reshaping the structure of fluidized beds - clarkson university

CEP July 2009 www.aiche.org/cep 49

Fluids and Solids Handling

Multiphase catalytic reactors are widely used throughout the chemical process industries. Sev-eral types of reactors are available for multiphase

reactions that employ solid catalyst, such as the packed bed, fl uidized bed, and slurry bubble column; many varia-tions on these three archetypes exist. Despite their frequent application, each design suffers from drawbacks. In packed beds, relatively large (millimeter-scale) particles are used to keep the pressure drop (and, thus, the energy consumption) acceptably low. This makes diffusion lengths long and leads to poor mass transfer. Moreover, packed beds are sensitive to fl ow maldistribution. This can lead to problems, such as hot-spot formation and runaway. Fluidized beds and slurry reactors couple short intrapar-ticle-diffusion lengths with good heat transfer. However, they can suffer from backmixing, catalyst attrition and particle-fl uid separation problems, and they are diffi cult to scale up. Increasing the gas fl owrate in a fl uidized bed typically increases the bubble size and reduces the rate of mass transfer between the bubbles and the solids, which is often the rate-limiting step in a fl uidized bed (Figure 1). One way to overcome the disadvantages of multiphase reactors is to structure the reaction environment. This introduces additional degrees of freedom and allows decoupling of confl icting design objectives, such as high mass transfer vs. low pressure drop in a packed bed, or high gas fl owrate vs. small bubble size in a fl uidized bed. Although structuring is more straightforward in reactors with a fi xed catalyst, for instance, by installing structured packings such as those com-

monly used in distillation columns, it is also possible to change the hydrodynamic structure of reactors containing a mobile catalyst (or no catalyst), such as fl uidized beds, bubble columns, and slurry bubble columns. An example of this is the extension of the homogeneous regime in bubble columns to high velocities by homogeneously injecting the gas at the bottom of the column (1). There are several reasons to impose structure on fl uid-ized beds: • It helps to limit bubble size, which leads to better mass transfer and, subsequently, higher conversion and bet-ter selectivity. • A smaller bubble size reduces erosion, attrition, and elutriation. • Structuring helps to fl uidize powders that are diffi cult to fl uidize because they are wet or the particles are very small. • A more-homogeneous gas pattern prevents channeling.

Adding structure to a gas-solid

fl uidized-bed reactor changes

the system’s hydrodynamics and

can improve performance.

J. Ruud van Ommen

John Nijenhuis

Delft Univ. of Technology

Marc-Olivier Coppens

Rensselaer Polytechnic Institute

Delft Univ. of Technology

Reshaping the Structure of Fluidized Beds

ParticlesParticle Surface

Pores

CatalyticSites

Fluidized Bed

m mm nmμm

Reactants

Dense PhaseBubble

(Dilute Phase)

Figure 1. Mass transfer in a fl uidized bed takes place over different scales. The main bottleneck is the mass transfer of reactants from the bubbles to the dense phase, which is indicated by the dashed arrow.

50 www.aiche.org/cep July 2009 CEP


• A structured fl uidized bed is easier to model and to scale up. • The increased number of independent variables facili-tates model validation. • A structured fl uidized bed offers more possibilities to adjust the process during operation. • Process intensifi cation by structuring reactors may lead to safer, cleaner and less-expensive operation with lower pressure drops and fewer post-reactor separation steps. Structuring does have its disadvantages, however. For example, it usually adds to the capital costs, and sometimes to the operating costs. It often requires additional internals,

which may suffer from fouling and erosion, incurring additional costs. The structuring of gas-solid fl uidized beds can be carried out by modifying the gas supply or by interfering in the particle phase. In both cases, either the dynamics can be changed or the geometry can be altered, yielding four different approaches (2). Table 1 lists several possibili-ties for each of the approaches, and Figure 2 illustrates a few of these.

Manipulating the dynamics of the gas supply The gas fl ow to a fl uidized bed is commonly adjusted on long time scales (hours), for instance, to change the throughput rate. Over very short times (seconds), fl uctua-tions might take place due to upstream variations, but normally attempts are made to limit these fl uctuations. A varying gas supply may be used to improve the performance of a fl uidized bed. One option is to include the gas supply in a control loop that acts on time scales smaller than seconds. The gas fl owrate is continuously adjusted to maintain the desired state of the fl uidized bed, which is determined from one or more measurements. Some steps have been taken in this direction (3, 4), but it is very dif-fi cult to obtain instantaneous information about the size and position of the hundreds or thousands of bubbles in an industrial fl uidized bed. It is likely to be a long time before

this kind of closed-loop feedback control can be applied to industrial installations. An alternative to feedback control is open-loop control, i.e., imposing a continuous periodic varia-tion on the gas fl owrate. This introduces additional degrees of freedom — amplitude, frequency, and wave shape can now be chosen. Pulsing the gas can cause considerable changes in the bed’s hydro-dynamics and signifi cantly improve reactor perfor-mance (5, 6). Two related methods are vibro-fl uidization and sound-assisted fl uidization. Vibro-fl uidization can be accomplished by vibrat-ing the gas-distributor plate or by a combination of vibration and gas pulsation (7). Magnetized particles or an agitator can also be used to fl uidize the par-ticles, but whether such systems should be consid-ered vibrated beds is open to debate. Reference 7 gives an overview of the different types of vibrated fl uidized beds. The use of sound waves causes less wear and tear on the equipment than vibration. Experiments indi-cate that low-frequency (50–500 Hz), high-intensity (>110 dB) sonic energy markedly improves the fl u-idization of fi ne-grained, cohesive powders such as plaster of Paris and pigments, but the fl uidization of

Distributed Gas InjectionOscillating Gas Flow

Electric Field to InduceInterparticle Forces

Optimization of DistributedParticle Properties

Par

ticle

s

SecondaryGas Flow

PrimaryGas Flow

Gas Flow

Gas

Live Electrodes

GroundElectrodes

Gas Flow

PrimaryGas Flow

SecondaryGas Flow

Dynamics Geometry

Figure 2. Structure can be introduced into a gas-solid fl uidized bed by four main approaches.

Table 1. Fluidized beds can be structured in a variety of ways that involve changing the geometry or dynamics of

the gas or particle phase.

Dynamics Geometry

Gas

Pulsation

Vibration

Sound Waves

Closed-Loop Control

Distributor Plate

Baffl es

Staged Injection

Membrane Tubes

Par

ticle

s Rotation

Magnetic Fields

Electric Fields

Particle Size Distribution

Shape Factor

Particle Mixtures


coarse, free-fl owing, granular materials was not improved (8). In general, very high sound levels are needed to affect the fl uidization behavior of nanoparticles (9). Alternatively, the gas fl ow can be oscillated at a lower frequency, typically below 10 Hz. By oscillating the gas fl ow introduced through a porous bottom distributor plate, bubble patterns in fl uidized beds may become ordered and periodic, as illustrated in Figure 3 (10, 11). It is important to note that this is not a linear resonance phenomenon: the pattern is formed for a range of frequencies, the pattern wavelength is not inversely proportional to the driving frequency, and the ordered patterns are propagated upward via the rising gas fl ow (2, 11). In experiments, regular bubble patterns are observed in 3-D cylindrical beds, but only for beds up to a few centimeters high (10, 12). These patterns are similar to those seen in shallower, vibrated granular layers (13), but to date, it has not been possible to stabilize such patterns for deeper beds. More research is needed before this approach can be applied to deep (considerably thicker than a few centimeters) 3-D beds. Additional work is needed to optimize the structure to achieve a signifi cant reduction in bubble size. In pulsed fl uidized beds, the ordered patterns are propa-gated upward via the rising gas fl ow, which differentiates these patterns from those observed in vibrated granular lay-ers, where all the energy is transmitted to the particles via the moving bottom plate. As a result, dissipation of energy is much stronger in vibrated granular matter than it is in gas-solid fl uidized beds, where it is possible to infl uence

the entire bed dynamics via a change in inlet gas dynamics. There are several ways to generate gas pulsations (14). One is to periodically interrupt the gas stream by means of a butterfl y valve installed in the gas inlet duct and rotated at a specifi c angular velocity. Other methods are based on sequential relocation of the gas stream to the bottom of the bed by using: a rotating distributor valve that makes the gas stream sweep across specifi ed chambers of a rectangular bed; a perforated rotating disc located below the circular bed; or a rotating slotted horizontal cylinder that directs the gas stream to different sections of the base of a circular bed. An important application of pulsed fl uidized beds is the drying of particles. Pulsing has the following advantages over a steady gas supply (14): • irregularly shaped particles or particles having a wide size distribution can be easily fl uidized • fl uidization-air requirements are 30–50% lower • fragile particles can be fl uidized without damage • pressure drops are lower • fl uidization is more uniform; channeling is reduced and mixing is improved. The last point in particular leads to shorter drying times, which makes the process more effi cient. An example of the improved mixing for pulsed fl uidized-bed drying is shown in Figure 4. Although true feedback control of the bed’s hydro-dynamics to control bubble size is still in its infancy,

Con

stan

t Fl

ow

t = 0 min t = 6 min

Pul

sed

Flo

w

Figure 4. Pulsation produces rapid mixing in a fl uidized bed, as demonstrated in these photographs of a cohesive material (a wet lactose-cellulose mixture), part of which has been colored to show the mixing. Source: Adapted from Ref. 43.

Figure 3. Oscillating the gas inlet fl ow to a fl uidized bed can turn the chaotic hydrodynamics into regular patterns of rising bubbles. Shown is a 15-cm-wide air-fl uidized bed of sand particles, illuminated from behind, with constant (left) and oscillating (right) gas fl ow. The driving frequency is 2 Hz.

Constant Gas Flow Oscillating Gas Flow



pulsing the gas fl ow is a useful method to structure fl uid-ized beds. Often, it does not decrease the bubble size, but it has been shown to drastically reduce processing times in fl uidized-bed dryers.

Manipulating the particle dynamics In a fl uidized bed, several forces act on a particle: buoyancy (which normally can be neglected in gas-solids fl uidization), drag, gravity, and interparticle forces (e.g., Van der Waals and electrostatic forces). The particle dynam-ics can be manipulated by changing the forces acting on the particles. Drag can be modifi ed by changing the gas veloc-ity, but that changes the complete hydrodynamics of the bed. The gravitational force can be reduced by operating the fl uidized bed during a parabolic fl ight or even in space (15, 16), but for most operations this is not very practical. A larger “gravitational” force can be imposed on the par-ticles by rotating the fl uidization chamber and injecting the gas radially inward. In a centrifugal fl uidized bed (17), much higher gas velocities can be used without blowing out the particles, allowing higher mass- and heat-transfer rates to be obtained. Most of the efforts on rotating fl uidized beds have involved drying. Recently, the rotating fl uidized bed has also gained interest as a way to fl uidize nanoparticles (18). A way to induce a rotating motion in a static geometry has also been proposed (19). In this setup, the fl uidiza-tion gas is injected tangentially via multiple gas inlet slots at the outer cylindrical wall of the fl uidization chamber. As a result of the tangential gas-solid drag force, the solid particles rotate as well and experience a radially outward centrifugal force. This design avoids problems related to a rotating chamber (such as mechanical vibrations and com-plicated sealing), as well as diffi cult feeding or removal of the solids to/from the fl uidization chamber. The forces between particles depend on, among other things, the properties of the particles, such as their size,

composition, and surface structure. Using magnetic (20) or electric fi elds, the interparticle forces can be manipulated without changing the particle properties and without adding auxiliary matter, such as liquids. To successfully apply a magnetic fi eld, the particles must be magnetically susceptible. When such particles in a bed are magnetized, interparticle forces will cause them to attract or repel one another. A disadvantage of magnetic structur-ing is that the power consumption is high, on the order of 100 kW/m3, whereas applying an electric fi eld requires only 40–80 W/m3 of fl uidized bed (21). To apply an electric fi eld, the particles must be electri-cally susceptible, i.e., they must have a dielectric response to an electric fi eld. The dielectric response (which occurs in many more materials than a magnetic response) manifests itself as a polarization of the particle to a dipole. As with magnetic fi elds, this leads to an interparticle force (Figure 5). The direction of the electric fi eld, whether an alternating (AC) or a constant (DC) fi eld is used, and the relative humid-ity of the system are important parameters because they determine whether or not particle movement is preserved. The electrical stabilization of fl uidized beds was fi rst mentioned in a patent more than 40 years ago (22). The applied fi eld was so high that gas ionization occurred and the bed collapsed into a packed bed. Later, researchers

+ + +

– – –

+ + +

– – –

+ + +

– – –

+ + +

– – –

Electric Field

Figure 5. Polarized particles in an electric fi eld are subject to attractive and repulsive forces.

Figure 6. Discrete particle simulation shows the infl uence of a vertical AC electric fi eld of 0 kV/cm (left), 7 kV/cm (center), and 30 kV/cm (right) at 30 Hz on the bubble and particle behavior in a fl uidized bed of 200-µm particles. Source: Adapted from Ref. 25.


moved from static agglomeration of particles (as in an elec-trostatic precipitator) to controlling truly fl uidized beds, and reducing bubble size became the focus. An interparticle-force model for a semi-insulating powder in alternating fi elds was able to predict (based on simple lumped circuit theory) fi eld-frequency trends consistent with experimental data (23). The trends were verifi ed experimentally on the basis of bed expansion, but the effect on bubble size was not determined. Using moderate-strength (up to 3 kV/cm) AC fi elds, a maximum reduction in bubble diameter of 25% can be achieved with small particles (77 μm), while for large par-ticles (700 μm) the bubble diameter can be reduced by as much as 85% (24). Simulations show that both horizontal and vertical fi elds lead to a better distribution of gas and a smaller mean bubble size (25). Simulations also show that particles become immobile at high fi eld strengths and form strings in the direction of the fi eld (Figure 6). These results suggest that of the methods that act on the particle dynamics, applying a rotating gas fl ow or an electric fi eld are presently the two most promising ways to intensify fl uidized-bed operation.

Manipulating the geometry of the gas supply In a fl uidized bed, the gas is normally introduced evenly via a bottom plate. However, this often creates preferred bubble pathways. By using different orifi ce sizes at different radial positions, it is sometimes possible to achieve a more-even distribution of the bubbles and mini-mize the development of preferred bubble pathways (26). Creating a swirling pattern by introducing the gas horizon-tally (27) or by using a rotating distributor (28) can also improve the fl uidization behavior. Baffl es or other internals can be installed to limit bubble growth and enhance bubble breakup (29). For instance, dividing a fl uidized-bed reactor into compart-ments using a small open area, slot-shaped openings, and baffl es with separate passages for the gas and the particles decreases the backmixing of gas and solids (30). A single wire-gauze baffl e (with holes of approximately 4 mm dia. and about 70% open area) hinders solids mixing by shed-ding the wake from bubbles, yet its effects on bubble size and velocity are negligible (31). Distributing the gas supply over the height of the bed is another way to structure fl uidized beds. Membrane tubes can be used to supply gas to (or remove gas from) a fl uid-ized bed (32, 33). However, studies have focused more on the use of staged injection to infl uence the concentrations of reactants and/or products than on infl uencing the hydro-dynamics of the bed. Recently, micro-jets (nozzle diameter < 500 μm) were found to be very effective in assisting nanofl uidization

(34). Downward-pointing jets are most effective, since the shear and turbulence they induce break up the large agglomerates near the bottom of the bed. As a result, the agglomerates become smaller, fl uffi er and fractal-like, and the bed is more-easily fl uidized. A hierarchical, tree-like fractal structure (Figure 7) that connects all secondary injection points has been proposed as a way to distribute the gas over the bed (11, 35). Gas fl ows from the trunk of this tree and exits through the tips of the branches, which are spread out over the reactor volume at optimized locations. Via the bottom plate, enough gas is fed to ensure at least minimum fl uidization throughout the bed. An important advantage of this fractal design is its intrinsic scalability — which is achieved simply by adding new generations of branches to serve larger reactor vol-umes. Because the length and diameter of all the branches of a particular generation are the same, the hydraulic path lengths and pressure drops from the inlet to all the outlets are also the same, and the fl uid leaves all the outlets at the same fl owrate. This avoids radial non-uniformity among the outlets lying in the same horizontal plane. In the vertical direction, spacing the outlets according to a designated pat-

Figure 7. This 3-D fractal injector design avoids radial non-uniformity, and by strategic placement of the outlets can compensate for axial gradients in the gas fl ow and reactant concentrations.



tern may compensate for axial gradients in the gas fl ow and the reactant concentrations. For both very fl at (pseudo-2-D) and 3-D columns with a fractal injector, bubble size decreases signifi cantly with increasing secondary injection at a constant total fl owrate (Figure 8). Even with constant gas fl ow through the bottom plate and increasing secondary fl owrate (and, thus, increased total fl owrate), most of the secondary gas does not contrib-ute to the bubble phase. The number of bubbles increases with secondary injection, but the bubbles are smaller. This indicates that bubble coalescence is reduced (36). In addi-tion, backmixing of gas in the reactor decreases, which increases the conversion for positive-order reactions. As shown in Figure 9 for ozone decomposition as the test reac-tion, the conversion reaches a maximum at higher secondary gas velocities, probably due to channeling with the injec-tor that was used in the experiment. Using wider injection outlets, or more of them, prevents this (37). We can conclude that vertically staging the gas fl ow — for example, by using a fractal injector — can considerably decrease the bubble size and thus improve the fl uidized bed’s performance.

Optimizing the distributions of particle properties The hydrodynamic behavior of a fl uidized bed strongly depends on the properties of the bed’s particles, including size and density (38), as well as shape, surface structure, and elasticity. Furthermore, not only is the average value of a given particle property important, but the property’s distribution also infl uences fl uidization behavior. This is well-known for particle size distribution —

the addition of fi nes (particles with a diameter <45 μm) improves fl uidization behavior and leads to faster mass transfer. A broader particle-size distribution leads to higher conversions for ozone decomposition, probably due to the disproportionate amount of fi nes in the dilute (bubble) phase (39, 40). It has also been speculated that fi nes act as a lubricant to lower the apparent viscosity of the dense phase, leading to smaller voids and more-uniform gas-solid distribution (41). However, current practice is that fl uidized-bed particles used to catalyze gas-phase reactions are mainly optimized up to the scale of single particles. Most attention is given to their pore size distribution, such that a high surface area is achieved and the active sites are easily accessible by the gaseous components. Little attention is paid to mass transfer from the gas in the dilute phase to the particles in the dense phase, which is essential to practical fl uidized-bed operation. We recently started research on optimizing the behavior of gas-solid fl uidized beds by tailoring the distributions of particle properties, such as size, density, shape and elasticity. A high-throughput approach (similar to the high-throughput screening that is often employed in catalysis and biochemical research) that is quite novel for hydrody-namics research was developed to allow fl uidization char-acteristics of a large number of mixtures to be measured (42). The automated set-up (Figure 10) consists of a robot that automatically loads the fl uidized-bed column (Figure 11) and then conducts automated experiments to determine relevant parameters, such as minimum fl uidization veloc-ity, voidage and bubble size (the latter two as a function of the superfi cial gas velocity). Because the measurement and data acquisition equip-

Incr

ease

in C

onve

rsio

n, %

Fraction of Gas through the Fractal Injector

Experimental Data Model

0 0.25 0.500

2

4

6

Figure 9. Conversion of ozone decomposition (relative to the conver-sion without secondary injection) increases as the amount of gas supplied through the fractal injector increases. The bed is fl uidized at four times the minimum fl uidization velocity; the model assumes that the dense phase fl ow is at minimum fl uidization and that half of the secondary gas mixes with the dense phase. For more details on the two-phase model used, see Ref. 36.

Figure 8. Increasing secondary air injection reduces bubble size. The air-fl uidized sand bed (2-D, 20 cm wide) on the left has no secondary injection; on the right, half the air is injected through the injector. The total gas fl owrate is three times the minimum fl uidization fl owrate. The red line indicates the top of the fractal injector.


ment is rather expensive, it is not practical to run parallel experiments in multiple fl uidized beds. Instead, a particle batch is fl uidized, assessed under several conditions, and automatically replaced by a new batch of particles. In the tests, catalyst supports such as silica and alumina are used as particles. Experiments are carried out in two industrially relevant fl uidization regimes: bubbling fl uidization and tur-bulent fl uidization. In the experiments, pressure measure-ments, optical probes, and video analysis are used to assess the hydrodynamics. Using this high-throughput technique, we found that manipulating the width of the particle size distribution of alumina powder can reduce the bubble size up to 40%. The addition of fi nes to a given particle size distribution also decreases the bubble size up to 40% (Figure 12), whereas the addition of coarse particles has little infl u-ence on bubble size. The opposite effect occurs at low gas velocities (which are industrially not very relevant): bubble diameters increase rather than decrease with fi nes content. It is very likely that there is no single explanation for the effect that fi nes have on the hydrodynamic behavior of a fl uidized bed, and in particular on the bubble size. The deviant behavior at low velocities is an indication that two or more effects may be counteracting each other. We conclude that by optimizing particle mixtures, mass transfer between the dilute phase and the dense phase in fl u-idized beds can be considerably improved. Taking this into account when designing or optimizing industrial fl uidized bed processes may lead to a considerable reduction in costs.

Application in practice These four different approaches to altering the hydrody-namics of a fl uidized bed yield different results. Therefore, the preferred method will depend on the specifi c application. When a fl uidized-bed process is to be intensifi ed, it is important to fi rst clearly determine the goals, such as increased mass transfer or reduced reactor volume. In prin-ciple, all four methods lead to improved gas-to-solids mass transfer. Particle-drying experiments indicate that pulsing

the bed also improves particle mixing, whereas reducing the bubble size by distributed gas injection or by applying additional particle forces (e.g., an electric fi eld) is likely to decrease particle mixing. To what extent this is acceptable will depend on the application. Moreover, it is important to determine the extent to which the equipment can be changed. Is installing addi-tional internals acceptable? A potential problem with adding internals to a fl uidized bed might be limited lifetime — fl uidization typically produces strong erosion. Especially for the thin wires used in the electric fi eld experiments, short lifetimes would be expected. Never-theless, the reduced bubble size reduces local shear rates, which should lead to less erosion. Furthermore, prelimi-nary experiments show that thin wires last much longer than would be predicted based on typical tube-erosion

Rel

ativ

e B

ubb

le S

ize

Percentage of Fines0% 10%

U02 cm/s4 cm/s6 cm/s10 cm/s16 cm/s24 cm/s

20% 30% 40% 50%0.3

1.0

0.9

0.8

0.7

0.6

0.5

0.4

1.1

1.2

Figure 12. Relative bubble size (normalized to the bubble size for a mix-ture without fi nes at the same velocity) is a function of the weight fraction of fi nes at varying superfi cial gas velocities. For this work, the standard bed material was alumina with a median diameter of 70 μm, and the bubble size has been determined from pressure fl uctuations. Source: (42).

PreparedBed Material

Robot Arm

Fluidized BedFreeboard Section

PC

PressureDrop

Sensors

PressureFluctuation

Sensors

A B C

Air

Figure 10. This novel apparatus was developed to allow automated high-throughput experimentation. The fl uidized-bed column is fi lled (A), the experimental measurement program is carried out (B), the column is emptied (C), and the procedure is automatically repeated with the next batch of particles.

Figure 11. A robot loads the bed mass into a column.



rates, even in the absence of a fi eld. The wires, which have a thickness comparable to the particle diameter, may be less susceptible to attrition because the particles move around them. In this respect, it is noteworthy that a new function can sometimes be integrated into an existing internal: heat exchanger tubes immersed in the bed could be used as electrodes to impose an electric fi eld. The scale and type of process are also important in choosing the structuring method. In a large-scale fl uidized bed used for a catalyzed gas-phase reaction (e.g., produc-tion of acrylonitrile or maleic anhydride), placing addi-tional internals in the bed is a major step, but modifying the particles’ properties is a less-demanding option. In a smaller fl uidized-bed unit used for coating, the particles are the desired product and modifying their properties would not be acceptable. In addition, because of the latter unit’s smaller size, adding internals to distribute the gas or to impose a fi eld will be more manageable. Instead of modifying the structure within the bubbling regime as discussed here, one might instead move to a different fl uidization regime. For example, increasing the gas velocity and operating in the turbulent regime can increase mass transfer from the voids to the dense phase. In fact, many industrial fl uidized-bed reactors operate in the turbulent regime. Therefore, it will be useful to com-

pare the improved operation of structured bubbling beds to unstructured turbulent beds.

Concluding remarks Of the four approaches discussed, pulsation of the gas fl ow leads to the clearest structuring (i.e., one that creates nicely regular bubble patterns), but, so far, yields the small-est reduction in bubble size. Although we have been able to impose stable bubble patterns only on pseudo-2-D and very shallow 3-D beds, we have shown that for a relatively deep bed, pulsation makes drying much more effi cient. Electric fi elds produce the largest reduction in bubble size at relatively low gas velocities, while staged injection is more effective at somewhat higher gas velocities. A rotat-ing fl uidized bed is also effective at higher gas velocities. Changing the particle size distribution works well at the highest gas velocities (up to near the turbulent regime). This technique leads to the least-clear structuring in the strict sense of the word: bubble diameters are decreased, but no regular patterns are formed. Nevertheless, this is the approach that can be most easily applied in industry if the particle size can indeed be changed, since it requires little or no change to the equipment. Manipulating the structure of a fl uidized bed indeed seems to be a promising way to achieve process intensifi cation.

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CEP


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J. RUUD VAN OMMEN, PhD, has been an assistant professor in the chemical engineering department (DelftChemTech) of Delft Univ. of Technology, the Netherlands, since 2001 (E-mail: [email protected]). In 2004–2005, he was a visiting researcher at Chalmers Univ. of Technology, Sweden, work-ing on computational fl uid dynamics (CFD) of gas-solid fl ow. His current research focuses on dispersed multiphase reactors, in particular, develop-ing novel methodologies to monitor and structure these reactors, and on chemical engineering approaches to make nanostructured particles using methods such as atomic layer deposition and electrospray deposition. Awards and grants include the Unilever Research Prize for his MSc thesis, the DSM Award for his PhD thesis, and the Veni grant of the Dutch National Science Foundation. He holds MSc and PhD degrees from Delft Univ. and is a member of AIChE, the Royal Institute of Engineers in the Netherlands, and the Royal Dutch Chemical Society.

JOHN NIJENHUIS is Technology Transfer Offi cer of the Faculty of Applied Sci-ences at Delft Univ. of Technology (E-mail: [email protected]). In this position, he manages the intellectual property position of the faculty, and is responsible for translating science into business and developing technology partnerships and acquisitions. He joined the TU Delft staff in 2000 as research engineer, managing the pilot plant facilities of the department DelftChemTech. His research combined multiphase CFD work with experiments in the fi eld of monitoring and manipulating multiphase

reactors. As a specialist in experimental research, he developed several novel high-end solutions and methods. He is an expert in chemical reactor engineering, atomic layer deposition, and laboratory safety. In 2008, he received the AIChE Particle Technology Forum award for the best paper in fl uidization. He holds an MSc degree in chemical engineering from Delft Univ. of Technology.

MARC-OLIVIER COPPENS, PhD, is a professor in the Isermann Dept. of Chemical and Biological Engineering at Rensselaer Polytechnic Institute (Troy, NY; E-mail: [email protected]) since 2006. He has been a visiting scholar at the Chinese Academy of Sciences, and a postdoctoral fellow at Yale Univ. and the Univ. of California Berkeley. He joined the faculty at TU Delft in 1998, was named Antoni van Leeuwenhoek Professor in 2001, and served as Chair of Physical Chemistry and Molecular Thermodynamics for 2003–2006. His multidisciplinary research combines fundamental theoretical work with experiments, centering on the theme of nature-inspired chemical engineer-ing, to design and build effi cient chemical processes, porous catalysts, and separation systems, guided by effi cient biological systems. His invited lectureships and awards include Young Chemist and PIONIER Awards from the Dutch National Science Foundation, and a Visiting Professorship at the Norwegian Academy of Science and Letters. He holds MSc and PhD degrees in chemical engineering from the Univ. of Ghent, Belgium, and is member of AIChE, ACS and the Belgian American Educational Foundation.

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