Download F_b_word

King saud university
Chemical Engineering Department
Fluidized bed
Name : Salman Alfihed
Number : 425104424
CHE 313
Dr. Malek Alahmed
# Introduction :
Fluidized beds are commonly used in chemical, biochemical and
petrochemical industries in processes such as hydrocarbon cracking, drying of solids,
combustion and gasification of coal and biomass, thermal treatment of metals,
recovery of energy from gases and hot solid particles, synthesis reactions and coating
of particles.
Gas-solid fluidized systems are characterized by temperature uniformity and high heat
transfer coefficients, due to the intense mixture of the solid material by the presence
of gas bubbles.
The phenomenon of fluidization :
A bed of lose particles offers resistance to fluid flow through it. As the
velocity of fluid increases, the drag force exerted on the particles increases.
If the fluid is flowing downward through a bed of particles it well tend to compact it.
However, if the fluid flow is upwards through the bed, the drag force well tend to
cause the particles to rearrange themselves within the bed to offer less resistance to
the fluid flow. Unless the bed is composed of large particles the bed will expand.
With further increase in the upward fluid velocity, the expansion on the particles well
be sufficient to support the weight of the particles.
In this state, the fluid / particles system begins to behave like a fluid and it well flow
under a hydrostatic head. This is the point of ( incipient fluidization ). The pressure
drop across the bed well be equal to the weight of the bed although it is likely that this
pressure drop well be exceeded just prior to the achievement of fluidization with gasfluidized system because the residual packing and interlocking of particles within the
bed must first be broken down. The peak is not present when fluidizing beds of large
mean particles diameter with a gas and bed expansion only begins to occur after the
point of incipient fluidization then. Whilst vibration or gentle tapping of the container
assists the particles to compact if the fluid is flowing downward through the bed of
particles it will also usually assists the uniform expansion of the fixed bed when the
fluid is fowling upwards through the bed and produce a pressure loss/velocity curve
without a 'hump'.
At the onset of fluidization the bed more or less uniformly expanded and up to this
point , it male little deference whether the fluid is a liquid or gas apart from the fact
that the velocity at which the bed become fluidized is less for a given bed particles
size and density when the fluid is a liquid. Beyond this point, however, the bed
behavior is markedly different, if the fluid as a liquid the bed continues to expand
uniformly with increase with the liquid velocity. If the fluid is gas the uniform
expansion behavior is soon lost except with fine particles, the system becomes
unstable and cavities containing few solids are formed.
These look like bubbles of vapor in a bulling liquid. The value of the gas flow rate at
which this happened depend on the particles of the fluidized solids, the design of the
bed and particulary on the type of gas distributor used.
Over the flow range between incipient fluidization and the onset of bubbling the bed
in a (quiescent) state. For large, closely sized spherical particles the ratio between the
bubble-point velocity and that for incipient fluidization will be small. For smaller,
irregular particles it will be large.
The bubbles are responsible for inducing particle circulation within the gas fluidized
bed and it is this circulation which has a most important bearing on the advantageous
heat transfer properties of gas fluidized system.
At high fluid velocity, whatever a fluid is a gas or liquid, a point is reached where the
drag force are such that the particles become entrained within the fluid stream and are
carried from the bed.
Smaller particles tend to become entrained at lower fluid velocities than larger ones
and the way that bubbles at the surface of gas fluidized bed and threw a spray of
particles into the space above the bed can considerably affect the rate lf loss of
particles from a gas fluidized bed. Between the extreme conditions of the packed bed
and that of solids transport-hydraulic or pneumatic conveying there lies the regime pf
fluidization. Throughout this regime the pressure drop across the bed remains
approximately constant and sufficient to support the weight of the bed.
Uses of fluidization :
The uses for fluidized beds are limited to our imaginations. Typical uses include :
- Reactors:
Cracking hydrocarbons.
coal gasification.
carbonization.
- heat exchange.
- Drying operations.
- Coating (example, metals with polymer).
- Solidification/Granulation.
- Growth of particles .
- Adsorption/desorption.
Theory in fluidized bed :
The term (fluidized bed) describes a finely granulated layer of solid material
(referred to as “the mass”) that is loosened by fluid flowing through to such an extent
that the particles of solid material are free to move to a certain degree. It is called
“fluidized” because the solid material takes on properties similar to those of a fluid.
Fluidized beds are used widely in engineering for applications such as combustion,
reactors, drying plants, and powder coating. To characterize a fluid bed, the pressure
loss of the fluid flowing through the bed can be used. As the fluid flows through the
solid material, the pressure below the mass initially rises with increasing air speed.
This occurs until the pressure forces match the weight of the mass and the material
becomes suspended. At this point, the layer reaches a fluid state. With further
increasing flow rate, the pressure loss is almost constant. After a certain flow rate, the
top particles no longer fall back into the fluidized bed; they are drawn along with the
fluid flow and removed. The characteristics of the transfer of heat from a heated body
to the surroundings also change on the formation of a fluidized bed. In the solid
bed, the transfer of heat is determined largely by the very low conductivity of the
mass of particles. Part of the heat is removed by the fluid flow; therefore, the heat
transfer slowly increases with fluid flow.
However, once the particles are in motion, the heat transfer is defined by the moving
particles. Due to the higher specify heat capacity of the particle material, the heat
transfer increases significantly. This allows for an extremely even temperature in the
fluidized bed.
# Heat transfer with fluidized bed :
Energy addition or extraction from fast fluidized bed are commonly
accomplished through vertical heat transfer surface in the form of membrane walls or
submerged vertical tubes.
Heat transfer coefficients:
Heat transfer coefficients for the vertical fluidized bed are higher than for the
horizontal bed at the same cross- sectional velocity. With no bed, the converse is
generally truer10 as shown in next Figure. When the two models are run with a bed,
the vertical coefficients are higher, due to more uniform fluidization . Most
experimenters agree that the increased heat transfer of a fluidized bed is due to
destruction of the boundary layer around the tubes. The boundary layer in a crossflow situation will probably be more difficult to uniformly minimize than in a parallel
flow situation in the presence of a fluidized bed. Therefore, the vertical unit provides
an upper limit for the coefficient achievable in the horizontal unit . Interpretation of
the data shown in the Figure is more complex in the case of the horizontal unit.
The cross-section velocity reported is based on the total open cross-sectional area and
neglects dead areas, since some flow does go through this non-moving bed. As the
flow rate increases, the dead areas at the edge of the horizontal unit become smaller
creating a more uniformly fluidized bed. This means that the actual cross sectional
velocity does not change linearly with increased flow rate. Bed flow patterns in both
units constantly shift which accounts for scatter in the data.
The heat transfer coefficient can be calculated from:
,
where P is the heater power, ΔT is the difference between the
heater and bed temperatures, and Ah is the surface area of the heater. The
heater surface area is the area of the cylindrical surface and the open face.
General characteristics :
Some of the generic characteristics were indicated by the early data of kiang
(1976), general characteristics maybe noted as :
-
magnitude of the heat transfer coefficient generally decreases with
elevations along the bed.
At low elevations the heat transfer coefficient generally decrees with
increasing gas velocity.
At higher elevations the heat transfer coefficient increases with increasing
gas velocity.
magnitude of the heat transfer coefficient is higher than that for equivalent
air convection, but lower than that found in dense bubbling beds.
These characteristics point to a correlation of the heat transfer coefficient with local
solid concentration, solid concentrations tend to be higher in the bottom region of fast
fluidized bed ; hens the higher heat transfer coefficient in that region,
Increasing gas velocity increases the upward acceleration of sold particles, causing a
decreases of sold concentration at the bottom and increasing concentration of the sold
at top regions with corresponding change in the local characteristics.
Heat transfer processes in bubbling fluidized bed combustion
boiler furnaces :
In furnaces of fluidized bed boilers with stationary, bubbling beds, specific
conditions for heat transfer exist. Compared to the conventional boilers there are two
major differences: combustion temperature is lower ( 800 – 900 °C ), and solid
particle concentration (not only in the fluidized bed) is much higher. So, heat transfer
by radiation is a less important process in FBC boilers. In conventional boilers heat
energy is transferred by two mechanisms—gas convection and radiation, while in
FBC boilers three mechanisms are in effect: radiation, gas convection and heat
transfer by contact of solid particles.
Average furnace temperature in FBC boilers is not dramatically changed along the
height, i. e., with the distance from the distribution plate. In the fluidized bed a
temperature of 800–900 °C is maintained, depending on the ash sintering temperature
and optimal conditions for SO2 bonding with CaO. Temperature is practically
uniform in the whole bed volume. Due to intensive heat transfer from the hot particles
of inert bed material to the cold combustion air entering through the distribution plate,
even in the immediate vicinity of the plate the local bed and gas temperatures are
equal to the average bed temperature. Temperature drop in the vicinity of the furnace
walls is also limited to very narrow regions. When burning fuels with high volatile
content (lignite's, biomass) freeboard temperature can be higher than bed temperature.
In furnaces used for production of high-temperature gases, with walls covered with
fire brick and other insulation materials, temperature in the freeboard can be up to
100–200°C higher than the bed temperature. Temperature immediately above the bed
in fluidized bed boilers with water-tube walls is not much different from that in the
bed. Further away from the bed’s surface, temperature differences along the height
and in the cross-section are much greater than in the fluidized bed.
In the fluidized bed region heat transfer by solid particles prevails, and in the
freeboard, far from the bed surface the radiation mechanism prevails. One should
have in mind that the glowing bed surface also radiates to the surrounding walls in the
region above the bed and to the other irradiated heat transfer surfaces in the furnace.
Intensity of the heat transfer process can also greatly differ along the furnace height.
In fluidized beds, heat transfer coefficients to the immersed surfaces range from 250
to 700 W/m2K . In the freeboard heat transfer coefficients are less than 100 W/m2K.
When considering heat transfer processes in fluidized bed furnaces it should be borne
in mind that in FBC boilers particles of type B, according to Geldart, are usually used.
Inert material particle size also ranges widely, with a significant percentage of
particles smaller than 1 mm, but also with a high content of very large particles when
fuels with large particles and a high content of tramp material (stones) are used.
In fluidized bed furnaces and boilers, fuels with high ash and moisture content can
also be burned, because cold fuel particles are rapidly heated in the fluidized bed of
inert material particles. Heat transfer from bed particles to fuel particles needs to be
analyzed and studied in order to correctly model the combustion process. This process
is especially important during furnace or boiler start-up.
Heat transfer in the fluidized bed has been the most studied process since the first
ideas for using fluidized beds in different technological processes. In the course of
several decades, an enormous number of mostly experimental works have been
published. In the last several years interest in heat transfer processes in bubbling
fluidized beds gradually decreased, although there are not enough reliable data for
beds of larger particles, which are important for fluidized bed combustion boilers.
Heat transfer between gas and solid particles in bubbling
fluidized beds :
The temperature field in the fluidized bed is practically homogenous due to
intensive mixing of particles in both horizontal and vertical directions. At temperature
levels of several hundred or even a thousand degrees Celsius, in conditions of even
fluidization, temperature differences do not exceed 2–5 °C. Gas temperature at the
exit from the fluidized bed is practically equal to the bed temperature, i. e.,
temperature of the solid particles. These facts speak of very high ability of the
fluidized bed of solid particles to exchange heat with the fluidizing gas. This ability to
exchange high quantities of heat between gas and particles is caused by a very large
specific surface used for heat transfer (3000– 45000 m2/m3), although average heat
transfer coefficients for gas to particle (or vice versa), calculated per m2 of particle
surface, are relatively small (6–25 W/m2K). The large heat capacity of solid particles
also contributes to small differences in gas and particle temperatures in the fluidized
bed. Gas temperature follows particle temperature and not vice versa. Heat transfer
from gas to particles is never the limiting factor in organization of processes in the
fluidized bed.
In solid fuel fluidized bed combustion, it is considered, based on sufficient
experimental data, that char combustion after devolatilization takes place in the
emulsion phase. Combustion of volatiles partially takes place in the bubbles as well.
Air used for combustion enters through the distribution plate at much lower
temperature than the bed temperature and partly traverses the bed in the bubbles.
Hence it is necessary to take into account heat and mass transfer between the bubbles
and emulsion phase when dealing with the combustion process in the fluidized bed.
As has already been remarked, heat transfer between bubbles and emulsion and also
between the gas and particles in emulsion, is very intensive and at small distances
from the distribution plate temperatures of gas in emulsion and bubbles and particles
are practically equal.
Because of that heat transfer between emulsion and bubbles is not crucial for fluidized
bed combustion processes.Oxygen consumption is always higher in the emulsion, so
combustion efficiency and necessary excess air greatly depend on oxygen transfer
between bubbles and emulsion.
Heat transfer between bubbles and the emulsion phase may be treated taking as the
basic assumption the two-phase fluidized bed model and gas exchange between
bubbles and emulsion. Described processes of gas and heat transfer between bubbles
and the emulsion phase as well as the expressions and models for calculating the
amount of transferred mass and energy enable the modeling of combustion to take
these processes into account in a physically correct manner.
Mechanisms of bed-to-surface heat transfer :
One of the most important properties of solid particle fluidized beds is very intensive
heat transfer to immersed surfaces. Since fluidized beds are at the same time very
suitable for numerous physical and chemical processes with heat consumption or
generation, it is very often the case that heat is removed or added, using heat transfer
surfaces immersed in the bed. These may be:
tube bundles, spiral pipe exchangers or pipes making up the walls of a reactor furnace
(water-tube walls).
Intensive heat transfer to the immersed surfaces is the consequence of great heat
capacity and mobility of particles. Particle heat capacity is around 1000 times greater
than the heat capacity of gases. Mechanisms by which heat is transferred from the
fluidized bed to immersed surfaces (or vice versa) are numerous and very complex.
Virtually all known heat transfer mechanisms are in effect.
heat transfer by particle motion and contact (particle convection), by which the heat is
transferred from the fluidized bed mass to the exchanger surfaces, heat transfer by
gas convection, by which the heat is transferred from the gas to the exchanger surface,
This process may be divided into components:
heat transfer by gas in bubbles, and heat transfer by gas in emulsion, and heat transfer
by radiation.
Due to the different physical nature of heat transfer mechanisms between the fluidized
bed and immersed surfaces, with the change in flow, geometrical and physical
parameters of the bed, great differences in heat transfer intensity occur. The heat
transfer process, except on the flow parameters, particle mixing characteristics and
overall, organized circulating flows in the bed, also depends on numerous physical
properties of particles and gas, and physical and geometrical characteristics of the
heat transfer surface and the fluidized bed.
# advantages and disadvantages of fluidized bed:
The advantages of fluidized bed :
- Liquid like behavior, easy to control and automate.
- Rapid mixing, uniform temperature and concentrations.
- Resists rapid temperature changes, hence responds slowly to changes in
operating conditions and avoids temperature runaway with exothermic reactions.
- Circulate solids between fluidized beds for heat exchange.
- Applicable for large or small scale operations.
- Uniform Particle Mixing.
- Uniform Temperature Gradients.
- Ability to Operate Reactor in Continuous State
- Heat and mass transfer rates are high, requiring smaller surfaces.
The disadvantages of fluidized bed :
- Bubbling beds of fine particles are difficult to predict and are less efficient.
- Rapid mixing of solids causes non-uniform residence times for continuous flow
reactors.
- Particle (breakup) is common.
- Increased Reactor Vessel Size.
- Pumping Requirements and Pressure Drop.
- Erosion of Internal Components.
- Particle Entrainment.
- Pipe and vessel walls erode due to collisions by particles.
# Design Recommendations :
A vertical exchanger requires tube bundle introduction from the top of the unit which
favors an even number of tube passes and makes it difficult to put several stages in a
single vessel. A disengagement section of approximately 15 cm must be left above the
bed to prevent particle carryover. This means that the top 15 cm of tubes will not be
covered with the fluidized bed. This short section of tubes will be susceptible to
increased corrosion rates and scaling.
Geometric considerations show that the horizontal design is more conducive to large
flow rates. This is due to a larger cross-sectional area presented by horizontal vessels
for the same length and number of tubes.
One possible way of increasing the flow capacity of the vertical unit is to supply a
larger shell than is needed for the tube bundle. Since the bed is isothermal and well
mixed, having portions of a vertical assembly with no tubes will not impair the heat
transfer efficiency .
A second possibility to increase flow rates through heat exchangers is to increase the
density of the bed material .
# Conclusions :
-
Smaller particles tend to become entrained at lower fluid velocities than
larger ones and the way that bubble.
the actual cross sectional velocity does not change linearly with increased
flow rate.
Fluidization is uses for reactor, heat exchanging, drying operations ..etc.
Heat transfer coefficients for the vertical fluidized bed are higher than for
the horizontal bed.
Fluidized bed has many advantage, also has many disadvantage .
Approximately 15 cm of each tube at the top of the unit must be
left uncovered by the bed.
In design the vertical tube is best because its give good heat transfer rate.
# References :
-
-
-
L. T. Cole, LIQUID-FLUIDIZED-BED HEAT EXCHANGER FLOW
DISTRIBUTION MODELS, 1979.
J. S. M. Botterill, FLUID-BED HEAT TRANSFER, academic press, 1975
.
Wen-Ching yang, FLUIDIZATION SOLIDS HANDLING AND
PROCRSSING, noyes publications usa,1999.
J. R. Howard, FLUDIZED BED COMBUSTION AND APPLICTIONS,
applied science publishers London and new york, 1983.
- E.J.Anthony, Fluidized Bed Combustion, MARCEL DEKKER, INC.,
1994.
http://www.pharmainfo.net/fluid-bed-granulation-articles.