2 function, as defined in textg Gravity, m/s2

G( g) solids stress modulushv heat transfer coefficient, kJ/s m °C2

h

290

310

330

350

370

390

0 50 100 150 200 250 300

Tem

per

atu

re (

K)

Time (s)

exp,v=5

exp,v=4

simulation,v=4

simulation,v=5

290

310

330

350

370

390

0 50 100 150 200 250 300

Tem

per

atu

re (

K)

Time (s)

center

surface

290

310

330

350

370

390

0 50 100 150 200 250 300

Tem

per

atu

re (

K)

Time (s)

d=3 mm

d=22mm

d= 5 mm

290

310

330

350

370

390

0 50 100 150 200 250 300

Tem

per

atu

re (

K)

Time (s)

density=1200

density=1500

density=900

m

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L Constant 2. Syahrul, S., I. Dincer and F. Hamdullahpur, 2003.n Constant Thermodynamic modeling of fluidized bed drying of

Moisture evaporation moist particles. International Journal of ThermalPr Prandtl number Sciences, 42(7): 691-701.p Pressure, kPa 3. Avci, A., M. Can and A.B. Etemoglu, 2001. ARe Reynolds number, as defined in text theoretical approach to the drying process ofr Radial distance from the centerline, m thin film layers. Applied Thermal Engineering,T Temperature, °C 21(4): 465-79.t Time, s 4. Midilli, A. and H. Kucuk, 2003, Mathematicalu Radial velocity, m/s modeling of thin layer drying of pistachio by usingv Axial velocity, m/s solar energy. Energy Conversation and Management,x Moisture content, kgw/kgs 44(7): 1111-1122.z Elevation, m 5. Fan, L.S. and C. Zhu, 1998. Principles of Gas-Solid

Greek symbols 6. Gidaspow, D., 1994. Multiphase Flow and

Gas–solid drag coefficient 7. Palancz, B., 1982. A mathematical model for° heat of vaporization, kJ/kg continuous fluidized bed drying, Chemical

Void fraction Engineering Science, 38 (7): 1045-1059.µ Viscosity, kg/m s 8. Lai, F.S. and Y. Chen, 1985, Modeling and simulation

Density, kg/m of a continuous fluidized bed dryer, Chemical3

Evaporation coefficient, kg/m s Engineering Science, 41(9): 2419-2430.2

Stress, kPa 9. Hajidavalloo, E. and F. Hamdullahpur, 2000, ThermalSpherically of a particle analysis of a fluidized bed drying process for crops.t

Subscripts of Energy Research, 24(9): 791-807.

Dz Drag in z-direction Thermal analysis of a fluidized bed drying processg Gas for crops. Part II: experimental results and modelic Inlet-cell verification, International Journal of Energyp Particle Research, 24(9): 809-820.pg Gas on the surface of a particle 11. Burgschweiger, J. and E. Tsotsas, 2002.r Radial Experimental investigation and modeling ofrr Radial-stress continuous fluidized bed drying under steady-states Solid and dynamic conditions, Chemical Engineeringsc Solid-critical Science, 57(24): 5021-5038.sz Solid-axial 12. Assari, M.R., H. BasiratTabrizi and M. Saffar-Avval,v Vapor 2006, Numerical simulation of fluid bed drying basedw Water on two-fluid model and experimental validation,wg Water-vapor Applied Thermal Engineering, 27(2-3): 422-429.z Axial 13. Ding, J. and D. Gidaspow, 1990. A bubblingzz Axial-stress fluidization model using kinetic theory of granular

REFERENCES 14. Basirat Tabrizi, H., M. Saffar-Avval and

1. Jelena, N.J., S. Branislav and S. Mladen, 2004. modeling of fluidized bed drying, 2 TrabzonThermal diffusivity coefficients by air International Energy and Environment Symposium,fluidized bed. Mechanical Engineering, Energy and the Environment, Begell house, Inc.,2(1): 125-134. pp: 63.

Flows, Cambridge University Press.

Fluidization, Academic Press.

Part I: mathematical modeling, International Journal

10. Hajidavalloo, E. and F. Hamdullahpur, 2000.

flow, AIChE Journal, 36(4): 523-538.

M.R. Assarie, 1998, Two dimensional mathematicalnd

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15. Hajidavalloo, E., 1998. Hydrodynamic and thermal 16. Dimattia, D.G., P.R. Amyotte and F. Hamdullahpur,analysis of a fluidized bed drying system,PhD. thesis, 1997, Slugging characteristics of group D particles inDalhousie University, Halifax, Canada. fluidized beds, Canadian Journal of Chemical

Engineering, 75(2): 452-459.

Persian Abstract

چكيدهمقادير دقيق پارامترهايي كه بر فرايندهاي صنعتي حاكم هستند، يك جنبه كليدي براي انجام يك فرايند و بهينه سازي آن داشتن

توسعه و اصالح شده اند. اين مطالعه در نظر دارد تا شبيه سازي جريان جهاني است. براي غلبه بر اين چالش، مدل هاي رياضي يك مبناجامد در خشك كن بستر سيال را ارائه دهد. حل عددي دوبعدي و مدل استوانه اي متقارن محوري براي دو فاز كه -گاز

ودي، قطر و چگالي ذرات طي فرايند انجام شده است. چند پارامتر فراگير شبيه سرعت و دماي گاز ور مي باشد، طالعهمدر اين و همچنين چالش هاي انتقال حرارتي در امتداد بستر، به طور جزيي بحث و بررسي شدند. دماي ذرات جامد در مركز خشك كردن

و سطح كه به شدت تحت تاثير زمان هستند، در نتايج نشان داده شده اند. در انتها، خروجي هاي مدل با داده هاي آزمايشگاهي قايسه شدند كه توافق خوب و منطقي را نشان مي دادند.م

DOI: 10.5829/idosi.ijee.2013.04.04.11