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Inductive heating of fluidized beds: Influence on fluidization behavior
Powder Technology 286 (2015) 90–97
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Inductive heating of fluidized beds: Influence on fluidization behavior Vesselin V. Idakiev a,⁎, Sebastian Marx a, Antje Roßau b, Andreas Bück a, Evangelos Tsotsas a, Lothar Mörl a a b
Thermal Process Engineering, NaWiTec, Otto von Guericke University Magdeburg, Universitätsplatz 2, 39106 Magdeburg, Germany GETEC heat & power AG, Albert-Vater-Straße 50, 39108 Magdeburg, Germany
a r t i c l e
i n f o
Article history: Received 18 February 2015 Received in revised form 23 July 2015 Accepted 7 August 2015 Available online 12 August 2015 Keywords: Fluidized bed Inductive heating Drying Heat sensitive materials
a b s t r a c t Fluidized beds are commonly used for solid processing. Required heat is usually transferred by conduction via the gas flow or by particle–wall contact with immersed steam-heated tubes. Both variants possess limitations with respect to maximum gas temperature or contact area. In this work, heating of fluidized beds by induction is investigated. This is realized by co-fluidization of conductive but chemically inert particles, yielding large moving heat transfer areas. The focus lies on the experimental study of the impact of the electro-magnetic field on fluidization behavior. Studied process parameters are induction power, air velocity, particle diameter, inert coating, and the ratio of conductive to nonconductive materials. Moreover, the time response of temperature to changes in the induction power is investigated. The obtained results show very fast temperature response. The most important factor influencing the fluidization behavior is the induction power which can cause strong fluctuations of the pressure drop which at high energy inputs changes the fluidization regime from bubbling to slugging regime. This can be prevented using a pulsating magnetic field or insulation of the conductive material with a coating. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Since the 1930s, fluidized bed technology has gained a steadily growing importance in industry with uses in drying, combustion, chemical reaction engineering in the field of catalysis, as well as classification. In a fluidized bed, a fluid (liquid or gas) is passed through the bulk material, so that the solid/fluid mixture behaves as a fluid. Through the intensive contact between the fluid and the particles, good heat and mass transfer rates are achieved. Thus, many efforts have been taken to explain their characteristics which, among others, depend on the gas flow and applied force fields [7,25,28]. Usually the heat energy is supplied by a hot fluidizing gas. But in some cases, for example to increase the heat and mass transfer by drying of brown coal in a fluidized bed, steam-heated pipes immersed in the bed are used for heat transfer. Due to economic and environmental reasons and to establish new areas of application, new ways are sought to provide an increase in efficiency and energy savings in heat transfer. One method is inductive heating in which the heat input is not realized by the heated fluid flow into the fluidized bed, but is transmitted directly by non-contact heating of electrically conductive but chemically inert particles in the fluidized bed. This yields large heat transfer surface areas and very quick heating and cooling. In various applications including annealing, bonding, brazing, forging, hardening, melting, plasma production etc., inductive heating ⁎ Corresponding author. E-mail address: [email protected] (V.V. Idakiev).
http://dx.doi.org/10.1016/j.powtec.2015.08.003 0032-5910/© 2015 Elsevier B.V. All rights reserved.
has proved itself superior to convective heating. There are many reasons why using induction heating for these processes is clearly beneficial. The heat is generated directly in the material of interest, minimizing heat loss and energy consumption. Besides, induction transfers more energy per square meter than the common heat source such as open flame, resulting in faster heating, which ultimately improves the throughput and quality. It is also a fast and controllable process [5]. These considerable economic benefits of induction can also be transferred to fluidized bed processes. The inductive technology can ensure short heating and cooling times of bed material, leading to high efficiency and better product quality. Therefore, it is intended to lower energy costs and thus enhance the complete production profitability by using highly effective inductive heating in fluidized beds. Since inductive heating is realized by applying an altering electromagnetic field with an integrated inductor, it is important to know the impact of the magnetic field on the fluidization behavior. As described in Section 2, although there have been various research studies on the influence of magnetic fields on fluidized beds, with the induction power identified to be an important influencing factor, additional work is necessary to understand exactly how induction may affect the pneumatic behavior. Therefore, the presented study focuses on this research aspect. By variation of several process parameters such as induction power, air velocity, particle diameter, kaolin coating, mass ratio of conductive to nonconductive material, their influence on fluidization behavior is investigated. The results of this study provide the basis for implementation of inductive technology in fluidized beds as well as suggestions for further research.
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2. State of the technology By inductive heating, conductive material can be heated without any contact to the heater. The energy conversion takes place mainly at the edge zones (surface) of the particle because of the skin effect. Skin effect is a tendency for alternating current to flow mostly near the outer surface of an electrical conductor, such as iron balls. It occurs with high frequency alternating current and describes the appearance of the current density. As a result of this effect, the current density is largest near the surface of the conductor and decreases towards the center because electric current flows mainly at the “skin” of the conductor. The higher the frequency is, the greater this effect. This is a desired effect by the inductive energy input, since only the particle surface must be heated for heat transfer [6,22]. Heating by induction is used in industry for many decades. In the 19th century, it was first introduced with induction melting furnaces, which were initially operated at network frequency. By achieving higher frequencies through generators, besides pure annealing and melting processes also surface heat treatment process could be realized. Further possible applications were quickly recognized and promoted the development. Even today new application areas are tested and developed [2]. As already mentioned magnetic fields are used to heat electrically conductive materials. The influence of external magnetic fields on magnetizable particles in a fluidized bed was first studied by Kirko and Filippov [16]. The authors used iron particles with water as a fluidizing medium and reported that the magnetic field strength had an impact on the bed properties. A significant change in bed pulsation was observed by the authors. In 1961, experiments were performed with liquid metals and magnetizable particles [10,11]. It was shown that the magnetic field had a positive effect on the behavior of fluidized beds with respect to reduced bubbling and channeling. In 1969, the term ‘magnetically stabilized fluidized bed (MFB)’ was introduced. By MFB the bubble formation can be reduced and the heat transfer can be increased significantly [19,20,27]. The correlation between fluidized bed pressure drop and strength of the magnetic field has been studied by many authors [15,23]. According to these studies, the increase in field strength causes a reduction in pressure drop. Moreover, many researchers investigated the formation of particle agglomerates due to the strong magnetic forces between ferromagnetic particles, favoring cluster formation [3,24,29]. The formed particle agglomerates provide greater resistance to the fluid flow. The effect of stagnation of electrically conductive bodies at their respective positions was observed by the authors and described in detail. Another study on the topic ‘Magnetic field assisted fluidization’ was conducted by Hristov [12–14]. Magnetically controlled fluidized beds are interesting because the magnetic particles can be easily removed from the fluid. This method can be used for many processes without damaging the product, such as filtration or centrifugal separation. The inductive heating of particulate materials in gas–solid fluidized beds was first investigated Roßau et al. [21,26]; they observed that the electro-magnetic field yields changes in the fluidization behavior. The interaction of the different process and material properties and their influence on the fluidization was not characterized. The observations are the starting point for the experimental investigations in this work. 3. Experimental setup 3.1. Experimental plant In order to investigate the inductive energy transfer into a fluidized bed and its influence on the fluidization behavior, more than 50 experiments were performed in two fluidized bed plants. The schematic representation of the experimental plants is given in Fig. 1, the technical specifications of each plant are listed in Table 1.
Fig. 1. Schematic design of both experimental plants.
The body of the experimental plants DN 139 and DN 300 consists of a steel cylinder with an inner diameter of 139 and 300 mm, respectively, and an overall plant height of 3000 and 1800 mm, respectively. The fluidized bed chamber is made of a heat-resistant borosilicate glass cylinder. It allows visual observation of the fluidized bed and prevents the chamber walls from being heated by induction. The fluidized bed chamber is surrounded by an inductor with 5 (experimental plant DN 139) and 9 windings (experimental plant DN 300), respectively, which generate the electromagnetic field in the fluidized bed, leading to heating of the electrically conductive particles in the fluidized bed chamber. The inductor is electrically powered by a generator from Hüttinger Elektrik GmbH + Co. KG (Big 40/100 and TruHeat MF 3040, respectively), that can transmit electrical power up to 40 kW. The inductor is made from copper of high purity and has a good electrical conductivity. In order to avoid heating of the copper, the inductor is cooled with water, thus keeping the electrical conductivity stable. A fan draws ambient air, which enters the fluidized bed through a perforated distributor plate and fluidizes the bulk material. After leaving the fluidized bed, the air first enters a cyclone separator and then flows into a filter. At several points in the fluidized bed, pressure, temperature and moisture are measured. Moreover, to precisely measure the bed pressure drop in the fluidized bed unit, a high frequency measurement device is used that can record the resulting pressure drop with frequencies up to 1000 Hz. 3.2. Experimental material In the present work, as an experimental material electrically conductive iron hollow balls (IHB) are used for the inductive energy input in fluidized beds (Fig. 2, right). The manufacturing process developed by the company Hollomet GmbH allows to produce nearly monodisperse particles of defined diameter and also allows varying the particle layer thickness [9]. Due to the possibility of producing hollow balls of conductive material with variable diameters and thicknesses it is thus possible to create particles with defined fluidization properties. In addition, the experimental material (IHB) can be coated with kaolin. The ceramic coating serves to insulate the iron hollow balls. It may prevent local overheating and sparks on particle surface, which can take place at higher energy input. The kaolin coated iron hollow balls are shown in Fig. 2. If the variables such as coating density ρI, the density of the medium enclosed in the hollow balls ρA, and the apparent density of the ball ρApp,
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Table 1 Technical specifications of experimental plant DN 139 and DN 300. DN 139
DN 300
Fluidized bed unit
Diameter: di = 139 mm height: 1 m (borosilicate glass), s = 7 mm 2 m (stainless steel), s = 3 mm Fluid bed distributor plate Diameter: dperforation = 1 mm Plate thickness: s = 2 mm Triangular distance: t = 4 mm Aperture ratio: ψ = 5.7% Material: FR4 ΔpBottom = 267.77 ⋅ w2F + 27.945 ⋅ wF Coil Winding: Number: 5, 10 × 29 mm, di = 173 mm, material: copper, water-cooled Generator Big 40/100 (f = 30–75 kHz, U = 400 V, Pmax = 40 kW) Ventilator RIBO model VS 9 (Pmax = 12.5 kW) cyclone + filter 4.4 l/min, circulation through pressure of tap water
Material separation Water-cooling
shown in Fig. 2 right, are known, the layer thickness of the particle s can be calculated as follows: 8 313 9 2 ρ > > = 1− ρApp d < I 5 ð1Þ s ¼ 1−4 > 2 > : ; 1− ρρA I
By the manufacturing method it is possible to produce nearly perfect metallic hollow balls having a sphericity close to 1, a diameter larger than 1.5 mm and a particle density lower than 1000 kg/m3 [1]. This type of inert particles would not sink, for example, in water. The properties of the iron hollow balls used in the present study are summarized in Table 2. To study the heating behavior in an inductively heated fluidized bed, different non-electrically conductive materials were also used as experimental material. Moreover, the non-conductive materials are meant to represent the product of a real process. For this purpose, glass, alumina and plastic beads with similar fluid dynamic properties (determined by similar Archimedes number, minimum fluidization and elutriation Reynolds numbers) as the iron hollow balls were chosen, which are characterized in Table 3. Mixtures of conductive and non-conductive materials in different ratios (see Table 4) were used in additional experimental studies. 3.3. Experimental design In order to determine the impact of individual process parameters on the inductive heating and the fluidization behavior, various
Diameter: di = 300 mm height: 0.5 m (borosilicate glass), s = 7 mm 1.3 m (stainless steel), s = 3 mm Diameter: dperforation = 2.3 mm Plate thickness: s = 1 mm Triangular distance: t = 4.1 mm Aperture ratio: ψ = 30% Material: steel ΔpBottom = 11.276 ⋅ w2F + 0.763 ⋅ wF Winding: Number: 3 × 3, 10 × 29 mm, di = 340 mm, material: copper, water-cooled Trumpf TruHeat MF 3040 (f = 20–100 kHz, U = 429 V, Pmax = 40 kW) HRD 7 FU-105/11.0
(Pmax = 1 kW, V fluid ¼ 1:920 cyclone + filter 12 l/min, active circulation
m3 ) h
parameters were systematically varied. First, the parameters such as applied electrical power and air velocity were examined for their impact on the efficiency of the inductive energy input and the behavior of the fluidized bed. In these studies, only monodisperse iron hollow balls are used as bed material. In further experiments, particle diameter and kaolin coating were varied to determine their effect on bed properties (see Table 2). Subsequently, the heating of a mixture of conductive iron hollow balls and non-conductive materials (NCM) is considered. The influence of the mass ratio of iron hollow balls to non-conductive material mIHB/mNCM on the fluidization behavior was determined by varying the quantity and the type of the non-conductive material, while all other remaining parameters were kept constant. The efficient and uniform heating of the non-conductive particles is targeted here. Table 4 summarizes the varied parameters over all experiments. All experiments are carried out in the same way to ensure comparability. First, the measurement must be started. After slowly increasing the air mass flow rate to create the fluidized bed, the bed is given time to stabilize itself. Next, the inductive power is applied. After a defined period of the experiment the inductive power is turned off and the system is let to cool down. During the whole experiment including the stabilizing and the cooling period the pressure drop is measured. In the period without induction treatment the pressure drop is defined as “at 0 kW” in the following graphical illustrations and accordingly during the induction as “at the associated values of induction power”.
Fig. 2. Iron hollow balls kaolin coated and non-coated (right) and (left) their manufacturing principle.
V.V. Idakiev et al. / Powder Technology 286 (2015) 90–97 Table 2 Overview of the properties of the iron hollow balls.
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Table 4 Experimental program.
Sauter mean diameter [mm]
Apparent density [kg/m3]
Kaolin coating %
3.22 3.24 3.33 6.16
598.6 592.9 721.8 687.6
– 5 25 –
Varied parameter
Applied electrical power [kW] Gas velocity [m/s] Ratio of conductive particle to bed material [%]
Range of variation DN 139
DN 300
2, 4, 6, 10, 14, and 18 2, 3, 4, and 5 ∗ wmf 20, 40, 60, and 80 percentages of IHB
2, 4, 6, 10, 14, and 18 2; 3; 4, an 5 ∗ wmf 20, 40, 60, and 80 percentages of IHB
4. Results and discussion In this section, the results of the experiments are presented and discussed. The main focus lies on the temperature response and the pneumatic behavior. 4.1. Temperature time response To find out the impact of the inductive heating on the temperature response of the process, the supplied inductive power is varied while the other parameters are kept constant and the outlet gas temperature was recorded. Fig. 3 shows the effect of the induction heating on the increase and decrease in the gas temperature by using iron hollow balls with a diameter of 3.22 mm. It can be seen that the induction technology massively reduces heating and cooling times. Therefore, temperature gradients are much more controllable and permit higher peak operating temperatures. According to the figure, after a few seconds the gas outlet temperature has already achieved a steady-state value. The heating and cooling process is very fast, which is beneficial for treatment of heat sensitive materials or biological substances demanding quick heating and cooling, e.g., canolol which forms by roasting of rapeseed meal or cake in a fluidized bed, but disintegrates again fast if heat is not removed within a short time after formation [18]. All conducted experiments show a similar time response of the temperature. The heating and cooling occurs equally quick, even at different induction powers. The more energy is supplied to the system, the higher the steady-state gas outlet temperature. However, with higher inductive energy, the temperature fluctuates strongly due to the bed pulsation, which indicates an inhomogeneous fluidization. The heat transfer into the fluidized bed to the iron hollow balls takes place until equilibrium is achieved between heated iron hollow balls and air flow and thus the temperatures do not change anymore. It should be noted that not all of the supplied electrical power is transferred to the iron hollow balls, but a part of the energy is consumed by the water cooling of the inductor as well as heat losses to the environment. As a result, the efficiency is approximately 75% in all experiments. 4.2. Fluidization behavior The literature has not yet been able to answer the question in detail as to what extent an externally applied magnetic field affects pneumatic behavior of fluidized bed by using iron hollow balls as a bed material and energy input of lower or higher power density. This was examined in a second series of experiments. In the following text, the field's influence on the fluidized bed during the induction heating is described. The pneumatic behavior of the fluidized bed is generally described by the pressure drop. Fig. 4 presents the total, bottom and bed pressure drop. Theoretically, after reaching the minimum fluidization velocity
and having a fluidized bed, respectively, the pressure drop should be constant. However, in reality, the pressure drop fluctuates due to the slugging behavior of fluidized beds as shown in the Fig. 4. Applying the induction field the fluctuations increase significantly as observed in the induction phase. Viewing the pressure drop as a function of time, it is difficult to graphically display the dependency of the pressure drop on the varied parameters. Therefore, in order to consider the fluidization behavior, the arithmetic mean is calculated over time from the measured bed pressure drops. The absolute difference of the measured pressures from the mean can be interpreted as how strong the bed fluctuates. The cumulative and relative frequencies of the differential pressures are calculated and are presented graphically. With strong pulsation, a wide distribution of differential pressures occurs. The dependency of the fluidization behavior on the process parameters are presented by means of the cumulative frequency distribution in the following presentation. Figs. 5 and 6 clearly show that the behavior of the fluidized bed depends on the transmitted induction power. In both experimental plants a strong influence of the field on the bed is observed. However, in the plant with smaller diameter (DN 139) this influence seems to be stronger mainly due to the apparatus configuration. In this configuration with relatively large height of bed and a relatively small diameter of apparatus, larger pressure drop fluctuations occur. By increasing the diameter of the plant to 300 mm, the influence of the alternating field on the fluidization behavior is smaller. In both cases the pressure fluctuations increase with increasing inductive energy input to the extent that above a critical load it leads to strong fluctuations or stagnation of the fluidized bed so that a correct functioning is no more possible, as can be observed in the accompanying video to this contribution. The visual observations agree very well with the measured pressure distribution. At induction powers below 2 kW no significant visual change in the fluidization regime is observed. For higher powers, a change in the fluidization behavior can be seen. The chaotic bubbling fluidized bed is replaced by a regular slugging fluidized bed. With further rising power (Pind N 14 kW), a piston-like motion of the entire bed material will be observed. Indeed an orientation of the individual particles towards the field lines can be seen. In some cases, it comes to a complete standstill of the bed. This phenomenon can be explained with the “field-induced” particle agglomeration. In an applied magnetic field ferromagnetic particles
Table 3 Overview of the properties of the non-electrically conductive materials. Material
Sauter mean diameter [mm]
Apparent density [kg/m3]
Glass beads Alumina beads Plastic beads
2.52 1.78 5.99
2395 678 1400
Fig. 3. Influence of the energy input on the gas temperature (DN 139; d32 = 3.22 mm; mbed = 1000 g; wgas = 3 ∗ wmf).
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Fig. 4. Influence of the electromagnetic field on the pressure drop (DN 139; d32 = 6.16 mm; mbed = 1000 g; wgas = 3 ∗ wmf; Pind = 10 kW).
such as iron particles develop north and south poles and align themselves with the direction of the electromagnetic field. Inside a fluidized bed exposed to the electromagnetic field, these particles can attract one another forming “pseudopolymeric” structures. Each particle induces a magnetic field gradient in its vicinity. The gradient increases with increasing field intensity. At high field intensity, interparticle attractive forces can cause ferromagnetic particles to form agglomerates. These agglomerates behave basically as one large particle and become much more resistive to flow than individual particles. Eventually, as the field intensity is further increased, agglomerated particles can be “frozen”. This bed-freezing phenomenon may appear even at very high gas flow rates which would usually result in a bubbling or a slugging fluidization regime if the external magnetic field was absent [17]. Indeed, in the present work, this bed-freezing phenomenon was also observed. At very high induction powers, it comes to a complete standstill of the bed. The influence of a magnetic field on the pressure fluctuations of the bed was examined by several authors. Ivanov and Grozev [15] found an increase in bed pressure drop at minimum fluidization velocity with increasing applied field intensity. This result is consistent with data of Hamby and Liu [8] from magnetofluidized beds in a gradient magnetic field. Shumkov et al. [23] and Bologa and Syutkin [4] reported that increasing the applied field intensity decreased the amplitude of bed pressure drop fluctuations. According to Bologa and Syutkin [4] the bed pressure drop decreases at high axial-field intensities. The authors explain this decrease by appearance of channels of reduced flow resistance along the lines of magnetic flux. However, Bologa and Syutkin [4] also found an increase in bed pressure drop when increasing the intensity of a magnetic field applied transversely relative to the fluidizing gas flow. This increase in bed pressure drop might be attributed to the field-induced particle agglomerations. However, the observation of Bologa and Syutkin [4] of a different effect of increasing applied field intensity on bed pressure drop in an axial and a transverse field is not in line with results reported by Hamby and Liu [8]. The authors found
Fig. 5. Influence of the energy input on the fluidization behavior (DN 139; d32 = 3.22 mm; mbed = 1000 g; wgas = 3 ∗ wmf).
Fig. 6. Influence of the energy input on the fluidization behavior (DN 300; d32 = 3.22 mm; mbed = 5500 g; wgas = 3 ∗ wmf).
that both the minimum fluidization velocity and the bed pressure drop at minimum fluidization velocity increased considerably with increasing field intensity applied both axially and transversely relative to the fluidizing flow. The authors ascribe this increase in increased field-induced agglomeration of iron particles along the lines of magnetic flux. The field-induced particle agglomeration becomes more significant with increasing applied field intensity. Agglomerated particles have larger effective diameters and higher resistance to gas flow. This results in an increase in bed pressure drop with increasing applied field intensity, because more energy (i.e., higher bed pressure drop) is needed to break particles apart and to cause them to fluidize. This result is consistent with our reported data. In the present study, it was shown that the pressure fluctuations increase at high induction powers most likely resulted from the increased field-induced particle agglomeration. Moreover, at higher induction power, the particles stay in the magnetic field lines and stagnate, which leads to lowered heat- and mass transfer and even local overheating. In order to avoid this negative effect, the power density of the magnetic field has been switched off for a short time and on again, which transferred the bed into the working state and prevents stagnation. Therefore, a pulsation control box is connected to the induction generator so that an automatic change of the magnetic field can be made at defined intervals (1 s without induction at 0 kW, and 3 s with induction at 6 or 18 kW). This allows to operate the fluidized bed well above the bed stagnation (Pind N 14 kW). Considering the cumulative frequency of 0.5, both curves at 0 kW in graphs 5 and 7 have the same deviation from the average bed pressure drop (approximately 50 Pa), so that the curves in these graphs can be compared with each other. As shown in Fig. 7, up to 18 kW energy input can be applied using a pulsating magnetic field. Pressure fluctuations are observed which are comparable to these of much lower induction power (see Fig. 5). Therefore, in order to achieve a higher power density without increasing pressure fluctuations, energy input should be realized by a pulsating magnetic field.
Fig. 7. Influence of the pulsating energy input on the fluidization behavior (DN 139; d32 = 3.22 mm; mbed = 1000 g; wgas = 3 ∗ wmf).
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Fig. 10. Influence of the particle diameter on the fluidization behavior (DN 139; mbed = 1000 g; wgas = 3 ∗ wmf). Fig. 8. Influence of the energy input on the fluidization behavior (DN 300; d32 = 3.22 mm; mbed = 5500 g; wgas = 3 ∗ wmf, measurements using the high frequency device, frequency at 1000 Hz).
To observe the fluid dynamic behavior closely, the bed pressure drop was measured with a high frequency device for 10 s. Fig. 8 presents the measured values of 2 s. The time distribution of the pressure drop at a different induction power indicates that a high energy input leads to stronger pulsation of the bed. This is due to the magnetization of the conductive particles, resulting in the formation of particle collectives. A very high induction power leads to stagnation of the collectives. These collectives are entrained by the gas flow, while at a lower induction power the particles are entrained in smaller collectives and move more uniformly. In addition to the energy input, other influencing factors such as air velocity, particle diameter, kaolin coating, ratio of conductive and non-conductive particles are investigated to determine the effect of the electromagnetic field on the pneumatic behavior of fluidized beds. Fig. 9 shows the impact of the gas velocity on the bed pressure drop. The graph illustrates that under a magnetic field, by changing the gas velocity, the pressure drop fluctuates considerably, while without exposure to the field the fluid dynamic of the fluidized bed is not strongly dependent on the gas velocity. In this case the pressure drops fluctuate minimally so that its influence can be neglected. It should be noted, that under the influence of the external electromagnetic field the higher gas velocity (five times higher than minimum fluidization velocity wmf) has a lower influence on the pressure drop compared to the lower gas velocity. This is due to the fact, that with increasing gas velocity the bed expands causing its height to rise. In this case the bed height is much higher than the height of the induction coil. Therefore, a part of the bed material is not exposed to the electromagnetic field, reducing the effect of the magnetic field. The same applies to the impact of the particle diameter on the fluidization behavior without magnetic field (Fig. 10). The particle diameter does not affect the distribution of pressure drop, but once the bed is
Fig. 9. Influence of the air velocity on the fluidization behavior (DN 139; d32 = 3.22 mm; mbed = 1000 g).
exposed to a magnetic field then the particle size seems to be more of a deciding factor. The larger the particle diameter, the lower is its influence: The particles with smaller diameters have a higher specific particle surface. Since by induction the energy conversion takes place mainly at the surface of the particles because of the skin effect, the magnetic field–particle interactions increase with lowering the particle diameter. A stronger influence of particles with diameter of 3.22 mm on the pressure drop distribution is therefore observed. These correlations are not observed if particles are coated with kaolin. The coating insulates the iron hollow balls and thus avoids local overheating and sparks on the particle surface. Fig. 11 shows that the kaolin coating has no negative influence on the pressure drop at the tested gas velocity (5*wmf), and even it appears to slightly stabilize the fluidization. Already 5 mass% of coating is enough to achieve this effect. Similar to the kaolin coating, the ratio of conductive and nonconductive materials has no significant effect on the pneumatic behavior (see Fig. 12). This means that even with a small mass fraction of iron hollow balls (20%), the bed shows similar behavior to a bed consisting of iron hollow balls only. Although the mixture ratio does not affect the fluidization behavior significantly, it is an important parameter from the point of view of heating behavior, which is not described in detail here, but it is remarked that for these experiments an induction power of 2 kW was used. However, at higher induction power stronger fluctuations of the pressure drop are expected with increase of the mass fraction of iron hollow balls. 5. Summary There are multiple advantages in the use of inductive heating of fluidized beds instead of convective heating. The heat exchanger is the electrically conductive particles in the fluidized bed. This leads to very high heat transfer surfaces and energy densities. There is also a very
Fig. 11. Influence of kaolin coating on the fluidization behavior (DN 139; mbed = 1000 g; wgas = 5 ∗ wmf).
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Greek letters
Symbol
Unit
Meaning
ρ ψ
[kg/m3] [%]
Density Aperture ratio
Indexes
Fig. 12. Influence of the mixing ratio on the fluidization behavior (DN 139; mbed = 1000 g; wgas = 3 ∗ wmf; Pind = 2 kW).
quick time response, minimizing time and energy consuming heating and cooling times and giving the possibility to quickly react to changes in the process. Large and expensive heating and cooling cycles and furnaces are not needed and energy loss in pipes is limited. Due to the fact that the inductive technology uses electromagnetic field to heat the electric conductive particles, the first question to be answered is how the electro-magnetic field influences the fluidization. This research work presented the impacts of the various parameters on the fluidization behavior under external electromagnetic field. The most important factor influencing the fluidization behavior is the induction power. With rising induction power the frequency of bed pressure drop fluctuations decreases and their amplitude increases. At very high induction power, a magnetic agglomeration and a stagnation of inert particles and accordingly a strong slugging bed motion were observed. However, the stagnation and resulting local overheating can be prevented with pulsating power input or kaolin coating of iron hollow balls. From a pneumatic point of view, the kaolin coating is not a significant parameter influencing the fluidization behavior. In contrast, the air velocity and the particle diameter have strong influence on the fluidization regime if an electromagnetic field is applied. An increase of the gas velocity results in lower pressure fluctuations despite a higher total pressure drop mainly due to the expansion of the bed and partial exposing of the bed to the electromagnetic field, respectively. Smaller particles influence the bed properties stronger because of the larger particle surface area interacting with the magnetic field. Concluding, the presented study explains in detail the considerable impact of the electromagnetic field on the fluid dynamic. Based on this knowledge, fluidized bed processes under electromagnetic field with predictable flow behavior can be designed. The next step is to investigate the heating behavior (including heating efficiency) and drying of particulate solids and suspensions in inductively heated fluidized beds. Latin letters
Symbol
Unit
Meaning
d d32 f m ṁ P p Δp s t U
[mm] [mm] [Hz] [kg] [kg/h] [kW] [Pa] [Pa] [mm] [mm] [V] [m3/h]
Diameter Sauter mean diameter Frequency Mass Mass flow Power Pressure Pressure drop Thickness Triangular distance Voltage Volume flow
[m/s]
Velocity
V w
Symbol
Meaning
A App F i I ind IHB max mf NCM p w
Air Apparent Fluid Inner Iron Induction Iron hollow balls Maximum Minimum fluidization Non-conductive material Particle Water
Abbrevations
Symbol
Meaning
DN FR4 IHB MIR TIR PB PdIR
Diametre nominal Flame retardant composite material Iron hollow balls Moisture indicator Temperature indicator Plastic beads Pressure indicator
Acknowledgments The authors gratefully acknowledge the funding of this work by the German Federal Ministry of Science and Education (BMBF) as part of the InnoProfile-Transfer project NaWiTec (grant numbers: 03IPT701A). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.powtec.2015.08.003. References [1] C. Augustin, W. Hungerbach, Production of hollow spheres (HS) and hollow sphere structures (HSS), Mater. Lett. 63 (13–14) (2009) 1109–1112. [2] G. Benkowsky, Induktionserwärmung, 5, Verlag Technik GmbH, Berlin, 1990. [3] M.K. Bologa, S.V. Syutkin, Magnetic field effect on the heat transfer in a fluidized bed, Electronna obrabotka materialov, 61976 61–66. [4] M.K. Bologa, S.V. Syutkin, The influence of an electromagnetic field on the structural– hydrodynamic properties of a fluidized bed, Electron. Obrab. Mater. 68 (1) (1977) 37–42. [5] EFD, Induction Heating Applications, Brochure2010. [6] M. Filtz, G. Birenbaum, Der Skineffekt in Bandleitern bei aufgeprägtem Wechselstrom, Arch. Elektrotech. (1987) 205–212. [7] D. Geldart, Gas Fluidization Technology, John Wiley & Sons, Chichester, 1986. [8] R.K. Hamby, Y.A. Liu, Studies in magnetochemical engineering: part VI. An experimental study of screen-packed and conventional fluidized beds in axial and transverse magnetic fields, Powder Technol. 64 (1–2) (1991) 103–113. [9] Hollomet®, Hollomet Brochure, 2014. [10] Herschler, A., 1965. Fluid treating method and apparatus. USA Patent 3,219,318. [11] Herschler, A., 1967. Method for the production and control of fluidized beds. USA Patent 3,304,249. [12] J. Hristov, Magnetic field assisted fluidization—a unified approach, Part 5. A hydrodynamic treatise on liquid–solid fluidized beds, Rev. Chem. Eng. 22 (4–5) (2006) 195–375.
V.V. Idakiev et al. / Powder Technology 286 (2015) 90–97 [13] J. Hristov, Magnetic field assisted fluidization — a unified approach. Part 8. Mass transfer: magnetically assisted bioprocesses, Rev. Chem. Eng. 26 (2010) 55–128. [14] J. Hristov, Magnetic field assisted fluidization — a unified approach. Part 9. Mechanical processing with emphasis on separations, Rev. Chem. Eng. 28 (4–6) (2012) 243–308. [15] D.G. Ivanov, G. Grozev, Hydrodynamics of a fluidized bed of iron–chromium catalyst for conversion of carbon oxide with water vapour in a magnetic field, Mineral Torve Nauch Tech, 391970. [16] I.M. Kirko, M.V. Filippov, Standard correlations for a fluidized bed of ferromagnetic particles in a magnetic field, Zn Tekhniko Fizika, 301960 1081. [17] Y.A. Liu, R.K. Hamby, R.D. Colberg, Fundamental and practical developments of magnetofluidized beds: a review, Powder Technol. 64 (1–2) (1991) 3–41. [18] F. Pudel, V. Habicht, T. Piofczyk, B. Matthäus, K.-W. Quirin, A. Cawelius, Fluidized bed treatment of rapeseed meal and cake as possibility for the production of canolol, OCL 21 (1) (2014) D103. [19] Rosensweig, R.E., 1979. Hydrocarbon conversion process utilizing a magnetic field in a fluidized bed of catalytic particles. USA Patent 4,136,016. [20] Rosensweig, R.E., 1978. Process for operating in magnetically stabilized bed. USA Patent 4,115,927.
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[21] A. Roßau, Induktiver Energieeintrag in eine fluidisierte SchüttungDissertation Otto von Guericke University Magdeburg, 2013. [22] V. Rudnev, D. Loveless, R. Cook, M. Black, Handbook of Induction Heating, Marcel Dekker Inc., New York, 2003. [23] S.H. Shumkov, D.G. Ivanov, K. Vekev, Pressure fluctuations in the boiling bed with magnetic field, God. Vissh. Khim. Teknol. Inst. 22 (3) (1977) 87. [24] S.H. Shumkov, D.G. Ivanov, Hydrodynamic characteristics of a fluidized bed in an electromagnetic field, J. Appl. Chem. 11 (1976) 2406–2409. [25] M. Stieß, Mechanische Verfahrenstechnik, 2, Springer Verlag, Heidelberg, 1993. [26] A. Stresing, L. Mörl, A. Khaidurova, M. Jacob, K. Walther, Bestimmung des Zeitverhaltes einer induktiv beheizten Wirbelschicht und deren Einflussgrößen, Chem. Ingenieur Tech. 85 (3) (2013) 308–312. [27] Tuthill, E.J., 1969. Magnetically stabilized fluidized beds. USA Patent 3,440,731. [28] H. Uhlemann, L. Mörl, Wirbelschichtsprühgranulation, Springer-Verlag, Berlin, Heidelberg, New York, 2000. [29] S.S. Zabrodskii, Yu.I. Tambovobtsec, Special features of interphase heat transfer in a ferromagnetic fluidized bed under the action of a magnetic field, Vetn. Akad. Nauk Beloruss, 691978.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Inductive heating of fluidized beds: Influence on fluidization behavior Vesselin V. Idakiev a,⁎, Sebastian Marx a, Antje Roßau b, Andreas Bück a, Evangelos Tsotsas a, Lothar Mörl a a b
Thermal Process Engineering, NaWiTec, Otto von Guericke University Magdeburg, Universitätsplatz 2, 39106 Magdeburg, Germany GETEC heat & power AG, Albert-Vater-Straße 50, 39108 Magdeburg, Germany
a r t i c l e
i n f o
Article history: Received 18 February 2015 Received in revised form 23 July 2015 Accepted 7 August 2015 Available online 12 August 2015 Keywords: Fluidized bed Inductive heating Drying Heat sensitive materials
a b s t r a c t Fluidized beds are commonly used for solid processing. Required heat is usually transferred by conduction via the gas flow or by particle–wall contact with immersed steam-heated tubes. Both variants possess limitations with respect to maximum gas temperature or contact area. In this work, heating of fluidized beds by induction is investigated. This is realized by co-fluidization of conductive but chemically inert particles, yielding large moving heat transfer areas. The focus lies on the experimental study of the impact of the electro-magnetic field on fluidization behavior. Studied process parameters are induction power, air velocity, particle diameter, inert coating, and the ratio of conductive to nonconductive materials. Moreover, the time response of temperature to changes in the induction power is investigated. The obtained results show very fast temperature response. The most important factor influencing the fluidization behavior is the induction power which can cause strong fluctuations of the pressure drop which at high energy inputs changes the fluidization regime from bubbling to slugging regime. This can be prevented using a pulsating magnetic field or insulation of the conductive material with a coating. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Since the 1930s, fluidized bed technology has gained a steadily growing importance in industry with uses in drying, combustion, chemical reaction engineering in the field of catalysis, as well as classification. In a fluidized bed, a fluid (liquid or gas) is passed through the bulk material, so that the solid/fluid mixture behaves as a fluid. Through the intensive contact between the fluid and the particles, good heat and mass transfer rates are achieved. Thus, many efforts have been taken to explain their characteristics which, among others, depend on the gas flow and applied force fields [7,25,28]. Usually the heat energy is supplied by a hot fluidizing gas. But in some cases, for example to increase the heat and mass transfer by drying of brown coal in a fluidized bed, steam-heated pipes immersed in the bed are used for heat transfer. Due to economic and environmental reasons and to establish new areas of application, new ways are sought to provide an increase in efficiency and energy savings in heat transfer. One method is inductive heating in which the heat input is not realized by the heated fluid flow into the fluidized bed, but is transmitted directly by non-contact heating of electrically conductive but chemically inert particles in the fluidized bed. This yields large heat transfer surface areas and very quick heating and cooling. In various applications including annealing, bonding, brazing, forging, hardening, melting, plasma production etc., inductive heating ⁎ Corresponding author. E-mail address: [email protected] (V.V. Idakiev).
http://dx.doi.org/10.1016/j.powtec.2015.08.003 0032-5910/© 2015 Elsevier B.V. All rights reserved.
has proved itself superior to convective heating. There are many reasons why using induction heating for these processes is clearly beneficial. The heat is generated directly in the material of interest, minimizing heat loss and energy consumption. Besides, induction transfers more energy per square meter than the common heat source such as open flame, resulting in faster heating, which ultimately improves the throughput and quality. It is also a fast and controllable process [5]. These considerable economic benefits of induction can also be transferred to fluidized bed processes. The inductive technology can ensure short heating and cooling times of bed material, leading to high efficiency and better product quality. Therefore, it is intended to lower energy costs and thus enhance the complete production profitability by using highly effective inductive heating in fluidized beds. Since inductive heating is realized by applying an altering electromagnetic field with an integrated inductor, it is important to know the impact of the magnetic field on the fluidization behavior. As described in Section 2, although there have been various research studies on the influence of magnetic fields on fluidized beds, with the induction power identified to be an important influencing factor, additional work is necessary to understand exactly how induction may affect the pneumatic behavior. Therefore, the presented study focuses on this research aspect. By variation of several process parameters such as induction power, air velocity, particle diameter, kaolin coating, mass ratio of conductive to nonconductive material, their influence on fluidization behavior is investigated. The results of this study provide the basis for implementation of inductive technology in fluidized beds as well as suggestions for further research.
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2. State of the technology By inductive heating, conductive material can be heated without any contact to the heater. The energy conversion takes place mainly at the edge zones (surface) of the particle because of the skin effect. Skin effect is a tendency for alternating current to flow mostly near the outer surface of an electrical conductor, such as iron balls. It occurs with high frequency alternating current and describes the appearance of the current density. As a result of this effect, the current density is largest near the surface of the conductor and decreases towards the center because electric current flows mainly at the “skin” of the conductor. The higher the frequency is, the greater this effect. This is a desired effect by the inductive energy input, since only the particle surface must be heated for heat transfer [6,22]. Heating by induction is used in industry for many decades. In the 19th century, it was first introduced with induction melting furnaces, which were initially operated at network frequency. By achieving higher frequencies through generators, besides pure annealing and melting processes also surface heat treatment process could be realized. Further possible applications were quickly recognized and promoted the development. Even today new application areas are tested and developed [2]. As already mentioned magnetic fields are used to heat electrically conductive materials. The influence of external magnetic fields on magnetizable particles in a fluidized bed was first studied by Kirko and Filippov [16]. The authors used iron particles with water as a fluidizing medium and reported that the magnetic field strength had an impact on the bed properties. A significant change in bed pulsation was observed by the authors. In 1961, experiments were performed with liquid metals and magnetizable particles [10,11]. It was shown that the magnetic field had a positive effect on the behavior of fluidized beds with respect to reduced bubbling and channeling. In 1969, the term ‘magnetically stabilized fluidized bed (MFB)’ was introduced. By MFB the bubble formation can be reduced and the heat transfer can be increased significantly [19,20,27]. The correlation between fluidized bed pressure drop and strength of the magnetic field has been studied by many authors [15,23]. According to these studies, the increase in field strength causes a reduction in pressure drop. Moreover, many researchers investigated the formation of particle agglomerates due to the strong magnetic forces between ferromagnetic particles, favoring cluster formation [3,24,29]. The formed particle agglomerates provide greater resistance to the fluid flow. The effect of stagnation of electrically conductive bodies at their respective positions was observed by the authors and described in detail. Another study on the topic ‘Magnetic field assisted fluidization’ was conducted by Hristov [12–14]. Magnetically controlled fluidized beds are interesting because the magnetic particles can be easily removed from the fluid. This method can be used for many processes without damaging the product, such as filtration or centrifugal separation. The inductive heating of particulate materials in gas–solid fluidized beds was first investigated Roßau et al. [21,26]; they observed that the electro-magnetic field yields changes in the fluidization behavior. The interaction of the different process and material properties and their influence on the fluidization was not characterized. The observations are the starting point for the experimental investigations in this work. 3. Experimental setup 3.1. Experimental plant In order to investigate the inductive energy transfer into a fluidized bed and its influence on the fluidization behavior, more than 50 experiments were performed in two fluidized bed plants. The schematic representation of the experimental plants is given in Fig. 1, the technical specifications of each plant are listed in Table 1.
Fig. 1. Schematic design of both experimental plants.
The body of the experimental plants DN 139 and DN 300 consists of a steel cylinder with an inner diameter of 139 and 300 mm, respectively, and an overall plant height of 3000 and 1800 mm, respectively. The fluidized bed chamber is made of a heat-resistant borosilicate glass cylinder. It allows visual observation of the fluidized bed and prevents the chamber walls from being heated by induction. The fluidized bed chamber is surrounded by an inductor with 5 (experimental plant DN 139) and 9 windings (experimental plant DN 300), respectively, which generate the electromagnetic field in the fluidized bed, leading to heating of the electrically conductive particles in the fluidized bed chamber. The inductor is electrically powered by a generator from Hüttinger Elektrik GmbH + Co. KG (Big 40/100 and TruHeat MF 3040, respectively), that can transmit electrical power up to 40 kW. The inductor is made from copper of high purity and has a good electrical conductivity. In order to avoid heating of the copper, the inductor is cooled with water, thus keeping the electrical conductivity stable. A fan draws ambient air, which enters the fluidized bed through a perforated distributor plate and fluidizes the bulk material. After leaving the fluidized bed, the air first enters a cyclone separator and then flows into a filter. At several points in the fluidized bed, pressure, temperature and moisture are measured. Moreover, to precisely measure the bed pressure drop in the fluidized bed unit, a high frequency measurement device is used that can record the resulting pressure drop with frequencies up to 1000 Hz. 3.2. Experimental material In the present work, as an experimental material electrically conductive iron hollow balls (IHB) are used for the inductive energy input in fluidized beds (Fig. 2, right). The manufacturing process developed by the company Hollomet GmbH allows to produce nearly monodisperse particles of defined diameter and also allows varying the particle layer thickness [9]. Due to the possibility of producing hollow balls of conductive material with variable diameters and thicknesses it is thus possible to create particles with defined fluidization properties. In addition, the experimental material (IHB) can be coated with kaolin. The ceramic coating serves to insulate the iron hollow balls. It may prevent local overheating and sparks on particle surface, which can take place at higher energy input. The kaolin coated iron hollow balls are shown in Fig. 2. If the variables such as coating density ρI, the density of the medium enclosed in the hollow balls ρA, and the apparent density of the ball ρApp,
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Table 1 Technical specifications of experimental plant DN 139 and DN 300. DN 139
DN 300
Fluidized bed unit
Diameter: di = 139 mm height: 1 m (borosilicate glass), s = 7 mm 2 m (stainless steel), s = 3 mm Fluid bed distributor plate Diameter: dperforation = 1 mm Plate thickness: s = 2 mm Triangular distance: t = 4 mm Aperture ratio: ψ = 5.7% Material: FR4 ΔpBottom = 267.77 ⋅ w2F + 27.945 ⋅ wF Coil Winding: Number: 5, 10 × 29 mm, di = 173 mm, material: copper, water-cooled Generator Big 40/100 (f = 30–75 kHz, U = 400 V, Pmax = 40 kW) Ventilator RIBO model VS 9 (Pmax = 12.5 kW) cyclone + filter 4.4 l/min, circulation through pressure of tap water
Material separation Water-cooling
shown in Fig. 2 right, are known, the layer thickness of the particle s can be calculated as follows: 8 313 9 2 ρ > > = 1− ρApp d < I 5 ð1Þ s ¼ 1−4 > 2 > : ; 1− ρρA I
By the manufacturing method it is possible to produce nearly perfect metallic hollow balls having a sphericity close to 1, a diameter larger than 1.5 mm and a particle density lower than 1000 kg/m3 [1]. This type of inert particles would not sink, for example, in water. The properties of the iron hollow balls used in the present study are summarized in Table 2. To study the heating behavior in an inductively heated fluidized bed, different non-electrically conductive materials were also used as experimental material. Moreover, the non-conductive materials are meant to represent the product of a real process. For this purpose, glass, alumina and plastic beads with similar fluid dynamic properties (determined by similar Archimedes number, minimum fluidization and elutriation Reynolds numbers) as the iron hollow balls were chosen, which are characterized in Table 3. Mixtures of conductive and non-conductive materials in different ratios (see Table 4) were used in additional experimental studies. 3.3. Experimental design In order to determine the impact of individual process parameters on the inductive heating and the fluidization behavior, various
Diameter: di = 300 mm height: 0.5 m (borosilicate glass), s = 7 mm 1.3 m (stainless steel), s = 3 mm Diameter: dperforation = 2.3 mm Plate thickness: s = 1 mm Triangular distance: t = 4.1 mm Aperture ratio: ψ = 30% Material: steel ΔpBottom = 11.276 ⋅ w2F + 0.763 ⋅ wF Winding: Number: 3 × 3, 10 × 29 mm, di = 340 mm, material: copper, water-cooled Trumpf TruHeat MF 3040 (f = 20–100 kHz, U = 429 V, Pmax = 40 kW) HRD 7 FU-105/11.0
(Pmax = 1 kW, V fluid ¼ 1:920 cyclone + filter 12 l/min, active circulation
m3 ) h
parameters were systematically varied. First, the parameters such as applied electrical power and air velocity were examined for their impact on the efficiency of the inductive energy input and the behavior of the fluidized bed. In these studies, only monodisperse iron hollow balls are used as bed material. In further experiments, particle diameter and kaolin coating were varied to determine their effect on bed properties (see Table 2). Subsequently, the heating of a mixture of conductive iron hollow balls and non-conductive materials (NCM) is considered. The influence of the mass ratio of iron hollow balls to non-conductive material mIHB/mNCM on the fluidization behavior was determined by varying the quantity and the type of the non-conductive material, while all other remaining parameters were kept constant. The efficient and uniform heating of the non-conductive particles is targeted here. Table 4 summarizes the varied parameters over all experiments. All experiments are carried out in the same way to ensure comparability. First, the measurement must be started. After slowly increasing the air mass flow rate to create the fluidized bed, the bed is given time to stabilize itself. Next, the inductive power is applied. After a defined period of the experiment the inductive power is turned off and the system is let to cool down. During the whole experiment including the stabilizing and the cooling period the pressure drop is measured. In the period without induction treatment the pressure drop is defined as “at 0 kW” in the following graphical illustrations and accordingly during the induction as “at the associated values of induction power”.
Fig. 2. Iron hollow balls kaolin coated and non-coated (right) and (left) their manufacturing principle.
V.V. Idakiev et al. / Powder Technology 286 (2015) 90–97 Table 2 Overview of the properties of the iron hollow balls.
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Table 4 Experimental program.
Sauter mean diameter [mm]
Apparent density [kg/m3]
Kaolin coating %
3.22 3.24 3.33 6.16
598.6 592.9 721.8 687.6
– 5 25 –
Varied parameter
Applied electrical power [kW] Gas velocity [m/s] Ratio of conductive particle to bed material [%]
Range of variation DN 139
DN 300
2, 4, 6, 10, 14, and 18 2, 3, 4, and 5 ∗ wmf 20, 40, 60, and 80 percentages of IHB
2, 4, 6, 10, 14, and 18 2; 3; 4, an 5 ∗ wmf 20, 40, 60, and 80 percentages of IHB
4. Results and discussion In this section, the results of the experiments are presented and discussed. The main focus lies on the temperature response and the pneumatic behavior. 4.1. Temperature time response To find out the impact of the inductive heating on the temperature response of the process, the supplied inductive power is varied while the other parameters are kept constant and the outlet gas temperature was recorded. Fig. 3 shows the effect of the induction heating on the increase and decrease in the gas temperature by using iron hollow balls with a diameter of 3.22 mm. It can be seen that the induction technology massively reduces heating and cooling times. Therefore, temperature gradients are much more controllable and permit higher peak operating temperatures. According to the figure, after a few seconds the gas outlet temperature has already achieved a steady-state value. The heating and cooling process is very fast, which is beneficial for treatment of heat sensitive materials or biological substances demanding quick heating and cooling, e.g., canolol which forms by roasting of rapeseed meal or cake in a fluidized bed, but disintegrates again fast if heat is not removed within a short time after formation [18]. All conducted experiments show a similar time response of the temperature. The heating and cooling occurs equally quick, even at different induction powers. The more energy is supplied to the system, the higher the steady-state gas outlet temperature. However, with higher inductive energy, the temperature fluctuates strongly due to the bed pulsation, which indicates an inhomogeneous fluidization. The heat transfer into the fluidized bed to the iron hollow balls takes place until equilibrium is achieved between heated iron hollow balls and air flow and thus the temperatures do not change anymore. It should be noted that not all of the supplied electrical power is transferred to the iron hollow balls, but a part of the energy is consumed by the water cooling of the inductor as well as heat losses to the environment. As a result, the efficiency is approximately 75% in all experiments. 4.2. Fluidization behavior The literature has not yet been able to answer the question in detail as to what extent an externally applied magnetic field affects pneumatic behavior of fluidized bed by using iron hollow balls as a bed material and energy input of lower or higher power density. This was examined in a second series of experiments. In the following text, the field's influence on the fluidized bed during the induction heating is described. The pneumatic behavior of the fluidized bed is generally described by the pressure drop. Fig. 4 presents the total, bottom and bed pressure drop. Theoretically, after reaching the minimum fluidization velocity
and having a fluidized bed, respectively, the pressure drop should be constant. However, in reality, the pressure drop fluctuates due to the slugging behavior of fluidized beds as shown in the Fig. 4. Applying the induction field the fluctuations increase significantly as observed in the induction phase. Viewing the pressure drop as a function of time, it is difficult to graphically display the dependency of the pressure drop on the varied parameters. Therefore, in order to consider the fluidization behavior, the arithmetic mean is calculated over time from the measured bed pressure drops. The absolute difference of the measured pressures from the mean can be interpreted as how strong the bed fluctuates. The cumulative and relative frequencies of the differential pressures are calculated and are presented graphically. With strong pulsation, a wide distribution of differential pressures occurs. The dependency of the fluidization behavior on the process parameters are presented by means of the cumulative frequency distribution in the following presentation. Figs. 5 and 6 clearly show that the behavior of the fluidized bed depends on the transmitted induction power. In both experimental plants a strong influence of the field on the bed is observed. However, in the plant with smaller diameter (DN 139) this influence seems to be stronger mainly due to the apparatus configuration. In this configuration with relatively large height of bed and a relatively small diameter of apparatus, larger pressure drop fluctuations occur. By increasing the diameter of the plant to 300 mm, the influence of the alternating field on the fluidization behavior is smaller. In both cases the pressure fluctuations increase with increasing inductive energy input to the extent that above a critical load it leads to strong fluctuations or stagnation of the fluidized bed so that a correct functioning is no more possible, as can be observed in the accompanying video to this contribution. The visual observations agree very well with the measured pressure distribution. At induction powers below 2 kW no significant visual change in the fluidization regime is observed. For higher powers, a change in the fluidization behavior can be seen. The chaotic bubbling fluidized bed is replaced by a regular slugging fluidized bed. With further rising power (Pind N 14 kW), a piston-like motion of the entire bed material will be observed. Indeed an orientation of the individual particles towards the field lines can be seen. In some cases, it comes to a complete standstill of the bed. This phenomenon can be explained with the “field-induced” particle agglomeration. In an applied magnetic field ferromagnetic particles
Table 3 Overview of the properties of the non-electrically conductive materials. Material
Sauter mean diameter [mm]
Apparent density [kg/m3]
Glass beads Alumina beads Plastic beads
2.52 1.78 5.99
2395 678 1400
Fig. 3. Influence of the energy input on the gas temperature (DN 139; d32 = 3.22 mm; mbed = 1000 g; wgas = 3 ∗ wmf).
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Fig. 4. Influence of the electromagnetic field on the pressure drop (DN 139; d32 = 6.16 mm; mbed = 1000 g; wgas = 3 ∗ wmf; Pind = 10 kW).
such as iron particles develop north and south poles and align themselves with the direction of the electromagnetic field. Inside a fluidized bed exposed to the electromagnetic field, these particles can attract one another forming “pseudopolymeric” structures. Each particle induces a magnetic field gradient in its vicinity. The gradient increases with increasing field intensity. At high field intensity, interparticle attractive forces can cause ferromagnetic particles to form agglomerates. These agglomerates behave basically as one large particle and become much more resistive to flow than individual particles. Eventually, as the field intensity is further increased, agglomerated particles can be “frozen”. This bed-freezing phenomenon may appear even at very high gas flow rates which would usually result in a bubbling or a slugging fluidization regime if the external magnetic field was absent [17]. Indeed, in the present work, this bed-freezing phenomenon was also observed. At very high induction powers, it comes to a complete standstill of the bed. The influence of a magnetic field on the pressure fluctuations of the bed was examined by several authors. Ivanov and Grozev [15] found an increase in bed pressure drop at minimum fluidization velocity with increasing applied field intensity. This result is consistent with data of Hamby and Liu [8] from magnetofluidized beds in a gradient magnetic field. Shumkov et al. [23] and Bologa and Syutkin [4] reported that increasing the applied field intensity decreased the amplitude of bed pressure drop fluctuations. According to Bologa and Syutkin [4] the bed pressure drop decreases at high axial-field intensities. The authors explain this decrease by appearance of channels of reduced flow resistance along the lines of magnetic flux. However, Bologa and Syutkin [4] also found an increase in bed pressure drop when increasing the intensity of a magnetic field applied transversely relative to the fluidizing gas flow. This increase in bed pressure drop might be attributed to the field-induced particle agglomerations. However, the observation of Bologa and Syutkin [4] of a different effect of increasing applied field intensity on bed pressure drop in an axial and a transverse field is not in line with results reported by Hamby and Liu [8]. The authors found
Fig. 5. Influence of the energy input on the fluidization behavior (DN 139; d32 = 3.22 mm; mbed = 1000 g; wgas = 3 ∗ wmf).
Fig. 6. Influence of the energy input on the fluidization behavior (DN 300; d32 = 3.22 mm; mbed = 5500 g; wgas = 3 ∗ wmf).
that both the minimum fluidization velocity and the bed pressure drop at minimum fluidization velocity increased considerably with increasing field intensity applied both axially and transversely relative to the fluidizing flow. The authors ascribe this increase in increased field-induced agglomeration of iron particles along the lines of magnetic flux. The field-induced particle agglomeration becomes more significant with increasing applied field intensity. Agglomerated particles have larger effective diameters and higher resistance to gas flow. This results in an increase in bed pressure drop with increasing applied field intensity, because more energy (i.e., higher bed pressure drop) is needed to break particles apart and to cause them to fluidize. This result is consistent with our reported data. In the present study, it was shown that the pressure fluctuations increase at high induction powers most likely resulted from the increased field-induced particle agglomeration. Moreover, at higher induction power, the particles stay in the magnetic field lines and stagnate, which leads to lowered heat- and mass transfer and even local overheating. In order to avoid this negative effect, the power density of the magnetic field has been switched off for a short time and on again, which transferred the bed into the working state and prevents stagnation. Therefore, a pulsation control box is connected to the induction generator so that an automatic change of the magnetic field can be made at defined intervals (1 s without induction at 0 kW, and 3 s with induction at 6 or 18 kW). This allows to operate the fluidized bed well above the bed stagnation (Pind N 14 kW). Considering the cumulative frequency of 0.5, both curves at 0 kW in graphs 5 and 7 have the same deviation from the average bed pressure drop (approximately 50 Pa), so that the curves in these graphs can be compared with each other. As shown in Fig. 7, up to 18 kW energy input can be applied using a pulsating magnetic field. Pressure fluctuations are observed which are comparable to these of much lower induction power (see Fig. 5). Therefore, in order to achieve a higher power density without increasing pressure fluctuations, energy input should be realized by a pulsating magnetic field.
Fig. 7. Influence of the pulsating energy input on the fluidization behavior (DN 139; d32 = 3.22 mm; mbed = 1000 g; wgas = 3 ∗ wmf).
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Fig. 10. Influence of the particle diameter on the fluidization behavior (DN 139; mbed = 1000 g; wgas = 3 ∗ wmf). Fig. 8. Influence of the energy input on the fluidization behavior (DN 300; d32 = 3.22 mm; mbed = 5500 g; wgas = 3 ∗ wmf, measurements using the high frequency device, frequency at 1000 Hz).
To observe the fluid dynamic behavior closely, the bed pressure drop was measured with a high frequency device for 10 s. Fig. 8 presents the measured values of 2 s. The time distribution of the pressure drop at a different induction power indicates that a high energy input leads to stronger pulsation of the bed. This is due to the magnetization of the conductive particles, resulting in the formation of particle collectives. A very high induction power leads to stagnation of the collectives. These collectives are entrained by the gas flow, while at a lower induction power the particles are entrained in smaller collectives and move more uniformly. In addition to the energy input, other influencing factors such as air velocity, particle diameter, kaolin coating, ratio of conductive and non-conductive particles are investigated to determine the effect of the electromagnetic field on the pneumatic behavior of fluidized beds. Fig. 9 shows the impact of the gas velocity on the bed pressure drop. The graph illustrates that under a magnetic field, by changing the gas velocity, the pressure drop fluctuates considerably, while without exposure to the field the fluid dynamic of the fluidized bed is not strongly dependent on the gas velocity. In this case the pressure drops fluctuate minimally so that its influence can be neglected. It should be noted, that under the influence of the external electromagnetic field the higher gas velocity (five times higher than minimum fluidization velocity wmf) has a lower influence on the pressure drop compared to the lower gas velocity. This is due to the fact, that with increasing gas velocity the bed expands causing its height to rise. In this case the bed height is much higher than the height of the induction coil. Therefore, a part of the bed material is not exposed to the electromagnetic field, reducing the effect of the magnetic field. The same applies to the impact of the particle diameter on the fluidization behavior without magnetic field (Fig. 10). The particle diameter does not affect the distribution of pressure drop, but once the bed is
Fig. 9. Influence of the air velocity on the fluidization behavior (DN 139; d32 = 3.22 mm; mbed = 1000 g).
exposed to a magnetic field then the particle size seems to be more of a deciding factor. The larger the particle diameter, the lower is its influence: The particles with smaller diameters have a higher specific particle surface. Since by induction the energy conversion takes place mainly at the surface of the particles because of the skin effect, the magnetic field–particle interactions increase with lowering the particle diameter. A stronger influence of particles with diameter of 3.22 mm on the pressure drop distribution is therefore observed. These correlations are not observed if particles are coated with kaolin. The coating insulates the iron hollow balls and thus avoids local overheating and sparks on the particle surface. Fig. 11 shows that the kaolin coating has no negative influence on the pressure drop at the tested gas velocity (5*wmf), and even it appears to slightly stabilize the fluidization. Already 5 mass% of coating is enough to achieve this effect. Similar to the kaolin coating, the ratio of conductive and nonconductive materials has no significant effect on the pneumatic behavior (see Fig. 12). This means that even with a small mass fraction of iron hollow balls (20%), the bed shows similar behavior to a bed consisting of iron hollow balls only. Although the mixture ratio does not affect the fluidization behavior significantly, it is an important parameter from the point of view of heating behavior, which is not described in detail here, but it is remarked that for these experiments an induction power of 2 kW was used. However, at higher induction power stronger fluctuations of the pressure drop are expected with increase of the mass fraction of iron hollow balls. 5. Summary There are multiple advantages in the use of inductive heating of fluidized beds instead of convective heating. The heat exchanger is the electrically conductive particles in the fluidized bed. This leads to very high heat transfer surfaces and energy densities. There is also a very
Fig. 11. Influence of kaolin coating on the fluidization behavior (DN 139; mbed = 1000 g; wgas = 5 ∗ wmf).
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Greek letters
Symbol
Unit
Meaning
ρ ψ
[kg/m3] [%]
Density Aperture ratio
Indexes
Fig. 12. Influence of the mixing ratio on the fluidization behavior (DN 139; mbed = 1000 g; wgas = 3 ∗ wmf; Pind = 2 kW).
quick time response, minimizing time and energy consuming heating and cooling times and giving the possibility to quickly react to changes in the process. Large and expensive heating and cooling cycles and furnaces are not needed and energy loss in pipes is limited. Due to the fact that the inductive technology uses electromagnetic field to heat the electric conductive particles, the first question to be answered is how the electro-magnetic field influences the fluidization. This research work presented the impacts of the various parameters on the fluidization behavior under external electromagnetic field. The most important factor influencing the fluidization behavior is the induction power. With rising induction power the frequency of bed pressure drop fluctuations decreases and their amplitude increases. At very high induction power, a magnetic agglomeration and a stagnation of inert particles and accordingly a strong slugging bed motion were observed. However, the stagnation and resulting local overheating can be prevented with pulsating power input or kaolin coating of iron hollow balls. From a pneumatic point of view, the kaolin coating is not a significant parameter influencing the fluidization behavior. In contrast, the air velocity and the particle diameter have strong influence on the fluidization regime if an electromagnetic field is applied. An increase of the gas velocity results in lower pressure fluctuations despite a higher total pressure drop mainly due to the expansion of the bed and partial exposing of the bed to the electromagnetic field, respectively. Smaller particles influence the bed properties stronger because of the larger particle surface area interacting with the magnetic field. Concluding, the presented study explains in detail the considerable impact of the electromagnetic field on the fluid dynamic. Based on this knowledge, fluidized bed processes under electromagnetic field with predictable flow behavior can be designed. The next step is to investigate the heating behavior (including heating efficiency) and drying of particulate solids and suspensions in inductively heated fluidized beds. Latin letters
Symbol
Unit
Meaning
d d32 f m ṁ P p Δp s t U
[mm] [mm] [Hz] [kg] [kg/h] [kW] [Pa] [Pa] [mm] [mm] [V] [m3/h]
Diameter Sauter mean diameter Frequency Mass Mass flow Power Pressure Pressure drop Thickness Triangular distance Voltage Volume flow
[m/s]
Velocity
V w
Symbol
Meaning
A App F i I ind IHB max mf NCM p w
Air Apparent Fluid Inner Iron Induction Iron hollow balls Maximum Minimum fluidization Non-conductive material Particle Water
Abbrevations
Symbol
Meaning
DN FR4 IHB MIR TIR PB PdIR
Diametre nominal Flame retardant composite material Iron hollow balls Moisture indicator Temperature indicator Plastic beads Pressure indicator
Acknowledgments The authors gratefully acknowledge the funding of this work by the German Federal Ministry of Science and Education (BMBF) as part of the InnoProfile-Transfer project NaWiTec (grant numbers: 03IPT701A). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.powtec.2015.08.003. References [1] C. Augustin, W. Hungerbach, Production of hollow spheres (HS) and hollow sphere structures (HSS), Mater. Lett. 63 (13–14) (2009) 1109–1112. [2] G. Benkowsky, Induktionserwärmung, 5, Verlag Technik GmbH, Berlin, 1990. [3] M.K. Bologa, S.V. Syutkin, Magnetic field effect on the heat transfer in a fluidized bed, Electronna obrabotka materialov, 61976 61–66. [4] M.K. Bologa, S.V. Syutkin, The influence of an electromagnetic field on the structural– hydrodynamic properties of a fluidized bed, Electron. Obrab. Mater. 68 (1) (1977) 37–42. [5] EFD, Induction Heating Applications, Brochure2010. [6] M. Filtz, G. Birenbaum, Der Skineffekt in Bandleitern bei aufgeprägtem Wechselstrom, Arch. Elektrotech. (1987) 205–212. [7] D. Geldart, Gas Fluidization Technology, John Wiley & Sons, Chichester, 1986. [8] R.K. Hamby, Y.A. Liu, Studies in magnetochemical engineering: part VI. An experimental study of screen-packed and conventional fluidized beds in axial and transverse magnetic fields, Powder Technol. 64 (1–2) (1991) 103–113. [9] Hollomet®, Hollomet Brochure, 2014. [10] Herschler, A., 1965. Fluid treating method and apparatus. USA Patent 3,219,318. [11] Herschler, A., 1967. Method for the production and control of fluidized beds. USA Patent 3,304,249. [12] J. Hristov, Magnetic field assisted fluidization—a unified approach, Part 5. A hydrodynamic treatise on liquid–solid fluidized beds, Rev. Chem. Eng. 22 (4–5) (2006) 195–375.
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