Note: Descriptions are shown in the official language in which they were submitted.
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Method and means for extracting heat from Aluminium electrolysis cells
Description
The present invention relates to a method and means for extracting heat from
an electrolysis
cell for production of aluminium. Specifically, it relates to the cooling of
the anode/stubs/yoke
assembly by heat conduction upwards along the anode stem, and the enhancement
and control
of this cooling effect.
The anode assembly in aluminium cells consists of the anode stem (rod), the
anode yoke with
stubs (studs), and the carbon anode block. The stem is attached at its upper
end to the anode
beam by means of a clamp, and its lower end is connected to the anode yoke.
The stubs are
integrated with the anode carbon block. The anode stem can be made of
aluminium or copper,
while the yoke is made of aluminium, copper or as normal made of steel. The
stubs are made
of steel. The electric and mechanic connection between the stem and the yoke
is constituted
by a bimetallic plate. One conventional way of fastening the stubs in holes in
the carbon block
is by means of cast iron.
Besides supplying the electrical current to the anode and providing the
mechanical connection
to the anode beam, thus fixing the anode in its correct position, the anode
stem plays an
important role in the energy balance of the cell. Approximately 50 percent of
the electrical
energy input to the cell is lost as heat. Up to 50 percent of the heat loss
takes place at the top
of the cell, and the major part of this again is through the anode.
Typically, for a 300 kA cell about 6-7 kW of heat is conducted through each
anode carbon
block from the electrolyte and upwards. Some of this passes through the anode
cover material
on top of the anode, but most of the heat (about 5 kW per anode) is conducted
through the
stubs and into the yoke. About 4 kW is then dissipated from the yoke and stubs
by
electromagnetic radiation and convective heat transfer, while the remaining 1
kW is
conducted into the anode rod. Part of the latter heat is dissipated into the
gas between the top
crust and the superstructure, and part of it is dissipated outside the
superstructure.
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The energy balance in an aluminium cell is very delicate. It is uttermost
important to keep the
energy balance right, since the cell operation heavily relies on having a
layer of frozen
electrolyte at the inner walls of the cell to protect the lining. When
increasing the amperage in
existing potlines, numerous actions must be taken in order to adapt to the
higher current.
Well-known measures are the use of cathode carbon with high electric
conductivity,
accommodation of larger (longer) anodes, increasing the dimensions of the cell
cavity by
using thinner sidewalls, and decreasing the anode-cathode distance (ACD).
However, there
are upper limits for the anode dimensions, and a lower limit for the ACD that
can be used
without excessive loss of current efficiency and without risking magneto-
hydrodynamic cell
instability. From a certain point on, further increase of amperage is only
possible by keeping
the ACD constant and taking measures to increase the heat flow out of the
cell.
Arguably, the easiest way to increase the heat losses is by increasing the
number of stubs in
each anode, or by increasing the diameter of the stubs. Besides increasing the
heat loss, this
has the inherent benefit of decreasing the electric resistance of the anode
assembly. However,
the increase in the heat loss through the stubs is less than proportional to
the increase in the
cross-sectional area, and the larger stub dimensions may give problems with
anode cracking.
Increased heat losses from the stubs/yoke will also lead to increased
temperature in the raw
gas. There are, at least, three reasons why this is not desired; 1) Increased
maintenance costs
related to the filter bags in the dry scrubber if the temperature increases
above their designed
operating temperature, 2) It is important to keep the temperature of the
superstructure below
certain limits due to the numerous electro-mechanical installations in this
area, and 3) There
may be increased heat stress on the operators working in the vicinity of the
cell. The extra
heat losses must therefore be compensated by increased air suction into the
cells. However,
the air flow in the exhaust ducts and the gas scrubbing system is the far
largest mass flow in
an aluminium plant (e.g., 80 t air/t Al), and the cost of transporting the gas
is approximately
proportional to the cube of the volumetric flow. Moreover, increased suction
rate may also
require a scaling up of the equipment related to the dry scrubbing system.
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One way of resolving the problem with increased raw gas temperature without
increasing the
suction rate, is to cool down the raw gas by spraying water mist into the raw
gas ducts, as
disclosed in WO 2004 064984. One probable disadvantage related to this way of
cooling the
raw gas is increased corrosion in the raw gas ducts. Furthermore, the moisture
content in the
alumina fed to the cells will increase, which probably gives higher HF
emissions to the
environment. A better way of decreasing the temperature in front of the dry
scrubber is
probably to place one or more heat exchangers in the raw gas flow. The
problems related to
fouling in the dusty and contaminated raw gas appears to be solved; see the
description in WO
2006 009459.
It was recently disclosed that a decrease in the raw gas temperature, as well
as a strongly
increased heat flow through the anode, can be achieved by active cooling of
the anode yokes
(WO 2006 088375). The potential of amperage increase, as well as the amount of
heat taken
out from the raw gas, appears to be extraordinarily high in this concept.
Still, the modification
of the anode yokes and the necessary installations at the cell's
superstructure may require
unacceptably high investments in some cases.
NO 318 164 B1 corresponds to WO 2004/018737 that discloses a method for
control of inert
electrodes in an electrolysis cell for aluminium production. The problem to be
solved is to
reduce dissolution of the anode material by transporting heat away from the
active anode
surface and to reduce deposit formation on the active surface of the cathode
by preferably
keeping the temperature of this surface higher than that of the electrolyte.
By solving this
problem, the electrolytic process based on inert electrodes can be enhanced.
One main purpose of cooling the anode assembly as described in accordance with
the present
invention, is to be able to raise the amperage on the cell while maintaining
the side and end
ledge (frozen bath) in the bath phase without reducing the ACD, without
increasing the
dimension of the stub and yoke and thereby without increasing the temperature
of the raw gas.
Removing heat from the anode with an active cooling will also increase the
efficiency of stub,
yoke and stem as a heat sink for heat leaving the interpolar distance where
most of the heat is
generated. The reason for this is because the specific electrical and thermal
conductivity of
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steel will increase and thereby leading to an increased heat loss through the
stub and yoke and
also because less internal heat will be generated in the material (steel).
Calculation on a heat
balance model with active cooling of the anodes has shown possibility for a 10
% increase in
the amperage maintaining the interpolar distance and keeping the side ledge
constant.
The basic idea in the present invention is to extract more heat from the
interior of the cell, as
well as reducing the heat dissipated into the raw gas, by increasing the
amount of heat
conducted from the cell along the anode stem. Enhancement of the heat removal
from the cell
can be achieved by improvement of the conduction along the stem or by
installing a
convective heat transfer circuit machined inside or fixed on the stem. The
heat transfer fluid is
circulated down to the yoke where it is heated up. It brings back this heat
outside of the
superstructure where the heat is released. Heat intake and release can be
enhanced by phase
transition of the refrigerant (boiling and condensation).
Further, the raw gas temperature can be reduced by applying thermal insulation
at least partly
at the anode stem inside the superstructure in that it limits the dissipation
of heat into the raw
gas.
In accordance to the invention, there can be removed heat in an amount that
influences the
overall thermal balance of the cell.
In accordance with one aspect of the invention, there is provided a method for
extracting heat
from an electrolysis cell for production of aluminium, the cell comprising a
superstructure
with an interior part with one or more suspended carbon anode(s), each anode
being
suspended by an anode yoke attached to one lower end of an anode stem which is
attached to
an anode beam at its upper end, the anode beam being arranged outside the
superstructure,
where heat is extracted via the anode stem, wherein the anode stem is cooled
so it extracts
heat from the interior to the exterior of the superstructure, and wherein that
thermal insulation
is applied at least at a part of the anode stem that is inside the
superstructure.
In accordance with another aspect of the invention, there is provided an
apparatus for
extracting heat from an electrolysis cell for production of aluminium, the
cell comprising a
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superstructure with an interior part with one or more suspended carbon
anode(s), the anode(s)
being suspended by an anode yoke attached to one lower end of an anode stem
which at its
upper end is attached to an anode beam arranged outside the superstructure,
wherein the anode
stem inside the superstructure is at least partly thermally insulated and is
further adapted for
5 being cooled by a cooling medium that is circulated along the anode stem
via pipes attached
to the outside of the stem, or in channels inside the stem, whereby heat is
extracted from the
inside of the superstructure to the outside of the superstructure by a closed
cooling circuit.
In accordance with still another aspect of the invention, there is provided an
apparatus for
extracting heat from an electrolysis cell for production of aluminium, the
cell comprising a
superstructure with an interior part with one or more suspended carbon
anode(s), the anode(s)
being suspended by an anode yoke attached to one lower end of an anode stem
which at its
upper end is attached to an anode beam arranged outside the superstructure,
wherein the anode
stem inside the superstructure is at least partly thermally insulated and is
further adapted for
being cooled by a cooling medium that is circulated along the anode stem via
pipes in
channels inside the stem, whereby heat is extracted from the inside of the
superstructure to the
outside of the superstructure by a closed cooling circuit.
These and further advantages can be achieved with the invention in accordance
to the
accompanying claims.
In the following, the invention shall be described further by examples and
figures where:
Fig. 1 discloses in general an anode assembly,
Fig. 2 a-b disclose two embodiments of cross sectional views of anode stems in
accordance
with the invention,
Fig. 3 discloses a diagram showing temperature gradients along an anode stem,
calculated for
four cases as discussed in the following text.
In Fig. 1 there is disclosed an anode assembly for an electrolysis cell that
comprises an anode
stem 1 which is connected to an anode beam 2 and an anode yoke 3 from which
stubs 4
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provide further electric contact to a carbon anode 5. The anode stem is cooled
by increasing
the surface area of the stem above the cell's superstructure 6, or by applying
a cooling
medium that circulates along the stem. The anode cooling can be combined with
the use of a
thermal insulation material 7 at the anode stem below (inside) the
superstructure.
In Fig. 2a and 2b there is shown two embodiments for arranging medium
transport inside the
anode stem 1. The Figures show possible technical solutions, which may also be
used in
combination with cooling of the anode yoke (WO 2006 088375).
In Fig. 2a, the anode stem 1 contains a longitudinal pipe 22 for the cold
fluid supplied or
recycled at the top, and another longitudinal pipe 23 for the hot fluid coming
from the bottom
of the stem or from the yoke and the bottom of the stem. The latter pipe is
thermally insulated
24 in order to avoid heating of the cold fluid or the anode stem itself. The
pipes can be made
two in parallel as in Fig. 2a or concentric as in Fig. 2b.
In Figure 2b the anode stem l' contains a longitudinal pipe 22' for cold fluid
supplied or
recycled at the top, and another longitudinal pipe 23' for hot fluid coming
from the bottom of
the stem or from the yoke and the bottom of the stem. The pipes are arranged
concentric with
a layer of insulation 24' between them.
The preferred technical solution should as earlier stated be a fluid that
evaporates at the lower
part of the stem or within the anode yoke, and is condensed at the upper part
of the stem.
Since there is a relatively large surface of contact between the anode beam
and the stem, the
heat from the top of the stem can be extracted by cooling the anode beam. This
eliminates the
extra work needed during anode replacement, if the fluid supply to and from
the stem or yoke
must be connected and disconnected.
The anode stem should be supplied with a relief valve, in case increasing
temperature should
lead to an unacceptable pressure build-up.
Circulation of the cooling medium can be forced by a pump or a compressor.
Circulation can
also be simply triggered by buoyancy. This is the classical concept of
thermosiphon. The heat
transfer fluid is heated at the bottom (yoke). It expands and flows to the top
(outside the
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electrolysis cell) where it is cooled. Its density increases and it falls back
to the yoke. In this
prospect CO2 based thermosiphon was found particularly promising. CO2 is an
inert gas
reducing safety issues, and heat exchange properties are very good.
Calculations showed that
0.014kg/s of CO2 at 50bars could carry 3kW between the hot side (yoke) at 300
C and the top
of the stem maintained at 100 C. If the heat transfer fluid is filled at a
pressure larger than the
critical pressure (70bars), the thermosiphon operates in transcritical mode.
Very large density
difference between the cold and hot sides, and then large flows can be
achieved without phase
transition which greatly reduces the risk of instabilities.
In order ensure a large heat extraction, the heat transfer fluid must be
cooled above the
superstructure. There are numerous ways of realising this cooling. The
simplest way, but not
the more effective, is to increase the surface area of heat transfer circuit
above the
superstructure with cooling fins. Those fins could for instance be sprayed by
water or by a
forced flow of air. The forced air flow can be provided by a fan, a lance
delivering pressurized
air, or by any other appropriate means.
A more advanced solution would be to couple the top the heat transfer circuit
with an external
cooling module. Heat exchange between the heat transfer fluid and refrigerant
could be
ensured by a proper heat exchanger. To increase cooling the vapour in the top
of the hanger,
the pipe that transport the warm gas upwards through the hanger is widened at
the top of the
hanger, i.e. to a small container. The container should be placed above the
area where the
current goes into the hanger from the anode beam.
However a solution that requires opening of the cooling circuit can be a
tedious operation.
Solid contact between fins of the heat transfer and the cooling circuit is
another possibility,
i.e. a cooling hood that is mounted on the top of each anode hanger will
ensure a large surface
area and good heat transmission to the cooling circuit.
An option that would solve all problems related to connection and
disconnection during
replacement of an anode would be to dissipate the heat into the anode beam by
conduction
across the electrical contact surface. This may require cooling of the anode
beam, which
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would lead to added benefits such as decreased ohmic resistance and better
mechanical
properties of the anode beam (increased creep resistance).
Ideally the heat extracted should be utilized for power production. The
cooling circuit would
then preferably be of Rankine type with an expansion turbine driving a
generator.
Heat extracted from several anode stems can be collected and led to an energy
conversion unit
conveniently arranged outside the pot room.
Recently thermionic materials have been developed. Such material installed on
the fin of the
heat transfer circuit would ensure cooling and convert the heat into
electricity without
complex connection.
As should already be clear from the descriptions and argumentation above, this
way of
extracting heat will add to the potential of amperage increase, as well as
reducing the demand
for higher rate of air suction following amperage increase. But it should also
be mentioned
that:
= By reducing the temperature in the yoke and anode hanger the electrical
conductivity
through the hanger and yoke is increased, i.e. saving energy.
= The invention will help stabilizing the temperature in the hanger and
yoke at a lower
level than today and make it possible to remove the bimetallic joint. If not
removing it, it will
live for a longer period.
= With a more stable temperature in the hanger and yoke, measurement of the
amperage
through the individual hangers can be measured indirectly more accurate than
today, by
measuring the voltage drop over a specified part of the hanger.
= Reduced temperature in the raw gas due to cooling of the anode assembly
will lead to
a lower pressure in the cell resulting in less Nm3 air needed to be sucked
from the cell
(reduced energy consumption on fans) to keep a certain under pressure in the
cell.
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= Less Nm3 sucked from the cell means less dimensions (reduced investment)
on the dry
scrubber system. Lower temperature on the raw gas means less maintenance
(reduced
maintenance cost) on the filter bags in the dry scrubber.
With less heat given away from the anode assembly to the cell, less heat will
be lead through
the hoods and into the working zone, in other words it will be less heat
stress on the operators.
This is especially important in the summer time or in parts of the world with
a high
temperature in the pot room.
= By regulating the cooling of the anode assembly it will be possible to
change the net
heat input into the cell. This can be used when the power in the pot line is
reduced for a
shorter or longer time by removing less heat from the hanger. In this way the
number of cells
that has to be shut down due to lack of enough power will be reduced. This is
not possible to
do, if a solution with increased stubs/ yoke/hanger dimension is chosen as a
mean to increase
the amperage.
= The proposed technical solution can also be used by regulating the effect
input to the
cell under normal operation instead of moving the anode up and down (power
pulsing). If the
cell needs more heat, less heat is removed from all or some of the anode
assemblies on the
cell, and if the cell needs less heat more heat could be removed from the
anode assemblies
than normal. In this way the need for upwards and downwards movements of the
anode to
increase or reduce the heat input to the cell will be less and therefore it
will be possible to
keep a more constant interpolar distance (ACD). By keeping the ACD more
constant the
fluctuation in the bath level will be reduced, and also the process control
will be improved
since movements of the anode normally will disturb the resistance signal to
the regulator
deciding the alumina addition.
= By cooling the yoke the need for long anode stubs (typical 30 cm) will be
reduced, and
thereby it will it be possible to reduce the specific energy consumption due
to lower voltage
drop in the stubs. A reduction of 10 cm should not be a problem. This will
also increase the
heat loss from the stubs.
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= Reduced length of the stubs will allow for higher anodes without
increasing the height
of the superstructure. (Reduced investment cost)
= A colder anode yoke will reduce the maintenance cost of the bimetallic
plate in the
hanger due to lower temperature in the bimetallic plate, and also reduce the
cowboy effect due
5 to less thermal expansion of the yoke, and thereby less expansion force
working on the stubs.
= If the temperature on the stubs is reduced, the possibility of anode
cracking due to a
higher thermal expansion on the stubs than on the anode will be reduced.
= A lower temperature on the yoke will also make it more easy to use other
materials in
the yoke than steel, by instance copper with a higher thermal conductivity and
higher
10 electrical conductivity than steel. Even an aluminium yoke could be
considered.
In order to illustrate and emphasize the main ideas and features of the
present invention, a
simplified model of the anode stem and its surroundings was made.
The model takes into account the thermal conduction along the anode stem and
the heat
dissipated from the stem. The heat transferred from the stem to the
surroundings was
calculated using a single heat transfer coefficient intended to contain both
the convectional
heat transfer and the electromagnetic radiation. As already indicated, the
model was not
intended to be very accurate, but still, the results should be regarded as
much better than
order-of-magnitude-estimates. In the calculations, the boundary between the
lower end of the
anode stem and the bimetallic plate was assumed to be constant (280 C).
Four cases were taken into consideration, as briefly explained below,
Case 1: No thermal insulation on stem, no extra cooling (reference
case,
today's standard).
Case 2: No thermal insulation on stem, stem cooled to 50 C 1 m
from the lower
end.
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Case 3: Stem thermally insulated below (inside) the superstructure,
and cooled to
50 C 1 m from the lower end.
Case 4: Stem thermally insulated below (inside) the superstructure,
but no extra
cooling.
The results of the calculation are given in Table I (heat flows) and in Figure
2 (temperature
gradients along the stem).
When comparing Case 2 and Case 1 (the reference case), one observes that the
cooling of the
stem outside the superstructure brings about an increase in the amount of heat
conducted into
the anode stem. This effect would, of course, be even more pronounced if the
anode stem was
cooled to a lower temperature, or cooled closer to the yoke.
Case 3 is comparable to Case 2, except that the stem is thermally insulated
below (inside) the
superstructure. In this case, the amount of heat conducted into the stem
becomes lower, but on
the other hand, the heat dissipated into the raw gas is eliminated. Insulating
the yoke is
therefore an effective means of reducing the raw gas temperature. When
comparing Case 3
and Case 4, however, it is clear that insulating the stem should only be done
in combination
with cooling, or else there will be a considerable decrease in the heat
conducted into the stem.
There are numerous ways of realising cooling of the anode stem. The simplest
way, but not
the more effective, is to increase the surface area of the stem above the
superstructure, i.e.,
supply the anode stem with cooling fins. Other ways of realizing cooling of
the anode stem
are described in previous embodiments. However, it should be understood that a
realization of
the invention is not limited to those embodiments.
Case 1 Case 2 Case 3 Case 4
Conducted from yoke to stem 1211 1404 1121 610
Dissipated outside superstructure 512 964 1121 610
Dissipated into raw gas 699 440 0 0
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Table I. Heat flows (in W) into and out from the anode stem in four different
cases as
described in the text above.
