The term fluidized bed (including the fluidized bed called english fluidized bed ) comes from the field of thermal and mechanical process technology . A bed of solid particles is called a fluidized bed , which is brought into a fluidized state by an upward flow of a fluid . The term “fluidized” in this context indicates that the (former) bed now has fluid-like properties. In some calculation models this is used as a basic approach. Since a fluidized bed is a fluid-solid mixture , this approach cannot describe all phenomena despite mostly good results.
Fluidized bed process is a major priority in fluidized bed dryers in the drying of granular media in furnaces of coal , alternative fuels or sludge in the fluid catalytic cracking -Kraftstoffherstellung or the coffee-roasting.
As part of the optimization of the Haber-Bosch process , the German chemist Fritz Winkler developed the first fluidized bed gasification system at BASF to produce synthesis gas from fine-grain lignite in the Winkler generator , for which he received a patent in 1922. In 1926 the first large-scale plant was put into operation. In 1929 four more plants went into operation. With the use of the fluidized bed in the United States for the catalytic cracking of mineral oils in the 1940s, extensive theoretical and experimental studies of the fluidized bed were quickly made. In the 1960s, the first power plant with a circulating fluidized bed was built in what was then the VAW Lippe Plant in Lünen to burn ballast-rich hard coal, later for the calcination of aluminum hydroxide . Fluidized bed systems are now used for many different purposes.
Properties of fluidized beds
A fluidized bed has liquid-like properties. As with water, a horizontal surface is always formed. If fluidized beds are generated in two interconnected containers, the same absolute height of the fluidized bed upper limit occurs in both ( principle of communicating tubes ). Furthermore, objects with a higher density than the bed sink in a fluidized bed, while objects with a lower density float ( Archimedes principle ). Since the 'density' (actually the solid volume fraction in the mixture) of the fluidized bed changes with the fluidization speed, objects with a density similar to the fluidized bed can be caused to sink or reappear by changing the fluidization speed.
In fluidized beds, there is close contact between the fluidized material (solid particles) and the fluidized medium (fluid: gas or liquid) and the individual particles move vigorously in all directions. In fluidized bed furnaces, this leads to good heat transport within the system and good heat transfer between the fluidized bed and the container wall or built-in heat exchangers. At the same time, the good heat transport and the (compared to gas) enormously high heat capacity of the bed inventory ensure a relatively homogeneous temperature field in the system, which results in low-pollutant combustion. The course of the solid volume profile, which can be calculated from the (apparent) pressure loss over the system height, is characteristic for the assessment of the fluidization state of fluidized beds.
Types of fluidized beds
A distinction is made between homogeneous fluidized beds with a spatially uniform distribution of the disperse phase and inhomogeneous fluidized beds with a spatially uneven distribution of the disperse phase. In addition, there are stationary or bubble-forming fluidized beds (BWS), in which the fluidized bed has a clear limit, from which only very few (fine) particles are discharged, and circulating fluidized beds (ZWS), in which the fluidized bed due to a greater flow velocity of the Fluids no longer has a clear upper limit and particles are largely discharged upwards. These then fall back into the fluidized bed in the form of clusters or are discharged into the gas cyclone, where they are separated from the gas flow and fed back to the bottom zone via a return, i.e. (re) circulated.
Fluidized bed conditions
Homogeneous fluidized bed
In the homogeneous fluidized bed, the solid (coarsely dispersed phase) is evenly distributed, and continuous expansion occurs when the fluid velocity is increased. Homogeneous fluidized beds appear in fine-grained, narrowly distributed and non-cohesive material systems with a low density difference and in fluidized fluidized beds.
Inhomogeneous fluidized bed
Inhomogeneous fluidized beds occur mainly in gas-solid fluidized beds and with a broad particle size distribution or large differences in density. This also applies to large and / or cohesive particles. The manifestations of inhomogeneous fluidized beds include:
- bubble-forming fluidized beds
- gas-forming fluidized beds
- abutting fluidized beds
Bubble-forming fluidized bed
A fluidized bed can take on different states. If one starts with a fixed bed through which a fluid or gas flow flows, and if this is continuously increased, the solid is carried through the flow from the so-called loosening point, the point of minimal fluidization (state A). The fluidization speed corresponding to this is generally referred to as. If the gas flow is increased further, bubbles will form in the fluidized bed (state B), which is now also referred to as the bubble-forming or stationary fluidized bed.
In a relatively wide range of gas velocities this state does not change significantly. Depending on the particle shape, their size, apparent density, etc., the fluidized bed retains its bubble-forming character (up to five to six times, for example ). As the gas velocity increases, the proportion of bubbles increases, as a result of which the proportion of the suspension volume becomes lower. As a rule, solids volume fractions of approx. 20% to 40% are found in bubble-forming fluidized beds.
However, the gas velocity in this state is significantly lower than the single-grain sinking velocity of the particles. Figuratively, one can speak of a swarm behavior of the particles. Some particles 'line up next to each other' so that their flow resistance ( ) is significantly higher than the flow resistance of the individual particle. At the same time, the particles lying above it 'slipstream' so that they are not exposed to the full gas flow and 'fall down' faster. The result is a compact suspension layer with a clearly defined surface that is churned up by bubbles bursting on the surface.
The bubble-forming fluidized bed is characterized by intensive mixing in the vertical direction. Depending on the cross-sectional area of the bed, large-scale circulation flows develop. In general, solid rises in the center of the fluidized bed and sinks again at the edges. With very large cross-sectional areas, several ascent and descent zones are formed. This effect can be intensified by specially arranged gas distributor trays in order to e.g. B. to improve the fuel mixing in the bottom zone in fluidized bed combustion.
With relatively coarse bed material and narrow pipe cross-sections, a “beating” fluidized bed can occur, as the bubble size in such beds extends over the entire cross-section (condition C). If the fluidization rate is increased, there is an increasing discharge of fine material in some processes that z. B. is returned to the bed by internal gas cyclone . (State D).
Circulating fluidized bed
A fluid flow that continues to grow leads to an increased discharge of initially finer particles from the fluidized bed (state E). To recirculate these particles (in the case of gases as a fluid), an aerocyclone for solids separation with a recirculation line is usually installed. This separates the discharged solid from the gas stream, which is then returned to the bottom zone of the circulating fluidized bed that is now being formed. In contrast to condition D, the amount of gas and solids to be handled is so large that the cyclone can no longer be integrated in the combustion chamber or in the riser (depending on the process application) of the system and has to be installed outside.
The advantages in the technical application of the circulating fluidized bed compared to the bubble-forming fluidized bed are the significantly higher gas velocity, which allows larger amounts of fuel to be added in combustion processes. In catalytic processes, e.g. B. "Fluid Catalytic Cracking" ( FCC ), the discharged catalyst can be transferred from the so-called cracker to the regenerator without mechanical installations.
The disadvantages of the circulating fluidized bed compared to the bubble-forming fluidized bed are the higher expenditure on equipment (→ higher production costs) and the higher energy consumption for the fan (→ higher operating costs).
The decision as to which fluidized bed (furnace) should be used depends on the size of the feed stream to be managed. Small (combustion) systems are often designed as bubble-forming, larger systems as circulating fluidized beds.
Circulating fluidized beds also have a dense bottom zone, which, however, usually no longer has a clear upper limit. Due to the high fluid flow , a relatively large number of particles are carried out of the bottom zone and (in the case of incineration plants) some of them are fed to the connected gas cyclone. However, a certain proportion of the solid still falls back into the bottom zone in the fluidized bed. In the so-called free space '( English freeboard ) above the dense zone, a flow pattern as the forms, the core ring structure (engl. Core-annulus ) is referred to. In the relatively wide core, the solid rises as a thin suspension, while directly at the edge of the system (on an industrial scale at a system height of 30 m to approx. 30 to 50 cm from the container wall) the solid material descends in clusters at high speed emotional.
A profile of the solids volume concentration is formed over the height of the system, the maximum of which is usually at the 'upper limit' of the dense zone and the minimum of which can be found at the top of the system. The proportion of solids by volume in the free space is less than 1% on average in modern incineration plants. There crackers have significantly higher solid volume fractions.
According to their different fluidization behavior, Derek Geldart has divided bulk goods for gas / solid fluidized beds into four so-called "Geldart groups". These are plotted in a double logarithmic diagram of density difference (fluid - solid) over particle size.
- Group A
- The particle size is between approx. 20 ... 100 μm, the particle density under approx. 1400 kg / m³. Before blistering occurs, the layer expands to two to three times the layer thickness at the point of loosening. Most of the powdery catalysts belong to this group .
- Group B
- The particle size is between approx. 40… 500 μm, the particle density is approx. 1400… 4500 kg / m³. Blistering starts immediately after the loosening point is exceeded.
- Group C
- These are extremely small and therefore very cohesive particles with a size of less than 20 ... 30 μm. Due to the strong cohesive forces, these particles are very difficult to fluidize (e.g. with the aid of mechanical stirrers).
- Group D
- The particle size here is over 600 μm with very high particle densities. A very large volume flow is required for mixing, which in turn involves the risk of abrasion.
If a fluid flows through a fixed bed, the pressure loss increases roughly proportionally with the flow velocity. At the vortex point ( ) the bed material is carried by the gas flow. The pressure loss then remains constant.
At the bottom of the system, the (apparent) pressure loss multiplied by the cross-sectional area of the fluidized bed is equal to the weight of the solid inventory (minus the buoyancy of the solid in the fluid).
The Reh diagram developed by Lothar Reh is suitable for estimating the state of fluidization .
Fluidized bed systems are used in technical processes to bring solids into close contact with gases, liquids or other solids. In the fluidized bed, among other things, the following basic characteristics of process engineering and chemical reaction engineering are used:
- high relative speeds between the continuous fluids and the disperse solid phase,
- frequent particle-particle collision and particle-wall collision
- intensive mixing of the particles
Fluidized bed systems are used for a large number of technical processes. At this point, the most important are briefly mentioned:
- Drying processes
- chemical reactions and in the field of energy conversion for gasification and combustion processes
- Classification of heterogeneous beds
The fluidized bed reactor has recently been used more and more in power plant technology , where it contributes to efficiency improvements. Operations such as powder coating can also be carried out in this way.
Power plants with fluidized bed combustion
- ZWS power plant Gardanne , France, 250 MW el
- Sewage sludge incineration in the Berlin-Ruhleben sewage treatment plant .
- Thermal substitute fuel recovery plant (TEV), Neumünster
- ZWS biomass power plant Lünen by Remondis - Lippe Plant for substitute fuels
- ZWS power plant Berlin-Moabit hard coal and brown coal 83 MW el 136 MW therm
- Stadtwerke Flensburg GmbH, three ZWS boilers for hard coal, substitute fuel (RDF) and wood chips
- ZWS power plant Berrenrath , RWE Power , lignite with sewage sludge and RDF, 2 × 230 MW therm
- ZWS factory Frechen - power plant , RWE Power , brown coal with sewage sludge 2 × 165 MW therm
- HKW Merkenich , ZWS power plant fluidized bed lignite, 240 MW therm , RheinEnergie , Cologne
Power plants with fluidized bed gasification
- Fluidized bed dryer with internal waste heat recovery (WTA) from RWE Power AG, z. B. in the Niederaussem power plant
- Pressurized fluidized bed evaporation drying (WVT, DDWT) with superheated steam drying of sugar beet pulp, NaWaRo ( CSD ) or pit-moist lignite (test facilities at the Brandenburg Technical University, Cottbus and in the industrial park Schwarze Pump )
In iron making
- FIOR process (stationary fluidized bed; only plant currently out of operation) (status 2000)
- FINMET process (stationary fluidized bed; plants with a capacity of 500,000 tons per year in operation (as of mid-2013))
- Circored and Circofer processes (circulating fluidized bed; only plant currently out of operation)
Other deployment methods
- Fluidized bed drying and granulation in the chemical or pharmaceutical industry
- Fluidized bed roasting of sulphidic ores (zinc, pyrite, copper)
- Fluidized bed roasting of coffee beans
- Fluidized bed calcination of gypsum
- Fluidized bed calcination of aluminum hydroxide
- Pickling regeneration (originating from the metallic pickling process, hydrochloric acid loaded with iron is converted back into pure hydrochloric acid)
- Basics of the circulating fluidized bed (cold model) (PDF file; 958 kB) ( Memento from September 28, 2007 in the Internet Archive )
- Chemietechnik, November 2013 , accessed December 11, 2013.
- Patent DE437970 : Method for producing water gas. Registered on September 28, 1922 , published on December 2, 1926 , applicant: IG Farbenindustrie AG, inventor: Fritz Winkler.
- Crowe, C., Multiphase Handbook , pp. 5-71
- M. Stieß: Mechanische Verfahrenstechnik 2 (Chapter 11 fluidized beds and pneumatic conveying). Springer publishing house. 1995.
- H. Schubert: Handbook of Mechanical Process Engineering. Volume 1 (Chapter 3.2.2 Fluidized Beds). Wiley Verlag GmbH & Co. KGaA, Weinheim. 2003.
- D. Geldart: Types of gas fluidization . In: Powder Technology . tape 7 , no. 5 , May 1973, pp. 285-292 , doi : 10.1016 / 0032-5910 (73) 80037-3 ( PDF ).
- depth study: Determination of the vortex point using dimensionless key figures
- Developments in the Venezuelan DRI Industry , p. 5
- What is FINMET Process (Technology) of Iron Making? ( Memento from April 2, 2015 in the Internet Archive )
- Outotec : Direct Reduction Technologies
- Outotec: Alumina Solutions
- Fluidized bed or spray grate: Advances in the development of hydrochloric acid regeneration systems