Electrolyzer apparatus
The invention which claims priority to EP patent application
N° 10191586.6 filed on November 17, 2010 the whole content of which is
incorporated herein for all purposes, concerns an electrolyzer apparatus and a
method of producing elemental fluorine.
Elemental fluorine is applied for a lot of purposes. It can be used for the
surface fluorination of polymers, for example to manufacture car tanks with
lower permeability for fuel. Highly pure elemental fluorine is applied as etching
agent or chamber cleaning agent in the manufacture of semiconductors,
photovoltaic cells, micro-electromechanical devices and flat panel displays.
Fluorine is widely produced electrolytically from HF. In the presence of
an electrolyte salt, HF releases fluorine if a voltage of at least 2.9 V is applied.
Practically, the voltage is often kept in a range of 8 to 10 or 11Volt.
A molten HF adduct of KF, often having the formula KF-(1.8-2.3)HF, is
the preferred electrolyte salt. HF is fed into the reactor containing the molten
electrolyte salt, and F2 is electrolytically formed from the HF according to the
equation (1) by applying a voltage and passing electric current through the
molten salt :
2HF H2 + F2 (1)
The electrolysis as known in the state of the art is performed in electrolytic
cells ; several cells are assembled in a cell room. The cathode of each cell is
presented by the cell vessel (also denoted as trough) which is made from metal or
metal alloys resistant to HF and F2, especially from stainless steel or nickel. The
cell vessel is connected to the (-) pole of a rectifier or it is further linked onto the
next anode bus bar in case of serial connection. Each cell often contains several
anodes, typically 20 to 30, which may be, for example, nickel anodes, carbon,
sintered material, diamond-coated anodes or comparable materials, but usually
are made from carbon. A single rectifier's (+) pole side (or a cathode in case of
serial connection) is connected to a (+) bus bar mounted onto the electrolytic cell
supplying different anodes in parallel. The electrolysis cells of a respective cell
room are preferably connected in series by a direct current (DC) conductor
system which loops from the rectifier (+) pole to the anode bus thus connecting
the different anodes in parallel.The DC current is imposed by a closed current loop control through a
single rectifier (commercially available) which supplies all the DC current into
the DC bus bar system. The electrolytic cells are connected in series from (+)
to (-), connecting the (+) pole of a main bus bar to the anode of the first cell and
the (-) pole of a main bus bar to the cathode of the last cell ; in between, the
respective cathode is connected with the respective anode by a short bus bar. In
such a serial connection individual cells may be bypassed by a short circuit
switch.
The distribution of electric current among parallel anodes of the described
cells shows various influencing parameters in respect to the ohmic resistance
between cathodes and anodes. The bus bars between the connecting points may
have a different size (length and/or width), there may be differing contact
resistances, the anode resistance may differ, the anodes may have differing
temperatures, the resistance between anode surface and the electrolyte may
differ, the electrolyte resistance may differ due to varying compositions of the
electrolyte, due to geometry, e.g. of the anodes, the cell or the arrangement of the
anodes, differing temperatures of it and possible fluctuations of the electrolyte in
the cell, the HF supply and/or the filling level of the cell may be different and
vary, there may be influences by electric field effects, the contact surface and the
resistance of the cathode vessel may differ etcetera. As a consequence, each
anode - cathode loop may have (and often has) an individual ohmic resistance.
The differences of the ohmic resistance of individual anodes cannot be
influenced or controlled by variation of the voltage, and the highest current will
pass through the anode with the lowest resistance between the connection point
on the anode side and the cathode side. It was observed that the current passing
through the anodes may differ strongly, in a ratio of 1.5 : 1. Consequently,
anodes conducting higher current may overheat, surface wear of the anodes may
vary, the anode may erode and break, and undesired reaction products may be
observed as a consequence, e.g. CF4, C2F6 or other perfluoro compounds,
especially if a worn carbon anode breaks or burns in the F2 atmosphere. Pieces
of broken anodes may provoke a short circuit inside the cell, with the risk of
heavy reactions inside the cell. It was mentioned above that electrolytic cells
often have 20 to 30 anodes, and it is difficult and time consuming to search for
the faulty anode after signs of malfunction is observed. The necessary shutdown
of the electrolytic cell is undesired of course, in view of economic efficiency.
Especially annoying is the increase of the CF4 content, because in certainapplications, e.g. in the manufacture of photovoltaic cells, TFTs or
semiconductors, highly pure F2 is required.
Problem to be solved by the invention is to provide an improved
electrolyzer apparatus for the manufacture of F2, especially, for the manufacture
of highly pure F2 as needed for the application in the electronic industry, for
example, in the manufacture of semiconductors, photovoltaic cells, micro-
electromechanical systems and TFTs.
The electrolyzer apparatus for the electrolytic manufacture of elemental F2
from an electrolyte comprises at least one electrolytic cell which contains at least
two anodes, a vessel for the electrolyte where the vessel also serves as cathode,
and at least two rectifiers such that one rectifier is allocated to one anode. The
term "one rectifier is allocated to one anode" means that each rectifier is
allocated to only one anode, and that each anode is allocated to only one rectifier.
Preferably, the electrolytic cell contains more than 2 anodes and more than
2 rectifiers. Thus, if the cell contains 26 anodes, the apparatus comprises
26 rectifiers, one rectifier for each of the anodes. As will be described in detail
below, two rectifiers can be combined to form a dual rectifier ; still, also in this
embodiment, always one individual rectifier in this dual rectifier is allocated to a
specific anode. The provision of dual rectifiers is especially advantageous for
plants producing F2 for semiconductor applications, especially for the use as
etchant and chamber cleaning agent in the manufacture TFTs and especially of
photovoltaic cells. The term "rectifier" denotes a single rectifier ; two combined
rectifiers are denoted as "dual rectifiers". If in the following description, a
certain number "x" of rectifiers are mentioned, then the reader will know that
alternatively, "x/2" dual rectifiers can be provided.
Brief description of the drawings
Figure 1 indicates the voltage variation for a cell having 26 anodes in
normal operation.
Figure 2 provides a scheme of an electrolyzer apparatus of the present
invention including a Basic Process Control System BPCS.
Figure 3 provides a scheme of an electrolyzer apparatus of the present
invention.
Figure 4 describes a plant for the manufacture of pure fluorine.
Detailed description of the invention
Figure 1 demonstrates the voltage variation in normal operation (no broken
anodes) for a cell with a total current of 5382 amperes and an averageof 207 amperes per anode. It shows a tolerance band and variations of voltage
for 26 anodes, the rectifiers grouped in 26 rectifiers in an electrolyzer apparatus
of the present invention. Dual rectifiers comprising two rectifiers inside one
housing may be applied ; this was tested successfully in pilot installations. The
data given in figure 1wee obtained using an apparatus with dual rectifiers ; the
term "left row" and "right row" denotes the arrangement of the anodes in the
form of two rows in the cell. As indicated in figure 1, the total range of variation
among the anodes is less than 6 % which is to be compared with the state of the
art systems where a variation of approximately ± 25 % can be observed.
Anodes' rectifiers are imposing exactly the same current per anode, so the
variation among anodes is expressed by voltage tolerance for each operating
anode (arranged in two rows).
Detailed description of the invention
In the following, figure 2 will be described in detail.
Figure 2 provides a scheme of an electrolyzer apparatus of the present
invention including a Basic Process Control System BPCS (consisting of a
Distributed Control System DCS referenced as A and a Programmable Logic
Controller PLC referenced as B), an electrolytic cell C, a multitude of rectifiers 1
and a respective multitude of anodes 2 .
The apparatus comprises a Basic Process Control System BPCS consisting
of a distributed control system DCS referenced as A, a programmable logic
controller PLC referenced as B and an electrolytic cell C. The programmable
logic controller PLC may be realized as discrete unit or could be a part of a
distributed control system DCS.
The distributed control system DCS A serves mainly three purposes : it
receives/generates the master current set point, it allows individual adjustments
per anode generating an individual set point for each anode rectifier individually,
and it sets technical limits as there are rectifier limit voltage and rectifier limit
current. It also indicates, preferably in respective instruments which give the
value in digits or in analogous form, actual values for current, voltage as
determined by sensors.
The PLC B contains programmable logics which compare the individual
settings of the anodes with the values provided by measurements and prompts an
increase or decrease of the voltage or increase of current of individual anodes in
case of a deviation from the settings. It also includes an "on/off logic which
provides a smooth start of the electrolysis process after shut down, and a shutdown control after operation. Programmable logic controller B preferably also
comprises programmable logic which may interact with plants emergency
shutdown system.
The apparatus of figure 2 also contains an electrolytic cell C.
The apparatus may comprise a multitude of rectifiers la, lb . . . ., preferably
20 to 30, or, alternatively, 10 to 15 dual rectifiers, and a multitude of anodes 2a,
2b, . . . ., for example, 20 to 30. Always one rectifier is connected with one anode.
In the apparatus of figure 2, two rectifiers are always grouped into one dual
rectifier, indicated in figure 2 as dual rectifier la, lb and l c ; but also when such
dual rectifiers are applied in the process of the present invention, each rectifier of
this dual set is connected to one individual anode. In the apparatus of figure 2,
only six anodes 2a . . . . 2f and only three dual rectifiers la, lb and l c are
displayed for the sake of simplicity ; it should be kept in mind, however, that
often, the number of rectifiers and anodes will be higher, e.g. from 26
to 30 rectifiers or 13 to 15 dual rectifiers per cell. If, for example, 26 anodes
2a . . . 2z are contained in the cell, 13 dual rectifiers la, lb, lc, . . . . Ik would be
present.
All rectifier (-) poles are connected to the vessel C (which is the cathode)
by a single bus bar 3 . Each rectifier la, lb . . . . is connected individually via a
conductor 4a, 4b, . . . . 4f to an anode 2a, 2b . . . . 2f.
The Distributed Control System DCS A comprises a unit 5 wherein
individual anode current correction factors can be set for the anodes 21, 2b, . . .
however the purpose is to distribute the current master set point entered in unit 8
to the individual rectifiers. Unit 5 also contains a display for the settings and an
indication of individual rectifier set points.
The Distributed Control System DCS A also comprises a unit 6 where the
measured individual current is displayed which passes through each anode la,
lb, . . . , and a unit 7 displaying the measured voltage at the individual anodes 2a,
2b Both displays 6 and 7 indicate alarms when limits the operation limit
curve of current and voltage would be exceeded.
The set points and correction factors are fed to the programmable logic
controller PLC 12 via data line 9, and transmitted through the lines 13, 14, 15
and other lines (not incorporated in figure 2) to other rectifiers (not incorporated
in figure 2 for the sake of clarity) from PLC to the rectifier control. The rectifier
control itself provides data relating to current and voltage measurements of theanodes through the lines 13, 14, 15 back to the PLC controller and PLC feds
back via lines 10 and 11 to the units 6 and 7 to be displayed.
Advantages of the invention, for example, avoiding short circuits and
avoiding total shut down of the electrolytic cell in case one anode causes
problems, relate to any type of electrode, e.g. nickel anodes, diamond coated
anodes and carbon anodes. Preferably, the anodes are carbon anodes.
Preferably, the electrolyzer apparatus of the invention contains equal to or
more than 3 electrolytic cells, preferably, equal to or more than 5 electrolytic
cells.
Preferably, the electrolyzer of the present invention contains equal to or
less than 15 electrolytic cells, preferably equal to or less than 10 cells.
Each electrolytic cell preferably contains equal to or more than 6 anodes,
more preferably, equal to or more than 10 anodes. Preferably, each electrolytic
cell contains equal to or less than 50 anodes. Preferably, each electrolytic cell
contains 20 to 30 anodes.
The apparatus further may comprise a programmable logic
controller (PLC) and/or a distributed control system (DCS). For a smaller and
simple apparatus, a PLC may not be necessary ; or, alternatively, no DCS is
necessary, but just a PLC is provided in the apparatus. Often, especially in larger
apparatuses, it is preferred to provide both a PLC and a DCS.
In a preferred embodiment, the electrolyzer apparatus of the invention
contains at least one distributed control system DCS. The distributed control
system DCS may be a mimic board or mosaic board or may be represented by a
computer system. The preferred distributed control system is realized with
computers. The programmable logic controller PLC serves to handle, monitor
and survey data concerning the set points assuring that this values cannot exceed
technological limits in normal operation (minimum/maximum level of voltage in
dependence of current of each individual anode), input for technical limits of set
values, input for correction factors for individual anodes, input of overall set
points, especially the minimum or maximum level of total current passing
through all anodes and cells, reception of measurements values related to
foresaid signals and alarm handling.
The apparatus further comprises a programmable logic controller PLC.
The programmable logic controller PLC receives information from rectifiers
delivering measured parameters (individual current and voltage) of the respective
anodes. The programmable logic controller PLC compares the measuredparameters with the settings and set points provided by distributed control
system DCS. Depending on the result of the comparison, the PLC does not
prompt any change or it detects an overrun of preset limits. If the comparison of
settings and measurements gives a deviation which is outside the correction band
- for example, if the deviation of setting value and determined value of current
and/or voltage is not in a predetermined tolerance band, but, e.g. greater
than 5 % -, the PLC will prompt the shutdown of the respective anode for
maintenance or repair. It may also send an acoustic or visual signal, e.g. to the
control board.
A broken anode is often indicated by a significant increase of voltage
operating the anode at preset current. In complement a short circuit of an anode
can be detected in the electrolytic cell when the current exceeds its limits and the
voltage drops below limits. Such short circuits are generated often by fragments
of anodes swimming inside the electrolyte, as these could be conductive carbon
pieces.
The term "tolerance band" denotes a specific acceptable minimum value of
each parameter, and a specific acceptable maximum value of each parameter. If
the parameter or parameters are within this tolerance band, no action of the PLC
is necessary. For example, the tolerance band for the DC voltage may be set to 4
to 12 V. The tolerance band for the current may be set to 5 to 250 Ampere.
Preferably, the tolerance bands for DC voltage and current are linked. For
example, for a voltage of 10 V, the tolerance band for the current of 100
to 240 A may be allocated. Or, for a current of 200 A, the tolerance band for the
voltage of 9 to 11.5 V may be allocated.
In the following table 1, preferred tolerance bands for a current (given in
Ampere) for a given voltage (in Volt) are compiled :
Table 1 : Preferred tolerance band for current for a given DC voltage
Voltage [V] Tolerance band for current
Minimum [A] Maximum [A]
4 > 0 8
6 2 40
7 10 80
8 40 120
9 65 190
10 100 240
11 160 240In table 2, preferred tolerance bands for the voltage for a given current are
compiled :
Table 2 : Preferred tolerance band for DC voltage (in Volt) for a given
current (in Ampere) :
It is preferred that the voltage remains in the indicated preferred tolerance
bands. It is especially preferred that the current remains in the indicated
preferred tolerance bands. If the current is outside the tolerance band for a given
voltage, the voltage of the respective will be increased or decreased so that the
current is then within the tolerance band. In the preferred case the current will be
controlled by a closed loop PID regulator for each anode (PID = Proportional,
Integral, Differential), comparing the current set-point and the measured current
and sending, in case of deviation, a re-adjusted current set-point to the rectifier.
The PID regulator can be realized inside the (commercialized) rectifier or outside
or inside in the PLC. In the preferred case the closed loop PID is realized inside
the rectifier controller which regulates the closed current loop.
It has to be noted that the tolerance bands for voltage and current may vary
slightly from anode to anode, e.g. from the geometric form of the anode, the
composition of the anode, and the surface of the anode or the geometrical form
of the cathode around the anode. The advantage of the electrolyzer apparatus of
the present invention is that the properties of each individual anode can be taken
into account when setting the tolerance bands and that specified operationconditions can be closely monitored and controlled inside their tolerances
obtaining a well specified electrolytic product.
In a preferred embodiment, the PLC is programmed in such a way that if
the measured parameter of a specific anode deviates by more than a preset level
from the upper or lower limit of the tolerance band, the respective anode will be
shut down. It is especially preferred that the anode will be shut down if the
measured parameter deviates by more than a preset level from the upper limit of
the tolerance band because this indicates a failure of the respective anode. For
example, the shutdown level of the deviation may be set to equal to or more
than 10 % from the upper limitation of the tolerance band ; this shutdown level is
called the "divergence factor" in the present application. Preferably, the
shutdown level is set to a deviation of equal to or more than 5 % of the upper
limit of the tolerance band. Thus, if the voltage or current must be decreased
by 5 % or more of the upper limiting value of the tolerance band, then the current
passing through the respective anode will be stopped, and the anode, the bars,
connections etc may be inspected for repair or substitution. Often, a broken
anode will be the cause if the current and voltage are outside the upper limit of
the tolerance band. As mentioned above, such irregularities may result in an
unacceptable production of side products like CF4. The electrolyzer apparatus of
the invention allows for the selective shutdown of single irregularly operating
anodes without the necessity of complete interruption of F2 production.
The PLC often will also contain an on/off logic. This on/off logic controls
the individual anodes and rectifiers to safeguard a smooth start up phase and a
smooth shutdown phase.
If desired, the PLC may also comprise again logics for other features, for
example a manual shutdown of a single anode when operator observes other
technical issues like contact surface overheating.
If desired, the PLC may also comprise again functionalities for other
features, for example comparing well proven operation conditions (in example
proven research results of operation conditions versus product quality) with
present measured operation conditions, again there could be parameter adapted
tolerance bands ( like electrolyte temperature adapting the tolerance band) to
improve the electrolyzer. Such logic can be realized preferably by calculated
reaction functions and comparators or a fuzzy logic.
The PLC preferably also comprises safety ramps, e.g. 1A/minit to prevent
the cell against spontaneous gazing effects.Each rectifier preferably comprises at least one device measuring
parameters of each anode wherein the at least one device is selected from the
group consisting of a DC measurement device, a current closed loop control
device, a voltage measurement device, a DC current short circuit protection, and
a DC voltage overvoltage protection. The parameters obtained by the respective
measuring device or devices are sent, preferably on-line, to the PLC needed to
determine if a correction must be prompted for any of the anodes or even a
shutdown as indicated above. In a preferred embodiment, the DC measurements
are realized inside the rectifiers ; such rectifiers are commercially available.
The electrolyzer apparatus of the present invention may further comprise
devices to measure safety-related parameters. For example, the apparatus may
comprise one or more pressure detectors ; one or more detectors for the ambient
temperature in the apparatus, the electrolyzer liquid, the anodes or lines for the
electric current ; one or more fire detectors or smoke detectors, e.g. one or more
"very early smoke detection apparatus" (VESDA). These safety related
parameters are preferably sent to the central control system or PLC which may
trigger an acoustic alarm, a visual alarm, the shutdown of single or all anodes,
single or all cells or even the complete electrolyzer apparatus, fire fighting or fire
preventing actions, e.g. flooding the apparatus with inerting gases, e.g. nitrogen,
carbon dioxide, or hydrofluorocarbons, e.g. C2HF5 or C3HF7, or mixtures
thereof.
The apparatus as described above provides a safe, steady and reliable way
of producing pure fluorine. If desired, the apparatus may comprise two
redundant central control systems. This guarantees safety and reliability even if
one control system should fail.
If desired, and to simplify the control system, two rectifiers can be
assembled in a dual rectifier housing ; in this embodiment, two anodes can be
addressed with one dual rectifier controller containing a single communication
port and in present example several dual rectifiers can be connected inside one
bus segment to the PLC's bus controller. If desired, the electrolyzer apparatus of
the present invention may comprise a bus, e.g. available under the name
Profibus DP®, which connects the rectifiers to the programmable logic
controller PLC. or to the distributed control system DCS when comprising such
PLC functions.
Some advantages of the electrolyzer apparatus of the present invention
(e.g. reliability, steady F2 production, decreased risk of contamination withundesired side products) are given above. Another advantage is that no anode
bus bar, no central bus bar system, short circuit switches, another rectifier for
conditioning the cells during starting phase of the electrolytic process or a central
rectifier system is necessary. Compared to the classical design with one
common rectifier for several cells in combination with a conditioning rectifier, in
present case each rectifier controls its own the cell conditioning procedure and
the normal operation mode.
The apparatus operates different from the apparatuses known from the art.
In the known apparatuses, overall settings were applied for the totality of
anodes ; the total current was observed, and it was regulated by the voltage
applied to all the anodes. According to the apparatus of the invention,
preferably, the level of the current of each individual anode is set and maintained
in a set level range or to a set level by varying the voltage. One further
advantage is that each of the multitude of rectifiers operates anodes at a well
defined current level without significant tolerances, whereas in classical design,
there are always anodes, due to variation of above mentioned resistances, which
take much more current then others. In the end the highest loaded anodes
determine the overall cell stream factor and the anode lifetime.
Thus current density at the anode surface can be better adjusted by present
optimized current control compared to classical installations.
Thus, the overall cell stream factor due better equilibrated to anode current
limits is expected to be higher.
The electrolyzer apparatus of the present invention can be applied in any
manufacturing unit for producing elemental fluorine. It is, as mentioned above,
especially suitable for the manufacture of pure fluorine applied as etching gas or
chamber cleaning gas in production plants for the manufacture of
semiconductors, MEMS, TFTs for flat panel displays and photovoltaic cells.
Often, it is desired to produce fluorine "on site" or "over the fence" of such
production plants. "On site" means that the fluorine producing apparatus is
integrated in the production plant. F2 is provided via respective lines to the point
of use. "Over the fence" means that the fluorine producing apparatus is close to
the plant, but separated from it, e.g. by fences. This enhances safety because
unauthorized persons can be kept off the premises easily. Further, transports
of F2 e.g. via road are not necessary as the F2 is produced directly besides the
consumer-plant, e.g. photovoltaic-plant.Figure 3 provides a scheme of an electrolyzer apparatus of the present
invention, comprising an electrolytic cell C, a multitude of rectifiers la, lb, . . .
to If A respective multitude of anodes 2a, 2b to 2f are individually connected
via lines 4a, 4b . . . to 4f to one rectifier two of which are grouped as dual
rectifiers. The distributed control system and a programmable logic controller
are combined in one housing B', which is a BPCS. Line 3 provides the
connection to the cell vessel which forms the cathode. The set points and
correction factors are
transmitted through the lines 13, 14, 15 and other lines (not incorporated in
figure 2) to rectifiers as also indicated in figure 2 .
A scheme for a plant for producing pure fluorine using the electrolyzer
apparatus of the present invention is given in figure 4 .
The plant described in figure 4 is especially suitable for the manufacture of
pure fluorine for the application in the manufacture of TFTs, MEMS,
semiconductors, photovoltaic cells, and the cleaning of chambers used especially
in these processes.
Liquid F F is stored in buffer tank 1. The liquid F F in the tank is
pressurized with N2 and transports the liquid FIF to the FIF-evaporator which is
located between buffer tank 1 and cells 2, but was not given an individual
reference in figure 4 . In the HF-evaporator, the liquid HF is evaporated and
being sent in gaseous phase to the electrolytic cells 2 . 4 cells are shown in
figure 4 but it must be kept in mind that the plant may comprise more cells.
In emergency cases, produced F2 can be passed via a hydraulic-seal-system
(filled with PFPE-oil = perfluoropolyether as sealing-liquid) or via the settling
boxes on the electrolytic cells (for the settling of electrolyte in the gas-stream) to
a decomposition unit 3 comprising a destruction tower, preferably a wet
scrubbing system where it is decomposed chemically, e.g. with alkali lye which
additionally may comprise alkali metal thiosulfate, (for the off gas lines
containing F2 and HF from the F2-side of the electrolytic cells), and another
scrubber in series for the off-gas from the F2 lines as redundant scrubber and for
the case of emergency.
H2 produced is advantageously passed through a settling-box 4 on the
electrolytic cells (for settling of electrolyte in the gas-stream) and cleaned in
abatement unit 5 (preferably a caustic water scrubbing system for HF in the H2
gas stream). The purified H2 may then be released to the atmosphere. The F2
produced is passed through a separator into a purification unit 6 where it is firstcontacted with cold liquid HF in a HF scrubber to remove entrained solids,
mainly entrained solidified electrolyte salt. After leaving the F F scrubber, the F2
is passed through a heat exchanger which is cooled to about -80°C to remove
entrained FIF by condensation. Any residual FIF is removed in two NaF
towers 7 . Highly pure F2 leaving the NaF towers 7 is collected in a buffer tank 8
from which it may be withdrawn though a filter 9 for solids.
The NaF towers 7a and 7b are redundant. The NaF-towers 7a and 7b
contain a pair of NaF-towers (two towers installed on a trolley for easy removal
and exchange out of the skid-installation). If one of them is loaded with
absorbed HF, it can be regenerated by passing N2 or other inert gas at elevated /
high-temperatures from line 10 through it.
The electrolyzer apparatus of the present invention is indicated by
reference sign 11. It includes the cells 2 including the anodes (not shown in
figure 4), the housings 12 for the rectifiers, a distributed control system DCS 13
and a programmable logic controller 14. The multitude of anodes in the cells 2
and the multitude of rectifiers in the rectifier housings 12, one rectifier connected
to one anode, are not shown for the sake of simplicity, but are part of the
electrolyzer apparatus, of course. If desired, two rectifiers may be joined in a
dual rectifier as mentioned above.
The electrolyzer apparatus can be assembled in the form of a skid. In this
embodiment, parts of the electrolyzer apparatus (for example, rectifiers and
electrolyzer vessel including the anodes, lines providing HF, and lines to
withdraw F2) are mounted in a skid.
According to a preferred embodiment, the electrolyzer apparatus of the
present invention is integrated in a fluorine producing plant according to the
"skid concept". The term "skid concept" denotes a plant wherein parts of the
plant are assembled in separate skids. The advantage is that the skids can be
manufactured and tested in a factory by experienced workers, are sent skid by
skid to the site where fluorine will be produced, and are assembled directly on
that site. Such a concept is described in co-pending patent applications
US 61/383533 and US 61/383204, later (on September 12, 201 1) re-filed as PCT
patent application having the filing N° PCT/ EP201 1/065773 the whole content
of which three applications is incorporated herein by reference for all purposes.
Such a plant comprises skid mounted modules including at least one skid
mounted module selected from the group consisting of- a skid mounted module comprising at least one storage tank for HF, denoted
as skid 1,
- a skid mounted module comprising at least one electrolytic cell to produce F2,
denoted as skid 2, which corresponds to the electrolyzer apparatus of the
present invention,
- a skid mounted module comprising purification means for purifying F2,
denoted as skid 3,
- a skid mounted module comprising means to deliver fluorine gas to the point
of use, denoted as skid 4,
- a skid mounted module comprising cooling water circuits, denoted as skid 5,
- a skid mounted module comprising means to treat waste gas, denoted as
skid 6,
- a skid mounted module comprising means for the analysis of F2, denoted as
skid 7, and
- a skid mounted module comprising means to operate the electrolysis cells,
denoted as skid 8 . This module corresponds to or comprises the central
control system.
The plant preferably also comprises skid modules which may be located
close to the skid modules 1 to 8 but may be separated from them, namely
- a skid module 9 which is an electrical sub-station mainly to transform
medium voltage to low voltage
- a skid module 10 which houses utilities (control room, laboratory, rest room).
At least, skids 1, 2, 3, 4 and 7, preferably all skids, comprise housings for
safety reasons.
Another aspect of the present invention is a method for the manufacture of
elemental fluorine wherein the electrolyzer apparatus of the present invention
can be applied.
The method of the present invention for the electrolytic manufacture of
fluorine by electrolysis of a molten composition comprising HF and an
electrolyte salt wherein an electrolyzer apparatus is used comprising at least one
electrolytic cell which contains at least two anodes, a metallic cathodic vessel,
and a number of rectifiers such that each anode is allocated to one rectifier ; and
each rectifier is allocated to one anode. The term "allocated" includes the
meaning "connected".
Preferably, the method of the invention is performed with preferred
embodiments of the electrolyzer apparatus as described above ; especially, it isperformed in a plant according to the skid concept as described in US 61/383533,
filed September 15, 2010, and US 61/383204, filed September 16, 2010, the
priority of which applications was claimed in PCT patent application filed on
September 12, 201 1 having the filing N° PCT/ EP201 1/065773.
Preferably, the molten composition to be electrolyzed has the approximate
composition KF-(1.8-2.3)HF.
Should the disclosure of any patents, patent applications, and publications
which are incorporated herein by reference conflict with the description of the
present application to the extent that it may render a term unclear, the present
description shall take precedence.
The invention, and specifically the use of the electrolyzer apparatus and
the method of the present invention in a method for the manufacture of fluorine,
will now be described in further detail in view of an example describing a
preferred apparatus and fluorine production method.
Example
The electrolyzer apparatus which may be mounted in a skid comprises 1
electrolytic cell which contains 26 anodes. The apparatus contains 13 dual
rectifiers with a capacity of 12V/250 A, each individual rectifier of the dual
rectifiers was connected to one anode. Each rectifier contains a DC current
measuring device (e.g., an ampere meter), a voltage measuring device (e.g. a
voltmeter) wherein current and voltage measurement may be provided by a
single device, a short circuit protection based on which may be set to a shut
down limit of, for example, 250 A, a DC voltage overvoltage protection which
may be set, for example, to 15 V, a digital bus connection between the anodes
and a central anode control unit exchanging rectifier set up parameters. For each
individual anode, the current loop control of the rectifier, be it closed or open, is
steered by the distributed control system DCS and the PLC via a current set point
and a manually set correction factor which are entered in the DCS. These data
are managed in the PLC, a central anode control unit, which was a programmed
Siemens S7-300 PLC controller. The DCS sets and the PLC controls the
required current level for each rectifier, applies the respective correction factor
for each anode, and allows reducing the electric charge of a particular anode,
compared to the charge of others.
The tolerance band preferably is set to a direct current voltage
(VDC) 12 V, and the direct current (IDC) is set to 240 A, as a reference.Electrolyte salt of an approximate composition of KF-2HF is filled into the
electrolytic cell vessels and is molten therein (at a temperature of about 85
to 100°C). Electric supply is switched on for the rectifiers, and the electrolytic
process is started. The PLC, the central anode unit, keeps voltage and current of
each individual anode with the tolerance band entered into the central control
unit for the specific anode. When electrolysis has started, the molten electrolyte
salt is heated, and respective cooling is advantageous. By feeding appropriate
amounts of HF into the vessel, the level of molten electrolyte is kept in a preset
range. During the electrolysis, F2 and H2 are formed and are withdrawn
separately from the cells. H2 from the cells is fed into a common line, is diluted
with inert gas and decomposed or passed into the atmosphere. Only during start
up-phase (conditioning-process of the electrolyte-mixture) or in case of
emergency, the F2 is sent to the F2 destruction unit / F2 abatement system. The
F2 formed is collected in a common line and purified. Often, it contains
entrained solids, mainly solidified electrolyte salt. The purification can be
performed as described in unpublished European patent application
N°10172034.0. According to the process for the fluorine purification described
therein, the fluorine is contacted with liquid hydrogen fluoride, which preferably
has a temperature equal to or higher than -83 °C, more preferably, equal to or
higher than -82°C, and preferably equal to or lower than -40°C. To provide
highly pure F2 as preferably applied in the manufacture as etching or chamber
cleaning gas as mentioned above, it is subjected to a further purification
treatment which preferably comprises at least one step of low temperature
treatment to provide highly pure fluorine and optionally, an additional step of
contacting the fluorine after the low temperature treatment with an adsorbent
for FIF, e.g. NaF, an additional step of passing the fluorine after the low
temperature treatment through a filter, or both. In the deep temperature
treatment, FIF entrained is removed from F2 by condensation or freezing it out.
The low temperature treatment is preferably performed at a temperature equal to
or lower than the freezing point of HF at the respective pressure ; often, at a
temperature equal to or lower than -82°C. The purified F2 is then filled into a
storage tank or forwarded to the tool where in it is used as etching gas or
chamber cleaning gas.
If the current or voltage measurement devices detect that current or voltage
are outside the preset tolerance band by a percentage which is higher than the
correction factor which preferably is set to 5 %, a digital shutdown command issent to the central anode control unit which distributes the shutdown command to
the respective anode rectifier or anode rectifiers connected with the respective
anode which shut down the current and thus stop the electrolytic process of this
or these individual anodes.
In this example, an alarm was sent to the operator identifying directly the
defective cell and anode. The faulty anode or anodes can then be identified and
repaired or substituted.
Another aspect of the present invention is a rectifier/anode system for the
manufacture of F2 by electrolysis of KF-(1.8 - 2.3)HF. The rectifier/anode
system comprises at least two carbon anodes and at least two rectifiers each of
which is individually connected with a rectifier.C L A I M S
1. An electrolyzer apparatus for the electrolytic manufacture of elemental
F2 from an electrolyte comprising at least one electrolytic cell which contains at
least two anodes, a metallic vessel, which serves as cathode, for the electrolyte,
and at least two rectifiers such that one rectifier is allocated to one anode.
2 . The electrolyzer apparatus of claim 1wherein the anodes are carbon
anodes.
3. The electrolyzer apparatus of claim 1 or 2 which contains equal to or
more than 5 electrolytic cells.
4 . The electrolyzer apparatus of anyone of claims 1 to 3 in which each
electrolytic cell contains 20 to 30 anodes.
5 . The electrolyzer apparatus of anyone of claims 1 to 5 wherein each
rectifier is connected to a control unit which individually controls each rectifier.
6 . The electrolyzer apparatus of claim 5 wherein the rectifier comprises
at least one device which measures parameters of each anode wherein the at least
one device is selected from the group consisting of a DC measurement device, a
current closed loop control device, a voltage measurement device, a DC current
short circuit protection, and a DC voltage overvoltage protection measured in the
rectifier and detected inside the PLC for each anode.
7 . The electrolyzer apparatus of anyone of claims 1 to 6 comprising a
programmable logic controller (PLC).
8 . The electrolyzer apparatus of anyone of claims 1 to 7 the apparatus
further comprises a distributed control system (DCS) and/or a safety shutdown
system.
9 . The electrolyzer of anyone of claims 1 to 8 which comprises a safety
controller connected to at least one device selected from the group consisting of
gas pressure detectors, temperature detectors, fire detectors, and smoke detectors.
10. The electrolyzer of claim 9 comprising at least two independent safety
controllers.11. The electrolyzer apparatus of anyone of claims 1 to 10 wherein two
rectifiers are grouped into one dual rectifier.
12. The electrolyzer apparatus according to anyone of claims 1 to 11
which is arranged in the form of a skid.
13. A method for the electrolytic manufacture of fluorine by electrolysis
of a molten composition comprising HF and an electrolyte salt wherein an
electrolyzer apparatus is used comprising at least one electrolytic cell which
contains at least two anodes, a metallic cathodic vessel, and a number of
rectifiers such that each anode is allocated to one rectifier.
14. The method of claim 13 wherein the molten composition has the
approximate composition KF-(1.8-2.3)HF.
15. The method of anyone of claims 13 and 14 wherein the electrolyzer
apparatus comprises at least one feature as set out in claims 2 to 12.
16. The method of anyone of claims 13 to 15 wherein the level of the
current of each individual anode is set and maintained in a set level range by
varying the voltage.