Homotopy excision theorem

Stationary or Bubbling Fluidized Bed is formed when bed of solid particles levitate due to an introduction of fluid that flows through the bed at low fluid velocity. In this state, the solid mass behaves and exhibits many of the characteristics of a fluid. This phenomenon is known as fluidization and results in a fluidized bed.

In a condition where the fluid velocity is at minimum fluidization velocity, the bed particles are relatively stationary. At fluid velocity just above the minimum fluidization velocity, the fluidized bed can be treated as if it consists of two phases with the formation of bubble and particulate (emulsion) phase.

Bubble movement promotes intense gas solids contact and heat transfer. Furthermore, near isothermal condition of the process is also possible due to the intense gas solids mixing. Therefore fluidized bed is ideal for various chemical reaction, drying, mixing, and heat transfer application.

History

Fluidized bed was only used to gasify coal in Europe until a significant breakthrough in the early 1940s. The need of better method in cracking long chain crude oil molecule into valuable gasoline and lubricant products rises due to widespread regional war during World War II. In 1938, Exxon Research joined a consortium of large oil and processing companies that came up with the concept of fluidized bed catalyst, which is then used for catalytic cracking of crude oil feed in gasoline production. Development of fluidized bed as a cracking unit significantly increases gasoline production and allows production of higher octane gasoline.

Operating principle

Fluidized solids state occurs when bed of solid particles is penetrated by fluid flowing vertically upwards with sufficient velocity to break up the bed. Bed of solid particles fluidized by using liquid is classified as homogeneous fluidization whereas heterogeneous fluidization occurs when the bed is fluidized by using gas.

Minimum fluidization velocity is the velocity required to break up the bed which varies with the type of solid particles. Stationary fluidized bed operates at a condition where the fluid velocity is at minimum fluidization velocity. At this stage, the bed particles are relatively stationary. As fluid velocity increases and just above the minimum fluidization velocity, bubbles begin to form. At this state, the fluidized bed can be treated as if it consists of two phases, bubble and particulate (emulsion) phase.

Bubbles are formed near the distribution plate, which rise and coalesce with other bubble to form large bubbles. Bubble continues to rise and erupt near the bed surface and ejecting particles away from its surrounding. Subsequently, solid particles are driven upward by the bubble movement, following the trail behind the bubbles. However, around and between the bubbles, and near the wall, particles are moving downwards. Therefore, the intensive particle circulation due to bubble movement promotes good gas and solid axial mixing. In addition, the intensive particle and bubbles movement makes the solid bed behaves and inhibits various fluid characteristic as the name “bubbling fluidized bed” suggested.

Application

Bubbling fluidized bed is commonly used in cracking and reforming of hydrocarbons, drying, adsorption, granulation, coating, freezing, heating, cooling and many other applications. In general,

• Bubbling fluidized bed can be used as a boiler in small scale applications, especially in fuels separation with low heat value and high moisture content. The efficiency that can be achieved is around 90%. Commonly used in combustion and gasification of coals since they can achieve high efficiency and low emission conversion, which means is more environmental friendly.
• Bubbling fluidized bed can be used as chemical reactor because it provides a good mixing condition and increases heat transfer rate, mass transfer rate and reaction rate of the process.
• Bubbling fluidized bed is also preferably used in pharmaceutical industry as drying, coating and granulation purposes due to the isothermal property of bubbling fluidized bed which promotes intense particle activity and good mixing.
• Since fluidized bed promotes heat and mass transfer rate due to the large fluid-solid contact surface, it is always used in cooling and heating of fluid or particles.

Since stationary and bubbling fluidized bed has very flexible operating principle, it has wide range of usage in many types of industry. With some modifications, it can be used as a reactor, boiler, dryer or heat exchanger as mentioned above. Some specific usage of bubbling fluidized bed is shown below,

• Catalytic cracking of petroleum
• Fluid catalytic reforming
• Fluid coking
• Fluid catalytic oxidation of ethylene
• Fluidized bed production of alkyl chlorides
• Fluidized bed iron ore reduction
• Fluidized bed roasting of pyritic ores, gold ores and limestone
• Fluidized sizing and drying
• Fluidized bed coal gasification and carbonization
• Fluidized bed hydrocarbon synthesis
• Fluidized bed production of phthalic anhydride

Advantages and limitations over competitive process

Advantages and limitations of bubbling fluidized bed boiler as compared to pulverized coal boiler

Advantages	Disadvantages
Simple fuel supply system	Larger cross section area of a furnace
Reduce the emission of nitrogen oxides	Higher surface heat loss
Fuel flexibility(able to burn low caloric fuel	higher carbon in ash level
In situ sulphur dioxide removal	Higher erosion rate
No slagging	Higher power of air van
Low corrosion rate

Advantages and limitations of bubbling fluidized bed reactor as compared to continuous stirred tanks reactor (CSTR)

Advantages	Disadvantages
High catalytic surface area	Scale up difficulty (especially for large diameter reactor)
Good mixing	Only suitable for dense and small catalyst or particles
High conversion
Back mixing can be avoided
Gas release during reaction can be handle easily
Transportation of large quantities of solid as part of the reaction process can be handle easily

Kinetic behavior of bubbling fluidized bed lies between continuous stirred tank and plug flow reactor (PFR)

Advantages and limitations of bubbling fluidized bed dryer as compared to drum dryer

Advantages	Disadvantages
High, uniform mass and heat transfer rate	Suited only for particles below 3 mm
Low weight and low floor area required	Limited drying air temperatures
Can use for coating, agglomeration and granulation	High electrical energy needed
Good control of drying condition	Expensive supplied air system
Low solid material temperatures difference
Short drying time
Can be used as batch or continuous operation

Stationary or Bubbling Fluidized Bed equipment design

Basic Fluidized Bed Components

Figure 1 illustrates the components of a common fluidized bed. This fluidized bed contains a fluidization vessel (freeboard, fluidized bed and gas distributor), a solid feeder/flow measurement, solid discharge, dust separator (for exit gas), gas distributor and gas supply.

Fluidization vessel – Most of the vessel has a common shape of a vertical cylinder. There will be adequate space in the vessel for solids to expand in the vertical direction and entrained solids. Freeboard is the height above the bed and the volume is known as disengaging space. The cross sectional area of the vessel depends on the volumetric flow rate and the allowable fluidizing velocity of the gas.

Bed – The bed height depends on gas contact time, length to diameter (L/D) ratio needed to provide staging, space needed for internal heat exchangers and solids retention time. Most bed heights are between 0.3 m to 15 m. The reactor is normally a vertical cylinder but there is no limitation on shape. The exact design features changes with operating conditions, existing space and use.

Freeboard – The freeboard is the height between the top surface of the bed and the nozzle where gas is exiting in the bubbling-bed unit. Classification of solids and the reaction between solids and gases take place in the freeboard.

Gas distributor – There are two types of grid plates, one is used when the inlet gas contains solids and the other one is used when the gas is clean. It is designed to stop solids from flowing back during normal operation and shut down. Pressure drops of gas or gas and solids flow are restricted from 0.5 kPa to 20 kPa in order to provide distribution. During normal and abnormal flow, the distributors must bear with the differential pressure across the restriction.

Convention Bubbling Fluidized Bed

Figure 2 shows a conventional bubbling fluidized bed and its components. Not all components are essential in the design, depending on the application. Individual components can also be different in each design. For instance, cyclone can be placed internal or external whereas heat transfer tubes can also be assembled vertically or horizontally

Minimum Fluidizing Velocity

Minimum fluidizing velocity is often used in fluid-bed calculations. For a fixed bed of spherical particles with diameter d, Carman-Kozeny equation expresses the relationship between the fluid velocity and voidage ε as shown in equation (1) and (2):

For laminar flow where Remf << 20,

$u_{mf}=\frac{({\rho }_p-\rho )g{d}_p^2}{150\mu }\frac{s^3}{(1-s)}$

For turbulent flow where Remf >>20,

$u_{mf}={\left[\frac{({\rho }_p-\rho )g{d}_p^2{s}^3}{1.75\rho }\right]}^{\frac{1}{2}}$

g is the gravity acceleration, µ is the viscosity of the fluid, ρp is the density of particles and ρ is the density of fluid.

Ergun equation (3) expresses the pressure drop along the length of a packed bed at a given velocity and this equation is used when particles are too big:

$\frac{\mathrm{\Delta }P}{L}=\frac{150\mu (1-s{)}^2{u}_0}{s^3{d}_p^2}+\frac{1.75(1-s)\rho u_0^2}{s^3{d}_p}$

∆P is the pressure drop, L is the bed height and u0 is the superficial velocity of fluid.

Bubbling Fluidized Bed Characteristics

In gas and solid bubbling fluidized bed, only partial bed expansion will occur when the gas velocity increase. Bubbling system takes place when gas velocity is slightly greater than the minimum fluidizing velocity umf. This contributes to a small expansion in the bed. Next, minimum bubbling velocity umb is the upper limit gas velocity of the particulate expansion and it depends on the type of the distributor and small obstacles in the bed. Additionally, the degree of the expansion can be determined by the ratio umb/umf in the equation below where light fine particles have high value and large dense particles have low value.

$\frac{u_{mb}}{u_{mf}}=\frac{4.125\times 10^4{\mu }^{0.9}{\rho }^{0.1}}{({\rho }_p-\rho )g{d}_p}$

Small bubbles tend to become adapted in the dense phase when it is injected into a stationary/non-bubbling bed. Subsequently when gas flow in the dense phase increases, larger bubbles tend to rise. If the bubble is larger than the critical size, the bed will start to expand in an amount which is same as the volume of the injected bubble. The bed will return to the level which is lower than the one before injecting bubbles when the bubble breaks the surface, hence resulting in a smaller voidage.

Geldart Categorization of Solids

The fluidization of solids by a gas highly depends on the properties of the particles. Geldart classified particles into four groups by the particle size and its characteristics. Table 1 shows the characteristics of fluidization solids for each category. Bubbling occurs at category B, the example for the solid material used in this fluidization is sand.

Table 1 Classification of solids and its fluidization characteristics.

Group	Particle size (µm)	Fluidization characteristics
A	30-100	Bed expansion occurs after minimum fluidization. Small particle size and low density.
B	100-800	Bubbling take place at velocity more than umf.
C	20	Solids are fine and cohesive, not easy to fluidize.
D	1000	Spouted beds are made. Particles are large and dense

The groups of solid are plotted in a particle density – particle size chart as shown in Figure 3.

Regime Diagram of Fluidization

Figure 4 depicts the performance of fluid bed at different regimes of fluidization. The flow of fluid becomes more turbulent as the velocity increases. Bubbling, slugging and turbulent regime are under aggregative fluidization. If the diameter of the bed and solids are small, bubbling transits to slugging. Likewise, bubbling becomes turbulent if bed diameter is large and solids diameter is small.

Bubbling Fluidized Bed Process Design

Bubbling Fluidized Bed Granulation

Granulation is a size enlargement procedure that a fine powder integrates into larger granules with a specific size and shape. The major role of the fluidized bed in this process is to mix the granulating powder as a mixer. The main distinction of fluidized bed mixer compared to other types of mixer is that the gas supplied to agitate the powder in fluidizing bed also causes binder evaporation as well as in charge of cooling or heating of the powder. As a result, this process allows drying and cooling stage to be achieved simultaneously with increase of size.

Bubbling fluidized bed granulator is often preferred based on its flexibility and the significant cost saving potential due to most of the essential steps in granulation process can be carried out in one single apparatus.

To achieve a certain size granule from the fluidized bed granulator, other parts of operations are combined to form the granulation circuits as showed in Figure 1 where the fluidized bed is located at the central. During the process, a liquid binder such as a solution or a melt is delivered through a pipe towards the top of the fluidized bed. Then the binder is sprayed onto an agitated bed containing different powders whose main role is to produce shearing forces in the powder mass. Air is pumped towards the bottom of fluidized bed from the fluidization air fan and heated through an air heater. The hot air is used to not only carry out the fluidization process but also evaporate the solvent in the binder. As solvent evaporates the powder particles stick together and consolidate under the shearing force. Inside the granulator, a grinding roller driven by a motor is equipped in order to operate granulation and grinding process simultaneously. On the other hand, a grinder can also be used to control the granule size for the process. The waste gas from the top of the fluidized bed then is passed through a gas cleaning system that consists of cyclones and filters so that the fines and granule discharges can be separated and recycled. Figure 5 shows the schematic representation of a fluidized bed granulation circuit.

Bubbling Fluidized Bed Combustion

Fluidized bed combustors are usually used in process heat supply, steam generation and electric power station. The key difference between fluidized bed combustors and other types of combustors is that coal fuel combustion can be reached at a relatively low temperature of 760 – 930 ℃ with a more uniform temperature distribution in fluidized bed combustors.

In bubbling bed combustor, carbonaceous fuel is fed towards the bottom of the fluidized bed. Limestone is also added in the bottom stream as the sulphur sorbent particles. Then atmospheric air is preheated and delivered to the bottom. The role of the air is not only in charge of fluidization but also contributes to combustion reaction process. The flow rate of the air through the bed directly influences the amount of fuel that can be consumed. As a result the velocity of fluidizing air has a range between 1.5–4 m/s.

The major equation of combustion can be presented as following:

Carbonaceous fuel (C,H,O,N,S)+air (O2,N2)→raw gas(CO2,H2O,N2)+by products

Bubbling bed combustors usually operate with Geldart group B particles which produce slow bubble behavior and good mixing. Almost all of the combustion process can be completed within the dense bubbling bed. Heat transfer can be achieved by converting water into steam in the combustor so that the bed temperature is high enough to ensure the heat efficiency as well as low enough to avoid bed agglomeration. The freeboard section is used to finish CO and volatiles combustion and to separate solids from the raw combustion gas. Furthermore small fuel solids in exiting gas will be captured by cyclones and recycled to the fluidized bed combustor.

Post-treatment System

The byproducts of the process contain SO2, SO3, NOX, halogen compounds and ashes. Sulphuric compounds are the main sources of acid rain and must be removed during the process. As mentioned limestone can be used in removal of sulphur oxides as following:

Limestone (CaCO3, MgCO3) + SO2 + O2→CaSO4, CaO, MgO, CO2

Finally the exiting gas will pass through the denitrification equipment in order to remove most of the nitrogen oxides. Figure 6 shows the schematic representation of a fluidized bed combustion circuit.

References

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