FLUIDIZATION

The Fluidized State and Its Early Application

When a gas or a liquid passes upward through a packed bed of granular particles, its motion creates friction against the particles, tending to lift them. This lifting force increases as the velocity of the gas or liquid increases, until, at some velocity, the fluid lifts all particles from contacting their neighbors to move freely, that is, they are “fluidized”. Segregated into a multiplicity of small particles, a solid presents more surface to transfer heat and mass or to react with the surrounding gas or liquid as compared to the original lumpy state. Since the 1930’s, fluidization has therefore become a significant technique in the processing of solid materials. In the earlier years, the Oxford dictionary[1] listed under the word “fluid” the verb “fluidify” meaning to make fluid, but nowadays there is a second verb, “fluidize”, meaning to make like fluid.

In the West, Georgius Agricola[2], a German philosopher-physician-scientist, was credited for the first description of the use of fluidization, in his book De Re Metallica (1556, Latin), to upgrade run-of-mine ores. Song Yingxin[3] 宋应星 (born 1587, and called by the noted sinologist, Joseph Needham, China’s Agricola) also illustrated in his work, Tiengongkaiwu 天工开物applications of fluidization, not only in ore dressing but also in winnowing of grain, as shown in Figure 1.

Figure 1. Early application of fluidization illustrated by Song Yingxin in Tiengongkaiwu天工开物

Left: in Volume 14, 《五金》, dressing of iron ore

Right: in Volume 4, 《粹精》，winnowing of grain

The Winkler[4] pulverized brown coal gasifier invented in the 1920’s to produce synthesis gas (H ₂, CO, CO ₂) was probably the first to use fluidization on an industrial scale, starting from an initial model of 2m dia.-13m h and producing some 2,000 m ³./h of gas, as shown in Figure 2 Up, and improved to 5.5m dia.-23m h processing some 700 t/d of coal, as shown in Figure 2 Down.

Up: Early Winkler gasifier (Amer. Gas Assoc.[7] 1945)

Down: Industrialized gasifier, (Nowacki, p. 204)

Nowacki[5] estimated that from its inception to the 1980’s there were some 63 Winkler gasifiers in 22 plants, distributed in 9 countries. The British Ministry of Fuel[6] and Powder estimated that from the downstream Fischer-Tropsch synthesis of the Winkler gas some 5,000,000 tons of petroleum were produced in 1944. From April to August of that year allied bombardment disabled most of these FT plants to some 120,000 t/y, contributing to victory in WW2.

Through distillation of virgin petroleum only about 20% of its weight could be recovered as gasoline. Although catalytic means to increase gasoline yield was started in the 1920’s, it was as late as 1938 that eight companies joined hands forming Catalytic Research Associates to study catalytic cracking of petroleum by using fluidization, leading to the first of a series of processes, SODI in Baton Rouge[8], as shown in Figure 3.

Figure 3. The first fluidized catalytic cracking.process, model SOD1 (Kunii and Levenspiel, 1969)

Squires[9] in 1982 found the gas velocity in the catalytic reactor to be 0.4m/s, thus confirming it to be a dense fluidized bed. This is a second example of early application of fluidization on an industrial scale.

Early Engineering Studies

The advent of fluidization took place at the time when the unit operations were scoring success, and scholars in chemical engineering began to study the basics of technology. For instance, Professor Wilhelm[10] and a student of his studied fluidization with a gas and with a liquid, leading to the following findings:

• Gas/solid fluidization is highly non-uniform, with much of the gas passing through the solids bed as bubbles, while for liquid/solid fluidization the solid particles are uniformly distributed. They named these two different phenomena, aggregative fluidization and particulate fluidization respectively.
• Solid particles start to fluidize when the velocity of the flowing fluid reaches some unique value at which the gravity force of a particle in the fluid is balanced by the frictional force of the flowing fluid on the particle. With further increase in fluid velocity, the solids bed increases in height to provide more interstitial voidage space for the moving fluid, until a single particle is suspended in an infinite volume of fluid with a fluid velocity corresponding to the free-fall velocity of the particle in that fluid. This phenomenon is expressed by a plot of solids bed pressure drop against fluid velocity on the lefthand side of Figure 4, and generalized to a plot of Archimedes number against Reynolds number on the right, in which the line on the left represents packed bed, the line on the right, particle free fall, and the intermediate lines, different voidages.

• On a log-log plot, for particulate fluidization, fluid velocity u (or its generalized form, Reynolds number) is found to be linear with voidage e, that is, u = e ⁿ , where n is an empirical parameter depending on solid and fluid properties, as shown in Figure 5.

Figure 5 Linear relation between voidage e and velocity u for liquid/solid systems

The last finding was generalized to the case for solids being continuously fed and removed from a fluidized bed at equal rates by simply substituting the relative velocity between the fluid and the solids for u, to provide a wealth of information for a multitude of operations, as shown in Figure 6[11].

Generalized fluidization is useful for fluidized leaching and washing, as shown in Figure 7[11], which operates hydraulically without mechanical parts, and is adaptable to extremely low flowing liquid-to-solid ratios, and occupies minimal floor space.

Bubbling and Bubbleless Fluidization

For gas/solid fluidization, the gas passes through the solids bed in two streams, one carried discontinuously by gas bubbles and the other through the relatively continuous matrix of aggregated solids. Phenomenological revelation of the flow was provided by Rowe’s X-ray [12] investigations, and the related hydrodynamics was analyzed by Davidson and Harrison[13] as shown in Figure 8.

On the basis of the Davidson model, Kunii and Levenspiel[7] constructed the fluid-bed model for the parallel and interactive flow through bubbles and through the dense solids matrix.

To obviate the non-uniform flow in gas/solid fluidization and to lower the unavoidable energy in bubbling flow, numerous efforts were spent in devising alternate means for bubbleless fluidization. An early attempt[11] was the use of dilute raining particles countercurrent to upflowing gas. The dynamics, including the initial accelerative motion of particles while starting to rain downward, and heat transfer between the particles and the upflowing gas, were analyzed, and experiments were conducted up to pilot scale for metallurgical processes[11] as shown in Figure 9.

Reh[14] noted in the 1960’s that Friend provided some data, contradictory to the Wilhelm-Kwauk findings, showing that very fine particles could be fluidized at gas velocities exceeding those for fee fall. From what he called such Abweichung, he found that by feeding solids continuously at the bottom of a vertical vessel while fluidizing with gas at velocities well above that of particle free fall, a new state of fluidization could be achieved, in which, instead of gas aggregating into bubbles, solids form strands floating in a continuum of a surrounding dilute suspension of solid particles. Such state has later been called fast fluidization, which has become the basis of the circulating fluidized bed. Reh devised a number of industrial applications among which the earliest and perhaps the most successful is that of calcining aluminum hydroxide to aluminum oxide, as shown in Figure 10.

Circulating fluidized beds (CFB) were soon accepted in many processes, including catalytic cracking of petroleum which has been modernized from Figure 3 to that of Figure 11[15].

To provide basic knowledge for designing CFB, Y. Li and Kwauk[16] soon devised a method of calculating the Z-shaped longitudinal voidage distribution in fast fluidization by considering the physical mechanism, as shown in Figure 12, employing four parameters determinable from the physical properties of the particles and the fluid.

And, based on the meso-scale entity of clusters, ubiquitous in fast fluidization, J. Li and Kwauk[17] further considered its physical mechanism as shown in Figure 13, in addition to the stability need for minimizing the energy of supporting and transporting the particles, proposed the energy-minimization-multi-scale (EMMS) model for three-dimensional analysis of fast fluidization. The EMMS model, has since its inception, been extended to other multi-phase systems, gas/liquid, liquid/liquid, as well as colloids[18].

The think-tank for bubbleless fluidization has by no means been exhausted. Other alternates already explored includes shallow fluidized beds, levitation and magnetic control.

Particle Behavior and Classification

Particles aggregate to form powder. Fluidization is affected not only by particles individually but collectively as powder. It was as late as 1973 that Geldart[19] classificed particles according to their group behavior in fluidization in terms of particle size and their effective density in a fluid, as shown in Figure 14. Groups B and D: relatively large particles; for B bubbles rise faster than gas velocity, whereas in D bubbles rise slower than gas velocity. Group A: relatively fine particles, with wide size distribution, easy to fluidize. Group C: fine particles, tending to agglomerate during fluidization, often leading to defluidized regions, with cohesive force between particles comparable to their weight.

Tung and Kwauk[20], Tung et al [21] and Qian and Kwauk[22] described a bed collapsing method for assessing the fluidizing performance of a powder in terms of a dimensionless time Q obtained from an automatic instrument they developed, FDAS (Fluidization Data Acquisition System), as shown in Figure 15.

For more details see a recent 1402-page handbook on fluidization edited by the author[23]