Circulating Fluidization Research at Cornell

Circulating Fluidized Bed (CFB) Research at Cornell

Circulating fluidization is a technology for carrying out gas-solid reactions with high solid throughputs, such as coal combustion or catalytic cracking. Excellent contacting is achieved as solids are entrained in a vertical riser column by a stream of reactive gases at high velocity.

In 1985, we recognized that limited understanding of circulating fluidized beds (CFB) rendered design extrapolations of pilot reactors to full-scale plants both empirical and expensive. In this context, we built at Cornell in 1986 the first cold circulating fluidized bed experimental facility capable of recycling any arbitrary mixture of inert gases at room-temperature (helium, air, CO2, SF6).

In doing so, we matched the dimensionless numbers of our cold facility to those of industrial units operating at higher temperatures. By changing the gas composition and particle size, we could simulate an increase in the size of the industrial unit, thus allowing us to study effects of scale-up on CFB hydrodynamics with a single facility. We also used the dense gas SF6 to simulate pressurized circulating fluidization.

The facility was decommissioned in 2002.

CFB_facility

Results

Our main publications on this subject include:

Chang H. and Louge M.: "Fluid dynamic similarity of circulating fluidized beds," Powder Tech. 70, 259-270 (1992).

In this work, we investigated effects of scale-up on the hydrodynamics of circulating fluidized beds (CFB) using a single cold laboratory facility with the ability to recycle fluidization gas mixtures of adjustable density and viscosity. By matching five dimensionless parameters, experiments employing plastic, glass and steel powders achieved hydrodynamic similarity with high-temperature CFB risers of 0.32, 0.46 and 1m diameter.

Comparisons of results obtained with the plastic and glass powders indicated that static pressure and its fluctuations scale with the riser and particle diameters, respectively. Experiments with the steel powder exhibited incipient choking behavior consistent with the greater analogous bed size that they simulate. The onset of choking with plastic and steel powders was well predicted by the correlation of Yang [Powder Technology 35, 143 (1983)].

Experiments with coated glass beads also showed that the magnitude of the Coulomb friction coefficient affects CFB hydrodynamics in the limit where this coefficient is small.

An excerpt of Chang and Louge (1992) is available here.

Louge M.Y., Bricout V. and Martin-Letellier S.: "On the dynamics of pressurized and atmospheric circulating fluidized bed risers," Chem. Eng. Sci. 54, 1811-1824 (1999).

We used our CFB facility to investigate the effects of gas density, scale and operating conditions on circulating fluidized bed risers. By matching five dimensionless parameters, experiments employing plastic and glass powders fluidized with mixtures of sulfur hexafluoride, carbon dioxide, helium and air near ambient temperature and pressure achieved hydrodynamic similarity with generic high-temperature risers of variable scale operating at pressures of 1 and 8 atm.

We interpreted results in the upper riser using steady, fully-developed momentum balances for the gas and solid phases. The analysis showed that, for a wide range of experiments, two parameters capture the dependence of the pressure gradients upon the ratio of the mean gas and solid mass flow rates. The first is the ratio of the mean particle slip and superficial gas velocities. The second represents the spatial correlation between the radial profiles of interstitial gas velocity and voidage. Variations of the first with dimensionless parameters indicated that the “atmospheric” and “pressurized” experiments conform to distinct viscous and inertial regimes.

An excerpt of Louge, Bricout and Martin-Letellier (1999) is available here.

V. Bricout and M. Louge: “Measurements of cyclone performance under conditions analogous to pressurized circulating fluidization,” Chem. Eng. Sci. 59, 3059-70 (2004).

We evaluate the performance of a relatively efficient cyclone operating under conditions analogous to a generic pressurized circulating fluidized bed at high solid loading.

Elizabeth Griffith and M. Louge: “The Scaling of Cluster Velocity at the Wall of Circulating Fluidized Bed Risers,” Chem. Eng. Sci. 53 (1998).

In this paper, we noted a simple relation for the speed at which particle clusters descend at the wall of circulating fluidized beds, U = 36√gd. The relation holds for a remarkable range of conditions.

An excerpt of Griffith and Louge (1998) is available here.

Elizabeth Griffith and M. Louge: “Convective heat transfer scaling at the wall of circulating fluidized bed risers,” AIChE meeting, Los Angeles (2000).

In this paper, we investigate experimentally the scaling of heat transfer at the wall of circulating fluidized bed using a unique capacitance probe fitted with a small platinum coil held at constant temperature.

V. Bricout and M. Louge: “A verification of Glicksman's reduced scaling under conditions analogous to pressurized circulating fluidization,” Chem. Eng. Sci. 59, 2633-38 (2004).

This paper provides evidence of the validity of Glicksman's simplified scaling laws for circulating fluidized beds.

A conference paper of Bricout and Louge on this subject is available here.

Louge M., Lischer D.J. and Chang H.: “Measurements of Voidage near the Wall of a Circulating Fluidized Bed Riser,” Powder Tech. 62, 269-76 (1990).

This papers reports the first measurements of solid volume fraction at the wall of a circulating fluidized bed.

An excerpt of Louge, Lischer and Chang (1990) is available here.

Louge M. and Chang H.: “Pressure and Voidage Gradients in Vertical Gas-Solid Risers,” Powder Tech. 60, 197-201 (1990).

In this paper, we pointed out that the customary practice of inferring average voidage from measurements of vertical pressure gradients can lead to significant errors at the transition between the dense and the dilute regions of a circulating fluidized bed riser. In this context, we developed a one-dimensional model to account for rapid variations of vertical voidage in these calculations. The model elucidated the discrepancy observed by Arena, et al. between the voidage profiles inferred from pressure gradients and those measured by a quick-closing valve technique.

An excerpt of Louge and Chang (1990) is available here.

Jeff Lischer in front of the Cornell circulating fluidized bed facility (July 2002).

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