Ind. Eng. Chem. Res. 2009, 48, 4611–4618
4611
Constrained Inverse Fluidization of Light Particles in a Draft Tube Airlift Reactor ˇ olovic´,† Bojan Petkovic´,‡ Aleksandar Zˇ. Selakov,§ Ivana M. Sˇijacˇki,*,† Radmilo R. C † Dragan Lj. Petrovic´, Miodrag N. Tekic´,† and Mirjana Ð. Ðuric´† Department of Chemical Engineering, Faculty of Technology, UniVersity of NoVi Sad, Bul. Cara Lazara 1, 21000 NoVi Sad, Serbia, Chemical and Biological Engineering Department, UniVersity of British Columbia, 2360 East Mall, V6T 1Z3, VancouVer, Canada, and Department for Computing and Automatics, Faculty of Technical Sciences, UniVersity of NoVi Sad, Trg Dositeja ObradoVic´a 6, 21000 NoVi Sad, Serbia
The objective of this work was to investigate the main fluidodynamic behavior of a constrained inverse fluidized bed in a draft tube airlift reactor (DT-ALR), with liquid in batch and air in continuous mode. The inverse fluidized bed was formed of relatively large particles (4.3-5.4 mm) that were composed of light polyethylene (PE), in the first case, and of particles (3.7-4.7 mm) that were composed of extremely light polystyrene (PS), in the second case. The influence of physical properties of liquid phase (surface tension and viscosity) on basic fluidodynamic characteristics was examined using the coalescence-inhibiting medium (1 wt % aqueous solution of ethanol) and viscous medium (46 wt % aqueous solution of sucrose). It is shown that the overall gas holdup, induced liquid velocity, minimum fluidization velocity, and bed expansion are dependent on the superficial gas velocity and liquid and solid properties. 1. Introduction 1
Inverse three-phase fluidization (mode II-a) is an operation in which solid particles with a density lower than the continuous liquid phase are suspended in liquid and gas media toward gravity by means of downward flow of liquid and/or gas bubbles.2 The inverse fluidized beds find their practical application in biochemical, food, chemical, petrochemical, and environmental processes, where fluidized solid particles are usually small, porous, and light. The solid phase primarily has the role of a biomass carrier or a catalyst and must be disposable entirely. Particles that are composed of polyethylene, polypropylene, polystyrene, and other organic materials are shown to be adequate for this operation. Reactors that involve inverse fluidization enable direct intimate phase contact, lower energy input, and abrasion, because of fluidization at lesser liquid velocities,3 high heat- and mass-transfer rates, the possibility of biofilm thickness control,4 and the possibility of controlled bed expansion by countercurrent gas- and liquid-phase flows.5 On the other hand, inverse fluidization has several disadvantages, because the gas flow can damage the particles, the biofilm layer on the solid carrier can be thinned by intense turbulence,6 the liquid circulation is necessary, and the systems have complex hydrodynamics. A review of studies conducted on inverse fluidization in bubble columns (BCs) and airlift reactors (ALRs) is presented in Table 1. Most of the research was conducted in BCs with water as the liquid phase and in a continuous mode of operation. As shown in Table 1, only Garnier et al. conducted their experiments in a draft tube airlift reactor (DT-ALR) with liquid in batch mode, but with water as the liquid phase and with very small particles. * To whom correspondence should be addressed. E-mail address:
[email protected]. † Department of Chemical Engineering, Faculty of Technology, University of Novi Sad. ‡ Chemical and Biological Engineering Department, University of British Columbia. § Department for Computing and Automatics, Faculty of Technical Sciences, University of Novi Sad.
Based on hydrodynamic research on three-phase inverse fluidized beds,7,8 four basic flow regimes were observed: (i) static or semifluidized bed with the presence of small bubbles, (ii) transition regime, (iii) bubble coalescence regime, and (iv) complete fluidized bed regime with bubble disintegration. The movement of particles can be restricted through the use of retaining grids at the top and bottom of the column.7 The grids affect the hydrodynamics of the entire system, as they take a role of additional gas distributor, introduce a resistance to the fluid flow, and enable accumulation of solids on or under them. This mode of operation is known as the constrained fluidized bed. It is well-known that the addition of surfactants, alcohols, electrolytes, acids, polymers, antifoam agents, and viscous liquids (both Newtonian and non-Newtonian) influences the gas holdup. The coalescence inhibiting systems, such as dilute alcohol and viscous solutions, increase the gas holdup. The addition of alcohols to water reduces the surface tension, resulting in smaller average bubble size, lower bubble rise velocity, an entrainment of bubbles into the downcomer, and, finally, higher gas holdups, compared to air-water systems.9 In liquids with viscosities greater than that of water (such as glycerin and sucrose solutions), the bubble rise velocity decreases because of additional hydraulic resistance, resulting in higher gas holdup.10 The effect of additives on liquid circulation and/or driving force for liquid circulation is also investigated. In dilute aqueous solutions of alcohols, the smaller bubbles are easily dragged in the downcomer, which reduces the driving force for liquid circulation, in comparison to that for water. In viscous solutions, the liquid velocity decreases, because of higher resistance to circulation.11-14 The presence of solids in a gas-liquid dispersion has a strong impact on the hydrodynamics, because of the interaction between the bubbles and the solid particles. This interaction is dependent on several factors: wettability; the shape, size, and density of solids; bed expansion; gas and liquid velocity; and the density, surface tension, and viscosity of the liquid phase.15 Small particles in a gas-liquid dispersion promote coalescence,16 because of the higher
10.1021/ie801915m CCC: $40.75 2009 American Chemical Society Published on Web 03/30/2009
4612 Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009
Figure 1. Experimental setup.
apparent density and viscosity of the liquid-solid suspension, in comparison with pure liquid.16,17 Larger particles in a gas-liquid dispersion induce intensive bubble disintegration, resulting in relatively small bubbles with a uniform size distribution.18 The transition between the bubble coalescence and the bubble disintegration regime occurs at a critical diameter of the solid particles.19 According to Lee et al.,18 the disintegration occurs when particles have a diameter that is similar or larger than that of the bubbles, because they have enough velocity to overcome the surface tension force and penetrate the bubble layer. In almost all of the research, the decrease in gas holdup, with the increase of solids loading, is noticed. Petrovic´15 concluded that the particle size and the particle concentration have strong influence on the liquid velocity in a three-phase DT-ALR. Before the fluidization, a static layer of solid particles almost completely hindered the liquid circulation; therefore, the fluidization began after a sudden increase in liquid circulation velocity. This research also showed that higher liquid velocities were achieved in systems with larger particles. Kawalec-Pietrenko20 determined the liquid circulation velocity in an external-loop airlift reactor (EL-ALR) with the inverse fluidized bed in the downcomer. This author has observed that the liquid circulation velocity decreases as the particle concentration increases, but increases linearly as the gas velocity increases toward the minimum fluidization velocity. When the minimum fluidization velocity was exceeded, the dependence is shown to be exponential.
The objective of this work was to investigate behavior and describe the phenomena in a DT-ALR with a batch of liquid phase and with a bed of larger, very light particles, which were brought to the inverse fluidization state. The contribution in this matter is made, in addition, by introducing liquid phases with different physical properties, as well as much useful data that are relative to the influence of light solid particles on the velocity of liquids within the DT-ALR. Consideration of the available investigations in such types of reactors (recall Table 1) involves systems that have not yet been investigated. 2. Experimental Setup The experiments were conducted at 20 ( 1 °C in a draft tube airlift column with two retaining grids used to center the draft tube, but also to hold the fluidized bed within the downcomer. The geometrical details of the experimental setup are shown in Figure 1. Air that was sparged into a draft tube through a single orifice with an inner diameter (ID) of 4 mm was used as the gas phase. Tap water, a 1 wt % aqueous solution of ethanol, and a 46 wt % aqueous solution of sucrose were used as the liquid phase. All experiments were conducted with the liquid phase in batch mode. Fresh solutions were used within a short set of experiments, to maintain the properties of liquids. At each gas flow rate, the system was brought to steady state before it was approached to the particular measurements, with
Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4613 Table 1. Review of Three-Phase Inverse Fluidized Bed Studies in Bubble Columns and Airlift Reactors reactor type and characteristics/mode of operation
ref Fan et al.7 Nikolov and Karamanev4 Garnier et al.21 Nikov and Karamanev22
Comte et al.2 Buffie´re and Moletta23 Briens et al.3
Buffie´re et al.5 Han et al.24 Kawalec-Pietrenko20 Loh and Liu6 Bandaru et al.25
Shin et al.26
system
Bubble column Liquid phase: continuous; H ) 2.73 m, D ) 0.0762 m Draft tube-airlift reactor Liquid phase: continuous; V ) 1.7 dm3, DC/D ) 0.46 Draft tube-airlift reactor Liquid phase: batch; H ) 1.5 m, DC/D ) 0.3 - 0.6 Bubble column Liquid phase: continuous; D ) 0.1 m Bubble column Liquid phase: batch; H ) 1.7-4 m, D ) 0.08-0.38 m Bubble column Liquid phase: continuous; H ) 1.5, 5 m, D ) 0.1, 0.25 m Bubble column Liquid phase: continuous; D ) 0.17 m Bubble column Liquid phase: continuous; H ) 0.8 m, D ) 0.08 m External loop airlift reactor Liquid phase: batch; H ) 1.3 m, D ) 0.06 m Bubble column Liquid phase: continuous; AD/AR ) 1, 2.11, 5.16 External loop airlift reactor Liquid phase: batch; V ) 4 dm3, D ) 0.03 m Bubble column Liquid phase: continuous; H ) 2.75 m, D ) 0.089 m Bubble column Liquid phase: continuous; H ) 2.5 m, D ) 0.152 m
dS (mm)
investigated parameters
air/water/polypropylene and polyethylene spheres
4.8-19.1
flow regimes, bed expansion, gas holdup
air/wastewater/polypropylene and polyethylene granules
0.8-3
wastewater treatment
air/water/polystyrene spheres
0.14-0.2
gas holdup, bed expansion, liquid velocity
air/water solution of polyethylene glycol/ polystyrene and polyethylene spheres air/water/ polyethylene spheres and discs
2.2-7.1
liquid-solid mass transfer coefficient
3-4
flow regimes
air/water/polypropylene pellets and Bio-HDC spheres air/deionized water with additives (inorganic salts, organic molecules)/ polypropylene spheres biogas/wastewater/ Extendospheres
0.175-4
flow regimes, liquid and gas holdups
4-6
gas holdup, minimum fluidization velocity
0.175
reactor start-up
air/water/polyethylene spheres
10
air/water/Ovipan spherical beads
2-6.5
gas holdup, minimum fluidization velocity, liquid velocity liquid circulation velocity
air/water and phenol solutions/polystyrene spheres air/water/polypropylene spheres
1-1.18
gas holdup, liquid velocity
6.1
air/water and aqueous solutions of CMC/ polypropylene and polyethylene spheres
4
flow regimes, minimum fluidization velocity, axial gas, liquid and solid holdups gas, liquid and solid holdup, bed porosity
Table 2. Physical Properties of Liquid Phase at 20 °C liquid tap water 1 wt % aqueous solution of ethanol 46 wt % aqeuous solution of sucrose
density, FL (kg/m3)
viscosity, µL (× 103 Pa s)
surface tension, σL (× 103 N/m)
998.2 996.3
1.003 1.037
71.5 65.5
1212
10.3
75.8
occurred within 10-15 min. Physical properties of the liquid phase are given in Table 2. The densities of the liquids used were measured by a densitometer (AP PAAR DMA46) with an accuracy of (0.1 kg/m3. The viscosities were obtained using Ostwald’s viscosimeter with an average error of 1%. Surface tensions of liquid phases were obtained using a tensiometer (Torsion Balance Model OS) with an accuracy of (0.0001 N/m. Polyethylene (PE) granules and polystyrene (PS) spheres were used as the solid particles. Their properties are summarized in Table 3. The PE granules were not chemically treated. Therefore, they showed low wettability. The experiments were conducted with 7.5 and 15 vol % of PE and 8 and 16 vol % of PS, calculated based on the downcomer volume between the two grids. The particle size range was defined by sieve analysis. The density of the particles was
obtained by pycnometry. Terminal rising velocity of a single particle in the downcomer was measured for all liquids used in the experimental work, to include the wall effect and different buoyancy effects. Five particles of both solid phases were chosen, as a representative sample. Each particle was brought to the downcomer bottom by means of a steel wire with a ring-shaped end and then it was released. The time required for a single particle to pass the distinct length, after it gained uniform velocity, was estimated, and, subsequently, the velocity was calculated. The measurements were repeated 10 times for each particle; therefore, the average velocity for a distinct solid phase was gained, with an average error of (7.4%, because of nonuniform particle size distribution. The minimum fluidization velocity was determined visually by applying the 1-s criterion. The gas flow rate was slowly increased until the moment when particles were moved apart from each other and brought to motion, with a stagnant state of
