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Chemical process opportunities for vibrated powders
Powder Technology 147 (2004) 10 – 19 www.elsevier.com/locate/powtec
Perspective
Chemical process opportunities for vibrated powders 2. In the field A.M. Squires* Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg VA 24061-0211, USA Received 18 July 2003; received in revised form 13 July 2004; accepted 13 July 2004 Available online 22 September 2004
Abstract Commercial opportunities are seen for chemical process applications employing vibrated powders: in reactors catering to non-steady-state gas–solid reaction kinetics; in non-fouling, non-corroding heat exchangers; in flash pyrolysis of coal working in tandem with fluid-bed combustion, yielding char for the combustion and a light oil useful (with little or no further treatment), for example, in a gas turbine generating peak-load power; in a system employing a Geldart Group A powder [D. Geldart, Powder Technol. 7 (1973), 285]; to carry heat over a relatively long distance (replacing less convenient heat carriers, such as mercury, molten salts and condensed-ring hydrocarbons); in counter-current heat exchange between a bgravelQ and a Group A powder; in flash distillation of shale oil. D 2004 Elsevier B.V. All rights reserved. Keywords: Vibrated bed; Non-steady-state heterogeneous reaction kinetics; Non-fouling; Non-corroding heat exchange; Flash coal pyrolysis; Flash shale oil distillation; Coal; Shale oil rock; Heat carrier; Fluid-bed combustion; Peak-load power
1. Introduction A first paper of a series on opportunities for vibrated powders [4] dealt with vibrated-bed microreactors, useful in the laboratory for simulating large-scale fluid bed reactors of several kinds. This second paper deals with opportunities for conducting chemical processes at the commercial scale in relatively conventional vibrated-bed equipment (or straightforward modifications thereof), requiring little new mechanical development.
2. Reactor for non-steady-state heterogeneous reaction kinetics Fig. 1 illustrates schematically a proposed reactor catering to non-steady-state heterogeneous reaction kinetics. The first paper of this series [4] described a bsliding-baffleQ vibratedbed microreactor capable of studying kinetics of a reaction * Tel.: +1 540 231 5972; fax: +1 540 231 5022. E-mail address: [email protected]. 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.07.013
regime wherein a powder (reactant or catalyst) bseesQ a primary gas for a brief interval and, in alternation, experiences a much longer bsoakQ in a secondary gas. The first gas may comprise a breaction gasQ supplied at high rate of flow; the second, a btreating gasQ, at low rate. Fig. 1 reactor [5] submits a Geldart Group A powder [1–3] to a regime of this nature. Spouts [6] carry the powder from a vibrated bed thereof into a bsupernatantQ space wherein the powder briefly encounters a horizontal flow of a primary gas; gravity returns the powder to its vibrated bed, in which it experiences a relatively prolonged btreatmentQ by a secondary gas. Table 1 gives examples of chemical processes for which Fig. 1 reactor may be advantageous. Fig. 2 shows an arrangement of vertical baffles impeding horizontal powder movement in Fig. 1 reactor’s supernatant space.
3. Non-fouling, non-corroding heat exchanger Fig. 1 can also be seen to constitute a non-fouling, noncorroding heat exchanger [5], the primary gas being a gas to
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Fig. 1. Vibrated-bed reactor concept catering to non-steady-state heterogeneous reaction kinetics [5]. Spouts emerging from small-bore vertical tubes [6] carry a Group A powder from its vibrated bed to the ceiling of a bsupernatantQ space wherein falling powder briefly contacts a primary gas (a breaction gasQ flowing horizontally at high rate of flow). Then, having fallen back into the vibrated bed, the powder comes into a much longer contact with a secondary gas (a btreating gasQ flowing at low rate). As section A–A illustrates, secondary gas reaches the vibrated bed via risers extending from a plenum to an elevation a little below the vibrated-bed surface. This arrangement minimizes primary gas penetration into the vibrated bed. Circles within the bed represent tubes carrying a heat-transfer medium (water, another fluid or a Group A powder acting in a system like that illustrated in Fig. 6).
be cooled. If this is relatively free of dust, as well as containing no corrosive gaseous constituent, no secondary gas need be supplied (the exchanger may omit plenum and risers that furnish this gas to the bed). Table 2, on the other hand, gives examples of hot gases difficult to cool in conventional heat exchangers, because they contain either an ultra-fine powder tending to deposit on surfaces (spoiling heat transfer) or a corroding gas, such as hydrogen chloride. For these examples, supplying an inert secondary gas protects heat-transfer surface within the vibrated bed from fouling or corrosion.
4. Flash pyrolysis of coal or other hydrocarbonaceous matter Fraas [7] proposed that flash pyrolysis of coal be conducted in concert with AFBC (fluid bed combustion at substantially atmospheric pressure). In his proposal, hot AFBC solids supply heat to the pyrolysis step; char from pyrolysis is fuel for the AFBC. A team at Virginia Polytechnic Institute and State University (bVirginia TechQ) joined Fraas in a study [8] examining his proposal in light of the advantageous reaction scene a vibrated bed offers, a scene closely meeting ideal features for coal flash pyrolysis:
prompt cooling of vapor, yielding oil and limiting the time in which oil vapor is subjected to the temperature of the pyrolysis; and minimal supply of a bforeignQ gas to the pyrolysis— preferably zero. First, slow heating of coal is undesirable: during heat-up, bregressiveQ reactions occur to an undue degree, reducing liquid yield in favor of production of a bsecondary charQ and, as well, rendering the liquid product bheavierQ (higher in molecular weight) and therefore less desirable. Second, there is evidence that char catalyzes regressive reactions in vapor held in contact therewith at temperature. Like slow heating, the contact diminishes value of an oil product and increases char yield. Third, rapid cooling avoids the regressive reactions that will occur (even in absence of contact with char) if the liquid remains at process temperature for a relatively long time.
rapid heating of coal to a temperature appropriate for pyrolysis; rapid separation of two products, vapor and char; Table 1 Examples of processes for which Fig. 1 reactor may be advantageous Reaction
Primary gas
Secondary gas
Fischer-Tropsch synthesis over iron catalyst Oxychlorination Methane oxidation
synthesis gas (H2+CO at 1:1)
hydrogen
hydrocarbon+chlorine methane
oxygen-containing gas oxygen-containing gas
Fig. 2. In the reactor of Fig. 1, vertical, V-shaped baffles impeding horizontal movement of powder. Upper diagram is a partial vertical crosssection showing suspension of baffles from the supernatant space ceiling; lower diagram, a horizontal cross-section.
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Table 2 Examples of hot gases difficult to cool by conventional indirect heat transfer Source of gas
Problem constituent
Current treatment of gas
Electric furnace producing ferrosilicon Fluid bed producing titanium oxide pigment Oxychlorination
silica fume
conventional boiler cleaned frequently by a bshowerQ of steel shot trombone cooler with an admixture of coarse matter
TiO2 product
hydrogen chloride
frequent replacement of cooling surfaces
Appendix A reviews coal chemistry applicable to the foregoing three features of an ideal coal flash pyrolysis. Fourth, supplying bforeignQ gas to the pyrolysis hurts thermal efficiency; heat required to carry the gas to pyrolysis temperature cannot be recovered. Vapor and gas from pyrolysis must be cooled far more quickly than could be accomplished by an indirect heat exchange recovering useful heat. Cooling by direct contact with a bcoldQ substance is mandatory. To give an example, in the approach studied by Occidental Research (following Sass’s lead [9]), a quench step inflicts a thermal loss amounting to ~8–9% of the feed coal’s heating value. Occidental employed a coal-contacting step in which a carrier gas conveyed coal at high velocity downward in the dilute phase together with hot char supplying heat for pyrolysis [10,11]. A cyclone separated char from vapor and gas, which were then quenched by contact with a recycle stream of cooled liquid product. Occidental’s thermal loss arises primarily from necessity to quench a large flow of gas whose only process function is to convey coal and char through the pyrolysis. Thermal loss for vibrated-bed pyrolysis (working in tandem with AFBC) can be less than 1% of the coal’s heating value. Too, heat exchangers, pumps and piping for recovering oil product are smaller; the recovery system costs less and, importantly, requires less space. The Fraas-Virginia Tech team prepared a detailed design for flash pyrolysis of moisture-and-ash-free (m.a.f.) Illinois No. 6 coal (assumed to contain 14% moisture) at a rate of 10.5 kg/s (1000 tons/day) [8]. The oil yield was taken to be 30% of the m.a.f. coal. Fig. 3 illustrates, schematically, the proposed flash-pyrolysis system; Table 3 lists flow rates. There are two vibrated beds: one for drying and preheating coal to 300 8C; a second for pyrolysis at 550 8C. Each bed was taken to be ~100 mm deep, 6 m wide and 3 m long (in direction of solid flow). The entire operation can fit into a footprint of ~611 m and a height of ~15 m. A comprehensive study yielded a capital cost (in 1989 dollars) of US$6,600,000 for this design when built concurrently with an AFBC boiler serving steam turbines generating ~60 Mw of electricity, i.e., the cost is ~US$110/kW of capacity. At a plant load factor of 80%, the estimated annual return on investment is 14.8% if coal enters the process at US$30/ton and if liquid product is valued at US$12.50/barrel. The
Fig. 3. System for flash pyrolysis of coal working in tandem with AFBC [8]. Hoppers are sized to provide for variable inventory, allowing a build-up sufficient to sustain flash pyrolysis for 15 min during an adjustment in operation of the AFBC.
simple payback time is ~7 years. Liquid product may advantageously be stored for use at a gas turbine in peakload service. Table 4 lists, very roughly, capital costs to back out one barrel per day of imported oil. Coal pyrolysis liquid produced at 80% load factor, by far, is the least expensive alternative. Even at 40% load factor, capital cost for flash pyrolysis is competitive with chain-grate or underfeed stokers. Table 3 Flows in Fraas-Virginia Tech flash coal pyrolysis system [8] Flow
Flow rate
Recycle stack gas used to lift AFBC solids Hot AFBC solids to drying-preheating vibrated bed Hot AFBC solids to flash pyrolysis bed Raw coal (m.a.f.) Steam from drying-preheating bed to AFBC Fuel gasa generated pyrolytically (to AFBC) Char to AFBC Oil productb
1.29 N m3/s 19.05 kg/s 27.2 kg/s 10.5 kg/s 1.71 kg/s 0.483 kg/s (~0.5 N m3/s) 6.867 kg/s 3.15 kg/s (~0.18 N m3/s)
Flows given here differ in a minor respect from the Fraas-Virginia Tech design [8]: here, there are no recycles of fuel gas to aerate the two vibrated beds. It can be doubted that aeration is required. a Molecular weight taken to be ~22. b Molecular weight taken to be ~400.
A.M. Squires / Powder Technology 147 (2004) 10–19 Table 4 Approximate capital costs (in 1989 dollars) to back out one barrel per day of imported oil Means for backing out imported oil
Capital cost
New supply of electricity from coal, replacing oil used for space heat at ~60% thermal efficiency Synthetic fuel from coal—either a synthetic oil feed for conventional refining or substitute natural gas or methanol or gasoline from methanol Synthetic oil via Occidental coal pyrolysis process Chain-grate coal stoker (9.5 Mw thermal at 4000 h/year) Underfeed coal stoker (4.5 Mw thermal at 2000 h/year) Fraas-Virginia Tech flash pyrolysis (at 7012.8 h/year) providing a fuel oil to gas turbines Fraas-Virginia Tech flash pyrolysis (at 3506.4 h/year)
US$100,000+ US$100,000+
US$50,000a US$14,000 US$13,000 US$6800 US$13,600
a
Basis: Occidental’s cost estimate of 1982 [11] with 30% escalation. Occidental’s process employs hydrogenation to upgrade raw pyrolysis liquid to 108 and 308 API gravity products; this step of course accounts for a large part of the capital cost. If work on Occidental’s process were to be revived, using a vibrated-bed reactor would reduce need for quench liquid roughly 10-fold, not only much improving thermal efficiency but also significantly reducing both equipment size and cost.
Fig. 4 is a schematic drawing illustrating features of a vibrated bed for flash coal pyrolysis. Gas and vapor from a pyrolysis bed move upward through risers penetrating a horizontal, well-insulated tray. Above the tray, gas and vapor encounter sprays of cold recycle oil, with condensation of oil product. Note that vapors do not encounter a cold surface until after the vapors enter the cooled space. Surfaces bseenQ by hot vapor (indicated by X’s in Fig. 4) are heated electrically to a temperature a bit higher than the
Fig. 4. Schematic diagram illustrating features of a vibrated-bed reactor for flash coal pyrolysis. Dried coal enters the reactor admixed with AFBC solids.
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pyrolysis. This feature prevents bpolymerizationQ of vapor species and lay-down of a secondary char. Experience teaches that a carbon deposit forms on a surface if this is cooler than the vapor even by a bit. Among practitioners of oil-refining arts, an ancient saying is that bcarbon begets carbonQ. A bit of carbon depositing on surfaces indicated by the X’s would lead to rapid, runaway production of additional carbon, blocking passages for flow of oil vapor to the quench zone in Fig. 4. Need for lateral mixing of solids in the vibrated bed of Fig. 4 can be met by providing a floor like that illustrated in Fig. 5. Serrations (seen in cross-section at the bottom of Fig. 5) drive granular material laterally (in directions seen at the top of Fig. 5). An attractive alternative for mixing vibrated matter in Fig. 5 is employment of bcomplex-mode vibrationQ [12,13]. This is achieved by a mechanical design, for example, that drives the bed’s vibratory motion along more than one axis, or in which the vibrating mass’s weight is unbalanced relative to a vibration axis. Such designs can create whirls, oscillations and rolling modes of motion in the vibrated powder. In the process design of Fig. 3, AFBC solids occupy roughly 70% of the vibrated bed for pyrolysis, while 30% of the bed is char. If necessary, when processing a caking coal, it can be diluted by recycling AFBC solids+char to the bed from the hopper seen beneath the pyrolysis bed in Fig. 3. It may be doubted that such a recycle will be required, since a
Fig. 5. Arrangement of optional serrations in floor of Fig. 4 reactor, creating lateral mixing of powder. Upper diagram is a floor plan, with arrows showing direction of powder motion. Lower diagram is a vertical crosssection at the floor. In the upper diagram, shading indicates the short side of a serration.
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Fig. 6. System employing a Group A powder as a heat carrier.
particle of bituminous coal, when heated binstantaneouslyQ to a pyrolysis temperature, remains btackyQ only a small fraction of a second [11]. Hot AFBC solids reaching the bed contain lime and very little carbon (generally less than 1% by weight). There is a high probability that presence of lime in the flash pyrolysis will reduce levels of sulfur in liquid product. A review of a large body of evidence [14], some of it from reports that appeared early in the 20th century, shows lime to be an active cracking catalyst, bcrackingQ sulfur away from hydrocarbonaceous matter while itself being converted to calcium sulfide. Small-scale fluid-bed coal-pyrolysis experiments in presence of AFBC solids [15] also suggested cracking activity: in presence of the solids, gas yield was less as well as yields of thiophenic, mercaptan and organicsulfide species. At a depth of 100 mm in the vibrated bed of Fig. 4, time of contact of vapor product with vibrated granular matter is ~1 s (less than the ~1–1/2 s time of contact with char in Occidental’s process). If the free-board height is ~0.6 m, the additional time at which vapor is at elevated temperature is less than 6 s. Gas-disengagement velocity from the dryingpreheating bed is ~0.24 m/s; gas velocity from the pyrolysis bed is ~0.11 m/s. Carryover of fine coal dust from the drying bed is not a problem; this dust returns to the AFBC. Higher drying-bed gas velocity is advantageous, tending to strip fines from the coal and thereby reducing carryover of fine char from pyrolysis. This is important, since a widely held opinion believes presence of fine carbon dust in a coal liquid greatly shortens the time during which it may feasibly be stored prior to use. (As well, fine solids are notoriously difficult to separate from heavy coal liquids. In addition, carryover from fluid-bed pyrolysis experiments has often frustrated attempts to upgrade liquids, for example, by catalytic hydrogenation [16].) In Occidental’s research, liquid yields declined if char-vapor contact exceeded 1–1/ 2 s and, as well, at higher ratio of char-to-coal throughputs; it was concluded [10] that char catalyzes either vapor cracking or, more probably, vapor bpolymerizationQ. The latter effect was hypothesized to relate to the aforementioned opinion that carbon limits oil storage life.
5. Use of a Group A powder as a heat carrier At sufficient vibrational intensity, vertical vibration of a Group A powder causes it to bimbibeQ gas, expanding the
powder and reducing its bviscosityQ to a low value [6]. Suppose powder were placed under pressure within a vibrating conduit. Imposition of a pressure gradient along the conduit will cause the powder to flow blike waterQ—for example, substantially obeying the Chenoweth-Martin correlation [17] for pressure drop in co-current flow of air and water. Fig. 6 illustrates, schematically, how this quality of a Group A powder can be exploited as a heat carrier in a system that conveys heat from a heat source to a heat sink. The former, for example, might be either a fluid-bed combustion or an exothermic chemical process. The latter might be an endothermic process or a boiler raising steam. The distance separating heat source and sink could be measured in kilometers. To give an example, a Group A powder losing 300 8C at 88 kg/s (350 tons/h) supplies on the order of 1300 kJ/s (~78 MMBtu/h). If the powder moves 1 km through a 300-mm pipe, an estimate of the pressure drop (one-way) is 43.3 kPa (~6.3 psi). Vibrated-bed heat exchange, for which heat transfer coefficients can be favorable [18], may be advantageous for a step in the Fig. 6 system in which the Group A powder is either heated or cooled.
Fig. 7. Counter-current heat exchange between a gravel and a Group A powder. The upper diagram is a vertical cross-section along the contrary directions of gravel and powder motion. The lower two diagrams are vertical transverse cross-sections. In the middle diagram, gravel and powder are in direct contact. In the lower diagram, heat transfer is indirect; vertical walls separate gravel and powder flows.
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The world’s petroleum industry discards each year many hundreds of tons of used fluid cracking catalyst (FCC) powder. This should be available at low cost and, accordingly, is a candidate Group A powder for employment as a heat carrier in chemical processes. Its virtues for this service, replacing molten salts, mercury vapor and condensed-ring aromatics, are obvious. FCC powder may be elevated in pressure by means of an aerated standpipe. If the necessary pressure for a particular application would require an unacceptably tall standpipe, the design might specify additional standpipes spaced at intervals along the powder-conveying duct, boosting the powder’s pressure along its path. In such an instance, direct displacement devices may also be considered (such as the Fuller-Kinyon pump, the Moyno pump and the like). Lock hoppers are another alternative. These should take the form of an improved design [19] that avoids the conventional arrangement’s bwasteQ of gas pressure and concomitant waste of energy. Conventionally, gas under pressure is vented from a hopper after it has been emptied, reducing its pressure to allow it to be filled with another batch of powder. In the improved design, water under pressure (or another suitable liquid) drives the high-pressure gas from the empty hopper, thereby retaining the empty hopper’s gas inventory for a constructive use at high pressure. Subsequently, hopper pressure is reduced and liquid vented, allowing an operator to introduce powder at low pressure into the hopper. Power to pump the gas-displacing liquid is small in comparison with power bwastedQ in the conventional arrangement, in which a comparable volume of gas must be compressed.
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6. Counter-current heat exchange between a bgravelQ and a Group A powder In a system employing a Group A powder as a heat carrier, a useful tool would be a device for countercurrent heat exchange between the powder and a coarser granular material (a bgravelQ). Fig. 7 illustrates a candidate heat-exchange device meeting this need. By virtue of the upward inclination of a duct fitted with a serrated floor, as illustrated, vibration of the duct can drive a gravel upward, while the same vibration expands the powder, rendering it fluid, thereby allowing gravity to carry it downward in a flow that is countercurrent to the upward motion of the gravel. Two cases are envisioned. In some applications, a direct heat-exchange contact between powder and gravel will be best. In other applications, when the two granular materials must be kept apart, the heat transfer can be indirect, as illustrated by the lowermost drawing in Fig. 7.
7. Flash distillation of oil shale rock Fig. 8 illustrates schematically a scheme for the flash distillation of shale oil from oil shale rock. Here, flash distillation enjoys much the same advantages [20] as those discussed earlier for the flash pyrolysis of coal. A vibrated-bed reactor for flash distillation of shale oil can exhibit the same general features as those illustrated in Fig. 4.
Fig. 8. System for flash distillation of shale oil. If attrition of the shale gravel generates a Group A powder, gravel-powder heat exchange should be indirect (see lower diagram in Fig. 7); optional powder flows indicated by the two dashed lines are for inventory control, whereby a powder discharge is essentially free of carbonaceous matter. If attrition does not generate a Group A powder, gravel-powder heat exchange may be direct (see middle diagram in Fig. 7); a suitable Group A powder must be furnished at a rate compensating for losses.
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8. Comparison, vibration or gas fluidization for treating a powder For treating a given granular material with a gas, the conventional vibrated bed, with aeration, can often enjoy an appreciable advantage over the conventional fluid bed. This arises from the fact that vibration (not an upward current of gas) creates powder mobility. Accordingly, for example, in a vibrated bed for drying service, gas flow can be set according to a requirement for heat, not (as for a fluid bed) a requirement that the flow be sufficient to buoy the powder. Vibrated-bed drying often requires a relatively small fraction of the power needed to operate a fluid-bed dryer [21], and Gutman argued [22], always requires less power.
9. Discussion Teams engaged in chemical process development can profitably consider opportunities for employing vibrated powders. Vibrated-bed installations for drying granular materials (for example, breakfast cereals) are mature products of manufacture, and teams should become familiar with the array of designs now commercially available. These include sizes of equipment at the scale required, for example, by vibrated-powder processes suggested herein: with relatively little modification, existing designs could provide a reactor affording opportunity for cyclic treatment of a powder, exploiting unsteady-state reaction kinetics; a heat exchanger for cooling a bdifficultQ hot gas (containing either a bstickyQ powder or a corroding gas); a reactor for flash coal pyrolysis or flash shale oil distillation; a pressurized duct conveying a Group A powder in service as a heat carrier in chemical processing; and apparatus for effecting heat exchange between a Group A powder and a bgravelQ. These suggestions, of course, do not exhaust the list of commercial-scale applications of vibrated powders that inventive imaginations may bring forth.
List of symbols AFBC atmospheric fluid bed combustion CFBC circulating fluid bed combustion FCC fluid catalytic cracking m.a.f. moisture-and-ash-free
Acknowledgments Vibrated powder studies at Virginia Tech were supported by U.S. Department of Energy (grant DE-FG07-831D1248), Virginia Center for Coal and Energy Research (Project No. 2057090) and National Science Foundation industry–university cooperative research grant CBT-8620244. I thank Arthur P. Fraas for many helps and stimulating discussions; Robert A. Graff for insights and data on flash pyrolysis
chemistry; Christian E. Raison, a Virginia Tech graduate student in Chemical Engineering who performed experiments relating to vibrated-bed coal flash pyrolysis; M.L. Myers, who assisted Fraas in developing an estimated capital cost for the Fraas-Virginia Tech flash coal pyrolysis system; Frederick A. Zenz, who collaborated in studies of flow of air and expanded (baeratedQ) FCC powder showing validity of the Chenowith-Martin pressure-drop correlation [17] for this mixture flowing horizontally (or upward at angles to 108); and Duayne D. Whitehurst (Mobil) for advice and ideas relating to shale oil distillation and the chemical process industry’s need for a new, more convenient heat carrier. I also thank those who supplied data, advice and stimulating questions relating to capital cost or feasibility of the Fraas-Virginia Tech pyrolysis system: R.L. Holcomb and H. Bowers of Oak Ridge National Laboratories; A. Cohn, S. Ehrlich and S. Alpert of Electric Power Research Institute; J. Selover and R. Duthy of Bechtel; C. Adams of E-Con; Hutchison of Harbison Walker Refractories Division of Dresser Industries; Espy of C-E Refractories Division of Combustion Engineering; Kelly of Alcoa Steel Fabricators; and J.R. Longanbach and T.H. Hand of Morgantown Energy Technology Center of U.S. Department of Energy.
Appendix A. Chemical background for flash coal pyrolysis That flash pyrolysis of coal, in comparison with a bslowQ coking process, yields a lighter liquid and at higher yield is readily understood. As a chemical species, coal may be likened to a cross-linked polymer—the bunitsQ, however, not comprising a few, repeating, monomers; rather, comprising a variety of chemical types. These exhibit both a distribution in molecular weight and an array of chemical functionalities. A few bunitsQ are simple hydrocarbons. More carry phenolic (acidic) functionalities. Many carry basic nitrogen functionalities and acidic groups other than phenolic. If coal is heated, it bdepolymerizesQ through rupture of bscissile bonds,Q the weakest bonds in the cross-linked structure. These are believed to be (1) hydrogen bridges, (2) simple ether linkages between aromatic structures and (3) benzylbenzyl carbon bonds—breaking in that order if coal is heated slowly [23,24]. A variety of moieties appear: a few are simple hydrocarbons; some are basic; some, acidic; some, amphoteric. Their mean molecular weight varies with the coal, but often is relatively low, between ~200 and ~500. Fig. 9 is a bpictureQ representing the reaction paths taken when coal is heated quickly [25]. Curved lines at the left of the diagram represent rapid scissile-bond cleavage, yielding prompt intermediates. These comprise a large number of distinct species forming a hierarchy of increasing chemical polarity toward the bottom of the diagram. Some of these have donor solvent properties, promoting take-up of hydrogen by their neighbors—i.e., sponsoring the bprogressiveQ
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Fig. 9. A working model of reaction paths for coal pyrolysis or donor solvent coal liquefaction [25]. In coal pyrolysis, bregressive reactionsQ (indicated by dotdash lines in the diagram) are the more significant, while in liquefaction, bprogressive reactionsQ (solid lines) predominate. In a bituminous coal, some progressive reactions come into play even in pyrolysis.
reactions indicated in Fig. 9 by unbroken lines, yielding an oil. In contrast, bregressiveQ reactions (indicated by dot-dash lines) form new cross-links by uniting basic and acidic functionalities, forming bonds far more stable than the scissile bonds initially present in the raw coal, and yielding a chemically non-reactive semi-coke. (Broadly speaking, bcharQ from a bituminous coal is a mixture of bprompt residueQ and a secondary product, semi-coke.) When a large coal particle is heated slowly, vapor formation raises its interior pressure. As vapor moves
outward, regressive reactions not only deposit semi-coke within the particle’s pores but also raise the remaining vapor’s molecular weight: stable cross-links form, converting the vapor to a relatively non-reactive heavy btarQ. Historically, studies of tar from the slow heating of coal gave researchers a false picture of coal chemistry, leading them to attribute to coal a far too low chemical reactivity. Fig. 10 summarizes data indicating yields to be expected from the vibrated bed of Fig. 4. Two data sets, from Occidental [10] and Australia’s Commonwealth Scientific
Fig. 10. Flash pyrolysis yields from bituminous coals. The X’s are data for a West Kentucky coal [10]; closed circles, for a Pittsburgh No. 8 coal [26]; triangles, for an Illinois No. 6 coal [27–29]: open, obtained from a bsemi-continuousQ experiment; closed, from a small fluid bed. In the foregoing data, vapor residence times at process temperature were controlled at a few seconds. See text for significance of the arrows. Open circles are data [30,31] with binstantaneousQ electrical heating of Pittsburgh Seam coal to the temperature indicated, coal being held at temperature for 2–10 s. In these data, there was no control of vapor residence times at temperature, which appear to have been a number of seconds (perhaps accounting for lower yields of liquid at ~650 8C).
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and Industrial Research Organization (CSIRO) [26], were obtained at a scale of many pounds per hour throughput (the best data, perhaps, on which to set one’s hopes). For pyrolysis at ~600 to ~650 8C, Occidental reported a liquid yield of about 35% by weight (1.9 barrels/ton) from a West Kentucky bituminous coal (m.a.f.). CSIRO reported similar but slightly lower yields from a Pittsburgh No. 8 bituminous coal. Other data in Fig. 10, obtained at a smaller scale by workers at City College of New York [27–29] and Massachusetts Institute of Technology [30,31], are confirmatory. Gas yields vary significantly with temperature, whether heating is slow [24] or rapid [30]. Molecular weight of gas (21.8) produced at City College by flash pyrolysis of Illinois coal (at 4.4% yield) agrees remarkably well with that of gas (22.8) produced by Occidental from West Kentucky coal (at 10.1% yield). Since liquid yields are relatively insensitive to temperature between ~480 and ~650 8C, a reasonable supposition is that the pyrolysis temperature can be adjusted to maintain a gas yield close to the hypothesized 4.4% while yet maintaining a liquid yield close to 30%. Residence time of coal appears not to be a major influence upon liquid yield (although Suuberg’s lower yields [31] for coal held at temperature for bzeroQ seconds, in comparison with coal held thereat for 2 s, demonstrate that some residence time is needed). Workers at City College of New York discovered that a 30-min bsteam soakQ at elevated pressure and a temperature around 350 8C produced a favorable alteration in the chemical nature of the coal [28,32]. (A bhelium soakQ at the same conditions had no comparable effect.) Flash pyrolysis of an Illinois No. 6 bituminous coal at atmospheric pressure after steam conditioning at 50 atm gave yields indicated by arrows in Fig. 10. Liquid yield was not perhaps so far outside scatter of other data to provide decisive evidence concerning liquid yield. What is decisive is the liquidTs lowered molecular weight: 318 versus 459 in oil from flash pyrolysis without steam conditioning. Assisted by workers at Exxon R&D, the City College of New York team concluded that the steam soak opened bscissileQ arylaryl ether bonds, with a concomitant increase in presence of hydroxyls [33]. The City College workers also conducted exploratory studies of a 15-min steam soak at atmospheric pressure [32]. Unfortunately for the present account, the studies did not include flash pyrolysis of the conditioned coal. For judging the conditioning’s chemical effect, pyridine extraction was both faster and easier. Steam-conditioning at atmospheric pressure increased yield of pyridine extract by ~40% (versus ~75% for steam-conditioning at 50 atm). Studies of donorsolvent coal liquefaction [34] provided further confirmation of a helpful chemical effect from steam treatment. An effort to develop the flash pyrolysis process of Fig. 3 should consider placing chambers for steam-conditioning between coal drying and flash pyrolysis. If further work
should show that a steam soak at high pressure is worth the extra cost, the chambers could take the form of lock hoppers. Two other modifications need consideration [8]. Vapor from flash pyrolysis could be treated by a donor-solvent vapor. Such a vapor could either aerate the vibrated bed of Fig. 4 or be injected into the bed’s free-board. A btrayQ housing a vibrated bed of a cracking catalyst could be interposed between the pyrolysis bed and vapor condensation.
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D. Geldart, Powder Technol. 7 (1973) 285. A.M. Squires, M. Kwauk, A.A. Avidan, Science 230 (1985) 1329. J.R. Grace, Can. J. Chem. Eng. 64 (1986) 353. A.M. Squires, Powder Technol. 147 (2004) 1. A.M. Squires, U.S. Patent 6,619,383 (September 16, 2003). B. Thomas, M.O. Mason, A.M. Squires, Powder Technol. 111 (2000) 34. A.P. Fraas, Production of motor fuel and methane via a flash pyrolysis process closely coupled to a fluidized bed utility steam plant, ASME Paper No. 80-WA/Fu-7 (November 1980). A.P. Fraas, A.M. Squires, B. Thomas, Circulating vibrated bed working in cooperation with an AFBC, Final Report of Phase I, Small Business Innovation Research Award from U.S. Department of Energy, Contract No. DE-AC05-88ER80629 (March 15, 1989). A. Sass, The Garrett research and development process for the conversion of coal into liquid fuels, Paper presented at 65th annual AIChE meeting, New York (November 1972), Chem. Eng. Prog. 70 (1974) 72. S.C. Che, E.W. Durai-Swamy, E.W. Knell, C.K. Lee, Flash pyrolysis coal liquefaction: process development. Final Report, Occidental Research Corporation (Irvine CA) to U.S. Department of Energy (July 1976–June 1978), Contract No. EX76-C-01-2244 (1979). K. Durai-Swami, S.C. Che, C.B. Chen, W. Deslate, S.S. Kim, Controlled flash pyrolysis, Final Report, Occidental Research Corporation (Irvine, CA) to U.S. Department of Energy (30 September 1980– 30 June 1982), Contract No. AC22-80PC30264 [DOE/PC/30264-21 (DE83005086)] (1982). A.P. Fraas, Mech. Eng. 120 (1) (1998) 76. Incisive Engineering, (A.P. Fraas), Density separation in complexmode vibration-fluidized beds, Final report, Federal Grant No. DEFG36-98GP10308, January 31, 2001. A.M. Squires, Fuel gasification, Adv. Chem. Ser. 69 (1967) 205. Y.D. Yeboah, J.P. Longwell, J.B. Howard, W.A. Peters, Ind. Eng. Chem. Process Des. Dev. 10 (1980) 646. EPRI, Coal pyrolysis: technical brief RP 2505, Electric Power Research Institute, Palo Alto, CA, 1987. a. J.M. Chenoweth, M.W. Martin, Petrol. Refiner 34 (10) (1955) 151; b. J.M. Chenoweth, M.W. Martin, Pet. Eng. 28 (4) (1956) C42. B. Thomas, M.O. Mason, R. Sprung, Y.A. Liu, A.M. Squires, Powder Technol. 99 (1998) 293. A.M. Squires, U.S. Patent 3,719,192 (March 6, 1973). D.D. Whitehurst, Personal communication. V.A. Chlenov, N.V. Nikhailov, Vibrofluidized Beds, Nauka, Moscow, USSR, 1972. R.G. Gutman, PhD thesis, University of Cambridge, Cambridge, United Kingdom (1974). H.R. Brown, P.L. Waters, Fuel (London) 45 (1966)1967 (17, 41). N.Y. Kirov, J.N. Stephens, Physical Aspects of Coal Carbonisation, Department of Fuel Technology, University of New South Wales, Sydney, Australia, 1967.
A.M. Squires / Powder Technology 147 (2004) 10–19 [25] A.M. Squires, Appl. Energy 4 (1978) 161. [26] I.W. Smith, New approaches to coal pyrolysis—CSIRO, Paper at EPRI Conference on Coal Pyrolysis, Palo Alto, CA (February 1981). [27] R.A. Graff, Status Report No. SR19 (for period 1 to 31 July), U.S. Department of Energy Contract DE-AC21-87MC23288 (1988). [28] R.A. Graff, S.D. Brandes, J. Energy Fuels 1 (1987) 84. [29] R.A. Graff, P. Zhou, S.D. Brandes, Techniques for coal gasification, Final Report (26 September 1984 to 25 September 1986), U.S. Department of Energy Contract DE-AC21-87MC23288 (1987). [30] J.B. Howard, Coal Pyrolysis—Product Yields and Properties: Survey, paper at EPRI Conference on Coal Pyrolysis, Palo Alto CA (February) 1981.
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[31] E.M. Suuberg, ScD thesis, Massachusetts Institute of Technology, 1977. [32] R.A. Graff, P. Zhou, S.D. Brandes, Steam conditioning of coal for synfuels production, Third Quarterly Report (for period 1 July to 30 September), U.S. Department of Energy Contract No. DE-AC2187MC23288 (1987). [33] S.D. Brandes, R.A. Graff, M.L. Gorbaty, M. Siskin, J. Energy Fuels 3 (1989) 494. [34] O. Ivanenko, R.A. Graff, V. Balogh-Nair, C. Brathwaite, Prepr. - Am. Chem. Soc., Div. Fuel Chem. 4 (2) (1996) 700.
Perspective
Chemical process opportunities for vibrated powders 2. In the field A.M. Squires* Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg VA 24061-0211, USA Received 18 July 2003; received in revised form 13 July 2004; accepted 13 July 2004 Available online 22 September 2004
Abstract Commercial opportunities are seen for chemical process applications employing vibrated powders: in reactors catering to non-steady-state gas–solid reaction kinetics; in non-fouling, non-corroding heat exchangers; in flash pyrolysis of coal working in tandem with fluid-bed combustion, yielding char for the combustion and a light oil useful (with little or no further treatment), for example, in a gas turbine generating peak-load power; in a system employing a Geldart Group A powder [D. Geldart, Powder Technol. 7 (1973), 285]; to carry heat over a relatively long distance (replacing less convenient heat carriers, such as mercury, molten salts and condensed-ring hydrocarbons); in counter-current heat exchange between a bgravelQ and a Group A powder; in flash distillation of shale oil. D 2004 Elsevier B.V. All rights reserved. Keywords: Vibrated bed; Non-steady-state heterogeneous reaction kinetics; Non-fouling; Non-corroding heat exchange; Flash coal pyrolysis; Flash shale oil distillation; Coal; Shale oil rock; Heat carrier; Fluid-bed combustion; Peak-load power
1. Introduction A first paper of a series on opportunities for vibrated powders [4] dealt with vibrated-bed microreactors, useful in the laboratory for simulating large-scale fluid bed reactors of several kinds. This second paper deals with opportunities for conducting chemical processes at the commercial scale in relatively conventional vibrated-bed equipment (or straightforward modifications thereof), requiring little new mechanical development.
2. Reactor for non-steady-state heterogeneous reaction kinetics Fig. 1 illustrates schematically a proposed reactor catering to non-steady-state heterogeneous reaction kinetics. The first paper of this series [4] described a bsliding-baffleQ vibratedbed microreactor capable of studying kinetics of a reaction * Tel.: +1 540 231 5972; fax: +1 540 231 5022. E-mail address: [email protected]. 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.07.013
regime wherein a powder (reactant or catalyst) bseesQ a primary gas for a brief interval and, in alternation, experiences a much longer bsoakQ in a secondary gas. The first gas may comprise a breaction gasQ supplied at high rate of flow; the second, a btreating gasQ, at low rate. Fig. 1 reactor [5] submits a Geldart Group A powder [1–3] to a regime of this nature. Spouts [6] carry the powder from a vibrated bed thereof into a bsupernatantQ space wherein the powder briefly encounters a horizontal flow of a primary gas; gravity returns the powder to its vibrated bed, in which it experiences a relatively prolonged btreatmentQ by a secondary gas. Table 1 gives examples of chemical processes for which Fig. 1 reactor may be advantageous. Fig. 2 shows an arrangement of vertical baffles impeding horizontal powder movement in Fig. 1 reactor’s supernatant space.
3. Non-fouling, non-corroding heat exchanger Fig. 1 can also be seen to constitute a non-fouling, noncorroding heat exchanger [5], the primary gas being a gas to
A.M. Squires / Powder Technology 147 (2004) 10–19
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Fig. 1. Vibrated-bed reactor concept catering to non-steady-state heterogeneous reaction kinetics [5]. Spouts emerging from small-bore vertical tubes [6] carry a Group A powder from its vibrated bed to the ceiling of a bsupernatantQ space wherein falling powder briefly contacts a primary gas (a breaction gasQ flowing horizontally at high rate of flow). Then, having fallen back into the vibrated bed, the powder comes into a much longer contact with a secondary gas (a btreating gasQ flowing at low rate). As section A–A illustrates, secondary gas reaches the vibrated bed via risers extending from a plenum to an elevation a little below the vibrated-bed surface. This arrangement minimizes primary gas penetration into the vibrated bed. Circles within the bed represent tubes carrying a heat-transfer medium (water, another fluid or a Group A powder acting in a system like that illustrated in Fig. 6).
be cooled. If this is relatively free of dust, as well as containing no corrosive gaseous constituent, no secondary gas need be supplied (the exchanger may omit plenum and risers that furnish this gas to the bed). Table 2, on the other hand, gives examples of hot gases difficult to cool in conventional heat exchangers, because they contain either an ultra-fine powder tending to deposit on surfaces (spoiling heat transfer) or a corroding gas, such as hydrogen chloride. For these examples, supplying an inert secondary gas protects heat-transfer surface within the vibrated bed from fouling or corrosion.
4. Flash pyrolysis of coal or other hydrocarbonaceous matter Fraas [7] proposed that flash pyrolysis of coal be conducted in concert with AFBC (fluid bed combustion at substantially atmospheric pressure). In his proposal, hot AFBC solids supply heat to the pyrolysis step; char from pyrolysis is fuel for the AFBC. A team at Virginia Polytechnic Institute and State University (bVirginia TechQ) joined Fraas in a study [8] examining his proposal in light of the advantageous reaction scene a vibrated bed offers, a scene closely meeting ideal features for coal flash pyrolysis:
prompt cooling of vapor, yielding oil and limiting the time in which oil vapor is subjected to the temperature of the pyrolysis; and minimal supply of a bforeignQ gas to the pyrolysis— preferably zero. First, slow heating of coal is undesirable: during heat-up, bregressiveQ reactions occur to an undue degree, reducing liquid yield in favor of production of a bsecondary charQ and, as well, rendering the liquid product bheavierQ (higher in molecular weight) and therefore less desirable. Second, there is evidence that char catalyzes regressive reactions in vapor held in contact therewith at temperature. Like slow heating, the contact diminishes value of an oil product and increases char yield. Third, rapid cooling avoids the regressive reactions that will occur (even in absence of contact with char) if the liquid remains at process temperature for a relatively long time.
rapid heating of coal to a temperature appropriate for pyrolysis; rapid separation of two products, vapor and char; Table 1 Examples of processes for which Fig. 1 reactor may be advantageous Reaction
Primary gas
Secondary gas
Fischer-Tropsch synthesis over iron catalyst Oxychlorination Methane oxidation
synthesis gas (H2+CO at 1:1)
hydrogen
hydrocarbon+chlorine methane
oxygen-containing gas oxygen-containing gas
Fig. 2. In the reactor of Fig. 1, vertical, V-shaped baffles impeding horizontal movement of powder. Upper diagram is a partial vertical crosssection showing suspension of baffles from the supernatant space ceiling; lower diagram, a horizontal cross-section.
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A.M. Squires / Powder Technology 147 (2004) 10–19
Table 2 Examples of hot gases difficult to cool by conventional indirect heat transfer Source of gas
Problem constituent
Current treatment of gas
Electric furnace producing ferrosilicon Fluid bed producing titanium oxide pigment Oxychlorination
silica fume
conventional boiler cleaned frequently by a bshowerQ of steel shot trombone cooler with an admixture of coarse matter
TiO2 product
hydrogen chloride
frequent replacement of cooling surfaces
Appendix A reviews coal chemistry applicable to the foregoing three features of an ideal coal flash pyrolysis. Fourth, supplying bforeignQ gas to the pyrolysis hurts thermal efficiency; heat required to carry the gas to pyrolysis temperature cannot be recovered. Vapor and gas from pyrolysis must be cooled far more quickly than could be accomplished by an indirect heat exchange recovering useful heat. Cooling by direct contact with a bcoldQ substance is mandatory. To give an example, in the approach studied by Occidental Research (following Sass’s lead [9]), a quench step inflicts a thermal loss amounting to ~8–9% of the feed coal’s heating value. Occidental employed a coal-contacting step in which a carrier gas conveyed coal at high velocity downward in the dilute phase together with hot char supplying heat for pyrolysis [10,11]. A cyclone separated char from vapor and gas, which were then quenched by contact with a recycle stream of cooled liquid product. Occidental’s thermal loss arises primarily from necessity to quench a large flow of gas whose only process function is to convey coal and char through the pyrolysis. Thermal loss for vibrated-bed pyrolysis (working in tandem with AFBC) can be less than 1% of the coal’s heating value. Too, heat exchangers, pumps and piping for recovering oil product are smaller; the recovery system costs less and, importantly, requires less space. The Fraas-Virginia Tech team prepared a detailed design for flash pyrolysis of moisture-and-ash-free (m.a.f.) Illinois No. 6 coal (assumed to contain 14% moisture) at a rate of 10.5 kg/s (1000 tons/day) [8]. The oil yield was taken to be 30% of the m.a.f. coal. Fig. 3 illustrates, schematically, the proposed flash-pyrolysis system; Table 3 lists flow rates. There are two vibrated beds: one for drying and preheating coal to 300 8C; a second for pyrolysis at 550 8C. Each bed was taken to be ~100 mm deep, 6 m wide and 3 m long (in direction of solid flow). The entire operation can fit into a footprint of ~611 m and a height of ~15 m. A comprehensive study yielded a capital cost (in 1989 dollars) of US$6,600,000 for this design when built concurrently with an AFBC boiler serving steam turbines generating ~60 Mw of electricity, i.e., the cost is ~US$110/kW of capacity. At a plant load factor of 80%, the estimated annual return on investment is 14.8% if coal enters the process at US$30/ton and if liquid product is valued at US$12.50/barrel. The
Fig. 3. System for flash pyrolysis of coal working in tandem with AFBC [8]. Hoppers are sized to provide for variable inventory, allowing a build-up sufficient to sustain flash pyrolysis for 15 min during an adjustment in operation of the AFBC.
simple payback time is ~7 years. Liquid product may advantageously be stored for use at a gas turbine in peakload service. Table 4 lists, very roughly, capital costs to back out one barrel per day of imported oil. Coal pyrolysis liquid produced at 80% load factor, by far, is the least expensive alternative. Even at 40% load factor, capital cost for flash pyrolysis is competitive with chain-grate or underfeed stokers. Table 3 Flows in Fraas-Virginia Tech flash coal pyrolysis system [8] Flow
Flow rate
Recycle stack gas used to lift AFBC solids Hot AFBC solids to drying-preheating vibrated bed Hot AFBC solids to flash pyrolysis bed Raw coal (m.a.f.) Steam from drying-preheating bed to AFBC Fuel gasa generated pyrolytically (to AFBC) Char to AFBC Oil productb
1.29 N m3/s 19.05 kg/s 27.2 kg/s 10.5 kg/s 1.71 kg/s 0.483 kg/s (~0.5 N m3/s) 6.867 kg/s 3.15 kg/s (~0.18 N m3/s)
Flows given here differ in a minor respect from the Fraas-Virginia Tech design [8]: here, there are no recycles of fuel gas to aerate the two vibrated beds. It can be doubted that aeration is required. a Molecular weight taken to be ~22. b Molecular weight taken to be ~400.
A.M. Squires / Powder Technology 147 (2004) 10–19 Table 4 Approximate capital costs (in 1989 dollars) to back out one barrel per day of imported oil Means for backing out imported oil
Capital cost
New supply of electricity from coal, replacing oil used for space heat at ~60% thermal efficiency Synthetic fuel from coal—either a synthetic oil feed for conventional refining or substitute natural gas or methanol or gasoline from methanol Synthetic oil via Occidental coal pyrolysis process Chain-grate coal stoker (9.5 Mw thermal at 4000 h/year) Underfeed coal stoker (4.5 Mw thermal at 2000 h/year) Fraas-Virginia Tech flash pyrolysis (at 7012.8 h/year) providing a fuel oil to gas turbines Fraas-Virginia Tech flash pyrolysis (at 3506.4 h/year)
US$100,000+ US$100,000+
US$50,000a US$14,000 US$13,000 US$6800 US$13,600
a
Basis: Occidental’s cost estimate of 1982 [11] with 30% escalation. Occidental’s process employs hydrogenation to upgrade raw pyrolysis liquid to 108 and 308 API gravity products; this step of course accounts for a large part of the capital cost. If work on Occidental’s process were to be revived, using a vibrated-bed reactor would reduce need for quench liquid roughly 10-fold, not only much improving thermal efficiency but also significantly reducing both equipment size and cost.
Fig. 4 is a schematic drawing illustrating features of a vibrated bed for flash coal pyrolysis. Gas and vapor from a pyrolysis bed move upward through risers penetrating a horizontal, well-insulated tray. Above the tray, gas and vapor encounter sprays of cold recycle oil, with condensation of oil product. Note that vapors do not encounter a cold surface until after the vapors enter the cooled space. Surfaces bseenQ by hot vapor (indicated by X’s in Fig. 4) are heated electrically to a temperature a bit higher than the
Fig. 4. Schematic diagram illustrating features of a vibrated-bed reactor for flash coal pyrolysis. Dried coal enters the reactor admixed with AFBC solids.
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pyrolysis. This feature prevents bpolymerizationQ of vapor species and lay-down of a secondary char. Experience teaches that a carbon deposit forms on a surface if this is cooler than the vapor even by a bit. Among practitioners of oil-refining arts, an ancient saying is that bcarbon begets carbonQ. A bit of carbon depositing on surfaces indicated by the X’s would lead to rapid, runaway production of additional carbon, blocking passages for flow of oil vapor to the quench zone in Fig. 4. Need for lateral mixing of solids in the vibrated bed of Fig. 4 can be met by providing a floor like that illustrated in Fig. 5. Serrations (seen in cross-section at the bottom of Fig. 5) drive granular material laterally (in directions seen at the top of Fig. 5). An attractive alternative for mixing vibrated matter in Fig. 5 is employment of bcomplex-mode vibrationQ [12,13]. This is achieved by a mechanical design, for example, that drives the bed’s vibratory motion along more than one axis, or in which the vibrating mass’s weight is unbalanced relative to a vibration axis. Such designs can create whirls, oscillations and rolling modes of motion in the vibrated powder. In the process design of Fig. 3, AFBC solids occupy roughly 70% of the vibrated bed for pyrolysis, while 30% of the bed is char. If necessary, when processing a caking coal, it can be diluted by recycling AFBC solids+char to the bed from the hopper seen beneath the pyrolysis bed in Fig. 3. It may be doubted that such a recycle will be required, since a
Fig. 5. Arrangement of optional serrations in floor of Fig. 4 reactor, creating lateral mixing of powder. Upper diagram is a floor plan, with arrows showing direction of powder motion. Lower diagram is a vertical crosssection at the floor. In the upper diagram, shading indicates the short side of a serration.
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A.M. Squires / Powder Technology 147 (2004) 10–19
Fig. 6. System employing a Group A powder as a heat carrier.
particle of bituminous coal, when heated binstantaneouslyQ to a pyrolysis temperature, remains btackyQ only a small fraction of a second [11]. Hot AFBC solids reaching the bed contain lime and very little carbon (generally less than 1% by weight). There is a high probability that presence of lime in the flash pyrolysis will reduce levels of sulfur in liquid product. A review of a large body of evidence [14], some of it from reports that appeared early in the 20th century, shows lime to be an active cracking catalyst, bcrackingQ sulfur away from hydrocarbonaceous matter while itself being converted to calcium sulfide. Small-scale fluid-bed coal-pyrolysis experiments in presence of AFBC solids [15] also suggested cracking activity: in presence of the solids, gas yield was less as well as yields of thiophenic, mercaptan and organicsulfide species. At a depth of 100 mm in the vibrated bed of Fig. 4, time of contact of vapor product with vibrated granular matter is ~1 s (less than the ~1–1/2 s time of contact with char in Occidental’s process). If the free-board height is ~0.6 m, the additional time at which vapor is at elevated temperature is less than 6 s. Gas-disengagement velocity from the dryingpreheating bed is ~0.24 m/s; gas velocity from the pyrolysis bed is ~0.11 m/s. Carryover of fine coal dust from the drying bed is not a problem; this dust returns to the AFBC. Higher drying-bed gas velocity is advantageous, tending to strip fines from the coal and thereby reducing carryover of fine char from pyrolysis. This is important, since a widely held opinion believes presence of fine carbon dust in a coal liquid greatly shortens the time during which it may feasibly be stored prior to use. (As well, fine solids are notoriously difficult to separate from heavy coal liquids. In addition, carryover from fluid-bed pyrolysis experiments has often frustrated attempts to upgrade liquids, for example, by catalytic hydrogenation [16].) In Occidental’s research, liquid yields declined if char-vapor contact exceeded 1–1/ 2 s and, as well, at higher ratio of char-to-coal throughputs; it was concluded [10] that char catalyzes either vapor cracking or, more probably, vapor bpolymerizationQ. The latter effect was hypothesized to relate to the aforementioned opinion that carbon limits oil storage life.
5. Use of a Group A powder as a heat carrier At sufficient vibrational intensity, vertical vibration of a Group A powder causes it to bimbibeQ gas, expanding the
powder and reducing its bviscosityQ to a low value [6]. Suppose powder were placed under pressure within a vibrating conduit. Imposition of a pressure gradient along the conduit will cause the powder to flow blike waterQ—for example, substantially obeying the Chenoweth-Martin correlation [17] for pressure drop in co-current flow of air and water. Fig. 6 illustrates, schematically, how this quality of a Group A powder can be exploited as a heat carrier in a system that conveys heat from a heat source to a heat sink. The former, for example, might be either a fluid-bed combustion or an exothermic chemical process. The latter might be an endothermic process or a boiler raising steam. The distance separating heat source and sink could be measured in kilometers. To give an example, a Group A powder losing 300 8C at 88 kg/s (350 tons/h) supplies on the order of 1300 kJ/s (~78 MMBtu/h). If the powder moves 1 km through a 300-mm pipe, an estimate of the pressure drop (one-way) is 43.3 kPa (~6.3 psi). Vibrated-bed heat exchange, for which heat transfer coefficients can be favorable [18], may be advantageous for a step in the Fig. 6 system in which the Group A powder is either heated or cooled.
Fig. 7. Counter-current heat exchange between a gravel and a Group A powder. The upper diagram is a vertical cross-section along the contrary directions of gravel and powder motion. The lower two diagrams are vertical transverse cross-sections. In the middle diagram, gravel and powder are in direct contact. In the lower diagram, heat transfer is indirect; vertical walls separate gravel and powder flows.
A.M. Squires / Powder Technology 147 (2004) 10–19
The world’s petroleum industry discards each year many hundreds of tons of used fluid cracking catalyst (FCC) powder. This should be available at low cost and, accordingly, is a candidate Group A powder for employment as a heat carrier in chemical processes. Its virtues for this service, replacing molten salts, mercury vapor and condensed-ring aromatics, are obvious. FCC powder may be elevated in pressure by means of an aerated standpipe. If the necessary pressure for a particular application would require an unacceptably tall standpipe, the design might specify additional standpipes spaced at intervals along the powder-conveying duct, boosting the powder’s pressure along its path. In such an instance, direct displacement devices may also be considered (such as the Fuller-Kinyon pump, the Moyno pump and the like). Lock hoppers are another alternative. These should take the form of an improved design [19] that avoids the conventional arrangement’s bwasteQ of gas pressure and concomitant waste of energy. Conventionally, gas under pressure is vented from a hopper after it has been emptied, reducing its pressure to allow it to be filled with another batch of powder. In the improved design, water under pressure (or another suitable liquid) drives the high-pressure gas from the empty hopper, thereby retaining the empty hopper’s gas inventory for a constructive use at high pressure. Subsequently, hopper pressure is reduced and liquid vented, allowing an operator to introduce powder at low pressure into the hopper. Power to pump the gas-displacing liquid is small in comparison with power bwastedQ in the conventional arrangement, in which a comparable volume of gas must be compressed.
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6. Counter-current heat exchange between a bgravelQ and a Group A powder In a system employing a Group A powder as a heat carrier, a useful tool would be a device for countercurrent heat exchange between the powder and a coarser granular material (a bgravelQ). Fig. 7 illustrates a candidate heat-exchange device meeting this need. By virtue of the upward inclination of a duct fitted with a serrated floor, as illustrated, vibration of the duct can drive a gravel upward, while the same vibration expands the powder, rendering it fluid, thereby allowing gravity to carry it downward in a flow that is countercurrent to the upward motion of the gravel. Two cases are envisioned. In some applications, a direct heat-exchange contact between powder and gravel will be best. In other applications, when the two granular materials must be kept apart, the heat transfer can be indirect, as illustrated by the lowermost drawing in Fig. 7.
7. Flash distillation of oil shale rock Fig. 8 illustrates schematically a scheme for the flash distillation of shale oil from oil shale rock. Here, flash distillation enjoys much the same advantages [20] as those discussed earlier for the flash pyrolysis of coal. A vibrated-bed reactor for flash distillation of shale oil can exhibit the same general features as those illustrated in Fig. 4.
Fig. 8. System for flash distillation of shale oil. If attrition of the shale gravel generates a Group A powder, gravel-powder heat exchange should be indirect (see lower diagram in Fig. 7); optional powder flows indicated by the two dashed lines are for inventory control, whereby a powder discharge is essentially free of carbonaceous matter. If attrition does not generate a Group A powder, gravel-powder heat exchange may be direct (see middle diagram in Fig. 7); a suitable Group A powder must be furnished at a rate compensating for losses.
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8. Comparison, vibration or gas fluidization for treating a powder For treating a given granular material with a gas, the conventional vibrated bed, with aeration, can often enjoy an appreciable advantage over the conventional fluid bed. This arises from the fact that vibration (not an upward current of gas) creates powder mobility. Accordingly, for example, in a vibrated bed for drying service, gas flow can be set according to a requirement for heat, not (as for a fluid bed) a requirement that the flow be sufficient to buoy the powder. Vibrated-bed drying often requires a relatively small fraction of the power needed to operate a fluid-bed dryer [21], and Gutman argued [22], always requires less power.
9. Discussion Teams engaged in chemical process development can profitably consider opportunities for employing vibrated powders. Vibrated-bed installations for drying granular materials (for example, breakfast cereals) are mature products of manufacture, and teams should become familiar with the array of designs now commercially available. These include sizes of equipment at the scale required, for example, by vibrated-powder processes suggested herein: with relatively little modification, existing designs could provide a reactor affording opportunity for cyclic treatment of a powder, exploiting unsteady-state reaction kinetics; a heat exchanger for cooling a bdifficultQ hot gas (containing either a bstickyQ powder or a corroding gas); a reactor for flash coal pyrolysis or flash shale oil distillation; a pressurized duct conveying a Group A powder in service as a heat carrier in chemical processing; and apparatus for effecting heat exchange between a Group A powder and a bgravelQ. These suggestions, of course, do not exhaust the list of commercial-scale applications of vibrated powders that inventive imaginations may bring forth.
List of symbols AFBC atmospheric fluid bed combustion CFBC circulating fluid bed combustion FCC fluid catalytic cracking m.a.f. moisture-and-ash-free
Acknowledgments Vibrated powder studies at Virginia Tech were supported by U.S. Department of Energy (grant DE-FG07-831D1248), Virginia Center for Coal and Energy Research (Project No. 2057090) and National Science Foundation industry–university cooperative research grant CBT-8620244. I thank Arthur P. Fraas for many helps and stimulating discussions; Robert A. Graff for insights and data on flash pyrolysis
chemistry; Christian E. Raison, a Virginia Tech graduate student in Chemical Engineering who performed experiments relating to vibrated-bed coal flash pyrolysis; M.L. Myers, who assisted Fraas in developing an estimated capital cost for the Fraas-Virginia Tech flash coal pyrolysis system; Frederick A. Zenz, who collaborated in studies of flow of air and expanded (baeratedQ) FCC powder showing validity of the Chenowith-Martin pressure-drop correlation [17] for this mixture flowing horizontally (or upward at angles to 108); and Duayne D. Whitehurst (Mobil) for advice and ideas relating to shale oil distillation and the chemical process industry’s need for a new, more convenient heat carrier. I also thank those who supplied data, advice and stimulating questions relating to capital cost or feasibility of the Fraas-Virginia Tech pyrolysis system: R.L. Holcomb and H. Bowers of Oak Ridge National Laboratories; A. Cohn, S. Ehrlich and S. Alpert of Electric Power Research Institute; J. Selover and R. Duthy of Bechtel; C. Adams of E-Con; Hutchison of Harbison Walker Refractories Division of Dresser Industries; Espy of C-E Refractories Division of Combustion Engineering; Kelly of Alcoa Steel Fabricators; and J.R. Longanbach and T.H. Hand of Morgantown Energy Technology Center of U.S. Department of Energy.
Appendix A. Chemical background for flash coal pyrolysis That flash pyrolysis of coal, in comparison with a bslowQ coking process, yields a lighter liquid and at higher yield is readily understood. As a chemical species, coal may be likened to a cross-linked polymer—the bunitsQ, however, not comprising a few, repeating, monomers; rather, comprising a variety of chemical types. These exhibit both a distribution in molecular weight and an array of chemical functionalities. A few bunitsQ are simple hydrocarbons. More carry phenolic (acidic) functionalities. Many carry basic nitrogen functionalities and acidic groups other than phenolic. If coal is heated, it bdepolymerizesQ through rupture of bscissile bonds,Q the weakest bonds in the cross-linked structure. These are believed to be (1) hydrogen bridges, (2) simple ether linkages between aromatic structures and (3) benzylbenzyl carbon bonds—breaking in that order if coal is heated slowly [23,24]. A variety of moieties appear: a few are simple hydrocarbons; some are basic; some, acidic; some, amphoteric. Their mean molecular weight varies with the coal, but often is relatively low, between ~200 and ~500. Fig. 9 is a bpictureQ representing the reaction paths taken when coal is heated quickly [25]. Curved lines at the left of the diagram represent rapid scissile-bond cleavage, yielding prompt intermediates. These comprise a large number of distinct species forming a hierarchy of increasing chemical polarity toward the bottom of the diagram. Some of these have donor solvent properties, promoting take-up of hydrogen by their neighbors—i.e., sponsoring the bprogressiveQ
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Fig. 9. A working model of reaction paths for coal pyrolysis or donor solvent coal liquefaction [25]. In coal pyrolysis, bregressive reactionsQ (indicated by dotdash lines in the diagram) are the more significant, while in liquefaction, bprogressive reactionsQ (solid lines) predominate. In a bituminous coal, some progressive reactions come into play even in pyrolysis.
reactions indicated in Fig. 9 by unbroken lines, yielding an oil. In contrast, bregressiveQ reactions (indicated by dot-dash lines) form new cross-links by uniting basic and acidic functionalities, forming bonds far more stable than the scissile bonds initially present in the raw coal, and yielding a chemically non-reactive semi-coke. (Broadly speaking, bcharQ from a bituminous coal is a mixture of bprompt residueQ and a secondary product, semi-coke.) When a large coal particle is heated slowly, vapor formation raises its interior pressure. As vapor moves
outward, regressive reactions not only deposit semi-coke within the particle’s pores but also raise the remaining vapor’s molecular weight: stable cross-links form, converting the vapor to a relatively non-reactive heavy btarQ. Historically, studies of tar from the slow heating of coal gave researchers a false picture of coal chemistry, leading them to attribute to coal a far too low chemical reactivity. Fig. 10 summarizes data indicating yields to be expected from the vibrated bed of Fig. 4. Two data sets, from Occidental [10] and Australia’s Commonwealth Scientific
Fig. 10. Flash pyrolysis yields from bituminous coals. The X’s are data for a West Kentucky coal [10]; closed circles, for a Pittsburgh No. 8 coal [26]; triangles, for an Illinois No. 6 coal [27–29]: open, obtained from a bsemi-continuousQ experiment; closed, from a small fluid bed. In the foregoing data, vapor residence times at process temperature were controlled at a few seconds. See text for significance of the arrows. Open circles are data [30,31] with binstantaneousQ electrical heating of Pittsburgh Seam coal to the temperature indicated, coal being held at temperature for 2–10 s. In these data, there was no control of vapor residence times at temperature, which appear to have been a number of seconds (perhaps accounting for lower yields of liquid at ~650 8C).
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A.M. Squires / Powder Technology 147 (2004) 10–19
and Industrial Research Organization (CSIRO) [26], were obtained at a scale of many pounds per hour throughput (the best data, perhaps, on which to set one’s hopes). For pyrolysis at ~600 to ~650 8C, Occidental reported a liquid yield of about 35% by weight (1.9 barrels/ton) from a West Kentucky bituminous coal (m.a.f.). CSIRO reported similar but slightly lower yields from a Pittsburgh No. 8 bituminous coal. Other data in Fig. 10, obtained at a smaller scale by workers at City College of New York [27–29] and Massachusetts Institute of Technology [30,31], are confirmatory. Gas yields vary significantly with temperature, whether heating is slow [24] or rapid [30]. Molecular weight of gas (21.8) produced at City College by flash pyrolysis of Illinois coal (at 4.4% yield) agrees remarkably well with that of gas (22.8) produced by Occidental from West Kentucky coal (at 10.1% yield). Since liquid yields are relatively insensitive to temperature between ~480 and ~650 8C, a reasonable supposition is that the pyrolysis temperature can be adjusted to maintain a gas yield close to the hypothesized 4.4% while yet maintaining a liquid yield close to 30%. Residence time of coal appears not to be a major influence upon liquid yield (although Suuberg’s lower yields [31] for coal held at temperature for bzeroQ seconds, in comparison with coal held thereat for 2 s, demonstrate that some residence time is needed). Workers at City College of New York discovered that a 30-min bsteam soakQ at elevated pressure and a temperature around 350 8C produced a favorable alteration in the chemical nature of the coal [28,32]. (A bhelium soakQ at the same conditions had no comparable effect.) Flash pyrolysis of an Illinois No. 6 bituminous coal at atmospheric pressure after steam conditioning at 50 atm gave yields indicated by arrows in Fig. 10. Liquid yield was not perhaps so far outside scatter of other data to provide decisive evidence concerning liquid yield. What is decisive is the liquidTs lowered molecular weight: 318 versus 459 in oil from flash pyrolysis without steam conditioning. Assisted by workers at Exxon R&D, the City College of New York team concluded that the steam soak opened bscissileQ arylaryl ether bonds, with a concomitant increase in presence of hydroxyls [33]. The City College workers also conducted exploratory studies of a 15-min steam soak at atmospheric pressure [32]. Unfortunately for the present account, the studies did not include flash pyrolysis of the conditioned coal. For judging the conditioning’s chemical effect, pyridine extraction was both faster and easier. Steam-conditioning at atmospheric pressure increased yield of pyridine extract by ~40% (versus ~75% for steam-conditioning at 50 atm). Studies of donorsolvent coal liquefaction [34] provided further confirmation of a helpful chemical effect from steam treatment. An effort to develop the flash pyrolysis process of Fig. 3 should consider placing chambers for steam-conditioning between coal drying and flash pyrolysis. If further work
should show that a steam soak at high pressure is worth the extra cost, the chambers could take the form of lock hoppers. Two other modifications need consideration [8]. Vapor from flash pyrolysis could be treated by a donor-solvent vapor. Such a vapor could either aerate the vibrated bed of Fig. 4 or be injected into the bed’s free-board. A btrayQ housing a vibrated bed of a cracking catalyst could be interposed between the pyrolysis bed and vapor condensation.
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