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Fluidization is the process where solid particles behave as fluid. This is achieved by passing a fluid such as air through the packed solid so that the frictional forces balance the weight of the particles until it eventually losses it's order and becomes fluidized. The earliest application of fluidisation occurred in the 16th century when a German scientist named Georgius Agricola described this process to upgrade ores. It was not developed until the 1930's with the progression of the Winkler coal gasification process that the realisation that fluidization could be used in an industrial scale. The desired reaction was a simple conversion of coal to synthesis gas and this was the first practical application of fluidization. Although Winkler generators were generally inefficient this began with fluidization technology and processes being increasingly used in different chemical processes.
One of the multiple benefits of using fluidisation is that once the solid has become fluidised it enhances heat and mass transfer. For example, a primary use is within dryers as the increase heat transfer results in lower heat rates which can easily be controlled letting it to be used widely in many industries which include the drying of multiple products which include maize and sugar. It has other applications which include the used as reactors again for the increase mass and heat transfer as well as good mixing compared with a conventional fixed bed reactor. In addition to this it there are low temperature gradients within such a reactor meaning no hotspot locations, less product degradation leading to less money lost. Prime example of this is production of pthalic anhydride developed in the 1960's. These are only a few of the applications which the phenomena fluidization is used for and this paper will investigate other's and give an overview of the benefits of the use of fluidization.
Despite the obvious advantages of using fluidization within industry there has been limited progress since it was first developed in the 1960's. The complicated mechanism behind fluidization and the lack of understanding has led to difficulties in modelling such a phenomena. This has gave additional problems when scaling up the model which gives rise to cases of defluidization increasing shut down time and decreasing profit. This along with problem of fluidization of smaller particles increasing agglomeration is an additional problem common within fluidized reactors. It remains one of the more difficult processes in industry to model due to lack of understanding of the mechanism and difficulties that accompany such a phenomena.
Although with limited progress work is being done today to overcome such problems. Whether it be to develop computational methods to models, cold models to observe within a section of the process or the use of additives to overcome defluidization, these methods are being investigated and developed daily in order to get a more in depth understanding of the complicated process which is fluidization.
This paper aims to explore what applications are currently using fluidization and if any recent emergence of new uses for fluidization. It will also give details of the most common complications which go hand in hand with fluidization and what exactly is being done presently to overcome such challenges. It will analysis the solutions and explore if they are the best solution or find if more work must be carried out if this problems won't to be kept to a mininium and to increase understanding of fluidization.
The attractive concept of fluidisation as both mass and heat transfer between the fluid and the solid is much better than say a conventional packed tower reactor. Such industries that widely use fludisation today would include drying, chemical synthesis, gasification, polymerisation ore beneficiation and coking.
Summary of Basic Principals of Fluidisation
To understand the applications of fluidization and its challenges it is important to get an overview of the process and get a understand of the mechanism of fluidization. If a solid is poured into a container the solid arrange itself to from a packed bed. However, there a space of gas between the solids, known as voidage, which explains why the volume of a packed bed will always be greater that the actually volume of the solid by itself.
If fluid is introduced at the bottom of the packed bed, it will begin to filter through the gaps of the solids particles. Initially, nothing occurs and the structure of the bed remains the same as the fluid can pass through the gaps, and the solid remains a fixed bed. This fluid velocity is then increased up to a point so that the weight per unit area of the bed matches the pressure drop across the bed. At this point there solid-solid interaction is no guaranteed and may no longer behave as a packed bed. This is known as the minimum velocity of fluidisation and once increased past this velocity then the bed becomes fully fluidised. Now the solid will truly act as a fluid, which means one could put their hand through it or in the chemical industry sense it becomes easy to transport or pass through an orifice.
The behaviour of the bed will depend if the fluid used is a gas or a solid. Generally, with a liquid there is a smooth increase of the bed with increase in flowrate, with no large-scel voids observed. This is not the case for gas and a major problem with using gas as the fluid is that bubbling is known to occur. This results in two phases within the bed and is known as aggregative fluidisation, which depends on both gas and particle properties. Bubbling can be a problem in industry as it leads to poor mixing and decreased fluidisation? This makes it essential for the conditions where bubbling occurs to be known. If the bubbles are big enough and the diameter of the vessel is small enough then slugging may occur. Problems with slugging is that it causes pressure changes along with poor heat and mass transfer.
Drying of Solids
Used throughout industry for a number of products which include polymers, coal, maize, coconut black tea, baker's yeast, fertilizers and pharmaceutical products are some examples of many. They are predominantly used due to low construction cost and high heat transfer. For example, iron and steel companies generally use fluidized beds to dry coal before feeding the coal burning ovens. Depending on the type of industry will affect the type of fluidized bed to use. For examples, if organic materials are in use such as blast furnace slag can be dried using a single stage fluidized bed as residence time within the reactor does not matter. However, if a pharmaceutical product needed to be dried this is a delicate material which needs equal drying times for each particle. This would use a drop particle bed which would drop each particle from bed to bed given even treatment of each particle. FIGURES BELOW
Recent work has been done in modelling the drying processes of wet granules, especially ones which can be used for pharmaceutical production, looking at how to model the process and also what equipment is available to carry out the production of pharmaceutical tablets. Modelling a reactive system which contains a homogenous catalyst is straightforward while the multi-phase modelling still remains difficult which is why even today work is being done to get it right.
Coating of Particles
This is generally done by spraying a solution into a hot fluidized bed of dry particles which wets the surface of the dry particles. This surface then dries which gives a coating to the particles surface. Thus is used extensively in the pharmaceutical industry. The flow and bulk density can be improves immensely allowing powders to be easily handled, sorted and processed. This is carried is usually done magnetically assisted in a dry coater for such particles such as ibuprofen , acetaminophen and ascorbic acid.
In addition to this is has been investigated recently in using near critical CO2 as a fluidizing medium for pharmaceutical applications. This process uses a specially designed glass chamber which allows for the fluidization of the dense CO2 to be controlled. CO2 can also remove any solvent from the coating solution sprayed onto the particle surface which gives a much smoother coating. Using this technique it particles have been successfully coated with thin polymer films for aesthetic purposes with degrading been avoided. Figure shows the internal details of the coating device
Multiple synthesis Reactions
Fluidized beds are used throughout such reactors where there is a need for control of temperature in the reactor with reasons for this include product sensitive to temperature fluctuations, small temperature range for explosion limits and catalyst deactivation. Such reactions are found in the table below:
For example for reactions such a phthalic anhydride production is highly exothermic and the use of a fluidized bed allows for easy control of the temperature.
Other advancements recently have included the effective production of Mg2Si using a fluidized bed reactor. There has an increase in demand for Mg2Si for structural materials due to its lightweight and beneficial thermodynamic properties, as well as the production of polycrystalline silicon materials. Similarly to the reactions already mentioned, it is beneficial to use a fluidized reactor as it is an extremely exothermic reactor. Recent results also show that Mg and Si have good fluidization properties in cold or thermal conditions for a whole range of gas velocities. The use of a fluidized bed has yielded a better reaction rate and shorter reaction time than a fixed bed.
Fluidization is used in various processes and one is used extensively for heat exchange due to the excellent transport of heat and maintains a uniform temperature throughout the bed. Such examples include using a fluidized bed for rapid quenching and tampering of a hot metawire to get desired properties of an alloy, which needs a large area of heat transfer which is provided by the fluidized bed. Another use may include a non-contact heat transfer between a hot fluidized solid to heat a cold gas, :
Some recent experiments have been focusing on using a fluidized bed to exchange heat between a combustion chamber and a CO2 sorbent regenerator. It uses a narrow fluidized bed which transfers heat from bed to bed through a wall which sepeartes them. This method has been investigated currently and makes use of solid regenerable sorbents to capture CO2, which has become of increased interest due to climate change
Breakdown of Hydrocarbons
The breakdown of hydrocarbons is dominated by two factors as the reaction is endothermic but also the large deposition of carbon on solid surfaces. This means that the reactions needs two locations one for the adsorption of heat for the reaction and the deposition of the carbon and another location where the heat is released by the burning of the carbon which has be deposited. This heat which is released needs to be circulated back into the first location to feed the reaction. This may be done effectively by the use of a fluidized bed which circulates the solid back into the first location which is the mode of heat transfer and is the process that is adopted widely in industry by companies such as Exxon and Texaco.
Fluid catalytic cracking has been used in the refinery industry to produce gasoline, kerosene, diesel and olefins from gas oil. Recently there has been the development of using palm oil to produce high quality of biofuels using zeolites as the catalyst to effective produce variety of liquid fuels. This would produce environmental friendly biofuels which are free of nitrogen and sulphur and has attracted more research to use such biofuels as an alternative energy source.
Incineration of Waste
It is beneficial to use a fluidized bed for this process as the waste is easily fluidized with ash from the bed deposited at the bottom of the bed. This can be used in conjunction with a waste heat boiler which can be installed to remove excess heat from the flue gas. They can deal with most forms of waste and good mixing within the incinerator means that there is less air requirement needed. Temperature is uniform throughout the bed as long as the air distribution is good which allows for the temperature within to be easily controlled. Common to incinerators are toxins which include chlorine, nitrogen oxides, sulphur and heavy metals such as Mercury need to be removed from the fuel gas which means that the clean-up of the flue gas must be efficient to be environmentally friendly. Although effective in burning waste however they can be prone to weeping, plugging and erosion. However, the decrease of landfill opportunities lead to increase importance to thermal treatment of waste. Using a fluidized bed which increased waste recycling and incineration with energy recovery can cut down CO2 emissions, with reduced emissions of between 21-40% in Europe by 2020.
It has been found that fluidization is one of the best techniques used for nanoparticles which can be used to disperse or to process nanoparticles. Such particles fluidized as porous agglomerates as they cannot be done individually. It has also been used to provide individual particles coating and the use of fluidization for mixing of two different types of particles. It is however difficult to model such behaviour and recent work has been geared in doing this. The approaches that have been used are to get an average size of the fluidized particle by either force balance or reversed version of the Richardson and Zaki equation while recent attempts have also be to apply computational fluid dynamics to model the system better and get an overall understanding of the process.
Combustion of Coal
The idea to use a fluidized bed for operation was in the 1970's where the "oil crisis" speared focus on using a fluidized bed as well as the need to increase the efficiency of conventional boiler furnaces. The fluidized bed used for combustion works by first fluidizing limestone or dolomite particles using air as the fluid with small particles of coal then injected within the bed, either below or horizontal to the bed. Large pieces of coal or a cake are then added into the bed by a spreader-stoker. Due to the large velocities of air used within the fluidized bed the smaller particles can be separated from larger particles. This unburned carbon can either be trapped and burned off or returned to fluidized bed using cyclone collectors. The feed is kept roughly at 850C by heat exchanger tubes within, as this is the most effective temperature that the CaO and MgO within the bed can capture sulphur. Alternatively, the bed can be recirculated and cooled within cyclone collectors. This recirculating bed is the most beneficial as it's use reduces the presence of large absorbent particles, with increased time for the solids in an uniform bed give almost complete combustion within the bed with very low emission of nitrogen and sulphur oxides.
Some recent advances have been to carry our experiments with coal combustion with metallic oxides such as nickel in order to model the combustion as a function of operating conditions to get reliable data which would help with scale up of the process. This process uses chemical looping combustion which is when an oxygen circulates between two fluidized beds. In the first reactor the oxygen carrier reacts with fuel and is reduced into CO2 and vapour. These can be easily separated by water condensation, and CO2 can be easily collected and stored which cuts down in emissions. The solid passes into the second reactor and reoxidized. Extensive work has been carried out in modelling this process currently and planned to expand oxygen carrier to NiO, NiAl2O4 for fuel applications.
One of the most common problems that have been discovered for each of the application of fluidization is the difficulty to model the process for scaling up in industry. Dr. John Matsen, who has done extensive work in modelling and scaling up fluidization, once said that "fluidized scale up is still not an exact science, but rather a mix of physics, mathematics, witchcraft, history and common sense that we call engineering." This quote highlights that even today that scaling up is indeed difficult and currently no set way in carrying it out.
Fluidization for drying as the drying specific material can only be scaled up using empirical pilot-pilot data, as in data that has been obtained in a laboratory experiment and not mathematical models. In this case mathematical models are too unreliable and therefore scaling cannot be reliably predicted until pilot-pilot test have been completed. This is because flow patterns within large fluidized bed reactors differ for solid and gas particle reactors. This is due to mixing differing with the size of reactor, with small reactor at low gas velocities mixing is due to large rising bubbles which is in contrast to larger equipment which has more vigorous mixing which is down to large scale toroidal circulation patterns, up the centre and down the wall. This is illustrated in Figure ()
The different types of mixing with different diameters means that the fluidized bed must be both testing in laboratory and pilot-plant scale along for proper scale-up procedure can be used to evaluated its performance. This therefore makes it difficult to only use such laboratory data to design the full-scale up fluidized dryer for industrial use. This along with the difficulty of estimating the heat and mass transfer coefficient with any accuracy make scale up increasingly difficult. Also, company's try and cut spending by collecting as little laboratory data as possible and no going to expensive pilot-plant trials which could affect the accuracy of data collected.
Scale up is also a challenge within the refining industry, which are generally large scale processes which are under severe conditions. These are complex phenomena which need to be considered to ensure proper design. Fundamentals of the fluid-particle phenomena are unknown which makes scale up for industrialisation a difficult and challenging task. Still within Research and Development in the refinery industry the multiphase flow systems is a key component that is today being investigated. More effort is needed to alter methods in place to specific systems and to develop data analysis methods to fully understand data obtained. The most important factor is that the phenomena must be understood and taken into consideration when modelling as it is pivotal for scale up and set up and it is depended on the process whether a simple system is enough or complex hydrodynamics are needed. It is clear that more effort is needed to get a better understanding of phase interaction in a multiphase systems so modelling can be improved which can make the task of scaling up easier.
Proper scale up within a fluidized reactor also has increased problems as the hydrodynamics and the chemical conversion on a bigger scale has many pitfalls, which can deteriorate the performance and the economy. Such models are incomplete and ignore effects such as wall effects and particle-particle interaction which cannot be ignored if an accurate model is to be used. The irregular shaped and dispersed particles, which exhibit a high gas velocity, also operate at high temperature and pressure further complicating the model.
A general problem with inappropriate scale up is that gas bypass into large bubbles which give a reduced mass-solid transfer. Challenging also occurs due to poor particle size distribution and inappropriate modelling. Such challenging can cause defluidization or particle agglomeration which can lead to downtime in order to clean. An example of the dangers of wrong scale up was present in a Fischer-Tropsch fluidized bed reactor in 1950, where two pressurised 5m 2-D reactors got constructed based on the results from a 0.305m pilot scale reactor under the same conditions. Slugging occured as bubble was the size of the diameter of the reactor. The slug rise velocity was much slower than the bubble rise velocity in the commercial scale reactor and corresponding decrease in gas residence time. This ended up giving a yield of 1500-2000 barrels/day rather than the predicted 7000 barrels/day. This illustrates the dangers of inappropriate scale up with incorrect estimates made which leads to decreased product therefore less money made by the company that uses the fluidized bed.
The size of the particles is also a problem within fluidization. For example, in bed dryers without internals it is extremely difficult to fluidize particles less than 50-100microm, or better known as Geldart's Group C particles. The fluidization of A and B particles is excellent compared to C particles. Not only are natural C particles a problem but due to bubble eruption on the surface and attrition between particles and particles and the wall lead to the fine group C particles being formed in a system. The fluidization of this fine and ultra-fine particles is because of the strong inter-particle force between particles which is difficult to overcome these forces which leads to poor fluidization behaviour along with challenging in the bed and agglomeration of fine particles. In addition to this problem, the chance of increased fine particles due to attrition can lead to a decrease in contact efficiency between solid and fluid phases, and a drop in performance of the fluidized bed. Fragile particles are another problem as attrition can break down these particles, and increases the chance of losing product.
It is not just Geldart C particles which are the problem. Size distribution of the particles can be varied which lead to increased problems which include agglomeration, and in the case of a dryer leads to lower drying rates and a decrease in the quality of product. It is not just a varied particle sizes that is a problem if the width of particles are varied this will also lead to lower fluidization quality therefore a lower product quality. Operational life of a fluidized bed dryer and the pipes and vessels that are involved with it are shorter than other dryers as the erosion caused by the particle-wall collisions. The higher pressure drop means that the process also needs a compactor which is more powerful, therefore increasing the operational costs to run such a dryer.
Agglomeration is quite a large problem in fluidization especially in solid fuel conversion process. Such conversion is carried out with sand or ash as the bed material and components from the fuel such as alkali's such as sodium and potassium form low-melting silicates with silica from the sand. Especially with low-grade coals were content of potassium and sodium is higher it is an increased problem. These low-melting silicates becomes coated with an adhesive layer. These particles have a sticky surface sick together forming larger agglomerates as the form permanent bonds during collisions. If this is not recognized this can led to defluidization, which can led to unscheduled shutdown of the plant.
Difficulties of Processing Solids and Start-Up Time
A study was carried out by Merrow in the 80's that identified some problems with two-phase industries. He found out that the plants that are processing solids operate at about 68% of its design capacity in its first year compared with ones which are using solids that operate at 90-95% of their capacity. The study also showed that 94% of plants that deal with solids in a two-phase system where shut off for a week or more causing potential millions in loss of profit. Other aspects of the study are summarised in the table below:
% of sample
Plants with performance problems
Non- Chemical Problems
Failure of mechanical equipment
Plugging of Reactor by solids
Handling of fines and dusts
Another problem is that processes which involve solids in them take longer to set up than allowed for in planning. Taking a look at Figure () below it can be seen that all process usually take longer to start up than anticipated, with gas/solid and refined solids taking almost double how much was allocated for it. However, comparing that with raw solids it can be seen that it takes almost five times the planned time. The knock on effect of such a process is that longer time planned, sets the workers under pressure to reach deadline and in turn might cause more money needing investing in order to get plant up in running with in the required time set.
Effect of charge
Within a fluidised bed as there is high particle-particle interaction as well as particle wall contacts this will lead to electrostatic charge generation. This particle charge tends to lead to particle-reactor wall adhesion, which is the particle clinging on to the wall of the reactor. It also leads to process equipment disruption and unwanted electrostatic discharge. This is a big problem in the polymerization industry, which is riddled with wall fouling as large chunks of particles fused onto the wall break off, which leads to the reactor becoming clogged and increases downtime to allow for cleaning. This is most common in the production of vinyl polycetate, polystyral and sand particles and within the polyethylene process as there are two types of particles that exist, the resin and the catalyst. The resin was small in size while the resin varies from the same as the catalyst to a few millimetres in size. This large range makes it difficult for the electrostatic charge to be measured. It has been identified that the particle size distribution plays an important role within the fluidized bed and affects the fouling within it. However, particle wall adhesion mechanism is complicated and has never been quantified. An experiment was carried out by Sowinski, Mayne and Mehrani to find the effect of particle distribution had on charge. A resin was sieved into five different narrowed particle size velocities and fluidized in two different velocities, one representing the bubbling and slugging regime.
The experiment found that within the bubbling regime that there was significant wall adhesion for particle size up to 600Î¼m, with very little happening bigger than this size. In the slugging regime showed wall adhersion for all particle sizes except the 600-700Î¼m range. The overall conclusion was that the smaller the particle size, lead to higher charge which resulting in increased wall fouling within the reactor.
With the study of two solid mixtures, which is the simplest of the multicomponent beds, the achievements to understand such a process are limited. The actually mechanism of the process is extremely complicated as both the phenomena solid suspension of both solids and modification of axial distribution occur simultaneously. It is thought that this could be modelled by expanding the theory of a monosolid system. This problem has been attempted to be tackled by many authors which try and modify existing equations to allow for the difference in multi-solid systems. The rising problem however is that most of these relationships are empirical and are depended on a laboratory test to get any relationship to find the minimum velocity of fluidization and the final fluidization velocity as each mixture is different, which gives different empirical results. An additional problem in using a monosolid approach is that the binary fluidization phenomena is overlooked, given inaccurate equations with inaccurate results. There are also a large number of variables to consider just a few to mention would be two different densities, diameters and shape factors each effecting the minimum and final fluidisation velocity.
Most studies seem to only focus on a homogenous mixture with very little study going into a non-homogenous mixture. This would lead to further complications and complicate the model even further. So, with an increase in the difficulty to model the system, and the vast amount of variables that exist then empirical methods are the best way forward to get value for the minimum and final fluidization velocity. However, it is very unlikely that every mixture has been covered experimentally so the fact that it is reliant on empirical methods is just unfeasible. However, there has been limited success when the actually phenomena has been accounted for and this would need to be fully studied to be able to expand and model adequately.
One of the most useful technique in solving troubleshoot problems involving solid transport in fluidization is to construct a cold model. Such a model can be constructed of clear materials which allows the solid movement to be observed. Flow patterns and problems such as stagnant regions can be easily observed which enhances understanding of any problem which is present. A helpful observation technique is to use a colour tracer which allows for monitoring of particle movement. This motion can easily captured using video equipment and transferred to a computer, which allows for time scale to be slowed down given a better visual representation of solid motion.
There are several ways to construct a cold model with the most common being either to construct all or part of the model with clear acrylic or clear PVC to allow for easy viewing. Sometimes a cold model would be made from metal such as steel or aluminium and ports added to the model so the flow can be viewed.
These models do not just give good visual aids but can also be used to solve problems that may be present in a plant. If a part of the plant can be simulated, then a model allows a systems set up or operating parameter to be varied and observations made to see if any such change can help overcome the problem. It is a disadvantage to make such a model to small as slugging will occur but large models are more expensive to build. Therefore, a compromise must be met allowing for the conditions to be accurately represented while maintaining the costs.
It is also beneficial to operate the cold model at an elevated pressure. This is more expensive but the effect of increased gas densities and the transition of flows can be fully understood decreasing such surprises with the actually process.
There are however negative aspects in using the cold models. Although the particle motions can be clearly observed with small diameters with a 3D fluidized motion which has a large diameter it is much more difficult to observe. This is due to the motion within a fluidized bed being that the solids travel up the centre and down through the wall which hides the view of the centre to the observer. This limits the usefulness of the model to understand the flow in view in a 3D motion in operation. This can be overcome by manufacturing a semi-circular column which allows the observer to see across the diameter of the bed.
It is also important not to manufacture the entire column out of plastic as this leads to a large build up of static charge which can be very dangerous to anyone operating the model and also leads to the agglomeration of particles in the model. These dangerous static charges can minimised by using an antistatic powder known as Larostat, which is a silic particle coated in a quaternary ammonium salt. However, for this to be effective the relative humidity of air coming into the model should be 15%.
There are additionally problems with scale up as a cold model does not accurately represent the larger problem. For example, to allow a model to represent and industrial process then a smaller particle and particle density must be used. For example, if Geldart Group B particles are ot b used within a large, hot unit then Geldart A should be used in the cold, small unit. This could lead to additional problems as the particles do not exhibt the same fluidization properties. This means that cold models would not be suitable for scale up purposes as there is too much risk for the data collected from the cold model to be not suitable for the larger unit. However, the model could be very useful if a piece of equipment within the plant needed to be changed.
Due to a number of things C particles are an increased problem within such dryers. One improvement which could be made to minimize the effect of C particles to the fluidization would be to install inner vertical baffles. These would break up large bubbles as well as decrease the cohesive force between particles.
Computational Fluid Dynamics Modelling
Computational Fluid Dynamic Modelling
One of the biggest in modelling the hydrodynamics of fluidization is the motion of the gas-solid is not known and the interaction between solid and fluid is understood for limited conditions. It therefore becomes helpful to get a better understanding to apply CFD modelling to the gas solid hydrodynamics.
There are two typical models used within CFD programming:
Eulerain-Lagrangian approach which is bases on the molecular dynamics and the motion of the interface between solid and fluid is not modelled. This model uses the continuous phase to be modelled by Eulerian framework while the trajectories of the particles are modelled by Legrangian framework. This approach is only efficient for systems with a low volume of solids.
Eulerian-Eulerian Method- This is based on continuous mechanics, which treats the two phases as interpenetrating continua and can be used in a multiphase process with large volume of solids.
As the Eulerian-Eulerian model can model large volumes of solids it is the most common one used. However, these just model the interaction between particles and fluids while additional models are used in conjunction with these to model the solid solid interaction within a bed which usually takes into the consideration the kinetic theory of the solid phase.
Experiments have been run recently comparing CFD packages which include MFIX and OpenFOAM, which are open access models where compared with a commercial software package called Fluent. All three were also compared with literature and experimental data to see if they can be used to model such phenomena. It was found that these models showed a acceptable qualitative agreement with the literature values. Values such as pressure drop and be expansion ratio were all acceptable using these three packages. A closer look into the flow field of the three shower good similarity between the MFIX and Fluent while not with the OpenFOAM package. This would come to the conclusion that to accurately model fluidization with a CFD package then MFIX and Fluent are two options that can be considered.
Better Reactor Design
Better design of equipment has also been investigated as a possible solution to the agglomeration within fluidization process. There has been one design put forward an integrated system which withdraws the agglomeration as they form. The system is made up of at least one grid-shaped chamber which withdraws the agglomeration below the fluidized bed, as the gas velocity gets increased it can be removed from the bottom of the bed. Other designs which have been mentioned in literature are to have higher gas velocities or to have internal stirring within the reactor both which are used to break up agglomerates. Each have been promising techniques however it still remains uncertain how they have been implemented within industry. It is also unclear which design method is most economically attractive and which is in working practice in industry. For this to become a reasonable solution to agglomeration more testing need to be done and it must undergo a economic evaluation for each one to decide which is the best value.
VIBRATED FLUIDIZED BED
Research in vibrated fluidized bed first started to occur in 1969 as an alternative to the conventional bed dryer. The vibration occurring upwards?? along with the forward flow of air allows for a smoother fluidization of particles that are difficult in fluidization such as C particles.
It also introduces high tensile stresses during the "transisent periods of the pressure pulses" and also allows the particles to have a higher particles acceleration which leads to break up of cohesive clumps of C particles into smaller more manageable agglomerates. Erosion will also decrease due to the minimum and complete fluidisation velocities being lowered. The fact that the vibrated fluidised bed has lower velocities means that fine particle entrainment is avoided and allows for more fragile, abrasive and heat sensitive particles to be fluidised.
For the problem with beds that have a varied particle size distribution, the vibration keeps coarse particles in a mobile stat and also allows for fluidization of finer particles. This increases the efficiency and effectiveness of both the heat and mass transfer.
Increased vibration gives better particle diffusivity and a better drying rate. In taken all this into account there is still little that is known how to model this. Therefore more work must then be done to have a full understanding of the phenomena.
AGITATED FLUIDIZED BED
If a bed incorporates agitation, then a homogenous fine particle fluidized bed will form which decreases the problems that are got from large bubbles. Deeper beds may also be used while still maintain the high quality of fluidisation. The drying rates increase with increase agitation as there is more particle-fluid interaction, so better mass and heat transfer and therefore better rates of drying. However, it is important to note that agitation can only be increased up until a certain point and then increasing it past this point will lead to decreasing drying rates. This is because with higher speeds of agitation the larger particles will go closer to the wall and further away from the flow of the fluid. This means less particle-fluid interaction, poor mass and heat transfer and therefore slower drying rates. It would be recommended that full understanding of what the maximum agitation speed is investigated and known before this is used extensively throughout the drying industry.
Current work has been carried out recently in investigating the benefits of using such a fluidized bed on the drying properties of CaCO3, which has an initial moisture content of 20%. Variables such as the inlet air velocity and speed of agitators was carried out with three different agitators which were straight blade, pitch blade and ribbon-type agitators. Results showed that the drying rate increased substantially with increase in inlet air temperature and slight increase due to increase agitator speed. It was also concluded that the drying rates in a ribbon-type agitator were better than the other two types of agitator investigated which is beneficial as it is the easiest to construct over a large cross sectional area.
CENTRIFUGAL AND ROTATING FLUIDIZED BED
This bed aims to balance the centrifugal forced by the chamber rotation with the particle drag force which is caused by radical fluidization. This allows for the gas velocity to be easily varied by varying the roatational speed of the chamber??
Big advantage of such a bed would be that it is more compact than conventional fluidized bed as the force of gravity can be increased by several times its value which increases the fluidization and its efficiency.
MAKE THIS PART BETTER. MORE INFROMATION NEED SIR
Addressing the C-particle problem that have already been stated, which included the problem of strong inter-particle force between fine particles which lead to poor fluidization. A suggestion would be to dry coat approach which involves the distribution of nano size particles on the surface of group C particles which aim to reduce the inter particle force and improve fluidisation. This approach works by using a coating material, commonly a polymer, which is in the form of a solution, suspension or melt sprayed through nozzles onto the fluidized particle surface and forms a shell size structure. A related gas, commonly air, is used to fluidized, mix and dry the particles.
This technique is available in a commercial fluidized bed, and has been investigated how factors such as polymer concentration and inlet temperature have an effect on the quality of coating on the particles. It was found that as concentration increases coating quality decreases while a lower inlet temperature of air, acting as the fluid, increases the quality of coating. A summary of the findings proved that particles as low down as 10microm could be coated without any notable agglomeration and even down to fivemicrom with some agglomeration. Technique could be investigated if to be used fully within industry.
Gaussian Special Pressure Distribution
In a process like particle coating defluidization can be prevented by both increasing and decreasing the gas velocity and the coating suspension flow rate respectively. However, for this to be affective the changes in the hydrodynamics of fluidization must be detected early. This can be extremely beneficial for industries as it allows for defluidization to be detected early which avoids unnecessary shut down of the process. A method to detect this is to use the Gaussian mean frequency which detects fluidization due to pressure fluctuations. An experiment was carried out by Parise, Silva, Ramazini and Taranto in a column of 0.143m in diameter and 0.71m length using coated microcrystalline cellulose as the fluidizing particles. Pressure readings were taken at a sampling rate of 400 Hz and stored using Labview software. Results showed that the defluidization region could be clearly identified using the Gaussian spectral pressure distribution technique as the central frequency value changes with bed moisture content and mass of the solids dectecting when defluidization takes place. This method could therefore be used in coating industry with an online-controller which would maintain the central frequency in an established range in order to keep the bed out of the defluidization region with the current work showing a central frequency of 6.0 or 7.0 Hz to be adequate. Figure () shows the results obtained from the experiment and gives a visual representation that this method could be used to detect defluidization
Low-rank coals which are high in sulphur and sodium content have caused increased problems once combusted. The problems are that the presence of sulphur, which is present in compounds such as sodium sulphur, are known to be "sticky" and cause fluidisation to be affected. In order to tackle such a problem additives which include limestone and dolomite can be added. This helps with the dilution of ash which leads to a less sticky layer and better fluidisation.
It was studied with two different types of Australian coal, Kingston and Lochiel, to determine if such additives did in fact have a positive effect on the fluidised bed. Rich-clay additives were used and the refreshing and removal of the bed without affecting the combustion processes was also investigated. It was found that with both coals they could run up to 30 hours without changing with Lochiel combustion happening up to 800Â°C with 10% clay additives of the feed while Kingston up to 850Â°C with clay additives of 5% of the feed each without defluidisation. However, it is important to note that to be fully incorporated into industry a full analysis of different clays must be carried out with attention to the effect of the clay particles to other parts of the plant.
Investigating the effect of Pulsed Bed on Mixing
The effect of pulsing the bed at different intervals was investigated to see if it could be used to improve the process of fluidisation. The pulse was investigated by varying the pulses between 0.5 Hz to 5Hz. A number of different variables were measured were pressure fluctuations, the average cycle frequency as well as the incoherent standard deviation and how these affect the bubble size and characteristics within the bed. In addition to this optical probe measurements were used to find the bubble fraction within the column and the bed height affect was also investigated. When the experiment was conducted it was found that when the total flow is oscillated with a frequency then the problem of defluidization can be prevented. In addition to this it was found that with the increase in bed height the increase in both bubble size and fraction for both constant and pulsed flow. Bubble size also increased when pulsed flow which was much greater than constant flow, given an optimum frequency was found to be 3.0 Hz which gave both the highest bubble frequency and size.
Generally, the size of bubble would want to be minimised as with an increase in bubble size introduces the whole idea of mass transfer limitations with increase in bubble size. However, larger bubble size could become beneficial in drying processes as gas challenging is reduced due to cohesiveness of particles and properly mixed. A recommendation would be to investigate the effect on pulsing a drying process to see if the drying rates can be improved by introducing a pulsed bed.
Effect of Plasma on fluidised beds
Plasma can be incorporated into a fluidized reactor and increase the benefits of such a reactor. When coupled with a fluidised bed it gives a higher rate of mass and heat transfer as it works by the generation of high bulk temperature using the plasma as the heat source. Additional advantages are that it can bring down the size of equipment and increase productivity. It has not been tested in a variety of applications and this needs to be eradicated before it is used throughout industry. As materials processing carried out in highly reactive high temperature zone, at a few thousand degrees. This high temperature is up to a few thousand degrees and there are problems with this in its own rights with selectivity, mass transfer and removal of by-products.
This Plasma fluidised bed can be used as a new generation of clean reactor and its advantages should be exploited and are attractive to investigations in core processes in industries such as metal and polymer. There has been relative success with the growth of films on solid granular polymeric materials and also success in ceramic nano-particles.
The bed reactor the use of plasma state of fluid is used instead of corresponding fluid as a carrier for the fluidised bed material. The additional advances of such are that there is a steadiness of solid circulation which gives better mass and heat transfer. In addition to this its gives a better uniform mixing which helps attain thermodynamic equilibrium in the species within the reactor.
Analysis of such solutions
As seen in the previous section a number of solutions have been put forward to overcome all the problems that come with using fluidization beds within industry. However, a key question is how useful are these techniques, can they be implemented throughout industry and what is the next step.
Solutions such as cold models may be beneficial to see the actually flow patterns within such a reactor however these cannot be used for ever process. Some processes cannot be modelled meaning that it still difficult to know exactly what is going on within the bed and how the process works. This makes accurate scale up almost impossible. The fact that when using such models smaller particles must be used than what is used in the process gives additional problems as these particles have different fluidization properties again making scale up even more difficult. Overall, such models are not suitable if accurate scale up is necessary, and it generally is for a fluidized bed to cut out unnecessary shut down of the process.
The computational programmes that are present have showed positive correlation between data collected when modelling and with experimental data from literature. This means that such programmes can be used to successfully model fluidization and should be used throughout industry. However, there have been limited amount of gas velocities used ranging just from 0-m/s. If a wider range of gas velocities are needed then it would be suggested to increase the gas velocity even further, collect such data as pressure drop and bed expansion data and compare with the model.
The use of improved fluidized bed's such as vibrated fluidized beds, pulsed beds, agitated beds or centrifugal beds all have shown to decrease the risk of defluidization and dealing with variable particle size. However, it has been mostly theoretical work with data received for a pulsed bed given an optimum frequency of 3 Hz. However, it has been showing that the other beds do improve the process however more work should be done to get a better understanding of the process and obtain optimum conditions for each one to get the best possible fluidized bed in industry.
Others ways such as the coating of particles and additives have showed very positive results with both decreasing the chance of shut down in the process due to defluidization. Coating has been beneficial within the polymer industry decrease agglomeration down to size 5microm while the use of clay additives have showed less time for shutdown with two different types of coal. To be used throughout industry a full analysis of each clay needs to be carried out while given additional detail to if such clay would affect other parts of the plant. Also, a more extensive analysis of more than two types of coal should be carried out. Additional work should also be carried out to see if it is just specific to the polymer industry or if it could be used throughout other industries. However, there has been very promising results that this could be used to tackle the problem of defluidization.
Using Plasma as the fluid medium within a reactor is a new generation of reactor. Theoretically it gives increased productivity while decreasing the size of the equipment, and excellent heat transfer. There has also been limited success in ceramic and polymeric material. Additional work needs to be done for this to be used throughout industry however if it is found to be successful then this could be a long term replacement of regular fluidized beds especially in industries such as metal and polymer.