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Chemical process opportunities for vibrated powders 3. Provisional design for an experimental sidewards flying bed
Powder Technology 175 (2007) 55 – 62 www.elsevier.com/locate/powtec
Chemical process opportunities for vibrated powders 3. Provisional design for an experimental sidewards f lying bed A.M. Squires ⁎ Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg VA 24061-0211, USA Received 24 October 2004; received in revised form 1 June 2006; accepted 1 June 2006 Available online 2 February 2007
Abstract At a scale suitable for laboratory experimentation, a provisional mechanical design illustrates a concept for a “sidewards flying bed” (SFB), a layer of particulate matter held upon the inner wall of a cylinder that rotates at varying speed. Acting upon the layer are centrifugal and tangential forces whose patterns of variation create time intervals during which the latter is the greater. During such intervals, the layer is postulated to respond by “flying sidewards.” In a laboratory SFB set-up, experiments can elucidate a new area of non-steady-state soil mechanics, with outcomes bearing upon a full-scale SFB's chemical processing potential. An important question is, what will be the delay in the SFB layer's taking flight after the moment tangential force just begins to exceed centrifugal? For the SFB design to be successful, this delay should be significantly shorter than a time interval during which the tangential force dominates. If experiments were to show this not to be so, a “what-then?” might be to install several stationary rakes near the cylinder's inner wall, parallel to its axis. Loosening the particulate layer as the cylinder turns, these should advance the timing of the layer's flight. Achieving adequate flight in an experimental SFB could point to a design at a commercial scale capable of promoting rapid local mixing — more rapid than that afforded by conventional vibrated or fluid beds. Such should become a strong candidate for applications requiring rapid heating of hydrocarbonaceous matter followed by quick separation of vapor product and carbonaceous residue (e.g., flash pyrolysis of coal or oilshale). The SFB's competencies should include ability to treat moieties of especially large size (e.g., for producing a smokeless solid fuel from coal, for heat-treating a metal part or a polymer, for flash-cooking foodstuffs without oil, for drying large particulates); ability to treat highly heterogeneous matter (e.g., for burning or gasifying comminuted biomass or solid wastes); and ability to promote excellent heat transfer between SFB and stationary surfaces embedded therein (e.g., in an air-cooled SFB steam-condenser). © 2007 Elsevier B.V. All rights reserved. Keywords: Sidewards flying bed; Rotating vibrated bed; Non-steady-state soil mechanics; Flash coal pyrolysis; Flash shale oil distillation; High-pressure flash hydrogenation of carbonaceous matter; Smokeless solid fuel; Polymeric monomer recovery; Polymer curing; Steel heat treatment; Flash-cooking food; Biomass combustion; Biomass gasification; Drying; Air-cooled condenser; Refrigerated storage of unstable oils
1. Introduction This is the third paper dealing with chemical process opportunities for vibrated powders. The first two papers [1,2] discussed laboratory and field applications, respectively, of the conventional vibrated bed, in which a substantially horizontal floor, subjected to substantially vertical cyclic displacement from a null position, supports a layer of powder. This third paper
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treats a new device for processing particulate matter, the “sidewards flying bed” (SFB), a shallow powder layer held by variable centrifugal force against a rotating cylinder's inside wall. Tangential force acting upon the layer also varies, becoming greater than the centrifugal force during certain time intervals. In such intervals, it is postulated, the layer takes “sidewards flight.” Exploration of the postulate requires experimental study of relevant non-steady-state soil mechanics. In principle, the SFB's rotating cylinder may be either upright or horizontal. An upright design will often be preferred: its footprint is smaller; and its orientation, usually, better facilitates employment of gravity-aided means for introducing
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Fig. 1. Provisional design for experimental “sidewards flying bed” (SFB): vertical cross-section at zero time. A motor rotates a left-hand eccentric arm in “forward” direction (toward a viewer of the drawing); a right-hand arm rotates “backward.” Transfer of momentum from eccentrics to support rod causes it to turn horizontally: first, clockwise (viewed from above); later, counter-clockwise. At the bottom of the rod, a bearing holds the rod in place against lifting. (Not shown are spring means needed to compensate for change in the torsion bar’s length as it twists.)
Transfer of momentum from eccentrics to rod causes the latter to turn horizontally in a back-and-forth motion. Sharing this motion are the motor, the two eccentrics with their horizontal shaft, and the upright cylinder. The latter's motion creates an intermittent centrifugal force pressing a layer of particulate matter against its inner wall. A suggested depth for the layer is 30 mm; a body of data is available [3–8] for conventional vibrated beds at this depth over a wide range in particle size. Three vertical posts (as seen in Fig. 2's section A–A drawing) carry wheels that stabilize the cylinder against horizontal deflection. The Fig. 1 drawing is for zero time. The section B–B drawing in Fig. 2 depicts the left-hand eccentric in Fig. 1 at zero time (as viewed from the left in the Fig. 1 drawing). A dot-dash circle depicts the travel of the weight at the end of the eccentric; the weight moves in clockwise direction through angle θ in a time interval set by the motor's speed. The second eccentric rotates counterclockwise (as viewed from the right in Fig. 1). At zero time, the eccentric of section B–B, as seen in Fig. 1, travels outward from the plane of the paper, toward the viewer. Transfer of momentum from eccentric to SFB cylinder causes the latter to turn inward from the plane of the paper, away from the viewer; i.e., the cylinder's rotation is clockwise as seen from above (see Fig. 2's section A–A). At zero time, as seen in Fig. 1, the right-hand eccentric travels away from the viewer; a point above it on the cylinder travels toward the viewer; i.e., again, the cylinder is seen to turn clockwise when viewed from above. When the two eccentrics rotate through the angle θ, momentum transfer causes the SFB cylinder to rotate through clockwise angle γ.
and withdrawing particulate matter. The SFB appears to have potential for rapid powder mixing as well as unusually high granular-matter processing capacity relative either to equipment volume or, especially, to equipment footprint. From standpoints of reliability and achievable scale, relative to the conventional vibrated bed, the SFB appears to share an advantage often enjoyed by rotary equipment (e.g., a turbine) over a competing design that employs reciprocating linear motion (e.g., a piston engine). 2. A provisional design for an experimental sidewards flying bed (SFB) 2.1. Mechanical design Fig. 1 is a vertical cross-sectional drawing of a provisional design for an upright SFB suitable for experiments providing an eyeball view of SFB behavior from above. In cooperation with a torsion bar (fixed in place at its top), a vertical rod (free to rotate) supports a horizontal electric motor and an upright cylinder. At constant speed, the motor rotates a shaft carrying two eccentric arms, each with a weight near its extremity. As seen in Fig. 1, the two arms are vertical, one up and one down.
Fig. 2. Provisional SFB design: horizontal section A–A; an alternative section A–A; and vertical section B–B. In section A–A, arrows within the SFB indicate the initial, counter-clockwise direction of flight, putatively commencing shortly after ~ 7 ms (see Fig. 5). In section B–B, a dot–dash circle indicates path of weight at end of the rotating eccentric.
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An opposite situation develops when right-hand eccentric reaches the top of its travel; the left-hand eccentric now points downward. In this configuration of the eccentrics, the SFB cylinder turns counterclockwise (viewed from above). As the cylinder turns, both a tangential force and a centrifugal force act upon the particulate layer. An SFB design should ensure existence of time intervals during which the former exceeds the latter. During at least a latter portion of such a time interval, the layer is postulated to “fly sidewards”; whether such occurs depends upon applicable unsteady-state soil mechanics (to be discussed in Section 2.3). When a layer takes flight, lateral particle velocity perhaps exhibits a gradient from a lower value near the wall to a higher value at the particulate layer's free surface. Such a gradient, if present, dilates the particle layer, drawing in a moiety of gas from the central region of the cylinder. Compaction of the layer, when centrifugal force again exceeds tangential, returns the imbibed gas to the central region. In some SFB applications, a designer may wish a more definite, more pronounced traffic of gas across the layer, reflecting periodic development and destruction of a gap next to the cylinder's wall, analogous to gap-formation in a conventional vibrated bed [8]. To accomplish this goal, the designer may specify a cylinder with vertical serrations, as seen in Fig. 2's alternative section A–A drawing. Here a tangential acceleration may be viewed as possessing two components facing one another at a right angle. One component is perpendicular to the plane of a serration facing in the direction of the acceleration. When this component is greater than a competing centrifugal-acceleration component pressing down upon this plane (and if particle size is not too small), a gap should form between particulate layer and plane, promoting flow of gas from the cylinder's central region, across the layer, and into the gap. Later, gap destruction returns gap gas to the central region. Except for direction of particle flight, particle action in the SFB with a serrated wall should resemble that of powder in a conventional vibrated bed. For example, recall that in a conventional bed of a Geldart Group B or D powder [9] operating at 25 Hz, up-and-down vibration of a floor causes particles resting thereupon to leap upward, en masse, roughly every 40 milliseconds (ms). At each upward leap, there occurs an interval of vertical free flight (like water from a fountain) lasting, typically, some 20-odd ms [8]. After its flight, the powder mass returns to the floor and experiences compaction. During the free flight (if the floor is non-porous), the leaping powder creates a negative pressure at the floor, drawing “supernascent” gas downward across the bed; at the end of flight, the floor pressure spikes upward, expelling the imbibed gas from the bed. For Group B or D powders, similar effects should be seen in the SFB with serrations. As in the design of a conventional vibrated bed, “tuning” the natural frequency of the torsion bar to the SFB's frequency can reduce the power required to drive the electric motor seen in Fig. 1. In Figs. 1, 2, the SFB cylinder's outer wall is non-porous. The wall may be porous, allowing introduction of a treating gas
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into the SFB from a surrounding plenum (e.g., a hot gas for drying wet particulate matter). See Sections 5–7 and 11 for examples requiring such an arrangement. In an SFB with serrations, however, for a process requiring only a small treating-gas flow, notice that such gas need not be fed from an outer plenum but can be introduced centrally, rapid exchange of gas between SFB and central space being adequate for the desired treatment of gas or particles. 2.2. Calculating accelerations For a provisional design, take motor speed in Fig. 1 to be 600 RPM. Then hðtÞ ¼ 2pft
ð1Þ
where t = time (s) and f = frequency = 10 Hz. dh=dt ¼ 2pf
ð2Þ
Take SFB cylinder height to be 60 cm and other design parameters to be: R1 R2 R3
M1
Radius of eccentric = 17 cm; Radius of SFB cylinder = 15 cm; Horizontal turning radius of an eccentric (length of horizontal shaft, from center of vertical rod to plane of eccentric) = 18 cm. “Effective” combined weight carried by the two eccentrics (each weight as if concentrated at an eccentric's radius, R1) = 50 kg;
Fig. 3. Provisional SFB design: instantaneous SFB speed of rotation.
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acceleration exceeds centrifugal between ∼7 and ∼43 ms and between ∼57 and ∼93 ms. Putatively, each of these time intervals includes a smaller interval in which the SFB particulate layer takes flight. Notice that direction of flight is different in the two intervals: viewed from above, counterclockwise in the first interval; clockwise in the second — in each case, opposite to the SFB's rotation. During an interval b1 ms, centrifugal acceleration is smaller than earth's gravity. During this brief interval, under the latter's influence, downward movement of the SFB particulate layer, in general, should be through a distance not greater than a few particle diameters. 2.3. New soil mechanics Fig. 4. Provisional SFB design: deflection of SFB from null position.
M2
“Effective” weight of SFB cylinder plus particulate layer (as if concentrated at cylinder's radius, R2) = 25 kg;
Let Ω⁎ = M1R1R3 / R2[M2R2 + M1(R32 + R12sin2θ)0.5] = Ω (1 + λ) / [1 +λ(1 + β2sin2θ)0.5] where Ω= M1R1R3 / R2[M2R2 + M1R3] = 50 × 17 × 18 / 15[25 × 15 + 50 × 18] = 0.8; λ = M1R3 / M2R2 = 50 × 18 / 25 × 15 = 2.4; and β2 = (R1/R3)2 = (17/18)2 = 0.892. In the first quadrant (θ = 0 to π/4), instantaneous rotational speed of the SFB is given by momentum transfer from rotating eccentrics to SFB cylinder: dg=dt ¼ ðdg=dhÞðdh=dtÞ ¼ ðX⁎coshÞð2pf Þ ¼ 2pf Xcoshð1 þ kÞ=½1 þ kð1 þ b2 sin2 hÞ0:5
To be answered through experiment is the question, before an SFB particulate layer takes flight, what time will pass following the crossover in Fig. 5 at ∼ 7 ms — i.e., following the moment at which tangential acceleration overtakes centrifugal? In a conventional vibrated bed at 25 Hz, a corresponding time interval (a delay from the moment at which the bed's vertically upward acceleration overtakes gravity) is, in general, relatively short in comparison with flight time [8]. Although this provides a degree of reassurance that delay in SFB flight will take up only a relatively small fraction of the ∼ 36 ms “available” for flight, seen in Fig. 5, such reassurance is only a warrant for performing experiments to determine what this fraction may be. Studies of the panel bed granular filter with puffback renewal of gas-entry faces also provides a degree of reassurance [10]. A puffback imposes action of a strong “puff” of gas in a direction toward these faces, this action creating body-motion of the
ð3Þ
Centrifugal acceleration acting on SFB (in g's): R2 ðdg=dtÞ2 =g ¼ ðR2 =gÞð2pf X⁎coshÞ2
ð4Þ
where g = acceleration of gravity = 980.7 cm/s2. Tangential acceleration acting on SFB powder is (in g's): R2 ðd2 g=dt 2 Þ=g ¼ −ðR2 =gÞfð2pf Þ2 Xð1 þ kÞ ½1 þ kð1 þ b2 sin2 hÞ0:5 g fsinh þ kb2 sinhcos2 h ½1 þ kð1 þ b2 sin2 hÞ0:5 ð1 þ b2 sin2 hÞ0:5 g ð5Þ Quadrant symmetries give SFB speed and accelerations at θ ≥ π/4, Fig. 3 displays the SFB cylinder's instantaneous speed of rotation, calculated from Eq. (3). The speed cycles between plus and minus 480 RPM. Fig. 4 plots angle γ, the SFB cylinder's deflection from its null position. The deflection cycles between about +43° and −43°. Fig. 5 plots centrifugal and tangential accelerations, from Eqs. (4) and (5), respectively. In a given 100-ms cycle, tangential
Fig. 5. Provisional SFB design: dashed curve = tangential acceleration; continuous curve = centrifugal acceleration. Sidewards flight is postulated to occur during time intervals in which tangential acceleration exceeds centrifugal. Arrows indicate directions of the putative flight: an arrow pointing to the left indicates counterclockwise flight (viewed from above); arrow to the right, clockwise.
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panel bed's granular material toward the faces within a timedelay on the order of 10 ms. Granular-matter “soil failure” in a panel bed under puffback comports with earlier knowledge of soil failure under sudden imposition of stress [11,12] (a knowledge acquired in response to desire to understand how a structure fails under impact of an atmospheric shock wave from a nuclear blast). When stress is imposed “instantaneously,” soil failure occurs everywhere, all at once. In contrast, in the more familiar situation, when stress slowly increases (or after it persists for a time at an unstably high level), soil tends to fail at a weakest point, individual particles having had time to shift their positions slightly, or to turn a bit, before the failure. A sharp, sudden stress leaves no time for particle rearrangement, and the mass of particles gives way everywhere at once. Acceleration curves in Fig. 5 suggest that a body-failure will occur, SFB particles taking flight everywhere at once, at a yetunknown time interval following the 7-ms and 57-ms crossovers. Finding this interval may interest students of soil mechanics. Prospects for the SFB's usefulness in chemical processing depend upon outcomes from this line of study: the shorter the time the better, although a delay in flight might be tolerated, perhaps, up to as much as ∼15 ms.
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possible, close positioning of their feeds on an SFB circumference can ensure intense local mixing. 3.2. Withdrawing granular material Seen in Fig. 6 is an arrangement for withdrawing pulverulent matter at the SFB's top. The material flows over the lip of a “weir” that establishes the SFB's surface level. Centrifugal force carries the withdrawn material outward, flinging it against the inner wall of a stationary vertical cylinder. Thereupon, the material forms a relatively concentrated “sheet,” which falls into a circular, conventional vibrated bed. This conveys the material circumferentially toward one or more down-comers (not seen in Fig. 6) for its removal and conveyance toward either disposal or further processing. In Fig. 6, at the SFB's bottom, material flows across a second weir establishing SFB surface level. As at the top, centrifugal force flings the material outward into a sheet that falls into a conventional vibrated bed carrying the material to one or more down-comers for final take-off. 3.3. Countercurrent heat exchange between gravel and powder Fig. 6 illustrates an arrangement whereby a designer may reasonably hope to achieve countercurrent heat transfer between
2.4. A “what-then?” if new soil mechanics data were to disappoint Suppose experiments were to show the delay to be much longer than 15 ms. A response might be to install 4 (or 5) stationary rakes near the SFB cylinder's inner wall, parallel to the cylinder's axis (the rakes not participating in the cylinder's rotation). Installed at 90° (or 72°) intervals, the rakes should permanently loosen the SFB particle layer. It should take flight, “instantaneously,” when tangential acceleration overtakes the centrifugal. 3. Feeding and discharging particulate materials 3.1. Downcomer feeds Fig. 6 illustrates provisional arrangements for feeding and withdrawing particulate matter from an SFB. A stationary cylinder, with top and bottom closures, surrounds the rotating SFB cylinder. Penetrating a top closure of the stationary cylinder are two stationary down-comers, one of which feeds material near the top of the SFB, the second, near the bottom. (A horizontal screen seen below the bottom feed is optional; Section 3.3 will discuss its use.) Preferably, a down-comer's cross-section should be sufficiently large such that, at a desired rate of feed, material drops through it in freefall. At its bottom, a short section at 45° to horizontal deflects the material laterally at a relatively high velocity toward a nearby point on the SRB's surface. If desired, a number of narrowly-set-apart down-comers may be arranged in a circle to provide a number of feeds at a given SFB elevation. If two materials must be mixed as rapidly as
Fig. 6. Vertical cross-section illustrating provisional arrangements for feeds and withdrawals of particulate material. Centrifugal forces fling withdrawn material into a circular vibrating trough, which conveys material circumferentially to a take-off down-comer (not shown in the drawing). (Not shown are means for vibrating the two circular troughs.)
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a “gravel” (Geldart Group B or D) and a Group A powder. Down-comers feed gravel close to the bottom of an upright SFB. Situated at an elevation just beneath the entry of gravel, a screen prevents its movement downward. A gradient in the SFB's surface, from screen to SFB top, sustains gravel upward motion. Subjected to an SFB's back-and-forth motion, a Group A powder can be expected to enter a liquid-like, “low-viscosity” state [8], in which gravity can be expected to pull the powder downward. Fed at a “sufficient” distance below a top outlet for gravel, a Group A powder should discharge primarily at the SFB bottom, below the screen. 3.4. An SFB lift for a “gravel” In Fig. 6, an SFB acts to lift gravel fed near its bottom. The arrangement does not, however, provide a practical means for lifting a gravel, since this entered the SFB at the top, via downcomers. What is needed to provide an SFB lift is an arrangement for feeding gravel at the bottom. Provisionally, Fig. 7 offers such an arrangement, a feature of which is a “mass lift” of the gravel from a conventional vibrated bed onto a rotating horizontal plate, the bottom closure of the rotating SFB. A gas-tight enclosure houses this conventional vibrated bed. To create the mass lift, a relatively small flow of gas creates a small overpressure within the enclosure, sustaining an upward gas flow
through a column of gravel originating from within the vibrated bed. The upward flow of gas should be sufficient to cause the gravel column to move upward in the gravel's dense, “settled” condition. Following the gravel's discharge onto the rotating plate, centrifugal forces carry it laterally onto the SFB surface. A relatively small gradient in this surface, declining toward the top, lifts the gravel toward removal (as seen in Fig. 6). 4. An SFB for flash pyrolysis of coal or shale-oil rock An SFB is a candidate device for flash pyrolysis of hydrocarbonaceous matter, such as coal or shale-oil rock [2]. This matter and a hot powder would be fed side-by-side via pairs of down-comers. A number of such pairs should be provided, several sets of pairs terminating at several elevations within a vertical SFB. For treating coal, a circulating-fluid-bed boiler in base-load service and burning char residue of pyrolysis can supply a steady flow of hot powder [2]. For treating shaleoil rock, a countercurrent heat exchange along the lines of Fig. 6, as described in Section 3.3, can be useful [2]. Condensation of vapor product can be effected in a central, stationary, vertical, perforated duct housing, near its top, either sprays of cooled, recycled pyrolysis liquid or a horizontal, perforated vibrating tray feeding refrigerated particulates (such as “MasterBeads” [2,8]) onto which vapors can condense. In an application in which a gas turbine will burn oil from pyrolysis to provide intermittent, peak-load electricity [2], low-temperature storage of particulates and oil condensate may protect the oil from reactions that unduly increase its molecular weight, rendering it difficult to pump at near-ambient temperature. 5. Flash hydrogenation of coal at high pressure An SFB is also a candidate reactor for high-pressure flash hydrogenation of coal or other hydrocarbonaceous matter [13– 15], an exothermic process capable of yielding (from coal) methane, ethane, benzene, toluene, and xylenes (with negligible yields of heavier hydrocarbons) together with a char residue. 6. A combustor for a small, biomass-fueled gas turbine
Fig. 7. Provisional design for feeding a “gravel” (a Geldart Group B or D powder [9]) to the bottom of an SFB lift. Gravel feed reaches a vibrating trough from a down-comer. The trough carries gravel laterally toward an up-comer. The trough, with its lid, forms a gas-tight enclosure. A relatively small supply of gas, elevating the enclosure’s pressure, sustains mass lift of gravel via the up-comer onto a horizontal “floor” at the bottom of the SFB cylinder, where centrifugal forces carry the gravel laterally into the bottom of the SFB lift. At the top of the lift, gravel withdrawal is by the arrangement seen in Fig. 6. (Not shown are {1} required flexible jointures connecting trough lid with down-and up-comers, {2} means for vibrating the trough enclosure, and {3} slat-works connecting SFB cylinder and support rod.)
An SFB is a candidate for service as a combustor burning biomass and supplying hot gas at ∼3 to ∼4 atmospheres to the expander of a “tiny” gas turbine. Turbines delivering as little as 40 kW are now available [16]. These operate at high speeds (reaching 90,000 RPM in some designs). Their development followed appearance of compact, inexpensive, solid-state devices capable of converting the ultra-high-frequency electricity generated at a high speed to conventional 50- or 60-cycle current. Thousands of small turbines now provide distributed power sources in large utility systems (saving transmission losses and postponing need to outlay capital for new transmission lines). Small turbines are also meeting small needs for power at locations where electricity does not reach. An ability to use biomass fuel could expand the usefulness of these small gas turbines. In an SFB combustor for biomass, the SFB advantageously comprises a “permanent” bed of an attrition-resistant Geldart
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Group B or D granular solid (such as the aforementioned “MasterBeads”). 7. An air-cooled condenser Embedding stationary heat-transfer surface within an SFB may give rise to attractive applications of the device. It is a practical certainty that coefficients for transfer of heat to or from such surface would be larger than those ordinarily seen in a conventional vibrated bed, since coefficients for conventionally vibrated powders increase when stationary surfaces therein experience a lateral flow [6]. Recall, too, that conventional vibrated beds afford heat-transfer coefficients comparable to those seen in fluid beds — coefficients on the order of 10 to 20 times larger than coefficients for transfer from air to a metal surface in absence of particulate matter. An air-cooled steam condenser might exploit heat transfer afforded by an SFB embedding stationary finned tubes; particulate matter should be an attrition-resistant granular solid (e.g., “MasterBeads”). 8. Gas-treating capacities: SFB versus conventional vibrated bed Consider processes in which a gas or vapor emerges from the surface of an SFB: for example, flash coal pyrolysis, biomass combustion, and air-cooled steam condensing. In such processes, the rate of flow of gas or vapor from an SFB surface can be greater than a flow rate appropriate for a conventional vibrated bed. In either device, generally speaking, a prudent specification of flow rate will be not far beyond the product, (surface area) × (minimum fluidization velocity). In general, specifying a much higher flow rate risks problems from carryover of finer bed material. For a given powder, minimum fluidization velocity varies roughly as the two-thirds power of an artificial gravity. Expressed as a factor times the earth's gravitational acceleration, g (cm/s2), the SFB's artificial gravity is given by ðD=2Þ½2pðRPM=60Þ2 =980:7; where D = diameter of the SFB (cm) and RPM = revolutions per minute. As Table 1 illustrates, the SFB's capacity advantage can be large. 9. The SFB environment That conventional vibrating beds provide a “gentle” environment accounts for the commercial role they play in Table 1 Gas-treating advantage of SFB relative to conventional vibrated bed Basis: SFB RPM = 450 SFB diameter, D (cm) Artificial gravity, g's (g's)2/3 ≈ SFB advantage a
300 ∼ 340 ∼ 49
100 ∼113 ∼ 23
30 ∼34 ∼10.5
15 ∼ 17 ∼ 6.5
a Approximate ratio, minimum fluidizing gas velocity in SFB versus that in conventional vibrated bed.
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drying delicate particulate materials, such as fragile breakfast cereals. A reasonable prediction is that the SFB environment will also be gentle. The prediction follows from a consideration of circumstances historically known to cause the degradation of particles (even when these are crafted to resist attrition). Consider how metallurgical coke, a material meeting a specification for “hardness,” tends to fragment in the raceway in front of an iron blast-furnace tuyere, where coke particles, accelerated to a high velocity, are subjected to high-momentum impacts. Consider the attrition-resistant ∼ 0.3-mm catalyst “beads” employed in historic gravitating-bed processes for cracking gas oils. In these processes, a gas lift circulates the beads. From the top of the lift, beads drop upon a stationary surface, having in their fall attained substantially their terminal velocity. (In one design, employing a lift pipe of large diameter, above the lift sits a suspended bead “cloud,” offering another target that beads strike at high velocity.) After a few passes, beads tend to split into two hemispheres; subsequent passes convert hemisphere edges to a fine powder, necessitating provision of means for its removal. In general, to distribute feed gas entering fluid beds large in size, designers have provided orifices through which gas moves at a relatively high speed, yet designers seldom allow highvelocity gas jets to enter the bed directly – thus creating “raceways” – since such jets can cause particle attrition and material losses as fines. In the blast furnace raceway; in a gravitating-bed catalytic cracker, following the gas lift; in jets of gas entering a fluid bed — in these situations, particles suffer break-up and attrition when, having achieved a high velocity, their motion is abruptly halted through collision. Like the conventional vibrated bed, an SFB will not expose particles to break-up from high-velocity impacts on stationary targets. In an SFB, sidewards flying matter is accelerated en masse. Its flight is stopped when artificial gravity causes the matter to “settle gently” against the SFB's inner wall. Accordingly, the SFB may become a high-capacity replacement for the conventional vibrated bed as a device for drying delicate particulate materials. 10. Opportunities for an SFB lift As Fig. 7 suggests, an SFB may be useful as a lift for a “gravel.” An SFB lift would bring to notice opportunities for new designs of the large numbers of two-step processes wherein, for example, a catalytic step must be followed by catalyst regeneration. The SFB, too, may be a candidate for use in either step. Deserving reconsideration are the aforementioned historic gravitating-bed catalytic gas–oil cracking processes. Although gasoline yields from these processes ran a few percentage points higher than yields from fluid catalytic cracking (FCC), they nevertheless, by the mid-1950s, lost their market for installations at large throughputs. Problems were (1) a necessity to build to a height on the order of 75 m (while FCC can come in below 30 m); (2) for capacities of ∼ 50,000 barrels/day and
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beyond, catalyst regenerator designs that were awkward to say the least (while FCC capacity could expand, it seemed, without limit); and (3) loss of catalyst beads through attrition in a gas lift. An SFB cracking process could avoid these problems and might compete with FCC. 11. Smokeless solid fuel from coal The SFB may become a candidate device to produce a smokeless solid fuel from coal. In the mid-1960s, before natural gas reach Britain from the North Sea, Great Britain's National Coal Board (NCB) produced such fuel commercially. NCB had made an important (and, for many, surprising) discovery: a bituminous coal can be rendered smokeless for combustion in a domestic stove or grate simply by reducing its volatile matter by just a relatively few percentage points. NCB accomplished adequate reduction by treating coal in a fluid bed at ∼ 400 C, the bed being fluidized by a gas containing ∼ 2% oxygen. Against NCB's expectation, briquetting fluid-bed product contributed significantly to capital and running costs, significantly increasing the latter through down-time for maintenance and repairs. In contrast with NCB's fluid bed, an SFB could treat lump coal in a volatile-matter-reduction step that directly yields a smokeless solid fuel in sizes suitable for domestic use, without need for briquetting. Heat for the SFB could be supplied by a flow of a hot powder (e.g., fine char from pyrolysis or partial combustion of the coal fines that inevitably accompany mining coal in lump-form). Typically, in Third World nations having made at least some progress in industrial development, many rich people burn kerosene for cooking and heat; the less-well-off, charcoal; and the poor, raw wood. In nations at earlier stages of industrial development, charcoal and wood are the usual fuels of choice. Foreign aid that the First World provides to the Third could advantageously include shipments of smokeless fuels manufactured from coals low in sulfur content (e.g., from coals of Virginia or Wyoming in the U.S.; or coals of Canada, Australia, New Zealand, Britain, Poland, etc.). Such foreign aid can be part of a program for delaying environmental effects of global warming. Release of carbon dioxide from manufacture and combustion of a smokeless fuel from coal can be considerably less than that arising from manufacture and combustion of charcoal, let alone from combustion of raw wood. In a comparison of the manufacture of a smokeless fuel from coal or wood, the energy content of feedstock is the smaller for coal; and substituting coal-derived fuels for charcoal would arrest destruction of forests valuable for their roles as a sink for carbon dioxide and protector of hillsides from devastation by, for example, a hurricane. 12. Treating solid moieties large in size The SFB's capability for producing smokeless fuel from lump coal illustrates a unique advantage of the SFB: its ability to deal with moieties at physical sizes so large that (1) gasfluidization is out of the question and (2) mixing in
conventional vibration of the moieties is negligibly small [8]. The SFB's mixing capability, it seems, is unique in the catalog of means for dealing with solids large in size (e.g., various designs for gravitating beds). Flash food cooking (rapid heating followed by prompt withdrawal of cooked product from a radiant heat source) could preserve flavors lost in slow cooking. For example, a tasty, fatfree substitute might compete with conventional French-fried potatoes. Flash pyrolysis of certain waste polymeric materials could recover valuable monomers. Curing of polymers (through heat treatment or exposure to a supply of gas — in some instances, a highly limited quantity of gas) is another candidate to consider. Heat treatment of small steel parts is yet another. Countercurrent heat exchange between such parts and a Group A powder could both preheat parts before treatment and recover heat after. 13. Conclusion The SFB's potential advantages (rapid local mixing, capability for flash heating, rapid separation of gas and solid products, ability to deal with moieties of large size) appear to make it a formidable candidate for study and development. List of AFBC D FCC g Hz NCB RPM SFB
symbols Atmospheric fluid bed combustion SFB diameter, cm Fluid catalytic cracking Earth's gravitational acceleration, cm/s2 Frequency, cycles per second National Coal Board of Great Britain Rotations per minute Sidewards flying bed
References [1] A.M. Squires, Powder Technol. 147 (2004) 1. [2] A.M. Squires, Powder Technol. 147 (2004) 10. [3] B. Thomas, Y.A. Liu, R. Chan, A.M. Squires, Powder Technol. 52 (1987) 77. [4] B. Thomas, M.O. Mason, Y.A. Liu, A.M. Squires, Powder Technol. 57 (1989) 267. [5] B. Thomas, A.M. Squires, Phys. Rev. Lett. 81 (1998) 574. [6] B. Thomas, M.O. Mason, R. Sprung, Y.A. Liu, A.M. Squires, Powder Technol. 99 (1998) 293. [7] B. Thomas, A.M. Squires, Powder Technol. 100 (1998) 200. [8] B. Thomas, M.O. Mason, A.M. Squires, Powder Technol. 111 (2000) 34. [9] D. Geldart, Powder Technol. 7 (1973) 283. [10] K.-C. Lee, R. Pfeffer, A.M. Squires, Powder Technol. 155 (2005) 5. [11] R.V. Whitman, K.A. Healy, Trans. Am. Soc. Civ. Eng. 128 (1963) 1553. [12] R.V. Whitman, E.T. Miller, P.J. Moore, Proc. Am. Soc. Civ. Eng. 90 (SM 4) (1964) 57. [13] A.M. Squires, R.A. Graff, S. Dobner, Science 189 (1975) 189. [14] R.A. Graff, S. Dobner, A.M. Squires, Fuel (London) 55 (1976) 109. [15] S. Dobner, R.A. Graff, A.M. Squires, Fuel (London) 55 (1976) 113. [16] M. Valenti, Mech. Eng. 122 (7) (2000) 50.
Chemical process opportunities for vibrated powders 3. Provisional design for an experimental sidewards f lying bed A.M. Squires ⁎ Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg VA 24061-0211, USA Received 24 October 2004; received in revised form 1 June 2006; accepted 1 June 2006 Available online 2 February 2007
Abstract At a scale suitable for laboratory experimentation, a provisional mechanical design illustrates a concept for a “sidewards flying bed” (SFB), a layer of particulate matter held upon the inner wall of a cylinder that rotates at varying speed. Acting upon the layer are centrifugal and tangential forces whose patterns of variation create time intervals during which the latter is the greater. During such intervals, the layer is postulated to respond by “flying sidewards.” In a laboratory SFB set-up, experiments can elucidate a new area of non-steady-state soil mechanics, with outcomes bearing upon a full-scale SFB's chemical processing potential. An important question is, what will be the delay in the SFB layer's taking flight after the moment tangential force just begins to exceed centrifugal? For the SFB design to be successful, this delay should be significantly shorter than a time interval during which the tangential force dominates. If experiments were to show this not to be so, a “what-then?” might be to install several stationary rakes near the cylinder's inner wall, parallel to its axis. Loosening the particulate layer as the cylinder turns, these should advance the timing of the layer's flight. Achieving adequate flight in an experimental SFB could point to a design at a commercial scale capable of promoting rapid local mixing — more rapid than that afforded by conventional vibrated or fluid beds. Such should become a strong candidate for applications requiring rapid heating of hydrocarbonaceous matter followed by quick separation of vapor product and carbonaceous residue (e.g., flash pyrolysis of coal or oilshale). The SFB's competencies should include ability to treat moieties of especially large size (e.g., for producing a smokeless solid fuel from coal, for heat-treating a metal part or a polymer, for flash-cooking foodstuffs without oil, for drying large particulates); ability to treat highly heterogeneous matter (e.g., for burning or gasifying comminuted biomass or solid wastes); and ability to promote excellent heat transfer between SFB and stationary surfaces embedded therein (e.g., in an air-cooled SFB steam-condenser). © 2007 Elsevier B.V. All rights reserved. Keywords: Sidewards flying bed; Rotating vibrated bed; Non-steady-state soil mechanics; Flash coal pyrolysis; Flash shale oil distillation; High-pressure flash hydrogenation of carbonaceous matter; Smokeless solid fuel; Polymeric monomer recovery; Polymer curing; Steel heat treatment; Flash-cooking food; Biomass combustion; Biomass gasification; Drying; Air-cooled condenser; Refrigerated storage of unstable oils
1. Introduction This is the third paper dealing with chemical process opportunities for vibrated powders. The first two papers [1,2] discussed laboratory and field applications, respectively, of the conventional vibrated bed, in which a substantially horizontal floor, subjected to substantially vertical cyclic displacement from a null position, supports a layer of powder. This third paper
⁎ Tel.: +1 540 231 5972; fax: +1 540 231 5022. E-mail address: [email protected]. 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2006.06.019
treats a new device for processing particulate matter, the “sidewards flying bed” (SFB), a shallow powder layer held by variable centrifugal force against a rotating cylinder's inside wall. Tangential force acting upon the layer also varies, becoming greater than the centrifugal force during certain time intervals. In such intervals, it is postulated, the layer takes “sidewards flight.” Exploration of the postulate requires experimental study of relevant non-steady-state soil mechanics. In principle, the SFB's rotating cylinder may be either upright or horizontal. An upright design will often be preferred: its footprint is smaller; and its orientation, usually, better facilitates employment of gravity-aided means for introducing
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Fig. 1. Provisional design for experimental “sidewards flying bed” (SFB): vertical cross-section at zero time. A motor rotates a left-hand eccentric arm in “forward” direction (toward a viewer of the drawing); a right-hand arm rotates “backward.” Transfer of momentum from eccentrics to support rod causes it to turn horizontally: first, clockwise (viewed from above); later, counter-clockwise. At the bottom of the rod, a bearing holds the rod in place against lifting. (Not shown are spring means needed to compensate for change in the torsion bar’s length as it twists.)
Transfer of momentum from eccentrics to rod causes the latter to turn horizontally in a back-and-forth motion. Sharing this motion are the motor, the two eccentrics with their horizontal shaft, and the upright cylinder. The latter's motion creates an intermittent centrifugal force pressing a layer of particulate matter against its inner wall. A suggested depth for the layer is 30 mm; a body of data is available [3–8] for conventional vibrated beds at this depth over a wide range in particle size. Three vertical posts (as seen in Fig. 2's section A–A drawing) carry wheels that stabilize the cylinder against horizontal deflection. The Fig. 1 drawing is for zero time. The section B–B drawing in Fig. 2 depicts the left-hand eccentric in Fig. 1 at zero time (as viewed from the left in the Fig. 1 drawing). A dot-dash circle depicts the travel of the weight at the end of the eccentric; the weight moves in clockwise direction through angle θ in a time interval set by the motor's speed. The second eccentric rotates counterclockwise (as viewed from the right in Fig. 1). At zero time, the eccentric of section B–B, as seen in Fig. 1, travels outward from the plane of the paper, toward the viewer. Transfer of momentum from eccentric to SFB cylinder causes the latter to turn inward from the plane of the paper, away from the viewer; i.e., the cylinder's rotation is clockwise as seen from above (see Fig. 2's section A–A). At zero time, as seen in Fig. 1, the right-hand eccentric travels away from the viewer; a point above it on the cylinder travels toward the viewer; i.e., again, the cylinder is seen to turn clockwise when viewed from above. When the two eccentrics rotate through the angle θ, momentum transfer causes the SFB cylinder to rotate through clockwise angle γ.
and withdrawing particulate matter. The SFB appears to have potential for rapid powder mixing as well as unusually high granular-matter processing capacity relative either to equipment volume or, especially, to equipment footprint. From standpoints of reliability and achievable scale, relative to the conventional vibrated bed, the SFB appears to share an advantage often enjoyed by rotary equipment (e.g., a turbine) over a competing design that employs reciprocating linear motion (e.g., a piston engine). 2. A provisional design for an experimental sidewards flying bed (SFB) 2.1. Mechanical design Fig. 1 is a vertical cross-sectional drawing of a provisional design for an upright SFB suitable for experiments providing an eyeball view of SFB behavior from above. In cooperation with a torsion bar (fixed in place at its top), a vertical rod (free to rotate) supports a horizontal electric motor and an upright cylinder. At constant speed, the motor rotates a shaft carrying two eccentric arms, each with a weight near its extremity. As seen in Fig. 1, the two arms are vertical, one up and one down.
Fig. 2. Provisional SFB design: horizontal section A–A; an alternative section A–A; and vertical section B–B. In section A–A, arrows within the SFB indicate the initial, counter-clockwise direction of flight, putatively commencing shortly after ~ 7 ms (see Fig. 5). In section B–B, a dot–dash circle indicates path of weight at end of the rotating eccentric.
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An opposite situation develops when right-hand eccentric reaches the top of its travel; the left-hand eccentric now points downward. In this configuration of the eccentrics, the SFB cylinder turns counterclockwise (viewed from above). As the cylinder turns, both a tangential force and a centrifugal force act upon the particulate layer. An SFB design should ensure existence of time intervals during which the former exceeds the latter. During at least a latter portion of such a time interval, the layer is postulated to “fly sidewards”; whether such occurs depends upon applicable unsteady-state soil mechanics (to be discussed in Section 2.3). When a layer takes flight, lateral particle velocity perhaps exhibits a gradient from a lower value near the wall to a higher value at the particulate layer's free surface. Such a gradient, if present, dilates the particle layer, drawing in a moiety of gas from the central region of the cylinder. Compaction of the layer, when centrifugal force again exceeds tangential, returns the imbibed gas to the central region. In some SFB applications, a designer may wish a more definite, more pronounced traffic of gas across the layer, reflecting periodic development and destruction of a gap next to the cylinder's wall, analogous to gap-formation in a conventional vibrated bed [8]. To accomplish this goal, the designer may specify a cylinder with vertical serrations, as seen in Fig. 2's alternative section A–A drawing. Here a tangential acceleration may be viewed as possessing two components facing one another at a right angle. One component is perpendicular to the plane of a serration facing in the direction of the acceleration. When this component is greater than a competing centrifugal-acceleration component pressing down upon this plane (and if particle size is not too small), a gap should form between particulate layer and plane, promoting flow of gas from the cylinder's central region, across the layer, and into the gap. Later, gap destruction returns gap gas to the central region. Except for direction of particle flight, particle action in the SFB with a serrated wall should resemble that of powder in a conventional vibrated bed. For example, recall that in a conventional bed of a Geldart Group B or D powder [9] operating at 25 Hz, up-and-down vibration of a floor causes particles resting thereupon to leap upward, en masse, roughly every 40 milliseconds (ms). At each upward leap, there occurs an interval of vertical free flight (like water from a fountain) lasting, typically, some 20-odd ms [8]. After its flight, the powder mass returns to the floor and experiences compaction. During the free flight (if the floor is non-porous), the leaping powder creates a negative pressure at the floor, drawing “supernascent” gas downward across the bed; at the end of flight, the floor pressure spikes upward, expelling the imbibed gas from the bed. For Group B or D powders, similar effects should be seen in the SFB with serrations. As in the design of a conventional vibrated bed, “tuning” the natural frequency of the torsion bar to the SFB's frequency can reduce the power required to drive the electric motor seen in Fig. 1. In Figs. 1, 2, the SFB cylinder's outer wall is non-porous. The wall may be porous, allowing introduction of a treating gas
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into the SFB from a surrounding plenum (e.g., a hot gas for drying wet particulate matter). See Sections 5–7 and 11 for examples requiring such an arrangement. In an SFB with serrations, however, for a process requiring only a small treating-gas flow, notice that such gas need not be fed from an outer plenum but can be introduced centrally, rapid exchange of gas between SFB and central space being adequate for the desired treatment of gas or particles. 2.2. Calculating accelerations For a provisional design, take motor speed in Fig. 1 to be 600 RPM. Then hðtÞ ¼ 2pft
ð1Þ
where t = time (s) and f = frequency = 10 Hz. dh=dt ¼ 2pf
ð2Þ
Take SFB cylinder height to be 60 cm and other design parameters to be: R1 R2 R3
M1
Radius of eccentric = 17 cm; Radius of SFB cylinder = 15 cm; Horizontal turning radius of an eccentric (length of horizontal shaft, from center of vertical rod to plane of eccentric) = 18 cm. “Effective” combined weight carried by the two eccentrics (each weight as if concentrated at an eccentric's radius, R1) = 50 kg;
Fig. 3. Provisional SFB design: instantaneous SFB speed of rotation.
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acceleration exceeds centrifugal between ∼7 and ∼43 ms and between ∼57 and ∼93 ms. Putatively, each of these time intervals includes a smaller interval in which the SFB particulate layer takes flight. Notice that direction of flight is different in the two intervals: viewed from above, counterclockwise in the first interval; clockwise in the second — in each case, opposite to the SFB's rotation. During an interval b1 ms, centrifugal acceleration is smaller than earth's gravity. During this brief interval, under the latter's influence, downward movement of the SFB particulate layer, in general, should be through a distance not greater than a few particle diameters. 2.3. New soil mechanics Fig. 4. Provisional SFB design: deflection of SFB from null position.
M2
“Effective” weight of SFB cylinder plus particulate layer (as if concentrated at cylinder's radius, R2) = 25 kg;
Let Ω⁎ = M1R1R3 / R2[M2R2 + M1(R32 + R12sin2θ)0.5] = Ω (1 + λ) / [1 +λ(1 + β2sin2θ)0.5] where Ω= M1R1R3 / R2[M2R2 + M1R3] = 50 × 17 × 18 / 15[25 × 15 + 50 × 18] = 0.8; λ = M1R3 / M2R2 = 50 × 18 / 25 × 15 = 2.4; and β2 = (R1/R3)2 = (17/18)2 = 0.892. In the first quadrant (θ = 0 to π/4), instantaneous rotational speed of the SFB is given by momentum transfer from rotating eccentrics to SFB cylinder: dg=dt ¼ ðdg=dhÞðdh=dtÞ ¼ ðX⁎coshÞð2pf Þ ¼ 2pf Xcoshð1 þ kÞ=½1 þ kð1 þ b2 sin2 hÞ0:5
To be answered through experiment is the question, before an SFB particulate layer takes flight, what time will pass following the crossover in Fig. 5 at ∼ 7 ms — i.e., following the moment at which tangential acceleration overtakes centrifugal? In a conventional vibrated bed at 25 Hz, a corresponding time interval (a delay from the moment at which the bed's vertically upward acceleration overtakes gravity) is, in general, relatively short in comparison with flight time [8]. Although this provides a degree of reassurance that delay in SFB flight will take up only a relatively small fraction of the ∼ 36 ms “available” for flight, seen in Fig. 5, such reassurance is only a warrant for performing experiments to determine what this fraction may be. Studies of the panel bed granular filter with puffback renewal of gas-entry faces also provides a degree of reassurance [10]. A puffback imposes action of a strong “puff” of gas in a direction toward these faces, this action creating body-motion of the
ð3Þ
Centrifugal acceleration acting on SFB (in g's): R2 ðdg=dtÞ2 =g ¼ ðR2 =gÞð2pf X⁎coshÞ2
ð4Þ
where g = acceleration of gravity = 980.7 cm/s2. Tangential acceleration acting on SFB powder is (in g's): R2 ðd2 g=dt 2 Þ=g ¼ −ðR2 =gÞfð2pf Þ2 Xð1 þ kÞ ½1 þ kð1 þ b2 sin2 hÞ0:5 g fsinh þ kb2 sinhcos2 h ½1 þ kð1 þ b2 sin2 hÞ0:5 ð1 þ b2 sin2 hÞ0:5 g ð5Þ Quadrant symmetries give SFB speed and accelerations at θ ≥ π/4, Fig. 3 displays the SFB cylinder's instantaneous speed of rotation, calculated from Eq. (3). The speed cycles between plus and minus 480 RPM. Fig. 4 plots angle γ, the SFB cylinder's deflection from its null position. The deflection cycles between about +43° and −43°. Fig. 5 plots centrifugal and tangential accelerations, from Eqs. (4) and (5), respectively. In a given 100-ms cycle, tangential
Fig. 5. Provisional SFB design: dashed curve = tangential acceleration; continuous curve = centrifugal acceleration. Sidewards flight is postulated to occur during time intervals in which tangential acceleration exceeds centrifugal. Arrows indicate directions of the putative flight: an arrow pointing to the left indicates counterclockwise flight (viewed from above); arrow to the right, clockwise.
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panel bed's granular material toward the faces within a timedelay on the order of 10 ms. Granular-matter “soil failure” in a panel bed under puffback comports with earlier knowledge of soil failure under sudden imposition of stress [11,12] (a knowledge acquired in response to desire to understand how a structure fails under impact of an atmospheric shock wave from a nuclear blast). When stress is imposed “instantaneously,” soil failure occurs everywhere, all at once. In contrast, in the more familiar situation, when stress slowly increases (or after it persists for a time at an unstably high level), soil tends to fail at a weakest point, individual particles having had time to shift their positions slightly, or to turn a bit, before the failure. A sharp, sudden stress leaves no time for particle rearrangement, and the mass of particles gives way everywhere at once. Acceleration curves in Fig. 5 suggest that a body-failure will occur, SFB particles taking flight everywhere at once, at a yetunknown time interval following the 7-ms and 57-ms crossovers. Finding this interval may interest students of soil mechanics. Prospects for the SFB's usefulness in chemical processing depend upon outcomes from this line of study: the shorter the time the better, although a delay in flight might be tolerated, perhaps, up to as much as ∼15 ms.
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possible, close positioning of their feeds on an SFB circumference can ensure intense local mixing. 3.2. Withdrawing granular material Seen in Fig. 6 is an arrangement for withdrawing pulverulent matter at the SFB's top. The material flows over the lip of a “weir” that establishes the SFB's surface level. Centrifugal force carries the withdrawn material outward, flinging it against the inner wall of a stationary vertical cylinder. Thereupon, the material forms a relatively concentrated “sheet,” which falls into a circular, conventional vibrated bed. This conveys the material circumferentially toward one or more down-comers (not seen in Fig. 6) for its removal and conveyance toward either disposal or further processing. In Fig. 6, at the SFB's bottom, material flows across a second weir establishing SFB surface level. As at the top, centrifugal force flings the material outward into a sheet that falls into a conventional vibrated bed carrying the material to one or more down-comers for final take-off. 3.3. Countercurrent heat exchange between gravel and powder Fig. 6 illustrates an arrangement whereby a designer may reasonably hope to achieve countercurrent heat transfer between
2.4. A “what-then?” if new soil mechanics data were to disappoint Suppose experiments were to show the delay to be much longer than 15 ms. A response might be to install 4 (or 5) stationary rakes near the SFB cylinder's inner wall, parallel to the cylinder's axis (the rakes not participating in the cylinder's rotation). Installed at 90° (or 72°) intervals, the rakes should permanently loosen the SFB particle layer. It should take flight, “instantaneously,” when tangential acceleration overtakes the centrifugal. 3. Feeding and discharging particulate materials 3.1. Downcomer feeds Fig. 6 illustrates provisional arrangements for feeding and withdrawing particulate matter from an SFB. A stationary cylinder, with top and bottom closures, surrounds the rotating SFB cylinder. Penetrating a top closure of the stationary cylinder are two stationary down-comers, one of which feeds material near the top of the SFB, the second, near the bottom. (A horizontal screen seen below the bottom feed is optional; Section 3.3 will discuss its use.) Preferably, a down-comer's cross-section should be sufficiently large such that, at a desired rate of feed, material drops through it in freefall. At its bottom, a short section at 45° to horizontal deflects the material laterally at a relatively high velocity toward a nearby point on the SRB's surface. If desired, a number of narrowly-set-apart down-comers may be arranged in a circle to provide a number of feeds at a given SFB elevation. If two materials must be mixed as rapidly as
Fig. 6. Vertical cross-section illustrating provisional arrangements for feeds and withdrawals of particulate material. Centrifugal forces fling withdrawn material into a circular vibrating trough, which conveys material circumferentially to a take-off down-comer (not shown in the drawing). (Not shown are means for vibrating the two circular troughs.)
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a “gravel” (Geldart Group B or D) and a Group A powder. Down-comers feed gravel close to the bottom of an upright SFB. Situated at an elevation just beneath the entry of gravel, a screen prevents its movement downward. A gradient in the SFB's surface, from screen to SFB top, sustains gravel upward motion. Subjected to an SFB's back-and-forth motion, a Group A powder can be expected to enter a liquid-like, “low-viscosity” state [8], in which gravity can be expected to pull the powder downward. Fed at a “sufficient” distance below a top outlet for gravel, a Group A powder should discharge primarily at the SFB bottom, below the screen. 3.4. An SFB lift for a “gravel” In Fig. 6, an SFB acts to lift gravel fed near its bottom. The arrangement does not, however, provide a practical means for lifting a gravel, since this entered the SFB at the top, via downcomers. What is needed to provide an SFB lift is an arrangement for feeding gravel at the bottom. Provisionally, Fig. 7 offers such an arrangement, a feature of which is a “mass lift” of the gravel from a conventional vibrated bed onto a rotating horizontal plate, the bottom closure of the rotating SFB. A gas-tight enclosure houses this conventional vibrated bed. To create the mass lift, a relatively small flow of gas creates a small overpressure within the enclosure, sustaining an upward gas flow
through a column of gravel originating from within the vibrated bed. The upward flow of gas should be sufficient to cause the gravel column to move upward in the gravel's dense, “settled” condition. Following the gravel's discharge onto the rotating plate, centrifugal forces carry it laterally onto the SFB surface. A relatively small gradient in this surface, declining toward the top, lifts the gravel toward removal (as seen in Fig. 6). 4. An SFB for flash pyrolysis of coal or shale-oil rock An SFB is a candidate device for flash pyrolysis of hydrocarbonaceous matter, such as coal or shale-oil rock [2]. This matter and a hot powder would be fed side-by-side via pairs of down-comers. A number of such pairs should be provided, several sets of pairs terminating at several elevations within a vertical SFB. For treating coal, a circulating-fluid-bed boiler in base-load service and burning char residue of pyrolysis can supply a steady flow of hot powder [2]. For treating shaleoil rock, a countercurrent heat exchange along the lines of Fig. 6, as described in Section 3.3, can be useful [2]. Condensation of vapor product can be effected in a central, stationary, vertical, perforated duct housing, near its top, either sprays of cooled, recycled pyrolysis liquid or a horizontal, perforated vibrating tray feeding refrigerated particulates (such as “MasterBeads” [2,8]) onto which vapors can condense. In an application in which a gas turbine will burn oil from pyrolysis to provide intermittent, peak-load electricity [2], low-temperature storage of particulates and oil condensate may protect the oil from reactions that unduly increase its molecular weight, rendering it difficult to pump at near-ambient temperature. 5. Flash hydrogenation of coal at high pressure An SFB is also a candidate reactor for high-pressure flash hydrogenation of coal or other hydrocarbonaceous matter [13– 15], an exothermic process capable of yielding (from coal) methane, ethane, benzene, toluene, and xylenes (with negligible yields of heavier hydrocarbons) together with a char residue. 6. A combustor for a small, biomass-fueled gas turbine
Fig. 7. Provisional design for feeding a “gravel” (a Geldart Group B or D powder [9]) to the bottom of an SFB lift. Gravel feed reaches a vibrating trough from a down-comer. The trough carries gravel laterally toward an up-comer. The trough, with its lid, forms a gas-tight enclosure. A relatively small supply of gas, elevating the enclosure’s pressure, sustains mass lift of gravel via the up-comer onto a horizontal “floor” at the bottom of the SFB cylinder, where centrifugal forces carry the gravel laterally into the bottom of the SFB lift. At the top of the lift, gravel withdrawal is by the arrangement seen in Fig. 6. (Not shown are {1} required flexible jointures connecting trough lid with down-and up-comers, {2} means for vibrating the trough enclosure, and {3} slat-works connecting SFB cylinder and support rod.)
An SFB is a candidate for service as a combustor burning biomass and supplying hot gas at ∼3 to ∼4 atmospheres to the expander of a “tiny” gas turbine. Turbines delivering as little as 40 kW are now available [16]. These operate at high speeds (reaching 90,000 RPM in some designs). Their development followed appearance of compact, inexpensive, solid-state devices capable of converting the ultra-high-frequency electricity generated at a high speed to conventional 50- or 60-cycle current. Thousands of small turbines now provide distributed power sources in large utility systems (saving transmission losses and postponing need to outlay capital for new transmission lines). Small turbines are also meeting small needs for power at locations where electricity does not reach. An ability to use biomass fuel could expand the usefulness of these small gas turbines. In an SFB combustor for biomass, the SFB advantageously comprises a “permanent” bed of an attrition-resistant Geldart
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Group B or D granular solid (such as the aforementioned “MasterBeads”). 7. An air-cooled condenser Embedding stationary heat-transfer surface within an SFB may give rise to attractive applications of the device. It is a practical certainty that coefficients for transfer of heat to or from such surface would be larger than those ordinarily seen in a conventional vibrated bed, since coefficients for conventionally vibrated powders increase when stationary surfaces therein experience a lateral flow [6]. Recall, too, that conventional vibrated beds afford heat-transfer coefficients comparable to those seen in fluid beds — coefficients on the order of 10 to 20 times larger than coefficients for transfer from air to a metal surface in absence of particulate matter. An air-cooled steam condenser might exploit heat transfer afforded by an SFB embedding stationary finned tubes; particulate matter should be an attrition-resistant granular solid (e.g., “MasterBeads”). 8. Gas-treating capacities: SFB versus conventional vibrated bed Consider processes in which a gas or vapor emerges from the surface of an SFB: for example, flash coal pyrolysis, biomass combustion, and air-cooled steam condensing. In such processes, the rate of flow of gas or vapor from an SFB surface can be greater than a flow rate appropriate for a conventional vibrated bed. In either device, generally speaking, a prudent specification of flow rate will be not far beyond the product, (surface area) × (minimum fluidization velocity). In general, specifying a much higher flow rate risks problems from carryover of finer bed material. For a given powder, minimum fluidization velocity varies roughly as the two-thirds power of an artificial gravity. Expressed as a factor times the earth's gravitational acceleration, g (cm/s2), the SFB's artificial gravity is given by ðD=2Þ½2pðRPM=60Þ2 =980:7; where D = diameter of the SFB (cm) and RPM = revolutions per minute. As Table 1 illustrates, the SFB's capacity advantage can be large. 9. The SFB environment That conventional vibrating beds provide a “gentle” environment accounts for the commercial role they play in Table 1 Gas-treating advantage of SFB relative to conventional vibrated bed Basis: SFB RPM = 450 SFB diameter, D (cm) Artificial gravity, g's (g's)2/3 ≈ SFB advantage a
300 ∼ 340 ∼ 49
100 ∼113 ∼ 23
30 ∼34 ∼10.5
15 ∼ 17 ∼ 6.5
a Approximate ratio, minimum fluidizing gas velocity in SFB versus that in conventional vibrated bed.
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drying delicate particulate materials, such as fragile breakfast cereals. A reasonable prediction is that the SFB environment will also be gentle. The prediction follows from a consideration of circumstances historically known to cause the degradation of particles (even when these are crafted to resist attrition). Consider how metallurgical coke, a material meeting a specification for “hardness,” tends to fragment in the raceway in front of an iron blast-furnace tuyere, where coke particles, accelerated to a high velocity, are subjected to high-momentum impacts. Consider the attrition-resistant ∼ 0.3-mm catalyst “beads” employed in historic gravitating-bed processes for cracking gas oils. In these processes, a gas lift circulates the beads. From the top of the lift, beads drop upon a stationary surface, having in their fall attained substantially their terminal velocity. (In one design, employing a lift pipe of large diameter, above the lift sits a suspended bead “cloud,” offering another target that beads strike at high velocity.) After a few passes, beads tend to split into two hemispheres; subsequent passes convert hemisphere edges to a fine powder, necessitating provision of means for its removal. In general, to distribute feed gas entering fluid beds large in size, designers have provided orifices through which gas moves at a relatively high speed, yet designers seldom allow highvelocity gas jets to enter the bed directly – thus creating “raceways” – since such jets can cause particle attrition and material losses as fines. In the blast furnace raceway; in a gravitating-bed catalytic cracker, following the gas lift; in jets of gas entering a fluid bed — in these situations, particles suffer break-up and attrition when, having achieved a high velocity, their motion is abruptly halted through collision. Like the conventional vibrated bed, an SFB will not expose particles to break-up from high-velocity impacts on stationary targets. In an SFB, sidewards flying matter is accelerated en masse. Its flight is stopped when artificial gravity causes the matter to “settle gently” against the SFB's inner wall. Accordingly, the SFB may become a high-capacity replacement for the conventional vibrated bed as a device for drying delicate particulate materials. 10. Opportunities for an SFB lift As Fig. 7 suggests, an SFB may be useful as a lift for a “gravel.” An SFB lift would bring to notice opportunities for new designs of the large numbers of two-step processes wherein, for example, a catalytic step must be followed by catalyst regeneration. The SFB, too, may be a candidate for use in either step. Deserving reconsideration are the aforementioned historic gravitating-bed catalytic gas–oil cracking processes. Although gasoline yields from these processes ran a few percentage points higher than yields from fluid catalytic cracking (FCC), they nevertheless, by the mid-1950s, lost their market for installations at large throughputs. Problems were (1) a necessity to build to a height on the order of 75 m (while FCC can come in below 30 m); (2) for capacities of ∼ 50,000 barrels/day and
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beyond, catalyst regenerator designs that were awkward to say the least (while FCC capacity could expand, it seemed, without limit); and (3) loss of catalyst beads through attrition in a gas lift. An SFB cracking process could avoid these problems and might compete with FCC. 11. Smokeless solid fuel from coal The SFB may become a candidate device to produce a smokeless solid fuel from coal. In the mid-1960s, before natural gas reach Britain from the North Sea, Great Britain's National Coal Board (NCB) produced such fuel commercially. NCB had made an important (and, for many, surprising) discovery: a bituminous coal can be rendered smokeless for combustion in a domestic stove or grate simply by reducing its volatile matter by just a relatively few percentage points. NCB accomplished adequate reduction by treating coal in a fluid bed at ∼ 400 C, the bed being fluidized by a gas containing ∼ 2% oxygen. Against NCB's expectation, briquetting fluid-bed product contributed significantly to capital and running costs, significantly increasing the latter through down-time for maintenance and repairs. In contrast with NCB's fluid bed, an SFB could treat lump coal in a volatile-matter-reduction step that directly yields a smokeless solid fuel in sizes suitable for domestic use, without need for briquetting. Heat for the SFB could be supplied by a flow of a hot powder (e.g., fine char from pyrolysis or partial combustion of the coal fines that inevitably accompany mining coal in lump-form). Typically, in Third World nations having made at least some progress in industrial development, many rich people burn kerosene for cooking and heat; the less-well-off, charcoal; and the poor, raw wood. In nations at earlier stages of industrial development, charcoal and wood are the usual fuels of choice. Foreign aid that the First World provides to the Third could advantageously include shipments of smokeless fuels manufactured from coals low in sulfur content (e.g., from coals of Virginia or Wyoming in the U.S.; or coals of Canada, Australia, New Zealand, Britain, Poland, etc.). Such foreign aid can be part of a program for delaying environmental effects of global warming. Release of carbon dioxide from manufacture and combustion of a smokeless fuel from coal can be considerably less than that arising from manufacture and combustion of charcoal, let alone from combustion of raw wood. In a comparison of the manufacture of a smokeless fuel from coal or wood, the energy content of feedstock is the smaller for coal; and substituting coal-derived fuels for charcoal would arrest destruction of forests valuable for their roles as a sink for carbon dioxide and protector of hillsides from devastation by, for example, a hurricane. 12. Treating solid moieties large in size The SFB's capability for producing smokeless fuel from lump coal illustrates a unique advantage of the SFB: its ability to deal with moieties at physical sizes so large that (1) gasfluidization is out of the question and (2) mixing in
conventional vibration of the moieties is negligibly small [8]. The SFB's mixing capability, it seems, is unique in the catalog of means for dealing with solids large in size (e.g., various designs for gravitating beds). Flash food cooking (rapid heating followed by prompt withdrawal of cooked product from a radiant heat source) could preserve flavors lost in slow cooking. For example, a tasty, fatfree substitute might compete with conventional French-fried potatoes. Flash pyrolysis of certain waste polymeric materials could recover valuable monomers. Curing of polymers (through heat treatment or exposure to a supply of gas — in some instances, a highly limited quantity of gas) is another candidate to consider. Heat treatment of small steel parts is yet another. Countercurrent heat exchange between such parts and a Group A powder could both preheat parts before treatment and recover heat after. 13. Conclusion The SFB's potential advantages (rapid local mixing, capability for flash heating, rapid separation of gas and solid products, ability to deal with moieties of large size) appear to make it a formidable candidate for study and development. List of AFBC D FCC g Hz NCB RPM SFB
symbols Atmospheric fluid bed combustion SFB diameter, cm Fluid catalytic cracking Earth's gravitational acceleration, cm/s2 Frequency, cycles per second National Coal Board of Great Britain Rotations per minute Sidewards flying bed
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