EXPERIMENT 7 FLUIDISATION 2011

amiy1.0 Objectives

The objectives of this experiment are to monitor the behaviour of particles in a bed with upward air flow and to study the correlation between bed height, bed pressure drop, bed temperature, particles size and upward velocity through a bed of granular material.

2.0 Introduction

Fluidisation is a process where a bed of solid particles are being brought to a mix with an upward fluid flow in a confined space. Above a certain fluid velocity (Figure 1), the solids will become suspended in fluid flow when the drag force from the fluid flow is stabilised by the weight for each particle. As the solid particles blends with the fluid particles, their movement behaves similarly to a fluid. The application of a fluidised bed includes separation, combustion and catalytic reactions.

Figure 1 – Initiating fluidisation on a packed bed(Saptoro 2011)

3.0 THEORY AND ANALYSIS

Fluidization is a concept, where packed bed of particles which is subjected to sufficient high

upward velocity (gas or liquid), and weight of particles is supported by the drag force exerted

by the fluid. Particles are loosened and are freely suspended (Subramaniam 2011). This can

be pictured by referring to Figure 4.1 and Figure 4.2.

Figure 4.1: Pressure gradient within bed against fluid velocity

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Increasing velocity

Fixed BedFluidised Bed

Increasin

Fixed Bed

Fluidis

Fluidised Bed

Increasing fluid velocity

Fluidization occur at this velocity

EXPERIMENT 7 FLUIDISATION 2011

Figure 4.1 above shows the relationship between the pressure gradient against superficial

velocity, using logarithmic coordinates. At the fixed bed region, gas/liquid moves slowly

upwards to the bed of particles, and linear relation between pressure gradient and flow rate

exist. Bed is said to be fluidized, when superficial velocity approaches the minimum velocity

Bed will start to expand with the increase of distance within particles, until particles has no

physical contact This increases the voidage between particles which results in decrement of

pressure drop. The fall in pressure drop will continue until the velocity is high enough for

transport of particles, and will later increase slowly due to the frictional drag of the tube walls

which becomes significant. When bed composed of larger sized particles, the flow will be

laminar, at very low velocities. The slope at the lower part of the curve will be greater and

may not be constant, especially if there are progressive changes in the flow regime as the

velocity increases (Richardson, Harker and Backhurst 2002).

Figure 4.2: Pressure drop across fixed or fluidized bed

Figure 4.2 shows the relationship between pressure across the whole bed instead of pressure

gradient, against velocity, by using logarithm coordinates. Linear expression can be obtained

up to point A, where expansion of bed starts to take place. As velocity is increased, the

pressure drop will reach until its maximum point and decreases to an approximate constant

value (Richardson, Harker and Backhurst 2002). This shows that porosity has reached a

stable or constant value, with larger space of porosity.

Porosity, is the ratio of free volume of a static bed to the overall volume of the fluidized bed

(Schreiberov and Kohout 2011). It is the fractional voidage of the bed and can be calculated

as follow (Saptoro and Chaudhary 2011):

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EXPERIMENT 7 FLUIDISATION 2011

where is the volume of particles

The air flow rate through the bed is controlled, to provide maximum indicated air flow, thus

allowing bed to mix thoroughly (Saptoro 2011). This can be expressed according to the

formula stated below:

Where: is the air flow rate (L/s)

is the bed material temperature ( )

is the air inlet temperature ( )

After obtaining the air flow rate through the bed, hence superficial velocity can be calculated.

Superficial velocity is also a hypothetical value calculated, in the absence of packing or

obstruction. This is expressed as follow:

To calculate pressure drop across the bed, there are few factors that needs to be considered,

such as fluid/gas velocity, diameter of the column, packing materials, etc. Ergun equation is

one of the most successful equation created, which describes both laminar or turbulent flow

region (Saptoro and Chaudhary 2011). This equation can also used to calculate the desired

pressure drop across the bed as explained below:

Ergun equation

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Reynolds number

Where: is the pressure drop across the bed (Pa)

is the equivalent spherical diameter of particle (m)

is the density of particle (kg/m3)

L is the bed length (m)

u is the superficial velocity (m/s)

is the dynamic viscosity (kg/ms)

is the fractional voidage

4.0 Apparatus Description

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EXPERIMENT 7 FLUIDISATION 2011

Figure 2– Fluidized Bed Apparatus(Saptoro 2011)

1. Digital Indicator To show the measurements

2. Manometer To measure the pressure drop

3. Air Flowmeter (Low flow) To measure the rate of low flow

4. Air Flowmeter (High flow) To measure the rate of the high flow

5. Air Regulator To regulate and control the air flow rate

6. Flow Control Valves To control the fluid flow

7. Pressure Probe To measure the pressure drop in the fluidised bed

8. Filter To filter the air flowing into the fluidized bed

9. Chamber Where fluidization occurs

10. Bed Temperature Sensor, T1 To detect the bed temperature across the bed

11. Heater To supply heat energy to the air flowing into the fluidization chamber

12. Air Distribution Chamber A section for air distribution

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13. Air Inlet temperature sensor, T2 To detect the temperature of the inlet air flow

14. Air Inlet A section where air flows into the fluidization chamber

Materials Description:

Granular material: Fused Alumina (White Aluminium Oxide)Density: 3770kg/m3

Grit size 46 60 80 100

Average particle size 326 250 177 125

Minimum particle size 250 177 125 74

Maximum particle size 600 390 274 194

*Pour density approx. Kg/m3 1720 1670 1620 1560

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5.0 Experimental Procedures

5.1 Changing Bed Material

First, the thermocouple probe, heating element and pressure probe are levered to the highest part of the bed chamber. Then, one hand is used to carry the bed chamber for support and the six nuts from the chamber supporting screws which pass through the mounting bracket are dismantled. The bed chamber is extracted cautiously and the loose filter should be taken care of. A required amount of granular material usually to a depth of 20 to 30 mm is transferred into the chamber. Next, the unit is rebuilt in backwards order and make sure the filter is perfectly placed in its location. The probes and element are lowered to the bottom of the chamber. Finally, the axis of the chamber is checked if placed vertically.

5.2 Investigations of the relationship between bed height, bed pressure drop, bed temperature, particle sizes and upward air velocity through a bed of granular material

Before starting the experiment, the manometer is reset to zero. Then, the heater is lifted to the uppermost position, the heater control is switched to zero and the temperature probe is descended into the bed. The chosen granular material is transferred into the bed chamber and all the components are reconstructed. The air flow control is rotated anti-clockwise to ensure the highest indicated air flow and the bed is left to blend evenly for about two to three minutes. Keeping the air

flow rate, constant, the air inlet temperature, T2, bed material temperature, T1, the bed height,

the pressure drop across the bed, the velocity of the air through the bed, by using the correlation

below, are measured.

The air flow rate is decreased in small intervals and the previous step is repeated. The air flow rate is lowered until zero. Then the bed chamber wall is tapped with flat hand until the bed achieves a minimum height. The procedure above is repeated and observations are done from zero air flow to maximum indicated flow at same values to the reducing flow. The behaviour of the bed at all stages is observed. Finally, the experiment is repeated by transferring a new set of chosen granular material into the bed chamber and with the experiment is extended further for various bed temperature and other sizes of particles of the same material density.

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6.0Table of Observations

1 2 3 4 5 …

Air flow rate recorded, (L/min)

Air flow rate recorded, (L/s)

Bed material temperature, T1 (°C)

Bed material temperature, T1 (K)

Air flow through bed, (L/s)

Superficial velocity, v (m/s)

Pressure drop across bed, ∆P (mm H2O)

7.0 Expected Outcomes

Figure 2 – Effect of superficial velocity on pressure drop (Saptoro 2011)

Theoretically, the results of the experiment will be observed as in the nature of Figure 2. As the experiment will use incoming air flow at a superficial velocity starting from a high value, its initial decreasing intervals will show almost no difference in pressure drop values. After a certain decrease in air flow, the pressure drop slightly increases and then decreases steadily until no more air flow is introduced. This implies that the bed of granular material starts to pack itself since the drag force from the air flow no longer overcomes the weight of the granules. In the other hand, as the air flow rate increases the upward velocity increases, the pressure drop increases as well, due to the fact that more force is needed to loosen the bed and to bring about movement inside the bed. The pressure drop inside the bed keeps increasing accordingly, until the air velocity reaches the point of minimum fluidizing velocity. The bed height remains the same with increase in the air flow rate, until

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the point minimum fluidizing velocity, where it starts to increase because the energy required to lift the bed is provided by the air velocity, and the increase in bed height does not have any effect on the pressure drop anymore. In the meantime, the bed temperature will increase with the decrease of the air flow since the dominant heat transfer mode in fluidized bed is by fluid convection (Roy and Sarma 1970), which depends on Nusselt number, a parameter proportional to the convective heat transfer coefficient and the air velocity inside the bed.

As particle size of granular material increases, the temperature of the bed is predicted to drop at a larger rate since the voidage within the bed increases with the particle size, allowing more air to pass through the bed. Hence, resulting lower pressure drop in the chamber, the bigger the particle, the larger the void fraction, thereby making the bed loosely packed and vice versa. The lower in void fraction will also decrease the superficial velocity as well as the pressure drop in the chamber.

(Subramaniam, 2011)

Referring to the equation above, it can be concluded that as the particle size increases (void fraction and diameter the superficial velocity is increased also. The pressure drop is also increased as the velocity increases; this is as a result of the increase in the diameter and void fraction of the bed due to the increase in the sizes of the particle. For the smaller particle size, the bed height will be increased this is due to the fact that they are easier to rise than the larger particles at constant pressure drop.

As the bed is heated up, the air inside the bed expands, thereby increasing the pressure drop. Due to the temperature rise, the volume of the air increases, thereby reducing its density. While the velocity and the pressure drop increase also. The height of the bed will also increase due to the increase in volume of the fluid (air) due to increase in temperature it occupies more space in the chamber.

From the experiment, there will be a certain errors that should be avoided. When adjusting or taking the measurement, eye should be levelled to the manometer, air flowmeter and etc. This will increase the accuracy of the data taken during experiment. After adjusting the fluid flowrate, it required a few minutes to stabilize the fluidflow in order to get accurate result.

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References

Roy, G.K., and K. J. R. Sarma. "Fluidized bed heat transfer." Department of Chemical Engineering, Regional Engineering College, Rourkela-8, 1970: 8.http://dspace.nitrkl.ac.in:8080/dspace/bitstream/2080/898/1/Gkroy1.pdf (accessed July 28, 2011)

Richardson, J.F, J.H Harker, and J.R Backhurst. 2002. Coulson and Richardson's Chemical Engineering Volume 2 - Particle Technology and Separation Processes (5th Edition). http://www.knovel.com.dbgw.lis.curtin.edu.au/web/portal/browse/display?_EXT_KNOV EL_DISPLAY_bookid=2997&VerticalID=0

Subramaniam, R. Shankar. 2011. Flow through Packed Beds and Fluidized Beds. Accessed July 26, http://web2.clarkson.edu/projects/subramanian/ch301/notes/packfluidbed.pdf

Schreiberov, Lenka and Martin Kohout. 2011. Fluidization. Accessed July 26, http://www.vscht.cz/uchi/english/ped/lab.chi.fl.eng.pdf

Saptoro. A and Deeptangsu Chaudhary. ChE 323 Transport Phenomena, External Flows: Flow in Porous Media, Curtin University Sarawak, 2011.

Saptoro, A. ChE 381 Process Laboratory Project Lab Manual, Curtin University of Technology, 2009.

Coulson, J.M., J.F. Richardson, J.H. Harker, and J.R. Backhurst. 2002. Particle Technology and

Separation Processes. England: Butterworth-Heinemann.

McCabe, W.L., Smith, J.C., and Harriot, P. 1993. Unit operations of chemical engineering. 5th ed., 154

- 166, Singapore: McGraw-Hill.

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