introductionchegrp5.wikispaces.com/file/view/prelab_4.docx/210557314/... · web viewfluidized bed...

Fluidized Bed University of Illinois

Fluidized Bed

Picture courtesy of: EPA

High velocity fluid and small solid particle packing are the key features of a fluidized bed reactor. The resulting fluid-particle mixture behaves as a fluid and allows for ample contact between the fluid and the

solid particles. These properties of fluidized bed reactors make them ideal reactors for many types of reactions including catalytic driven reactions. In this lab fluidized bed reactors will be modeled using

sand and silica as the solid packing and air as the high velocity fluid.

1Unit Operations ChE 382 Group 5 Spring 2011 3/14/2011Damo, Duffy, Guerrero, Hsu, Kosak, Qamar, Tyska


Lab Prep Report

Unit Operations II Lab 4

March 14, 2011

Group 5

Andrew Duffy

Daniyal Qamar

Jeff Tyska

Bernard Hsu

Ryan Kosak

Tomi Damo

Alex Guerrero



Contents1. Introduction.........................................................................................................................................4

2. Literature Review/Theory....................................................................................................................4

3. Experimental.......................................................................................................................................4

3.1. Apparatus....................................................................................................................................4

3.2. Materials and Supplies.................................................................................................................4

3.3. Experimental Procedure..............................................................................................................4

4. Anticipated Results..............................................................................................................................4

5. References...........................................................................................................................................4

6. Appendix I: Job Safety Analysis............................................................................................................4



1. Introduction

A fluidized bed reactor consists of a column filled with a solid packing. High velocity fluid flowing

through the column unseats the solid packing causing the subsequent fluid-particle mixture to then act

as a fluid itself (Lab Packet 1). The fluid used can be either gas or liquid. The most common purpose of a

fluidized bed reactor is to ensure that there is ample contact between the solid particles, reactor fluid,

and the reactor walls. This allows for virtually uniform temperature distribution even when highly

exothermic reactions are occurring. Often times the solid particles are used to catalyze the reaction

occurring within the column. In many cases the solid packing is a support for a catalyst, and fluidization

of the particles allows for significant contact between reactants catalytic particles. The most common

uses for fluidized beds are catalytic cracking in the petroleum industry, as well as coal combustion,

catalyst regeneration, solid-gas reactors, ore roasting and gas adsorption operations (Lab Packet 1).

Though much more complex than a simple packed bed reactor, a fluidized bed reactor also includes

a void fraction for its packing. Because of the void fraction of the packing material, there is a maximum

velocity at which the fluid can move through the reactor without fluidizing the solid packing. The

particles are usually ≤ 500µm in length. However, in order for fluidization to occur, the fluid has to have

a minimum velocity in order for the particles to flow easily and rapidly. The observation of bubbles of

fluid passing through the solid packing of the bed is how one would determine that this minimum

velocity has been reached. The bed sits upon a distributor and this allows for an even fluid distribution.

Located at the gas outlet is a disengaging section which prevents solid particles from flowing out of the

system.



Two different fluidized bed columns will be evaluated in this lab. The first will use sand as the solid

packing material. This sand will be of varying grain sizes and air will be the fluid. A second column will

be evaluated with silica as the solid packing material and air as the fluid. The second column will also be

heated. Flow rates and pressure drops throughout the column will be measured and recorded and then

the superficial velocity and minimum fluidization velocity of the system will be found using these

measurements. A relationship between gas velocity and pressure drop can then be established.

2. Literature Review/Theory

Fluidized beds are commonly used in chemical engineering process for multiple purposes, such

as catalytic reactions, solid-gas reactions, coal combustion, roasting ores, drying, and adsorption

operations (McCabe 3). Even though fluidized beds also contain a packing material to give good contact

between the phases, they have several major differences from packed beds. First, the packing is

supported by the upflowing phases and behaves much like a liquid, hence the name. The packing phase

is in constant motion within the fluidized bed. Such a unit is more complex than a simple fixed bed and

has several advantages which can be important in industrial applications: (1) the rapid mixing motion in

the bed gives a high heat transfer rate between the bed and the shell of the unit; thus, heat can be

easily transferred to or away from the bed, (2) the bed unit tends to be quite uniform in concentration

when compared to the non-mixed packed bed; this can be an advantage or a disadvantage for a given

separation or chemical reaction, and (3) the packing can flow out of the unit for separate treatment and

back into the unit. (McCabe 145)



At low gas velocities, the pressure drop through the bed can be described by the Ergun

equation. However, as the flowrate increases a point is reached where the pressure drop becomes

constant and does not change with gas flowrate. This is defined as the point of fluidization. The point of

fluidization is where the solid particles and the gas are in a state of equilibrium. When particles in the

bed become suspended on the upward following gas the system becomes known as a fluidized bed.

Fluidization can only be used with relatively small particles, <300 μm with gases (Sinnott, Towler 667). In

the case of this lab experiment sand and silica are used to test the affect of particle size on point of

fluidization and pressure drop. Visually, the bed does not appear like a fixed bed but there is random

motion of the particles within the bed; if a valve is opened on the side of the unit the packing will flow

out much like a liquid. Further increases in gas flowrate result in violent mixing within the bed and the

formation of large gas bubbles passing through the bed. Upon decreasing the gas flowrate, the pressure

drop versus gas flowrate does not exactly follow the previous curve as seen in figure 1 below. There is a

significant hysteresis effect due to the frictional forces in the initial packed bed. The reason for this path

dependence is due to a change in the orientation of the packing particles as they become looser than

before due to the air flowing through the bed; this results in a lower friction factor and lower pressure

drop.



The behavior of the particles based on pressure drop with relation to upward superficial velocity

can be seen in figure 1 below.

Figure 1: Pressure Drop vs. Superficial Velocity (McCabe 3)

In this figure it can be seen as the pressure drop is increased the superficial velocity increases somewhat

exponentially until it reaches a plateau where the minimum fluidization point is. Along the first section

of the graph (area A) the pressure drop can be related to the velocity using the Ergun Equation

(equation 1 below). This equation can only be used in this section because of the small pressure drop

and velocity where it is considered a packed bed (Bird, Stewart, Lightfoot 191).



(( P0−PL)∗ρ

G02 )∗(

Dp

L)∗( ε 3

1−ε)=150∗( 1−ε

D pG0

μ )+ 74 (1)

Where:

P0 = initial pressure [Pa]

PL = pressure at the end of the column [Pa]

G0 = mass flux [kg/m2*s]

Dp = particle diameter [m]

L = height of the bed [m]

ε = void fraction [dimensionless]

ρ = density of the particle [kg/m3]

μ = viscosity [kg/s*m]



However, as the velocity and pressure drop increase the Ergun Equation (1) cannot be used in

the determination of fluidized bed values. Figure 2 below also shows this relation but with the overall

bed height verses the superficial velocity in the column.

Figure 2: Bed Height vs. Superficial Velocity (McCabe 3)

Point C on both figures is the point of minimum fluidization velocity, V f, and can be caluculated by the

following equations. (Note: The small frictional force exerted on the wall was ignored). First the

determination of the upward force by the gas on the bed can be calculated using equation 2 below.

Upward forceon the bed=∆ P∗A (2)

Where:

ΔP = pressure drop across the bed [Pa]

A = cross-sectional area of the bed [m2]



Next the volume of the particles can be solved for using equation 3 below.

Volume of particl es=(1−ε )∗A∗L (3)

Where:




From here we can determine the net weight of the particles in the column by using equation 4 below.

Net weight of the particles= (1−ε )∗( ρp−ρf )∗A∗L∗g (4)

Where:




ρp = density of the particle [kg/m3]

ρ f= density of the fluid (or gas) [kg/m3]

g = acceleration due to gravity [m/s2]



The theoretical pressure drop can be determined by using equation 5 below.

∆ P=(1−ε )∗( ρp−ρf )∗L∗g (5)

Where:




Typically for a bed with small particles (Dp < 0.1 mm), the flow conditions are such that the Reynolds

number (Re) is relatively small (Re < 10) meaning that the Kozeny-Carmen Equation can be used to find

the velocity of fluidization (Vf) (McCabe 5). This Kozeny-Carmen Equation can be seen as equation 6

below.

V f =

( ρp−ρ f )∗g∗Dp2

150∗μ∗ε3

1−ε(6)

Where:

Vf = fluid velocity [m/s]



ρ f = density of the fluid (or gas) [kg/m3]






When the superficial velocity (Vs) is equal to the fluidized velocity (Vf) this state is known as

“incipient fluidization” (McCabe 5). Next the settling velocity can be determined by restricting the size of

the particle to be small like before so that Stokes Law can be used to calculate this velocity. This can be

seen in equation 7 below.

V Settling=( ρp−ρf )∗g∗Dp

2

18∗μ(7)

Where:

Vsettling = settling velocity [m/s]


ρ f = density of the fluid (or gas) [kg/m3]




Once the velocity of settling and the fluidization velocity are determined a ratio of the two can

be formed relating the void fraction back to both the velocities, which can be seen in equation 8 below.

V settling

V f=

253

∗1−ε

ε3(8)

Where:

Vf = superficial velocity [m/s]

Vsettling = settling velocity [m/s]




In cases where the particles a very small it is likely that they may be carried out of the bed system.

Therefore, filters or cyclones must be emplaced to recover these particles at high superficial velocities.

Bubbling fluidization will occur in this experiment because this is strictly a gas-fluidized bed which will be

seen as large pockets of gas moving through the free particles.

3. Experimental

3.1.Apparatus

Equipment Manufacturer Purpose Description

Rotameter Nupro Company To measure air flowrate on column on left

Max flow = 2.2 L / min.

Pressure meter Omega Engineering Measures pressure of air into rotameter mentioned above

0 – 100 psi

2 air valves Crane Company Changes air flowrate into rotameter mentioned above

No. 1. Cranite Disk

2 Half turn valves Smith Company Changes air flow rate into fludizied beds

On bottom of each fluidized bed

2 PVC columns None listed Used as fluidized beds Two are given

2 Tops None Listed Tops to fluidized beds Funnels, so that sand is easy to pour

3 Pipette bulbs None Listed Used to keep sand from flowing out of the fluidized beds

Normal pipette bulbs

1 U-Tube Manometer Meriam Instruments Used to measure 0 – 25 psi



pressure drop in fluidized beds

2- 250 mL Erlenmeyer flasks

Pyrex Used to collect sand from the fluidized beds

Tubes hooked up to fluidized beds and U-Tube manometer

Plastic Tubes None Listed Used to link up the fluidized beds and U-Tube manometer to the Erlenmeyer flasks

4 in total

Set of Ceramic Spheres None Listed Packing in the bottom of each column

1 set in each column

Thermocouple Fluke Used to show data from thermometers in a fluidized bed

2166A digital thermometer

Thermometers Omega Hooked up to thermocouple, measure temperature in a fluidized bed

3 in total

Heater SE Engineering Used to heat the fluidized bed

In the middle of the apparatus

Rotameter F&P Co Used to measure the air flowrate in a fluidized bed

13.9 CFM maximum

Pressure Gauge None Listed Measures pressure of air to a fluidized bed (on the right)

Made in China, 0 – 200 psi

Valve Apollo To main air supply Yellow

Valve Willerson Corp Used to change the air flowrate into the rotameter on one of the fluidized beds

Near the pressure gauge without a manufacturer name



Figure 3 Key:

1. Tops to fluidized beds

2. Fluidized bed columns

3. Ceramic spheres

4. Rotameter (Nupro)

5. Pressure gauge (Omega

Eng.)

6. Valve (Cranite)

7. Valve (Wilkerson)

8. Pressure gauge

(No

manufacturer listed)

9. Rotameter (F&P Co)

10. Valve (Cranite)

11. Pipette bulbs

12. U-Tube manometer

13. Half turn valves (Smith)

14. Heater

15. Thermocouple

Figure 3: Fluidized Bed Lab Station



3.2.Materials and Supplies

Equipment Manufacturer Purpose Description

1 Scooper None listed Used to scoop out sand

Made of plastic

1 yardstick None listed Used to measure the height of the sand

Also used for other labs

Air None Used to create the fluidized bed

From the main compressed air line

Water None Used to help calculate the density of sand

From a faucet

Wet-Dry Vac Dayton For vacuuming up loose sand and silica

120 V, 60 Hz

Sieves Us Standard Sieve Series Used to separate the sand into different size distributions

Multiple sieves, 4750 – 28 microns

Graduated Cylinder 250 mL Used to store sand and water in for the density calculations

TC/TD at 20 degrees Celsius

Beaker Pyrex Used to store sand in for the density calculations

40 mL

Funnel None listed Used to funnel sand into beaker and graduated cylinder

1 in total

Sea Sand Fischer Scientific For use as packing in fluidized bed

Normal sand, various particle sizes

Silica Fischer Scientific For use as packing in Looks like crushed



fluidized bed quartz, similar particle sizes

Figure 4: Solid Particle Sifters



Figure 5: Solid Particle (sand)

3.3.Experimental Procedure

Start Up

1. Check that the apparatus and all required materials are present and in working order. Clean out

columns (2) if not already clean.

2. Clean, by taping, and stack the sieve trays in descending order of grain diameter. Set up a

container to catch the fly off sand

3. Record the diameter range of the two decided sand samples as well as the silica.

4. Using a 100 ml graduated cylinder, add 25 ml of sand sample.

5. Add an equal amount of water to the graduated cylinder.

6. Allow sufficient time to pass for the water to penetrate the sand layer then measure the final

volume.

7. Use these measurements to calculate the void fraction. Repeater for each sample.



Sand Column Procedure

8. Load the first sand sample into the left column (2). Add roughly 10 cm of sand to the column.

Tap the column to even out the sand and accurately measure the sand’s height.

9. Make sure all valves are in the proper starting position

10. Fully open the valve at the base of the column (13), and then slowly open the flow meter valve

(5) to allow air in.

11. For each increment of change in the air flow record: the air flow rate, the pressure drop, height

of the packed bed also any notable observations with the packed bed.

12. Keep incrementally increasing the flow rate until the maximum is achieved. Maximum is noted

by a plateau of pressure drop with increasing air flow.

13. Repeat recording for decreases in airflow.

14. Remove all the sand with the Shop-Vac and repeat steps 8 to 13 with the second sample of

sand. Empty out the sand into separate container so not to mix it with the silica later on.

Silica Column Procedure

15. Load silica in the right column (2). Add enough silica to cover the top thermal sensor by 1.5

inches

16. Open the air valve at the bottom of the right column (13) (have the left one closed) then slowly

open the flow meter valve to let in air.

17. For each incremental change in air flow record: the air flow rate, the pressure drop, the height

of the packed bed and any observations.

18. Once the maximum is reached record it.

19. Start to slowly decrease the air flow rate and record the corresponding measurements.



20. Turn of the air and the turn on the heater (14). Wait till packed material reaches steady state

temperature.

21. With the heated bed repeat steps 16 to 19. Along with the standard measurements also record

both top and bottom temperatures.

Shut Down and Clean Up

22. Turn of air supply (6), heater (14) and digital thermometer (15).

23. Remove funnels (1) for the top of both columns and use Shop-Vac to clean the column and

bench area.

24. Organize sieve trays and place inside the cabinet.

4. Anticipated Results

5. References

Bird, R. Byron, Warren E. Stewart, and Edwin N. Lightfoot. Transport Phenomena. New York: J. Wiley, 2007. Print.

"Fluidized Beds." University of Illinois at Chicago - UIC. Web. 25 Jan. 2010. <http://www.uic.edu/depts/chme/UnitOps/che381-2005f-frame.html>.

Sinnott, Ray, and Gavin Towler. Chemical Engineering Design. Amsterdam: Elsevier, 2009. Print.

W.E. McCabe, J.C. Smith, and P. Harriott 2001. Unit Operations of Chemical Engineering, McGraw Hill, New York.

6. Appendix I: Job Safety Analysis


http://www.uic.edu/depts/chme/UnitOps/che381-2005f-frame.html


What is the purpose of this experiment?

The purpose of this experiment is to determine the gas pressure drop across a bed of particles as

a function of the gas velocity. The fluidization behavior of a bed of small particles will be

studied.

What are the hazards associated with the experiment?

The spilling of sand and silica on the floor, which will occur during operation of the column and

during the sifting of the sand in order to obtain the right particle size range, makes for a slippery

work environment. Compressed air is used to fluidize the bed. Since there is air passing through

small sand and silica particles, it is possible that these particles could enter the eye, nose and/or

mouth. A heater adjacent to the fluidized bed apparatus is used and could possibly expel

exuberant amounts of heat and burn the operator.

How will the experiment be conducted in a safe manner?

A vacuum is included with the station to maintain a clean working and walking area when sand

and silica are being used. Eye protection will be worn so that sand and silica particles do not

enter the operator’s eyes. Care should be taken not to inhale either of these particles and hands

will be washed after working with the silica.

What safety controls are in place?



The air inlet regulator has an internal diaphragm that will maintain constant inlet pressure of 40

psig. Should the pressure increase erratically, the regulator valve will relieve the pressure to

maintain the inlet pressure. The vacuum cleaner has a filter adapter attached which prevents

excessive dust accumulation when cleaning up the area. The columns are fitted with a

disengaging section to prevent solid particles from leaving the system.

Describe safe and unsafe ranges of operations.

In order to prevent blowing sand or silica out the top of the column, operate at a superficial

velocity below the settling velocity. Carefully controlling increments air flowing through the

column will prevent the discharge of the bed out the top of the column. Silica column cannot be

heated to temperatures greater than 110°F. Should the Plexiglas reach temperatures of 265ºF,

then it will melt.

I have read relevant background material for the Unit Operations Laboratory entitled:

“Fluidized Beds” and understand the hazards associated with conducting this experiment. I have

planned out my experimental work in accordance to standards and acceptable safety practices

and will conduct all of my experimental work in a careful and safe manner. I will also be aware

of my surroundings, my group members, and other lab students, and will look out for their safety

as well.

I have read relevant background material for the Unit Operations Laboratory entitled:

“Fluidized Bed” and understand the hazards associated with conducting this experiment. I have



planned out my experimental work in accordance to standards and acceptable safety practices

and will conduct all of my experimental work in a careful and safe manner. I will also be aware

of my surroundings, my group members, and other lab students, and will look out for their safety

as well.



Electronic Signatures:

Bernard Hsu

Daniya l Qamar

Jeff Tyska

Alex Guerrero

Tomi Damo

Ryan Kosak

Andrew Duffy


introductionchegrp5.wikispaces.com/file/view/prelab_4.docx/210557314/... · web viewfluidized bed...

Documents