US2667706A - Solids-gas contacting - Google Patents

Description

Feb. 2, 1954 R. D. MORSE ET AL SOLIDS-GAS CONTACTING 2 Sheets-Sheet 1 Filed Sept. 9, 1950 INVENTORS EDUA RD E VON WE TTBERG, JR.

and ROLL/N 0. MORSE 4%.

ATTORNEY 1954 R. D. MORSE ET AL 2,667,706

SOLIDS-GAS CONTACTING Filed Sept. 9, 1950 2 Sheets-Sheet 2 95c CLES sEc ND IOO souuo PRESSURE, DECIBELS (REFERRED TO 2 Xl0' DYNES/CM 6 IO 20 3O 4O 5O 6O HElGHT ABOVE SUPPORT SCREEN-INCHES iNVEN T0125 EDI/A PD F. VON WETTEERQJR.

an d ROLLIN D. IMO/73E ATTORNEY Patented Feb. 2, 1954 UNITED STATES T GFFICE 2,667,706 SOLIDS-GAS CONTACTING 'p'any, Wilmington, ware Pa., and Wilmington, DeL, Nemours & Com- Application September 9, 1950, Serial No. 183,998

4 Claims. 1

This invention relates to improvements in solids-gas contacting, and more particularly to the efficient contacting of solids and gases by fluidization.

Fluidized solids-gas contacting has gained favor over the years as a chemical manufacturing operation because of the intimate association of the'process materials and the uniformity of reaction temperature which can thereby be secured. Fluidization is effected by introducing the gaseous material into a mass of particulate solids at such a-rate as to expand the volume occupied by the mass and separate the individual solid particles by gas-filled voids. When fluidized, the solid particles are thoroughly agitated and the mass takes on the appearance of a boiling liquid, and for this reason the fluidized state has sometimes been referred to as pseudo-liquid. In general, fluidization depends upon the velocity of gas flow and the size of the solid particles. In practice,

some materials, which as a class might be designated fluent, are relatively free flowing and fluidize readily over a relatively wide range of gas velocities and particle sizes. Other materials do not fluidize satisfactorily under most conditions, or will fiuidize only within a narrow range of gas velocities and particle sizes, and these materlais are hereinafter termed non-fluent.

The reasons for non-fluency are not fully explainable by present knowledge; however, materials which have a high angle of repose and the particles of which agglomerate readily into balls, will sometimes not fluidize at all, or in any event will-not fluidize satisfactorily. Particle shape is probably also an important consideration, since particles which are flat and plate-like, or needlelike, are not usually as fluent as those which are spherical. There are possibly other unknown factors responsible for non-fluent behavior and no explanation of the phenomenon is therefore ofiered here.

A primary object of this invention is to provide a method for the fluidization of materials which are normally non-fluent.

A further object of this invention is to provide a method for the fiuidization of normally non-- uent materials which is suitable for operation in conjunction with commercially available fluidization equipment.

A further object of this invention is to provide aneconomical method for the fiuidization of materials which are normally non-fluent.

Other objects of this invention will become apparent from the detailed description and the drawings, inwhich:

materials and Figure 1 is a partially diagrammatic sectional view of one embodiment of apparatus constructed according to this invention.

Figure 2 is a plot'of sound pressure versus location for several sound frequencies in apparatus constructed according to Figure 1.

'In general, the objects of this invention are attained by applying sonic vibrational energy 'to the non-fluent solids and flowing gases in contact at such a pressure level as to cause fluidization to take place to the same-degree as would'occur with materials which are normally fluent. In all cases, the decibel pressure levels set forth in this specificationare based on a reference pressure of 2 10- dynes/cmfi. Sonic vibrational energy in the range of about 40 to about 800 cycles per second with transmitted sound pressures above about decibels have been found to be entirely satisfactory, although it Will be understood that these requirements may vary with different'solid also with the configuration of the contacting vessel. The sonic vibrational energy is preferably introduced directly into the reactants with the interposition of a minimum of barrier material therebetween. Appreciable advantage is secured, howeveneven if the reactor walls or other-equipment elements are maintained in vibration within the "sonic frequency range.

Referring to Figure 1, the fluidization reactor may comprise a cylindricalenclosure I0,-only the lower end of which is shown, for the reason that the remaining structure is conventional in the art. The lower end of the reactor is closed ofi by a gas-permeable, solids-impermeable screen II, which permits the substantially free passage of fiuidizing gas therethrough but prevents the passage of solid material into the gas chest l2. Screen H supports a bed of solid particles indicated-at 13, and enclosure I9 is fixedly supported on chest l2 by the common collar piece M, which may be attached to both elements by set screws, spot welding or in other well-known manner. Fluidizing gas is supplied to chest [2 by conduit 15 at suflicient static pressure so that iiuidizing velocity is attained by the gas at all'points in its transit through solids bed I3. Other suitable conduitsfor gas and solids withdrawal from enclosure 10 may be provided but are not shown in Figure 1, because they form no part of this invention.

Chest 1 2 opens into the inner confines 0i housing IS in which is mounted the sound energy source it and sound directing cone 20, which in the drawing is shown in longitudinal section. Chest I2 may be supported fromv cover l6 by atmaterial loaded into tachment of its lower flange 22 thereto in any suitable manner, and bolts l1 secure the cover to the upper flange of housing 18. The lower end of housing I8 is closed off by a plate 23 which may be made removable, if preferred. An open conduit 24 is preferably connected between chest 12 and the inside of housing H! to equalize the static pressures existing at these points. Sound source I9 is supported in axial alignment with the bore of enclosure ill by clamping its support arms 2| between cover l6 and the upper flange of housing 18. A flexible peripheral diaphragm 30 joined along the inner edge to the open end of cone 20, with its outer edge clamped between cover l6 and the upper flange of housing 18, is preferably employed to prevent thedissipation of sonic energy from source [9 as turbulence. A resilient annular gasket 25 may be employed to seal housing 18 against leakage to the atmosphere and a similar gasket (not shown) may be provided for plate 23. Electric power conductors 26 connect sound source :9 with power sources 23 and 29 through leakage-preventing plugs 21. When material of fine particle size is being processed, it may be preferable to protect sound source 19 against any contact therewith by interposing a vibratable, sound-transmitting diaphragm betweenthe entrance to chest l2 and housing l8, in which case a rubber or flexible metal sheet (not shown) may be sealed across this opening by joinder with flange 22. v

In a laboratory size column constructed according to Figure 1, enclosure was a clear glass tube 3% inches in diameter, 60 inches long. Solid the tube was supported by a thin, finely-woven cloth screen which was stiffened by sandwiching it between two pieces of wire screen of about 10 mesh. Sound source IS, incorporating cone 20 and support arms 2|, was provided by a Western Electric Co. Model 728-13, 12-inch loud speaker with a power rating of watts. This speaker was powered from an audio amplifier 28 with a power rating of watts, which was in turn powered by an audio signal generator 29 with adjustable frequency output overthe range of 40 to 20,000 cycles per second. With this apparatus, it was possible to secure measured sound pressures of up to 155 decibels in the empty tube and somewhat lower levels when fiuidization was being conducted, due to absorption of some of the sound energy by the process materials. Figure 2 is a plot of the sound pressures existing at various levels in the 3%-inch x -inch column hereinabove described while empty. 7 As the periodic nature of the plotted curves indicate, standing waves were set up in the tube upon the transmission of sound therethrough, and it was possible to tune signal generator 29 to a frequency utilizing this effect to the maximum advantage in power economy.

The laboratory size column hereinabove described was employed for the fluidization of plaster of Paris with air, the average particle size being about 15,11 but with a substantial proportion of 4,1 material. The shape of the plaster of Paris particles, as determined by microscopic examination, was flat and plate-like. The bulk density was 61.4 lbs/ft the specific gravity 2.66, the percentage voids after compacting was 63%, and the material compressed readily when force was applied to it. The angle of repose of this material was between about 30 and 85, as measured by confining a portion within a glass bottle which was slowly tipped at an increasing angle bottom static with the horizontal. When some of the material was poured into a heap on a horizontal plate, the edges of the pile were inclined at about the same to angle.

When the column was loaded with plaster of Paris to a height of between about 10 inches and 30 inches, and fluidizing air was supplied at a superficial velocity of 0.43 ft./sec., the air passing through the bed appeared to lift the solid particles and pack them along the sides, until stable channels of sufiicient diameter opened to permit the air to escape without suspending the solids. The air flow was thus confined to these channels and no fluidization of the solid material occurred, the bed remaining completely static except for the erratic agitation of a very small proportion of the particles aroundthe peripheries of the channels.

When sonic energy of sufiicient pressure was applied through sound source [9, the static condition of the solids bed was immediately altered, material adjacent to the channels moving laterally to close off these passages and the entire bed being expanded. During the brief expansion period, it was observed that the air formed a multitude of small pockets which appeared to be rather uniformly distributed throughout the bed. These pockets moved upwardly, collapsing the solid material around their walls and exploding into small geysers on reaching the top of the bed. Within a very short time, the rise of air pockets could no longer be observed and the surface geysers were replaced by a surging, boiling interface characteristic of the true fluidized state.

Measurements of sound pressure above the solids bed confirmed the fact that a minimum sonic energy pressure was necessary before stable fluidization took place. Thus, with a 14/z-inch unfluidized plaster of Paris bed, a sound pressure of 10 decibels at a frequency of 130 cycles/ second was reached at a height of 30 inches above support screen ll before the transitional geysering stage of movement was observed. When the sound pressure was increased to slightly ,over 115 decibels at the same frequency, the material became fully fluidized with a marked expansion in volume. Expansion to a height of 21 inches and an increase in volume of about 31% was completed in about one minute, after which a sound pressure measurement disclosed that the sound pressure (with constant electrical energy input) had dropped to about decibels at the 30-inch height. This decrease in energy transmitted to the 30-inch level was probably due in part to the absorption of an appreciable amount by the expanded fluidized bed. It appears also that there may have been a shift of the resonant frequencies of the apparatus resulting from the expansion, with an accompanying shift of energy level similar to the periodicity illustrated in Figure 2. A further increase of sound input to a measured value of decibels at the 30-inch level and 130 cycles/second frequency resulted in increased expansion of the bed to about 45%, the increase occurring within about 5 seconds time with the quality of fiuidization remaining excellent. When the input of electrical energy was decreased to that value corresponding to decibels sound pressure in the empty tube, the solids immediately commenced to re-stagnate, returning to an essentially static state within one or two minutes, although at higher air velocities more time Was required for complete settlement. Stagnation commenced with the formation of a layer pierced by one or more open channels through which the air escaped. These escape channels filled progressively with balls of solid material which withdrew the balance of the solids from fluidization, until th entire bed resumed the static state with the air blowing through somewhat smaller open passages traversing the bed from top to bottom.

The effects resulting from vibrating the column per se were studied with the plaster of Paris-air system employing a Syntron vibrator attached to the '3%-inch tube at a height of about inches, the solids depth in the unfluidized state measuring12 inches. The sound source operated transversely at a frequency of about 60 vibrations per second. This vibration also caused collapse of the stagnant solids bed with accompanying expansion and fluidization, although the degree of fluidization appeared to be inferior to the case where the sonic energy was applied directly to the entering air stream.

A second series of tests was made with a commercial grade of marble dust commonly known as Georgia White. This material was an extremely fine, non-gritty, pigment-grade ground limestone which was highly non-fluent and so lacking in flowing properties that it Was necessary to charge the 3%-inch tube by scooping and rodding the material in. An attempt was made to fiuidize an l-inch settled bed by passing an air stream therethrough at a superficial velocity of 0.4 ft./sec., but the bed merely lifted and broke to open a number of free escape channels. When sonic energy was applied to the air stream through a loud-speaker l9 at frequencies of 40 to 450 c. p. s. and sound pressures in the neighborhood of 110 to 112 decibels measured at the bed surface, good fiuidization rapidly ensued.

Other experiments with non-fluent, high angle of repose materials having particle sizes in the range of from about 2 [L to about 30 ,u, and particle shapes ranging from spheres and plates to relatively long needles gave similar results to those described for plaster of Paris and marble dust. In some instances a very high uniformity of fiuidization resulted which was seldom achieved even with highly fluent materials, but in all cases at least satisfactory fiuidization resulted from the introduction of suitable amounts of sonic energy.

Since the beneficial efiects of sonic energy are secured only when certain minimum sound pressures are maintained, in some cases it may be preferable to provide the reaction vessel with a number of individual sound sources located at such points that the output of one reinforces or supplements the outputs of others. The most effective arrangement pattern and the total en-' ergy requirement for particular installations will depend upon the equipment dimensions and the specific process materials involved. A variety of sound sources are commercially available, including gas whistles, sirens and vibrating diaphragm units, the latter being particularly preferred because they do not require gas flows for operation and thus do not complicate control of the fluidizing s stream.

It will be understood furthermore that the introduction of sonic energy is not limited to the point of entry of the fluidizing gas but may be effected at one or more points within the body of the solids mass if desired.

From the foregoing detailed disclosures it will be apparent that this invention is capable of wide modification without departing from th spirit of the invention, and it is not our intention to be limited in the scope thereof except as specifically indicated by the appended patent claims.

W e claim:

1. In the iiuidizing of solids by a stream of gas flowing therethrough, the steps of improving the fluency and reducing the agglomeration and angle of repose of particles which comprises applying sonic vibrations through the gas medium to the solids within the audible range at a sound pressure level greater than about 100 decibels to produce collapse of the solids bed, and continuing the sonic vibrations to create and maintain uniform flow within the mass of said solids.

2. The method of fluidizing a bed of normally nonfiuent particulate solid materials in a stream of gas flowing therethrough which comprises applying sonic vibrations to the solids through the medium of the gas at a frequency varying from about 40 to about 800 cycles per second, with sound pressures of said vibrations within the solids bed at a level of above about 100 decibels to produce collapse of the solids bed and create substantially uniform fluidization within the mass of said solids.

3. In the method of fluidizing a bed of normally nonfluent particulate solids susceptible to agglomeration and channelling, the steps which comprise subjecting the solids to sonic vibrations through a gas medium at a frequency below about 900 cycles per second, with sound pressures within the solids bed at a level of greater than about to decibels to produce substantially uniform fluidization within the mass of said solids substantially free of agglomeration and channelling of the fluidized bed of particulate material.

4. The method of claim vibrations are applied at 40 to about 800 cycles.

ROLLIN D. MORSE. EDUARD F. von WETTBERG, JR.

References Cited in the file Of this patent UNITED STATES PATENTS 3 in which the sonic a frequency varying from OTHER REFERENCES High Intensity Sound Waves, R. W. Porter. Chemical Engineering, March 1948, pages 100, 101, and 115.

Claims (1)

1. IN THE FLUIDIZING OF SOLIDS BY A STREAM OF GAS FLOWING THERETHROUGH, THE STEPS OF IMPROVING THE FLUENCY AND REDUCING THE AGGLOMERATION AND ANGLE OF REPOSE OF PARTICLES WHICH COMPRISES APPLYING SONIC VIBRATIONS THROUGH THE GAS MEDIUM TO THE SOLIDS WITHIN THE AUDIBLE RANGE AT A SOUND PRESSURE LEVEL GREATER THAN ABOUT 100 DECIBELS TO PRODUCE COLLAPSE OF THE SOLIDS BED, AND CONTINU-

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