An examination of Wen and Yu’s formula for predicting the onset of fluidisation

Gibson, I.A.; Zheng, Yaoyao; Scott, Stuart A.; Davidson, John; Hayhurst, A.N.

doi:10.1016/j.cherd.2018.05.009

Cited by 14 publications

(8 citation statements)

References 28 publications

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“…3.175 mm, stainless steel) feeding liquid glycerol of medicinal purity. The bed contained alumina particles (sieved to be from 355 to 425 μm [9], with a geometric mean of 388 μm). They were angular with some very sharp points [9].…”

Section: Figure 1 Hereaboutsmentioning

confidence: 99%

“…The bed contained alumina particles (sieved to be from 355 to 425 μm [9], with a geometric mean of 388 μm). They were angular with some very sharp points [9]. The flowrate of air to the bed was controlled by a rotameter, for rates up to 48 litres/min, as measured at ambient pressure and temperature.…”

Section: Figure 1 Hereaboutsmentioning

confidence: 99%

“…When the flowrate of air through a bed of particles is increased gradually above zero, the particles remain stationary until U reaches a critical value, U mf , when all the particles begin to move. This is the onset of fluidisation; U mf was measured at elevated temperatures in the bed in a separate study [9], where further details can be found. When U = U mf , the pressure drop across the bed is just sufficient to support the submerged weight of the particles.…”

Section: Figure 1 Hereaboutsmentioning

confidence: 99%

“…A thermocouple was sometimes attached to the sampling tube to measure the temperature of the sampled gas, as described in more detail below. With the bed at 800 o C, U mf was 0.114 m/s [9], so that with U/U mf = 2.5, U (the velocity in the freeboard) was 0.285 m/s. This indicates that the interval between gas leaving the bed and reaching the sampling probe was 0.325/ 0.285 ≈ 1.1 s.…”

Section: Figure 1 Hereaboutsmentioning

confidence: 99%

“…As Fig. 1 shows, the bubbles are most probably spherical caps, for which a wake-fraction of 0.15 is appropriate [20] for the angular particles of alumina [9] fluidised here. In addition, the rise-velocity (with respect to the bed's exterior) of a bubble of diameter, d b , can be estimated [20] from: bubbles of glycerol, when rising up the bed, is ~ 19 mm.…”

Section: Visual Observationsmentioning

confidence: 99%

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The continuous combustion of glycerol in a fluidised bed

Gibson

Zheng

Davidson

et al. 2019

Combustion and Flame

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It is difficult to burn a liquid fuel inside a fluidised bed. For the first time, liquid glycerol has been burned, when continuously injected into the bottom of an electrically heated bed of alumina particles (sieved to 355-425 μm), fluidised by air. The temperature in the bed was held at 700, 800 or 900 o C; usually (U/U mf) was 2.5. The bed's depth was varied, as also were (U/U mf) and the ratio of fuel to air supplied to the bed. Measurements were made of the concentrations of CH 4 , O 2 , CO and CO 2 , and also of the temperature, in the freeboard well above the bed. On entering the bed, the liquid glycerol, rapidly formed bubbles of vapour, which quickly decomposed thermally, yielding mostly CO and H 2. These gases then mixed with the other gases in the bed. It appears that the diffusive H 2 mainly burns between the fluidised particles. With the bed at 700-900 o C, no CO was detected far downstream of the bed, provided the equivalence ratio, θ, was below 0.7, i.e. with more than 43 % excess air. Under these fuel-lean conditions, all the carbon in the glycerol was oxidised to CO 2. However, in a more fuel-rich situation, with θ > 0.7, CO was detected well above the bed, particularly with a deeper bed, at a lower temperature and operating more fuel-rich. Thus, with the bed at 900 o C, CO was mostly oxidised inside the bed, but occasionally some CO burned on top of the bed. When a fuel-rich bed was below  850 o C, not all the CO burned in the bed. Achieving complete combustion inside a fluidised bed is partly a problem of mixing the products of glycerol's thermal decomposition with the fluidising air, which on entry exists mainly in bubbles. Consequently, increasing (U/U mf) promoted both mixing and combustion in a bed. In addition, in-bed combustion requires the bed to be sufficiently deep, hotter than  850 o C and θ to be less than a critical value. The effects of other variables are discussed. Nomenclature d b diameter of a bubble, m g acceleration due to gravity, 9.806 m s-2 G volumetric flowrate of glycerol vapour into a bed, ml s-1 T bed temperature of a bed, K or o C U superficial velocity of fluidising air through a hot bed, m s-1 U mf value of U at incipent fluidisation, m s-1 U rise rise velocity of a bubble, m s-1 V b volume of a bubble, m 3 x i mole fraction of species i in the gas phase ε mf voidage in a bed at minimum fluidisation θ equivalence ratio = (fuel supplied / fuel for a stoichiometric mix with the O 2 supplied) θ crit minimal value of θ for producing CO in the off-gases ψ x CO /(x CO + x CO2)

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Section: Figure 1 Hereaboutsmentioning

confidence: 99%

Section: Figure 1 Hereaboutsmentioning

confidence: 99%

Section: Figure 1 Hereaboutsmentioning

confidence: 99%

Section: Figure 1 Hereaboutsmentioning

confidence: 99%

Section: Visual Observationsmentioning

confidence: 99%

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The continuous combustion of glycerol in a fluidised bed

Gibson

Zheng

Davidson

et al. 2019

Combustion and Flame

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Properties, Minimum Fluidization, and G eldart Groups

Grace

2020

Essentials of Fluidization Technology

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Experimental methods in chemical engineering: Reactors—fluidized beds

et al. 2019

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Industry relies on fluidized beds to synthesize chemicals (acrylonitrile, maleic anhydride, titanium dioxide, vinyl chloride), combust coal, dry powders, and treat waste. Fluidized bed folklore declares that they are hard to scale‐up and the gas phase is backmixed. Commercial failures that disregard standard design criteria around powder management, gas/solids injection, and mixing reinforce this belief. However, engineers select fluidized beds for processes that are impractical with conventional technologies to achieve economies of scale for highly exothermic, endothermic, or explosive reactions, for catalysts that deactivate in seconds (or minutes), and for chemistry that requires multiple dosing cycles. Failures are more frequent for these challenging applications. For this reason, researchers study reaction kinetics in fixed beds despite internal mass transfer limitations and axial and radial temperature and concentration gradients. Fluidized bed hydrodynamics vary with powder properties (particle diameter, size distribution, density, sphericity), operating conditions (gas density, viscosity, temperature, pressure), reactor geometry (diameter, height, mass, grid geometry). The minimum fluidization velocity (Umf) is a property that identifies the transition from the fixed bed regime to the fluidized bed regime and equals the gas velocity at which the upward drag force equals the weight of the powder. At the experimental scale, fluidized beds operate isothermally, solids are completely backmixed, and the gas phase is close to plug flow (Ug<-0.25em3Umf). Here, we describe the relationship between powder properties and fluidization quality, list experimental techniques, describe recent applications, and gas phase hydrodynamics and uncertainties.

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An examination of Wen and Yu’s formula for predicting the onset of fluidisation

Cited by 14 publications

References 28 publications

The continuous combustion of glycerol in a fluidised bed

The continuous combustion of glycerol in a fluidised bed

Properties, Minimum Fluidization, and G eldart Groups

Experimental methods in chemical engineering: Reactors—fluidized beds

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