Yang Fluidization, Solids Handling, and Processing

788Fluidization, Solids Handling, and Processing

10.0SPECIAL CASES

There are instances in specific industries, or in unusual applications, which would appear to exhibit major deviations from all the foregoing “best design practice.” This is most frequently associated with unconventional characteristics of the solids being handled. Figure 14 illustrates a particular case in point. This unit has: (a) essentially no barrel beyond the inlet height, (b) a gas discharge tube with half of its perimeter cut away over about half its length (this portion covered with a coarse screen), (c) a relatively high ratio of inlet height to inlet width (80"/18" = 4.44 as compared to the conventional ~2.2–2.7), and (d) a relatively high outlet to inlet area ratio of 3.14 (compared to 0.4 to 1.5).

This unit operates with air under a positive inlet pressure and discharges to ambient. It is fed shredded paper ranging in size from 1/2" × 1/2" to 1/2" × 6". It constitutes essentially a single traverse cyclone, which is sufficient for such large “particles.” The screen on the gas discharge tube allows escape of the air on the inside radius of the first turn and thus “shrinks” the usual natural vortex length to zero. The 24" I.D. cone apex is open to an unsealed collection or baling bin and hence some amount of gas underflow augments discharge, rather than reentrainment or hang-up. The so-called “tangentials” offered for example by American Air Filter Corp. illustrated in Fig. 15 constitutes, in effect, a horizontally oriented version of Fig. 14.

The European patent office recently granted two applications to the A. Ahlstrom Corp. (Hyppanen, et al., 1991) relating to what is described in the text, and in the apparatus and method claims, as a centrifugal separator of “square cross-section.” The patent applications make no mention, nor give any examples, of what separation efficiencies can be anticipated. The suggested advantages lie in the fabrication convenience and economy afforded by the flat, as opposed to conventionally curved, water walls. The efficacy of Ahlstrom’s separator lies in the solids, at high loadings, piling up in the corners so that the effective inside diameter approaches the contours of a cylinder. This follows the principle of using TEEs or mitered elbows in pneumatic conveying lines as introduced over 50 years ago and as employed in the separator developed for their TRC process jointly by Stone & Webster Engineering Corp. (Gartside, 1985) and Gulf Oil (now Chevron). That separator, illustrated in Fig. 16, was essentially a square section box, all dimensional design ratios of which were optimized in a modeling program at PEMM-Corp studying 32

Cyclone Design 791

Figure 17. Curved “ARM” collector.

11.0BED PARTICLE SIZE DISTRIBUTION AND CYCLONE DESIGN

In a fluidized bed reactor, entrained particles leaving in a dilute phase stream are conventionally and desirably either partially or wholly condensed into a bulk stream and returned to the bed via a centrifugally driven cyclone system. At equilibrium, or when steady state operation is attained, any particle loss rate from the cyclones, as well as the remaining bed particle size distribution, are functions of (a) the rate of any particle attrition within the system and (b) the smallest particle size that the cyclone system was designed to completely collect (i.e., with 100% efficiency), or conversely the largest size which the system cannot recover. These two functions result in an interdependency between loss rate and bed particle size distribution, eventually leading to an equilibrium state (Zenz & Smith, 1972; Zenz, 1981; Zenz & Kelleher, 1980).

Case A. To illustrate, consider a situation in which a reactor is charged with a catalyst having a size distribution ranging from 1 to 150

792 Fluidization, Solids Handling, and Processing

microns. Suppose that the cyclone system was designed to retain all particles 10 microns or larger in size and that the particles are so “strong” that there will be no attrition. As time progresses after start-up, the bed will approach a size distribution from 10 to 150 microns. All particles smaller than 10 microns will have been lost, only the fluidizing off gases will leave through the cyclone system. No additional catalyst will need to be charged to make up for the loss from the cyclone system because no further particle loss occurs once the 0 to 10 micron fraction of the charge has left the system. The mean particle size of the equilibrium bed will be larger than that of the original catalyst charge, since the weight percent of all sizes smaller than the largest will have decreased in proportion to the lost weight fraction of the 0 to 10 micron particles in the original charge.

Case B. Suppose, more realistically, that the catalyst undergoes a known, experimentally determined, rate of attrition as a function of particle size (Zenz, 1971; Zenz & Kelleher, 1980). The particle loss rate from the cyclone system will now approach and finally equal the rate of production of 0 to 10 micron particles by attrition from all the larger sizes. To maintain reactor inventory, this loss rate will be replaced, at an equal rate, with fresh catalyst. Since the rate of attrition of any size particle depends on its concentration in the stream subjected to the attrition (as finer particles effectively “cushion” the coarser), and since the loss is replaced with fresh catalyst (containing the coarsest), the bed size distribution will reach a steady state between 10 and 150 microns in which the mean size, as well as all sizes smaller than the largest, will now be decreased from what would have prevailed under conditions of zero attrition.

Case C. Now suppose that in order to maintain conversion in the reactor the equilibrium bed must be further increased in catalytic surface area, or in other words must maintain a greater “fines” content under steady state operation. This would require the cyclone system to now retain all particles smaller than say 5 microns in diameter. Since the existing system design could not retain particles smaller than 10 microns, the cyclone system must be modified to meet the new and more stringent specification. This can be accomplished either by replacing the final collection stage with a larger number of smaller cyclones in parallel, still operating at the same inlet and outlet velocities, or by modifying the existing cyclone system’s final stage to operate at higher inlet and outlet

Cyclone Design 793

velocities. Both changes are designed in principle to reduce the time required for particles to reach and “condense” on the barrel walls. Replacing the final stage with a multiplicity of smaller units is likely the more costly modification, but would advantageously not increase the catalyst attrition rate. The less costly expedient of simply modifying the internal dimensions of the inlets and outlets of the existing cyclones would simultaneously increase the catalyst attrition rate, so that when the new steady state is reached, the bed would have a size distribution from 5 to 150 microns, a mean particle size smaller than in all previous cases, a greater weight fraction of all sizes smaller than the largest, and a loss rate of 0 to 5 micron particles likely greater than the weight rate of loss of 0 to 10 micron particles in case B. This will require more fresh catalyst makeup to maintain reactor inventory. The time required to reach the new steady state equilibrium will be principally a function of the entrainment rate and the reactor catalyst inventory. There are cases illustrated in the literature in which this could take as long as a year (Zenz & Smith, 1972).

The choice of cyclone modification, from an operating point of view, becomes a balance of incremental profit from increased conversion, versus catalyst makeup charges, and from a capital cost point of view, the price of either of the cyclone modifications, which must be depreciated. In many instances, there is an additional background time element, involving ongoing development of more attrition resistant and/or active catalyst.

12.0CENTRIFUGAL VERSUS CENTRIPETAL CUT POINT PARTICLE SIZE

Though the efficiency of a cyclone is relatable to the smallest particle size it will collect with 100% efficiency (or conversely the largest particle that appears in its loss), it must be borne in mind that any such evaluation, or comparison among designs, must be conducted under identical conditions of loading and feed particle size distribution as well as gas density, viscosity and inlet velocity. In operation a cyclone, losing a substantial amount of fine particles when fed a high concentration of fines is not necessarily to be regarded as an inherently poor design. It should principally be evaluated in terms of the largest particle that it loses (i.e., is unable to collect). In a similar situation in which such a cyclone acted as a second stage where its preceding unit recovered a reasonable amount of

794 Fluidization, Solids Handling, and Processing

small particles or where it was preceded with less attrition, the loading to this now second stage would have a lower percentage content of small particles and hence would show a higher total efficiency. This higher efficiency simply reflects a feed containing less losable material; the maximum size lost would still be the same as when fed material with a greater concentration of the unrecoverable particle sizes, where it had then been considered a poorer efficiency cyclone. The differentiation between collection efficiency and maximum unrecoverable or lost size has never been clearly established.

Maximum unrecoverable particle size is overshadowed at high loadings where bypassing to the outlet tube can cause larger particles to be lost though the overall efficiency may be high. Fundamentally the cut point diameter or maximum size lost is observable only at low loadings less than about 50 grains/cu.ft. and reasonably predictable by equating the centrifugal force of the exiting vortex (impeding particle loss) to the net or average radial inflow (centripetal force) to the exiting vortex. The low loading (exiting) cut point diameter differs in its derivation and hence its significance from the higher loading (“inlet catch”) Dth of Figs. 1 and 2. A lack of appreciation between these two distinct, albeit seemingly interrelated, performance criteria is likely one of the causes of the 5-fold variance in the intercepts of experimental fractional efficiency curves, at 100% collection, illustrated in Fig. 2.

13.0 CYCLONE DESIGN EXAMPLES

With consideration to the domestic industrial practitioner, the following illustrative examples utilize conventionally recognizable units as viscosity in centipoise, density in lbs/ft3, velocity in ft/sec, vessel dimensions in inches, and particle diameter in microns.

Example A. Suppose it was desired to design a cyclone to collect all particles larger than 97 microns and release no more than 60 lbs/hr from a 6328 ACFM gas stream bearing 80 grains of solids per cubic foot of gas. The particle size distribution and density of the solids is given in Fig. 18; the gas has a density of 0.1 lbs/ft3 and a viscosity of 0.02 centipoise. The pressure drop across the cyclone is not to exceed 10" H2O. Note from Fig. 1 of the text that 100% collection efficiency of 97 micron particles would require a Dth of 9.7 microns which would be collected at an average 50%