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36 Mehul Soni, Anand Laddha, Harshitkumar Chudiwal, O. K. Mahadwad

International Journal of Engineering Technology Science and Research

IJETSR

www.ijetsr.com

ISSN 2394 – 3386

Volume 2 Issue 5

May 2015

Comparison of a Fluidized Bed Dryer with Conventional

Dryers Used in Food Processing Industries

Mehul Soni1, Anand Laddha

2, Harshitkumar Chudiwal

3, O. K. Mahadwad

4

Department of Chemical Engineering

MGM’s Jawaharlal Nehru Engineering College, Aurangabad-431003

ABSTRACT

In the rapidly developing food sector, food processing industries in India are picking up pace. This

has led to development of technologies, processes and equipments which not only reduce time but

also increase the output. There is particular interest in drying operation as it takes up long periods of

time and on the other hand, is of extreme importance. Hence, the proposed work is an attempt

towards encouraging use of fluidized bed dryer over dryers used conventionally in food processing

industries.

Drying of wheat, millet and oats is carried out in a lab-scale fluidized bed dryer designed to study its

performance. The comparison is done with fixed bed dryer and tray dryer which have commonly

been employed. The fluidized bed dryer is operated at room temperature in bubbling fluidization

regime. The results obtained show that the drying time and the rate of drying reduced by 8 to 10

times when fluidized bed dryer was used as compared to tray dryer.

Keywords: Fluidized Bed Dryer, Food Processing, Drying Time, Rate of Drying

1. INTRODUCTION

Fluidized beds have characteristic properties that are extremely advantageousand have therefore

found extensive application in traditional industries, as well as in metallurgical processes and in the

manufacturing, chemical and pharmaceutical industries. For the very same characteristic properties

that have made fluidized beds a practical and attractive option in these cases can be used in designing

novel processes, although always with new issues to be addressed [1][2]. Fluidized beds are one of

the preferred modes of contact between gas-solid, gas-liquid and gas-liquid-solid operations in

industries with the application ranging from simple adsorbers, wastewater systems to complex

reactors. These fluidized bed technologies have found increasing application in the drying operations

of agricultural material although they are widely used in industries for drying of fertilizers,

chemicals, pharmaceuticals and minerals. The increasing application of fluidized bed drying for

agricultural materials is due to the evolving designs of fluidized bed for fluidization of coarse

material, which is rather difficult to fluidize. Fluidized beds, as compared to other modes of drying,

offer advantages such as high heat capacity of the bed, improved rates of heat and mass transfer

between the phases, and ease in handling and transport of fluidized solids [3].

1.1 Objectives:

1. To reduce moisture content in food items.

2. To increase and multiply the amount of food items being processed in a given span of time.

3. Attain uniform drying throughout.



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4. Design a lab scale fluidized bed dryer and compare its performance with the conventional dryers

used in the food industries.

2. LITERATURE SURVEY

The fluidization technology is commonly used in drying agro-food materials & other materials. It is

also used in freezing systems. Fluid bed drying has been recognized as a gentle, uniform drying

method capable of drying down to very low moisture content with a high degree of efficiency

[4].This process is characterized by high moisture and heat transfer rates and excellent thermal

control capacity compared to conventional drying process [5]. It is also a very convenient method for

drying heatsensitive food materials as it prevents them from overheating due to mixing [6]. A

complex transport phenomenon takes place during drying process including unsteady state heat and

mass transfer which occur simultaneously [7][8].Heat necessary for evaporation is supplied to the

particles and moisture vapour is removed from the material into the drying medium. Heat is

transported by convection from the surrounding to the particle surfaces and then by conduction from

there into the inside of particles. Moisture is transported in the opposite direction as a liquid or

vapour on the surface evaporates and passes on to the surroundings by convection [9].The heat and

moisture transfer rates are related to drying air temperature and Reynolds number as a function of

velocity of circulating air. The developments of the regime of fluidization and subsequent design

modifications have made fluidized bed drying a desirable choice among other driers. However the

efficiency of the conventional drying system is usually low. It is therefore desirable to improve the

efficiency of the drying process. Drying process has usually three typical drying rate periods [10]

namely pre-warming period, constant rate and falling rate period. The diffusion controls the drying

process especially in the falling rate period [11]. Tanfara et al. [12] studied the effect of particle size

distribution on local voidage in a bench-scale conical fluidized bed dryer containing dried placebo

pharmaceutical granule.

3. METHODOLOGY

3.1 Phase I - Designing

The parameters for the dryer that were finalized were as follows:

a. Food Items Chosen for Drying: Wheat, Millet and Oats

b. Shell Diameter Chosen: 100mm i.d., 5mm thickness

c. Shell Height: 2 ft.

d. 100mesh screen used.

The equations used are [13]:

1. Pressure Drop across the Bed:

∆𝒑𝒃 = 𝟏 − 𝜺𝒎𝒇 𝝆𝒔 − 𝝆𝒈 . 𝒈. 𝑳𝒎𝒇 … (3.1)

2. Minimum Fluidization Velocity:



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3. 𝒖𝒎𝒇𝟐 =

𝒅𝒑 𝝆𝒔−𝝆𝒈 .𝒈

𝟏.𝟕𝟓𝝆𝒈. 𝜺𝒎𝒇

𝟐 . ∅𝒔 … (3.2)

Check for: (𝑅𝑒𝑝 ,𝑚𝑓 > 1000) 𝑤𝑕𝑒𝑟𝑒, 𝑅𝑒𝑝 ,𝑚𝑓 =𝑑𝑝 .𝑢𝑚𝑓 .𝜌𝑔

𝜇

4. Terminal Fluidization Velocity (Haider & Levenspeil’s Equations):

𝒖𝒕 = 𝒖𝒕∗

𝝁. 𝝆𝒔−𝝆𝒈 .𝒈

𝝆𝒈𝟐

𝟏𝟑

… (3.3)

Where,

𝒖𝒕∗ =

𝟏𝟖

𝒅𝒑∗

𝟐 +𝟐.𝟑𝟑𝟓−𝟏.𝟕𝟒𝟒∅𝒔

𝒅𝒑∗

𝟎.𝟓

−𝟏

… (3.4)

𝒅𝒑∗ = 𝒅𝒑

𝝆𝒈. 𝝆𝒔−𝝆𝒈 .𝒈

𝝁𝟐

𝟏𝟑

… (3.5)

The above given equations were used along with the data of physical properties of wheat provided in

Table 3.1 below. The results obtained using the above data and equations are as follows in Table

3.2.

Table 3.1 Physical Properties of Wheat

Parameter Wheat

∅𝐬(Spherecity) 0.85

𝐝𝐩(𝐄𝐪𝐮𝐢𝐯𝐚𝐥𝐞𝐧𝐭 𝐃𝐢𝐚𝐦𝐞𝐭𝐞𝐫)(m) 4.06*10-3

𝛍(𝐀𝐢𝐫 𝐕𝐢𝐬𝐜𝐨𝐬𝐢𝐭𝐲) (Kg/m.s) 1.8*10-5

𝛒𝐛(𝐁𝐮𝐥𝐤 𝐃𝐞𝐧𝐬𝐢𝐭𝐲) (Kg/m3) 800

𝛒𝐠(𝐀𝐢𝐫 𝐃𝐞𝐧𝐬𝐢𝐭𝐲) (Kg/m3) 12

𝛆𝐦(Void fraction) 0.40

𝐋𝐦𝐟 (Bed Height) (m) 0.05

Table 3.2Values Obtained on Calculation

Calculated Values Wheat

∆𝒑𝒃 231.908 Pa

𝒖𝒎𝒇 0.450 m/s

Area 7.85 ∗ 10−3 𝑚2

Flowrate 12,715 LPH

𝑹𝒆𝒑,𝒎𝒇 1218 (∴ 𝑅𝑒𝑝 ,𝑚𝑓 >1000)

𝒅𝒑∗ 267.589

𝒖𝒕∗ 19.09

𝒖𝒕 1.887 m/s

The material used and the cost of respective items have been mentioned the Table 3.3. The

mentioned costs are to be considered only for the Phase I.



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Table 3.3Materials Used and its Respective Cost

Sr.No. Material Cost (₹)

1 Acrylic Cylinders ( 100 mm i.d. , thickness 5 mm) 1500

2 Acrylic Flange 500

3 G.I. Flange 250

4 Reducer 400

5 Gasket 40

6 Stand 500

7 Miscellaneous 100

Total= 3290

3.2 Phase II – Experimentation and Comparative Study

To obtain the rate of drying curves, the procedure has been mentioned below.

1. Select the sample.

2. Take 100g of dry sample and add water such that only 20g is retained.

3. Take this 120g of sample and place it in the dryer.

4. Let the sample dry. The drying time interval is chosen as 90 seconds.

5. The sample is removed and the reduced weight is measured and noted.

6. Continue this procedure until no further reduction takes place in sample weight.

7. The obtained data should be tabulated in the following form shown in Table 3.4.

Table 3.4Sample Table

Time Wt. of

Sample

ΔƟ Δx Δx/ΔƟ Free

Water

Rate of

Drying

Free Water

Dry Stock

0 120 - - - 20 - 0.2

Where,

ΔƟ = Time interval between two successive readings

Δx = Moisture content lost in ΔƟ

8. Now, plot a graph of Drying Time vs. Wt. of Sample.

9. Once this graph is plotted, plot another graph of Drying Time vs. Δx/ΔƟ. The slope of this graph

will give dx/dƟ (not shown in above table).

10. Once dx/dƟ is obtained, the following equations are to be used to calculate the rate of drying and

subsequently, the rate of drying curve.

Drying Rate = [ 𝒅𝒙 /(𝒅Ɵ) ∗(60/1000)] / (Area of Tray) … (3.6)

Free Water = Weight of Sample – Weight of dry Sample … (3.7)

Free Water Dry Stock = 𝑭𝒓𝒆𝒆 𝑾𝒂𝒕𝒆𝒓

𝑾𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒂𝒏𝒚 𝒔𝒂𝒎𝒑𝒍𝒆 … (3.8)

11. After obtaining the rate of drying values, plot another graph of Rate of Drying vs. Free Water

Dry Stock.Use the above obtained graphs to study the difference in performances of various

dryers.



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4. RESULTS AND DISCUSSIONS

The results obtained were tabulated and depending on which three types of graphs have been plotted

namely:

a. Drying Time v/s Wt. of Sample

b. Drying Time v/s ∆𝑥/∆𝜃

c. Rate of Drying v/s Free Water Dry Stock

Out of these 3 graphs, type (a) and type (c) are provided here as they are the ones that are going to be

used as the basis for comparison. All the readings were taken using following parameters for all 3

samples:

Dry Sample Weight = 100g

Moisture Content = 20g

Total Sample Weight = 120g

Area (Fluidized &Fixed) = 0.00785 sq.m.

Area (Tray) = 0.045 sq.m.

The results obtained have been shown in following graphs.

Figure 4.1 Plot of Wt. of Sample vs. Drying Time (Wheat)

90

95

100

105

110

115

120

125

0 9 0 1 8 0 2 7 0 3 6 0 4 5 0 5 4 0 6 3 0 7 2 0 8 1 0 9 0 0 9 9 0 1 0 8 0 1 1 7 0 1 2 6 0 1 3 5 0 1 4 4 0 1 5 3 0

SAM

PLE

WEI

GH

T (G

)

DRYING TIME (S)

WHEAT (WT. OF SAM PLE VS. DRYING TIM E)

Fluidized Drying Fixed Bed Drying Tray Drying



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Figure 4.2Rate of Drying Curve (Wheat)

Figure 4.3 Plot of Wt. of Sample vs. Drying Time (Millet)

Figure 4.4Rate of Drying Curve (Millet)

90

95

100

105

110

115

120

125

0 9 0 1 8 0 2 7 0 3 6 0 4 5 0 5 4 0 6 3 0 7 2 0 8 1 0 9 0 0 9 9 0 1 0 8 0 1 1 7 0 1 2 6 0 1 3 5 0 1 4 4 0 1 5 3 0

SAM

PLE

WEI

GH

T (G

)

DRYING TIME (S)

M ILLET (WT. OF SAM PLE VS. DRYING TIM E)




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Figure 4.5Plot of Wt. of Sample vs. Drying Time (Oats)

Figure 4.6 Rate of Drying Curve (Oats)

5. CONCLUSIONS

According to the experiments carried out, it can be concluded that fluidized bed dryers show superior

performance as compared to the other two dryers. Even today, most of the industries opt to use tray

dryers. Fluidized bed dryers have reduced the drying time by as much as 8-10 times as compared to

the tray dryer in the bubbling fluidization mode and similar trends have been observed for all the

samples. Major differences have not been observed between fixed and fluidized bed as the air

velocities values were close to each other. When air velocity is increased in fluidized bed dryers,

appreciable differences are expected to be achieved.We, hence, can imagine how rapid would drying

operation be, if it is carried out in turbulent fluidization mode or fast fluidization mode.

90

95

100

105

110

115

120

125

0 9 0 1 8 0 2 7 0 3 6 0 4 5 0 5 4 0 6 3 0 7 2 0 8 1 0 9 0 0 9 9 0 1 0 8 0 1 1 7 0 1 2 6 0 1 3 5 0 1 4 4 0 1 5 3 0

SAM

PLE

WEI

GH

T (G

)

DRYING TIME (S)

OATS (WT. OF SAM PLE VS. DRYING TIM E )




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REFERENCES

[1] Grace J. R., “A Perspective on Development of Novel Fluidized Bed Processes for a more

Sustainable Global Future”, Proceeding of 13th International Conference on Fluidization, (2010).

[2] Westerink E. J., Westerterp K. R., “Safe Design & Operation of Fluidized Bed Reactors: Choice

Between Reactor Models”, Chemical Engineering Science, Vol. 45, No. 1, pp. 333-354, (1990).

[3] Srinivasakannan C., Balasubramanian, N., Shoaibi, A. A.,“Continuous Fluidized Bed Drying

With and Without Internal: Kinetics”,Chem. Biochem. Eng. Q., 26(2), pp. 97–104 (2012).

[4] Borgolte, G. and Simon, E.J., “Fluid bed processes in the manufactureof snack products”, CEB

review for chocolate, confectionary and Bakery, 6(2), pp. 7-8, 10, (1981).

[5] Vanecek, C., Markvart, M.and Drbohlar, R., Fluidized bed drying, Lenard Hill London, UK

(1996).

[6] Gibert, H., Baxerres, J. L., and Kim, H., “Blanching time in fluidized beds. In food processing

Engineering”, Applied science publishers, London (1980).

[7] Geldart, D. and Kelsey, J.R., “The influence of the gas distributor on the bed expansion, bubble

size and the bubble frequency in the fluidized beds”, In Fluidization: Chem.E. Symposium Series, (J.

M. Pierie eds) pp. 114- 125. Institution of Chemical Engineers, London,

(1968).

[8] Husain, S., Akihiko H. and Naoto H., “Heat and mass Transfer Analysis of Fluidized bed Grain

Drying”, Memoriesof the faculty of engineering, Okayama University, Vol.41, pp.52-62, (2007).

[9] Mowla, D. and Montazeri, H., “Drying of particles in batch Fluidized Beds”, J. Aerosol Sci., 31

(Suppl. 1), pp. 793-794, (2000).

[10] Vojtech, V. C. Sc, Markvart, M. and Drbohlav, R., Fluidized bed drying, Leonard Hill, London

(1996).

[11] Wang, H.G., Dyakowski, T., Senior, P. R. S and Yang, W. Q., “Modeling of batch fluidized bed

drying of pharmaceutical granules”, Chemical Engineering Science, 62, pp. 1524-1535, (2007).

[12] Tanfara, H., Pugsley, T. and Winters, C., “Effect of Particle Size Distribution on Local Voidage

in a Bench Scale Conical Fluidized Bed Dryer”, Drying Technology Journal, 20 (6), pp. 1237-1289,

(2003).

[13]Kunii D., Levenspiel O., Fluidization Engineering 2nd edition. Butterworth-Heinemann Corporation, (1991).

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