Fluidized Bed Reactor

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Oct 30, 2013 (3 years and 8 months ago)

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Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
1

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala





Fluidized Bed Reactor


Lab Prep Report

Unit Operations Lab 2

March 02
, 2011


Group 6

Liliana Gutierrez

Linda Quan

Lipi Vahanwala

Priya Chetty

Sana Buch

Vijeta Patel


Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
2

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala

1. Introduction


A fluidized bed reactor is defined as a reactor in which gases or
liquids flow through
an unconstrained bed of particles at a certain fluid velocity to cause the particles to move in
a fluid
-
like manner. These units are much more complex than fixed packed be
ds and
require a much larger

vessel than a packed bed reactor.

In addition, a fluidized bed reactor

requires a lot of pumping power depending on the size and density of the solid material.
Furthermore, when using fine particles, the particles themselves can easily be entrained in
the fluid or gas a
nd carried out of
the reactor; therefore, a
separation unit to extract the
particles

is required. In addition, these fine particles can
cause erosion and damage to
internal components of a
system which can be expensive and

difficult to control
(Perry).
On the other hand, f
luidized beds provide a greater heat transfer rate between the bed and
the shell of the unit
due to the rapid mixing motion. T
he concen
tration of the fluidized bed
is
homogeneous which can be good for separation, and the reactor can be operated at a
contin
uous state.

Fluidized bed reactors can be used for both catalytic and non
-
catalytic processes.
Some catalytic processes include the oxidation of naphthalene to phthalic anhydride, the
production of polyethylene and ammoxidation of propylene to acrylon
itrile. Some non
-
catalytic uses of fluidized beds include roasting of sulfide ores, coking of petroleum
residues, calcinations of ores, combustion of coal, and the incineration of sewage sludge
(Perry). These systems can also be used for heat transfer be
tween gases and solids,
temperature control, solid mixing, gas mixing, drying of solids and gases, adsorption,
desorption, heat treatment, and coating.

In this experiment, a vertical cylindrical column is used as the platform for the
experiment and t
he s
et up is similar to
Fig. 1.1.

Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
3

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala


The column will be filled with various grain sizes and air will be used as the gas which will
flow into the column from the bottom. The grain sizes, air flow rate, pressure drop, and the
bed depth will be recorded and used to determine the minimum fluidiz
ation which is the
point at which the particles are suspended. As the flow rate of the gas through the particles
increase, the drag force on the particles increase. When this drag force is equal to the
weight of the particles in the bed, the particles be
come fully suspended in the column. The
particles are then known to be “fluidized”. If the gas flow rate is further increased, bubbles
will begin to form and the particle behavior begins to resemble that of fluids (Perry). The
experiment will be conduct
ed again under the same conditions, but at different
temperatures to observe the effects of temperature on the minimum fluidization.


2. Literature Review/Theory

In
a

fluidized bed, the packing is supported by the up
-
flowing phases and thus
behaves much like a liquid. This packing phase is in constant motion within the contactor.
In the fluidized bed:

Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
4

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala


(1)
T
he rapid mixing motion in the bed gives a high heat transfer

rate between the bed and
the shell

of the unit; thus, heat can
easily trans
fer

to
wards

or away from the bed.

(2) The bed unit tends to be quite uniform in concentration when compared to t
he non
-
mixed packed bed. This can be an advantage or
disadvantage fo
r a given separation or
chemical reaction
.


(3) The packing can
recycle. In other words, the packing can flow out of the unit for
separate

treatment
s

and back into the unit.


The

main object
ive

of this experiment is to measure the hydrodynamics such a
s,
p
ressure drop of the fluidized bed

by performing one phase experiment using air

as the gas
.

The
calculated

data will
then
be used to
compare

with existing correlations.

The
Ergun equation can be used to describe the pressure drop
of

the bed at low gas
veloc
ities. However, as the flow
-
rate
increases
the pressure drop becomes the consta
nt and
does not change with
increasing gas flow
-
rate. This point is defined as fluidi
zation. As the
gas flow
-
rate increases

further, violent mixing within the bed

begins to occu
r
. This
causes
the

formation of large gas bubbles passing thro
ugh the bed. I
f the gas flow
-
rate is
decreased, the graph of pressure drop
verses

the flow
-
rate does not exactly foll
ow

the
curve. This is due to significant hysteresis effect due to the fricti
onal forces in the initial
bed.



Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
5

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala

Figure 2.1
: relationship of superficial velocities with pressure drop and bed height

(
Subramanian)



Figure 2.1

describes the behavior of the bed particles as the upward superficial fluid
velocity is gradually increased from zero to the point of fluidization and back down to zero.
In the
initial state of the
fluidized bed
,

there is an initial gas flow
-
rate in which

the pressure
drop is zero an
d the bed has a certain height.
As the superficial velocity increases (path
ABCD
with arrows going to the right
), the pressure drop gradually increases while the bed
height remains constant. This is the region where the Ergun
equation for the packed bed
can
be used to relate the pressure drop to
the velocity. In the Figure 2.1
when the point B is
reached, the bed
begins to increase

in height while the pressure drop levels off

and ceases
to increase.
At this point, the upward f
orce exerted by the fluid on the particle is sufficient
to balance the net weight of the bed and the particles
begin to
float in the fluid

(fluid here is
understood to be the gas or air in this case)
. As the velocity of the
gas

is increased further,
the be
d continues to increase in the height, but the pressure drop
is
constant.


As the superficial velocity starts to decrease (in the reverse direction), the behavior
of the bed particle follows th
e curve DCE. The pressure drop stays
constant w
hile the bed
se
ttles back down. When point

C is reached
, the pressure drop begins to decrease
. The bed
height becomes the constant while the pressure d
rop follows the curve CEO. P
oint C in
Figure 2.1

is the minimum fluidization velocity
, V.
After the
point of
fluidiza
tion, the bed
particles
begin to
settle back into
a

loosely packed state. Therefore,

the constant bed height
in the reverse direction
is larger tha
n the bed height in the forward direction
.


The fluid mechanic
s of fluidized beds are
very complex. Therefore
, semi
-
empirical
correlations for the minimum velocity of fluidization have been developed based on the
particles’ Reynol
ds number. The basis of

the
se

correlation
s

is that the minimum
fluidization velocity for the uniform particles can be predicted by usin
g the
force balance.
Therefore, at

minimum fluidization
,

the drag force extorted on the bed of particles by the
fluid is equal to the force of gravity on the bed.

The upward force on the fluidized bed can be described as:

And the volume of

the particle can be described as: (1
-
ϵ
)*AL

Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
6

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala

Hence, the net gravi
tational force on the particle can be described as

(
Subramanian
)
:

(1
-
ϵ
)*AL * (ρ
s



ρ
g
) g

This yields the following equation:


Where,


A = bed cross section area (ft^2)


ΔP= pressure drop
across the bed (lbf/ft^2)


V= bed volume at minimum fluidization (ft^3)


= bed void fraction at minimum fluidization (no unit)



= density of the solid (lbm/ft^3)



= density of the gas (lbm/ft^3)




When the
Reynolds

number,

Re, is low and Pé
clet number, Pe, is high, both inertial
a
nd thermal elects are negligible. Furthermore, the
hydrodynamic interactions lead to
highly complex behaviors that have

defied

explanation. For the particles with small
diameters (D≤0.1mm)
, the Reynolds number is very
small

(Re≤ 10) which
requires the use
of the
Ko
zney
-
Carmen Equation. It is important to n
ote that
a small
Reynolds number
for
the

fluidized

bed implies that there is a steady parabolic
flow

across the thickness of the
bed, wi
th the velocity vanishing on the walls of the cell

(Tee)
.



For this experiment, air flow
-
rate and pressure drop across the bed will be
measured in order to obtain the Reynolds number. The Reynolds number can be measured
using the equation below:



In or
der to correlate this Reynolds number with the Ergun Equation, the following
equation has been derived by Wen and Yu that will be used for this experiment


= (33.7
2


0.0408Ar)
1/2



33.7

Where,

Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
7

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala



Ar= Archimedes number=



µ= visco
sity of the fluid

)



= diameter of the particle (ft)




= minimum fluidization velocity (ft/s)



The pressure obtained from this equation may be compared to the experimental
pressured drop.


3. Experimental


3.1
Apparatus




Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
8

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala




Figure 1. The Fluidized Bed apparatus


Following are the major parts of the apparatus:


Component Description

Purpose

2

3

4

5

6

7

8

10

11

12

13

14

15

16

1

9



Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
9

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala

1

½ “ Smith Ball valve

Control
air flow rate into the sand column

2

½ “ Crane Co. Globe valve

Control air flow rate into the Gilmont flow
meter

3

0
-
100 psig Omega Engineering, Inc pressure
gauge

Monitor the pressure of the air entering
into the sand column

4

0
-
100 % Gilmont flow
meter

Monitor the flow rate of air into the sand
column

5

Plexiglas column with I.D. of 9.8cm

Houses the sand fluidized bed reactor

6

0
-
50 in H
2
O Monometer

Monitor the pressure drop across the
fluidized beds in both columns

7

Powerstat variable
transformer

Control the temperature of the silica
fluidized bed

8

Fluke 2166A Digital Thermometer

Monitor the temperature of the silica
fluidized bed

9

½ “ Smith Ball valve

Control air flow rate into the silica column

10

½ “ Crane Co. Globe valve

Control air flow rate into the F & P Co.
flow meter

11

Type K thermocouple

Used in conjunction with the Fluke digital
thermometer to monitor the silica
fluidized bed

12

Plexiglas column with I.D. of 9.8cm

Houses the silica fluidized bed reactor

13

½ “
Wilkerson air inlet pressure regulator

Maintains a constant inlet pressure of air
into the system

14

0
-
200 psig Wilkerson pressure gauge

Monitor air inlet pressure going through
the air inlet pressure regulator

15

0
-
100 % F & P Co. flow meter

Monitor the

flow rate of air into the silica
column

16

½ “ Ball valve

Control the inlet of external air to the
system


Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
10

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala

3.2 Materials and Supplies


Following is the list of materials and supplies used in the experiment:


Component Description

Purpose

1

Ottawa
Standard Sand 20
-
30 mesh

Used in the column for the one of the
fluidized beds

2

Silica

Used in the column for the one of the
fluidized beds

3

(2) 250 ml Erlenmeyer flasks

Prevent sand from entering the manometer

4

(2) Rubber stoppers with holes

Allow
tygon tubing to the Erlenmeyer flasks
and then connection to the manometer

5

Tygon tubing

Connect the column, Erlenmeyer flask, and
manometer to each other

6

Vacuum

Cleaning out the column and spills on the
floor

7

100 ml Graduated Cylinder

Accurately
measure the void fraction of the
sand and silica

8

USA standard testing sieve ranging 3360
-
25 μm

Get a consistent fluidized bed by filtering out
the larger products

9

Ruler

䵥asure the height of the fluidized bed
during each flow rate



䵩cro Pipette
bulbs

䍲eate a seal on the column walls



External air supply

Used throughout the system for data
analysis


3.3 Experimental Procedure


1. Determine the average particle size as follows:

A.

Arrange several sieve trays with the largest micron value at the
top and the smallest
at the bottom.

Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
11

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala

B.

Place packing material into the top sieve tray and allow it to trav
el through the sieve
trays by shaking
.

C.

Weigh and record the mass of material in each sieve tray.

2. Determine the Packing material void fraction as
follows:

A.

Place small amount of dry particles in a graduated cylinder and record the apparent
volume.

B.

Measure out 50.0 ml of water in another graduated cylinder.

C.

Slowly add dry particles to the water and wait until the particles settle.

D.

Record the volume of

combined water and dry particles.

E.

Obtain the void fraction of packing material.

F.

Load the fluidized bed with fine silica up to 6 inches.

G.

Allow all particles to settle down by gently tapping the fluidized bed column.

H.

Turn on the main air supply.

I.

Slowly tur
n on the air flow by carefully opening valve corresponding to the silica
fluidized bed.

J.

When operating with one fluidized bed, make sure the air flow to the other fluidized
bed is completely closed.

K.

Increase the air flow rate by 5
-
10% and with each increme
nt record th
e gas flow
rate, pressure
drop, height of the bed that
the
silica

or sand

occupies, and visual
changes that occur in the fluidized bed.

L.

Do not decrease the air flow rate until minimum fluidization is reached.

M.

After the minimum fluidization is r
eached, begin to decrease air flow rate by 5
-
10%
and with each increment record the same characteristics as in step K.

N.

Do not increase the air flow rate until the zero velocity is reached.

O.

Increase silica temperature by using Powerstat variable transformer
.

P.

Repeat steps G through I.

Q.

Load the second fluidized bed with Ottawa sand up to 7 inches.

R.

Repeat steps G through I for the second fluidized bed.

S.

Load the fluidized bed with Ottawa sand up to 5 inches.

T.

Repeat steps G through I.

U.

Shut down system by turning
off air supply and closing all valves.

Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
12

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala

V.

Vacuum sand particles in the two columns and the area of experiment.


4. Anticipated Results

In the fluidized bed reactor experiment, the behavior of the fluidized bed is examined by
varying the type of particles util
ized and finding the minimum fluidization velocity. Furthermore,
the reactor bed is fluidized when it starts to bubble and maintains bubbling without losing the
sand or silica grains. Graphical analysis can be used to find a correlation between air flow ra
te,
pressure drop, and the minimum fluidization velocity. Graphically, as the pressure profile
decreases, the slope increases and the minimum fluidization velocity can be determined.
Additionally, the Ergun equation can be used to determine the theoretica
l minimum fluidization
velocity when the particle’s bulk density and quality of the air are specified. Theoretically
fluidizing the bed particles and allowing the particles to return to their stationary phase creates
looser packing between the particles.

This leads to an increase in void fraction and an increase in
bed height.

Tighter packed bed results in an increased friction factor because the bed impedes the air
flow throughout the bed while looser packed beds with larger void fractions decrease the

friction
factor. Consequently, superficial air flow velocity increases as the friction factor decreases.


The Reynolds number in a fluidized bed is independent of the height and temperature of
the bed. Only the void fraction and the diameter of the par
ticles are used to determine the
Reynolds number. Finally, the heat exchanger in the silica bed generates heated air to enter the
packed bed column resulting in heat exchange between the silica grains. Consequently, as the
superficial velocity of the air
increases, the pressure drop in the bed increases resulting in the
particle suspension at the minimum fluidization velocity. Past the minimum fluidization velocity
point, any increase in gas velocity results in turbulent bubbling.

5. References


1)

Perry, R
. H., and D. W. Green.
Perry's Chemical Engineers' Handbook.

New York:
McGraw
-
Hill, 2008. Print.

2)

Subramanian, R. Shankar. "Flow through Packed Beds and Fluidized Beds." Clarkson
University. Web. 1 Mar. 2011.
Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
13

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala

<http://web2.clarkson.edu/projects/subramanian/c
h301/notes/packfluidbed.pdf>
.

3)

Tee, Shang
-
You, P. J. Mucha, M. P. Brenner, and D. A. Weitz.
"Velocity fluctuation
s in a
Low
-
Reynolds
-
number fluidized Bed."

J. Fluid Mech

596 (2008): 467
-
75. Cambridge
University Press, 18 Oct. 2007. Web. 1 Mar. 2011.
<
http://www.amath.unc.edu/Faculty/mucha/Reprints/JFMfluidized.pdf>.


Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
14

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala

6. Appendix I: Job Safety Analysis (formerly called WP &C)

What is the purpose of this experiment?

The purpose of this lab is to increase air flow incrementally through a bed of fine part
icles of
Ottawa sand, sea sand, a
nd silica with varying dimensions

until the bed is fluidized. The
volumetric air flow
-
rate will be regulated
and the
pressure drop
s will be recorded
. There is a
heater to heat the silica bed from ambient temperatures while

the sand bed will be
maintained at ambient temperature.

What are the hazards associated with the experiment?

The sand and silica in the columns are known to spill onto the floor around the column
under careless operation. The floor around the u
nit is sli
ppery as a result. Constant
maintenance of the laboratory area must be conducted to prevent
slip
ping

and fall
ing while
walking
near the unit.

Compressed air
is used to fluidize the bed. C
are must be taken to wear eye protection so
that suspended partic
les do not blow into the e
yes of the operator. The heater

is located
directly adjacent to the fluidized bed lab apparatus which could expel excessive amounts of
heat

which can cause burns. Silica is

hazard
ous

if inhaled and can cause damage to the eyes,
skin, and digestive tract.

How will the experiment be conducted in a safe manner?

The floor needs to be kept clean by using the vacuum cleaner to remove particles that spill
onto the floor. Eyewear must be worn at all times to protect the eyes from sand or silica,
which
can be expelled
from the column or through leaks in the column by
the compressed
air. Individuals handling the silica should be sure to wash their hands prior to eating or any
other activity. Care should be taken not to inhale silica dust particles.

What safety controls are in place?

The air inlet regulator has an inte
rnal diaphragm to maintain a constant inlet pressure of 40
psig. If the pressure surges, the regulator valve will relieve the pressure to maintain the
inlet pressure. The vacuum cleaner has a filter adapter attached to it in order to prevent
excessive du
st accumulation when cleaning up the lab area and adjusting the height level of
the bed in the column.

Fluidized Bed Reactor


University of Illinois


Unit Operations ChE
-
382 Group No. 6

p.
15

Spring 2011
10/30/2013

Buch, Chetty, Gutierrez, Patel, Quan, Vahanwala

Describe safe and unsafe ranges of operations.

Operating at a superficial velocity below the settling velocity will prevent blowing sand or
silica out
th
e top
of the column. Controlling the increments of increasing air flow
ing

through
the column will also prevent the discharge of the bed out the top of the column. The silica
column cannot be heated to temperatures greater than 110°F. Plexiglas melts at i
s
approximately 265 ° and operating conditions should never reach these temperature ranges.


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

Fluidized Bed Reactors
” and understand the hazards associated with conducti
ng this
experiment. I have plan
n
ed 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, a
nd other lab
students, and will look out for their safety as well.


Signatures:
_
Sana Buch________
____
_______
__________________




_
Priya Chetty
____________________________________




_
Liliana Gutierrez
_
_______________________________
_





_
Vijeta Patel
_____________________________________




_
Linda Quan_____________________________________



_
Lipi Vahnwala__________________________________