Abstract page
One of the most important assets of the industrial application these days is the Fluidized bed reactors. When gasses, solids, and liquids contact simultaneously, a large number of industrial process reactions are involved. All these operations need a multiphase reactor to be carried out. Since a variety of multiphase chemical reactions can be carried out in a Fluidized Bed Reactor, it is interesting to learn about its functioning, various types, application, advantages, and disadvantages. Initially, fluidized bed reactors were built for the petrochemical and oil industries. Now, they are widely used for producing Gasoline and other fuels, plastics, polymers, nuclear power and chemical intermediates. Fluidized Bed Reactors offers a very efficient and cleaner process than other previously used technologies.
The present study aims to learn about the normal functioning of the fluidized bed reactors, about its history, its basic principle of working and about its applications. Also, to know about what are various types of fluidized bed reactors, how they work, what are their various applications and what are the advantages and disadvantages of all these various types of reactors. The detailed analysis of the fluidized bed reactors along with its various types has been provided in the report.
The type of reactor device in which a variety of multiphase chemical reactions can be carried out is called a Fluidized Bed Reactor. The most important feature of the Fluidized bed Reactor is that due to the upward flow of the reacting fluid, solids are held in suspension which promotes heat-transfer rates and high mass thereby enabling good mixing. In this reactor, the fluid which can be either liquid or gas is passed through a catalyst which is a granular solid material at very high velocities so that solids are suspended. In the FBR, the jackets and internal coils are able to obtain heat-transfer coefficients of the order of 200 W/m^2 degree centigrade. The fluidized bed reactor can be used in many industrial applications.
History of the Fluidized Bed Reactors
The first fluidized bed reactor was developed by Fritz Winkler in the 1920s in Germany. The first commercial fluidized bed reactor, Powdered Catalyst Louisiana, also called PCLA#1 was created in the Baton Rouge Refinery, New Jersey by the Standard Oil Company on 25th May 1942. This bed allowed the cracking of heavy gas oils efficiently to meet the growing demand for high octane-fuels. Thus, many FBR were built for the petrochemical and oil industries. With the invention of this technology, the production of various fuels in the United States was significantly increased.
These days Fluidized bed reactors are widely used for producing Gasoline and other fuels, plastics and chemical intermediates. Using FBR technology, many industrially produced polymers like rubber, styrene, polypropylene, vinyl chloride etc. are produced. FBR's are also used for nuclear power plants, coal gasification, waste and water treatment settings. Thus, Fluidized Bed Reactors offers a very efficient and cleaner process than other previously used technologies.
Principle of working of Fluidized Bed Reactor
In a fluidized bed, the granular solid material is subjected to up flow fluid through the immobilized enzyme bed which is sufficient to support its weight. At this point, solid behaves like a fluid and can be transferred from and to the reactor easily. The velocity of the solid substrate is not that high that the enzymes may sweep away from the reactor entirely. Thus, the mixing is done which is complete mixing as in the CSTR model. In the fluidized bed, the nature of the reaction is such that, on the catalyst, carbon is deposited which deactivates it rapidly. Thus, a catalyst can be removed continuously and send to the second fluidized bed where carbon can be burnt off by fluidizing with air.
One of the common examples of the fluidized bed reactor is the catalytic cracker which gained much of the importance in the 1930s. The fluidized bed reactors have most recently found considerable interest in the combustion of coal as when powdered limestone is entered into the bed, solid CaSO4 is formed by the conversion of Sulphur present in the coal which helps in reducing atmospheric pollution. So, in exothermic reaction fluidized bed are very advantageous as the efficient mixing within the bed helps in eliminating the hot-spots and heat is also transferred to the walls of the bed.
Types of Fluidized Bed Reactors
There are three types of Fluidized Bed Reactors which are mentioned below:
1. Stationary or Bubbling Fluidized Bed: Bubbling Fluidized Bed is formed when fluid is introduced through the bed at a low fluid velocity which forms the bed of the solid particles. Thus, many of the characteristics of the fluid are exhibited by the solid mass. This results in the formation of the fluidized bed and the phenomenon are called the fluidization. The bed particles are relatively stationary when the fluid velocity is at minimum fluidization velocity. When the fluid velocity just reaches above the minimum fluidization velocity, two phases of the fluidized bed are created: bubble phase and emulsion phase.
With the bubble movement, heat is transferred as the intense gas and solid starts contacting. Also, due to the intense mixing of the gas and the solids, the isothermal condition of the process is created. Thus, stationary fluidized bed reactors are very suitable for various chemical reactions, heat transfer applications, mixing and drying.
Applications of Bubbling Fluidized Reactors:
In small scale industries, these reactors can be used as a boiler. These can also be used in fuel separation with high moisture content and low heat value. Since these reactors can achieve an efficiency of around 90 percent and have low emission conversion, these are commonly used in combustion and gasification of coals as they are environment-friendly.
In pharmaceutical industries, for granulation, coating and drying purposes due to the isothermal properties of these reactors which allow good mixing and intense particle activity.
In heating or cooling of particles or fluids. They are also used in catalytic cracking of petroleum.
Advantages of Bubbling Fluidized Reactors:
The Bubbling fluidized bed reactors offer many advantages as compared to pulverized coal boiler as in bubbling reactor the fuel supply system is very simple. It has low corrosion rate and reduces the emission of nitrogen oxides. The low caloric fuel can also be burnt in bubbling reactors and thus it offers fuel flexibility. Also, there is no slagging in the bubbling bed reactor.
As compared to continuous stirred tanks reactor, bubbling reactors have the high catalytic surface area, high conversion and provides good mixing. It is easier to handle the gas released during reaction and transportation of large solid quantities is also very easy.
The Bubbling bed reactor can be used for coating, granulation, and drying. It has low weight and low floor area is required. The temperature difference between the solid materials is also very low.
Disadvantages of Bubbling Fluidized Reactors:
The cross section area of the furnace of the Bubbling bed reactor is quite large and heat loss from the surface is very high. In ash, the level of carbon is also very high. The erosion rate is quite high.
This reactor is only suitable for dense and small particles or catalyst. For reactors, having a large diameter, there are scaling up issues.
The reactor needs high electrical energy and expensive air system. This is only suitable for particles having a size less than 3mm.
2. Circulating Fluidized Bed Reactors: These reactors are relatively new technology in which emission of pollutants is very less. In these reactors, the solid fuels are suspended by the upward jets of air in the combustion phase. Thus, better chemical reactions and heat transfer are obtained as solids and gas mix together turbulently. In order to ensure that nitrogen oxide is not formed, the fuel is burnt at 1400 degree Fahrenheit to 1700 degree Fahrenheit of temperature. Though Sulfur dioxide is released during the burning of the fuel but due to sulfur absorbing chemicals, almost 95 percent of the pollutants are absorbed which promises environmental friendly characteristics.
The recycling of the sulfur absorbing chemicals and fuel can be done to produce high- quality steam and lower the pollutants emission. Thus, in comparison to other traditional bed reactors, circulating fluidized bed reactors can be used to burn fuel in a very environment-friendly method. Due to this, in different areas ranging from gas and oil to power stations, these bed reactors can be applied.
Applications of Circulating Fluidized Reactors:
It is most commonly used in combustion of carbon-containing fossil fuels and for the production of energy as it is environmentally friendly.
It is also used in dry-scrubbing of the process. Circulating fluidized bed scrubber is the most common application of this reactor in which flue gas enters the reactor from the bottom of the vessel. It is used in power stations to reduce pollutants like HCl, HF, SO2 and SO3 in a gas stream.
Circulating fluidized bed gasification system is also one popular application of circulating bed reactor. In this, the power plant waste gas materials are converted into the synthetic gas without any combustion.
Advantages of Circulating Fluidized Reactors:
This reactor offers excellent heat and mass transfer as there is high mean relative velocity between solids and gas.
The reactor circuit always maintains the temperature uniformity by continuous recycles of hot solids at a high rate.
As fine-grain solids circulate at a highly expanded bed, it provides short diffusion paths and high specific exchange surface.
The retention time of solids is adjustable as it can be changed by varying bed inventory. The retention time of gasses is also adjustable up to several seconds.
One or more gaseous reagents can be safely added at different levels.
Since the operating temperature is lower, the nitrogen oxide production is also low which is a contributor to smog.
Disadvantages of Circulating Fluidized Reactors:
The circulating fluidized bed scrubber can be operated only up to 400 MW per unit. The products used in it like limestone are expensive and cannot be kept in a pile but, either in steel or concrete silo.
The bed reactor produces by-product like CaCl which is of not much use.
Because of the low operating temperature, it is not possible to achieve high gasification yield for less reactive fuels like pet coke and anthracite.
The flow of gasses is multiphase complex and different scaling is needed for every distinct particle.
3. Vibrator Fluidized Bed Reactors: The vibrator fluidized bed function when heated air is passed through a bed consisting of perforated plate mounted on a plenum chamber. Vibrating fluidized beds are normally plug-flow type. Two major parameters that should be considered while designing a vibrator fluidized bed are amplitude which should be a few millimeters and frequency which should be in the range of 5-25 Hz. Various flow patterns can be used in vibrator fluidized bed depending on the configuration and the type and number of chambers that will be used. In these reactors, the pressure drop between the bed and the distributor plate is much less than the traditional fluidized bed reactors.
The vibrator fluidized bed are often shallow as, more the distance from the vibrating grid, lesser is the effect of the vibration. These bed reactors can operate in both continuous and batch modes.
Applications of Vibrator Fluidized Bed:
The vibrator fluidized bed reactors can be applied to abrasive, fragile and heat sensitive materials as when vibration is combined with the upward flow of air, the particles can pseudo fluidize themselves and smoothly. The attrition caused by vigorous actions between particle-wall and particle-particle is very less
It can be used in polydisperse powders as the mobile state of the coarse particles can be kept with the help of vibration.
Vibratory beds can be used for processing the food matrices that pass through a cohesive or softening phase as it helps in maintaining the live fluidized state of such materials when they are transferred from one point to another. The food products when not processed through vibrating beds fluidize poorly due to high temperature, fragile and sticky nature etc.
These are used in drying of whole milk powder due to improved solid interaction and circulation.
Advantages of Vibrator Fluidized Bed:
In Vibrator fluidized bed, gas velocity is lowered by 8-25 percent to avoid excessive separation of particles based on their shape, size etc. while vibrations are used for keeping large particles in motion.
In these bed reactors, the pressure drop between the bed and the distributor plate is much less than the traditional fluidized bed reactors.
Reduction in airflow on applying vibration helps in reducing product transport by the flow of the fluid.
The cost and size of gas-cleaning equipment can be reduced considerably without any effect to the drying kinetics.
Disadvantages of Vibrator Fluidized Bed:
Vibrator Fluidized Beds normally have acoustic noise associated with them.
Conclusion
Fluidized bed reactors have been proven to be very reliable, useful, cost-effective and well-suited for industrial applications. From nuclear power plants to coal gasification to waste and water treatment settings and producing various polymers, fluidized bed reactors have been found to be very useful. Fluidized Bed Reactors offers a very efficient and cleaner process than other previously used technologies. FBR takes only 10 percent of the electricity that was consumed by traditional rod reactor. Presently, Fluidized bed reactors designs depend mainly on plant observations, engineering, and correlations. More attractive options may be available with the computational fluid dynamic approach. A Significant gain in the solar polysilicon's market is expected to be made by the fluidized bed reactor technology.
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