Note: Descriptions are shown in the official language in which they were submitted.
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Heat Exchanging Arrangemen-t including Cells for
electrometallurgical Purposes
in particular Aluminum Electrolysis.
BACKGROUND OF THE INVENTION
During electrolysis in the electrometallurgical
industry, for example in the aluminum melting industry, large
amounts of power are lost in the form of heat from the cells
employed in the processes. As far as the running and the
efficiency of the actual process is concerned, it is also very
important to take into account the cooling conditions.
Particularly, in recent time there has been a growing interest for
energy economy and recovery, and thus there have been put Eorward
various proposals for heat recovery in the above industry.
Examples of known proposals in this direction may be
found in Published British Patent Applications No. 2,076,428 and
No. 2,047,745. In the former British application there is des-
cribed removal of heat by means of a number of cooling elements in
the sidewall of the cell. The cooling is controllable, inter alia
~0 by means of valves for the flow of cooling medium in each element.
These cooling elements consist of pipes. The control takes place
in response to heat sensors provided in the sidewall. The speci-
~; fication, however, does not give any e~planation as to whether the
purpose bf the arrangement is to recover energy. The arrangement
proposed aims at controlling the temperature in the cell, and more
particularly in the cell bath.
~ On the other hand British Patent Application ~o.
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2,047,745 describes recovery of energy with heat exchangers pro-
vided above the bath and in the sidewalls respectively, possibly
also in the bottom. The purpose of this is to produce steam or
electricity at the same time as -the side coating (crust) shall be
secured or maintained. The cell walls shall be well insulated.
~here is provided a cover above the bath so that the cell will be
closed. A temperature sensor measures the electrolyte tempera-
ture. Air is used as a cooling medium, which is highly hazardous
in the environment concerned.
In the practical operation of control, cooling and heat
recovery in the electrometallurgical industry it is of substantial
significance to take into account the need for individual control
of the temperature distribution at the side and bottom surfaces oE
cell boxes of the various types found within the electrometal-
lurgic industry. Moreover, the high and diverse stresses to which
cells and auxiliary equipment are subjected to within this indus-
try, make it necessary that all equipment installed near the pro-
cess either stand up to the stresses concerned or that the equip-
ment at least cannot cause any significant damage if it should
~ail. This also applies to media used in the operation, such as
coolants, for example.
In aluminum electrolysis for example cells are construc-
ted with a cell box having an internal refractory lining in bottom
and walls. ~he structure of the bottom and walls is to a sub-
stantial degree aimed at withstanding the high temperatures and
strong corrosive forces which occur by contact with the molten
bath. Corresponding stresses act also on the bottom faces of the
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anode. These contact surfaces or parts of the cell which
essentially delimit the bath sideways, downwards and upwards, are
decisively siynificant to the above heat and temperature
conditions.
SUMMARY OF TH~ INVENTION
An object of the present invention is to satisfy khe
requirements which are imposed on control systems and equipmen~ in
the electrometallurgical industry as discussad above. On the one
hand it is a question of making the operation of each cell more
effective, and, on the other hand, to be able to utilize the heat
output from the cell for recovering power.
Cell arrangement for electrometallurgical purposes
electrolysis comprising a cell box having an internal refractory
lining in the bottom and walls thereo~, said inkernal refractory
lining providing contact surfaces against a cell bath; an anode
partially immersed within said cell bath; and a heat exchanger
associated with at least one of said contact surfaces, said heat
exchanger comprising a plurality of cooling chambers, each of said
cooling chambers having a base area covering a small proportion of
the area of the contact surface and said plurality of cooling
cha~bers together covering a substantial proportion of the area of
the contact surface without any significant space between said
cooling chambersr cooling chambers being adapted to rece1ve a
throu~h-flow of a cooling medium which is controlled individually
for each cooling chamber in response to a system for temperature
control connected to temperature sensor devices within said
cooling chambers; wherein said heat exchanger i5 directed
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incorporated in a closed circuit with an expansion engine and said
cooling medium in said heat exchanger is a working medium in said
expansion engine.
Primarily it is oi interest to provide a heat exchanger
in the sidewalls or the botto~ of the cell or in both the sidewall
or the botkom of the cell box. However, in certain si~uation~ the
heat exchanger may be located ln the anode, in particular whan
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contemplating new anode designs which may be developed. Advan-
tageously the controllable heat exchanger can serve to secure a
desired side coating or crust layer in the cell.
As will be described more closely below it is an advan-
tage that the heat exchanger comprises cooling chambers each hav-
ing a base area which covers only a small proportion of the area
of ~he contact sur-face concerned, and which together cover a sub-
stantial proportion of the area of the contact surface without any
significant space between the cooling chambers, and that the cool-
1~ ing chambers are adapted to have a through-flow of a cooling
medium being controlled individually for each cooling chamber.
Concerning in particular the cell walls and the bottom respect
ively, at the parts being covered by cooling chambers, the struc-
ture can have a signiiicantly reduced total thickness and heat
: transfer resistance compared to what would be required when the
cooling chambers were not present.
With this sub-division of the cooling or heat recovery
system by means of the comparatively small chambers, there are
for~ed separate flow or recircu.lation circuits which by suitable
control makes it possible to adapt the cooling and the heat output
respectively, at the dif~erent portions of the cell with high
accuracy according to the local temperature conditions therein, in
particular in -the cell walls and bottom. Thereby it will be
possible to obtain a cooling of the various portions of the cell
; so that there is obtained a desired temperature distribution in
the cell itself and in particular in the cell walls, and also
obtain an optimal heat recovery. In this way there is also
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obtained an advantageous effect to the cell operation as such,
since portions thereof having a tendency to for example undesire-
ably increased temperatures, may be eliminated~ The cell design
itself can thereby also be carried out simpler and cheaper than
according to the manner of construction now being common, because
the cooling and heat recovery system takes care of the heat
developed in a more favourable way than what has been the case
hitherto. ~ot the least the arrangement according to the inven-
tion involves a possibility of operating with a significantly
increased amperage and thereby an increased production, with the
same cell design. This is due to the much more effective cooling
effect which is obtained. This is to a substantial degree due to
the direct circulation and control of the coolant and working
medium, as will be described below. Since that part of the cell
box which is between the cooling system and the process or melt
bath, has a low heat capacity and a low thermal resistance, the
cooling can be controlled quickly so that a cell row can be
regulated in a short time for a lower or a higher current.
DETAILED DESCRIPTIO~ OF THE INVE~TIO~
The invention will be further described with reference
to the accompanying drawings showing, by way of example, an
embodiment of the invention. The particular illustrated embodi-
ment is a cell for recovery of aluminum but it will be appreciated
that the invention can be used in electrometallurgical cells other
than cells for the recovery of aluminum.
Of the drawings:
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FI~. 1 shows a simplified cross-section of a part of the cell
wall and bottom as well as the anode in an aluminum
electrolytic cell included in an arrangement according
to the invention,
FIG. 2 is a simplified elevation view of a sidewall module or
block which can be included in the arrangement of FIG.
1, and
FIG. 3 shows highly schematically a recirculation circuit for a
cooling medium included in a system for temperature
control with the arrangement according to the inven-
tion.
In accordance with common design practice the electro-
lytic cell in FIG.l has an internal refractory lining which com-
prises a bottom lining 1' and a wall lining 1. Suitably the lin-
ing can consist of a material having good properties with respect
to the ability to resist corrosive attacks from the electrolyte
~; and from molten aluminum, as well as reasonably good properties
with respect to thermal and electrical conductivity. Nowadays it
is common practice to use carbon based materials such as anthra-
cite or graphite, but other materials can also be used for this
Eunction. Possibly there may be applied a steel plate enclosure
outside the lining, but this is not regarded as necessary in
-~ connection with this invention, since the practical construction
o~ such a cell intended for an arrangement according to this in-
vention, can take place more effectively without such a continuous
plate structure which is common in conventional aluminum electro-
lytic cells.
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Above the cell bottom there is shown a layer of molten
aluminum 4 and on top of this an electrolyte layer 3 consisting of
molten alumina and cryolite. Moreover, there is shown a side
coating 5 and a crust layer 5' consisting of solidified cryolite.
As known the side coating 5 has an important function in the cell
operation, and it is very significan-t to effect control of the
temperature conditions in the cell so that there is formed such a
side coating 5 of suitable shape and -thickness. The side coating
serves inter alia to protect the wall lining 1 against the strong
corrosive effect which may be caused by the electrolytic bath 3.
In this connection the temperature gradient through the various
layers from the melt bath 3, 4 out through the side coating 5 and
the lining 1 is very important. The same also applies in part to
; the heat transfer conditions through the bottom structure of the
cell.
The cell design according to FIG. 1 is specific in so
far as the cell walls and bottom respectively, have a significant-
ly reduced thickness of the lining and a low thermal resistance
through the lining, compared to what has been used earlier in cell
structures for electrometallurgical purposes, for example aluminum
electrolysis. In this branch of inaustry there has been a very
conservative attitude to the dimensioning of such cell boxes,
perhaps in particular because of the expensive and potentially
dangerous consequences which may occur when a cell box is molten
through so that the molten coDtents may flow out. By providing a
cooling system as described here it will be possible to reduce to
a high degree the dimensions and the material requirement for
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~constructing the.se cell boxes, since the necessary control and
local cooling is ~ffected in a new and advantageous manner which
is to be described in the following.
As will appear from FIG. 1 there is provided a heat
exchanger system comprising side cooling chambers 6A, 6B and GC
engaging the wall lining 1 and bottom cooling chambers 6' beneath
lining 1'. There are shown cooling chambers 51 in the anode 50 of
the cell.
The cooling chambers 6A, 6B and 6C on the cell wall have
a base area or surface of engagement covering a comparatively
small proportion o~ the sidewal} of the cell. The base o~ the
cooling chambers can advantageously have an approximate square
æhape. The cooling chambers are located with an unsignificant
spacing and are adapted to receive a through-flow of a cooling
medium with individual control for each cooling chamber.
As seen from the interior of the cell the cooling
chambers (heat exchanger elements) 6A, 6B, 6C lie behind the
linlng 1 and further beh1nd the chambers there is mounted a heat
distributing plate 16 which in the first place has a safe~y
function. The plate 16 shall distribute the heat to adjacent
chambers i~ one of the chambers should fail, possibly at
connections thereto. Einally a highly insulating material can be
pxovided behind the heat distributing plate 16.
FIGS. 1 and 2 illustrate somewhat more in detail the
~; cooling system for the cell wall. The cooling system (Fig. l)
comprises supply plpes 7A, 7B, 7C having a com~on supply as
indicated at 7. In FIG. 2 correspondiny supply pipes are denoted
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7A', 7B' and 7C'. For each cooling chamber 6A, 6B, 6C (FIG. 1)
there are inserted control valves 8A, 8B and 8C respectively, in
the corresponding supply pipes. Moreover, for these chambers
there is shown a common return conduit 9 with short pipe sections
to each of the chambers, of which the pipe section 9A for chamber
6A has been indicated specifically.
As essen~ially parts of the sys~em for temperature
control of the cell shown, there is illus~rated in FIG. 1 in a
purely schema~ic and simplified manner, a control unit 40 which
suitably can be a computer, and which delivers a setpoint through
outputs indicated at 41, to a number of control devices 10 which
in their turn actuate the above mentioned valves 8A, 8B and 8C.
In addition to a setpoint from the control unit 40 there is
applied to the control devlces 10 one or more measurement value~
relating to the heat conditions in and in association with the
cooling chamber 6A, 6B and 6C. Thus, in chamber 6C there is shown
a te~perature measuring element 18 and besides a heat flux meter
;.,
19, the measurement values from these elemen~s being lead each to
a separate control device 10 as shown. Thereby the flow of
cooling medium can be controlled individually for each cooling
chamber. In accordance with conventional control methods the
control uni~ or computer 40 can calculate the respe~tive setpoints
on the basis of desired cell operation parameters and measurement
values from dif~erent parts of the system or cell lnstallation.
In connection wi~h FIG. 1 there is only mentioned three
cooling chambers 6A, 6B and 6C above (in FIG. 2 thxee chambers
6A', 6B'and 6C') but it is evident that a higher number of such
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27~84-2
cooling chambers are provided along the whole length of an
electrolytic cell in order ~o cover a subs~antial portion of the
wall surface. Cooling chambers are mounted over all those parts
of the wall surface which is of significance for the cooling and
control during operation of the cell.
According to the invention an adva,ntageous embodiment
consists therein that the cell wall is built up sectionally by
moclular blocks, ~f which one block or module i5 shown in FIG~ ~.
Thiæ figure shows corresponding three cooling chambers 6A', 6B'
and 6C', as in FIG. 1, with associated supply pipes 7A', 7B' and
7C' respectively. For simplicity the valves in these pipes are
not included in FIG. 2. Possibly the valves can be located
outside the modular hlock so that the structure thereof will be
somewhat simplified. For each cooling chamber 6A', 6B' and 6C'
there is indicated an associated square lining part lA, lB and lC
which can either be composed oX separate lining parts or may
constitute a continuous element ~or the block. The cooling
chambers are shown in FIG. 2 with a circular basic shape and have
a central entry of the supply pipes 7A', 7B' and 7C'. The
connection of a return conduit (not shown) from each of these
chambers is indicated at 9A, 9B and 9C respectively. Like the
supply pipe 7A', 7B' and 7C' the return conduit from each chamber
can be extended vertically upwards for connection to the remaining
circulation system at the upper edge of the cell wall, as
lndicated in FIG. 1.
In order to obtain a favourable circulation and
distribution of the cooling medium in each cooling chamber these
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can be provided with internal distribution walls, as shown
specifically in the cooling ~ha~ber 6C' in FIG. 2. Thus, in
relation to the
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circ~lar shape of the cooling chambers shown therein, the distri-
bution wall 29 in the chamber 6CIhas a spiral shape which leads
the cooling medium in a spiral shaped flow path from the center
out towards the connection to the return conduit at 9C adjacent
the periphery of the chamber.
The measuring elements 18 and 19 are not shown in FIG.
2, but the location thereof will be in accordance with known
principles for instrumer.tation. In addition to pure temperature
measuremznt in the cooling medium, possibly in the wall lining,
there can also be provided ~or measurement of heat flow in the
chambers theat flux meters 19).
The modular block 20 as shown in FIG. 2 can be mass
produced with all associated elements and plpe fittings ready for
mounting and coupling in connection with the construction of a new
cell or restoration o~ a cell which has been in operation and
initially based on a system as described here - possibly also as a
replacement Oe the lining in a cell which has been based on
earlier technology.
An arrangement of cooling chamber~ on the cell wall~ has
been described above. FIG. 1, however, also Rhows a heat exchang-
er with cooling chambers 6' und2rneath the bottom lining 1' of the
cell, with associated circulation pipes for a cooling medium. As
th~ temper~ature and heat conditlons in thc bottom ar~ not ~o
critical as they are along the cell walls, the cooling chambers 6'
under the bottom do not have to be as small as explained in con-
nection with the wall structure. Thus, the chambers 6' in the
bottom can extend across a larger portion of the cell or possibly
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over the whole length thereof. ~evertheless it may be an advan-
tage to have a heat distributing plate 16' included.
For a more complete heat recovery and possibly a desired
cooling effect, there is also in the anode S0 shown cooling cham-
bers 51 provided with corresponding conduits, valves and control
devices corresponding more or less to those discussed above in
relation to the sidewall of the cell. Also in the anode there can
~e provided a heat distributing plate 56 behind the cooling cham-
bers. The provision of such cooling chambers in the anode re-
quires a modified design thereof in relation to what is conven-
tional techniques. With such cooling of the anode in aluminum
electrolytic cells great advantages can be obtained.
As a cooling medium, according to the present example,
helium is used, as on the one hand it has favourable flow proper-
ties and on the other hand it is a favourable medium for heat
transportO Moreover, since helium is a one atom, inert gas it is
not dangerous when employed in connection with electrolytic cells
; comprising high temperatures, electric current and other risk
factors. The use of helium is particularly advantageous when the
temperature control is also intended for heat recovery and not
only for a pure cooling effect for purposes of the cell operation.
When the arrangement according to the invention is
included in a system for heat recovery it is an important feature
that the helium circulatlon takes place in a closed circuit for
direct heat exchange to the high pressure side of a thermodynamic
engine (expansion engine), for example a turbine, which utilizes
heat recovered from the cell.
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Helium is a one atom gas having a high Cp/Cv ratio and a
low viscosity. This makes helium well suited as a working medium
- in a thermodynamic engine.
The principle for production of electric power by means
of a closed gas circuit and a compressor, a high temperature heat
exchanger, a gas turbine and a cooler is well known, and is desig-
nated Joule's ideal gas cycle~ The theoretical maximum efficiency
is lower than for Carnot's cycle, but it is not much lower. The
equation for efficiency is given by:
N = l-(Pl / P2) (k-l) /k
Pl = Minimum pressure
P2 = Maximum pressure
K = Cp/Cv
~;~ Cp = Specific heat at constant pressure
Cv = Specific heat at constant volume~
For helium K is practically independent of temperature and
pressure and equal to 1.67.
As shown by the equation, the efficiency increases with
increasing pr~ssure ratio. The problem is that the temperature
; 20 in the gas increases strongly with an increasing degree of com-
pression~ and this involves that less heat can be absorbed per
.
cycle when the maximum temperature is given.
The principle of the heat r~covery is shown schemati-
cally and simplified in FIG. 3~ FIG. 3 shows a heat exchanger 32
which comprises an arrangement of several cooling chambers as
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described above. From this heat exchanger 32 helium circulates to
the high pressure side 30A of a turbine which drives a generator
31, for example for producing electric power. Moreover, helium
circulates to a second heat exchanger 33 at the low pressure side,
with a possible subsequent control valve 34 and then to the low
pressure side (the compressor part) 30B of the turbine. From
there the helium flow goes back to the heat exchanger 32 on the
electrolytic cell or cells. This direct heat exchange from the
cell to the high pressure side of the turbine aggregate involves a
strong simplification of the whole heat recovery system and has
been made possible inter alia by employing helium as the cooling
medium, which permits a lower maximum pressure in the circulation
system.
The secondary heat exchanger 33 makes it possible to
utilize still further portions of the waste heat, for example for
water heating.
When the generator 31 shall supply electric alternating
current at a substantially constant frequency, for example 50 Hz,
the rotational velocity of the turbine 30A should be kept constant
with a varying heat transfer to the high temperature heat exchang-
er 3~. Such variations will occur during normal operation of
aluminum electrolytic cells. The regulation thereof takes place
; through changes in the amount of circulating helium, i.e. by
pressure changes in the closed circuit. Introduction of helium
increases the pressure, whereas extrac-tion of helium from the
circuit will lower the pressure therein. This is preferably done
at point 39 in which there is a comparatively low pressure and low
temperature,i.e., behind the low temperature heat exchanger 33.
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Control of the pressure or amount of helium can be
effected in various ways, but it is preferred to avoid a
consumption or loss of helium in this connection. Thus, in FIG. 3
there is shown a pressure tank or accumulator 61 for helium and an
associa-ted valve 63 which permits of a controlled supply of helium
from the tank 61 to the circulation circuit at point 39.
Moreover, there is provided a compressor 62 which through another
valve 64 serves to control the lowering o~ pressure in the
circuit, by transferring (compress) helium to the tank 61. During
such a pressure lowering operation valve 63 is obviously closed.
The regulation described here can take place under the
control of a calculating unit 40' which suitably can be constitu-
ted by or can be included as a part of the computer 40 in FIG. 1,
whereby the relevant input signals for controlling the helium
circulation will be obvious to an expert, the amperage at which
the electrolytic cells are operated, being an important para-
meter.
~; The regulation arrangement with the pressure accumulator
tank 61 and compressor 62 and associated valves can be common to a
number of or all cells in an electrolysis plant, or such arrange-
ment can be provided for each cell.
Control for obtaining a substantially constant rotation-
al veloclty as mentioned, is also advantageous with most interest-
ing ty~es of expansion engine (turbine~ 30A and the associated
compression engine (compressor) 30B~ These types of engine1 as a
/ rule have a relatively narrow range of rotational velocity with
maximum efficiency.
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It is to be understood that the cell arrangement of the
present invention can comprise a plurality of cells.
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