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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, NO. 2, JUNE 1999
Continuous Superconducting-Magnet Filtration System
Norihide SAHO, Hisashi ISOGAMI, Take0 TAKAGI, and Minoru MORITA
Mech. Eng. Res. Lab., Hitachi Ltd., 502 Kandatsu-machi,Tsuchiura, Japan
Abstract- We developed a new water purification
system using a continuous superconducting-magnet
filtration system that removes phytoplankton. The
system consists of twin-type magnets with a helium
refrigerator: a reciprocating magnetic main filter, and a
rotating magnetic sub-filter. This system removes
phytoplankton from the lake water by separating
coagulated magnetic flocks with the magnetic filters. The
filters can be continuously cleaned and regenerated
under low magnetic fields while flocks’ are being
separated under high magnetic fields. This system
removed more than 93% of the phytoplankton from lake
water at flow rates of 400 m3/day.
I. INTRODUCTION
Lakes and reservoirs are crucial for humans and animals,
providing sources of drinking and agricultural water as well
as recreational uses. At present, however, lakes and reservoirs
are suffering from progressive eutrophication mainly due to
the inflow of household waste water and other agricultural
runoff. In particular, blue-green algae films, which are
produced in summer, appear on lake surfaces, causing a foul
smell and taste and degrading the quality of the water. For this
need alone, the development of direct, rapid purification
devices is needed. The concept behind magnetic filtration
systems is that ferromagnetic particles and flocculant are
added to the influent while applying a high gradient magnetic
field. The suspended solids are non-magnetic, but stirring the
influendmagnetic-particle mixture produces magnetic flocks
consisting of non-magnetic and seeded particles[ 11-[4]. This
slurry is passed through a magnetic separator, which consists
of a matrix of ferromagnetic filaments magnetized by a strong
applied magnetic field. The magnetic flocks are then trapped
on the edges of the magnetized fibers of the matrix and the
influent is purified. When the matrix becomes loaded with
magnetic flocks, they are washed from the matrix by reducing
the magnetic field to zero and by backwashing. Such systems
are called batch-type magnetic filtration systems. This
interruption for backwashing, however, is an obstacle for
practical use of such batch-type systems. We therefore
developed a magnetic filtration system that can operate
continuously and does not require backwashing interruption.
First, we produced a prototype [5] of the batch type system
with a superconducting small magnet cooled by a helium
refrigerator. We demonstrated 95 % removal of blue-green
algae. We did not, however, demonstrate continuous
operation or the stability of the cooling system when using
practical superconducting magnet. In this paper, therefore,
we describe modifications to the system and report the
performance under continuous-flow operation at 400 m3/day.
We also describe the cooling characteristics of the
superconducting magnets.
n . SYSTEM DESIGN
Fig. 1. Structure of a continuous magnetic separator
Manuscript received September 14, 1998
A. Filtration System
A schematic of the continuous
superconducting magnetic separator is shown in Figure 1.
The magnet is a split-type magnet, divided into two air-core
solenoid coils made of NbTi. The influent flows through the
magnet bore. The main magnetic filter consists of a
ferromagnetic matrix and is inserted between the two
magnets. High magnetic gradients are created in the
ferromagnetic matrix by the magnetic field. The strong forces
created by the magnetic field gradient remove the suspended
particles by separating the magnetic flocks. The main filter
moves reciprocally, so that when the matrix becomes loaded
with magnetic flocks and the filtration efficiency drops below
a critical level, the main filter is automatically replaced with a
clean filter. The loaded filter is then backwashed. By cycling
two filters between filtration and backwashing, this system
can be operated continuously. To remove the load on the
main filter, a sub-filter is used to remove bulk quantities of
easily filtered material. In the same manner as the main filter,
sub-filter is rotated between filtration and backwashing,
1051-8223/99$10.00 0 1999 IEEE
399
Table 1. The diameter of the bore through which the seeded
water flows is 310 mm, and the center magnetic field of the
sub-filter is 0.6 T and that of the main filters is 1.0 T.
The calculated magnetic field strength in the sub- and main
filters is shown in Figure 3. The x-axis shows the radial
position, R, from the center axis of the magnet, and the
vertical axis shows the absolute value of the corresponding
magnetic flux density, B. For a given radial position, there is a
distribution of B in the axial direction of each filter, but the
value of B shown in Figure 3 corresponds to the middle of
each filter. Figure 3 indicates that the magnetic field in the
sub-filter is in the range of 0.5 to 0.6 T and that in the main
filter is 0.7 to 1.0 T. It also shows that beyond a radial
distance of about 0.5 m, the magnetic field is less than about
0.02 T. Backwashing occurs in this region of low magnetic
field, so that Figure 3 indicates that backwashing can be done
effectively beyond a radial position of about 0.5 m. The main
filter consists of an active filter with dummy filters on either
end. The dummy filters separate the main filter from the filter
support, reducing the force on the support. Figure 4 shows the
theoretical relationship between the magnetic force acting on
the filter support, F, and the dummy filter length, XD.The
vertical axis in Figure 4 is a non-dimensionalmagnetic force,
F*(=F/Fo), where Fo is the magnetic force for moving no
dummy filter (i.e., XD =O). For XD = 200 mm, Figure 4
indicates that the magnetic force on the supports is reduced to
10 % of that without the dummy filter.
Fig. 2. Photograph of a continuous magnetic separator
Table 1 Magnet characteristics
room temperature bore
main filter magnetic field
sub-filter magnetic field
3 10 mm
1.OT
0.6 T
1.2
I-main
1
filter
I
E 0.8
sbn
5 0.6
b
-z
ro
Tl
0.4
.
Y
e
s
$ 0.2
0
0
0.2
0.4
0.6
0.8
I
radial distance (m)
Fig. 3. Magnetic field at each filter
permitting continuous operation. A photograph of a magnetic
separator capable of filtering 400 m3/day at 93% efficiency is
shown in Figure 2, and its magnet characteristics are listed in
B. Cooling System A schematic of the cooling system for
the superconducting magnets is shown in Figure 5. (For
clarity, the filter and the duct for seeded water are omitted.)
The cooling system consists of a helium refrigerator to cool
the magnet to about 5 K, and a magnetic separator with a
superconducting magnet. The helium refrigerator consists of
a Gifford McMahon, Joule Thomson (GM.JT) refrigerating
cycle, with a Gifford McMahon (GM) expander for the precooler, and Joule Thomson (JT) valves. The cryostat has a
heat shield cooled in the first stage of the GM expander, and
this shield prevents radiation heating from the side walls to
the low-temperature parts. The superconducting magnet is
cooled to the minimum temperature of the two-phase helium
flow expanded in the second JT valve in the cooling tube,
because the tube is connected thermally to the magnet bobbin.
The Joule heat load of the current leads connected to the
cooling tube is reduced by making them from High-T,
superconducting (HTS) material. We show the heat load for
each cooling part in Table 2. These results indicate that
refrigeration capacity of less than 1 W is required for the
cooling tube and less than 30 W is required for the heat
shield. Using a single GM.JT cooling unit, we achieved a
maximum cooling capacity of 4 W at the cooling tube and a
heat load of 30 W at the first stage of pre-cooler. But to
prevent a temperature difference between the heat shields,
another single-stage GM expander was installed to cool the
heat shield on the magnet side.
400
1.2
I
I
magnetic SePWatiOn part
helium refrigerator
GM ex
(for heat
0.05
0.1
0.15
0.2
0.25
length of dummy-filter part XI, (m)
Fig. 4. Support force on main filter
Fig. 5. Cooling system for the twin-type superconducting magnet
Table 2 Heat load of each cooling part
heat shield
cooling tube
radiation
current leads
supports
(total
radiation
current leads
supports
(total
,
6.0
w
1w
14
12 W
27 W)
0.2
0.3 W
0.1 W
0.6 W)
w
5.5
h
525 5.0
a
Y
E
4--c
2
B 4.5
I..I..
-.I-----
a. TEST RESULTS
We tested both the performance of the cooling subsystem and the filtration efficiency of the entire system. The
cool-down time for the cooler was about 150 hours from near
room temperature. Under steady-state conditions, the
temperature of the upper superconducting magnet was 4.72 K
and the temperature of the lower magnet was about 4.74 K.
The temperature difference between the magnets was
therefore under 0.02 K. This small temperature difference
shows that both magnets were cooled uniformly. The
electrical consumption of the GM.JT refrigerator was 9.1 kW
(2.4 kW for the liquefaction circuit and 6.7 kW for the precooler unit) plus 3.3 kW for the GM expander for heat shield
cooling(for a total electrical consumption of 12.4 kW).
Figure 6 shows the relationship between the power-lead
current and the cooling temperature of the magnet. The
temperature was measured in each power-lead under steadystate conditions. The temperature of the cooling part of the
HTS power lead increased with increasing current, but the
+ -m-
m-
I
-
I
-
--...I.-...-.
4.0
0
20
40
60
80
100
120
current (A)
Fig. 6. Temperature of various parts cooling-system vs lead current.
magnet temperature did not change significantly. This
indicates that the temperature of the current lead did not
influence the magnet temperature. The measured cooling
characteristics of the cooling system are listed in Table 3.
The flow through the purification system is shown in Figure
7. First, the influent is chemically neutralized with NaOH.
Then flocculant and ferromagnetic particles are added to the
influent. This mixture is then agglomerated to form magnetic
flocks. These four steps constitute the flocculation part of the
filtration system. The resulting magnetic flocks are attracted
to the magnetic filter wires in the main part of the filtration
system and removed from the influent. We successfully
401
operated this filtration system continuously, automatically
rotating and backwashing the filters, demonstrating the
ability of this system to filter water continuously. Table 4
lists the removal efficiency of algae, biochemical oxygen
demand (BOD), total nitrogen (T-N), and total phosphorus
(T-P) when filtering water from a lake near our laboratory at a
flow rate of 400 m3/day. The concentration of chlorophyll-a
(including algae) in the influent was 27.4 mgL, and
decreased to 2 m g L in the effluent, for a removal efficiency
in excess of 93%. The removal efficiency of BOD and T-P
was greater than 78%. Using the removal efficiency of
chlorophyll-a as an index for the removal efficiency of
phytoplankton, we demonstrated this system can remove
more than 93% of phytoplanktonunder continuous operation.
Because phytoplankton take up nitrogen and phosphorus as
nourishment, the simultaneous removal of nitrogen and
phosphorus occurs when removing phytoplankton.
Table 3 Performance of magnet cooling system
coil temperature
cool-down time
electric power consumption
4.1 -4.7 K
150 hours
12.4 kW
influent
PISUMMARY
flocculation part
To improve the water quality of lakes and reservoirs, a
continuous-flow, superconducting-magnet filtration system
for removing phytoplankton has been developed. The system
consists of a twin-type superconductingmagnet with a helium
cooler, a reciprocating main magnetic filter, and a rotating
magnetic sub-filter. The reciprocating and rotating
assemblies permit the magnetic filters to be alternated
between filtration of the water (in the presence of a strong
magnetic field) and a backwashing cycle in the presence of
low magnetic fields; thus, the system can be operated
continuously. This system removed more than 93% of the
phytoplankton from lake water at flow rates of 400 m3/ day.
REFERENCES
[l] Ralph Mitchell, Gabriel Bitton, Christopher de Latour, and E. Maxwell:
Magnetic Separation: A New Approach to Water and Waste Treatment:
J.P.W.T-A, V01.7 (1975), 349-355
[2] G. Bitton , J.L. Fox, and H. G. Strickland.: Removal of Algae from
Florida Lakes by Magnetic Filtration: Applied Microbiology, Vo1.30,
No.6 (1975), 905-908
[3] Christopher de Latour: Seeding Principles of High Gradient Magnetic
Separation: JOURNAL AWWA, August, (1976), 443-446
[4] R. R. Oder and B. Ihorst: Wastewater Processing with High Gradient
Magnetic Separators: 2nd National Conference on Complete WaterReuse, Chicago, (1 975), 887-897
[ 5 ] Hisashi Isogami, Norihide Saho, Take0 Takagi, and Minoru Morita: A
Superconducting Magnetic Separator with An Integral Refrigerator for
Blue-Green Algae, Proc. of the 16th Int. Cryo. Eng. Conf., (1996).
1125-1 128
,E
3
sluge
neutralization
2 ferromagneticparticles
3 flocculant
@ polwr
1
Fig. 7 . System flow of filtration
Table 4 Test results
influent effluent removal
efficiency
chlorophyll-a (,U g/L)
27.4
c2
>93%
BOD (mg/L)
4.2
0.7
83%
1.86
1.29
31%
total-Nitrogen(mg/L)
total-Phosphorus(mg/L) 0.09
c0.02
>78%