Epstein N., Grace J.R. Spouted and Spout-Fluid Beds: Fundamentals and Applications
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Spouted bed electrochemical reactors 277
Figure 16.4. Copper particles grown from 0.4 mm to 5 mm; seed particles were cut copper wire.
cells or ones with SBEs. In contrast, the standard electrode potential of zinc is less than
that of hydrogen, implying that thermodynamically zinc should not deposit on a cathode
from aqueous solutions, but rather hydrogen should be generated. The fact that zinc
can be electrowon is due to the slow kinetics of hydrogen deposition on a zinc surface;
nevertheless, current efficiency is a major concern in zinc electrowinning, and care must
be taken to minimize hydrogen evolution (e.g., by using highly purified electrolytes to
avoid impurities that would otherwise catalyze hydrogen evolution).
As Table 16.1 shows, the electrowinning of copper at an SBE has been quite practical
on a laboratory scale.
3,6–8,12,14,20–23
Experiments at Berkeley and in Prague have shown
current efficiencies comparable with those of conventional cells, at current densities
(and therefore space-time yields) an order of magnitude greater than in conventional
cells. This has been achieved with specific energy consumptions comparable with those
of conventional cells. In electrowinning using an SBE, “seed particles” are fed to the
cell and grown to some final size. This is vividly shown in Figure 16.4, in which are seen
0.4-mm seeds (commercially available cut wire particles) g rown to 5 mm, an increase in
mass by a factor of about 2400. The copper deposits were dense and shiny. Scaleup of
copper electrowinning to a large pilot reactor has been described by Robinson et al.
11,12
Electrolytes used in conventional copper electrowinning are generated by leaching of
ores, followed by concentration and purification of the solution by solvent extraction.
An alternative technology using copper matte (a solid copper–iron sulfide produced in
the alternative pyrometallurgical route to copper) as the starting point was explored by
Jiricny and coworkers.
7,8
In one part of that study, the matte was leached by a ferric-
ion-containing solution produced on the anode side of a cell. The resulting solution
(containing copper and ferrous ions) then passed through the spouted bed cathode of
the same cell to deposit copper. The ferrous-ion-containing solution then flowed into the
anode side for oxidation of the ferrous ions to ferric ions. Low energy consumptions at
high current densities were reported.
278 J. W. Evans and Vladim
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100
(a) (b)
95
90
85
80
1000 2000 3000
Flat bed
Cylindrical bed
4000 5000 6000 7000
Current density [A/m
2
]
Current efficiency [%]
Current density [A/m
2
]
4.0
3.5
3.0
2.5
Specific energy [kWh/kg]
1000 2000 3000 4000 5000
Flat bed
Cyclindrical bed
6000 7000
Figure 16.5. (a) Current efficiency in electrowinning of zinc. Zinc concentration reduced by 50
g/L, with neutral initial catholytes; (b) Specific electric energy consumption of SBEs for same
zinc concentration reduction.
2
With permission of Springer Science and Business Media.
The use of SBEs to electrowin zinc has proven to be more challenging. In conventional
electrowinning, the whole of the zinc electrode is electrically connected to the power
supply and zinc deposition effectively competes with hydrogen generation. In the case of
the SBE, many particles, such as those in the draft tube or spout, are electrically isolated
and subject to dissolution if the electrolyte is acidic. (Many readers will recall chemistry
experiments in which hydrogen is generated by adding zinc particles to dilute sulfuric
acid.) Figure 16.5a, from Salas-Morales and colleagues,
2
illustrates the consequence.
Current efficiency is seen to drop in these batch experiments as zinc is stripped from
solution and acid is generated (by reaction at t he anode) as the amount of charge passed
increases with time. Figure 16.5b from this same investigation illustrates the consequence
for specific electrical energy consumption.
As the current density is raised, the current efficiency increases because there is
more metal deposition to compete with the (current-insensitive) metal dissolution. This
increase is overwhelmed by an increase i n cell voltage, however, so the specific energy
consumption exceeds conventional values at a few thousand amps/m
2
. These current
densities are much higher than those of the conventional cell (Table 16.1), implying a
much higher space-time yield.
Although the use of the SBE for electrowinning zinc from the usual acid elec-
trolytes
9,13,24
appears to be problematic, its use for electrodeposition from alkaline
electrolytes
5,10,25–28
seems more promising, because the near absence of hydrogen ions
in the electrolyte makes hydrogen deposition or chemical dissolution of zinc less of an
issue and high current efficiencies can be expected. Alkaline electrolytes are not encoun-
tered in the present production of zinc from ores, but technology for recycling zinc from
galvanized steel
25
(sodium hydroxide leaching) or spent electrolyte from zinc-air bat-
teries (potassium zincate solutions) entail such solutions. Figure 16.6 shows particles
Spouted bed electrochemical reactors 279
Seed particles Intermediate stage
Intermediate stage Final particles
(a) (b)
Figure 16.6. (a) Zinc particles grown from 0.5 mm diameter to 1 mm from alkaline electrolyte;
(b) SEM micrograph of cross-section of zinc final size par ticle.
27
With permission of Springer
Science and Business Media.
electrowon from alkaline electrolyte in an SBE. The morphology of the deposit is not
as smooth or as dense as that of the copper in Figure 16.4, but this might be acceptable
for some applications. As detailed in Table 16.1, the cell voltage and specific energy
consumption for this use of the SBE are low.
In the majority of metal electrowinning experiments using SBEs, the anode has
differed from the lead alloy anodes of conventional electrowinning. A typical anode is
DSA
R
, a titanium mesh anode with a proprietary coating that is catalytic for oxygen
evolution. The higher cost of such an anode might be justified by the high current density
typical of a cell with an SBE.
16.3.2 Wastewater treatment
Waste solutions containing ionic species are common in semiconductor processing,
electroplating, and photofinishing. Although chemical precipitation may be used to
precipitate ions from solution, this typically introduces other species into solution, so
that disposing or recycling of the water from the wastestream is inhibited. Electrolytic
deposition of ions may overcome this difficulty. Because ion concentrations are typically
low, mass transfer rates to electrode surfaces are low and large electrode surface areas are
necessary if equipment is to remain compact. This s uggests a three-dimensional electrode
of the sort seen in the lower half of Figure 16.1.
29–31
Scott
32
has examined various three-
dimensional electrodes for this application, including the SBE. One difficulty is that
the electrolyte flow rate needed for particle circulation results in a low fraction of ions
extracted from solution per pass. Hence batch operation, a number of units in series, or
incorporation of additional steps, such as ion exchange, are necessary.
29,32
280 J. W. Evans and Vladim
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Use of the SBE for treatment of organic pollutants has been suggested by Spiegel
et al.
33
16.4 Concluding remarks
The SBE appears to be a promising tool for electrochemical production, particularly
of metals that can be deposited from aqueous solutions. The electrode results in cells
that can be operated at much higher space-time yields than conventional cells with
planar electrodes and, thereby, a plant with a much smaller “footprint.” This should
be achievable while maintaining specific energy consumptions comparable to those of
conventional cells. The electrowinning of metals in conventional cells requires that the
cell remain open because starter sheets/cathode blanks are inserted, and finished cathodes
removed, through the top of the cell. Seed introduction and particle removal from a cell
with an SBE occurs as a suspension in electrolyte flowing into or out of the cell. The
cell can therefore be closed, which presents the opportunity for control or capture of cell
emissions, such as oxygen generated at anodes.
The particulate nature of the metal product from an SBE is both an advantage (ease
of material handling compared with meter-square slabs of metal) and a disadvantage
(current metal processing technology is designed for handling slabs, e.g., in melting
furnaces, rather than particles). However, the principal impediments to commercial
implementation of the SBE appear to be the high capital cost of the sophisticated cell
(compared with the simple tanks of conventional cells), the lack of engineering know-
how necessary for plant construction, and the longevity of the separators in the cells that
employ them.
It seems likely that the SBE will see its first commercial application in removal of
metal ions from wastewater streams. Because of low concentrations in solution, simple
batch operation becomes possible with the bed of particles being replaced en masse only
at long intervals. In such application, the large surface area per unit volume offered by
a three-dimensional electrode is particularly advantageous.
References
1. D. E. Hadzismajlovic, M. G. Pavlovic, and K. I. Popov. The annulus of a spouted bed as a
3-dimensional electrode. Hydrometallurgy, 22 (1989), 393–401.
2. J. C. Salas-Morales, J. W. Evans, O. M. G. Newman, and P. A. Adcock. Spouted bed elec-
trowinning of zinc. 1. Laboratory-scale electrowinning experiments. Metal. & Mat. Trans.
B—Proc. Metal. & Mat. Proc. Sci., 28 (1997), 59–68.
3. P. A. Shirvanian and J. M. Calo. Copper recovery in a spouted vessel electrolytic reactor
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namics in a rectangular spouted vessel. Internat. J. Multiph. Flow, 32 (2006), 739–753.
5. Eveready Battery Company, Inc., Alkaline electrochemical cell with reduced gassing, J. L.
Stimits (Inventor), U.S. Patent 0226976 (A1) (2008).
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TMS, 1999), pp. 629–642.
7. V. Jiricny, A. Roy, and J. W. Evans. Copper electrowinning using spouted-bed electrodes:
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Metal. & Mat. Proc. Sci., 33 (2002), 669–676.
8. V. Jiricny, A. Roy, and J. W. Evans. Copper electrowinning using spouted-bed electrodes:
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Trans. B—Proc. Metal. Mat. Proc. Sci., 33 (2002), 677–683.
9. O. M. G. Newman, P. A. Adcock, M. J. Meere, P. Freeman, J. W. Evans, and S. Siu. Inves-
tigations of spouted bed electrowinning for the zinc industry. Presented at 5th International
Symposium on Hydrometallurgy in honour of Ian M. R itchie (Vancouver, BC, Canada, 2003).
10. A. Roy, S. Siu, V. Jiricny, J. W. Evans, and O. M. G. Newman. Electrowinning of zinc from acid
and alkaline electrolytes using a spouted bed electrode. Presented at International Symposium
on Zinc and Lead Processing (Calgary, Alberta, Canada, 1998).
11. De Nora Elettrodi S. P. A., Spouted bed electrode cell for metal electrowinning, D. J. Robinson,
S. A. MacDonald, V. Jiricny, D. Oldani, F. Todaro, L. Carrettin, G. N. Martelli, and D. Scotti
(Inventors), WO Patent 007805 (A2) (2004).
12. D. Robinson, S. MacDonald, and F. Todaro. Commercial development of a descending packed
bed electrowinning cell, part 2: Cell operation. Presented at 5th International Symposium on
Hydrometallurgy in honour of Ian M. Ritchie (Vancouver, BC, Canada, 2003).
13. A. Verma, J. C. Salas-Morales, and J. W. Evans. Spouted bed electrowinning of zinc. 2.
Investigations of the dynamics of particles in large thin spouted beds. Metal. Mat. Trans.
B—Proc. Metal. Mat. Proc. Sci., 28 (1997), 69–79.
14. De Nora Elettrodi S. P. A., Method for copper electrowinning in hydrochloric solution, D. J.
Robinson, S. A. MacDonald and V. J iricny (Inventors), U.S. Patent 163082 (A1) (2006).
15. J. S. Newman and K. E. Thomas-Alyea. Electrochemical Systems, 3rd ed. (Hoboken, NJ: John
Wiley and Sons, 2004).
16. J. S. Newman and C. W. Tobias. Theoretical analysis of current distribution in porous elec-
trodes. J. Electrochem. Soc., 109 (1962), 1183–1191.
17. P. A. Shirvanian and J. M. Calo. Hydrodynamic scaling of a rectangular spouted vessel with
a draft duct. Chem. Eng. J., 103 (2004), 29–34.
18. F. M. Doyle. Teaching and learning environmental hydrometallurgy. Hydrometallurgy, 79
(2005), 1–14.
19. F. Habashi. Principles of Extractive Metallurg y, Volume 4: Amalgam and Electrometallurgy
(Sainte-Foy, Qu
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etallurgie extractive Qu
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ebec, 1998).
20. B. M. Dweik, C. C. Liu, R. F. Savinell, C. D. Zhou, E. J. Taylor, E. Stortz, and R. P. Renz.
Circulating particulate bed electrode: Electrochemical extraction of copper ions from dilute
aqueous solutions. In Electrochemistry in Mineral and Metal Processing IV, ed. R. Woods,
F. M. Doyle, and P. Richardson (Pennington, NJ: The Electrochemical Society, 1996),
pp. 416–428.
21. D. McKay, C. Hayes, and C. Oloman. Electrowinning on a spouted bed. Presented at 32nd
Canadian Chemical Engineering Conference (Vancouver, BC, Canada, 1982).
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S. A. MacDonald, and F. Todaro (Inventors), U.S. Patent 0102302 (A1) (2007).
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24. J. W. Evans, A. Roy, and C. Allen. Spouted bed electrowinning of zinc from chloride elec-
trolytes. Presented at Lead-Zinc 2000 Symposium (Pittsburgh, PA, 2000).
25. V. Jiricny, A. Roy, and J. W. Evans. Spouted bed electrowinning in the recovery of zinc from
scrap galvanized steel. Presented at EPD Congress 1998, TMS Annual Meeting (San Antonio,
TX, 1998).
26. V. Jiricny, A. Roy, and J. W. Evans. Electrodeposition of zinc from sodium zincate/hydroxide
electrolytes in a spouted bed electrode. Metal. Mat. Trans. B—Proc. Metal. Mat. Proc. Sci.,
31 (2000), 755–766.
27. V. Jiricny, S. Siu, A. Roy, and J. W. Evans. Regeneration of zinc particles for zinc-air fuel
cells in a spouted-bed electrode. J. App. Electrochem., 30 (2000), 647–656.
28. University of California, Efficient electrowinning of zinc from alkaline electrolytes. S. C. Siu
and J. W. Evans (Inventors), U.S. Patent 5958210 (A) (1999).
29. K. Juttner, U. Galla, and H. Schmieder. Electrochemical approaches to environmental prob-
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30. V. Stankovic. Metal removal from effluents by electrowinning and a new design concept in
wastewater purification technology. Presented at 4th Croatian Symposium on Electrochem-
istry (Primosten, Croatia: 2006).
31. V. D. Stankovic and S. Stankovic. An investigation of the spouted bed electrode cell for the
electrowinning of metal from dilute-solutions. J. Appl. Electrochem., 21 (1991), 124–129.
32. K. Scott. A consideration of circulating bed electrodes for the recovery of metal from dilute-
solutions. J. Appl. Electrochem., 18 (1988), 504–510.
33. Eltron Research, Inc., Three dimensional electrode for the electrolytic removal of contami-
nants from aqueous waste streams, E . F. Spiegel and A. F. Sammells (Inventors), US6298996
(B1), October 9, 2001.
34. Eveready Battery Company, Inc., Alkaline electrochemical cell with reduced gassing, J. L.
Stimits and J. S. Dreger (Inventors), US2008102360 (A1), September 18, 2008.
35. B. M. Dweik, C. C. Liu, and R. F. Savinell. Hydrodynamic modelling of the liquid-solid
behaviour of the circulating particulate bed electrode. J. Appl. Electrochem., 26 (1996),
1093–1102.
17 Scaleup, slot-rectangular, and
multiple spouting
John R. Grace and C. Jim Lim
17.1 Introduction
Scaleup of test results from smaller to larger equipment is an important aspect in
the design and operation of spouted bed units for physical and chemical processes of
practical importance. A key need is the ability to use data obtained on smaller units
in the design of full-scale equipment. Similar considerations pertain to the application
of results obtained at atmospheric pressure and room temperature to columns to be
operated at elevated pressures and/or temperatures. Despite the inclusion of words such
as “scaleup” and “scaling” in the titles of a number of articles dealing with spouted
beds, there are no established procedures for scaleup and considerable uncertainty about
how best to achieve it. The question of the maximum practical size of spouted beds is
also unresolved. As noted with reference to fluidized beds,
1
scaleup is “not an exact
science, but remains the province of that mixture of mathematics, witchcraft, history
and common sense which we call engineering.”
This chapter considers a number of interrelated questions and issues:
r
Is there a maximum practical size of spouted beds, and if so, what is it?
r
What is the effect of column diameter on spouting behavior? What are the effects of
pressure and temperature?
r
Can one use small-scale results reliably to predict spouting behavior in larger columns?
In particular, can one use dimensional similitude to scale up (or scale down) hydrody-
namics and other experimental results obtained in equipment of a given size to predict
with confidence behavior and performance of a unit of a different size but similar
geometry?
r
Can multiple spouts, slot-rectangular spouted beds, or some combination of the two
(rectangular vessels with multiple gas-entry slots) facilitate scaleup?
r
How does scaling of spouted beds compare with scaleup of fluidized beds?
In this chapter, we consider only cases in which the spouting fluid is a gas. In addition,
we consider only cases in which the containing vessel is vertical at the spouted bed
surface (i.e., we do not cover the scaleup of conical beds [Chapter 5]). Although one
of the claims sometimes made for adding draft tubes to spouted beds is to improve
Spouted and Spout-Fluid Beds: Fundamentals and Applications, eds. Norman Epstein and John R. Grace.
Published by Cambridge University Press.
C
Cambridge University Press 2011.
284 John R. Grace and C. Jim Lim
scaleup, we do not treat draft tubes here, as they are covered in Chapter 7. We barely
touch on scaleup of spout-fluid beds. In addition, specific processes such as drying,
combustion, and gasification are treated in Chapters 11, 12, and 15, so they are not dealt
with specifically in this chapter. Finally, we note that, just as for fluidized beds,
1
serious
operability challenges (e.g., related to agglomeration, attrition, and vibration) are often
encountered in scaleup, but are not treated in this chapter.
17.2 Maximum size of spouted beds
Spouted bed and spout-fluid bed studies have been performed predominantly on rela-
tively small units, 0.3 m in diameter. In applications of spouted and spout-fluid beds,
the diameter is normally less than 1 m. This compares with fluidized beds, for which
there are many applications in which the containing vessels are tens of meters in diam-
eter. Key questions with respect to spouted beds are, therefore, how they can be scaled
up (and scaled down for test purposes) and whether there is any upper limit on spouted
bed diameter.
Consider a cylindrical or conical-cylindrical column of diameter D with a single
central orifice of diameter D
i
at the bottom. For stable spouting over the entire column,
the optimum range of D/D
i
is said
2
to be 6 to 10, and the maximum is likely to be ∼12
to avoid substantial dead regions. For stable spouting, it has been shown
3
that D
i
/d
p
should not exceed 25. For a single-orifice spouted bed, to have steady coherent spouting
that covers the entire cross-sectional area of the column, this suggests that
D
d
p
=
D
D
i
×
D
i
d
p
≤ 300. (17.1)
This is similar to the maximum ratio of 200 proposed by Nemeth et al.
2
Hence ∼300
particle diameters is at or near the upper limit of what one should adopt as the diameter of
single-orifice spouted beds of circular cross-section for optimal hydrodynamics. Since
most particles processed in spouted beds are approximately 4 mm in size or smaller,
this then cor responds to a maximum column diameter of the order of 1 m. If columns
of similar single-orifice geometry, but larger diameter, are provided, they are likely to
operate in unsteady and suboptimal modes. Operating with larger equipment requires
either multiple orifices or alternative equipment configurations, as will be discussed
later.
17.3 Influence of column diameter on hydrodynamics
The column diameter is included as a parameter in a number of the theoretical, empirical,
and semiempirical correlations and equations used for predicting the behavior of spouted
beds (see Chapter 3). If column geometry and such operating variables as D
i
, d
p
, particle
density, and bed depth (H) are held constant, then, for example, the well-known Mathur-
Gishler
4
equation predicts that the minimum spouting velocity, U
ms
, ∝ D
−4/3
, based on
Scaleup, slot-rectangular, and multiple spouting 285
small columns, although, as indicated in Chapter 3, the dependence decreases toward a
0-power for columns of D > 0.5 m. The maximum spoutable bed depth, H
m
, ∝ D
8/3
,
according to the correlation of McNab and Bridgwater.
5
There is a much lesser, though
still appreciable, effect of D on average spout diameter, with exponents
6,7
of 0.68 and
0.59. On the other hand, neither the total pressure drop nor the pressure drop at minimum
spouting are expected to be affected by D, providing that the other specified variables are
maintained constant. In cases in which there are “dead regions” around the lower outer
part of the column, there is evidence
8,9
that the volume of the dead zone relative to the
bed volume is likely to be larger as the column diameter and bed depth are increased.
Given these very different dependencies on D, the effect of column diameter is clearly
complex.
Relatively few spouted bed columns of diameter or width exceeding 0.61 m have been
reported in the open literature. Those in which there was a single inlet orifice are:
r
A dryer of D = 0.914 m at the National Research Council of Canada, reported in a
personal communication from W.S. Peterson to Mathur and Epstein.
10
r
Green and Bridgwater
11,12
:30
◦
-sector column of radius 0.78 m.
r
Fane and Mitchell
13
: full column of D = 1.1 m.
r
Lim and Grace
8
: half column of D = 0.91 m with a flat base.
r
He et al.
9
: same half column of D = 0.91 m as in the previous entry, but now with a
conical base, and operated as both a spouted bed and a spout-fluid bed.
r
Kalwar et al.
14
: rectangular column, 0.75 m wide, with dimensions up to 0.47 m i n
the orthogonal direction.
r
Priolo granulator, Italy, 3.5 m diameter column sketched by Piccinini.
15
Only the last of these is substantially l arger than 1 m in diameter, and there is no
indication of how well, or in what hydrodynamic mode, this unit operated.
17.4 Application of dimensional similitude to scaling of spouted beds
A powerful technique for simulating and scaling physical systems is to build a geometri-
cally similar physical “cold” model in which all the impor tant independent dimensionless
groups match (i.e., are identical) for the model and prototype units. The set of dimen-
sionless groups to be matched can be obtained in several different manners:
r
By writing all the governing differential equations (e.g., equation of motion) in dimen-
sionless form;
r
By applying the well-known Buckingham Pi theorem, after intuitively listing all
quantities likely to affect the phenomena of interest;
r
When hydrodynamics alone are involved, by considering “force balances,” such as
balances of gravity and drag on particles, again including all forces likely t o be
important in the given problem.
Dimensional similitude methodology is widely employed in aircraft and ship design,
and has become popular in scaling of fluidized beds.
16
He et al.
17
applied this approach
286 John R. Grace and C. Jim Lim
Table 17.1. Properties of the four columns used in the scaling study of
He et al.
17
Diameter,
mm
Temperature,
◦
C
Pressure,
kPa Particles
0.051 25 101 Steel shot
0.076 25 135 Steel, glass
0.152 25–500 101–312 Glass, sand
0.914 25 101 Polystyrene
Table 17.2. Properties of particles used to test scaling criteria.
17
Material D
p
,mm ρ
p
, kg/m
3
ε
0
,– φ
s
,– ϕ,
◦
Glass beads 2.18 2400 0.41 1 26
Glass beads 1.09 2450 0.42 1 27
Steel shot 1.09 7400 0.42 1 28
Steel shot 0.73 7400 0.42 1 29
Sand 2.18 2400 0.44 0.88 38
Sand 1.80 2490 0.45 0.88 39
Sand 0.54 2490 0.45 0.88 41
Polystyrene 3.25 1020 0.44 0.87 40
to scaling of the hydrodynamics of spouted beds, leading to the following dimensionless
groups for a bed of fixed geometry:
gd
p
U
2
,
ρd
p
U
μ
,
ρ
p
ρ
,
H
d
p
,
D
i
d
p
,φ
s
,ε
0
,ϕ,PSD, (17.2)
where φ
s
denotes the par ticle sphericity, ϕ is the angle of internal friction of the partic-
ulate material, and PSD is a dimensionless parameter (e.g., standard deviation divided
by mean particle diameter) describing the particle size distribution. These groups are
the same as those of Glicksman
16
for fluidized beds, except for the inclusion of the
loose-packed bed voidage (ε
0
) and ϕ. These extra terms are added because of more
frequent particle–particle contacts and greater transmission of compression and friction
forces in spouted beds relative to fluidized beds.
He et al.
17
also carried out experimental hydrodynamic tests with narrow size distri-
butions of regular particles to test the sufficiency of these relations for four different
semicylindrical columns, differing in diameter, as well as being operated at different
temperatures and pressures. The diameters of these columns and operating temperature
and pressure ranges are provided in Table 17.1. Properties of the particles tested appear
in Table 17.2. Dependent variables compared were the maximum spoutable bed depth,
fountain height, spout diameter, axial pressure profiles, and dead zone boundaries. When
all the groups in (17.2) were very nearly matched, the dimensionless hydrodynamic
behavior was nearly identical. Deliberately mismatching the dimensionless groups, one